System for interchanging messages in real time between stations interconnected by a loop link, in particular between stations in a telecommunications exchange

The interchange system comprises stations connected to a loop link having two rings (A1, A2) making use of a token access method and operating in load sharing mode, with each ring providing all interchanges in the event of the other ring breaking down. Each station has one adapter (1, 2) per ring, a coupler (C), and a terminal (T). Each adapter performs level 1 procedures and lower layer level 2 procedures only. The coupler includes a processor and uses a protocol at the upper layer of level 2 and at level 3 for managing load sharing between the rings, for switching the station onto one ring in the event of the other ring breaking down, and for providing protection against transmisson errors, against messages being duplicated, and against messages being out of sequence, against breakdowns in the adapters and the rings, and against short traffic interruptions due to a station being inserted or withdrawn.

The invention relates to interchanging information between stations 
connected via a loop link. 
BACKGROUND OF THE INVENTION 
Various different loop links exist having ring topology and whose operating 
and connection characteristics are defined in the standard IEEE 802. 
Within this document, standard 802-5 is applicable to ring topologies using 
a token access method, and it defines level 1 (physical level) and a 
portion of level 2 referred to as Medium Access Control (MAC), i.e. the 
lower layer of level 2. This is the data link level for defining the 
control of access to the ring. 
Standard 802-2 relates to defining logic link control (LLC), i.e. a 
different portion of level 2, referred to as the upper layer. 
Standard 802-1 relates to higher levels. 
On the ring, a pattern of several bytes, called a token, circulates 
permanently. If no station is transmitting, then the token is free. When a 
station A desires to transmit, it seizes the token, marks it as being 
occupied, and transmits data in the form of a message to a destination 
station B. No other station can then transmit. As the message goes past, 
station B recognizes its own address, copies the data intended for itself, 
and marks an acknowledgement. On return of the message, the transmitting 
station A recognizes the acknowledgement and deletes the transmitted data 
together with the token busy state. The token is thus freed for all 
stations. 
Using standard 802-5 has the following advantages: 
a large number of stations, up to 256, can be connected to the ring, and 
stations can easily be added without interrupting traffic; 
links between stations are asynchronous; 
a single station can broadcast simultaneously to a plurality of other 
stations or to all of them; 
the quality of message transmission between stations is excellent; and 
depending on requirements procedures are available for operating and 
protecting the ring. 
With respect to quality of transmission and operating security, application 
of standard 802-5 makes the following possible: 
automatic declaration of each station merely by being physically plugged 
onto the ring; 
diagnosis of the access interface at each connection; 
automatic declaration of a station having the task of token surveillance, 
and automatic replacement thereof by a different station in the event of 
the first station failing; and 
reconfiguration of the ring in the event of failure. 
The characteristics of a token ring are the following: 
At the physical level: 
asynchronous point-to-point transmission enabling long physical connections 
to be used; 
choice of physical medium depending on performance requirements, e.g. 
screened telephone pair, coaxial cable, or optical fiber; 
various data rates: 4 Mbit/s; 16 Mbit/s; 100 Mbit/s; 
an encoding law (Manchester) providing a first level of transmission 
protection; 
data protecion by means of a cyclic redundancy check for detecting errors; 
and 
standardized components and protocols for the 4 Mbit/s version (standard 
802-5); 
At link level: 
the lower layer MAC of level 2 is standardized (standard 802-5) and 
integrated in circuits of the adapters, with an example of such a circuit 
being sold by Texas Instruments under the reference TMS 380; 
the main services provided by the MAC lower layer are: 
variable message length; 
integration of level 2 protection; 
point-to-point dialog or broadcast; and 
managing various different priority levels. 
A disturbance on the ring may be of limited duration, as occurs, for 
example, when a station is inserted or withdrawn, or it may be of longer 
duration in the event of a breakdown. Mechanisms are provided at the MAC 
lower layer for protecting the ring, and indeed for completely eliminating 
the station responsible for a breakdown. If detection and confinement of a 
breakdown take several milliseconds to several seconds or even tens of 
seconds, that is not compatible with the requirements of data transmission 
operating in real time, as is the case, for example, when switching in a 
telecommunications exchange. The LLC upper layer of level 2 is generally 
implanted in each adapter and is not suitable for real time operation. 
The object of the invention is to allow stations to interchange messages in 
real time by means of a token ring type loop and to allow them to continue 
by means of a token ring type loop and to allow them to continue 
interchanging messages during a disturbance of said loop. 
Another object of the invention is to avoid losing any messages during a 
disturbance and to avoid messages getting out of sequence. 
SUMMARY OF THE INVENTION 
The present invention provides a system for interchanging messages in real 
time between stations interconnected by a loop link having two rings, each 
of which uses a token access method, each station having one adapter per 
ring, a coupler fitted with a processor and connected to the adapters, and 
a terminal fitted with at least one processor and connected to the coupler 
via a bus, said bus conveying messages received via an adapter and the 
coupler to the terminal, and conveying messages delivered by the terminal 
for transmission purposes to the coupler, wherein the two rings operate in 
load sharing mode, with messages circulating in the same direction on both 
rings, wherein, for each adapter, the coupler includes transmission 
waiting queues for storing messages to be transmitted, wherein each 
adapter performs procedures relating to level 1 and the lower layer of 
level 2 only, and wherein the processor in each coupler uses a protocol 
for the upper part of level 2 and for level 3 to manage load sharing 
between the rings, switching the station over onto one of the rings in the 
event of a breakdown on the other ring, providing protection against 
detection errors detected by a transmitting station, against messages 
being out of sequence, against messages being duplicated, against adapter 
and ring breakdowns, and against short traffic interruptions due to a 
station being inserted or withdrawn.

DETAILED DESCRIPTION 
In FIG. 1, stations Sl to Sn are interconnected by two token rings A1 and 
A2 with tokens circulating in the same direction on both rings. Each 
station comprises a terminal T which is the core of that station, and 
which is interconnected by bus BSM to a coupler C which is in turn 
connected to two adapters 1 and 2, with each adapter being connected to 
one of the rings. For example adapter 1 is connected to ring A1 and 
adapter 2 is connected to ring A2. Each adapter includes message reception 
memories and transmission memories, and a processor circuit providing 
level 1 and MAC lower layer 2 processing of IEEE standard 802, only. The 
circuit may be constituted by a Texas Instruments TMS 380 circuit, which 
is designed specifically for this purpose. The coupler C includes a 
processor for managing interchanges between the station and the rings 
which are operated in load sharing mode. To this end, the processor in the 
coupler implements a protocol on the upper layer of level 2 and on level 
3. 
Each processor circuit in an adapter includes a processor, and as soon as a 
transmission frame has been loaded into one of the transmission memories 
of the adapter and is therefore waiting to be transmitted or is being 
transmitted, the processor of the adapter prepares for the following 
transmission by loading the following frame to be transmitted into one of 
the transmission memories of the adapter. In the following description, 
the term "adapter processor" is used to designate the processor in the 
processor circuit. Similarly, each terminal of a station comprises one or 
more processors and these are referred to as the "station processor(s)". 
The set of adapters connected to a ring is protected by three logical 
entities: 
the ring error collector (i.e. "ring error monitor") which has the purpose 
of collecting all of the errors detected by the adapters; 
the supervisor (i.e. the "network manager") which serves to monitor and to 
change the individual state of each adapter and to manage the 
configuration of the ring; and 
the parameter server (i.e. the "ring parameter server") which serves to 
provide the various parameters required for operation of the ring, and is 
used only during initialization. 
Each of these three logical entities has a functional address and is 
implemented in a station processor. 
A station coupler needs to know about the logical machines and the various 
logical entities that may be implemented in the station only for the 
purpose of directing the messages and the end-of-transmission 
acknowledgements it receives. In this respect it needs to know about the 
configuration of the station in which it is located, i.e. for information 
interchanging purposes it needs to know the descriptors of the various 
queues that it is called on to manage. 
FIG. 2 is a block diagram of a station coupler C. A processor P is 
connected by a bus D and a control link LC to an adapter interface IA and 
to a terminal interface IOC. The adapter interface IA is connected to the 
adapters 1 and 2 which are in turn connected to the rings A1 and A2. The 
terminal interface IOC is connected to the terminal T of the station via 
the bus BSM. The interface IOC includes a programmable read only memory 4 
of the EPROM type, a set of registers 5, and a read/write memory 6, all 
connected to the bus D and to the bus BSM and to the control line LC. The 
memory 6 contains queues waiting to be transmitted, e.g. T queues per 
adapter, i.e. 1 queue for each of the 4 possible processing priority 
levels available to a terminal, plus a queue for messages that have been 
rerouted after being subjected to an attempt at transmission via the other 
adapter. On reception, received messages are stored in 4N terminal queues 
at the terminal coupler interface, where 4 is the number of possible 
processing priority levels and N is the number of logical machines in the 
terminal. The set of registers 5 contains information relating to coupling 
to the bus BMS. The read only memory 4 contains all of the information 
required for coupler operation, and in particular the protocols for the 
upper layer of level 2 and for level 3. 
In the description below, the term "data logical link" is used to designate 
a transmitter-destination pair, where the transmitter is a station and the 
destination is either one station or a group of stations. There are two 
data logical links pair transmitter-destination pair, one of which uses 
one of the rings as first choice and the other of which uses the other 
ring as first choice. 
The main functions relating to conveying messages as provided by upper 
layer level 2 and by level 3 are the following: managing the ring; 
detecting and correcting transmission errors; and providing protection 
against wrong sequencing, duplications, and breakdowns. 
Managing the Ring 
The two rings operate on a load-sharing basis. At each station, half of the 
data logical links use one of the rings on a first choice basis, and vice 
versa. The messages on a data logical link include a forward sequence 
number NSA, with each message being given a forward number modulo M, where 
M is equal to 256. 
When returning to operation on both rings after a breakdown, traffic is 
re-balanced. In normal operation, i.e. when no errors occur, a data 
logical link has only one physical path, i.e. one of the rings. This 
ensures that messages do not get out of sequence. 
Detecting and Correcting Transmission Errors 
Errors are detected by the transmitter (i.e. the transmitting station) by 
making use of the level 1 acknowledgement as provided by the adapter: 
error flag, frame not copied flag, address not recognized flag, and also 
by timing the messages. These categories of failure should give rise to a 
message loss rate which is less than 10.sup.-7 since these faults are 
detected by the transmitter and they are therefore capable of being 
correctly processed since the terminal of the transmitting station is 
informed and the coupler provides explicit co-ordinates concerning the 
message in question. This message loss rate is obtained by ensuring that 
the receive buffer memories of the adapter are sufficiently large and by 
retransmitting erroneous messages over the other ring, with a single 
retransmission sufficing. 
Providing Protection Against: Wrong Sequencing, Duplications, and 
Non-Detected Faults 
Messages can get out of sequence during any of the following abnormal 
conditions: 
breakdown at an adapter or a station (coupler, terminal); 
insertion of or withdrawal of a station; 
transmission faults affecting a ring or an adapter; and 
congestion of an adapter. 
By sharing traffic over the two rings (one ring per data logical link) 
activation of this protection function is limited to abnormal conditions, 
with the rate at which messages get out of sequence being less than 
10.sup.-7. 
Messages may be duplicated each time there is a transmission fault, 
estimated to occur at a rate of 10.sup.-5, or each time an as-yet 
undetected breakdown occurs, as happens typically when messages are 
retransmitted. In practice, it suffices for there to be a fault on the 
return path back from the destination to the transmitter, i.e. one fault 
in two, on average, for messages to be duplicated. 
Faults are detected and corrected under such abnormal conditions by the 
receiver, i.e. the destination station of the message. For non-broadcast 
messages, the procedure used is the following. The receiver monitors the 
sequence of forward sequence numbers, NSA, for each data logical link, and 
if it detects a gap in the sequence or if it receives two identical 
numbers, then there is a fault. 
On detecting that a message is missing from a data logical link (two 
forward sequence numbers NSA more than unity apart), the procedure is as 
follows: the, or each, message that has arrived too soon is stored in a 
temporary queue in the coupler while waiting for the possible arrival of 
the late message, thereby making it possible to continue supplying the 
terminal with messages in sequence. This waiting period is protected by a 
time-out of a few milliseconds, after which the messages are delivered to 
the terminal. If the missing message arrives after the time-out, the 
coupler does not deliver it. The rate at which this occurs is estimated to 
be less than 10.sup.-10. 
The temporary queues should become saturated at a rate of less than 
10.sup.-3. 
On detecting a duplicated message, the coupler should deliver only one copy 
of the message to the terminal. 
The above-mentioned abnormal conditions which are potential causes for 
switching over to the other ring, are now examined and solutions are 
mentioned for limiting unnecessary switchovers. 
Adapter Faults 
The protective measures required for avoiding a breakdown disturbing 
overall traffic flow are examined without assuming particularly 
high-quality efficiency and speed in the detection means specific to 
providing such protection. 
Depending on the location of the breakdown, the type of breakdown, and the 
detection means, there are two types of breakdown which can have 
repercussions on overall traffic, and these two types of breakdown are 
examined below. 
The breakdown may cause a token to be absent. For example the currently 
active monitor does not manage to send a ring purge signal for one second 
and decides to enter into the monitor contention stage. In this type of 
fault, none of the adapters connected to the ring receives a signal since 
the various time-outs used for standard protective purposes are about 1 
second in duration, which time is much longer than the time required to 
saturate the waiting queues. Ring switchover must therefore have taken 
place long before that, with the criterion for deciding to switch over 
being a transmission time-out which is shorter than the time required to 
saturate the queues, e.g. 16 ms or 32 ms, but greater than the time 
typically required for a ring to recover after a station has been inserted 
or withdrawn. This transmission time-out is started by the processor in 
the coupler at the moment it instructs an adapter to transmit a message, 
and it continues until it has received the transmission status. If the 
transmission time-out is exceeded, then it is time to switch over to the 
other ring. This type of breakdown causes all of the stations to switch 
over to a single ring substantially simultaneously, and while this is 
happening it is essential neither to loose messages nor to get messages 
out of sequence. On receiving messages, the receiver coupler processors 
correct possible wrong sequencing by means of the forward sequence numbers 
NSA, as described above. 
The breakdown does not give rise to token absence, but the messages are 
never copied by the adapter of the destination station. The transmitter, 
i.e. the station transmitting a message continues to receive an "address 
not recognized" or a "frame not copied" flag. In order to ensure good 
traffic flow, it is necessary to provide protection against faults of this 
type since they can penalize transmitter couplers heavily. The 
transmission of a message to a station whose adapter is not functioning is 
followed by retransmission of the same message in an attempt to reach the 
destination station. This is what typically happens when an adapter breaks 
down without disturbing the circulation of tokens, however it may also 
occur if there is congestion in the destination station or in its adapter. 
For this type of breakdown, the detection criterion is overflow of a 
counter for counting the number of failures to transmit to a given station 
via one of the rings. The transmitter then decides to switch over to the 
other ring in an attempt to reach the station. 
Insertion or Withdrawal of an Adapter 
This operation stops the flow of traffic on the ring corresponding to the 
adapter: signal is momentarily lost, and the token disappears. So long as 
the duration of the disturbance remains acceptable, i.e. so long as 
messages can be stored in transmission queues (i.e. about 10 ms to 15 ms), 
it is not desirable to switch rings. The length of time for which a ring 
is interrupted due to insertion or withdrawal of an adapter may vary as a 
function of whether or not the active monitor is present on the station 
corresponding to the adapter (there is a hold up of at least 50 ms on a 
ring if an active monitor is removed therefrom, since it is the monitor 
which regenerates the clock signal), and as a function of the method used 
for removing the adapter (switching off its power supply or giving it a 
deinsertion instruction). 
Consequently, the same station is never the active monitor on both rings. 
In most cases, and in particular when inserting an adapter, the insertion 
or withdrawal of an adapter takes place in less than the 10 ms which 
corresponds to the duration of the disturbance caused by the insertion 
relay of the adapter. In other cases, the transmission time-out of 10 ms 
to 15 ms described above for various types of breakdown serves to decide 
whether or not to switch over: a transmitting station may decide to 
transmit all of its messages via the other ring. 
Transmission Faults Affecting a Ring or an Adapter 
For faults of this type which are transient, retransmitting the message 
over the other ring should ensure that transmission takes place reliably, 
and there is no need to continue the switchover for following messages. 
Adapter Congestion 
Adapter congestion should not, of itself, give rise to a switchover. When 
adapter congestion occurs, the procedure is as follows: after a first 
failure to send a message to a station via one of the rings due to adapter 
congestion in the destination station, a second transmission attempt is 
made on the other ring. 
All failures relating to one of the data logical links, regardless of 
whether they are due to adapter breakdown or to adapter congestion, are 
counted in the station couplers by a counter on each data logical link for 
counting the number of failures thereon, and a switchover is invoked only 
when the counter overflows. Naturally, failures realting to a data logical 
link are counted only in the transmitting station. 
In each station, these counters are used by the software for protecting the 
coupler in order to detect faults quickly and in order to ensure that they 
are brought up to date. 
When the counter relating to one of the data logical links overflows, the 
coupler decides that the link in question is unaccessible and informs its 
local protection means. The local protection means compares information 
from both data logical links and declares that the destination station is 
unaccessible. The local protection means is a software entity which draws 
conclusions from all of the faults or anomalies detected by a station. 
This type of detected message loss is acceptable and this type of 
operation serves essentially to reduce the probability of switchover 
merely because of congestion. The next message will be given the same 
forward sequence number as the number that could not be transmitted. 
Regardless of the reason for a changeover, the changeover itself should not 
cause any message to be lost and should not get messages out of sequence. 
If messages do get out of sequence, this is corrected by the receiving 
stations so long as it occurs within the limit set by the time-out, with 
each receiving station seeking to correct wrong sequences only during the 
10 ms to 15 ms, for example, following a switchover. 
The forward sequence number NSA is used for detecting wrong sequencing or 
message loss over a data logical link. Any break in the sequence of 
forward sequence numbers causes messages that have arrived too soon to 
wait in a receive waiting queue for a maximum length of time fixed by the 
time-out. When the time-out expires, it is assumed that any missing 
message is lost and the messages in the receive queue are delivered to the 
terminal. 
Switchover is accompanied by sending information to the local protection 
means which is then put into operation for returning to the normal ring. 
Such a return is effective after a return test procedure on the normal 
ring has been successful and after a time-out has elapsed whose value is 
greater than the time required to detect breakdowns. 
An attempt at transmission is considered as having failed either on 
receiving an error (frame not copied, transmission fault, etc.), or else 
because the transmission time-out has overflowed after being started in 
the adapter by the coupler processor at the moment the message 
transmission command was enabled at the adapter. 
After K failed attempts at transmitting a message via both rings, P 
attempts on one ring and Q attempts on the other ring where K=P+Q, then 
the message is lost. However final switchover from one ring to another 
still does not take place. On rings having a data rate of 4 Mbit/s, K=2, 
and P=A=1. At higher data rates, the value of K may be increased by 1 or 
2, but should not be greater than 4. 
As mentioned above, all failures relating to a data logical link are 
counted by a counter with there being one counter per data logical link, 
and each message transmission is protected by a transmission time-out of 
about 10 ms to 20 ms, with the duration thereof being greater than the 
mean temporary nonavailability time of a ring that occurs when a station 
is inserted or withdrawn, and less than the time required for transmission 
waiting queues to saturate. If the transmission time-out overflows, that 
means that the ring is completely unavailable. 
After the counter which counts the number of failures on a data logical 
link has overflowed, then said data logical link is switched over to the 
other ring. On the transmission time-out expiring, the coupler of a 
station decides to switch over to the other ring all of the data logical 
links it is handling for transmission purposes on the ring whose 
transmission time-out has just overflowed, which ring is considered as 
being unavailable. A test prior to returning on the said unavailable ring 
takes place later on, at the initiative of the local protection means, in 
order to allow traffic from the station to be put back into balance on 
both rings. 
Message Broadcasting Is Now Considered 
Broadcasting is a facility offered by virtue of the "ring" notion and which 
makes it possible to considerably reduce the number of messages that are 
interchanged, but it is not treated specially by the adapters. So long as 
one station accepts a message, it then appears that a broadcast has been 
successful since the frame copied bit FCI and the address recognized bit 
ARI are at value zero on transmission. When broadcasting a message, the 
transmitting station can never find out whether or not the broadcast 
message has been received by all of the destination stations merely by 
observing the FCI and ARI bits it receives. This has the following 
consequences: 
it is necessary to guarantee an error loss rate in the event of adapter 
congestion which is less than 10.sup.-10, since any lost message is 
equivalent to a non-detected error; and 
the mechanism for obtaining protection against adapter breakdowns based on 
counting the number of faults per data logical link is not suitable for 
broadcasting since an adapter breakdown will not be observed. 
However transmission faults are detected and automatically cause the 
message to be rebroadcast. In all other cases, broadcast messages are 
treated in the same way as other messages. 
Like other messages, broadcast messages are given a forward sequence 
number. 
FIG. 3 shows a telecommunications exchange in which the various units each 
constituting a station are connected together via ring links of the 
invention. 
The stations SMB, SMC, SMS, and SMM are control stations interconnected by 
an inter control station loop MIS constituted by two rings A1 and A2. 
The stations SMA and SMT1 to SMTn are respectively: m stations fitted with 
auxiliary and tone equipment, and n connection units connected to 2 Mbit/s 
PCM multiplex links. Each of these stations is connected to a switching 
network SMX via respective multiplex links 31, 32, and 33. The switching 
network and the stations are connected to loop links MAS1 to MAS4 giving 
access to the control stations, with each loop link comprising two rings 
A1 and A2. The switching network SMX is connected to each of the loop 
links and to both rings in each of them, and each station is connected to 
both rings of any loop link to which it is connected. The number n of loop 
links lies in the range 1 to 4 depending on the capacity of the 
telecommunications exchange and on the data rate of the links, with 
stations being distributed over the loop links so as to spread the traffic 
load. Each semaphore station SMS is connected via a multiplex link 35 to 
the switching network SMX. 
The number of control stations SMB, SMC, and SMS is a function of exchange 
capacity. Each control station SMC is connected to all of the loop links 
MAS. 
The control stations support logical machines implanted in the terminal of 
each station. Each logical machine corresponds to a function implanted in 
a processor of the terminal, and a station supports one or more logical 
machines. The various logical machines are the following: 
a marker logical machine (MQ) which performs the following operations: 
switching messages between the switching network SMX and the stations, and 
also between the connection units SMT and the other stations; 
controlling and monitoring connections in the switching network; 
managing the multiplex links 31 to 33 to which the switching network is 
connected; 
managing operator positions; and 
acting as a bridge between the loops MIS and MAS. 
A multiregister machine (MR) for setting up and clearing down calls, and 
also for making test calls. 
A logical charging machine (TX) whose functions are to calculate call 
charges, to establish detailed billing and manage subscriber accounts, to 
perform temporary traffic observation, and to supervise charged 
subscribers. 
A translator logical machine (TR) whose purpose is to provide the 
multiregister, charger, and user (see below) logical machines with the 
characteristics of the subscribers and the circuits required for setting 
up and clearing down calls. 
A No. 7 semaphore logical machine (PE/PU) whose purpose is to perform the 
following operations when processing CCITT No. 7 semaphore signalling: 
managing semaphore channels (level 2); 
processing signalling messages, in particular message discrimination and 
distribution (level 3); 
managing telephone resources (level 4); and 
routing signalling messages (level 3). 
A No. 7 central logical machine (PC) for performing: 
management of the semaphore network, i.e. management of traffic, routes, 
and semaphore channels; 
tests and maintenance functions; and 
a function of centralizing observations. 
A stations logical machine (SM) which performs the system functions for 
each station and comprises a network manager for managing the 
configuration of the stations SMB, SMC, SMS, SMA, SMT, SMX, and SMM. 
A switching network logical machine (GX) for managing the switching network 
SMX. 
A central logical machine (OC) for controlling the switching of messages 
relating to maintenance, and for providing access to the maintenance logic 
machine (OM) of a maintenance and operating unit. 
A connection logical machine (URM) having the function of managing circuit 
status, time slot by time slot, and managing PCM multiplex links with 
remote electronic satellite concentrators and with digital satellite 
exchanges. 
An auxiliary logical machine (ETA) for handling tones and the status of 
auxiliary equipment. 
A switching logical machine (COM) for setting up, supervising, and clearing 
down connections through the switching network SMX. 
And a maintenance logical machine (OM) for performing maintenance functions 
for all of the stations, and also for keeping archives. 
In general, the logical machines are implanted in the connection stations 
as follows: 
station logical machines are implanted in all stations; 
multiregister logical machines (MR), charging logical machines (TX) and 
marker logical machines (MQ) are implanted only in SMC control stations; 
No. 7 semaphore logical machines (PE/PU) are implanted only in semaphore 
control stations SMS; 
auxiliary logical machines (ETA) are implanted only in the stations SMA; 
the connection logical machines (URM) are implanted only in SMT connection 
stations; 
the switching logical machine (COM) is implanted only in the station SMX, 
i.e. the switching network; 
the central logical machines (OC) and the maintenance logical machines (OM) 
are implanted only in the SMM control stations; 
the translator logical machine is implanted either in an SMB control 
station or else in an SMC control station; and 
the No. 7 central logical machine (PC) is implanted in any one of the 
control stations SMB, SMC, and SMS. 
The way in which the logical machines are spread amongst the control 
stations and the number of stations are functions of the capacity of the 
exchange. 
For example, the spread and the number of stations could be as follows: 
in a small capacity exchange, there may be two stations SMB fitted with 
logical machines PC, two station SMC fitted with translator, marker, 
charging, and multiregister logical machines, two semaphore stations SMS 
each fitted with a No. 7 semaphore logical machine, and a station SMM 
fitted with the central and maintenance logical machines. 
In a medium capacity exchange, there may be two stations SMB fitted with PC 
logical machines, two stations SMC fitted with multiregister logical 
machines, two stations SMC fitted with marker, translator, and charging 
logical machines, and a plurality of semaphore stations SMS fitted with 
No. 7 semaphore logical machines, and a station SMM fitted with central 
and maintenance logical machines. 
For a large capacity exchange, there may be two stations SMB fitted with 
translator logical machines, two stations SMB fitted with PC logical 
machines, two stations SMC fitted with charging logical machines, two 
stations SMC fitted with marker logical machines, a plurality of stations 
SMC fitted with multiregister logical machines, a plurality of semaphore 
stations SMS fitted with No. 7 semaphore logical machines, and a station 
SMM fitted with central and maintenance logical machines. 
The stations SMB are connected solely to the inter control station loop 
MIS. The stations SMC are connected to the inter control station loop MIS 
and to all of the loop links MAS. The stations SMC therefore have one 
coupler for the loop link MIS and one coupler per loop link MAS, whereas 
the stations SMB and SMS only have a coupler for the inter control station 
loop link MIS. 
Loop links MIS and MAS have data rates of 4 Mbit/s or of 16 Mbit/s, for 
example. Since each loop link is constituted by two rings, these data 
rates naturally apply to each of the rings. 
It is mentioned above that the number of MAS loop links lies in the range 1 
to 4 depending on the capacity of the exchange and on the data rate of the 
links. For example, one 4 Mbit/s MAS loop link suffices for 512 multiplex 
links each at 2 Mbit/s, and 4 MAS loop links suffice for an exchange 
switching traffic over 2048 multiplex links. With MAS loop links operating 
at 16 Mbit/s, a single MAS loop link suffices for 1024 2 Mbit/s multiplex 
links, so 2 MAS loop links suffice for an exchange switching traffic over 
2048 multiplex links. 
The system for interchanging messages in real time between stations is 
applicable to any set of stations interchanging messages in real time, 
with a telecommunications exchange merely constituting an example. Local 
networks using the token method are widely used in many industries, e.g. 
for controlling industrial manufacturing processes, or for controlling 
operating systems, thus constituting other applications for an interchange 
system of the invention.