Olicom 8600, 8605 manual Congestion Control, Three Switching Modes, Cut-Through, Store and Forward

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Block destination address at a specific port—this prevents the specified port from sending frames to a specified destination.

Allow destination address at specific ports—this indicates that the specified port must send frames to the specified destinations only.

Force destination address to a specific port—this allows forwarding to a unicast address that has not been learned. It can also be used to limit the forwarding of Multicast addresses to a subset of ports. This last filter applies to non-source-routing frames only.

Congestion Control

At regular intervals, the switch CPU inspects the queues on all output ports. If a queue size is above a certain threshold, the port is instructed to:

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Set the transmit priority for low priority frames to a specified high level

Delete old frames from the queue until it reaches a specified size

When the queue size again comes below a normal threshold size the port is instructed to set the transmit priority back to the normal level.

Three Switching Modes

Cut-Through

In this mode the switch starts forwarding the packet to the output port as soon as the destination address or the source-route of the incoming packet has been resolved. This technique ensures very low latency, typically in the range of 30-100 µs. However, if errors occur on the input port during the reception of a packet, the error will still be forwarded to the output port. Note that cut-through can only be used in transmissions between ports which operate at 16 Mbps.

Store and Forward

In this mode, the switch receives the total packet from the input port, checks it for any errors and then starts forwarding the packet to the destination port. This technique will ensure that no faulty packets are transmitted by output port. The negative impact however, is higher latency, typically in the range of 40–2,000 µs depending on the packet size. Though slower than cut-through mode, this is still much faster that conventional bridges.

Auto (Adaptive Cut-Through)

This is a technique whereby the switch will automatically swap between store-and- forward and cut-through modes based on an error threshold. If the number of received faulty packets is low, then cut-through mode is used; if the number of

CrossFire 8600/8605 Token-Ring Switches v. 1.2, P/N: 710001641 Switch Theory of Operation