GB2255259A

GB2255259A - Switching arrangement and method

Info

Publication number: GB2255259A
Application number: GB9204445A
Authority: GB
Inventors: Richard John Proctor
Original assignee: GPT Ltd; Plessey Telecommunications Ltd
Current assignee: GPT Ltd; Plessey Telecommunications Ltd
Priority date: 1991-03-06
Filing date: 1992-03-02
Publication date: 1992-10-28
Also published as: GB9104712D0; FR2673786A1; JPH05103003A; DE4207027A1; GB9204445D0

Description

-1 2 5 5 -j _) -1 -I- SWITCHING ARRANGEMENT AND METHOD The present

invention relates to a way of switching large Synchronous Transmission Mode (STM) data streams using an Asynchronous Transmission Mode (ATM) switch as a STM space switch for payloads such as the Synchronous Digital Hierarchy (SDH) Synchronous Transport Module I (STM- 1) Auxiliary Unit 4 (AU4), this could also switch the Virtual Container Number 4 (VC4) or any other whole payload for existing (non SDH) transmission systems.

This type of cross connect function is used for high level transmission reconfiguring.

The application of this type of switching is where there is a small requirement for AU4 switching at a node that has an ATM switch.

The conventional way of handling this type of switch is to space switch the data between the ports, using STM methods. This is an effective way of doing the switching provided that the switch is only doing this type of cross connect operation. The conventional way is probably cheaper when required in quantity. But when only occasional use is required, the following may be more suitable.

In ATM all information is carried in small fixed sized packets called cells, these have a 5 octet header that identifies the cell, and 48 octets of information that is carried end to end across the network.

To adapt services for ATM a function called the ATM Adaptation Layer (AAL) is provided. There are a number of types of AAL for different types of services, for instance there is a type 1 AAL that is used to carry streams of data, this uses one octet in the information portion of the cell. This octet is used to carry a sequence number and some protection for this number. This allows the cell to carry 47 octets of the data stream.

An SDH stream operating at 155M carrying 150M of ATM could carry about 132M of data at full load, allowing for the cell headers and the type 1 AAL. However because of the statistical nature of ATM it may only be loaded to about 80%, thus carrying about 106M of data.

Because of these limitations ATM switching can not be used conventionally for switching data at 155M.

In accordance with the present invention there is provided a method of switching STM data streams with an ATM switch comprising the steps of dividing the data stream into blocks which fill the header and information fields and disabling the header translation unit in the header decode unit of the ATM switch, whereby the data block may be reconstituted at the output port thereof.

There is further provided an ATM switch for a telecommunications system comprising an ATM switch and a header decode unit including disabling means for disabling the header translation unit thereof.

The present invention will now be described, with reference to the accompanying drawings, in which:- Figure I shows diagrammatically a functional model of an ATM switch; Figure 2 shows the structure of a 62 octet cell as used with an ATM switch; Figure 3 shows a block diagram of a typical header decode unit;.

Figure 4 shows a block diagram of a typical header decode unit when modified for use in accordance with the present invention.

It is proposed to use an ATM switch as a space switch, and use this to carry the data between the ports using the full 53 octet capacity of ATM, that completely fills the cells, both header and information carrying capacity. As all the data can be routed the same way and does not need to be separately identified there is no need to use the header to provide an identity for the data. As the data is not being mixed with other traffic there is no need for statistical multiplexing and therefore the load can approximate to 100%. Thus an ATM switch capable of handling traffic at 155.52M can switch AU4s of 155.52M. All that is required is to disable the header translation that would normally be applied to the cells.

The functions of an ATM switch can be broken down into three main components as shown in Figure 1, any switch design having one or more of each function.

The functions are:

a) b) Header decoder units 11, 12 --- 1N on each port which translate the incoming circuit identity into an outgoing circuit identity and port number. This unit is also able to police the utilisation of a particular circuit against its negotiated traffic level limits and where necessary discard cells in order to avoid overloading the switch which could impact on other traffic being carried. To transfer the cells from the incoming ports to the outgoing ports according to the physical port routing information derived by the header decoder unit. This is essentially a spatial routing function 20. To statistically multiplex the cells transferred across the routing function onto the designated outgoing port traffic streams 31, 32 --- 3N. Due to the peaks in traffic which exceed the capacity of the outgoing stream it will be necessary to queue some of the cells in output queuing units 21, 22 --- 2N. This outgoing multiplexing and queuing function may be likened to the operation of a time switch in synchronous circuit switching, but does not obey a predefined cyclic allocation of virtual circuits in the time domain. For this type of traffic, the cell transfer and multiplexing functions can be used without change, the header decoder function 11, 12 --- 1N needs to have the translation function disabled, deriving the outgoing port number locally. The policing would also be disabled.

The method described allows for data to be sent across the switch, but does not have any protection, as provided by the AAL. The description that follows shows how this can be provided using a particular ATM switch design, this is not the only way, as for other switch designs the AAL equivalent can be carried in other ways, such as the provision of per cell CRCs, and using multiple synchronous planes.

A typical basic ATM switch design and its use in broadcast services are described in U.K. Patent Applications Nos. 9019340.0 and 8917530.1 which are incorporated herein by reference thereto.

Within the switch a cell is routed as a 62 octet cell with routing information and a sequence number (with protection) for error detection and the complete 53 octet cell as shown in Figure 2.

The housekeeping octet is equivalent to the sequence number protection provided by AAL type but it has a longer sequence and is therefore better than the AAL type 1 protection.

The 53 octet complete cell carries the information.

The FCS check sum octet is equivalent to the sequence number protection provided by the type 1 AAL but it is a larger field and therefore is better than the AAL type 1 protection.

The design of ATM switch that will be used, uses this internal cell of 62 octets, to allow routing information, and protection to allow two planes of the switch to operate with confidence.

(a) (b) The system would operate as follows:The data stream on input to the switch is divided into 53 octet chunks, has a sequence number and routing information added. This is then switched across the switch in the same way as for ordinary ATM traffic. The data on output from the switch is extracted from the 62 octet internal cells, checked using the sequence number and FCS protection and then sent out of the switch. In Figure 3 is shown a typical header decode unit 40. A signal coming from the line 42 is checked in a cell reception and checking unit 44 and the sell header removed and sent to a policing unit 46 and a header translation unit 48. The header translation unit 48 takes the existing header and produces the outgoing circuit identity and port number. This identity and port number are then combined with the cell contents in the Construct Internal Cell unit 50 and passed to the switch.

In Figure 4 a modified form of the header decode unit shown in Figure 3 is shown. In normal operation the unit 40' operates as the unit 40 shown in Figure 3. In the AN mode the Policing Unit 46' and the header translation unit 48' are disabled by a signal 52 from a central control unit not shown. As a result the cell header is passed to the Construct Internal Cell unit 50' in the unmodified form and recombined with the cell contents for passing to the switch, so re-establishing the original data cell.

ATM switches usually generate jitter as a function of load on the data stream, due to the statistical multiplexing of traffic together. However in this case it uses so much bandwidth that it must be routed on its own without any other traffic. This has the affect that the jitter would be very small, also the delay could be minimal.

This application of an ATM switch, would be a cost effective way of providing a limited amount of AU4 level cross connection using an ATM switch, requiring only a minimal change to the behaviour of the ports.

The provision of an ATM switch solely to carry out this function is unlikely and unreasonable, but the occasional use of an existing ATM switch in this manner would be advantageous.

The description relates to a particular form of ATM switch but a similar approach could be used for other forms of ATM switches.

Claims

1. A method of switching STM data streams with an ATM switch comprising the steps of dividing the data stream into blocks which fill the header and information fields and disabling the header translation unit in the header decode unit of the ATM switch whereby the data block may be reconstituted at the output port thereof.

2. A method as claimed in Claim 1 further including the step of disabling the policing unit in the header decoder unit.

3. A method of switching STM data streams with an ATM switch substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 4 of the accompanying drawings.

4. An ATM switch for a telecommunications system comprising an ATM switch and a header decode unit including disabling means for disabling the header translation unit thereof.

5. An ATM switch for a telecommunication system substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 4 of the accompanying drawings.