Cabletron Systems STS16-20R, STS16-20D manual Three Switching Modes, Cut-Through, Store and Forward

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Three Switching Modes

This section describes the three switching modes available on the switch.

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 faulty packets is high, the store and forward mode is used. This provides optimized performance but introduces variable latency.

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