Synchronous transmission system adapted to carry both synchronous and asynchronous traffic

Abstract: A synchronous transmission system is adapted to carry both synchronous and asynchronous traffic and has a number of nodes providing ingress and egress of the traffic. Each node includes first and second ATM/SDH identifiers whereby the switch can separate the synchronous and asynchronous traffic. Separate processing of synchronous and asynchronous traffic is performed at each node.

Description: This invention relates to communications networks and in particular to synchronous networks carrying both synchronous and asynchronous (ATM) traffic. BACKGROUND OF THE INVENTION A recent development in communications technology has been the introduction of synchronous networks commonly referred to as synchronous digital hierarchy (SDH) or, in North America, as SONET. Synchronous networks have the significant advantage of simplifying the process of demultiplexing channels and thus avoid the need for the so-called multiplexer/demultiplexer mountains that were associated with previous plesiochronous techniques. Synchronous networks are now being used to carry both synchronous and asynchronous/ATM traffic. The asynchronous or ATM traffic is carried in cells which are carried over the synchronous network in standard SDH virtual containers (VCs). The cells are recovered from the virtual containers into the standard ATM transmission format at the egress from the synchronous network. Arrangements for carrying both synchronous and ATM traffic are described for example in US patent No 4,926,416, and IEE Conference Publication No 39, October 1990, pages 73-78. These conventional systems suffer from the disadvantage that they are of complex construction and require a relative large number of ATM ports to handle the ATM traffic. Further, many of these systems are inefficient in their use of bandwidth. GB2,287,857 relates to a synchronous switch architecture for switching virtual containers containing 64kbit/s traffic, ATM traffic and mixed traffic to corresponding destinations. However, it gives no indication of how the switch architecture recognizes which kind of traffic a virtual container carries, so that the switch architecture can switch the virtual container to the correct destination. SUMMARY OF THE INVENTION An object of the invention is to minimise or to overcome these disadvantages. A further object of the invention is to provide an improved arrangement and method for carrying ATM traffic over a synchronous network. According to the invention there is provided a synchronous transmission system (11) adapted to carry both synchronous and asynchronous traffic in virtual containers with each virtual container containing asynchronous or synchronous traffic, the system having a number of synchronous network nodes (12) providing ingress and egress of said traffic, wherein means (22) are provided at said nodes for performing separate switching of the synchronous and asynchronous traffic at that node, characterized in that: the virtual containers are provided with a respective signal label byte or path trace byte in the path overhead identifying the traffic as synchronous or asynchronous, and means are provided at said nodes (23a, 23b) for retrieving said identifying byte so as to identify the synchronous and asynchronous traffic from said path overheads to enable the separate switching to be performed. According to another aspect of the invention there is provided a method of processing communications traffic comprising synchronous traffic and asynchronous traffic carried in virtual containers with each virtual container containing asynchronous or synchronous traffic and received at a synchronous network node (12), the method including providing separate switching of the synchronous and asynchronous traffic at the node, characterized in that the method additionally includes providing each said virtual container with a path overhead, providing in said path overhead a signal label byte or a path trace byte identifying the traffic as synchronous or asynchronous, retrieving said identifying byte at said node, and, responsive to said retrieval carrying out the separate switching. In a further embodiment, the path trace (J1) can be employed in conjunction with a look-up table provided at the node to determine the traffic type. The arrangement and method facilitate the handling of synchronous and asynchronous traffic and provide a more effective use of the transmission bandwidth. Further, cell consolidation from sources with a high peak and a low average rate can be achieved in a single VC stream. The technique is compatible with existing transmission standards. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described with reference to the accompanying drawing in which:- Figure 1 is a schematic diagram showing an SDH ring network carrying synchronous and asynchronous/ATM traffic between network nodes; Figure 2 shows the construction of a node of the network of figure 1; and Figure 3 illustrates the path overhead of a synchronous virtual container. DESCRIPTION OF PREFERRED EMBODIMENT Referring to figure 1, communications traffic enters and leaves the SDH ring network 11 via nodes 12 each of which provides a multiplexing/demultiplexing function. Asynchronous/ATM traffic arriving at a node is already in or is packaged into SDH virtual containers for transmission over the SDH network 11 to an appropriate egress node. Typically the ring 11 carries four VC4 streams, although other synchronous formats can of course be employed. Further, although a ring network is shown in figure 1, it will be appreciated that other network constructions are envisaged. The construction of a network node or multiplexer/demultiplexer is shown in figure 2. Traffic is carried on the ring in a STM4/1 format, but it will be appreciated that other standard SDH formats are equally applicable. The node is controlled by a network manager interfaced by the control function 21 and includes a switch 22, e.g. a 4/4 or 4/3 switch which interfaces with the ring via ATM/SDH identifiers 23a, 23b. These identifiers alert the switch as to the type of traffic that is being handled. Received synchronous traffic is directed or demultiplexed via a 4/1 cross connect 24 to a synchronous interface 25 which is in turn coupled to 2Mbit, 34/35Mbit and 140/155 Mbit tributaries. Synchronous traffic received from the tributaries is multiplexed up for transmission to the ring. Each virtual container arriving at the node is examined to determine whether it contains ATM or SDH traffic. This is effected by use of the path overhead associated with that virtual container. The most general mechanism for distinguishing between VC3/4s containing different types of traffic involves the use of the signal label (C2) byte within the VC3 or VC4 path overhead. The VC3/4 path overhead, which is shown schematically in figure 3, comprises a number of preallocated fields each having a set number of bytes and each reserved for carrying a particular type of information. For example, the J1 byte contains the path trace, the B3 byte provides bit error monitoring and the C2 byte indicates the composition of the payload. The ITU-R G.707 recommendation for the C2 byte specifies hex codes relating to different types of traffic as illustrated in the table below. C2 byte hex code Traffic type 00 unequipped 01 equipped non-specific 02 TUG structure 03 locked TU 04 asynchronous mapping of 34 and 44 Mbit/s into container-3 12 asynchronous mapping of 140 Mbit/s into container-4 13 ATM 14 MAN (DQDB) 15 FDDI Those virtual containers containing asynchronous/ATM traffic are identified by the identifiers 23 by reading the cell C2 headers described in the above table and the ATM cells contained therein are directed to a cell switch 26 for output via an asynchronous interface 27. The interface maps received ATM cells into a VC4 /34 or 2Mbit/s PDH format or maps outgoing ATM cells to these formats or other standard formats. Similarly, those virtual containers containing synchronous traffic are also identified from the C2 header information and the synchronous traffic contained therein is routed to the cross-connect 24. In an alternative embodiment, the path trace J1 byte is employed to identify the type of traffic within each virtual container. The path trace, which uniquely identifies a particular path, is assigned by the network manager to each virtual container that is created by any of the network elements under its control. The network manager also informs each network element as to which path traces are to be expected with incoming virtual containers. In the present arrangement, this information is stored, together with an indication of the traffic type corresponding to each path trace, in a look-up table 29 that is accessible by the identifiers 23. On arrival of a virtual container, the identifier matches the path trace against the corresponding look-up table entry to determine whether the traffic is synchronous or asynchronous. Preferably, as a default option, traffic for which no matching path trace can be found in the look-up table is routed to the cross-connect 24 In an further embodiment, the switch 22 may be controlled by the network manager which maintains a record of the traffic being transmitted and is thus aware of its precise arrival time the switch. In this way the switch can be instructed by the network manager to route the SDH and ATM traffic to the respective switches and the identifiers may then be dispensed with. It will be appreciated that, although the system and method have been described above with particular reference to the European SDH protocol, they are in no way limited to that protocol. Thus, with appropriate minor adaptation, the technique may be adapted e.g. to the North American SONET protocol. It will also be apreciated that the technique is applicable to both unidirectional and bi-directional rings.

Claims: A synchronous transmission system (11) adapted to carry both synchronous and asynchronous traffic in virtual containers with each virtual container containing asynchronous or synchronous traffic, the system having a number of synchronous network nodes (12) providing ingress and egress of said traffic, wherein means (22) are provided at said nodes for performing separate switching of the synchronous and asynchronous traffic at that node, characterized in that : the virtual containers are provided with a respective signal label byte or path trace byte in the path overhead identifying the traffic as synchronous or asynchronous, and means are provided at said nodes (23a, 23b) for retrieving said identifying byte so as to identify the synchronous and asynchronous traffic from said path overheads to enable the separate switching to be performed. A synchronous transmission system as claimed in claim 1, wherein the identifying byte is a path trace byte and the means further comprises a look-up table providing a matching between path traces and respective traffic types. A system as claimed in claim 1, wherein each said node (13) incorporates a switch (22) , said switch being interfaced to the network via first and second asynchronous/synchronous identifiers (23a, 23b) whereby in use the switch is enabled to differentiate between the synchronous and asynchronous traffic. A system as claimed in claim 1, and including network manager means (29) whereby in use the switch is enabled to differentiate between the synchronous and asynchronous traffic. A system as claimed in claim 4, and including means for demultiplexing synchronous traffic via a cross-connect (24) to a synchronous interface (25). A system as claimed in claim 5, and including a cell switch (26) coupled to an asynchronous interface (27) for processing asynchronous traffic. A method of processing communications traffic comprising synchronous traffic and asynchronous traffic carried in virtual containers with each virtual container containing asynchronous or synchronous traffic and received at a synchronous network node (12), the method including providing separate switching of the synchronous and asynchronous traffic at the node, characterized in that the method additionally includes providing each said virtual container with a path overhead, providing in said path overhead a signal label byte or a path trace byte identifying the traffic as synchronous or asynchronous, retrieving said identifying byte at said node, and, responsive to said retrieval carrying out the separate switching. A method as claimed in claim 7, wherein said identifying byte is a path trace byte and said retrieving step further comprises providing a matching between path traces and respective traffic types from a look-up table.

Inventors: HOLLAND PETER, HOLLAND, PETER
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1997
Main classification: H04Q11
IPC codes: H04J3, H04L12, H04Q11