Time-locking method for stations which form part of a local loop network, and local loop network for performing this time-locking method

A method for use in a local loop network comprising a plurality of stations which are distributed along a bus, each station being connected to the bus via a coupler which is inserted in the bus. The time-locking method for these stations comprises a transmission phase during which a looping unit which is also inserted in the bus transmits a frame (transmission frame) which consists of a synchronization word and one or more slots which initially do not contain data and which correspond to the time position occupied by each of the stations, a receiving phase during which the demodulation of the transmission frame by the master clock of the looping unit is performed after the retransmission in the form of a frame which is referred to as the receiving frame, of this frame to the coupler of each of the successive stations and, after the receiving phase, new transmission and receiving phases until the insertion of the data on the bus by each station during a transmission phase takes place with the desired accuracy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a time-locking method for a local loop network 
comprising a plurality of stations which are distributed along a bus, each 
station being connected to the bus by way of a coupler, said bus 
comprising a looping unit inserted in the bus, said method comprising 
inter alia an initialization phase for allocating to each station a time 
reference with respect to a master clock of the looping unit. 
2. Description of the Prior Art 
A method of this kind is used in a local loop network which is described in 
U.S. Pat. Ser. No. 235,291, filed Feb. 17, 1981, now U.S. Pat. No. 
4,430,699. Information transmitted by an arbitrary station forming part of 
the local loop network circulates on the bus from coupler to coupler. The 
looping unit provides the main synchronization of the network and each of 
the stations synchronizes itself during the initialization phase, for 
example, with the aid of a phase-locked loop which enables the station to 
find this synchronization. The looping unit acts as the "master" and the 
couplers act as "slaves". The transmission of the information between the 
coupler and the station itself requires a given period of time because the 
information must travel through the link which connects the station to the 
coupler. The following problem occurs: a station which is coupled to the 
bus has access to data circulating on the bus only during a period t.sub.1 
after their passage at the level of the coupler. Information transmitted 
by the station requires a period t.sub.2 in order to travel from the 
station to the coupler (t.sub.2 represents the propagation time in the 
link between the station and the coupler). This means that during the 
period t.sub.1 +t.sub.2 the bus will be unoccupied, so that the bus is 
used less efficiently. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a time-locking method for the 
stations in order to determine the correct instant at which a station 
should transmit, thus reducing the idleness of the bus. 
A method in accordance with the invention is characterized in that the 
initialization phase forms part of a transmission phase which comprises 
the following steps: 
(a) the transmission of a frame, referred to as the transmission frame, by 
the looping unit, said transmission frame comprising: 
a synchronization word which initializes the transmission and 
initialization phase, 
at least one slot which initially does not contain data and which has for 
each station a different position with respect to the position of the 
synchronization word, said position corresponding to the time position 
occupied by each station in the transmission frame; 
(b) the generating of a transmission command by at least one station after 
the transmission of the synchronization word; 
(d) the insertion, by the station and under the control of the transmission 
command, of a word into the empty slot allocated to the station; 
said transmission phase being followed by a receiving phase which comprises 
the following steps: 
(a) the demodulation of the transmission frame by the looping unit; 
(b) the retransmission, in the form of a frame which is referred to as the 
receiving frame, of said demodulated transmission frame by the looping 
unit; 
(c) the measurement by the station of the delay, with respect to the 
beginning of the slot allocated thereto, with which the word has been 
inserted into the allocated slot by the station during the transmission 
phase; 
(d) the storage by the station of the measured delay value in order to 
enable the station to advance the insertion of a word into the allocated 
slot during a next transmission by a period which corresponds to the delay 
value. 
The allocation of at least one slot which initially does not contain data 
to a station during the transmission phase enables this station to insert 
a word into the allocated slot. Due to the propagation time in the link 
between the coupler and the station, the word will be inserted with a 
delay with respect to the beginning of the slot. During the receiving 
phase, the station will again read the word it has inserted into the 
allocated slot and will measure the delay, with respect to the beginning 
of the slot, with which the word has been inserted. Knowing the value of 
this delay enables the station to take into account this delay value for a 
next transmission in order to correctly determine the instant at which it 
should transmit. 
The advantages of this method are essentially the ease of connection for 
the user who wishes to connect his station to the bus via, for example, 
optical fibers. Actually, the time-locking device allows for a substantial 
variation of the length of the fibers without any precision being required 
over this length. A further advantage of the method is that locking is 
automatic (no manual intervention, automatic locking in the case of 
repairs or replacement of fibers, or variations in propagation time due to 
temperature variations or due to the ageing of the components). 
A first preferred embodiment of a time-locking method in accordance with 
the invention is characterized in that a word inserted during the 
insertion step of the transmission phase comprises several bits, said word 
being inserted with a delay of several bits with respect to the beginning 
of said empty slot, said measurement of the delay during the receiving 
phase being a measurement based on the number of bits of delay, said 
receiving phase being followed by a further transmission phase which 
comprises the following steps: 
(a) the transmission of a new transmission frame, by the looping unit, said 
new transmission frame comprising: 
a synchronization word which initializes the transmission phase; 
at least one said slot which initially does not contain data; 
(b) the determination, taking into account the measured value of the delay, 
of the instant at which the transmission command must be generated; 
(c) the generating of the transmission command; 
(d) the insertion of a word into the empty slot allocated to the station 
with a delay which is less than or equal to one bit with respect to the 
beginning of said empty slot and under the control of the transmission 
command; 
said further transmission phase being followed by the receiving phase. 
Thus, a time-locking method is obtained in which the correct instant at 
which a station should transmit is determined with an accuracy which is 
equal to or smaller than one bit. 
A second preferred embodiment of a time-locking method in accordance with 
the invention is characterized in that the receiving phase following the 
further transmission phase is followed by so many further transmission 
phases and receiving phases that the first bit of the word inserted int 
the allocated slot occupies a correct position with respect to the 
beginning of the slot and that the complete word is inserted in the slot 
allocated to the station. Thus, a time-locking method is obtained in which 
the correct instant at which a station should transmit is determined with 
an accuracy which is substantially smaller than one bit. 
Preferably, a time-locking method in accordance with the invention is 
characterized in that said measurement of the delay is performed by 
counting the clock pulses which separate the beginning of the allocated 
slot and the beginning of the word inserted in the slot. Due to the 
presence of a master clock in the looping unit, the counting of the clock 
pulses offers a simple solution for the measurement of the delay time. 
The invention also relates to a local loop network for performing the 
time-locking method in accordance with the invention. A local loop network 
of this kind is notably characterized in that each station comprises: 
(a) a slot control device which serves to verify, during the transmission 
phase as well as during the receiving phase, whether the slots appearing 
are indeed those which have been allocated; 
(b) a counter for counting the delay between said beginning of the slot and 
the beginning of the word inserted in the slot, an input of said counter 
being connected to the slot control device in order to receive a signal 
indicating the beginning of the allocated slot; 
(c) a locking device which comprises a first input which is connected to 
the output of said counter in order to receive the delay value and a 
second input which is connected to the slot control device in order to 
receive a signal which indicates the beginning of the allocated slot, said 
locking device comprising a generator for generating transmission 
commands, an output of said locking device being connected to the coupler. 
A first preferred embodiment of a local loop network in accordance with the 
invention is characterized in that the generator of said locking device 
comprises: 
(a) a first shift register which comprises a parallel input and a series 
output for the storage of the word to be inserted; 
(b) a second shift register which comprises several parallel, shifted 
outputs which are connected, by way of a first multiplexer, to a control 
input of the first register; 
(c) a pulse generator for generating pulses at the signalization frequency, 
an output of said generator being connected to the first and to the second 
shift register and also to the counter; 
(d) a memory for storing said delay value, an input of said memory being 
connected to the counter in order to receive the delay value, an output of 
said memory being connected to the first multiplexer in order to select a 
parallel output of said second register. 
The use of shift registers with a multiplexer which is controlled by the 
pulse generator enables determination of the correct instant at which the 
station should transmit a message, taking into account the measured delay 
value which is stored in the memory. 
A second preferred embodiment of a local loop network in accordance with 
the invention is characterized in that the slot control device comprises a 
first and a second counter which count the word frequency, an input 
thereof being connected to the coupler while an output is connected to a 
first and to a second verification circuit, respectively. A control device 
having a comparatively simple construction is thus obtained. 
A further preferred embodiment of a local loop network in accordance with 
the invention is characterized in that the series output of the first 
shift register is connected to an input of a delay line having a series 
input and parallel outputs shifted through a fraction of the maximum delay 
of said delay line, said parallel shifted outputs of said delay line being 
connected to a second multiplexer, an output of which is connected to the 
coupler and a control input of which is connected to the counter via the 
memory. Thus, a time-locking is obtained in which the word is inserted 
with a delay which is smaller than a fraction of a bit, for example, one 
tenth of a bit in the case of a delay line which introduces a delay by one 
bit and which comprises ten shifted parallel outputs. 
The bus of the local loop network in accordance with the invention is 
preferably an optical bus. 
The invention is preferably used in a microcomputer network with only a 
small geographical dispersion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The data processing system illustrated in FIG. 1 comprises several local 
systems SL.sub.a, SL.sub.b, . . . SL.sub.i, . . . SL.sub.n. This data 
processing system comprises an assembly of three functional "layers" which 
are concentrically shown and which are successively referred to as the 
coordination layer 10, the communication layer 12, the transport layer 14 
and, in the center of the three layers, the actual communication network 
or data circulation bus 16. 
The coordination layer 10 is managed by intercommunication processors 11 
which comprise special hardware and software which provide the various 
coordination, communication, control initialization functions for the 
associated local systems SL.sub.a, SL.sub.b, . . . SL.sub.i, . . . 
SL.sub.n. The communication layer 12 is managed by communication modules 
13 which are associated with the various local systems and which also 
comprises special hardware and software which provide the management of 
the communication protocols between these local systems, that is to say, 
the management of the means for establishing the logic links, for 
controlling the output of data, for presenting general events at the level 
of each local system SL.sub.i, and for the detection of errors. The 
transport layer 14 comprises transmission modules 15, a looped optical bus 
16 and a looping unit 17; the modules 15 themselves comprise special 
hardware and software for maintaining the synchronization between the 
communication modules 13 and the optical bus 16, for performing the 
electro-optical and opto-electronic conversions, and for checking for 
parity errors; the looping unit itself comprises special hardware and 
software for the encoding and decoding of each transmission frame on the 
optical bus 16, thus ensuring correct transmission and reception of the 
data in the time slots allocated to each local system SL.sub.i, and for 
managing the initialization procedure enabling the synchronization of the 
transmission of the bus 16. A looping unit is described in U.S. Pat. No. 
4,430,699. To this end, the looping unit notably comprises a master clock. 
In the described embodiment the bus 16 is provided in order to establish 
the interconnection in a loop organization in which each station SL.sub.i 
operates with an output of 350 kwords of 16 bits per second; however, such 
an embodiment does not restrict the invention in any way. The transmission 
on the bus is synchronous and is organized according to the time multiplex 
principle. In the example shown in FIG. 2, each of the transmission 
modules 15a through 15n is connected to the bus 16 by means of couplers 
20a through 20n. A transmission frame transmitted by the transmission 
module of the station is inserted by passive coupling on the bus, 
indicating that the words constituting the transmission frame are inserted 
one after the other in time slots. Each coupler 20i has a time slot 
available for the transmission of data. Depending on the number of 
stations connected and their relevant requirements, it is possible to 
assign one or more time slots to the same coupler so that maximum use of 
the bus is achieved. The assignment of one or more time slots to the same 
coupler is performed by the looping unit 17. In the chosen example the 
time slot is the same for each station. The configuration of the system is 
formed, prior to its being put into operation, by the allocation in 
advance of the time slots in accordance with the real number of stations 
present with respect to the maximum number possible. 
In order to ensure that the allocation of the time slots of necessarily 
limited length to each coupler 20i is correctly performed, time-locking is 
absolutely necessary for each station. According to the present invention 
such locking is automatically performed as follows: the exchange of 
information between two arbitrary stations requires two cycles, taking 
into account the unidirectional organization of the looped bus 16; these 
two cycles are a transmission phase or write phase E, during which all 
stations have the opportunity to transmit their words, and a receiving or 
read phase R by these same stations. During these exchanges, the main 
synchronization of the network is ensured by the looping unit 17 and each 
of the stations performs its own synchronization with the aid of a 
phase-locked loop which permits the station to find this main 
synchronization (the looping unit acts as the "master" and the couplers as 
"slaves"). 
During the transmission phase (see the FIGS. 3a through 3n, part E), the 
looping unit 17 in the looped bus 16 transmits a frame, referred to as the 
transmission frame, which comprises first of all a synchronization word SW 
which is followed by several time slots which are void of data but which 
comprise for the stations a specific pattern which allows these stations 
to find the main synchronization by maintaining the receiving clock phase 
in each transmission module throughout the operation. The synchronization 
word serves on the one hand to inform the stations that the transmission 
phase prevails and on the other hand to impart a time reference 
successively to each of the stations via their relevant coupler 20i. The 
time slots serve to permit the insertion, by passive coupling on the bus 
16, of different words which are to be transmitted by the stations and 
which are referred to as W.sub.1, W.sub.2, W.sub.3, . . . , W.sub.i, . . . 
W.sub.n. 
During the receiving phase, provided by demodulation of the transmission 
frame by the master clock contained in the looping unit 17 and the 
retransmission of this frame, the frame thus retransmitted (referred to as 
the receiving frame) is successively received by each of the stations 
which perform a read operation. The FIGS. 3a through 3n (part R) which 
correspond to the state of the frames in front of each of the relevant 
stations suitably illustrate the progressive writing, during the phase E, 
of the words transmitted by the stations in the slots allocated thereto 
and, during the phase R, the reading of these words after demodulation and 
recopying by the looping unit 17. 
Upon its return to the looping unit 17, the receiving frame is erased and 
replaced by a new transmission frame which consists of a new 
synchronization word and slots which are again void of data and which are 
progressively filled again during the new transmission phase. This new 
transmission phase will be fofllowed by a new retransmission phase, which 
itself is followed by a new receiving phase, and so on. 
Supposing, for example, that there is a network in which each station 
operates with an output of 350 kwords (of 16 bits per word) per second. 
Because, after encoding, each 16-bit word becomes a 24-bit word, taking 
into account the addition of a procedure bit and a parity bit enabling the 
detection of errors, and supposing that there are, for example, eight 
stations, an effective frame thus comprises 24.times.17=408 bits (because 
it contains 17 words: a synchronization word, eight transmission words, 
eight receiving words). The signalization frequency f.sub.s then equals 
408 bits times the frequency of the words, which itself is equal to 
approximately 344 kHz, so the value of the frequency f.sub.s announce to 
140.4 mHz. 
The following description concerns the use of these transmission and 
receiving frames for performing automatic time-locking of the stations. 
The problem is as follows: an arbitrary station (st.sub.i) which is 
coupled to the bus 16 does not have access to the data which circulate on 
this bus at the instant of their passage at the level of the corresponding 
coupler, but only after a time interval t.sub.1 (see the FIGS. 4a and 4b 
which show this shift on the slots i, j, k) and, conversely, the insertion 
of its own data on the bus by the station can be correct only if the 
station transmits these data with an advance equal to t.sub.2 (see the 
FIGS. 5a and 5b) with respect to the beginning of the time slot j 
allocated thereto on the bus. These time intervals t.sub.1 and t.sub.2 
represent the propagation time through L.sub.1 meters of optical fiber, 
increased by the propagation time t.sub.r in the receiver, and the 
propagation time through L.sub.2 meters of optical fiber, increased by the 
propagation time t.sub. e in the transmitter, respectively. If v is the 
speed of propagation in the optical fiber, these time intervals are 
expressed as: 
EQU t.sub.1 =L.sub.1 /v+t.sub.r 
EQU t.sub.2 =L.sub.2 /v+t.sub.e 
Thus, for each station SL.sub.i the correct instant of transmission must be 
determined in order to ensure that the data are inserted in the relevant 
time slot without overlapping into neighboring slots. 
This determination is performed as follows. The initialization of the 
transmission is performed sequentially according to a "handshake" 
procedure, for the looping unit/transmission module. The looping unit 
first transmits the synchronization word SW, after which it allocates an 
empty slot to the first station. The remainder of the frame is occupied by 
the looping unit which transmits, (by way of its master clock) or repeats, 
depending on whether a transmission or a receiving phase is concerned, a 
particular pattern in all other slots, corresponding to a maximum energy 
of the Nyquist frequency, in order to ensure perfect synchronization of 
the stations. 
The first station is temporarily locked in several frames. A particular 
word despatched by the station at the end of locking informs the looping 
unit that this operation has been terminated. The looping unit 
subsequently indicates the next station and displaces its empty slot by 
one word (24 bits). If there is no response from a station, either because 
it is defective or because no station is connected, the looping unit 
itself acts as the station and imposes the same pattern as previously. A 
quasi-constant continuous component is thus preserved, regardless of the 
number of connected stations. 
In the case where not all stations need be interconnected, it is possible 
to assign two time slots to the same station, depending on the 
configuration and the requirements of a system or an application. A time 
slot preferably has the same duration for each station. Thus, initially 
the reception of the synchronization word may be considered to give a time 
reference to the station during the receiving phase. 
Taking into account the maximum and minimum forward/return propagation 
times between a coupler and the corresponding station (the real value is 
not yet known during the initialization, because it depends on the length 
of the fiber used and on the exact propagation time in the circuits) each 
station (except when it is absent, that is to say when it is not connected 
or out of service, the looping unit 17 immediately adapting itself to such 
a situation as mentioned above) transmits the word to be transmitted so 
that it is inserted into the first empty slot with a delay. After 
measurement of this delay during the subsequent receiving phase, the 
correct instant of transmission for the other stations is deduced 
therefrom during the subsequent transmission phase. 
During the next cycle, the stations should transmit their respective slots 
in advance with respect to the synchronization received because of the 
propagation time between the associated coupler and station. This locking 
is realized as will be described in detail hereinafter. 
FIG. 6 shows an automatic time-locking circuit of a transmission module. 
This circuit comprises an opto-electronic conversion module 66 for the 
transmission and an opto-electronic conversion module 50 for the reception 
of a word W to be transmitted by a station i via its coupler 20i. The word 
W to be transmitted is loaded into a shift register 63 whose command 
defines the transmission instant for said station i. The signal for this 
command is despatched by an n-bit shift register 60 which is activated at 
the signalization frequency f.sub.s and whose n outputs are multiplexed in 
a multiplexer 62. The signalization frequency f.sub.s is generated by a 
pulse generator 59. The generating of f.sub.s does not take place in an 
autonomous clock unit. The Great unit comprises a unique master clock 
whereto each of the stations is successively locked while adapting the 
phase of its own clock (slave) to the data received during the receiving 
phase. The generation of f.sub.s may thus be provided by a phase locked 
loop which adapts the phase to the signalization frequency desired for the 
looping unit. This phase locked loop is formed during the initialization 
phase. 
The shift start command is applied to the register 60 by a control signal 
which verifies whether the slot allocated to the station is the correct 
one and which comprises for this purpose a transmission word counter 53 
and a test circuit 54 in order to perform a comparison with the address of 
the preceding station. 
To this end, the transmission word counter 53 counts the transmitted words 
at the word frequency (presented to the input f.sub.m) and verifies, by 
way of the test circuit 54, whether the slot allocated is indeed the 
correct one. Said comparison with the address of the preceding station is 
performed, for example, by storing the address of the preceding station in 
the memory in advance. The counting of the counter for the transmission 
words starts upon the reception of the synchronization word from the 
output of the opto-electronic conversion module 50. During the circulation 
of the first transmission frame, the station selects for the control of 
the register 63 the n.sup.th output of the register 60 so that the 
transmission is performed with a maximum delay. During the circulation of 
the first subsequent receiving frame, a control circuit which comprises 
for this purpose a words received counter 52 and a test circuit 55 for the 
address of the station verifies whether the station is actually the one 
which should receive. Therefore, the words received counter 52 counts at 
the frequency of the words (presented to the input f.sub.m), received and 
verifies, by way of the test circuit 55, whether the station is indeed the 
one which should receive. The verification is performed, for example, on 
the basis of the address. The counting of the counter for the words 
received starts upon reception of the first word of the first receiving 
frame from the output of the opto-electronic conversion module 50. When 
said control circuit (52, 55) has determined that the station is indeed 
the one which should receive, it despatches a command to start the 
counting by the counter 56. The counter 56 thus starts to count, at the 
signalization frequency (f.sub.s), the number of clock pulses which 
separate this start of counting from the beginning of the word transmitted 
by the station in the slot allocated during the first transmission frame. 
The first bit of this word in the allocated slot is detected by the 
detection circuit 51 which then generates a control signal which stops the 
counter 56. Assume that, for example, C.sub.i is the number of clock 
pulses counted by the counter 56. During the next cycle, during the second 
transmission frame, the station selects the signal on the output 
(n-C.sub.i +1) of the register 60 for the control of the register 63, so 
that a first provisional locking operation can be performed with an 
accuracy of one bit; the bit counter 56 indicates the value 1 during the 
second receiving frame and a memory 61 which is connected between this 
counter 56 and the multiplexer 62 stores the command of this multiplexer. 
For suitable operation of the bus it is necessary to use a locking accuracy 
of better than one bit; therefore, a final locking operation is performed 
as follows. The output of the shift register 63 is applied to a delay line 
64 whose total delay equals one bit and which comprises ten intermediate 
outputs with steps of 1/10 bit which are multiplexed in a multiplexer 65. 
For each transmission frame, a frame counter 57 which starts only when the 
counter 56 has passed 1 selects the preceding intermediate output of the 
delay line 64, so that the transmission is advanced by each time 1/10 bit 
(see FIG. 7). For as long as the first bit of the transmitted slot does 
not occupy a correct position with respect to the clock signal (in order 
to be taken into account by a clock signal), a signal must precede this 
clock signal by at least a given time interval; this minimum time Tm 
before which the clock signal does not "see" said signal is referred to as 
the set-up time; the signal thus taken into account does not appear until 
after a period t.sub.pd which represents the propagation time of the 
signal with respect to the clock), the looping unit 17 repeats only (24-1) 
bits of the word during its demodulation and retransmission phase. When 
this phase is finally correct, the bit counter 57 of the station assumes 
the low level "0" (the command of the multiplexer 65 again being stored by 
means of a memory 58 which is connected between the counter 57 and this 
multiplexer) and despatches command to the counter 57 in order to 
terminate counting. The automatic time-locking of this station is then 
terminated. 
It is to be noted that the present invention is by no means limited to the 
embodiment described above; many alternatives are possible within the 
scope of the present invention. Notably when the links are formed by 
cables instead of optical fibers, the invention can be used equally well 
without requiring modifications other than the use of components other 
than opto-electronic components.