Note: Descriptions are shown in the official language in which they were submitted.
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WDM NETWORK WITH CONTROL WAVELENGTH
Field of the invention
This invention relates to an optical communications system, and to terminal
equipment forming part of such a system. More particularly, this invention
relates to
an optical communications system comprising a plurality of terminals,
interconnected
by a light path (particularly but not exclusively, an optical fibre cable) and
arranged
to signal using selected wavelengths from a plurality of possible simultaneous
signalling wavelengths.
Background Art
Optical communications systems are suitable for applications in which large
volumes of data are to be communicated between stations, such as local area
networks
(LANs), or Metropolitan area networks (MANs). It is known to provide such
networks using optical fibre cable to interconnect network stations, and
signalling on
selected ones of a plurality of wavelengths using wavelength division
multiplexing, or
(WDM).
One example of such a system, and specifically a terminal station, for such a
system, is described in the Proceedings of the Nineteenth European Conference
on
optical communication (ECOC'93) Volume 2, paper TuP4.4, pages 121-124, Sep.
12-16 1993, I Chlamtac et al "A Multi-Gbit/s WDM Optical Packet Network with
Physical Ring Topology and Multi-subcarrier Header Encoding" . In the system
there
described, each terminal station consists of a laser tuned to operate at a
single discrete
frequency (different to the frequencies of all other lasers of all other
stations in the
system), a subcarrier receiver tuned to operate at a single discrete
subcarrier frequency
(different to the frequencies of all other receivers of all other stations in
the system),
and a tuneable wavelength selector capable of selectively tuning to any of the
transmitter wavelengths. All the stations are connected by a single optical
fibre cable.
Data is communicated in packets, all having the same predetermined length.
When a
station wishes to transmit a packet, it transmits a header on the subcarrier
of the station
to which the packet is to be sent, and then sends the data on its transmit
wavelength by
using its laser diode, the output of which is then coupled to the fibre. At
the
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destination station, the header on a subcarrier is detected. The header
includes an
indication of the transmitting station, and therefore the transmitting
wavelength, and
this is used to tune the wavelength selector to the correct receiving
wavelength, and the
packet is received ("dropped") via the wavelength selector.
Because of the high bandwidth of optical fibres (or optical paths in general),
it
is possible to provide a reasonably large number of stations each operating at
a high
data rate using this type of wavelength multiplexing system.
Further, because every station has its own transmit wavelength, there is no
possibility of collision between data from different stations on the same
wavelength.
However, the system does have several drawbacks. Firstly, it requires every
station to have a different transmitting frequency, and this means either
manufacturing
a very large number of fixed frequency laser diodes of different frequencies,
or
providing a tunable laser at every station (which would require accurate
wavelength
stabilisation equipment at each station to avoid cross-talk between
wavelengths). The
same applies to the need for a separate subcarrier receiver for each station.
Finally,
the total number of stations must inevitably be limited to the total number of
available
wavelengths (and/or subcarriers).
A similar WDM system having stations provided on a bus, each having a fixed
frequency receiver and a tunable transmitter is disclosed in Journal of
Lightwave
Technology, vol. l l, no. 5/6, May 1993, New York US, pages 1104-1111,
XP396738,
S. Banerjee et al. ' FairNet: A WDM-based Multiple Channel Lightwave Network
with
Adaptive and Fair Scheduling Policy' .
A WDM system in which terminal stations utilise wavelength multiplexers and
demultiplexers is disclosed in IEEE infocom' 90, 3 Jun. 1990, San Francisco
US,
pages 1030-1037, K. Yamaguchi et al, ' A Broadband Access Network Based on
Optical Signal Processing: The Photonic Highway'
A star-connected WDM system is described in optoelectronic Interconnects,
vol. 1849, 18 Jan. 1993, Los Angeles US, pages 172-183, K. Ghose ' Performance
Potentials of an Optical Fiber Bus using Wavelength Division Multiplexing' .
In this
system, each station has a fixed transmission frequency (shared by several
stations) and
a receiver which receives all frequencies.
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EP 0497005 discloses a WDM system in which terminal stations are provided
on a ring bus. A supervisory station is also provided. Each terminal station
has a fixed
Frequency light source and receiver operating at a control wavelength, a
tunable light
source, and a receiver with a tunable optical filter.
Each terminal station wishing to transmit sends a reservation signal on the
control channels and the supervisory station checks for conflicts, allocates a
signalling
wavelength, and sends a signal indicating the wavelength to the transmitting
and
receiving terminal stations via the control channel.
IEEE Communications Magazine, vol. 31, no 2, February 1993, New York
US, pages 78-88, XP334606, R. Ramaswami ' Multiwavelength Lightwave Networks
for Computer Communication' , is a review paper disclosing various WDMA
protocols. Of these protocols, DT-WDMA (Dynamic Time - Wavelength Division
Multiple Access) is stated to provide terminal stations each having a fixed-
wavelength
light source and a tunable receiver. A common signalling wavelength is used by
each
terminal station to indicate subsequent transmission of data on the terminal's
fixed
transmission wavelength. This arrangement would lead to destination conflict,
and so
an arbitration algorithm is required. It is stated that, in this class of WDM
network,
typically at least as many wavelengths as there are terminal stations
required.
Journal of Lightwave Technology, vol. 10, no. 11, November 1992, New York
US, pages 1688-1699, XP355283, K. Boginemi et al, ' A Collisionless Multiple
Access
Protocol for a Wavelength Division Multiplexed Star-Coupled Configuration:
Architecture and Performance Analysis' discloses a WDM network coupled in a
passive star configuration, comprising a plurality of star-connected terminal
stations
each employing a tunable optical transmitter and a tunable optical receiver,
together
with a fixed optical receiver for monitoring a common control channel. The
access
protocol is referred to as ' TDMA-C' . Each terminal station transmits an
indication
on the control channel of a future data packet transmission on one of the WDM
wavelengths. All terminal stations monitor the control channel, to detect
messages
intended for themselves, and to avoid channel conflict or destination
conflict.
EP 0452895 discloses an optical network system which comprises a plurality
of terminal stations interconnected by an optical fibre cable. In a first
embodiment, a
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base station transmits a plurality of different wavelengths. A first
wavelength variable
filter continually scans all the wavelengths, to attempt to fmd a free
wavelength. When
a free wavelength is found, the current setting of the first wavelength
variable filter is
used to set a second wavelength variable filter which extracts the free
wavelength. The
extracted free wavelength is modulated by an optical modulator, and recombined
with
the other wavelengths in a multiplexes. The initial part of the data
transmitted by the
modulator is an indication of the destination station for the data. All
stations, therefore,
also scan all the wavelengths to attempt to locate such a header indicating
that data is
addressed to them. When such a header is located, the second wavelength
variable
filter is set to the wavelength on which the header occurred, and the
subsequent data
is demodulated using a photosensor.
In the second embodiment, the method of reception of data is as in the first
embodiment. The method of transmission of the data from a station differs,
however,
in that, instead of using an optical modulator to modulate the extracted free
wavelength, two laser diodes are employed to generate free wavelengths which
are
modulated by optical modulators and multiplexes into the signals on the
optical fibre.
As before, a wavelength variable fitter sweeps the available wavelengths to
search for
a free wavelength, and the laser diodes are set to the or each free
wavelength. The
laser diodes are stabilised by the transmission, from the base station, of a
reference
wavelength which is extracted by a separate wavelength variable filter, and
used to
control the laser diodes at each station.
Both embodiments thus avoid the need for every station to have a different
transmitting frequency and a different receiving frequency, and hence either
large
numbers of laser diodes or temperature stabilisation at each station
(although, in the
second embodiment, some wavelength stabilisation circuitry is needed).
However, this is achieved only by sacrificing a major advantage of the
Chlamtac system above; namely, its immunity from collision. In the system of
EP 0452895, collision is highly likely because all stations are simultaneously
scanning
the free wavelengths in order to be able to transmit data. Thus, several
stations may
simultaneously detect that the same wavelength is free, and attempt to
transmit data at
the same time. Obviously, in this instance, all the transmitted data on that
wavelength
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will become corrupted. For this purpose, EP 0452895 proposes to use the
transmission
protocol known as carrier sense multiple access/collision detection (CSMA/CD),
in
which stations detect collision of data and attempt re-transmission. However,
this in
turn can lead to repeated collisions, as the re-transmissions themselves
collide; and, in
any case, leads to delay in the transmission of data, and the need for further
complicated circuitry to deal with the control of the collision protocols.
Furthermore, because each station needs continually to scan all frequencies to
determine the wavelength on which data for that station may be transmitted,
the rate
of transmission is limited by the rate to scanning of the wavelength variable
filter and
the number of wavelengths to be scanned; since, if a destination indicating
header is
only scanned part way through, the receiving station may not correctly decode
the
destination, and accordingly may not decode the signal. For this reason, some
form
of acknowledgment signalling, and associated re-transmission of data, would
appear
to be increasingly necessary as the speed of
transmission or the number of wavelengths employed in this system increases.
Summary of the Invention
In one aspect, the present invention provides a wavelength division multiplex
transmission system utilising a plurality of data transmission wavelengths,
and a
dedicated control channel (which may be on a predetermined one of said
wavelengths,
which may be separate of all said data transmission wavelengths) in which a
plurality
of terminal stations are each arranged to signal forthcoming transmissions of
data on
one of the data transmission wavelengths by a signal on the dedicated control
channel,
and to respond to signals on the dedicated control channel to initiate
reception of a
signal, in which a head station is provided which
comprises a light source generating a wavelength division multiplexed optical
signal
including said plurality of data transmission wavelengths, and each of the
terminal
station consists of a modulator arranged to modulate a selected said data
transmission
wavelength.
Thus it is possible to provide a wavelength multiplexed communications
network in which the number of stations is not limited by the number of
wavelengths
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employed, and the number of light sources required in the system is not
limited by the
number of stations present. For example, it had been calculated to be possible
to
provide 1000 terminal stations using only four different data transmission
wavelengths
supported by a single head station comprising four light sources. This is to
be
compared with the above referenced prior art systems, where for DT-WMDA 1000
different signalling wavelengths (and hence differently tuned
transmitters) would be required, and for TDMA-C 1000 different tunable
transmitters
(one at each station) would be required. Thus, the need for expensive
wavelength
stabilisation equipment at every terminal station is avoided, and the channel
degradation caused by wavelength inaccuracy is reduced. Further, it is
possible to
co-locate the light sources at one head station (or a small number of head
stations),
which increased the ease with which wavelength stabilisation may be performed.
At
the same time, communication is conducted in a deterministic fashion, and
collision can
be substantially avoided. The stations are not required to have a tunable
receiver
arranged continually to scan all available wavelengths of a plurality of
wavelengths,
or a separately tuned subcarrier receiver.
In another aspect (which is preferably employed in the first aspect) each
terminal station or a WDMA network includes a wavelength separator means and
an
optical switching means arranged to switch a selected wavelength from the
wavelength
separator means to an optical data sender (for example a modulator) or
receiver.
Thus, in this aspect, wavelength demultiplexing and switching technology is
employed rather than a wavelength-tunable filter to route the selected
wavelength to
and from a receiver or sender. Such demultiplexing and switching technology is
stable, and fast in operation compared to several types of tunable optical
filter.
In another aspect, the invention provides a WDM network in which terminal
stations are linked by a bus (which may be connected in a ring) consisting of
a first
optical bus carrying light in a first direction to the sequence or terminal
stations along
the bus, and a second optical bus carrying light in the reverse direction
along the
sequence of terminal stations, each terminal station having a first side for
communicating with the first bus and a second side for communicating with the
second
bus.
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Thus, signalling access by a terminal station at one end of the bus is as good
as access at the other end of the bus where, for example, a signalling
protocol as in the
first embodiment or the invention enables successive terminal stations in the
bus to
progressively reserve wavelengths.
A terminal station for a bus of this kind preferably comprises, in a further
aspect of the invention, a cross connection means for linking the two buses.
Thus, in
the event of failure of one or more or the buses, or one or more of the
terminal
stations, the network can be reconfigured to connect the two buses as a single
looped
bus at one or both sides of the failure.
In a yet further aspect of the invention, a plurality of bus structures (for
example according to earlier aspects of the invention) are connected to share
common
light sources (i.e. the buses are connected in a star configuration).
Thus, the number of light sources required to operate a very large number of
terminals is reduced yet further. Preferably, in this embodiment, separate
buses are
arranged to communicate with each other via one or more lateral connection
optical
buses.
According to a further aspect of the invention, a connection station for
interconnecting two optical buses consists of a first optical add/drop
terminal in
communication with a first of the buses and a second optical add/drop terminal
in
communication with a second of the buses, each of the terminals having an
electrical
input port and an electrical output port, in which the electrical input port
or one
terminal station is electrically cross connected to the electrical output port
of the other,
and vice versa.
By providing electrical interconnection between the two terminal stations, the
requirement for an optical memory buffer to buffer data passing between the
two buses
is avoided.
Other aspects, embodiments and preferred features of the invention are
substantially as described or claimed hereafter.
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Brief Descriptions of the Drawings
The invention will now be described in greater detail, by way of example, with
reference to the accompanying drawings, in which:
Figure 1 is a block diagram showing schematically a bus arrangement of a first
embodiment of the invention;
Figure 2a is a block diagram showing schematically the elements of a head
station according to the first embodiment;
Figure 2b is a block diagram showing schematically the elements of a terminal
station according to the first embodiment;
Figure 3 is a diagram indicating the wavelengths used for transmission in the
first embodiment;
Figures 4a - 4f are diagrams showing the contents of each of the wavelengths
of Figure 3 over time, at progressive stages through the system of the first
embodiment;
Figure Sa is a block diagram corresponding to Figure 2a, and showing a head
station of the first embodiment in generalised form;
Figure Sb is a block diagram corresponding to Figure 2b, and showing a
terminal station of the first embodiment in generalised form;
Figure 6 is a block diagram showing schematically the construction of a head
station of a second embodiment of the invention;
Figure 7 is a block diagram showing schematically the structure of a terminal
station of a third embodiment of the invention;
Figure 8 is a block diagram showing schematically the structure of a terminal
station of a fourth embodiment of the invention;
Figure 9 is a block diagram showing schematically the structure of a terminal
station according to a fifth embodiment of the invention;
Figure 10 is a block diagram showing schematically the structure of a terminal
station according to a sixth embodiment of the invention;
Figure l la and l lb are diagrams showing the contents of the wavelengths of
Figure 3 over time at progressive intervals in a network according to the
embodiment
of Figure 10;
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Figure 12 shows a double bus network arrangement according to a seventh
embodiment of the invention;
Figure 13 shows a double looped bus arrangement according to an eighth
embodiment of the invention;
Figure 14 shows a single looped bus arrangement according to a ninth
embodiment of the invention;
Figure 15 is a block diagram illustrating schematically the employment of a
terminal station according to a tenth embodiment of the invention to link
between
several different communications networks;
Figure 16 is a block diagram illustrating schematically a double bus network
arrangement according to an eleventh embodiment of the invention;
Figure 17a and 17b are block diagrams of the eleventh embodiment showing
the operation of that embodiment in isolating a defective terminal station;
Figure 18 is a block diagram of a star network arrangement according to a
twelfth embodiment of the invention; and
Figure 19 is a block diagram of a star network comprising a thirteenth
embodiment of the invention.
Description of the Preferred Embodiments
First Embodiment
Referring to Figure 1, in a first embodiment, a wavelength division multiplex
(WDM) local area network (LAN) system comprises a plurality of terminal
stations la,
lb, . . . In and head station 2. The head station 2 and the terminal stations
1 are
connected in a chain configuration by an optical cable 3 comprising lengths of
optical
fibre 3a, 3b, 3c. . . In this embodiment, each station 1 is capable of passing
data to
each downstream station. Thus, this embodiment is useful where the terminal
station
la nearest to the head station 2 is to broadcast data to all other stations lb
. . . ln; or
where a plurality of stations la, lb . . . are all to transmit
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data to a single station in (the furthest from the head station 2), as for
example where
a number of sensors are to communicate data to a data logging station, or
where a
plurality of computer terminals are to communicate with a single printer.
Connected to each of the terminal stations la, lb . . . In is a respective
data
utilising apparatus 100a, 100b, . . . 100n (for example, a computer, a printer
or a
sensor) .
The terminal stations 1 each communicate with their respective terminal
apparatus 100 by electrical input and output ports 11, and each terminal
station carries
an optical input port for receiving an optical cable 3 and an optical output
port for
receiving an optical cable 3, (except that the last terminal station in,
furthest from the
head station 2, only requires an optical input port).
Referring to Figure 2a (in which only one terminal station 1 is shown for the
sake of clarity), in this embodiment, the head station 2 comprises a plurality
(e.g. five)
laser diodes 6-0 to 6-4, each generating light at a corresponding frequency
~.o - ~.4 as
indicated in Figure 3. Each of the laser diodes 6 generates continuous wave
(CW)
light, and a control circuit 4 is provided to modulate the output of the laser
diodes 6.
The wavelength ~,o is used as a signalling wavelength for control signals, and
the wavelengths ~,, - ~,4 are used as data transmission wavelengths.
The outputs of the laser diodes 6 are combined by a combining device 7,
functioning as a wavelength multiplexer, which preferably comprises a grating
device
(e.g. a grating filter) receiving the light from the lasers at different
incident angles,
such that all the light leaves at a common diffraction angle, which light is
launched
into a light path containing an optical amplifier 8, for example an erbium
doped fibre
amplifier as described in Fourth Optoelectronics Conference OEC ' 92 (Japan)
Invited
paper 1733-1, Technical Digest pages 281-283, B. J. Ainslie; "Erbium doped
fibre
amplifiers" . This amplifier 8 may be omitted if the output of the wavelength
multiplexer 7 is reasonably high.
A portion of the combined optical signal is tapped by an optical coupler 9,
and
fed back to a wavelength stabiliser circuit 10 which stabilises the wavelength
of the
laser diodes 6 and may be, for example, as described in IEEE Journal of Wave
Light
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Technology, Vol. 8, No. 3, pages 396-405, March 1990 S. Yamazaki et al; "A
Coherent Optical FDM CATV Distribution System" .
After passing through the coupler 9, the combined light signal is injected
into
the optical fibre 3a, through which it arrives via other terminal stations 1
(not shown)
and the optical fibre 3m, at the terminal station lm illustrated in Figure 2b.
After the fibre 3m is coupled to the optical input port (not shown) of the
station
lm, the light beam is amplified, by an optical pre-amplifier 12 (e.g. an
erbium doped
fibre amplifier) to compensate for signal losses in the fibre 3m, and is
wavelength
demultiplexed (i.e. separated into spatially distinct beams of different
wavelengths ~,o -
~,4) by a wavelength demultiplexer 13. The wavelength demultiplexer 13 is, for
example, a grating device which may be identical to the wavelength multiplexer
7 of
the head station 2, but in reverse configuration, so as to receive a signal
beam, and
split it into component wavelength beams.
As the optical fibre 3m will, in many cases, apply a chromatic dispersion to
the
light beam so as to delay different wavelengths by different amounts, there is
provided
a dispersion compensator 14 which consists, for each beam to different
wavelength,
of a length of optical fibre or other optical transmission medium. As the
longest
wavelengths are those most delayed by the chromatic dispersion of the fibre
optic cable
3m, the lengths of fibre within the dispersion compensator 14 are inversely
related to
the wavelengths of the beams ~,~, - ~,4
For example, if the optical fibre optic 3mhas a standard dispersion of 18
ps/nm/km and a length of 40 km, and if a wavelength spacing of 1 nm is used, a
delay
difference of 720 ps (equivalent to 44 mm of fibre) is necessary between
adjacent
wavelengths.
Thus, after each separated wavelength has passed through the respective length
of fibre in the dispersion compensator 14, the signals at all the wavelengths
~.o - ~,4 are
time aligned.
The signalling wavelength ~,o generated by the laser 6-0 at the head station
2,
is received at a photodiode (or other photosensor) receiver 17, where it is
converted
to an electrical signal which is supplied to a processor 18 (which may be a
microprocessor or an ASIC).
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The data transmission wavelengths ~,o - ~,4 are passed to a 4×4
non-blocking optical switch 15, so that any one of the four wavelengths at the
input
ports of the switch 15 can be switched to any one of the four output ports.
The
non-blocking optical switch 15 is a commercially available product described,
for
example, in IEEE proceedings--J, Vol. 139 No. 1, February 1993; J. E.
Midwinter;
"Photonics in Switching; the Next 25 Years of Optical Communications." The
switch
is controlled by a switch driver circuit 16 supplying electrical control
signals to the
switch 15 and the switch driver circuit 16 is controlled by the processor 18
in
dependence upon data received from the receiver 17.
The processor 18 in this embodiment is connected to a laser diode 19 which
emits light at the signalling wavelength ~,o.
On one of the output lines from the optical switch 15 is a coupler 20 (e.g. a
fused fibre coupler) arranged to tap a small part (e.g. 10% ) of the power of
the signal
on that line, which is supplied to a photodiode (or other optical) receiver
21, the
electrical output signal of which is supplied, via a gating circuit 22, to an
electrical
output (drop) port connected to the terminal apparatus 100.
Connected to another of the output lines of the optical switch 15 is an
optical
modulator 25 (which can, for example, simply have the structure of a two port
optical
switch, only one input and one output of which are connected) for modulating
the
optical signal on that output port in accordance with an electrical signal
supplied by an
electrical driver circuit 24 supplying a serial bit stream in accordance with
parallel data
held in a memory 23 connected to an add (input) electrical port 11 of the
terminal
station lm, for receiving signals from the terminal equipment 100.
The optical signal from the modulator 25 is combined with the signal from the
transmitter 19, the signal from the coupler 20, and the signals from the other
two ports
of the switch 15, after passing through a bank of attenuators 48 arranged to
compensate
for the drop in power caused by the modulation and tapping, so that all the
combined
wavelengths ~,o - ~.4 have equal power levels. The data transmission
wavelengths and
the signalling wavelength are then recombined by a combiner 26 (which may be
an
optical coupler) and the combined optical signal is amplified by an optical
amplifier 30
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(e.g. an erbium doped fibre amplifier) to take account of the losses of the
attenuator
bank 48, switch 15 and other components.
In this embodiment, one of the wavelengths is routed through the coupler 20
whether or not it contains data to be received. The processor 18 controls the
gate
circuit 22 to block the received data from the receiver 21 except where data
is being
received.
The operation of this embodiment will now be explained in greater detail.
Referring to Figure 2a, the control circuit 4 at the head station 2 generates
regular pulses of duration Tp separated by a guard time Tg so that the pulse
repetition
interval is T=TP +Tg. The pulses are applied to switch on and off the outputs
of the
data transmission wavelength laser diodes 6-1 to 6-4 (e.g. through modulators,
not
shown) so as to produce a slotted continuous wave signal from each as
indicated in
Figure 4a. The control circuit 4 also generates a digital code which modulates
the
signalling wavelength laser diode 60, the code indicating that all wavelengths
are free
for signalling. In this embodiment, this simple digital code is generated on
all
occasions except where, for example, one of the laser diodes 6-1 to 6-4 is
damaged;
in which case, the code indicating the identities of the wavelengths which are
available
for signalling is transmitted instead.
The transmitted code relates to the availability of wavelengths in subsequent
time slots; typically, the next time slot but possibly the next but one or
next but two
time slots (to allow the terminal stations la-lc more time to configure
themselves).
Referring to Figure 4b, when the optical signal arrives at the fourth terminal
station lm, the three preceding stations have already commenced sending data.
In the
first time slot (0 <_ time s T) the first station (la) has transmitted a data
signal to the
mth station lm of Figure 2b (in a manner which will be discussed in greater
detail
below) on wavelength ~,1 and the second station (lb) has transmitted a data
signal to
the pth station (lp) on wavelength ~.2. In the second time slot, (T <_ time ~
2T) the
third station has transmitted a message to the nth station on wavelength ~,, .
In the third
time slot, (2T <_ time <_ 3T), the second terminal station (lb) has
transmitted a further
data signal to the pth terminal station on wavelength ~., and the third
terminal station
(lc) has transmitted a message to the mth terminal station on wavelength ~.Z.
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It will be seen that, due to the chromatic dispersion of the fibre 3m to which
the
terminal station lm in Figure 2b is connected, the time slots in the different
wavelengths have become misaligned.
Referring to Figure 4c, after passing through the dispersion compensator 14,
the time slots are shown to be substantially realigned.
At this time, the terminal station lm of Figure 2b has in its memory a packet
of data to be sent to terminal station lp and a packet of data to be sent to
the terminal
station ln, both stored in the memory 23, having previously been received from
the
terminal equipment 100.
The receiver 17 in the terminal station 1 reads the digital signal carried by
the
signalling wavelength ~,o in the time period between t=-T and t=0, labelled
control 0
(m-1) in Figure 4c. This contains four digital words; one for each of the
transmission
wavelengths ~,, - ~,4 in the next time slot. The first word comprises an
indication of the
mth terminal station (for example a five bit signal) and a corresponding
indication of
the destination terminal station for the first wavelength ~,1 and in this case
indicates that
the first terminal station (la) is transmitting a message to this station lm.
Similarly the
second word indicates that the second station is transmitting a message to the
pth
station on ~,2.. The third and fourth words indicate that ~,3 and ~,4 are free
for message
transmission in the time slot extending between t=0 and t=T.
The processor 18 receives the electronic digital data generated by the
receiver
17, and extracts from it timing signals for synchronising the readout from the
memory
23 and read-in to the gate 22. The processor 18 studies the destination parts
of each
word, and matches these against its own stored terminal station number m. In
this
instance, a match is found in the word relating to the first wavelength ~,, .
Accordingly,
the processor 18 controls the switch driver 16 to set the switch 15 to route
the
wavelength ~,1 from its input port at the switch 15 through to the first
output port of the
switch 15, at which is located the coupler 20. Then, in the following time
slot from
t=0 to t=T, the processor 18 generates a gating signal 28 lasting the duration
of a time
slot to switch the electrical output signal of the receiver 21 to the
electrical output port
connected to the terminal equipment 100, so as to drop the data packet on the
wavelength ~,, .
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At the same time, the processor 18 has an indication that data is awaiting
transmission in the memory 23. Accordingly, the processor 18 notes the
occurrence
of the first word indicating a free wavelength, in this case ~.3 in the
following time
slot between t=0 and t=T.
However, since the packet to be transmitted in the memory 23 is destined for
terminal station lp and since the processor 18 has decoded the signalling
wavelength
~.o and decoded an indication that wavelength ~,2 already contains a data
packet
transmitted on the free wavelength in the next time slot, the terminal station
lp would
receive two messages simultaneously and only be able to decode one in this
embodiment.
Accordingly, to avoid this collision at the destination terminal station lp
the
processor 18 does not transmit on any of the free wavelengths in the next time
slot.
The processor 18 then controls the transmitter 19 to re-transmit the received
header data, in exactly the same form, on the signalling wavelength ~,o, in
the timeslot
between t=-T and t=0.
Since the total volume of information signalled on the signalling wavelength
~.o
is relatively low compared to that on the data transmission wavelengths, the
incoming
data will be received during a relatively early portion of the time slot
between t=0 and
t=T.
Because the processor 18 takes a finite processing time (TR) to read and
regenerate the signalling data (although the time may in fact be quite short,
since the
volume of data is low), it cannot write the data to exactly the right time
position in the
time slot. Accordingly, in this embodiment, a delay (5-1 to 5-4) of length
equal to TR
is positioned in the path of each data wavelength to bring them into time
alignment
with the (TR delayed) signalling wavelength ~.o. The delays (5-1 to 5-4) may
be lengths
of fibre, and could alternatively conveniently be provided combined with the
dispersion
compensator 14.
During the next time slot between t=0 and t=T, whilst the incoming data on
~,, is being dropped, the processor 18 reads the signalling channel ~,o and
detects a free
wavelength ~.2 in the following time slot from t=T to t=2T. Moreover in this
time
slot there is no conflicting message to terminal station lp. Accordingly, as
the
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CA 02239913 2000-08-O1
processor 18 will transmit data on this wavelength, it must re-write the word
on the
signalling wavelength ~,o. The processor 18 controls the laser diode 19 to
generate the
control signal indicating that wavelength ~.1 is occupied with a message from
terminal
station 3 to terminal station n; this portion of the message transmitted by
the diode 19
is identical to that received by the receiver 17. However, the processor 18
also adds
a message that the wavelength ~,2 will contain a message
from this terminal station lm to terminal station lp, and that only the
wavelengths ~,3
or ~,4 are now free.
Referring now to Figure 4d, during the time slot from t=T to t=2T, the
processor 18 controls the switch 15 to route the selected wavelength ~.2 from
its input
port of the switch 15 to the second output port of the switch 15 at which is
located the
modulator 25. The continuous wave signal input to the modulator 25 is then
modulated
in accordance with data packet stored in, and supplied from, the memory 23
which data
packet is read out as a serial bit stream under control of the processor 18
through the
driver circuit 24. Thus, as shown in Figure 4d, during the second time slot,
the
wavelength ~.2 is occupied by this transmitted data packet. The modulated
wavelength
is recombined with all the others through the combiner 26 for retransmission.
Whilst the data packet is being transmitted, in the time slot from t=T to 2T
the
receiver 17 decodes the control signal (control 2(m)) during the same time
interval and
notes that wavelength ~.3 is free in the following time slot from t=2T to
t=3T, and that
there is an incoming packet for that terminal station on wavelength ~,2.
Accordingly, the processor 18 transmits, via the laser diode 19, an amended
signal on the signalling wavelength ~,o indicating (in addition to the
previous data
received by the processor 18), that a packet from station lm to station In
will be
transmitted on ~,3 in the next time slot and that this wavelength is thus not
free.
In the next time slot between 2T and 3T, the processor 18 controls the switch
driver 16 to route wavelength ~,2 to the coupler 20, so that the packet on
that
wavelength can be dropped and routes the continuous wave modulated signal on
wavelength ~.3 through to the modulator 25, which modulates onto the
wavelength the
data packet for station n, the combiner 26 for transmission to the next
station.
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In this embodiment, as well as sending single packets of data from one
terminal
station to another, it is possible to broadcast data from one terminal station
to all
terminal stations further down the fibre (or from the head station 2 to all
terminal
stations 1).
Referring to Figure 4e, in this case, the control word on the signalling
wavelength ~.o in time t=0 to t=T indicates that in the following time slot, a
message
from the first station (la) to all stations is being transmitted. Accordingly,
the
processor 18 of each station will control the switch 15 to route wavelength
~.1 to the
coupler 20, to drop the wavelength at each station.
Further, if (as shown in Figure 2b) a selectable feedback path is provided
from
the gate 22 to the memory 23, wavelength translation may be performed at each
terminal station by receiving a data packet, routing the packet from the gate
22 to the
memory 23, and retransmitting the packet in a subsequent time slot. This may
be
useful where, for example, a failure of the switch 15 makes it impossible to
connect
one of the wavelengths (for example ~.,) to the coupler 20, so that packets on
the
wavelength ~,1 cannot be received at that terminal station. Accordingly, a
preceding
terminal station may perform wavelength translation to move the packet to one
of the
other wavelengths.
The effect of this is illustratively indicated in Figure 4f in which a data
packet
in the time slot T-2T from the third terminal station to the nth terminal
station has been
received on wavelength ~,, and retransmitted on wavelength ~.3 in the
following time
slot. Such wavelength translation may, for example, be instructed by a control
signal
on the signalling wavelength ~,o from the head station 2, or another terminal
station 1,
or may be preprogrammed into the processor 18.
Variations to the First Embodiment
Various modifications or substitutions can be made in the structure and
function
of the first embodiment. For instance, the wavelength multiplexers and
demultiplexers, instead of being grating interferometer devices, may be Mach-
Zender
devices, dielectric multiple thin film filters, or even optical couplers. The
optical
amplifier 8 may, instead of being a fibre amplifier, could be a semiconductor
optical
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CA 02239913 2000-08-O1
amplifier. Optical amplifiers in the head station 2 and terminal stations 1
may be
omitted if the losses in the wavelength multiplexers and fibres 3 are
sufficiently low,
or if the fibres 3 are of the self amplifying type (e.g. erbium
doped fibres), or if separate repeater stations are provided to amplify the
signals.
The laser diodes 6 may be substituted by light emitting diodes, gas lasers,
fibre
lasers or other suitable light source.
Naturally, other numbers of wavelengths are possible; Figure 5 illustrates the
generalised structure of the first embodiment for k wavelengths.
The guard time Tg is provided to allow for switching of the optical devices
(e.g.
switch 15) in the terminal stations 1, but it is possible to omit the guard
time and
provide continuous wave light, if no data transmission is actually performed
during the
switching time.
The signalling wavelength ~,o can furthermore be used to carry other
signalling
and control information concerning the operation, administration and
maintenance of
the network, or even low bit rate communication between terminal stations.
Data may
be transmitted on the signalling wavelength in the synchronous digital
hierarchy (SDH)
format, as described in "Transmission Networking: SONET and Synchronous
Digital
Hierarchy"; M. Sexton & A. Reid, 1992 published by Artech House, USA, ISBN
0-89006-551-9.
Accordingly each processor 18 of each terminal station may write status
information concerning its operation (e.g. including any component failures)
to the
signalling channel.
The optical modulator 25 may for example be an electro-absorption modulator,
or a Mach-Zender modulator, and may use amplitude modulation, or frequency
modulation, phase modulation or some other modulation system.
The dispersion compensation unit 14 may, rather than comprising an array of
fibre optic cables of different length, comprise a single dispersion
compensation fibre
(i.e. a fibre having the reverse chromatic dispersion behaviour to ordinary
fibre), as
described in Proceedings of the Nineteenth European Conference on Optical
Communications (ECOC'93) Sep. 12-161993, Vol. 2, paper WeC8.3, pages 349-352,
A. Belov et al; "The Realisation of Broadband Dispersion Compensation using
the
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CA 02239913 2000-08-O1
Multicladding Waveguide Structure", or in the same Conference Proceedings at
paper
WeC8.5 at pages 357-360, M. Onishi et al; "High Performance
Dispersion--Compensating Fibre and its application to Upgrading of 1.31 ~,m
optimised
system" .
In this case, the dispersion compensator 14 would be positioned before the
wavelength demultiplexer 13. Alternative devices such as optical rings, Fabry
Perot
resonators, or compensators using optical phase conjugation techniques could
also be
employed. Rather than employing a parallel bank of delays, different
wavelengths
could be separated out one at a time and a ladder structure of differential
delays in
series could be employed between separation points.
The dispersion compensator 14 might be omitted under exceptional
circumstances, where the cable 3 comprises very short lengths of fibre; or
where the
chromatic dispersion of the fibre 3 is low. It would also, of course, be
possible to
position the dispersion compensator 14 between terminal stations, rather than
at
terminal stations, or to position it at the output side of the head station 2
and terminal
stations 1, so as to pre-distort the multiplexed signal such that the
dispersion of the
fibre 3 will result in an undistorted signal arriving at the terminal stations
1.
The wavelength stabiliser 10 of the head station 2 may be omitted if a
sufficiently large wavelength spacing between adjacent wavelengths (for
example more
than 1 nm) can be provided. In this case, rough temperature stabilisation for
each light
source 6 is sufficient to avoid crosstalk. Of course, such an arrangement
makes less
efficient use of the bandwidth of the fibre.
It would be possible to omit some or all of the optical attenuators 48, the
optical
power difference between different wavelengths is sufficiently small, and the
phrase
"attenuator" will also be understood to encompass an optical amplifier having
a gain
of greater than unity in some or all of the wavelength paths.
Although Figure 2b shows a 4X4 non-blocking optical switch which is realised
as a 2X3 array of 2X2 non-blocking optical switch elements, it will be
apparent that
other configurations for non-blocking optical switches could equally be
employed.
This embodiment may operate in the 1.55 ~.m wavelength domain, at 155Mbit/s
or 2.5 Gbit/s per wavelength data rates, for example.
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CA 02239913 2000-08-O1
In summary, the first embodiment shows an arrangement in which each terminal
station 1 has a coupler device for tapping and receiving data from a selected
wavelength and a modulator device for modulating data onto a selected
wavelength,
and the data transmission wavelengths are spatially separated and selectively
switched
to the coupler or the modulator by a (non-blocking) optical switch. The
signalling
wavelength is received, and the contents thereof are retransmitted by a light
source in
the terminal station 1.
Second Embodiment
Referring to Figure 6 (which corresponds to Figure 2a in the first embodiment)
in the second embodiment, all details are the same as in the first embodiment
described
above and will not be repeated here, except that in the head station 2,
instead of
providing a single wavelength multiplexes 7, there are provided a first
wavelength
multiplexes 7a, which combines the data transmission wavelengths ~,1 - ~,4
from the
light sources 6-1 to 6-4 to provide a single combined light beam, and a second
wavelength multiplexes 7b which combines this data transmission light beam
with the
signalling beam ~,o from the light source 6-0 to produce the same output as
the
wavelength multiplexes 7 in the preceding embodiment. Rather than modulating
the
output of each laser diode 6-1 to 6-4 separately, the control circuit 4 in
this
embodiment can supply a single modulation pulse train to operate a modulator
device
7c located in the combined data signalling beam between the output of the
wavelength
multiplexes 7a and the input of the wavelength multiplexes 7b so as to
modulate all
data transmission wavelengths simultaneously. As in the earlier embodiment,
the
modulator 7c may be for example an electro-absorption modulator or a Mach-
Zender
modulator. Since only one modulator is necessary, the head station structure
may be
simpler than in the first embodiment.
Third Embodiment
Referring to Figure 7, in the third embodiment, all details of the system are
the
same as in the first or the second embodiments except as discussed below.
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CA 02239913 2000-08-O1
In this embodiment, two drop paths for dropping data from two wavelengths
simultaneously are provided, and two add paths for adding data to two
wavelengths
simultaneously are provided in a terminal station 1.
Each of the add paths comprises a packet (buffer) memory 23a (23b), a driver
circuit 24a (24b) and a modulator 25a (25b). The two modulators 25a, 25b are
connected to different output ports of the switch 15.
Likewise, each of the two drop paths comprises a receiver 21a (21b) and a
gating circuit 22a (22b). Additionally, in this embodiment, packet memories
23c, 23d
are provided in each drop path to retain the incoming packets, so that one
packet can
be retained while the other is output to the terminal equipment 100.
Furthermore, in this embodiment, an electrical switch 31 is provided for
selectively routing the output of one of the memories 23d either to the
terminal
equipment 100, or on the wavelength translation path to the add paths. Also
provided
in this embodiment is a 2×2 electrical switch 32 receiving at its input
ports one
output of the switch 31 and one output signal from the terminal equipment 100,
and
routing these to a selected one of the memories 23a, 23b. The switches 31, 32
are set
under control of the Processor 18.
Other details of this embodiment are as disclosed in relation to Figure 2b.
Thus, the processor 18 can selectively route any two of the data transmission
wavelengths ~., - ~,4 the two modulators 25a, 25b and thus two data jackets
can
simultaneously be transmitted. The processor in this case is arranged to write
corresponding information to the signalling wavelength indicating that two
packages
are transmitted.
Likewise, the processor 18 can route two incoming wavelengths containing data
packets to the two couplers 20a, 20b for simultaneous reception of two data
packets.
The memories 23a, 23b and the switch 31 are arranged so that memories 23c, 23d
are
coupled in sequence to the electrical output port connected to the terminal
equipment
100.
Thus, the data transmission rate in this embodiment can be higher. Since each
terminal station 1 can receive two packets simultaneously, unlike the first
embodiment,
if the processor 18 detects a packet is already being transmitted to a
terminal station
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CA 02239913 2000-08-O1
lp and the processor itself has a packet to be transmitted to that station, it
can proceed
to do so without fear of collision. However, if the signalling wavelength data
indicates
that two packets are already being simultaneously transmitted to that station
lp on
different wavelengths, the processor 18 in this embodiment will not cause a
further
packet to be sent to that station so as to avoid overloading the station with
three
packets.
It will be apparent that other numbers of add and drop paths, functionally
identical to those shown in Figure 7 can be employed, up to the number of data
transmission wavelengths employed (in this case four). For example, four add
and two
drop paths may be provided, or three drop and two add paths.
Thus, in this embodiment, several add and/or drop paths are provided in
parallel to permit simultaneous adding and/or dropping a plurality of
wavelengths, and
wavelengths are selectively routed to the add and/or drop paths by an optical
spatial
switch.
Fourth Embodiment
Referring to Figure 8, in this embodiment, all components are the same as in
the preceding embodiments except where otherwise indicated below.
In this embodiment, the optical switch 15 is omitted, and there is a direct
optical path between the wavelength demultiplexer 13 and the combiner 26 for
each of
the data transmission wavelengths ~,, - ~,4. In each of the paths is a
respective coupler
20a-20d and a respective modulator 25a-25d. The four outputs of the four
couplers
20a-20d are received at respective input ports of a 4X1 optical switch 33,
which (under
control of the Processor 18) selectively routes one of the
outputs (i.e. one of the data transmission wavelengths) to the receiver 21 and
gate 22,
which operate as in the first embodiment.
Likewise, the four control inputs of the modulators 25a-25d are connected to
respective output ports of a 1X4 electric switch (selector) 34, which
selectively
connects the control signal from the driver 24 and memory 23 to one of the
modulators
25a-25d (and hence one of the data transmission wavelengths ~,1 - ~,4 under
control of
the processor 18.
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CA 02239913 2000-08-O1
Thus, when the processor is aware that a data packet for the terminal station
is
arriving on a wavelength (say ~.3) the processor 18 controls the switch 33 to
connect
the output of the coupler 20c through to the receiver 21 to receive the
packet.
Likewise, when the processor has located a free wavelength (say ~,4) on which
to transmit ("add") a packet held in the memory 23, the processor controls the
switch
34 to route a signal from the driver 24 to the modulator 25d. In each case,
the other
modulators 25 are set to an inactive condition, in which they provide a
straight-through
path for the wavelengths concerned.
Various changes could be made to this embodiment. For instance, the optical
switch 33 could be omitted and the single receiver 21 could be replaced by
four
receivers 21a-21d, one in each of the output lines of the couplers 20a-20d. In
this
case, a 4X1 electrical switch would be provided for selectively routing the
output of
one of the receivers 21a-21d to the gate 22.
If the optical switch 33 has a null state in which none of the inputs thereto
is
connected to the output, then the gate circuit 22 can be omitted, as setting
the optical
switch 33 to the null state will effectively gate the signal therethrough.
In this embodiment, the combines 26 is preferably a wavelength multiplexes
(e.g. of the diffraction grating type, or any of the other types mentioned
above) since
this structure has a lower insertion loss than a conventional coupler. This is
possible
because, in this embodiment, the oath followed by each wavelength is
predetermined
and does not vary over time.
It will be apparent that, although in Figure 8 only a single drop oath and a
single add path are shown, the arrangement could be expanded in a similar
manner to
that discussed with reference to Figure 7 to allow for simultaneous add and
drop of
packets. For example, the switches 33 and 34 could be omitted and separate
receivers
21a-21d and gates 22a-22d could be provided in each output path of each
coupler
20a-20d, connected to respective temporary buffer memories as in Figure 7 to
retain
received packets, and likewise four separate driver circuits 24 and packet
transmission
memories 23 could be provided in the respective input paths to the four
modulators
25a-25d, the processor 18 selectively operating up to four of the couplers and
modulators simultaneously.
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The modulators in this embodiment may, for example, be Lithium Niobate
modulators as disclosed in the above referenced Midwinter paper.
In summary, in this embodiment, a modulator and a coupler are provided in
each of the demultiplexed data transmission wavelength paths and consequently
the
optical switch 15 may be omitted (together with the switch driver circuit 16).
Accordingly, the insertion loss associated with the optical switch 15 is
omitted, and the
overall loss in the terminal 1 in this embodiment is therefore lower.
Furthermore, the
control circuitry for controlling the switches 33 and 34 can be made somewhat
simpler
than the switch driver circuit 16 required for the optical switch 15.
Fifth Embodiment
Referring to Figure 9, in the fifth embodiment, in the terminal station 1
shown
in Figure 9 the optical data transmission wavelengths ~.1- ~,4 are not
demultiplexed and
spatially separated as in the earlier embodiments. Instead, optical tunable
filter devices
are employed to selectively tune to the desired wavelength for dropping or
adding data
packets. Other details of this embodiment are as described in the earlier
embodiments
unless otherwise indicated below.
Accordingly, in this embodiment, the received optical signal is dispersion
equalised by a dispersion compensator 35 (which is preferably a single length
of
dispersion compensation fibre as discussed in the above disclosed Belov and
Onishi
papers). The dispersion compensated, wavelength multiplexed optical signal is
then
fed to a wavelength demultiplexer 36 which merely separates the signalling
wavelength
~,o on to one spatial path and leaves all four data transmission wavelengths
~,, - ~,4 on
a second path. A coupler 20, the output of which is connected to a receiver 21
and
gate circuit 22 as in the first embodiments, is located in the combined data
transmission
wavelength path.
Interposed between the coupler 20 and the receiver 21 is a tuneable bandpass
filter 37, for example an acoustically tuneable optical filter as described in
Applied
Physics Letters Vol. 56 (3), 15 Jan. 1990, D. A. Smith et al; "Polarisation -
Independent Acoustically Tuneable Optical Filter" , or in IEEE Photonics
Technology
Letters Volume 1 (2) pages 38-40, February 1989, K. Cheung et al; "Electronic
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CA 02239913 2000-08-O1
Wavelength Tuning Unit Acousto-Optic Tuneable Filter with Broad Continuous
Tuning
Range and Narrow Channel Spacing" , (referenced in the above mentioned
Chlamtac
paper). The tuneable bandpass filter 37 is controlled by the processor 18 to
pass only
one of the wavelengths ~,, - ~,4, which has been detected by the processor 18
to be free
on the basis of data on the signalling channel ~.o.
The optical path followed by the combined wavelengths ~,, - ~.4 also passes to
a further tuneable bandpass filter 38 (which may be of the same type as the
filter 37).
The tuneable optical filter 38 splits the combined optical signal into a
bandpass output
which is fed to a modulator 25 (as in the preceding embodiments), and a band
reject
output which is fed to an attenuator 48 having the same attenuation
characteristic as the
modulator 25. The outputs of the attenuator 48 and modulator 25 are then
recombined,
together with the signalling wavelength output ~,o of the transmitter 19, in a
combiner
26 (e.g. a coupler) and output via an amplifier 30.
Thus, when data packet is to be added in this embodiment, the processor 18
controls the filter 38 to select its passband to correspond to the desired
wavelength (for
example ~.4) which is accordingly modulated by the modulator 25, all other
wavelengths passing through the reject output of the filter 38 and being
recombined
with the modulated wavelength in the combiner 26.
It will be apparent that variations may be made to the structure of this
embodiment. For example, the tuneable bandpass filter 37 could be replaced
with a
wavelength demultiplexer receiving the output of the coupler 20 and providing
for
wavelength demultiplexed light paths one containing each of the transmission
wavelengths, which can then either be routed to a single receiver 21 using a
4X1
optical switch as in the preceding embodiment, or fed to four respective
receivers
21a-21d the output of one of which is selected by a 4×1 electrical
switch as in the
above embodiment, or the outputs of the four receivers 21a-21d could be
provided to
separate memories to allow up to four data packets on different wavelengths to
be
simultaneously dropped. This arrangement could be used also in the above
embodiments .
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CA 02239913 2000-08-O1
The gate 22 can be omitted if the bandpass filter 37 is controlled by the
processor 18 to switch to a wavelength other than ~,, - ~.4, thus effectively
blocking the
transmission of all data therethrough.
The tuneable optical filters 37 and 38 could instead be grating filters,
dielectric
thin film filters, fibre Fabry-Perot filters, or filters of the type disclosed
in EP
0452895.
Multiple filters 38 could be provided to allow more than one data packet to be
added simultaneously.
As in the above embodiments, it may be possible to dispense with the optical
amplifiers 12, 30, the dispersion equaliser 35, and the attenuator 48, or to
provide
instead of the attenuator 48 an optical amplifier in the path of the modulator
25.
In other respects, this embodiment may involve features of the above described
embodiments .
In summary, in this embodiment, a tuneable bandpass filter is used in the add
path and/or the drop path, of the terminal station 1 to separate out the
desired
wavelength. This embodiment offers greater flexibility than the above
described
embodiments, because the bandpass filters 37, 38 can have continuously
variable or
controllable characteristics and so the terminal station 1 can be utilised
without
changing hardware when the data transmission wavelengths are changed or added
to,
merely by changing the control signals supplied by the processor 18 to the
filters 37,
38. Thus, this embodiment offers the potential for greater flexibility than
the above
described embodiments.
Sixth Embodiment
Referring to Figure 10, this embodiment differs in two respects from the first
embodiment.
Firstly, in this embodiment, the signalling wavelength transmitter 19 is
omitted,
and the signalling wavelength receiver 17 of the first embodiment is replaced
by a
coupler 17a coupling to the signalling wavelength light path, the output of
which is
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CA 02239913 2000-08-O1
supplied to a receiver 17b (which functions identically to the receiver 17 in
the first
embodiment) .
In this embodiment, the signalling wavelength path is continuous and leads
through to the combines 26.
Referring to Figures lla and llb, in this embodiment, the head station 2
operates the signalling wavelength laser diode 6-0 to generate the signalling
wavelength
~,o having an initial portion in each time slot in which any signalling data
is present,
followed by a continuous wave portion extending for the rest of each time
slot.
In this embodiment, the signalling wavelength light path in the terminal
stations
1 passes through a modulator 49 controlled by the processor 18, which is
arranged to
modulate part of the previously continuous wave portion of each time slot,
when the
terminal station 1 has data to transmit in the next time slot. Thus, as the
signalling
wavelength ~,o passes through successive terminal stations 1, the continuous
wave
portion in each time slot may progressively be filled by successive terminal
stations in
the network. The operation of the processor 18 in this embodiment will be
described
in greater details below.
The second difference between this embodiment and the first embodiment is in
the structure of the routing means whereby the demultiplexed wavelengths are
routed
to the modulator 25 and coupler 20 (the component 24 is omitted from the
diagram for
clarity). Instead of the non-blocking optical switch 15 of Figures 2b and Sb,
a bank
50 of optical switches is provided, having k inputs (where k is the number of
wavelengths) and three groups of k outputs.
The bank 50 of switches comprises a first array S la-S lk of 1X2 optical
switches
each receiving one of the output optical paths of the wavelength demultiplexer
14 and
providing two output paths. Each of the output ports of the switches S la-S lk
is routed
to a respective input port of a wavelength multiplexes 53 (e.g. a grating
filter device
or any of the other types of wavelength multiplexes disclosed above), the
output of
which passes to the modulator 25.
The other output port of each of the switches S l a forms the input to a
respective
one of a second array of optical 1X2 switches 52a-52k. One of the output ports
of
each of the second array of switches 52a-52k is supplied to a respective input
port of
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a wavelength multiplexes 54, the output of which is supplied to the coupler
20. The
other output port of each of the switches 52a-52k is connected to the
respective inputs
of a further wavelength multiplexes 55, the output of which passes straight to
the
combines 26.
Thus, when a data packet is to be transmitted the switch driver circuit 16
(omitted for clarity in Figure 10) is controlled by the processor 18, to
switch one of
the switches 51 to pass the respective input wavelength to the wavelength
multiplexes
53 and modulator 25, where it is modulated. All the other first switches 51
are
controlled to route their outputs through the respective second switches 52,
all or
which are set to route their outputs to the wavelength multiplexes 55. The
modulated
wavelength is recombined with the others from wavelength multiplexes 55 at the
combines 26.
When the terminal station 1 is to receive a data packet, the processor 18
controls all of the first switches 51 to route their outputs to the respective
second
switches 52. The processor 18 causes the switch control circuit 16 to set one
of the
second switches 52 to route its output to the wavelength multiplexes 54, to
the output
of which the coupler 20 is connected, and to set all the other switches 52 to
route their
outputs to the wavelength multiplexes 55.
The tapped wavelength from the coupler 20 is recombined with all the others
from the wavelength multiplexes 55 in the combines 26.
In this embodiment, the gate circuit 22 is unnecessary because no data passes
through the coupler 20 except when a data packet is to be dropped.
In this embodiment, it will be seen that if the terminal station 1 is
inactive, i.e.
the terminal station 1 is not either dropping a data packet or transmitting a
data packet,
all the data transmission wavelengths ~., - ~,4 are routed through identical
paths through
the first switches 51 and second switches 52, and the wavelength multiplexes
55 and
combines 26, and accordingly all receive identical attenuation in the terminal
station
1 (in contradistinction from the first and either the coupler 20
or modulator 25 even when the terminal station 1 is inactive).
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CA 02239913 2000-08-O1
Referring once more to Figure l la and Figure l lb, in Figure l la it will be
seen
that the head station 2 in this embodiment generates a continuous wave signal
on each
of the data signalling wavelengths ~,, - ~,k and generates on the signalling
wavelength
~,o a signal which contains signalling information at the start of each time
slot (to
synchronise the terminal stations 1) and continuous wave modulation
thereafter.
In Figure llb, the corresponding contents of the signalling and data
transmission wavelengths are indicated for an arbitrary terminal station lm.
In the first
time slot (t=0 to T) the processor 18 has previously learned (from data on the
signalling wavelength ~,o in the previous time slot) that a data packet from
the first
station will be arriving for that terminal station lm on the wavelength ~.,.
Accordingly,
the processor 18 sets all the switches 51 to pass their outputs to the
switches 52, and
sets switch 52a to route its output to wavelength multiplexes 54 to route
wavelength
~,, to the coupler 20. All the other switches 52b -52k are set to route their
outputs to
the wavelength multiplexes 55. Accordingly, as previously described with
regard to
the earlier embodiments, the packet is read by the terminal station lm.
Assuming now that the terminal station lm contains in its memory 23 a data
packet to be transmitted to a terminal station ln. As in the first embodiment,
the
processor 18 observes the existence of a packet destined for station In in the
first time
slot and accordingly does not transmit in the first time slot. In the second
time slot, the
processor 18 decodes the signalling information from the signalling wavelength
~.o and
notes that all wavelengths are unoccupied in the second time slot (t=T to
t=2T).
Accordingly, the processor 18 controls the modulator 49 to modulate part of
the
continuous wave portion in the signalling wavelength in the first time slot,
to write an
indication that a data packet will be transmitted on wavelength ~,1 from
station lm to
station ln.
During the guard time Tg after the end of the data packet in the first time
slot
and before the start of data packet transmission in the second time slot, the
processor
18 controls the switch control circuit 16 to set switch 51a to route its
output to the
wavelength multiplexes 53 and modulator 25. All other first switches 51 are
set to
route their outputs to respective second switches 52, and all second switches
52 are set
to route their outputs to the wavelength multiplexes 55. Accordingly, in the
second
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CA 02239913 2000-08-O1
time slot, whilst the processor 18 is receiving the signalling wavelength ~,o
the
wavelength ~,~ is modulated to contain the packet from the memory 23 and all
other
wavelengths are passed without alteration.
It will be apparent that various modifications can be made to this embodiment.
For example, the wavelength multiplexers 53/55 could be replaced by couplers
(although the insertion loss would be higher). Likewise, multiple add and drop
paths
could be provided as in the earlier embodiments. The feature of modulating,
rather
than rewriting, the signalling channel could be employed without the switch
structure
described in this embodiment (and vice versa).
Thus, to summarise, in a first aspect of this embodiment the signalling
wavelength is not terminated and rewritten at each node with a separate laser
diode or
other transmitter, but instead is passed and modulated (where necessary) at
each
terminal station 1 so as to progressively use up the continuous wave power
thereof.
This avoids the need for a stabilised light source at each terminal station 1.
In a second aspect, this embodiment provides routing of the data signalling
wavelength to either an add path or a drop path, by spatially separating the
data
signalling wavelengths and employing an optical switch to route them, in which
when
the terminal station 1 is inactive all the data signalling wavelengths may be
routed so
as to by pass the drop path and the add path.
Seventh Embodiment
Referring to Figure 12, in this embodiment the network illustrated in Figure 1
is improved by providing a double bus structure comprising a first bus 103a
and second
bus 103b. A plurality of terminal stations lOla, lOlb . . . are provided, each
interconnected by optical fibre cable forming part of the bus 103a and optical
fibre
cable forming part of the bus 103b. The two buses 103a, 103b communicate data
in
opposite directions. At either end of the chain of interconnected terminal
stations
lOla-lOlm, before and after the first and last terminal stations, are a pair
of head
stations 102a, 102b.
Each of the terminal stations 101 comprises, essentially, two terminal
stations
as described in any of the foregoing embodiments; one for receiving and
transmitting
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CA 02239913 2000-08-O1
packets of data on the bus 103a, and one for receiving and transmitting
packets of date
on the bus 103b. Thus, in this embodiment, data can not only be signalled from
a
terminal station to any station downstream (i.e. further away from the head
node) but
in both directions. Accordingly, in this embodiment, the terminal equipment
100 is
connected to both halves of the terminal station 101, and a simple decision
circuit is
provided to route data for transmission to one of the two halves depending on
the
location of the destination terminal station. In this embodiment, the
structure of the
terminal stations 101 is preferably somewhat simplified by providing only a
single
processor 18 shared by, and controlling, both
halves of the terminal station, and making the decision as to which bus 103a
or 103b
to transmit a message on.
Of course, the furthest station along the bus 103a can only transmit data on
the
bus 103b, and the furthest station along the bus 103b can only transmit data
along the
bus 103a.
In this embodiment, the last terminal station lOla on the bus 103b is
preferably
arranged to communicate the signalling wavelength ~,o to the processor 4 of
the head
station 102a of the bus 103a, and the last station lOlm along the bus
103a is likewise preferably arranged to transmit the signalling wavelength of
the bus
103a to the processor 4 of the head station 102b of the bus 103b. In this
manner, any
information on component failures or traffic conditions along one of the buses
can be
transmitted back to the head station of that bus via the other bus.
Thus, this double bus arrangement can be used to signal between terminal
equipment 100 of equal status (for example a plurality of computer terminals).
Rather than employing two head end stations (one at either end of the bus), at
the last terminal station lOlm, the end of the bus 103a may be simply
connected to the
beginning of the bus 103b (in other words, the output optical port of one half
of the
terminal station lOlm is connected by a loop to the input optical port of the
other half),
so that the buses 103a, 103b form a single continuous looped bus. In this
arrangement,
data can be transmitted in either direction along the looped bus (although
stations
further along the bus have less access to free wavelengths).
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CA 02239913 2000-08-O1
Eighth Embodiment
Referring to Figure 13, the structure of the bus of the preceding embodiment
is modified to connect the last station on each bus with the first station on
that bus. It
would be possible to provide two head stations in this embodiment, as in the
preceding
embodiment. However, since the head station 2 in each case may simply be
generating
continuous wave wavelengths for each bus, a single head station 2 can be used
to join
both buses as shown in Figure 13. The last terminal station (lOlm) along bus
103a is
connected to the processor 4 of the head station, as in the preceding
embodiment, to
signal back any status information from terminal stations along the bus 103a.
Likewise, the last terminal station lOla on the bus 103b is connected to the
processor
4 for the same purpose. The wavelength division multiplexed signal generated
by the
head station 2 in this embodiment is supplied to a coupler 40 which splits the
signal
into two parts for transmission to the first station lOlm of the bus 103b and
the first
station lOla of the bus 103a. In all other respects, this embodiment functions
in the
same manner as the preceding embodiment.
It will also be apparent that the single bus arrangement of Figure 1 and the
first
embodiment could be connected in a loop, so that signalling information from
the
terminal stations of the bus 3 may be returned to the head station 2. In this
case, it
would be possible also for terminal stations (for example the last terminal
station in the
bus) to transmit data intended for earlier stations in the bus, if a further
terminal station
is provided within the head node 2 and data packets are decoded and wavelength
translated for retransmission by the head station 2.
This embodiment may be made more efficient than the preceding embodiment,
in that a single head node 2 can be employed to operate the two buses 103a,
103b,
rather than a pair of head nodes 102a, 102b as in the preceding embodiment.
Ninth Embodiment
In this embodiment, referring to Figure 14, the bus 3 is connected in a ring
configuration. The terminal stations in this embodiment may be as described in
any
of the first to fifth embodiments. The head stations 102a, 102b in this
embodiment are
as described hereafter.
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CA 02239913 2000-08-O1
In this embodiment, the two head stations 102a, 102b use different data
transmission wavelengths. The head station 102a generates continuous wave
signals
on data wavelengths ~,, and ~.2 using laser diodes 6-1 and 6-2; the other
components
of the head station 102a are as described in the second embodiment.
The head node 102b generates data transmission wavelengths ~,3 and ~,4 with
laser diodes 6-3 and 6-4; the other components of the head station 102b are as
described in the second embodiment.
The wavelength multiplexed continuous wave output of the head station 102a
is coupled onto the bus 3 by a combiner 43a, and that of the head station 102b
by a
combiner 43b. Just before the combiner 43a is a band reject filter 41 which
cuts the
wavelengths ~,, and ~.2 so that the preceding data modulated on the ring on
these
wavelengths is terminated and does not continue to recirculate round the ring.
Likewise, just before the combiner 43b in the bus 3 is a band pass filter 42
which
rejects wavelengths ~,3 and ~.4, so that these wavelengths do not continue to
circulate
around the ring.
In this embodiment, since each of the terminal stations 1 includes means 19
for
generating the signalling wavelength ~,o, neither of the head stations 102a or
102b
requires such means.
At the start of the operation of this embodiment, the station lOlm generates a
signalling wavelength signal indicating that wavelengths ~.1 and ~,2 are free
for
transmission. It a terminal station lOla wishes to communicate with a station
101(L+1) it can therefore generate a packet on wavelength ~,, as discussed in
the
foregoing embodiments, and regenerate the signalling wavelength ~,o to
indicate that
this wavelength is not free in the next time slot.
Upon reaching the filter 42, the wavelengths ~., and ~,z continue to circulate
and
so messages transmitted by preceding stations on these wavelengths continue to
pass.
Wavelengths ~.3 and ~,4 are blocked. At the station lOIL the processor 18 of
the station
lOIL generates on the signalling wavelength ~.o an indication that wavelengths
~,3 and
~,4 are free for signalling, and downstream of this station lOlL,
continuous wave data transmission wavelengths ~.3 and ~.4 are inserted
combiner 43b
for use by the subsequent stations 101(L+1) etc.
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CA 02239913 2000-08-O1
Timing signals from the terminal stations lOlL and lOlm are transmitted to the
processors 44a and 44b of the head stations 102a and 102b to synchronise the
time
slots in the wave lengths ~,4 with those in the wavelengths ~,,, ~,2.
In this embodiment, new continuous wave wavelengths free for transmission are
added to the bus 3 part way along the bus, at the station lOIL. Thus, in this
embodiment, the terminal stations following the station lOlL have the same
likelihood
of access to a free data transmission wavelength as those earlier in the bus;
this is an
improvement over the bus of the first embodiment, in which the data
transmission
wavelengths become progressively filled along the length of the bus.
It would be possible, as in the above described embodiments, to provide for
the
ring to consist of two parallel buses running in opposite directions and for
each
terminal station likewise co consist of parallel circuits for operating on
each bus.
Tenth Embodiment
Referring to Figure 15, in this embodiment, connection between several
different optical WDM networks is disclosed. A single terminal station 201 may
act
as a link between two buses 203a, 203b, which may use different communications
wavelengths. In this case, the electrical input and output ports of a station
la on the bus
203a and on station lb on the bus 203b are, rather than being routed directly
to
terminal equipment 100, routed via switches 46a-46d and input memory buffers
47a,
47b. One terminal of the output port switch 46a of the terminal station la is
linked to
the other switch port of the input switch 46b of the terminal station lb, and
vice versa.
Thus, a data packet received at the terminal station la of the bus 203a can be
routed
to the terminal station lb of the bus 203b, and vice versa; incoming signals
from
terminal equipment 100 in this case are accordingly stored in the buffer
memories 47a,
47b for transmission after the link between the buses 203a, 203b has been
completed.
The two terminal stations la, lb may be physically collocated in a single
terminal station 201 linking the two buses 203a, 203b, or they could be
separated and
interconnected by communications lines. In the former case, a single Processor
18 may
be provided to operate both the terminal stations la and lb ; otherwise, the
two
terminal stations la, lb are appropriately arranged to exchange timing
signals.
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CA 02239913 2000-08-O1
Eleventh Embodiment
Referring to Figure 16, in this embodiment, the double bus structure of
Figure 12 and Figure 13 is employed. However, each of the terminal stations
101 is
able to communicate with any other terminal station, because the head station
2a in this
embodiment includes a switch 51a on the bus 103a and a switch 51b on the bus
103b,
the switch having a position in which the wavelength multiplexed optical
signal is
transmitted straight through the head station and a switch position in which
the head
station supplies continuous wave data transmission wavelengths as in the above
described embodiments.
In operation, initially each of the switches 51a, 51b is set to connect the
output
of the optical power splitter 40 (as described above in relation to the eighth
embodiment) to supply continuous wave optical signals to each of the optical
fibre
buses 103a, 103b, which run in opposite transmission directions round the ring
of
terminal stations lOla, lOlb, . . . lOlm . . . lOlz.
After generating one time slot of data transmission wavelengths and signalling
wavelength ~,o, the head station 2 closes the switches 21a , 51b and thus
permits the
wavelength multiplexed optical signals from the last station lOlz to pass
through the
head station 2 to the first terminal station lOla. Thus, the station lOlz (or
any other
terminal station) can transmit data through the head station 2 to any terminal
station
(e.g. lOla ) earlier in the ring. The control circuit 4 thus alternately opens
and closes
the switches 51a, 51b.
In this embodiment, a single bus 103a is thus sufficient to communicate data
from any one terminal station to another in the ring. However, a further bus
103b is
provided to allow for fault tolerance in the event of failure of a terminal
station or
optical fibre cable between terminal stations. Also, as shown, a second head
station
2 (identical to the first) is provided, the switches 51a, 51b thereof being
normally
closed so that the terminal station 2 is transparent to transmissions on
either bus 103a
or 103b.
In this embodiment, each of the terminal stations is of the form of the
terminal
station lOlm, which comprises a first terminal station portion la connected
within the
bus 103a and a second terminal station portion lb connected within the bus
103b.
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CA 02239913 2000-08-O1
Each of the terminal station portions la, lb is the same as that in any one of
the first,
third, fourth, fifth or sixth embodiments, except that they lack a processor,
and a single
processor 18 as provided to control both.
Each terminal station also comprises a pair of cross coupling switches 56a,
56b,
both controlled by the processor 18. The switch 56a routes the output of the
first
terminal station portion la either to the bus 103a or the bus 103b, and the
switch 56b
routes the multiplexed optical output of the second terminal station portion
lb to either
the second bus 103b or the first bus 103a. Normally, the processor 18
maintains both
switches 56a and 56b open, so that the output of the first terminal portion la
is
connected to the first bus 103a and that of the second terminal portion lb to
the second
bus 103b.
Referring to Figures 17a and 17b, when a terminal station 101(m+ 1) fails, the
switch 56b of the terminal station lOlm is operated to connect the bus 103a on
to the
bus 103b at that station, and the switch 56ain the mode 101(m+2) is operated
to
connect the bus 103b on to the bus 103a at that terminal station. Thus, the
failed
terminal station 101(m+1) is isolated, and the two buses now form one single
continuous ring 103, along which any terminal station can communicate with any
other.
In this embodiment, preferably there are provided two head stations 2a, 2b.
One
of the stations 2a is normally active, and the other is normally inactive
(i.e. acts as a
transparent link in the buses 103a and 103b). In the event of failure of the
light
sources in the first head station 2a, the role of the two head stations can be
reversed
so that the second station 2b becomes active.
In the event of a failure affecting the switches S l a, S lb in the first head
station
2a, so as to break the path through the station, the switches 56a, 56b of the
adjacent
terminal stations lOla, lOlz can be operated as described above with reference
to
Figures 17a and 17b to isolated the defective head station 2a. The system can
then
continue to operate with the remaining head station 2b in a single ring.
In this embodiment, the terminal stations 101 are arranged to transmit
information indicating station failure, for example on the signalling
wavelength ~,o.
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CA 02239913 2000-08-O1
The propagation time, in this embodiment, of the signal around the ring needs
to exceed the length of each time slot T.
It will be apparent that the two features of this embodiment (namely,
providing
a head station which can pass data transparently, so that data can
recirculated twice
round a ring, and providing a pair of buses together with isolation switches
enabling
the selective isolation of defective stations) can be used separately of each
other, and
separately of the distinguishing features of other embodiments. They might
also be
used with other types of WDM communication system (for example that of
EP0452895).
Twelfth Embodiment
Referring to Figurel8, a twelfth embodiment of the invention will now be
described.
In preceding embodiments, a bus or ring structure has been disclosed. In this
embodiment, the number of terminal stations which may be used is further
increased,
without increasing the number of light sources or available wavelengths, by
connecting
several such structures in a star arrangement.
In Figure 18, a head station 1002 comprises a plurality of light sources (for
example 5) indicated as 1006 generating different wavelengths ~.o - ~.k, the
outputs of
which are multiplexed together as in the above embodiments to provide a
wavelength
division multiplexed signal which is amplified by a optical amplifier 1008.
The amplified optical signal is fed to a sputter 1240 (for example a 1:128
splitter) which splits the WDM signal evenly between a plurality (for example
128) of
optical output ports. The gain of the optical amplifier 1008 is such as to
compensate
for the sputter loss (which may for example be around 24 dB).
Connected to a plurality (for example 120) of the optical output ports of the
splitter 1240 are a corresponding plurality of optical fibre cables 1103a,
1103b, 1103c,
1103d which each provide the input to a respective terminal station 1101a,
1101b,
1101c, 1101d forming half of a respective bus head end station 1201a, 1201b,
1201c,
1201d.
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CA 02239913 2000-08-O1
Each of the terminal stations 1101a-1101d corresponds to the terminal stations
101 of Figure 12 or Figure 13 and consists of two halves each half for
signalling in a
different direction. The optical cable 1103a is routed through a first side of
the first
terminal station 1101a, and forms an optical bus 1003a which interconnects a
plurality
of equivalent terminal stations 1101e . . . 1101f (for example 30 terminal
stations).
At the endmost terminal station 1 lOlf of the bus 1003a, the bus 1003a is
routed
back from the output port of the first side of the terminal station 1001a to
the input port
of the second side of the terminal station so that, unlike the bus structure
shown in
Figure 12, no head end station beyond the final terminal station 1101f is
required.
Thus, in this embodiment, the bus 1003a forms a loop starting (at the first
side) and
ending (at the second side) at the terminal station 1101a. The same is true of
the
optical buses 1003b, 1003c, 1003d etc.
Also comprised within each bus head end station 1201a-1201d is a head end
terminal station 1901a-1901d, respectively. Each of the head end terminal
stations
1901a-1901d also corresponds to the terminal stations 101 of Figure 12 or
Figurel3
and comprises two sides, one for communicating in each direction. The head end
terminal stations 1901a-1901d (equal in number to the number of buses, for
example
120) are interconnected by first and second optical connection buses 1903
running in
opposite directions; in this case, a first group of 60 bus head end stations
1901a-1901b
are interconnected by a first bus 1903a running in a first direction, and a
second bus
1903b running in a second direction, and a second group of 60 bus head end
stations
is interconnected by a first optical bus 1903c running in a first direction
and a second
optical bus 1903d running in the opposite direction.
Each of the bus head end stations 1901a-1901d corresponds in structure to the
station 201 shown in Figure 15, there being provided electrical connection
paths
between the electrical input port of the terminal stations 1101 and electrical
output port
of the head end terminal stations 1901, and vice versa. Thus, data can be
communicated from a terminal station on one bus 1003a to a terminal station on
another bus 1003b, via the bus head end stations 1201a, 1201b and connection
bus
1903a.
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CA 02239913 2000-08-O1
A head end terminal station 1901b connected to the first and second connection
buses 1903a, 1903b is similarly electrically connected to a second head end
terminal
station 1901c on the third and fourth connection buses 1903c, 1903d.
Continuous wave modulated light is supplied to the connection buses
1903a-1903d via cables 1904a-1904d from optical output ports of the splitter
1240.
Thus, in this embodiment, terminal stations 1101 are interconnected by optical
communication buses 1003, the communication buses themselves being
interconnected
by one or more connection buses 1903. A single set of stabilised light sources
6
provides optical power for all the optical buses 1103, 1903 via a splitter
1240.
Addressing logic is provided at each head end node station 1201, to enable it
to drop
data from a connection bus and add data to a communication bus 1103, and vice-
versa.
This embodiment is able to interconnect about 3600 terminal stations 1101.
Various
modifications may be made to this embodiment. For example, the connection
buses
1903 may be omitted and each pair of optical buses 1003a -1003d may be
directly
connected to its neighbours by stations of the type disclosed in the tenth
embodiment.
More than one set of stabilised light sources 6 may be provided.
In this embodiment, the length of each of the communication buses (i.e. the
number of terminal stations in the bus affects the loss along the bus, and
hence for
longer buses, more optical power is required to be supplied to each bus.
Treating the
optical power supplied from the light sources 1006 as a fixed constraint,
reducing the
number of terminal stations in each bus increases the losses which are
acceptable in
the splitter 1240, and hence the number of stages which the sputter can have
(which
is exponentially related to the number of buses which can be supported by the
sputter).
Thus, relatively short buses and a relatively high number of buses may be a
preferred
structure for some applications.
Thirteenth embodiment
This embodiment is based on the preceding embodiment, and includes
additional features providing security against failure of a system components
(e.g.
breaks in the cable, or failures of light sources or terminal stations).
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CA 02239913 2000-08-O1
Referring to Figure 19, in this embodiment, terminal stations 1101 are
interconnected by a double ring bus structure, as in the eighth and eleventh
embodiments, so that a terminal station 1101a can communicate with a terminal
station
1101b either via a first optical fibre ring 1913a, or via a second optical
fibre ring
1913b in the opposite direction round the ring to the first.
Each terminal station 1101 has the same structure as in the eleventh
embodiment, so that if both fibre rings 1913a and 1913b break between two
terminal
stations, or if a terminal station fails, the two terminal stations to either
side of the
defect can be reconfigured to interconnect the two rings 1913a, 1913b into a
single
looped bus, maintaining full communication between all undamaged terminal
stations.
If only one ring breaks, communication can proceed normally on the other
without
such reconfiguration.
In this embodiment, the head end terminal station 1101a of a communication
ring bus therefore receives two optical fibre cables 1904a, 1904b (not shown
in Figure
19) from two ports of the splitter 1240a. The head end station 1101a
corresponds in
structure to head end station of Figure 13, but without the light source 6 and
splitter
40 thereof (these being replaced by the light sources 1006a and sputter
1240a).
Additionally, to safeguard against failure of the light sources 1006a,
splitter
1240a, cables 1904a and 1904b or station 1101a, a secondary optical power
supply
system is provided comprising secondary light sources 1006b identical to the
primary
light sources 1006a (which correspond to those 1006 of the twelfth
embodiment); and
secondary amplifier and sputter components 1008b, 1240b and optical supply
cables
fibres (not shown). Two ports of the secondary (protective) splitter 1240b are
connected via optical fibre cables to a secondary head end station 1101a (on
the same
ring as the first head end station 1101a), which is normally configured as a
transparent
stage in the buses 1913a, 1913b. In the event of failure of the primary
optical power
supply system comprising the light sources 1006a, sputter 1240a, and primary
head
end node 1101 a, the secondary
head end node 1101c is activated (e.g. by a predetermined data signal) to
supply optical
signals from the light sources 1006b to the optical rings 1913a, 1913b.
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CA 02239913 2000-08-O1
Interconnection between the terminal stations on the double ring bus 1913a,
1913b and terminal stations on another double ring bus (for example, that
comprised
by optical fibre rings 1913c, 1913d) is provided via interconnection ring bus
structures
1903a, 1903b; 1903c, 1903d; 1903e, 1903f.
Connection between the communication ring bus 1903a, 1903b and the
interconnection ring bus 1903a, 1903b is provided by a pair of interconnection
node
stations 1201a , 1201b, one of which is normally inactive (in which condition
it
functions as a transparent node on the communication double ring and the
interconnection double ring). Each interconnection station 1201a, 1201b
consists of
a pair of terminal stations, one on each bus, electrically interconnected as
in the tenth
embodiment. On failure of one interconnection node station 1201a,
communication is
maintained by activating the second connection node station 1201b.
Each of the interconnecting ring buses 1903a, 1903b is supplied with a WDM
optical single at a primary head node 1101d (functionally identical to the
primary head
node 1101a) receiving light via a pair of optical fibre cables 1904c, 1904d
from the
primary splitter 1240a, and a secondary head end node 1101e (functionally
equivalent
to the secondary head node 1101c) receiving light via a pair of optical fibre
cables
1904e, 1904f from the secondary splitter 1240b.
Preferably, a number of such interconnection double rings are provided (
1903c,
1903d; 1903e, 1903f), and, as in the preceding embodiment, adjacent rings are
interconnected by interconnection stations 1201 e, 1201 f, 1201 g
(functionally equivalent
to the interconnection stations 1201a, 1201b). A pair of such interconnection
stations
(not shown) may be provided, to give redundancy.
Thus, in this embodiment, the arrangement of discrete buses or rings into a
star
arrangement, sharing a common light source, is further improved by the
provision of
a duplicate ring structure in each of the communication and interconnection
buses,
giving protection against failure of one or both optical fibre cables making
up each
double ring, or against failure of a terminal station. Duplicate (redundant)
interconnection between each communications ring and each interconnection ring
protects against failure of the interconnection stations between the two.
Provision of
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CA 02239913 2000-08-O1
duplicate optical WDM signal sources gives protection against light source or
optical
cable failure.
It will be apparent that these features may individually be employed
separately
of each other, and that they are illustrated together in Figure 19 merely for
convenience.
Other Modifications and Embodiments
It will be clear from the foregoing that many other modifications,
substitutions
and embodiments are possible. For example, other network configurations than
those
described are possible (e.g. as disclosed in Canadian Patent 2,123,220
(W093/21706),
of British Telecom.
Rather than using a separate signalling wavelength ~,o,it would be possible to
use subcarriers (e.g. TDM positions of the each of the data signalling
wavelengths
modulated by a radio frequency, or the like) as disclosed in the above
referenced
Chlamtac paper, or TDM portions of the data signalling wavelengths, to
transmit the
signalling information discussed above.
Rather than transmitting the signalling information in one time slot to
indicate
the contents of the next time slot, the signalling information in one time
slot could
indicate the contents of the next but one or subsequent data transmission time
slots.
Equally, it would be possible to insert a one time slot delay line in each
terminal
station 1, as disclosed in the above referenced Chlamtac paper, and to
transmit the
signalling information simultaneously with the data transmission information
to which
it relates, the signalling information being extracted prior to imposition of
the delay.
Rather than employing a plurality of light sources 6-0 to 6-k at each head
station, it would be possible to use the structure described in EP 0452895 in
which a
single laser component is used to generate a plurality of different
wavelengths.
Although it is preferred to use passive modulation of the data transmission
wavelengths as disclosed above, the possibility of using controlled
retransmission with
data transmission light sources at terminal stations is not excluded in all
aspects of the
invention.
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CA 02239913 2000-08-O1
In the foregoing, unless expressly indicated to the contrary, each feature of
each
embodiment may be combined with those of each other embodiment in a manner
apparent to the skilled person.
In this document, the terms "light" and "optical" are intended not only to
refer
to the visible spectrum but also to any wavelengths which obey the laws or
optics in
substantial fashion.
It will be understood from the foregoing that the invention comprehends each
and every novel feature and subcombination of features disclosed in the
foregoing,
together with all obvious variants and modifications thereof. Accordingly, the
scope
of the invention will be understood not to be limited by the above examples
but to
extend to all equivalents thereof, whether or not within the scope of the
accompanying
claims.
Connectivity
The maximum number of terminal stations in this invention is not limited to
the
number of different light wavelengths available. It is affected by the number
of light
sources, however. In fact, it may be found that a larger number of data
signalling
wavelengths can actually reduce the maximum number of terminal stations which
can
be supported by a single head station, because the fraction of the total
optical power
on each wavelength is reduced and consequently the signal to noise ratio is
likewise
reduced.
To give some concrete examples, making sensible assumptions about the loss
of various components, it is estimated that using four different data
transmission
wavelengths (k=4), 1,000 terminal stations can be supported by a single head
station
at 155 Mbit/s data rate with an achieved capacity of 620 Mbit/s; 61 terminal
stations
can be supported at 2.5 Gbit/s data rate with an achieved capacity of 10
Gbit/s; and 15
terminal stations can be supported at 10 Gbit/s with an achieved capacity of
40 Gbit/s.
When the number k of data signalling wavelengths is increased to 16, the
number of
terminal stations supported decreases to 292, 16 and 2 respectively. The
number of
terminal stations which can be supported may be increased by reducing the loss
at each
terminal station, and through other measures.
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CA 02239913 2000-08-O1
It will thus be apparent from the foregoing that the invention can enable the
use
of a relatively small number of wavelengths to support a much larger number of
terminal stations.
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