Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a system for transmittingdigital
information between two stations connected by a two-wire line,
e.g. a coaxial line or a symmetrical balanced line.
In order to exchange coded information such as pulse code
modulation (P.C.M~) signals or digital data between twostations,
known transmission systems generally use a transmission channel
such as a four-wire line, a two-wire channel being used for
transmitting information in one direction and the other two-wire
channel being used for transmitting information in the other
direction. The cost of the wired cable containing the four-wire
channels is an important element in the cost of a digital trans-
mission system.
An object of the invention is to reduce by half the number
of wires used for bilateral transmission of information, even at
the expense of increasing the complexity of the end station and of
the intermediate repeaters, if any, inserted along the
transmission line if required owing to its length.
Some known bilateral transmission systems use two-wire
transmission supports, e.g. in the "N+N" system, in which infor-
mation is transmitted in the form of analog signals locatedin
different frequency bands for each transmission direction. This
method does not appear easy to adapt to the transmission of
numerical signals.
It is also known from French Patent Spec. 2 244 315 dated
12 September 1974 to use two-wire lines for connecting telephone
subscribersreceiving and transmittingP.C.M. signals to a subscriber
concentrator connected to a digital exchange by a time-division
multiplexing line. In this system, the concentrator is connected
to the numerical exchange by a two-directional time-division
30 multiplexing channeltransmitting 32 octets in a 125u sframe, l.e.
at a flow rate or rhythm of 2.048 Megabitsper second. The octets
received from the exchangearestored in areceivingstore in the
concentrator, whereas the octets intended for a particular
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subscriber's station are re-transmitted by the two-wire line of
the subscriber's station at a lower rate, e.g. 256 kilobits/sec..
After receiving an octet, the subscriber's telephone set transmits
an octet to the concentrator at a low rate, and the octet is stored
in a transmission store in the concentrator and re-transmitted at
a high rate to the exchange.
The preceding method cannot solve the general problem of
transmitting digital information of any nature between two stations
connected by a two-wire line. This is firstly because the method
mainly relates to the network formed by the subscriber's lines
connected to a concentrator, the network having a star structure
in which the flow rate transmitted along each arm is much less than
the rate of the main flow entering the concentrator, whereas the
invention relates to a connection between two stations. Another
reason is that the method applies only to digital information in a
time-division multiplexing telephone network, which means that the
duration of the multiplexer scanning frame (125 ~s) has to be taken
as the duration of the full transmission cycle on the two-wire line.
According to the invention, there is provided a bidirec-
tional digital transmission system between two end stations inter-
connected by a single two-wire line along which regenerative repea-
ters having a controllable direction of amplification are inserted,
in which each end station comprises transmission equipment including
a transmission store unit, means for writing incoming digital infor-
mation into the transmission store unit at a rate of _ bits per
second, means for reading out information written into the trans-
mission store unit at a rate greater than 2d bits per second, means
for transmitting the read out information to the line, and means
for adding thereto a signal controlling the amplification direction
of the regenerative repeaters, and in which each end station
further comprises receiving equipment including a receiving store
unit, means for writing incoming digital information into the
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receiving store unit at a rate greater than 2_ bits per second, and
means for reading out the digital information written in the recei-
ving store unit at a rate of _ bits per second.
According to another feature of the invention, the trans-
mission system also comprises means for selecting one or the other
direction of transmission in the end stations and, if required, in
the repeaters and/or regenerative repeaters inserted in the two-
wire line, the said means comprising switching means and transmit-
ters and receivers of one or more special signaIs having the same
nature as the signal to be transmitted or having a different nature,
i.e. either one or more digital or one or more analog control si-
gnals, the transmission system also comprising means in the end
stations for controlling the selection means according to a periodic
sequence.
According to another feature, the means for controlling
the means for selecting the transmission direction comprise a clock
controlling a transmission logic circuit and a receiving logic cir-
cuit in each end station. The control means are autonomous in one
of the end stations, which is considered as the control station,
whereas in the other end station, which is considered as the con-
trolled station, they are themselves controlled by the signals
received from the control station.
According to another feature of the invention, interfaces
are inserted between the stations and the general network and com-
prise stores in which the information bits coming from or en route
to the general network are written in or read out continuously at
a rate of d bits per second, and the information bits going to or
coming from the two-wire line of the device of the invention are
discontinuously read out or written in at a rate greater than 2d
bits per second.
According to still another feature, the writing-in and
reading-out operations of the stores forming the interfaces bet-
ween the system according to the invention and the general
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network are controlled by ~he logic assembly of the end stations
of the device according to the invention.
Other features of the digital transmission system according
_ to the invention will be clear from the following description,
which is illustrated by the accompanying drawings, in which :
Fig. 1 is a diagram of the bilateral transmission system
according to the invention, comprising a two-wire line;
Fig. 2 is a time diagram of transmission, when the duration
of transmission is the same for the two stations;
Fig. 3 is a similar diagram, when the duration of transmission
is different for the two stations;
Fig. 4 shows an end station in the digital transmission system;
Fig. 5 shows a regenerative repeater inserted in the two-wire
line; and
Figs. 6 and 7 show two ways of arranging the stores disposed
in the end stations.
Referring firstly to Fig. 1, references 1 and 2 denote two
stations A and B connected by a two-wire line 3. Regenerative
repeaters 5 are inserted in line 3.
Starting from a time to (Fig. 2), station A transmits digital
information for a period of I seconds to station B via transmission
line 3 operating in the direction from 1 to 2. The numerical
information arrives at 2 between (to + ~) and (to +T +~). At the
end of the holding time tgb~ i.e. at the time (to +T +~ + tgb)
station B transmits digital information for a period of T seconds
to station A via transmission line 3 operating in the direction
from 2 to 1, the digital information arriving at 1 between
(to +T + 2~ + tgb) and (to + 2T + 2~ + tgb). After a holding time
tga, station 1 can again transmit to 2, and so on. The times at
which transmission from 1 to 2 starts are :
to + iT = to + i( 2T + 2~ + tga + tgb)
and the times at which transmission from 2 to 1 starts are :
, . . . , .: . . . . . , . . . , . .
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to + T + ~ + tgb + iT = to ~ r + ~ + tgb ~ i(2T + 20 + tga + tgb)
If tga = tgb = tg, the times at which transmission begins
are for 1 to 2 :
to + 2i ( T+~+tg)
and, for 2 to 1 :
to + t2i+1)(T+~+tg)
If the flow rates in the two directions are different (Fig. 3),
the transmission periods are Ta from l to 2 and Ib from 2 to 1 and
the period T becomes :
la + Tb + 2~ + tga + tgb
It can be seen that, if the propagation time and the holding
time are neglected, T = 2~. Consequently, the transmission rate of
data on the two-wire lines must be at least 2d, i.e. at least twice
the transmission rate d of date on the four-wire line. Owing to
the propagation time, the holding time and the fact that synchro-
nization bits and auxiliary bits can be inserted, the "two-wire"
rate must be more than twice the "four-wire" rate, i.e. equal to
(2d + ).
Referring now to Fig. 4, 11, 12 denote the four-wire line
connected to the end station on the side of the general network,
the two-wire line 11 being the incoming line and the two-wire line
12 being the outgoing line. A two-wire line 3 connects the end
station shown to another end station in the system. Line 11 is
connected to an inverting circuit 14 having two outputs connected
to the inputs of two transmission stores 15, 16 respectively. The
outputs of stores 15, 16 are connected to the two inputs of an
inverting circuit 17, and the output of circuit 17 is connected
to a time-division multiplexer 18. The multiplexer i5 adapted to
add a synchronization word to the digital information to be
transmitted, the synchronization word being preceded if required
by a preamble and followed by various auxiliary information
depending on the particular application, e.g. service-channel bits,
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tele-control bits, tele-signalling bits or quality control bits.
It is advantageous to use this facility of inserting supplemen-
tary bits in order to give the group a predetermined length in
spite of the nature (usually plesiochronic) of the digital infor-
mation sources, by adding stuffing bits and a word indicating
the variable length of the useful part of the group; this faci-
lity is show in Fig. 4. A transmission control unit 20 controls -
inverters 14 and 17, writing-in and reading-out of transmission
stores 15 and 16, and the insertion of the supplementary bits of
the synchronization word, of themessage-length word and of the
stuffing bits.
The digital train formed in multiplexer 18 is sent to a
coder 19 which converts the code used in the station to the code
used on the line. An analog control signal produced by generator
21 is added to the digital train and the resulting composite
signal is amplified by amplifier 22 and applied to the two-wire
line 3.
In the reception direction, the two-wire line 3 is connect~
ed to a bandpass filter 31 which transmits the control signal.
The filter is followed by a detector 32, and the detected signal
is applied to a transmission control unit 20 and a reception
control unit 40. Line 3 is also connected to a circuit comprising
a locking amplifier 33, an equalizer 34, a variable-gain ampli-
fier 35, a band cut-off filter 36 cutting off the frequency of the
control signal, a regenerator 37 which also decodes in inverse
manner to coder 19, a demultiplexer 38 and an inverting circuit
39. Demultiplexer 38 is used for extracting the synchronization
bits and the stuffing indication bits from the digital train and
applying them to the reception control unit 40.
The two outputs of circuit 39 are connected to two recep-
tion stores 41, 42. The outputs of stores 41, 42 are connected
to the inputs of the inverting circuit 43, whose output is
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connected to the outgoing two-wire line 12. The reception
control unit 40 controls inverters 39 and 43, writing-in and
eading-out cf the receltlon
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stores 41, 42 and demultiplexer 38.
The entire end station is controlled by a clock 10. Since
stores 41 and 42 are written in discontinuous manner and since
reading-out must be continuous, it is controlled by a rhythm
smoothing device 44.
Fig. 5 shows a regenerative repeater. On each side of the
repeater, the two-wire line 3 is connected to blocking amplifi.ers
51 and 61 respectively, to bandpass filters 52 and 62 respectively
and to amplifiers 53 and 63 respectively. Filters 52, 62 are
respectively adjusted to the frequencies Fl, F2 of the control
signals transmitted by stations 1 and 2. They are connected to
detectors 54, 64 respectively, which in turn are connected to a
logic control unit 50. Unit 50 controls the alternate blocking
and unblocking of amplifiers 51, 61, inverters 58, 68 and control
signal regenerators 59, 69, which will now be described.
Amplifiers 51, 61 are each connected to a circuit respectively
comprising control cut-off filters 55 and 65, equalizers 56 and
66 and variable-gain amplifiers 57 and 67. The outputs of amplifiers
57, 67 are connected to the two inputs of an inverter 58 and the
inputs of amplifie7s 53 and 63 are connected to the two outputs
A of an inverter ~. A regenerator 60 is disposed between inverters
58 and 68. ~
A regenerator~for the control signal at frequency Fl is
inserted between filter 52 and amplifier 53 ànd a regenerator~of
the control signal F2 is inserted between filter 62 and amplifier 63.
It can easily be seen that, when the regenerative repeater 5
receives the control signal Fl, the control unit 50 unblocks
amplifier 51, blocks amplifier 61, places regenerator 60 between
the output of amplifier 57 a,nd the input of amplifier 53, unblocks
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the control-signal ~e~e~r~=~ 59 and blocks the control-signal
regenerator 69.
When repeater 5 receives the control signal F2, the control
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unit 50 unbloc~s amplifier 61, blocks amplifier 51, places
regenerator 60 between the output of amplifier 67 and the input
of amplifier 63, unblocks the regenerator of control signal 69
and blocks the regenerator of control signal 59.
If ~he system according to the invention is applied to
high-speed transmission, series~parallel and paxallel-series
conversions have to be made at the store inputs and outputs
respectively, so as to use store units such as MOS or CCD stores
operating at speeds less than the transmission speed. If do is
the minimum write-in or read-out rate of the store units, the
required number k of units :in parallel must be at least (2d+E)/d
where (2d+F) is the flow rate along the line.
Advantageously, the following use may be made of the need
for series-parallel conversion in order to use high-capacity stores
operating at reduced speed. Instead of a digital train at d bits/s,
it may be preferable to have k digital trains at the rate of
(d/k)bits per second at the transmitting station since, during
high-speed digital transmission, the digital train is usually
obtained by multiplexing a number of low-speed plesiochronic
digital trains. For example, a digital train at 34.368 Mb/s is
obtained by multiplexing 16 digital trains at 2.048 Mb/s.
Owing to the need to use store units in parallel, it is
unnecessary to dispose a multiplexer upstream of the transmission
end station and a demultiplexer downstream of the receiving end
station in order to make the transmission from 2 to 34 Mb/s and
34 to 2 Mb/s respectively. Thus, the multiplexer operation is
incorporated with store operation.
Two possible methods of arranging stores 15, 16 and 41, 42
are described with reference to Figs. 6 and 7. In both cases, it
is assumed that there are 16 incoming digital channels and 16
outgoing digital channels at 2.048 Mb/s, i.e. an incoming or
outgoing flow rate of the order of 34 Mb/s allowing for stuffing,
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and that the flow rate on the two-wire line 3 is of the order of
72 Mb/s and the groups contain 40,000 bits per incoming channel,
i.e. a total of about 640,000.
In Fig. 6, 16 incoming channels 700 to 715 are applied to
input inverters 740 to 7415 which send 16 groups, each of 40,000
bits, to the 16 store parts 750 ~ 7515. Writing-in the store occurs
at the rate of 2 Mb/s. Read-out occurs at the rate of 4.2 Mb/s.
The 16 outputs of output inverters 770 ~ 7715 are connected to the
inputs of a multiplexer 78 operating bit by bit. The output of mul-
tiplexer 78 is connected to the input of multiplexer 18 in Fig. 4.
It can be seen that the store read-out rate remains acceptable,
since the high rate along the two-wire line 3 results from the ac-
tion of multiplexer 78 which interlaces the 16 channels in a single gro~.
In a manner analogous to the two stores 15, 16 in Fig. 4,
the two stores 75, 76 shown in Fig. 6 operate in opposition, one
store being written in when the other is read out. The switching
period is of the order of 20 milliseconds.
In Fig. 7, the bits from the 16 incoming channels 800 ~
8015 are not interlaced into a single group as before, but distribu-
ted among 16 successive sub-groups of 40,000 bits, i.e. one sub-
group per channel. Multiplexer 88 disposes the sub-groups in series,
and they are successively delivered by multiplexers 890 ~ 8915 at
an output rate of about 72 Mb/s. Owing to the high rate, high-order
multiplexing of the stores is necessary so that each fraction of a
store can be read at a speed compatible with its technology. In
Fig. 7 stores 85 and 86, which are alternately written-in and
read-out, are each divided into 16 stores 850 to 8515 and 860 to
8615 associated with one channel. Each channel store is subdivided
into 40 parts, e-g. 850 is made of 8500, 850 1 -- 850 39- It is
preceded by a demultiplexer 83 having 40 outputs and followed by
a multiplexer 89 having 40 inputs. The digital train on channel 800
is written in store 850 as follows: The first bit following a
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command from the transmission control unit 20 is written in 8500,
the second in 850 1' the third in 850 2 ~ the fortieth in 850 39' -'
the forty-first in 8500, the forty-second in 8501 and so on up to
about the 40,000th bit, and control unit 20 changes over store
85 to the read-out phase and store 86 to the write-in phase.
During the write-in phase of 85, all the channels are simultane-
ously written in the respective stores, i.e. channel 800 in 85
channel 801 in 851, ... channel 8015 in 8515. During the read-
out phase of 85, the stores for channels 850 to 8515 are succes-
sively read, beginning with 850. The bits are read in the orderin which they were written and are placed in series bit by bit at
72 Mb/s by multiplexer 890 in the case of the bits read in 850,
then by multiplexer 891 in the case of the bits read in 851,...,
,and by 8915 in the case of the bits read in 8515.
The advantage of dividing the group sent from one station
to another into a number of sub-groups equal to the number of
; incoming digital trains is that each sub-group can be made inde-
pendent and can be provided by multiplexer 18 (Fig. 4) with a
preamble, a synchronization word, and specific auxiliary bits
such as the departure address and the arrival address, so that
the sub-groups are,independent of the other sub-groups and
intermediate,stations can be produced, with insertion and extrac-
tion of 2 Mb/s channels at the level of the sub-groups which
represent them.
The two store arrangements which have been described with
reference to Figs. 4, 6 and 7 differ with regard to the manner
in which the bits from the various incoming channels are distributed
(i.e.,dispersed or in groups) in the transmitted group. Figs. 4,
6, 7 have a common feature, in that two-stores equal capacity
are used alternately for writing-in and reading-out.
" Other known methods of using stores may also be used ac-
cording to the invention. More particularly appreciable reductions
in store capacity and in the transmission time of the system can be
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obtained by using a single store per 2 ~lb/s track, divided into
secto'rs which are written-in and read-out in sequence, a small-
capacity margin being left so as to prevent any sector from being
simultaneously written-in and read-out.
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