Integrated telecommunications line systems and cross-connect

Abstract: An integrated line system/cross-connect arrangement for switching traffic carried on first and second paths in a synchronous telecommunications network reduces the quantity of cross-site cabling and switch complexity required at higher transmission bit rates The traffic is loaded into virtual containers according to whether that traffic is or is not to be switched at the cross-connect. Switching is performed at the cross-connect at the virtual container level.

Description: This invention relates to telecommunications systems and in particular to the provision and operation of cross-connects in synchronous transport networks. BACKGROUND OF THE INVENTION Traditionally, synchronous (SDH or SONET) transport networks incorporate backbones that have been designed using cross-connects as flexibility points linked by line systems. In European SDH networks, the primary function of the cross-connects in the backbone is VC4 management for routing and restoration. In North American SONET networks, 3x3 cross-connects serve a similar function to provide management at the sts-1 level. In these networks, line systems comprise either; unprotected 1+1 or 1:N protected line systems interconnecting cross-connects in a mesh arrangement; or ring systems with either unidirectional (DPRing) or bi-directional (MS-SPRing) rings. In Europe, the most common network strategy has been the use of the mesh arrangement with unprotected STM16 (2.5 Gbit/s) line systems. In North America, long distance transport employs meshing with 1:N OC48 (2.5 Gbit/s) line systems, while regional operating companies employ ring systems. In a synchronous network, digital traffic is transported as the payload in virtual containers to which overheads are attached and which are then incorporated in frames. The international synchronous network standards define different combinations of virtual containers that can be used to fill the payload area of a frame. The process of loading containers and attaching overheads is repeated at a number of multiplexing levels A description of a SONET cross connect is given by F G Noser in specification No. EP-A2-0 559 091. A description of a cross connect architecture for SDH signals is given by R Ahola et al. in specification No WO 95/32599. With the constantly increasing traffic demand, operators are now installing higher capacity transport systems, the new standard being 10 Gbit/s which corresponds to the SDH STM64 standard or the SONET OC192 standard. The large increase in network bandwidth that is provided by these high capacity transport systems has introduced a consequent need for growth in the size of cross-connects, particularly in meshed networks. At a cross-connect, the incoming signal is demultiplexed into its components which are individually switched before being re-multiplexed for onward transmission. However, the cross-connect sizes that are now being demanded for these high capacity systems exceed the capabilities of the equipment suppliers. Using current technology, a cross-connect of the required capacity would require numerous interface shelves and a large amount of cross-site cabling between the line systems and the cross-connect. This lack of availability of a suitable cross-connect at an acceptable cost has placed constraints on the introduction of higher bit rate systems. GB2298767 discloses a cross-connect node for a synchronous communications network. The cross-connect node has an interface unit for converting a traffic signal carried in a first synchronous communications network transmission format (SONET OC-12/STS-12) from the optical domain to the electrical domain and to de-multiplex it into a plurality of signals at a second, lower bit-rate transmission format (STS-1). Of the plurality of second transmission format signals, those that are to be switched are passed to a cross connection unit where they are switched whereas those of said plurality of second transmission format signals that are not to be switched are passed through the cross-connect node at the second transmission format bit-rate and then multiplexed up to the first transmission format bit-rate for onward transmission in the synchronous communications network. Consequently, all traffic signals received at the cross-connect node are de-multiplexed prior to dropping that part of the traffic to be switched and passing through that part that is not to be switched. SUMMARY OF THE INVENTION An object of the invention is to minimise or to overcome this disadvantage. A further object of the invention is to provide an improved cross-connect for a synchronous transport network. A further object of the invention is to provide an improved method of handling traffic at a cross-connect in a synchronous network. According to one aspect of the invention, there is provided a method of processing traffic carried on first and second paths at a cross-point in a synchronous communications network, the method including: transporting the traffic on each path in virtual containers encapsulated in a first transmission format of the synchronous communications network; for each said path, determining from overheads attached to the virtual containers the traffic that is to remain on that path through the cross-point and the traffic that is to be switched from that path at the cross-point, dropping only the traffic to be switched at the cross-point, de-multiplexing the traffic to be switched to a second transmission format of the synchronous communications network, said second transmission format having a lower bit-rate than the first transmission format; and switching the dropped traffic in a switching fabric of the cross-point; the method being characterised in that for each path the traffic remaining on that path is transmitted through the cross-point in the higher bit-rate first transmission format. According to another aspect of the invention, there is provided a cross-connect node for processing traffic carried on first and second paths at a cross-point in a synchronous communications network, the cross connect node having: means for transporting the traffic on each path in virtual containers encapsulated in a first transmission format of the synchronous communications network; means for determining for each said path from overheads attached to the virtual containers the traffic that is to remain on that path through the cross-point and the traffic that is to be switched from that path at the cross-point, means for dropping only the traffic to be switched at the cross-point, means for de-multiplexing the traffic to be switched to a second transmission format of the synchronous communications network, said second transmission format having a lower bit-rate than the first transmission format; and a switching fabric for switching the dropped traffic at the cross-point; the cross connect node being characterised in being adapted to transmit the traffic remaining on each path through the cross-point in the higher bit-rate first transmission format. In this system and method, traffic is loaded into appropriate virtual containers according to the route that it will take at a cross-point, i.e. to be switched from one path to another or to continue on the same path. Switching at the cross-connect is performed at the virtual container (VC4 or STS1) level. By dropping out only the traffic that needs to be switched at the cross-point, the demand on the switching equipment is significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which; Figure 1 is a schematic diagram of a synchronous transport network; Figure 2 shows for comparative purposes the general configuration of a conventional cross-connect arrangement. Figure 3 is a schematic diagram of an integrated line system/cross-connect according to a preferred exemplary embodiment of the invention; Figure 4 shows the integrated line system/cross-connect of figure 3 in further detail; Figure 5 illustrates the equipment configuration of the cross-connect of figures 3 and 4; and Figure 6 is a schematic diagram illustrating traffic paths through a node incorporating the cross connect of figures 3 and 4. DESCRIPTION OF PREFERRED EMBODIMENT Referring first to figure 1 which is introduced for explanatory and comparative purposes, this shows a synchronous transport system in highly schematic form. As shown in figure 1, the network comprises a number of rings 11 each of which may have a number of nodes 12 and which are interconnected by cross-connects 13. Each ring node can receive synchronous traffic from either ring direction and the system thus provides path protection in the event of a failure at any one point in a ring. The cross-connects 13 provide both an interconnection and switching function to route traffic between adjacent rings or to drop traffic to the metropolitan and access networks at the node. The construction of a conventional cross-connect site is shown in schematic form in figure 2 which is also introduced for explanatory and comparative purposes. For clarity, only the input half of the cross-connect is shown and it will be understood that the output half is a mirror image of the input. Further, although this figure is described below with particular reference to SDH terminology, it will be appreciated that the equivalent SONET terminology is equally applicable. The cross-connect site comprises line termination equipment (LTE) racks 21 incorporating a number of line trunk equipments 210 each of which terminates a number of e.g. STM16 channels so as to provide an interface between the optical transmission medium and the electrical switching medium. The line trunk equipment provides a demultiplexing function. In the example given of twelve STM16 channels, these will be demultiplexed down one hundred and ninety two STM1 channels which are fed via corresponding STM1e (STM1 electrical) coaxial cable links to the cross-connect interface racks 22, which effect conversion from STM1 to the cross-connect internal signalling and the switch matrix 24 comprising a further set of cross-connects 241. The switch matrix can connect any input channel to any output channel. After the STM1 channels have been switched by the matrix, they are multiplexed back up to STM16 for further transmission. It will be appreciated that a large volume of cross-site cabling will be required to provide the coupling between the LTEs and the interface racks, and that as higher transmission rates, e.g. STM16 and STM64 are introduced, this volume of cabling will become excessively large. Effectively, the system is concerned with demultiplexing the STM16 signals to access the cross-connect matrix. Figures 3 and 4 illustrate an integrated line system/cross connect according to a preferred embodiment of the invention for use in the network of figure 1. Referring first to figure 3, this shows the integrated line system/cross connect in highly schematic form. As shown, the cross connect has a number of incoming and outgoing e.g. STM 64 (or OC192) channels on first and second paths which are coupled via the cross connect. The paths may also carry a number of STM16 channels (not shown). Switching of traffic between the paths is performed at the VC4 level for SDH traffic or at the equivalent STS1 level for SONET traffic. Local traffic is added and dropped at the cross connect via an add/drop multiplexer. Referring now to figure 4, this shows the construction of the integrated line system/cross connect in more detail. In essence, the arrangement comprises a shelf 33 housing STM1, STM4 or STM16 Transmit/receive I/F units. Typically, while STM1 is required, those external access shelves linked to the ISDX (Integrated Services Digital Exchange) via STM16 would be used. This is not to be confused with a transitional cross-point where all connections are at STM1. In this case, STM1 is needed only for traffic dropping down from the backbone to legacy equipment. The arrangement further comprises a line shelf 34 housing up to four STM64 line I/F interfaces, and the matrix, and a shelf 35 housing STM1, STM4 or STM16 Transmit/receive I/F units. The integrated line system/cross-connect shown in figure 4 is achieved by incorporating a large switching matrix 331 with several line terminating interfaces 332 within the same equipment. Advantageously, multi-wavelength optical repeaters and wave division multiplex (WDM) couplers (not shown) may also be fitted to the equipment for applications where several systems share fibres. All of these system components are managed from the same management system (not shown) and can be controlled via control shelf 36 via the internal protected Ethernet channels. To provide a high availability for the equipment, the switching matrix is 1+1 protected by comprising identical matrices (Matrix A and Matrix B) which perform the switching function in parallel and which are fed from dual separately fused power supply feeds. Fast hitless reconfiguration is provided by storing the switch connection maps in an on-line working memory and a number of off-line memories. OAM (operations, administration and maintenance) control or protection switching can trigger automatic exchanging of the maps. As will be described below, the effective size of the matrix as a double CLOS non-blocking matrix is five hundred and twelve VC4s. Thus, the equivalent of five hundred and twelve STM1 inputs can terminate at the equipment and be switched to any of five hundred and twelve STM1 or STM1 equivalent outputs. Typically, the terminations will be STM4, STM16, STM64 or combinations thereof. The switching matrix 331 is shown in further detail in figure 5, and is constructed from a three stage (Clos/Benes) array of e.g. 20 Gbit/s switching ASICs 311 in a 4,4,4 arrangement. For clarity, only switch matrix A is shown as switch matrix B is identical. The granularity of switching may be at the sts-1 level for SONET applications or at the equivalent VC4 level for SDH applications Using e.g. 20 Gbit/s switches in stages I and III, and 40 Gbit/s switches in stage II, a double non-blocking matrix of five hundred and twelve STM1 (VC4) equivalent capacity is provided, i.e. five hundred and twelve VC4 inputs can terminate at the equipment and be individually switched each to any one of five hundred and twelve VC4 outputs. The VC4 virtual containers can arrive at the cross-connect via STM1, STM4, STM16 or STM64 interfaces with no necessity for separate line systems. The switching matrix 331 performs the following functions: Switching any input VC4 to any output VC4 under OAM control. Reading SDH overhead data upon which protection switching decisions are made and for alarm and performance monitoring reporting. Protection switching (SNCP, 1+1, 1:1, 1:N MSP and MS-SPRing). Switching SDH overhead. The line system/cross connect incorporates a range of traffic interface cards 35 (e.g. STM1, STM4, STM16 and STM64) which terminate SDH multiplex and regenerator sections. The VC4 payloads are extracted and sent via 622 Mbit/s synchronous buses to the switching matrix 331. The system further includes a control shelf arrangement 36 housing a shelf controller, a communications maintenance interface, dual internal communications controllers, a parallel telemetry unit, an order wire unit and an optional element manager. Referring now to figure 6, this illustrates the operation of a system node incorporating the cross connect of figures 3 and 4. The node 43 forms a cross-point between first and second traffic paths (41 and 42) intersecting at the node. The traffic on either path that is to be switched is dropped out at the node and is switched by the switch matrix 331. The straight-through traffic on each path is not dropped out but is allowed to remain on that path. The traffic is transported in virtual containers each of which contains either traffic that is to remain on that path at the cross-point or traffic that is to be switched to the other path at the cross-point. Thus, switching at the cross point can be effected at the virtual container level. By demultiplexing or dropping out only the traffic that requires switching at the node, the switching load imposed on the switch matrix is considerably reduced thus allowing very high transmission rates, e.g. STM16 and STM64 to be employed without the need for an excessively large switching installation. The arrangement of figures 3 to 5 can be employed to perform the following functions: The cross-connection, line termination and amplifying functions of a mesh. An add/drop multiplexer (ADM) which can interconnect two or more rings. By the use of shared protection rings, add/drop multiplexers can be used to minimise the size of cross-connects required at flexibility points. Further, by incorporating the cross-connect function within the add/drop multiplexer, the problem of flexible interconnection of adjacent or stacked rings is overcome. The arrangement provides the network with a high degree of robustness. Provided dual homing between rings is used, any failure is restored typically within 50 milliseconds. This compares with a conventional recovery time which is of the order of seconds. By way of comparison, in a conventional system this interconnection would require the provision at the interconnecting node A of two standard STM16 ADMs (add/drop multiplexers) to terminate the subtending ring, a cross-connect with 160xVC4 capacity and an STM 64 ADM. The STM16 ADMs would in turn each require sixteen STM1 tributary interface units, and the STM64 ADM would require one hundred and twenty eight STM1 tributary interface units. In order to set up a connection through the node A, the network elements (NEs) would need to be managed at this node. This would require the installation of either three hundred and twenty STM1 cables or eighty STM4 interfaces if all three NE types support STM4 tributaries. In contrast, use of the integrated line system/cross-connect described above, with reference to figures 3 to 6, requires two pairs of STM16 tributary interface units for the subtending rings, one pair of STM 64 line interface units for the backbone ring, and an appropriate number of STM1/STM4 tributary interface units to connect traffic being added or dropped at the node. Preferably, the subtending rings and are dual homed to the backbone ring via respective STM64 nodes (not shown). In a further application where the synchronous system incorporates nodes that require switching functions of more than five hundred and twelve VC4s, the equipment can be expanded using ports to provide an intra-station mesh or ring to provide a virtual cross-connect. It will be appreciated that the technique is not limited to use with the synchronous STM16 or STM64 (or their North American SONET equivalents), but can of course be extended to accommodate higher levels of multiplexing and the corresponding higher bit rates when these become available.

Claims: A method of processing traffic carried on first and second paths (41, 42) at a cross-point (43) in a synchronous communications network, the method including: transporting the traffic on each path (41, 42) in victual containers encapsulated in a first transmission format of the synchronous communications network; for each said path (41, 42), determining from overheads attached to the virtual containers the traffic that is to remain on that path through the cross-point (43) and the traffic that is to be switched from that path at the cross-point (43), dropping only the traffic to be switched at the cross-point (43), de-multiplexing the traffic to be switched to a second transmission format of the synchronous communications network, said second transmission format having a lower bit-rate than the first transmission format; and switching the dropped traffic in a switching fabric (331) of the cross-point (43); the method being characterised in that for each path (41, 42) the traffic remaining on that path is transmitted through the cross-point in the higher bit-rate first transmission format. A method as claimed in claim 1, wherein said virtual containers are SDH VC-4 (SONET STS-1) containers. A method as claimed in claim 1 or 2, wherein the first transmission format of the synchronous communications network is any one or a combination of STM-1 (STS-3/OC-3), STM-4 (STS-12/OC-12) or STM-64 (STS-192/OC-192). A method as claimed in any of claims 1 to 3, wherein the switching of dropped traffic is performed via a three stage switch process. A method as claimed in claim 4, wherein the switching of dropped traffic is performed at a SDH VC-4 (SONET STS-1) transmission rate. A cross-connect node (33, 34, 35) for processing traffic carried on first and second paths (41, 42) at a cross-point (43) in a synchronous communications network, the cross connect node having: means (332) for transporting the traffic on each path (41, 42) in virtual containers encapsulated in a first transmission format of the synchronous communications network; means for determining for each said path (41, 42) from overheads attached to the virtual containers the traffic that is to remain on that path through the cross-point (43) and the traffic that is to be switched from that path at the cross-point (43), means (33, 35) for dropping only the traffic to be switched at the cross-point (43), means for de-multiplexing the traffic to be switched to a second transmission format of the synchronous communications network, said second transmission format having a lower bit-rate than the first transmission format; and a switching fabric (331) for switching the dropped traffic at the cross-point (43); the cross connect node being characterised in being adapted to transmit the traffic remaining on each path through the cross-point (43) in the higher bit-rate first transmission format. A cross connect node as claimed in claim 6, wherein said virtual containers are SDH VC-4 (SONET STS-1) containers. A cross connect node as claimed in claim 6 or 7, wherein the first transmission format of the synchronous communications network is any one or a combination of STM-1 (STS-3/OC-3), STM-4 (STS-12/OC-12) or STM-64 (STS-192/OC-192). A cross connect node as claimed in any of claims 6 to 8, wherein the switching of dropped traffic is performed via a three stage switch process. A cross connect node as claimed in claim 9, wherein the switching of dropped traffic is performed at a SDH VC-4 (SONET STS-1) transmission rate. A communications system including a cross connect node (33, 34, 35) as claimed in any of claims 6 to 10.

Inventors: BETTS MALCOLM CHARLES, DIPERNA DINO, MARTIN DAVID, NICHOLSON DAVID JOHN, PURSE CHRISTOPHER MICHAEL, SOLHEIM ALAN, TATAR MOHAMMED ISMAEL, BETTS, MALCOLM CHARLES, DIPERNA, DINO, MARTIN, DAVID, NICHOLSON, DAVID JOHN, PURSE, CHRISTOPHER MICHAEL, SOLHEIM, ALAN, TATAR, MOHAMMED ISMAEL
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1999
Main classification: H04J3
IPC codes: H04J3, H04Q11