Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL NETWORK
This invention relates to an optical network.
Currently, in the United Kingdom, the telecommunications network
includes a trunk network which is substantially completely constituted by
optical
fibre, and a local access network which is substantially completely
constituted by
copper pairs. In future, it would be highly desirable to have a fixed,
resilient,
transparent telecommunications infrastructure all the way to customer
premises,
with capacity for all foreseeable service requirements - or at least to points
(e.g.
the kerb) closer to such customer premises. One way of achieving this would be
to create a fully-managed fibre network for the access topography. An
attractive
option for this is an optical tree access network, such as passive optical
networks
(PONS) which incorporate single mode optical fibre and no bandwidth-limiting
active electronics.
In a PON, a single fibre is fed out from a head-end (exchange), and is
fanned out via passive optical splitters at cabinets and distribution points
(DPs) to
feed optical network units (ONUs). The ONUs can be in customers' premises, or
in
the street serving a number of customers. The use of optical splitters enables
sharing of the feeder fibre and the exchange-based optical line termination
(OLT)
equipment, thereby giving PONs cost advantages. At present, simplex deployment
of PONs is the preferred option, that is to say separate upstream and
downstream
PONs are provided whereby each customer has two fibres. Although simplex
working increases the complexity of the infrastructure due to the two fibres
per
circuit required, it benefits from a low optical insertion loss fdue to the
absence of
duplexing couplers), and a low return loss, since such systems are insensitive
to
reflections of less than 25dBm with separate transmit and receive paths.
However, duplex PONs where one single fibre carries traffic in both directions
are
also possible. Typically, a PON has a four-way split followed by an eight-way
split,
so that a single head-end fibre can serve up to 32 customers.
In a known arrangement - TPON (telephony over a passive optical
network) - a head-end station broadcasts time division frames to all the
terminations on the network. The transmitted frames include both traffic data
and
control data. Each termination recognises and responds to appropriately-
addressed
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portions of the data in the broadcast frames, and ignores the remainder of the
frames. In the upstream direction, transmission is by time division multiple
access
(TDMA) where each termination transmits data in a predetermined timeslot, so
that the data from the different terminations are assembled into a TDMA frame
of
predetermined format.
The present applicant has developed a bit transport system (BTS) for use
in a PON which operates using TDMA. The BTS is described in our European
patent specifications 318331, 318332, 318333 and 318335.
One feature necessary to such a network is the provision of compensation
for the differing delays and attenuations associated with the different
distances of
the various terminations from the head-end station. To this end, each
termination
is arranged to transmit a ranging pulse timed to arrive in a respective
predetermined portion of the upstream TDMA frame. The head-end station is
arranged to monitor the timing, i.e. the phase and the amplitude of the
arrival of
the pulse from each of the terminations, and to return servo-control signals
to the
terminations to retard or advance their transmissions as appropriate, and to
adjust
their launch power. This ranging and levelling process is particularly
important
during set-up of a PON system, or when a PON system is upgraded, or when a
PON system is returned to use after a fault has been repaired. In such cases,
the
ranging and levelling process takes a finite time (the round trip delay) which
is
dependent upon the distance from the head-end station to the terminations.
This
round trip delay from the terminations to the head-end station and back to the
terminations to effect ranging and levelling is known as the dead zone. This
is
because the dead zone represents the time during which PON customers can get
no service as the PON is being used exclusively for ranging and levelling. For
a
simple PON of the type described above, in which a head-end station is
connected
to up to 32 terminations over a distance of typically 6-8km, the dead zone is
only
60-80rrs, and this does not represent a major problem.
Recently, however, the PON principle has been expanded to form what is
known as the SuperPON concept, in which high power optical amplifiers are used
to allow very large, high split PONs to be built. For example, the use of
optical
amplifiers (such as fibre amplifiers) permits up to 3500 customers to be
connected
to a single head-end station over distances of up to 200km. In this case, the
dead
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zone is of the order of 1 ms to 2ms, and this does give rise to significant
loss of
service to customers of such a SuperPON.
Unfortunately, optical amplifiers can only be used on a downstream
SuperPON, as the use of amplifiers on an upstream SuperPON would cause noise
problems resulting from the superposition of amplified stimulated emissions
(ASEs)
from the amplifiers. One way of providing amplification in an upstream
SuperPON
is to replace the last level of split (that is to say the level of split
nearest the head-
end) by a repeater. This device converts incoming optical signals to
electrical
signals, amplifies them, and converts the amplified electrical signals to
optical
signals for onward transmission. Note that such networks are often loosely
referred to as PONs, even though they may include electronic amplification and
are
not, therefore, strictly speaking, "passive".
The present invention provides an optical network comprising a head-end
station connected to a plurality of optical network units via an optical fibre
network having a plurality of split levels, wherein one level of split in the
upstream
direction is constituted by a n:1 repeater provided with monitoring means for
ranging and levelling transmissions from the optical network units.
Preferably the repeater is connected to the head-end station by a single
optical fibre.
Typically, said single optical fibre has a length of up to 200km (e.g 100 to
200km), and the optical network units are spaced from the repeater by
distances
up to about 8km.
In a preferred embodiment, the repeater includes n receivers and a
transmitter, the transmitter being connected to the head-end station, and each
of
the receivers being connected to a respective optical fibre forming part of
the
network between the repeater and the optical network units. Conveniently, the
repeater may be provided with a multiplexer for multiplexing signals received
by
the receivers for transmission to the head-end station by the transmitter, and
with
supervisory means for monitoring the functions of the receivers, the
multiplexer
and the transmitter.
Preferably, each receiver is constituted by a pair of parallel receiver
boards, and the transmitter is constituted by a pair of parallel transmitter
boards.
In this case, the supervisory means may be such as to disconnect one of the
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receiver boards of each pair from its associated optical fibre and to connect
the
other receiver board of that pair to said optical fibre upon detection of a
fault in
said one receiver board, and is such as to disconnect one of the transmitter
boards
from said single optical fibre and to connect the other transmitter board to
said
single optical fibre on detection of a fault in said one transmitter board.
In another preferred embodiment, the repeater is constituted by a plurality
of repeater modules each of which is connected to a single optical fibre
leading to
the head-end station.
For example, each repeater module may include a plurality of receivers and
a transmitter, the transmitter being connected to said single optical fibre,
and each
of the receivers being connected to a respective optical fibre forming part of
the
network between the repeater and the optical network units, the arrangement
being such that the total number of receivers in the repeater modules equals
n.
Each repeater module may be provided with a multiplexes for multiplexing the
signals received by the receivers of that module for transmission to the head-
end
station by the associated transmitter, and with supervisory means for
monitoring
the functions of the associated receivers, the associated multiplexes and the
associated transmitter.
Preferably, each receiver is constituted by a pair of parallel receiver
boards, and each transmitter is constituted by a pair of parallel transmitter
boards.
In this case, the supervisory means of each repeater module may be such as to
disconnect one of the receiver boards of each associated pair from its
associated
optical fibre and to connect the other receiver board of that pair to said
optical
fibre upon detection of a fault in said one receiver board, and is such as to
disconnect one of the associated transmitter boards from said single optical
fibre
and to connect the other transmitter board of that pair to said signal optical
fibre
on detection of a fault in said one transmitter board.
A SuperPON incorporating a ranged and levelled repeater constructed in
accordance with 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 schematic representation of the SuperPON;
Figure 2 is a block diagram of the repeater of the SuperPON;
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Figure 3 is a block diagram of a receiver board forming part of the repeater
of Figure 2; and
Figure 4 is a schematic representation of a SuperPON including several
repeaters.
5 Referring to the drawings, Figure 1 shows a SuperPON having a head-end
station 1 connected to a plurality of ONUS (not shown) via a single fibre 2, a
repeater 3 and a network 4 /shown schematically) including two or more levels
of
split. The SuperPON shown is an upstream SuperPON, that is to say it is a
simplex network carrying signals from the ONUs to the head-end station 1. A
downstream PON is also assumed to be present to carry signals from the head-
end
station 1 to the ONUs, though not shown in the drawings; though the invention
may also be applied to duplex PONs, if desired. Typically, the distance
between
the head-end station 1 and the repeater 3 lies in the range of from 100km to
200km. The SuperPON is arranged to use the BTS.
The repeater 3 fsee Figure 2) replaces the last level of split of a standard
upstream SuperPON, that is to say the level of split nearest the head-end
station 1,
the other levels of split being formed by passive optical splitters, in
conventional
manner. This last level of split is 12:1, so that the repeater 3 connects the
single
fibre 2 from the head-end station 1 to twelve fibres 5 (only two of which are
shown) forming part of the network 4. Each of the fibres 5 carries TDMA
traffic at
155 Mbit/s, and the fibre 2 carries traffic at 1860 Mbits.
The repeater 3 includes twenty-four receiver boards 6 which are
connected in pairs to the twelve fibres 5. One receiver board 6 of each pair
forms
a back-up for the other receiver of that pair, thereby providing one-to-one
receiver
protection for each of the fibres 5. The repeater 3 also includes a
transmitter/packetiser block 7 for packetising and transmitting data coming in
from
the fibres 5. In order to provide one-to-one transmitter protection, the block
7
includes two transmitter/packetiser devices. A multiplexer/supervisories block
8 is
positioned between the receiver boards 6 and the block 7. The multiplexer of
the
block 8 multiplexes the signals from the twelve active receiver boards 6 prior
to
these signals being packetised and transmitted by the block 7; and the
supervisories control the ranging and levelling functions of the repeater (as
is
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described below). Any convenient method of multiplexing may be used over the
fibre 2; it is not necessary to follow the TDMA framing used on the fibres 5.
Each receiver board 6 (see Figure 3) is constituted by a 155Mbit/s receiver
chip 9, a ranging ASIC 10, a levelling ASIC 1 1, a supervisories chip 12, a 16-
bit
micro controller chip 13, a packetiser/depacketiser chip 14, an interface to
multiplexer chip 15, and a power control chip 16. The chips 9, 14 and 15 are
connected together by a high data rate bus 17, and all the chips 9-16 are
connected together by a low data rate control bus 18.
The receiver chip 9 of the receiver board 6 receives data signals from its
input fibre 5, and transfers this data to the packetiser/depacketiser chip 14
and to
the interface to multiplexer chip 15 via the high data rate bus 17. The
interface to
multiplexer chip 15 interfaces with the multiplexer of the block 8, and the
packetiser/depacketiser chip 14 filters out any operations and maintenance
(0&M)
signals and passes these to the 16-bit micro controller chip 13. The O&M
signals
are sent regularly from the ONUs (say every nth frame of the BTS), and it is
important to prevent these 0&M signals being returned to the head-end station
in a
SuperPON as head-end processing could be overrun under certain problematical
situations. Thus, the repeater 3 is effective to carry out the control of O&M
signals that is usually carried out by the head-end station of a PON.
The ranging ASIC 10 and the levelling ASIC 1 1, under the control of the
16-bit micro controller chip 13, carry out the ranging and levelling functions
normally carried out at the head-end of a PON. Thus, the ranging ASIC 10 and
the
levelling ASIC 1 1 monitor the timing, i.e. the phase and the amplitude of the
arrival
of the ranging pulses from the ONUS, and return servo-control signals to the
ONUs
to retard or advance their transmissions as appropriate, and to adjust their
launch
power. As there is a fixed delay between the repeater 3 and the head-end
station
1, the only uncertainty in ranging and levelling arises from the delays
between the
ONUS and the repeater 3. However, as the network 4 is such that the ONUs are
typically 6-8km from the repeater 3, the dead zone for ranging and levelling
is only
60-80rrs, and this does not cause a problem. Here again, therefore, an
important
control function has been taken over by the repeater 3 from the head-end
station
1. The return of these servo-control signals is preferably transmitted from
the
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repeater, on the link: 2, to the head-end 1 and then forwarded by the head-end
via
the downstream PON to the ONUS.
The supervisories chip 12 monitors the functions of the receiver chip 9,
the multiplexer of the block $ and the transmitter of the block 7, and is
effective to
switch to the paired receiver board 6 if a problem is sensed with its own
receiver
board. Thus, the receiver boards 6 are paired to have 1:1 redundancy with hot
stand-by. The rapic switch over between the two receiver boards 6 of each
pair,
possibly without lo:;s of service, helps with the identification of faults on
the
distribution side (that is to say the downstream side of the repeater 3) of
the
SuperPON. In other Words, the supervisories chip 12 can be used to identify
faults
in parts of the network associated with specific fibres 5. This enables
remedial
work to be carried out on the faulty branch without having to shut down the
entire
SuperPON. This is to be contrasted with known SuperPONs which do require
complete shutdown for fault eradication. Similarly, the two transmitters of
the
block 7 have 1:1 redundancy with hot stand-by. Alternatively, the load could
be
interleaved between the two transmitters so that, if one fails,- the other
takes on
the faulty transmitter's load.
The 16-bit micro controller chip 13 controls the other chips 9-12 and 14-
16, and the power control chip 16 controls the power supplies to all the other
chips.
Figure 4 shows a modified form of upstream SuperPON, in which a head-
end station 21 is connected to three repeaters 23a, 23b and 23c by a single
fibre
22. The repeater ~:3c is equivalent to the repeater 3 of Figure 1, in that it
is
situated between 100km and 200km from the head-end station 21 . The repeaters
23a and 23b are, however, situated much nearer to the head-end station 21. A
respective PON 24a, 24b and 24c is associated with each of the repeaters 23a,
23b and 23c. In this case, the three repeaters 23a, 23b and 23c collectively
constitute the final split in the upstream SuperPON, i.e. they provide a
distributed
repeater function in which, say, the repeater 23a has six pairs of receiver
boards
(not shown but similar to the receiver boards 6 of Figure 2), and the
repeaters 23b
and 23c each have three pairs of receiver boards. Obviously, each of the
repeaters 23a, 23b and 23c has a transmitter/packetiser block (not shown but
similar to the block 7 of Figure 2) having two transmitter/packetiser devices
for
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providing one-to-one transmitter protection, and a multiplexer/supervisories
block
(not shown but similar to the block 8 of Figure 2). This type of SuperPON,
having
a distributed repeater function, is suitable for more rural areas of the
country,
where the customers are more widely spaced. Apart from this distributed
repeater
function, the embodiment of Figure 4 works in the same way as that of Figure
1.
The repeaters 23a, 23b and 23c can be thought of as parts of a single repeater
forming the last level of split of the upstream SuperPON leven though the
first two
repeaters 23a and 23b are joined to the main fibre 22 leading to the head-end
station 21 by splitters 25a and 25b respectively). Thus, each of the repeaters
23a, 23b and 23c controls the ranging and levelling functions for its own PON
24a, 24b and 24c. As the dead zone in each of these PONs 24a, 24b and 24c is
only of the order of 60-80rrs (the PONs being such that their ONUs are only 6-
8km from their repeaters 23a, 23b and 23c), this does not give rise to
significant
loss of service to customers of this type of SuperPON. Note however that if
the
repeaters are located at different distances from the head end and TDMA is
used
on this part of the network then a ranging and levelling function will need to
be
provided (either at the repeaters or at the head end) to range and level the
part of
the network between the head end and the repeaters. Alternatively the link
from
the repeater (or repeaters) to the head end could itself be part of a separate
PON
with its own ranging and levelling facilities. This separate PON could be a
conventional PON, or one in accordance with the present invention.
It will be apparent that the type of ranged and levelled repeater described
above greatly reduces the dead zone in an upstream SuperPON. Moreover, ranging
and levelling to the repeater's receiver reduces the power budget of the
system,
that is to say dynamic range and receiver sensitivity requirements are
reduced.
Thus, in prior art upstream SuperPONs it is necessary to combine, at the head-
end
receiver, up to 3500 customer signals, and this leads to the problem of
additive
noise. By moving the ranging and levelling function downstream to the
repeaterls),
the number of customers to be dealt with is reduced to 288 per receiver board
6,
and this leads to a reduction in the required power budget. Moreover, each of
the
receiver boards 6 only needs to take account of signals coming in from ONUs at
distances of up to 6-8km, and this leads to the reduction in dynamic range
requirements. With reduced dynamic range, it should be possible to consider
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implementing the system with no levelling. In this case, the receiver boards 6
of
the repeaterls) could be simplified.
Another advantage of using the type of ranged and levelled repeater
described above is that the upstream data rate can be far higher than
300Mbit/s
' S (which is the maximum for normal upstream SuperPONs). This is because each
of
the twelve branches leading to a repeater can carry 300Mbit/s, and the data
streams can be multiplexed for onward transmission. The advantage of this is
that
a SuperPON can now offer the same bit rate of 1.2Gbit/s in both the upstream
and
downstream directions.
Other advantages of this arrangement are greater flexibility in the link to
the head-end station, as the distribution and transport sections of the
SuperPON
are separated, so that duel parenting of sections of the PON to two or more
head-
end stations is possible. Also, the filtering of 0&M messages by the repeater
has
the advantage of avoiding overload at the head-end station. A further
advantage,
particularly with the embodiment of Figure 4, is the accommodation of
temperature
changes. Thus, the BTS normally monitors temperature changes by monitoring the
ranging pulses in the packet headers, appropriate correction signals being
sent to
the ONUs when temperature changes are sensed. In this connection, it should be
noted that, with a long reach PON, the temperature change which will shift
data
by one bit is as low as 0.075°C. This temperature monitoring function
can,
however, only be carried out where there is no overlap of bits from different
customers. This overlap of bits is a function of the distance between the
customers' ONUs and the monitoring centre. Thus, where this type of repeater
is
used to monitor ONUs for distances of only up to 6-8km, there is considerably
less
chance of overlap of bits from different customers, and so improved
temperature
monitoring and correction can be accomplished.
