DE3507582A1 - Telecommunications system having a plurality of subscriber stations which access a transmission medium with a ring structure - Google Patents

Abstract

In a local area network having a plurality of stations which access a unidirectional ring in which each station which wishes to transmit initially listens in to the ring line to determine whether a transmission process is already taking place there and, if the ring line is busy, waits for access until it receives access authorization by means of a token, if the ring line is free, the station which wishes to transmit accesses the ring line, in competition with other stations, with the risk of collision, if it does not already have an information packet in the ring which has been transmitted earlier and has been terminated by a token. If a collision is detected, the transmission of the information packet is interrupted and a collision packet is transmitted; on reception of such a collision packet, the ring is closed again and the station which wishes to transmit does not start to listen in to the ring line again until a randomly determined waiting-time interval has elapsed, unless it has already recognised a token and gained access.

Description

Telecommunication system with a plurality of on one

Transmission medium with ring structure accessing subscriber stations.

For fast transfer of digital signals between frequently changing remote stations (subscriber stations), for example between telecommunication terminals and / or data processing facilities, in a spatially relatively narrowly limited area Telecommunication systems known as local area networks (LAN) are often found in this area Application in which the associated subscriber stations in accordance with an access control Have access to a common transmission medium. Such local networks are usually according to the type of transmission medium, according to its topology and according to the type of access method differed from each other (Clark et al .: "An Introduction to Local Area Networks ", Proc. IEEE 66 (1978) 11, 1497 ... 1517): Common transmission media of local networks are shielded (twisted pair) cables with two twisted cores, Coaxial cable or fiber optic cable; in topological terms one differentiates in principle between star-shaped systems, typically with central switching, and (non-directional) bus and (directional) ring systems, both typically with distributed mediation.

From the large number of access methods, only two are intended here Methods are mentioned (Leinweber: "Local Networks (LAN)", Signal + Draht 76 (1984) 7/8, 119 ... 124): As a typical bus access method, the so-called. CSMA / CD (Carrier Sense Multiple Access with Collision Detection) method should be mentioned: Before a station starts transmitting, it listens to the bus to see if it is already there a transfer process is taking place. If so, it will wait for it first End from; then the station in question begins to transmit, initially continuing "listens" until it is certain after a certain period of time (round trip delay time) is that all other stations could notice their sending. Did in the same way If another station starts transmitting at about the same time, both stations will detect a collision of their digital signals with those of the other station, whereupon they cancel their sending process after a random period of time to start over.

This is outlined below as a typical ring access method Token-Passing-Procedure called: A certain signal word ("token"), which the statement "free" or "occupied" circulates constantly in the ring. Before a Station begins to send, it must recognize the ("free") token and put it in the "occupied" token (hereinafter also referred to as "Connector") to which you can then send the sending message to adds. The message that goes through the rest of the stations and can be copied there in whole or in part, but only from the through the associated address of the specific station is finally received, is after complete Passing through the ring is deleted again by the sending station, and at the same time the "occupied" token is converted back into the "free" token.

The access methods, however, are not adapted to a specific LAN topology bound; a ring network can also be operated as a so-called contention ring, in which a station willing to send first listens to the ring line to see whether it is already there a transfer process takes place and, if applicable, with access to the Ring line waits until it recognizes a token with access authorization which it first converts the token into a connector to perceive and then a news packet (or a series) by opening the loop of message packets separated from each other by a connector) with sends out a token that completes the (last) message packet, and in which the message packets transmitted in the ring only after they have passed through the whole Ring line when the respective transmitting station is reached again from the ring line be removed again (Tanenbaum: "Computer Networks", Prentice Hall, 1981, p.311).

It has been shown, however, that such a ring system is still relatively Is prone to collision, and the invention is now the task of a telecommunications system to be specified with increased collision security.

The invention relates to a telecommunication system with a plurality of subscriber stations accessing a transmission medium with a ring structure, in which the message packets transmitted in the ring only after they have passed through of the entire ring line when the respective transmitting station is reached again from the Ring line to be removed again and in which a station willing to send initially listens to the ring line to see whether a transmission is already taking place there, and, if applicable, waits to access the loop until it with Recognition of a token receives the access authorization for the exercise of which it first converts the token into a connector and then opens the loop a message packet or a series of each through a connector from each other separated message packets with a (last) message packet terminating Sends out tokens; this telecommunication system is characterized according to the invention that with a free ring line the station willing to send is in collision-prone competition to other stations only takes access to the ring main when it is about message packet previously sent out and terminated with a token again removed from the ring main.

The invention brings with it the avoidance of several of one and the same Station originating (non-contiguous) packets on the ring line a reduced Collision probability and thus a higher collision security with itself.

In a further embodiment of the invention, each sending station, based on a - at least partial comparison with her on the ring line incoming message packets removed from the loop with the corresponding Copies of the previously sent message packets detects a possible collision or which receives a collision packet indicating a collision via the ring line, send out such a collision packet with the message packet being aborted and when such a collision packet is received, the removal of incoming packets quit to then only after a waiting period of time when aborting of the message packet transmission already wholly or partially located on the loop Resend message packets.

Without the need for packages of a certain minimum length, this enables a reliable detection of a collision for everyone involved Participant stations and at the same time quick collision resolution.

Further particularities of the invention will become apparent from the following detailed explanation of an embodiment of a telecommunication system according to of the invention based on the drawings. 1 shows an exemplary embodiment a telecommunication system according to the invention; FIG. 2 ff. Illustrate certain Operating situations in such a telecommunication system.

The drawing FIG 1 shows schematically in an understanding of the Invention required scope a telecommunication system with a by a Ring line R given transmission medium to which a plurality of subscriber stations T has access. The subscriber stations T are for this purpose each by means of a connection device (Ring interface) J connected to the ring line R, the connection device J on the one hand, the ring line section leading to it constantly with the input E connects the relevant station T and on the other hand depending on the respective switching state of a switch S either the ring line R at this point via a 1-8it delay element V closes or the ring line R interrupts and output A instead the relevant station T with the continuing ring line section connects. Such a connection device J, which essentially meets the same requirements suffices as a connection device for so-called token rings, normally holds the Ring R closed via its switch S, so that originating from other stations T Message packets are retransmitted in the ring R; at the same time enables the connection circuit J the station T belonging to it listening to the ring line R.

If the associated station T itself sends message packets, the ring switch S of the relevant connection circuit J is switched over, see above that the ring R is interrupted at this point; the ring remains interrupted until the packet (s) sent out has (have) completely traversed the ring R and thus no longer appears (appear) on the ring.

In the event of an error, as usual in comparable cases, it must be ensured that that the ring remains closed or

is closed, which is not to be pursued further here, since this is not necessary to understand the invention.

Message packets sent from a station T like this is also indicated in Figure 1, the station output A bit-serial from a memory WS are fed forth, in which message packets pending for transmission are in a queue can form; Copies of the packages sent in each case remain in the queue memory for the time being or, as is also indicated in FIG. 1, in a separate one associated therewith Memory MS stored until it is certain that they have been transmitted without errors.

As can also be seen from FIG. 1, may each station T a with have the station input E connected receive buffer register PR, which for Intermediate storage of message packets that are received via the ring line R is used are, however, of all received packets only the packets, their recipient address matches the ring address of the station currently being viewed, not discarded will. In Figure 1 it is indicated that the recipient address of a message packet an address detector AD is connected to the receiving part of the buffer register PR, the wired or otherwise stored address of the relevant station T compares with the recipient address contained in the respective received message packet and only unlocks the output of the buffer register PR if they match.

Simultaneously with the recording of one at the station just viewed T incoming message packets in their buffer register PR is the relevant The message packet is also forwarded from the associated connection device J in the ring R, provided that the station you are looking at was not the sender of the package itself. Was the station T currently under consideration, on the other hand, is the sender of one in this station T just received packet, the packet is in the connection device J from the ring R, which it has just passed through completely, is removed again; this happens in that the connection device J with the help of its switch S the ring R Holds interrupted until all of the station T currently being viewed has been sent Message packets from the station T have been received again. To do this, how this is also indicated in FIG. 1, a packet comparator PV each in the buffer register PR received message packet with the earliest one sent and compare the message packet still stored in the memory MS in order to find it in the event of a match to delete; the memory MS is empty, - what if necessary also by one of the respective Up / down counter indicating memory content can be displayed, - so are all the message packets just sent out by the station T in question after completely passing through the ring R again removed from the ring R, and in the Connection device 3, the switch S can close the ring R again. In FIG 1 is indicated that an input side to corresponding memory cells of the Memory MS connected OR gate TO determines that the last one in the memory MS Message packet was deleted, and then the delivery of a reset pulse causes a bistable flip-flop SK, which will be returned to below.

From further detailed explanations of the above Outlined basic mode of operation of the telecommunications system outlined in FIG. 1, as given by a station T when message packets are sent is, can be disregarded here, since this is not necessary for an understanding of the invention is; Rather, the invention is based on known working principles of telecommunication systems with a plurality of on a transmission medium with ring structure accessing subscriber stations.

However, before sending message packets through When a station T comes, the station T willing to send first listens to the ring line R in whether a transfer process is already taking place there. In Figure 1 is this indicated that to the input E of the station in question T and thus to the ring line R a monostable flip-flop HK is switched on via a switching device HS, that through each "1" signal element (or "O" signal element) is one message packets arriving via the ring line R at the station T under consideration be restarted and then a certain, the ring rotation time corresponding Time to stay in his working state. If this is the case, i.e. finds on the ring line R already takes place a transmission process, so the willing to send is waiting Station T with access to the ring line R until it recognizes a token receives the access authorization. In FIG 1 it is indicated for this purpose that at the output of the monostable flip-flop HK connected to one input of an AND element 1A whose other input is connected to that part of the buffer register PR is that, if necessary, a token that terminates a received message packet records. When such a token is received, the coincidence condition of the AND gate 1A met, which means that the station T currently being considered is authorized to access the ring line R.

To exercise the access authorization is in the station T first the ("free") token is converted into a connector ("occupied" token), which differs from the Token usually only differs in its last bit. In FIG 1 is accordingly indicated that the AND element 1A is a monostable flip-flop element BK is started, the working time span of which may correspond to a bit time span and for this period of time via an OR gate SO the ring switch S to the station T belonging connection device J switches and at the same time a connector bit generator CG unlocked, so that instead of the last bit of the - otherwise so far via the Ring switch S in ring R transferred - tokens transfer an inverted bit and so a connector is transmitted instead of the token.

Then the interruption caused by the ring switch S is maintained the ring line R a message packet - or a series of each through a Connector separate message packets - with one the news package (or the last news package in such a series of message packets) final tokens are sent. In Figure 1 is indicated for this purpose, that from the output of the monostable trigger element BK via an OR element ZO a bistable Flip-flop SK is activated, the output of the queue memory from its output WS accordingly unlocks and at the same time the ring switch via the NAND element SO S holds in its switching state, in which it connects the station output A with the outgoing Ring line section connects.

As can be seen from the explanations above, in the telecommunication system described here, in contrast to a packet format suitable for token-passing protocols, according to which the token (or the connector) is located in the packet header, each message packet is replaced by a token (or a Connector), resulting in the following packet format: Recipient sender number Text cToOkneector "Receiver" and "sender" are bit sequences of z. B. 8 bit length, which clearly identify the recipient and the sender of the package in the ring. The sequential number, for example 4 bits long, is used to number the packets that are transmitted from a specific sender to a specific recipient consecutively, whereby the numbers can be reassigned cyclically.

The text contains the information to be transmitted, whereby the text length from about 100 to 10,000 bits are not subject to any restrictions on the part of the access protocol subject.

Finally, token and connector are two bit sequences of, for example 8 to 16 bits, which are otherwise not allowed to appear in message packets, for example can be achieved by so-called bit stuffing; Token and connector are different usually exactly in the last bit.

As already mentioned, each packet is transmitted by a token or a connector completed, with a connector indicating that the packet just transmitted another packet follows, while a token indicates that another is not immediately Package follows. With the described listening into the loop and - through a transmission process on an occupied ring line - waiting for a message packet The final tokens that bring with them the right to send will be issued in the Connection to the previous transmission process of the station picking up the token a collision-free access to the ring line R enables.

If, however, the first listening into the ring line R is willing to send Station finds the ring line R free, the station T willing to send takes it in collision-prone competition with other stations that are also willing to broadcast Access to the ring line R, but only if you have one of her earlier sent and terminated with a token message packet again from the Ring line R has removed. This ensures that one and the same station does not contribute to a collision possibility more than once. In Figure 1 is indicated for this purpose, that when listening into the ring line when the ring line is free in its idle state remaining monostable flip-flop HK in this case now instead of the AND element 1A a NOR gate 1B prepared for the coincidence case; this occurs if at the same time one on the input side to corresponding memory cells of the memory MS connected OR gate TO determines that in the memory MS none with a token completed message package is no longer included. With the fulfillment of its coincidence condition is the NOR gate 1B from an output signal through which the following OR gate ZO the bistable flip-flop SK is activated so that it is in one of the operations already described above corresponding to a switchover of the ring switch S and to unlock the queue memory on the output side WS is coming.

At this point, take a look at the drawings of FIG. 2 to FIG. 7 thrown, which is somewhat simplified in comparison to the illustration in FIG Representation of a ring system make certain operating situations clear how they can arise in a telecommunication system according to the invention.

Here, FIG 2 shows how an individual sent by a station Ts Message packet Ps of - based on the ring cycle time - of medium length in one occupies the ring R at a certain moment; the package is closed by a token To. In this very situation, in which the sent out by her and completed with the token To The package is still completely (or only partially) on the ring line R, may the station Ts do not send another package.

FIG. 3 shows a series of packets ..., Ps2, sent by station Ts Ps3, Ps4, ..., each separated from one another by a connector Co; the last package of such a series, which, as indicated in FIG. 3, also - as well as a single packet - longer than the round trip time can be, is in turn through a token must be completed, and as long as this token does not return to the station Ts will have arrived, no further package will be sent from the station Ts.

The operating situations outlined in FIG. 2 and FIG. 3 can arise both result if the station currently sending Ts previously had access to the ring line R initially occupied by a previous transmission process had waited until it received the last packet of this previous transfer process final tokens recognized and picked up, as well as when they hit the had accessed the ring line R recognized as free when initially listening, without other stations also accessing the ring line R at the same time to have.

In FIG. 4 and FIG. 5, two operating situations are now outlined, such as they can arise when two stations Ts and Tt willing to transmit are in competition to each other at about the same time to the ones found as free during the initial listening Access ring line R: According to FIG. 4, the station Tt begins to send its packet Pt, while the packet Ps already completely dispatched by the station Ts is in the ring R is located; According to FIG. 5, both stations Ts and Tt transmit practically at the same time off their packages Ps, Pt. In both cases, from the two stations Ts, Tt to The respective packet is terminated by a token To in the manner explained above at least as long as the respective package is still wholly or partially on the Ring line R is located, no further package sent.

Both transmission situations still lead to a collision, regardless of whether this - what of the location of the respective recipient and the respective Package length depends, - the packages from the intended recipients in whole, in part or not received at all.

Likewise, the operating situations outlined in FIG. 6 and FIG. how it appears when three stations Ts, Tt, Tu are accessed at approximately the same time the Ring R can result in collisions.

It should be noted here that a collision occurs when one Station (e.g. the station Tt in FIG. 4 to FIG. 7) which receives a packet (Pt in FIG. 4 to FIG 7) has sent or is still sending, then a different package (Ps in FIG 4 and FIG 5; Pu in FIG 6 and FIG 7) of another sender (Ts in FIG 4 and 5; Tu in FIG 6 and FIG 7) receives without first receiving its own packet to have.

In the operating situations outlined in FIG. 4 and FIG. 5, each can of the two simultaneously transmitting stations Ts and Tt the collision based on a at least partial comparison, including at least the packet headers, of the comparison with her arriving via the ring line R - and removed from the ring line in the process - Message packets with corresponding copies of the previously sent message packets determine. In FIG 1 it is indicated that with the output of the packet comparator PV is connected to the blocking input of a blocking element KU, which is connected to two further inputs each via an OR element VOe, VOs with the two-sided inputs of the packet comparator PV is connected. The blocking element KU for is via the two OR elements VOe, VOs the coincidence case is prepared as soon as at both inputs of the packet comparator PV one package is pending at a time; if both packages do not match, see above the coincidence condition for the locking element KU is met.

As soon as a station recognizes that a collision has occurred, interrupts they send their parcels. In Figure 1 is indicated in this respect that when meeting the Coincidence condition for the blocking element KU from its output via an OR element KO the bistable flip-flop SK is reset, with which the output of the queue memory WS is blocked.

If exactly two stations are involved in a collision, then necessarily both stations recognize the collision of the sent packets. However, if three or more stations are involved in the collision, this is not the case absolutely the case; as can be seen in FIG 6, the ring system could then be in a get into an undefined state. To prevent this, each station sends the recognizes a collision, a collision packet K (in FIG. 7), which is from each different message packets as well as tokens and connectors, and holds by means of their ring switch S (in Figure 1) interrupted the ring R until they have a collision packet received and removed from the ring. In Figure 1 it is indicated that at the same time with the resetting of the bistable flip-flop SK, a bistable flip-flop KK is activated so that the ring switch S continues to be in its toggle state via the NAND gate SO remains, and that a collision packet generator KG is also unlocked, so that via the output A of the relevant station T and its ring switch S a collision packet is fed into the ring R.

Such an operating situation for the two stations is shown in FIG Ts and Tt are sketched, which accordingly each have a collision packet Ks or Send out Kt.

In the operating situation outlined in FIG. 7, the station Tu has up to now not recognized that a collision has occurred; but one recognizes from the drawing 7 shows that the station Tu will soon receive the collision packet sent out by the station Ts Ks is received.

A station, as in the example of FIG 7, the station Tu, at least has a package wholly or partially on the ring, but has not yet had its own collision package sent because she couldn't see a collision, also cancels an ongoing transmission process as soon as it receives a collision packet receives. Then it in turn gives a collision packet to the next station in the Ring R further.

In the drawing FIG 1 is indicated for this purpose that with the buffer register PR a collision packet detector KD is connected to the reception of a collision packet responds and thereby emits an output signal, which via the OR gate KO the triggers the same processes as they are also triggered by a in the manner explained above Output signal of the blocking element KU are triggered. After such a transfer a received collision packet becomes the ring from the station T concerned R closed again; FIG. 1 indicates in this regard that after one for the collision word forwarding required period of time by an appropriately dimensioned by the collision packet detector KD started timer KL, the bistable flip-flop KK is reset.

The station T in question already had a collision itself detected and accordingly already sent out a collision packet, so If a collision packet is received, it is only removed from ring R, whereupon the ring is closed again by means of the associated ring switch and thus the Incoming packet removal is terminated. In FIG 1 it is indicated for this purpose that in such an operating situation the coincidence condition for an AND element KKU is met, one input of which is connected to the output via a bistable flip-flop KS of the blocking element KU is connected and the output side also to the reset input of the bistable flip-flop KK leads.

For the sake of completeness, it should be noted that in all stations, in which the ring R (in Figure 1) closed via the respective associated ring switch is, a collision packet is retransmitted unchanged like any other packet.

Every station affected by a collision sends - if it does not recognized and picked up a token beforehand - yours when the message packet was aborted message packets already wholly or partially located on the ring line R. only turns off again after a randomly determined waiting period. The respective Station one e.g. with the help of a random number generator, one from a pulse generator with a pulse period corresponding to the ring cycle time with counting pulses Pulse counter and a comparator comparing random number and counter reading or realized with the help of a microprocessor, randomly determined in length Run through waiting time loop VS, after which the switch HS (in Figure 1) of the relevant Station T (in Figure 1) receives a renewed actuation signal, so that the relevant Station T again in one of the processes already explained above Way begins by first listening into the ring line R.

The waiting period w is expediently dependent on this determined by the number z of collisions that the first packet of a sequence of from the station in question sent packets that were at the time of the collision detection was on the ring, has already learned about. This collision number z can, like this is also indicated in FIG. 1, determined with the aid of a collision counter KZ that of the OR gate KO with counting pulses is applied and which is reset to zero in each case by the output of the packet comparator PV. The waiting period w is then expediently with (v for z <lb N w = ((2LRi for z alb N, where LR is the cycle time, i is the serial number (position) the station under consideration in the ring and N is the number of stations in the ring and where v is a random variable that uniformly distributes values from the value interval I assumes 0, 2Z LR1.

As long as one of the station you are currently viewing (possibly

again) the packet to be sent experienced fewer than 1b N collisions has, i.e. the counter KZ shows a smaller collision number z, the waiting time becomes determined equally distributed from the interval 10, 2Z LR1. With a larger number of collisions z is one of its serial numbers (position) in ring R for the station concerned corresponding fixed waiting period assigned to the product of the station number i and twice the ring cycle time LR. In this way it is achieved that the waiting period is not even when unfavorable circumstances coincide can be any size.

After in the above the invention based on one with in each station T provided special circuits implemented embodiment explained was finally noted that a realization with the help of microprocessors provided in the stations is possible.

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Claims (3)

Telecommunication system with a plurality of on a transmission medium (R) with a ring structure accessing subscriber stations (T), in which the message packets transmitted in the ring (R) only after they have passed through of the entire ring line (R) when reaching the respective transmitting station (T) again be removed from the ring line (R) and in which a station willing to transmit (T) first listens to the ring line (R) to see whether there is already a transmission process there takes place and, if applicable, waits to access the ring line (R), until it receives the access authorization with recognition of a token to perceive it they first convert the token into a connector and then open the Ring line (R) a message packet or a series of each through one Connector separate message packets with one the (last) message packet Sends final token, characterized in that when the ring line is free (R) the station wishing to transmit (T) in collision-prone competition with other stations (T) Access to the ring line (R) only takes place if it was sent out earlier and a token-terminated message packet back from the loop (R) removed. 2. Telecommunication system according to claim 1, d a d u r c h g e k It is noted that each sending station (T), based on at least one partial comparison of the incoming via the ring line (R) and thereby message packets removed from the ring line (R) with corresponding copies the previously sent message packets detects a possible collision or the receives a collision packet indicating a collision via the ring line (R), sends out such a collision packet with the message packet being aborted and when such a collision packet is received, the removal of incoming packets terminated, and that every such sending station (T), which in the meantime has not recognized a token and has picked up, only after a waiting period of time when the message packet transmission was canceled Message packets already wholly or partially on the ring line (R) resends. 3. Telecommunication system according to claim 2, characterized in that that the waiting period w is measured with (v for to ob N w = ((2LRi for z alb N, where L R is the cycle time, i is the serial number of the station in the ring and N is the Number of stations in the ring are and where v is a uniformly distributed value from the Values interval 10, 2z LR 1 is a random variable assuming.