Network protection scheme

Abstract: A communication system has first and second rings for passing information in opposite directions. Each ring is provided with input and output ports. A receive port receives information to be added to the network and a transmit port transmits information received from the network. A control circuit responsive to the detection of a fault is arranged to reconfigure the rings by connecting the input ports to the transmit port and the output ports to the transmit port. This provides protection of the system such that there is capacity for return traffic even in the event of failure of the system and in the presence of broadcast traffic.

Description: The present invention relates to a method and apparatus for protecting communication networks. It is common for communications equipment to be connected together in a configuration commonly known as a ring. In this type of configuration, it is necessary to provide some protection for messages being passed between pieces of equipment so as to prevent the whole ring failing in the event that either one of the pieces of communication equipment fails or else a link between two pieces of communication equipment fails. A particular problem arises in situations where a number of pieces of communication equipment wish to receive signals from a single head end site. The site may commonly house an operator's local switch, ATM edge switch, a video server or some other transmission equipment. Currently, there are two basic types of protection which are being used. In the first type of protection, all the pieces of communication equipment (nodes) are connected to the head end by two communication paths for circulating information in opposite directions and information is passed from the head end to the nodes down both parts simultaneously. Communication equipment at each node is then free to choose the information from one of the two paths. While such an arrangement is secure, it does not provide any return bandwidth in the event that the head end is broadcasting traffic. Also, there is no re-configuration of any part of the ring in the event of a fault. The second protection scheme will be described in relation to Figure 1 which explains a scheme described in CA-A-2,081,051. In this figure, the ring head end 10 normally communicates with remote communication equipment 11, 12 and 13 using a communication ring 14. A protection ring 15 indicated in broken lines is also provided but is not used in normal operation. In the event that a fault arises, the location of the fault is detected in some convenient manner and the communication ring is re-configured by causing nodes on either side of the fault to cause the ring 14 to be connected to the ring 15 as will be described in more detail Let us assume that there is a fault in the ring between nodes 12 and 13 as indicated by the large X. Nodes 12 and 13 detect this and, in the case of the node 12, the input port for ring 14 is connected through the node 12 to the output port for the ring 15, thus bringing the portion of the ring 15 between the node 12 and the head end 10 into operation. Likewise, the node 13 causes the output port for the ring 14 to be connected to the input port for the ring 15 bringing into operation the portion of the ring 15 between the node 13 and the head end 10. In the re-configured state, information destined for node 13 travels firstly by means of the ring 14 through nodes 11 and 12, by the ring 15 from node 12 through node 11, head end 10 and into node 13. While this arrangement is an improvement on the previously proposed method, messages from the head end are sent only one way from the head end and thus the arrangement is still very inefficient because there is no return bandwidth available in the event that there is broadcast traffic from the head end 10. A further protection scheme is described in US Patent No. 5,159,595 which comprises nodes coupled in a ring via two multiplexed transmission paths having opposite transmission directions. Bidirectional communications between any two nodes are provided in normal operation using both paths around a first most direct part of the ring. Within this system each path between nodes can simultaneously carry both normal and protection switched signals. It is known from WO 96/32787 to provide an optical network with nodes in a ring, with the ring normally broken in one place by an open optical switch, to form a bi-directional bus. Each node transmits and receives in both directions simultaneously around the ring. In the event of a fault, the normal break in the ring is temporarily reconnected as follows: "the node originally having the blocking (off) switch resets its switch to a transmitting (on) position and the node switch (or switches) adjacent to the cable break shuts off, the latter to ensure a break in both directions. In this way, a bus is provided that can change its starting and stopping point depending on where a cable break occurs." It is an object of the present invention to provide a method of protecting communication networks in such a way that there is capacity for return traffic even in the event of failure of the network and in the presence of broadcast traffic. SUMMARY OF THE INVENTION The present invention provides a method of transmitting information as set out in claim 1. The present invention also provides communication equipment as set out in claim 4. The invention also provides a communications network as set out in claim 7. An advantage of the above method and apparatus is that when the head end is broadcasting traffic, even in the event of a fault, there is bandwidth available for return messages to the head end. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a diagram depicting a prior art protection scheme; Figure 2 shows a diagram representing a protection system according to the present invention; Figure 3 shows a diagram similar to that shown in Figure 2 but modified due to the presence of a fault; and Figure 4 shows a block diagram of a piece of communications equipment suitable for use in the system shown in Figure 2. In the preferred embodiment of the present invention, the network may be thought of as two concentric rings with traffic being capable of flowing in the rings in opposite directions. In normal operation as shown in Figure 2, one ring 24 runs from a head end 20 to nodes 21, 22 and 23 and will carry traffic in a counterclockwise direction. The other ring 25 extends to the head end 20 from the nodes 23, 22 and 21 and is capable of carrying traffic in a clockwise direction. The portions of both rings 24,25 between the node 23 and the head end 20 are not normally used. In the event of a fault, the ring is re-configured to form two loops, one on either side of the head end 20 and the head end is arranged to transmit into both loops and receive return information from both loops. The way in which this is achieved will now be described. The head end and each node all have the same port configuration, which is as follows:- Port A is a receive port for receiving information to put into the network; Port B is an output port connected to the ring 24; Port C is an input port connected to the ring 24; Port D is a transmit port for transmitting information from the network to another device or devices; Port E is an input port connected to the ring 25; and Port F is an output port connected to the ring 24. In normal operation, each node 21, 22, 23 has its receive port A connected to the output port F and its transmit port D connected to the input port C. Each node except the last node in the ring 24 is capable of passing information not destined for itself from its input port C to its output port B and from its input port E to its output port F. The head end 20 is configured differently in normal operation in that its receive port A is connected to its output port B while its input port E is connected to its transmit port D. Each piece of equipment also has the following circuitry as shown in Fig. 4, namely a fault detector circuit 41 which, when activated causes a control circuit 42 to carry out the appropriate port to port connections via a switch circuit 43. The fault detector circuit 41 may operate in a number of ways and be sensitive to the following:- a loss of signal on an input port, loss of a part of a signal, and excessive amount of errors in data received, and the presence of a fault flag from an upstream node. The control circuitry detects the type of fault signal and, if appropriate causes the switching circuit to operate. It also inserts a fault flag into the control/data it adds to the ring so as to signal the fault to other nodes. In the presence of a fault, as shown in Fig. 3 the nodes on either side of the fault operate in similar but opposite manners in order to isolate the faulty section of the network. This is achieved by one of the nodes 22 connecting the ring input A to the ring output B while the other node 21 on the other side of the fault connects the ring input port C to the ring output port D. Neither node will pass information from its input to its output ports in either direction. This brings the previously unused parts of the rings 24 and 25 into operation and creates two loops one on either side of the head end. The head end itself also has to modify its connection scheme. In the presence of a fault, the receive port A remains connected to the ring output port B but also is connected to the ring output port F. This means that information destined for the network is transmitted to both of the loops which have been formed by the re-configuration of the ring. Likewise, the transmit port D is connected to the receive port C and the receive port E. This allows return information to be received from the ring and transmitted to other devices remote from the network. It is noted that node 23 has also changed its connection configuration. In the presence of a fault, it now passes information not destined for itself from its input to its output ports and has its receive port A connected to its output port B while its transmit port D is connected to its receive port E. For traffic added to the ring from nodes and destined for the head end, the working assumption is that before the occurrence of a fault, all traffic added must have been connected and administered to fit inside the return bandwidth of one chain or ring, also inside the transmit port bandwidth of the head end 20. After the occurrence of a fault, traffic added which is destined for the head end, has potentially more bandwidth available but provided it obeys the previous restriction it must still fit inside the head end transmit port bandwidth. Therefore, the head end only has to unintelligently sum the traffic added from both chains knowing that it "fitted" before the occurrence of the fault. The way in which the head end combines traffic coming from both directions from two chains or loops formed as a result of re-configuration of the network upon the occurrence of a fault, although relatively simple, requires some explanation. The exact way in which the head end operates will depend on the type of traffic being handled by the network. One example is traffic of the virtual container level 4 (VC-4) type. It is assumed that when nodes "turn around" in response to a fault, that dropped and inserted time slots remain the same. In normal operation, the transmit port from the head end would select outgoing traffic only from the clockwise ring or loop 25. In the presence of a fault, the head end transmit port would be required to select some traffic from the clockwise ring or loop 25 and some traffic from the anti-clockwise ring 24. Numerous criteria are available for this selection including checking for correct path trace, having nodes which break through paths set the paths unequipped, or having nodes which break through paths set them AIS. Node behaviour could be chosen such that the head end only requires a normal SNC/path protection selector. The above scheme has been described in relation to synchronous traffic but it could be applied to ATM traffic which is a synchronous or to a mixture of synchronous and ATM traffic in which case then subsidiary nodes will be performing a cell drop and insert function. In this case the complexity of the traffic combining at the head end applicable to this type of traffic depends on the nature of the service the ring offers. For instance, if the peak rates of node ingress traffic at the receive port A are all controlled to a total value of less than the total available ring bandwidth, then after re-configuration of the ring, the sum of cell rates arriving at the additional section of the head end will not increase. They will also fit in the transmit port, so cells just have to be multiplexed into the transmit port. If, on the other hand, some measure of ATM switch functionality is incorporated in the drop and insert nodes, in order to permit an ATM physical interface capable of burst operation or to support some network statistical gain, then the traffic combining function will also require ATM switch functionality. Not all networks need to have all nodes protected. One such example is where the VC-4 scheme has added to it an ATM or SDH service where the ATM cells and the SDH traffic can be contained with the VC-4 scheme. It is therefore desirable to select a protocol which permits leaving certain nodes unaffected and which could ease the upgrade of the other nodes. It is reasonable to assume that existing nodes which should be left unaffected can transparently pass VC-4 traffic. Therefore, the path overhead of the VC-4 traffic can be used to receive a protection protocol. The preferred protocol conforms with the following rules: i) Nodes need to know which port is clockwise and which port is anti-clockwise. ii) If a span between nodes fails in one direction then it is assumed that the corresponding span in the opposite direction has also failed. iii) Nodes which see a failure in one direction with respect to themselves take no action. The above described arrangement offers efficiency advantages for broadcast traffic applications, in that bandwidth available for return traffic is not reduced by the broadcast. This advantage can be used with SDH, Sonet, ATM or mixed traffic types. The precise messaging protocol which will be necessary to initiate a fault or to control the switching should be suited to or selected for the specific application.

Claims: A method of transmitting information between a number of pieces of communication equipment in a communication network (20; 21; 22; 23), the pieces of communication equipment each having receive and transmit ports (A-F) for inputting and outputting information to and from the network and being arranged in a ring circuit system form of first and second rings (24, 25) operated in opposite directions, the pieces of communications equipment dividing said ring circuit system into ring sections, wherein a defined ring section is not normally used and provides protection capacity, and wherein the method comprises the steps of i) detecting the occurrence of a fault in the ring circuit system; ii) reconfiguring the ring circuit system to utilize the ring section normally not used to form two loops extending from the fault in the ring circuit system in opposite directions, and iii) operatively connecting both loops to the receive and transmit ports of one of the pieces of communication equipment, and the method is characterised by the pieces of communications equipment (22,23) normally transmitting in one direction and receiving from one direction around the ring avoiding using the defined ring section, and by reconfiguring those pieces of communications equipment (22,23) cut off by the fault to alter their direction of receiving and transmitting around the ring to make use of the defined ring section. A method according to claim 1, wherein the pieces of communication equipment have first and second output ports and first and second input ports for connection to the first and second rings respectively, the connecting step of connecting both loops to the transmit port including the step of multiplexing the information from both loops on to the transmit port of said one of the pieces of communications equipment. A method according to claim 1 wherein the pieces of communication equipment have first and second output ports and first and second input ports for connection to the first and second rings respectively, the connecting step of connecting both loops to the transmit port including the step of adding the information from one loop to the information from the other loop. Communication equipment for use in a communications network including first and second rings (24, 25) adapted to operate to pass information in opposite directions, the rings being divided into ring sections by communication equipment, wherein a defined ring section is not normally used and provides protection capacity, the equipment comprising first and second input ports (C, E) for connection to the first and second rings respectively and first and second output ports (B, F) for connection to the first and second rings respectively, a receive port (A) for receiving information to be added to the network, a transmit port (D) for transmitting information received from the network, in normal use, only the first input port (C) and the second output port (F) being connected ,to the rings, the equipment also having a switching circuit (43) for altering the connections between the ports, and control means (42) responsive to detection of a fault in the network for controlling the switching circuit, characterised in that said control means (42) is adapted to connect the second input port (E), not the first to the transmit port (D) and the first output port (B) not the second to the receive port (A) Equipment according to claim 4, and including a multiplexing device for receiving information from the first and second input ports and feed multiplexed information to the transmit port. The equipment of claim 4 or 5, the network being arranged as a broadcast network having a head end node, the equipment being arranged to communicate to and from the head end node. A communications network comprising: a plurality of pieces of communication equipment (20, 21, 22, 23) arranged to be coupled to each other by first and second rings (24, 25) adapted to operate to pass information in opposite directions, the pieces of equipment each having receive and transmit ports (A-F) for inputting and outputting information to and from the rings (24,25) wherein the rings are divided into ring sections by the communication equipment and wherein one section between two adjacent pieces of communication equipment is not normally used and provides protection capacity, characterised in that the pieces of communications equipment (20, 21, 22, 23) normally transmit in one direction and receive from one direction around the rings so as to avoid using the defined ring section, and in that those pieces of communications equipment cut off by a fault in the rings are able to be reconfigured to alter their direction of receiving and transmitting around the rings to make use of the defined ring section. The network of claim 7, the network being arranged as a broadcast network having a head end node, the pieces of equipment being arranged to communicate to and from the head end node. The network of claim 8, the head end node being arranged at one side of the defined ring section. The network of claim 9, the head node being arranged to be normally transmitting in one direction and receiving in one direction around the ring so as to avoid using the defined ring section, and arranged to be reconfigured in the event of the fault to transmit and receive in both directions around the rings to make use of the defined ring section.

Inventors: GOODMAN DAVID MICHAEL, GOODMAN, DAVID MICHAEL
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1998
Main classification: H04J3
IPC codes: H04J3, H04Q11, H04L12