The switches were sourced from ECI Telematics. The NCX1E6 ATM Switch consists of a non-blocking 3.5Gbps switch fabric which is connected to 16 I/O modules, as shown:
The Durham testbed is fitted only with 155Mbps STM1 cards, electrical and optical. Each IOC provides a single port in this case. The buffering system currently available consists of a group of four buffers (termed a ‘Myrtle') as shown below:
Each queue is divided into 8 'discard zones'. Each connection has a configured discard level, 0 to 7. When a cell arrives from that connection at its output queue, its discard level is encoded in its internal cell header. If the current level of the queue exceeds the discard level then the cell is discarded. The cell's discard level may be modified ('skewed') by the policing function (if enabled) on its arrival at a switch input port. A discard level of 7, for instance, means that cells are only discarded at the output queue when the queue is full. A level of 3 would mean that cells would be discarded here when the queue is more than half full.
A 256 entry scan list is maintained in the IOC. Each entry in this list specifies a queue to be checked. When the queue is checked, if a cell is available to go, it is extracted and transmitted. The relative frequency with which a queue is checked (ie the frequency of its occurrence in the scan list) is a configurable parameter for the IOC: the ‘scan ratio'. Ratios are configured for each of the four queues. An algorithm within the node (not at present known to us) distributes the queues amongst the 256 entry list as evenly as possible.
This mechanism gives a means for the operator to assign priorities: higher scan ratios are given to queues which are assigned to higher priority traffic, ie the queues require more frequent service in order to guarantee a lower cell delay variation. However, with this technique, even the lowest priority queue is guaranteed some service. Thus all queues are guaranteed some minimum bandwidth (in fact, 1/256 of the total available bandwidth).