GB2332832A

GB2332832A - Path protection with flexible routing

Info

Publication number: GB2332832A
Application number: GB9727242A
Authority: GB
Inventors: John Paul Russell; David Michael Goodman
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1997-12-23
Filing date: 1997-12-23
Publication date: 1999-06-30
Anticipated expiration: 2017-12-23
Also published as: GB9727242D0; GB2332832B

Abstract

To ensure that a communication path between nodes (104, 122, 136, 142) of a network is maintained in the face of data interruptions caused by infrastructure failures (200, 210), network intelligence (170-173) is arranged to control switching circuits that selectively engage primary (106-108, 126-128) and standby (110-112, 130-132) paths within the network. Although only a single end-to-end circuit connection typically exists at any one time, each node sends data, such as regularly transmitted SDH overhead, on both a primary and a standby circuit. Network intelligence associated with interconnected nodes can then respond to the receipt (or not, as the case may be) of incident SDH overhead by selectively switching portions of intervening primary or standby paths to maintain at an operational connection. Typically, the network is realised by an interconnected combination of an SDH ring (102) and at least one arc or dual homed-spur (120), while the end-to-end connection is typically realised via a series of nodes that are located intermediate between subscriber terminals. The network may be wireline or optical fibre.

Description

COMMUNICATION SYSTEM AND METHOD OF ROUTING INFORMATION THEREIN Background to the Invention This invention relates, in general, to both a communication system and a method for routing information in such a communication system, and is particularly, but not exclusively, applicable to a communication system operating synchronous digital hierarchies (SDH), such as used in high bandwidth optical fibre communication networks.

Summary of the Prior Art When designing and subsequently deploying telecommunication systems that are wireline or optical fibre based, it is important that multiple distinct routes exist between end-to-end connections within the network. More particularly, since information transfer is reliant upon the physical properties of the wireline or fibre system (as opposed to the intangible, multi-path ether associated with radio frequency transmissions). It Is essential for network operators to be able to maintain physical end-to-end connections. The network operator therefore necessarily builds redundancy into the deployed cabling scheme to provide "path-protection" against cabling breaks that disrupt certain paths.

In the context of the present application, the term "path-protection" Is Intended to also include techniques such as sub-network connection protection (SNCP), while both techniques are sometimes collectively referred to by the term "trail protection"). Indeed, as will be understood, the generic term "trail protection" describes the protection of a path between two points that are not necessarily directly connected or are connected at or across an edge of a communication network.

Unfortunately. while separate physical paths can be used to route system traffic across the network, a single fibre break or a single network element failure on either of the alternative paths substantially limits the ability of a network operator to provide an effective service. Moreover, network operators are generally reticent to invest in redundant infrastructure equipment because it is expensive and its use must be restricted to a standby function (since failure of a particular communication link having no back-up both significantly effects service and diminishes the goodwill of subscribers). Additionally, In the event that a network operator deploys a redundant standby architecture, then the costs levied against services supported on the network are (initially) artificially high in order that the network operator can recoup, In a reasonable period of time, its significant investment in Infrastructure With specific regard to protection schemes for SDH networks, such protection schemes are defined in lTU-T (International Telecommunications Union Telecommunications) Standards G.841 and G.842 and ETSI standard DTR/TM 03041. More particularly, these three alternative connection mechanisms utilise dual-homed spurs, arcs and matched nodes to provide path-protection in an SDH ring. By way of brief introduction, it will be appreciated that, in relation to prior art structures having either dual-homed spurs or arcs, both these structures have Intermediate multiplexers that are common to both the SDH ring and either the arc or the dual-homed spur. In contrast, a matched node does not contain common infrastructure equipment, i.e. interconnected SDH rings have distinct and separate multiplexers that are only interconnected via a fibre bridge, and is hence relatively expensive In the cases of dual-homed spur and arc architectures. two failures (one In each of the alternative paths) will render the service unavailable However matched nodes will maintain service with one failure In each of the SDH rings and a failure in the interconnecting routes. Matched nodes therefore provide higher resilience against path failures, but this higher resilience Is obtained at a cost of both having to provide more network elements (i e greater infrastructure cost) and requiring more protection bandwidth to be made available. As will be understood, the term "protection bandwidth" relates to bandwidth (or communication resource) that is not normally used for traffic, but which is held in reserve to provide path-protection in instances when a fault in the main path occurs. As such, if the path-protection is shared between multiple main paths, then the amount of protection (reserved) bandwidth nominally allocated per main path Is reduced proportionally.

It would therefore be desirable to provide an improved mechanism for infrastructure interconnection that accentuates reliability of wireline or fibre networks without imposing additional expense on the network operator.

Furthermore, it would also be desirable to develop an architecture for a wireline or fibre network that optimises available channel resources while providing path-protection. Indeed, it would be more desirable for a network to be able to continue to provide a service even in the face of multi-element failure or multiple breaks in the wireline or fibre network.

Summary of the Present Invention According to a first aspect of the present invention, there is provided a communication network comprising a plurality of nodes interconnected via a plurality of wireline connections that support at least two alternative communication paths, the plurality of nodes having switches coupled within the alternative communication paths and operationally arranged to selectively engage the at least two alternative communication paths to provide an endto-end connection between two nodes. the communication network further comprising network intelligence coupled to the nodes and arranged to control the operation of the switches, the nodes comprising: means for sending a control message on the alternative communication paths; and wherein the network intelligence associated with a first node responsive to the control message is arranged to determine the presence of the control message to select a condition for the switch associated with the first node. thereby to support the end-to-end connection via a preferred path.

In a second aspect of the present invention, there Is provided a communication system comprising interconnected first and second networks each containing an input-output device for providing direct access to primary and standby transmit paths and selective access to primary and standby receive paths, each input-output device comprising a switch for selectively engaging one of the primary and standby receive paths, each input-output device further including a control processor arranged to send control messages on the primary and standby transmit paths and wherein the control processor is further arranged to detect the presence of the control messages on incident primary and standby receive paths, the control processor operationally coupled to the switch to toggle the switch to select an alternative receive path in response to the control processor failing to detect the control messages.

In a further aspect of the present Invention there Is provided a method of providing path-protection in a communication network comprising a plurality of nodes interconnected via a plurality of wireline connections that support at least two alternative communication paths, the plurality of nodes having switch circuits coupled within the alternative communication paths and operationally arranged to selectively engage the at least two alternative communication paths to provide a connection between two nodes, the communication network further comprising network Intelligence coupled to the nodes and arranged to control the operation of the switch circuits, the method comprising the steps of: sending a control message on the alternative communication paths; at the network Intelligence associated with a first node responsive to the control message, determining the presence of the control message; and selecting a condition for the switch circuits associated with the first node dependent upon the presence of the control message.

In another aspect of the present invention there Is provided a method of providing path-protection in a communication system comprising interconnected first and second networks each containing an input-output device for providing direct access to primary and standby transmit paths and selective access to primary and standby receive paths. each input-output device comprising a switch and a control processor operationally coupled to the switch for the control thereof, the method comprising the steps of: during an initial operational phase, selectively engaging one of the primary and standby receive paths; having the control processor send control messages on the primary and standby transmit paths; having the control processor detect the presence of the control messages on incident primary and standby receive paths; and in a subsequent phase and in response to the control processor failing to detect the control messages, having the control processor toggle the switch to select an alternative receive path.

In still yet another aspect of the present invention there is provided an Inputoutput device for a wireline communication system supporting a synchronous digital hierarchy, the input-output device having: primary and secondary transmission terminals; a selectable receive Interface having a switch arranged to selectively coupie the input-output device to one of a primary receive terminal and a secondary receive terminal; and a processor coupled to the switch; wherein the processor is arranged to simultaneously provide data messages to the primary and secondary transmission terminals for regular transmission across the wireline network and wherein the processor is further coupled to the receive interface, the processor further being arranged to identify the presence and absence of the data messages on the primary receive terminal and the secondary receive terminal and to toggle the switch in the absence of the data messages to select an operational communication path.

The present Invention therefore advantageously provides an enhanced form of path-protection In a wireline or optical fibre communication network which path protection is commensurate in performance with a matched node SDH arrangement but which costs significantly less to implement. Indeed, existing infrastructure may be easily modified through the provision of a processorcontrolled switch and an appropriate software up-grade.

Brief Description of the Drawings An exemplary embodiment of the present Invention will now be described with reference to the accompanying drawings. In which: FIG. 1 shows a prior art SDH ring network with dual-homed spur; FIG. 2 shows a prior art SDH ring network and arc; FIG. 3 shows a prior art architecture for matched nodes between two SDH ring networks; FIG. 4 shows a network architecture according to a preferred embodiment of the present invention; FIG. 5 illustrates, In accordance with a preferred embodiment of the present invention. routing of communication traffic through the architecture of FIG. 4 in which a single break exists In a communication resource: FIG. 6 illustrates, in accordance with a preferred embodiment of the present invention, routing of communication traffic through the architecture of FIG. 4 in which multiple breaks exist in a communication resource; and FIG. 7 depicts a flow chart of a network routing protocol and operating method of a preferred embodiment of the present Invention.

Detailed Description of a Preferred Embodiment Before discussing a network architecture and operating method of a preferred embodiment of the present invention, reference Is first made to Figs. 1 to 3 of the accompanying drawings In order for the skilled addressee to appreciate fully the differences between the path-protection mechanisms employed in the prior art and the present Invention. respectively.

Turning to FIG. 1, there is shown a prior art SDH ring network with dualhomed spur 10. Basically, an SDH ring 12 supports many communication channels having narrowband and/or broadband capabilities, which SDH ring 12 is interspersed with multiplexers 14-20 that serve to add or drop communication channels into the SDH ring 12. The multiplexers 14-20 may act as an opportune location to provide a communication relay function in which channel signals, in segments of the SDH ring 12, are boosted or amplified; amplification/relay circuitry is not shown for the sake of clarity.

Access to the SDH ring 12 may be provided via two alternative routes to a solitary ring-multiplexer, such as multiplexer 14. More particularly, an entryexit multiplexer 28 may be coupled to provide alternative paths 30-32 to the ring-multiplexer 14 from a single incident branch 34. To add a spur 36 to extend the original (basic) SDH ring network 12. a spur-multiplexer 38 is positioned at an appropriate service location outside the SDH ring 12. which spur-multiplexer 38 is independently coupled via wireline or fibre 40-42 to two different interface multiplexers (e.g. multiplexers 18-20) of the SDH ring 12. In this way, two alternative traffic paths 44-46 exist between any multiplexer in the SDH ring 12 (e.g. the entry-exit multiplexer 28) and the spur-multiplexer 23 of the spur 22. More especially, a first traffic path 44 Is provided between entry-exit multiplexer 28 and spur-multiplexer 23 via multiplexers 14 16 and 18, while a second traffic path 38 is provided through multiplexers 14 and 20.

In the context of the present invention, the terms "wireline" and "optical fibre" are interchangeable and are considered to be functionally equivalent. As such, the use of one term should not be restricted in any way, but should be considered to be a reference to both the stated alternative and any functional equivalent.

Referring now to FIG. 2, an architecture for an SDH ring network 12 with an arc 50 is illustrated. Basically, the configuration of the SDH ring network and arc of FIG. 2 is identical to the configuration of an SDH ring network and dual-homed spur of FIG. 1, except that the addition of an arc 50 to an existing SDH ring network includes the provision of intermediate multiplexers 52-56 located between the interface multiplexers 18 and 20 and the spur-multiplexer 38. The provision of an arc (rather than a spur) therefore provides a network operator with an ability to expand further the network, while also including path-protection. More especially, pairs of intermediate multiplexers 52-56 within the arc 50 can be used as jump-points to additional arcs or spurs.

The provision of an arc 50 having Intermediate multiplexers 52-56 therefore ensures that a network can be extended with path-protection, while a spur 36 merely provides access to a single multiplexer and potentially inhibits extension of the network through the apparent Incapacity of the spur to support path-protection. In other words. for a network operator to extend a spur, the network operator must either provide an additional multiplexer In a communication path 44-44 between the spur-multiplexer 38 and the interface multiplexers to produce an arc, or must provide wireline or fibre coupling from an interface multiplexer and the spur multiplexer 38 to another multiplexer located In a service area.

With reference to FIG. 3, a prior art architecture for matched nodes between two SDH rings is illustrated. Such matched nodes occur when, for example, a network operator acquires a second. physically distinct SDH ring network. and integration of the two SDH rings Is required by the network operator for service convenience. A first SDH ring 12 may therefore comprise a plurality of interconnected multiplexers 14-20 while a second separate SDH ring 70 also comprises a plurality of Interspersed multiplexers 72-78 In order that the first SDH ring 12 and the second SDH ring 70 can be Interconnected, a matched node arrangement 80 Is deployed between the separate SDH rings The matched node 80 provides distinct wireline or fibre connections 82-84 between pairs of Interspersed multiplexers (e.g multiplexers 18-20, and multiplexers 72-74) otherwise located in two distinct SDH rings. Specifically, a first route 82 (provided by a fibre or wireline connection) is coupled between the multiplexer 18 of the first SDH ring 12 and the multiplexer 72 of the second SDH ring 70, while this first route 82 is supplemented and hence protected by a second route 84 provided by a wireline or fibre link between multiplexer 20 of the first SDH ring and multiplexer 74 of the second SDH ring 70.

Each multiplexer In FIG. 3 is seen to be coupled to an operations, administration and maintenance centre (OAM) 88, with this arrangement also fundamental to the construction of FIGs. 1 and 2 (although omitted for the sake of clarity). Moreover, each multiplexer in a transfer route (i.e. a transmission path) has the ability to cross-connect between sets of virtual ports in different pools of resources available to each multiplexer. This crossconnection Is controlled by the system manager, e.g. OAM 88, and is semipermanent inasmuch as the connection is not released after a connection has been terminated but instead remains in tact until the OAM requests a change in the set-up of a particular multiplexer. The OAM 88 is therefore located at a higher system level than multiplexer of the actual transfer network.

Furthermore, the OAM in an SDH system provides SDH control overhead information on a control channel, which control overhead information identifies a path-trace to both ends of a connection In operation, the prior art either selects the primary route or the standby route, with this selection based entirely on whether information (such as SDH overhead information) Is being received. There is no ability to combine paths to produce a hybrid link between different multiplexers.

It has now been identified that the prior art dual-homed spur and arc architectures share a common routing mechanism by which an entry-exit multiplexer Is coupled to a spur multiplexer through alternative paths, which alternative paths are direct routes having the fewest number of Intermediate multiplexers. In other words, if one considers the situation in which an original SDH ring has been extended by a dual-homed arc or spur. then the two distinct fibre connections (e.g. cabling 40-42) provided to the two interfacemultiplexers in the SDH ring by the arc or spur. render redundant or eliminate the original interconnecting segment of the SDH ring between the two interface-multiplexers of the SDH ring. In fact, implementation of a prior art spur or arc terminates the use and requirement of the Interconnecting segment between the two interface multiplexers located In the original SDH ring.

Turning now to FIG. 4. an architecture of a preferred embodiment of the present invention is illustrated. For the sake of explanation only, the following description will refer to the interconnection of an arc to an SDH ring, although it will be appreciated that the concepts of the present Invention can be applied to an SDH ring having an adjoining dual-homed spur.

A first subscriber terminal 100 is coupled to receive and transmit data into a first SDH ring 102 via an input-output interface 104. such as a first multiplexer. The first SDH ring 102 contains primary (or main) paths 106-108 and secondary (or standby) paths 110-112 for both the up-link and the downlink. More particularly the subscriber terminal 100 Is able to transmit information via a primary transmit path 106 or a secondary transmit path 110. and is able to receive information on the primary receive path 108 or a secondary receive path 112. The subscriber unit 100 Is permanently coupled to both the primary transmit path 106 and the secondary transmit path 11 0. and hence (during operation) transmits data on both of these circuits.

When full system operation is available, the subscriber terminal 100 Is usually coupled to a main receive path 110, but may be selectively routed to a standby receive path 112 via a first processor-controlled switch 114 located within the input-output interface 104.

For the sake of explanation (as indicated), an arc 1 20 also contains a second input-output interface 122 (such as a second multiplexer) that couples information signals to and from a second subscriber unit 124. The arc 120 is coupled to the first SDH ring 102, which interconnection will be described subsequently.

In a similar fashion to the structural configuration of the first SDH ring 102, the arc 120 contains primary (or main) paths 126-128 and secondary (or standby) paths 130-132 for both the up-link and the down-link. More particularly, the second subscriber terminal 1 24 Is able to transmit Information via a primary transmit path 126 or a secondary transmit path 130, and is able to receive information on the primary receive path 128 or the secondary receive path 132. In a similar fashion to the first SDH ring 102, the exact routing of the paths is selectable, with routing of information to be received by the second subscriber terminal 124 controlled by a second processorcontrolled switch 134 located within the input-output device 122. The second subscriber unit 124 is permanently coupled to both the primary transmit path 126 and the secondary transmit path 130, and hence (during operation) attempts to transmit data via both of these circuits.

The interconnection of the SDH ring 102 and the arc 120 occurs at two intermediate infrastructure nodes (reference numerals 136 and 142) that are shared between the SDH ring 102 and the arc 120. Specifically, a main node 136 Is arranged to selectively and independently switch both the primary transmit path 106 and the primary receive path 108 of the SDH ring 102 (and the corresponding primary transmit path 126 and the primary receive path 128 in the arc 120) to appropriate and corresponding secondary paths of the SDH ring 102 and arc 120. Similarly. a standby node 142 Is arranged to selectively and Independently switch both the secondary transmit path 110 and the secondary receive path 112 of the SDH ring 102 (and the corresponding secondary transmit path 132 and the secondary receive path 130 in the arc 120) to appropriate paths of the SDH ring 102 and arc 120. As such (and unlike the prior art). the intermediate nodes contain processorcontrolled switches that are selectively operable to provide alternative routes within the network.

More particularly. in relation to the primary paths, third and fourth independently operable. processor-controlled switches 138 and 140 (located within the main node 136) control the routing of Information through the primary paths of the SDH ring 102. the arc 120 or both. Specifically. In the primary transmit path 106, fourth switch 140 (under fully functioning system conditions) usually routes Information from the primary transmit path 106 of the SDH ring 102 directly to the primary receive path 128 of the arc 120, while the third switch 138 couples the main transmit path 126 of the arc 120 to the main receive path 108 of the SDH ring 102.

The standby node 142 also controls two Independently operable processorcontrolled fifth and sixth switches 144-146. In a fully functional system (i.e. in a system without any equipment or fibre failures), the fifth switch 144 couples the standby transmit path 1 30 of the arc 1 20 to the standby receive path 112 of the SDH ring 102. However. since the processor-controlled first switch 114 of the first multiplexer 104 Is usually open-circuit. signals transmitted from the second subscriber terminal 124, via these standby-to-standby paths. are prevented from being connected to the first subscriber terminal 100 at the first multiplexer 104.

The processor-controlled sixth switch 146 in the standby node 142, also under normal operating conditions, couples the standby transmit path 110 of the SDH ring 102 to the standby receive path 112 of the arc 120. However, since the processor-controlled second switch 134 In the second multiplexer 122 is usually open-circuit, information transmitted from the first subscriber terminal 124, via the standby-to-standby paths, is terminated In the second multiplexer 122 in the arc 120 and therefore never reaches the second subscriber terminal 124.

The main node 136 and the standby node 142 are also coupled together via the fibre or wireline (termed the "efficiency route") that would otherwise be disregarded in the aforementioned prior art systems. More explicitly. third and fourth switches 138-140 and fifth and sixth switches 144-146 are typically two-position switches. A standby terminal 150 of the third switch 138 is coupled to a standby terminal 152 of the fifth switch 144 through a first efficiency route 154. During normal (full system) operation. the first efficiency route 154 is isolated from both the primary paths and the secondary paths.

Similarly, a standby terminal 156 of the fourth switch 140 Is coupled to a standby terminal 158 of the sixth switch 160 through a second efficiency route 160. During normal (full system) operation, the second efficiency route 160 Is isolated from both the primary paths and the secondary paths Complementary operation of the third switch 1 38 and the fifth switch 144 therefore connects the main transmit path 126 of the arc 120 to the secondary receive path 112 of the SDH ring 102 via the first efficiency route 154. Similarly. complementary operation of the fourth switch 140 and the sixth switch 146 therefore connects the secondary transmit path 110 of the SDH ring 102 to the main receive path 128 of the arc 120 via the second efficiency route 160. In other words, the fourth switch 140 serves to make or break the primary transmit path 106 to primary receive path 128 connection between the SDH ring 102 and the arc 120, while the third switch serves to make or break the primary transmit path 126 of the arc 120 to primary receive path 108 of the SDH ring 102. Also, the fifth switch 144 goes to make to break the secondary transmit path 1 30 from the arc 120 to the secondary receive path 112 in the SDH ring 102 although (as previously stated) the first switch 114 Is usually open-circuit. Finally, the sixth switch 146 goes to make to break the secondary transmit path 110 from the SDH ring 102 to the secondary receive path 132 in the SDH ring 102. although (as previously stated) the second switch 134 Is usually open-circuit. In this way, the switched within the main node 136 and the standby node 142 can co-operate to selectively route and re-route information (or data) through either one or both of the first efficiency route 150 and/or the second efficiency route 160.

Processors 170-173 within the network of FIG. 4 are arranged both to control the switches and to monitor communication activity on the primary paths and secondary paths In both the SDH ring 102 and the arc 120 Indeed, in a fully operational system. it will be appreciated that the processors should detect continuous SDH overhead information sent on both the primary and secondary paths, with such SDH overhead information (namely control and system status bytes) sent irrespective of the presence of useable speech or data. Typically. SDH overhead information will contain bytes assigned to identify, for example. alarm conditions and path-trace information. The processors 170-173 are arranged to detect a 'Far-End Receiver Failed" (FERF) scenario. an "Alarm Indication Signal" (AIS), a Loss Of Signal" (LOS) and/or a "Payload Defect Indicator" (PDI). Furthermore, it will be appreciated that the function of the processor within each node also extends to the provision and compilation of control messages. such as the SDH overhead messages, with the node having circuitry arranged to suitable encode and transmit these SDH overhead messages.

The detection of a fault by a processor downstream of a fibre break or infrastructure element failure therefore causes the processor to selectively switch between main paths and standby paths in both the SDH ring 102 and the arc 120. In this way, and by virtue of providing dual transmit outputs from Interconnected subscriber terminals, the system of the present invention can route and re-route communications via any of the available circuit paths, and can mitigate the effects of element failure or fibre breaks by selectively rerouting information.

FIG. 5 illustrates the revised interconnection of primary paths and secondary paths within the SDH ring 102 and the arc 120 during specific failures of the interconnecting infrastructure (irrespective of whether this is a break in the fibre or a failure of an intermediate multiplexer, for example).

An equipment failure or fibre break in the main path of the SDH ring has been illustrated by a cross 200. Since the downstream node (I.e the processor 171 associated with the main node 136) in the main transmit path of the SDH ring 102 is now be unable to detect SDH overhead, the processor 171 causes the fourth switch 140 to engage the second efficiency route 160 to produce a new open-circuit connection to the standby terminal 146 of the sixth switch 146.

The processor 173 associated with the second multiplexer 122 reports LOS with respect to the primary receive path 128 and therefore causes the second switch 134 to engage the standby receive path 132. The processor 173 also reports a FERF by sending an appropriate and will switch to receive signals on the secondary receive path 112. In other words, the first switch 114 selects connection of the secondary receive path 112 in the SDH ring 102 and hence isolates connection to the SDH ring via the primary receive path 108.

In the event that the state of the third switch 138 Is altered, the third switch therefore couples the primary transmit path 126 of the arc 120 to the standby terminal 152 of the fifth switch 144 (which Is still open-circuit) via the first efficiency route 150.

The configurations of the fifth switch 144 and the sixth switch 144 remain unaltered from the default (fully operational) condition, I.e. the secondary transmit path in the SDH ring Is coupled directly to the secondary receive path in the arc 120 and the secondary transmit path In the arc 120 Is coupled directly to the secondary receive path in the SDH ring 102).

In accordance with FIG. 5, therefore. Information addressed to the first subscriber terminal 100 and transmitted from the second subscriber terminal 124 can therefore be routed via the main or standby paths, while transmitted information from the first subscriber terminal 100 to the second subscriber terminal 124 is routed via the standby paths.

In relation to the switching algorithm used for path-protection In the present invention the decision criteria and strategies based on a received matched efficiency node assumes that both receive and transmit paths have failed simultaneously (I.e. dual ended protection) and. as such. if the receiving node receives FERF or AIS it will switch the traffic onto the standby path. At the receiving node, if a path-protection switch is required. the node will determine whether traffic is present on the efficiency route. (I.e the standby path) and if so will check the path-trace byte (J1 or J2) of the protection path to determine whether the protected traffic has come from the required source. If it has. then the protection switch can be made and the network will recover from a second failure.

Referring now to FIG. 6, a multi-point (two-point) failure is now shown to have occurred within the system of FIG. 4. Again, for the sake of clarity, the first subscriber terminal 100 and the second subscriber terminal 124 (as well as the OAM 88) have been omitted for the sake of clarity. The second failure is also depicted by a cross (identified by reference numeral 210), with this second failure 210 occurring in at least one of the standby paths of the arc 120; thereby severing the two-way connection of FIG. 5.

The architecture of FIG. 6 (and specifically the configuration of switches) generally follows that of FIG. 5, other than that the polarity of the fifth and sixth switch 144-146 in the standby node 142 have been reversed and the polarity of second switch 134 in the input-output device 122 (in the arc 120) has also been reversed. In this way, the primary paths 126-128 of the arc 120 and the standby paths 110-112 in the SDH ring 102 are utilized to route transmitted and received information between the first subscriber terminal 100 and the second subscriber terminal 124. Moreover, routing of information between the subscriber terminal 100 and the second subscriber terminal 124 now occurs via both the main node 136 and the standby node 142, rather than via the mutually exclusive arrangement of FIGs. 4 and 5 where only one of these two nodes actively routes information between the first subscriber terminal 100 and the second subscriber terminal 124 In more detail, the interruption in the data flow caused by the second fault 210 causes the processor 172 to register LOS (if the fault effects the transmit connection from the second node 122). Consequently, the processor 172 causes the fifth switch to change polarity to engage the first efficiency route 1 54. It will be appreciated that this may not be required if the third switch has otherwise remained in its default condition, since a direct connection would already exist from the arc 120 and the SDH ring (as realised by the primary paths). In the event that the fifth switch is toggled, transmission from the arc 120 to the SDH ring 102 is therefore via the main transmit 126 of arc 120, the first efficiency route 1 54 and the secondary receive path 112 of the SDH ring 102. Since the second fault 210 interrupts data flow in the secondary receive path 132 of the arc 120, the processor 173 therefore identifies an LOS condition and substantially contemporaneously causes the second switch 134 to engage the primary receive path 128 of the arc 120. The processor 173 also sends FERF SDH overhead, which overhead is received and Interrupted by the processor 172 associated with the standby node 142. Consequently. processor 1 72 causes the sixth switch 146 to change state such that the second efficiency route 160 Is engaged and information Is relayed from the secondary transmit path 110 of the SDH ring 102 (via the second efficiency route 160) to the primary receive path 128 of the arc 120.

The processors associated with the various nodes therefore act In an autonomous fashion from one another, with each processor selecting the path offering best quality service (and preferably the path without any associated alarm message).

The present Invention therefore maintains a higher availability of path routes than is presently attainable In the prior art arc and dual-homed spur structures, while the present Invention affords the same degree of network protection as a matched node configuration. However, the configuration of the present invention does not require additional infrastructure and therefore has a lower cost associated with its implementation than a matched node architecture. Furthermore, the present Invention uses bandwidth more efficiently than matched nodes because (referring to FIG. 3) both clockwise and anti-clockwise traffic on each nng Is passed between each of the matched nodes, thus potentially reducing the total ring bandwidth available to traffic (i.e. between multiplexers 18-20 and 72-74) Indeed. In the preferred embodiment of the present invention, the shared (efficiency) route Is only used for traffic that is already on the ring, hence only half the bandwidth needs to be provisioned when compared to that required for a matched node arrangement.

In summary (and with reference to the flow diagram of FIG. 7). intelligence that is incident to received signals (either in the upstream or downstream path) is able to detect either a fibre break or a network element failure by virtue of the lack of SDH overhead or the presence of a fault identification byte contained within the SDH overhead. The intelligence (typically realised by individual processor assigned to each node) is arranged to monitor (step 250) alternative transmission paths (step 252) and then to cause re-routing of information via an alternative protection path when loss of signal occurs (step 254). Like the prior art, the transmitting end (In both an upstream and a downstream sense) automatically broadcasts on both available paths (step 252). The matched efficiency scheme provided by the structure of the present invention therefore enhances the resilience of interconnecting communication resources by virtue of the independently operable pairs of switches (step 256) in the nodes. Where necessary, the nodes also communicate failure messages (step 258), generated in response to an LOS determination, to indicate to a succeeding node that a change In its switch configuration may be required. As such, the system performance of the present invention is comparable to that of a matched node configuration. which system performance Is achieved at significantly reduced expense (as compared against a matched node configuration). Furthermore, in relation to the efficiency routes between the main node and the standby node, only one provisioned path is required, which path is also provided by an existing SDH ring. Consequently, no additional bandwidth is required in either logical ring.

In these respects, therefore, the present invention guarantees service even if there is a break in the main path and the standby path Once a node has detected a fault condition the node may also inform the OAM 88 by sending suitable control messages to the OAM 88 (step 260 of FIG. 7). In this way, the OAM 88 can assist in Identifying the segment of wireline in which the fault has occurred.

It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention. For example, while the present invention assumes that a main path in either of the SDH ring or the arc has failed for both the up-link and down-link paths, it will be appreciated that separate fibres may be provided for each of these paths. As such. the Independent switching mechanism of the present invention can potentially maintain an upstream path even in the face of a break in the corresponding downstream path (and vice versa) by appropriately configuring switches in the main node and the standby node.

Additionally, while the present invention has been described in relation to an SDH architecture, it will be appreciated that the Invention can be applied to other forms of network provided that signalling messages are both regularly communicated between the various nodes In the network and that the nodes generally communicate the signaliing messages on two alternative paths.

Furthermore, while the processor 170-173 used by the present invention are shown to be exterior to the nodes (I.e. the multiplexers), it will be appreciated that the processors are typically located with the nodes. Furthermore, the present invention should not be considered as being limited to having processors dedicated on a per node basis, since a system wide controller could also be Implemented. However. It Is desirable to have separate controllers In order to avoid the necessity for long Interconnecting control lines (typically realised by wireline connections)

Claims

Claims 1. A communication network comprising a plurality of nodes interconnected via a plurality of wireline connections that support at least two alternative communication paths, the plurality of nodes having switches coupled within the alternative communication paths and operationally arranged to selectively engage the at least two alternative communication paths to provide an end-to-end connection between two nodes, the communication network further comprising network intelligence coupled to the nodes and arranged to control the operation of the switches. the nodes comprising: means for sending a control message on the alternative communication paths; and wherein the network intelligence associated with a first node responsive to the control message is arranged to determine the presence of the control message to select a condition for the switch associated with the first node. thereby to support the end-to-end connection via a preferred path 2. The communication network of claim 1, wherein the network intelligence of the first node further includes means for generating and sending. in response to the absence of the control message. a far-end receiver fail (FERF) message.

3. The communication network of claim 2. wherein the FERF message Is sent in a backwards direction relative to a direction of the incident control message.

4. The communication network of claim 2 or 3. wherein the network intelligence, in response to receipt of a FERF message, is further arranged to modify a switch position to alter the communication path of the end-to-end connection.

5. The communication network of claim 2, 3 or 4, wherein the at least two alternative communication paths initially comprise an active path and a standby path, and wherein a data interruption In the active path Is reported on the standby path in the FERF message.

6. The communication network of any one of claims 2 to 5, further comprising a system manager coupled to the plurality of nodes, the system manager having means for providing path-trace Information to nodes at an end of the communication path.

7. The communication network of any preceding claim, wherein the network intelligence regularly sends the control message.

8. The communication network of any preceding claim, further comprising a first input-output node in a first communication loop and a second Inputoutput node in a different communication loop, the first communication loop and the different communication loop coupled together through main and standby intermediate nodes having switches that selectively engage the at least two alternative communication paths and which switches connect together the main and standby intermediate nodes through efficiency routes.

9 The communication network of claim 8. wherein the network intelligence engages at least one of the efficiency routes in response to an interruption of data flow In an Initially active communication path.

10. The communication network of any preceding claim, wherein downlink and uplink directions between nodes are supported on different wireline connections, such that communication paths for the downlink and uplink are independently operational 18 The communication network of any preceding claim, wherein each node contains a dedicated processor that controls the operation of associated switches within each node and wherein the processor determines the presence of the control message.

12. The communication network of any preceding claim, wherein each processor provides an autonomous switch-management function within each node.

13. The communication network of any preceding claim, wherein the communication network is a synchronous digital hierarchy.

14. The communication network of any preceding claim, wherein the plurality of nodes are realised as multiplexers.

15. A communication system comprising interconnected first and second networks each containing an input-output device for providing direct access to primary and standby transmit paths and selective access to primary and standby receive paths, each input-output device comprising a switch for selectively engaging one of the primary and standby receive paths, each input-output device further including a control processor arranged to send control messages on the primary and standby transmit paths and wherein the control processor is further arranged to detect the presence of the control messages on incident primary and standby receive paths, the control processor operationaily coupled to the switch to toggle the switch to select an alternative receive path in response to the control processor failing to detect the control messages.

16. The communication system of claim 15, further comprising a plurality of intermediate nodes located at an interface between the first and second networks. each intermediate node containing: a primary switch for selectively interconnecting the primary transmit path of the first network to the primary receive path of the second network; and a secondary switch for selectively interconnecting the standby transmit path of the first network to the standby receive path of the second network; a third switch for selectively connecting the primary transmit path of the second network to the standby receive path of the first network; a fourth switch for selectively connecting the standby transmit path of the first network to the primary receive path of the second network; and first and second efficiency routes selectively operable to provide additional communication paths between the first and second networks, the first efficiency route coupling the primary switch to the secondary switch and the second efficiency route coupling the third switch to the fourth switch.

17. A method of providing path-protection In a communication network comprising a plurality of nodes Interconnected via a plurality of wireline connections that support at least two alternative communication paths, the plurality of nodes having switch circuits coupled within the alternative communication paths and operationally arranged to selectively engage the at least two alternative communication paths to provide a connection between two nodes, the communication network further comprising network intelligence coupled to the nodes and arranged to control the operation of the switch circuits, the method comprising the steps of: sending a control message on the alternative communication paths: at the network intelligence associated with a first node responsive to the control message, determining the presence of the control message: and selecting a condition for the switch circuits associated with the first node dependent upon the presence of the control message.

18. The method of providing path-protection according to claim 17, further comprising the step of in response to the absence of the control message at the network intelligence of the first node, generating and sending a far-end receiver fail (FERF) message.

19. The method of providing path-protection according to claim 18, wherein the FERF message is sent in a backwards direction relative to a direction of the incident control message.

20. The method of providing path-protection according to claim 19, further comprising the step of: in response to receiving a FERF message, modifying a switch position of the node to alter the communication path of the end-to-end connection.

21. The method of providing path-protection according to any one of claims 18 to 20, wherein the at least two alternative communication paths initially comprise an active path and a standby path, and wherein the step of sending a FERF message further includes the step of: in the event of a data interruption occurring in the active path, sending the FERF message on the standby path and vice versa.

22. The method of providing path-protection according to any one of claims 17 to 21, wherein the step of sending the control message occurs regularly.

23. A method of providing path-protection In a communication system comprising interconnected first and second networks each containing an input-output device for providing direct access to primary and standby transmit paths and selective access to primary and standby receive paths, each input-output device comprising a switch and a control processor operationally coupled to the switch for the control thereof. the method comprising the steps of: during an Initial operational phase, selectively engaging one of the primary and standby receive paths, having the control processor send control messages on the primary and standby transmit paths; having the control processor detect the presence of the control messages on incident primary and standby receive paths; and in a subsequent phase and in response to the control processor failing to detect the control messages, having the control processor toggle the switch to select an alternative receive path 24. The method of providing path-protection according to claim 23, wherein the communication system further comprising a plurality of intermediate nodes located at an Interface between the first and second networks, each intermediate node containing: a primary switch for selectively interconnecting the primary transmit path of the first network to the primary receive path of the second network; a secondary switch for selectively interconnecting the standby transmit path of the first network to the standby receive path of the second network; a third switch for selectively connecting the primary transmit path of the second network to the standby receive path of the first network; a fourth switch for selectively connecting the standby transmit path of the first network to the primary receive path of the second network; and first and second efficiency routes selectively operable to provide additional communication paths between the first and second networks, the first efficiency route coupling the third switch to the fourth switch the second efficiency route coupling the primary switch to the secondary switch, and wherein the method further comprises the steps of: in the presence of a first data Interruption In the primary transmit path of the first network: breaking the connection between the primary transmit path of the first network and the primary receive path of the second network, whereby the first switch establishes an open-circuit connection across the first efficiency route, and breaking the connection between the primary transmit path of the second network and the primary receive path of the first network, whereby the third switch establishes an open-circuit connection across the second efficiency route; and establishing an end-to-end connection across the standby transmit path of the first network and the standby receive path of the second network.

25. The method of providing path-protection according to claim 24, further comprises the steps of: in the presence of a second data interruption in the standby receive path of the second network, terminating the end-to-end connection in the input-output device of the second network: establishing a connection between the primary transmit path of the second network and the standby receive path of the first network via the second efficiency path; and establishing a connection between the standby transmit path of the first network and the primary receive path of the second network via the first efficiency path.

26. An input-output device for a wireline communication system supporting a synchronous digital hierarchy, the input-output device having: primary and secondary transmission terminals; a selectable receive interface having a switch arranged to selectively couple the input-output device to one of a primary receive terminal and a secondary receive terminal; and a processor coupled to the switch: wherein the processor is arranged to simultaneously provide data messages to the primary and secondary transmission terminals for regular transmission across the wireline network and wherein the processor is further coupled to the receive interface, the processor further being arranged to identify the presence and absence of the data messages on the primary receive terminal and the secondary receive terminal and to toggle the switch In the absence of the data messages to select an operational communication path.

27. A communication network or system substantially as hereinbefore described with reference to FIGs. 4 to 6 of the accompanying drawings.

28. A method of providing path-protection in a communication network or communication system. substantially as hereinbefore described with reference to FIGs. 4 to 7 of the accompanying drawings.

29. An input-output device substantially as hereinbefore described with reference to FIGs. 4 to 6 of the accompanying drawings.