A Core Competence Product Evaluation

Benefits of switching over bridging

The Desktop Switch and Switch 1000 combine multi-port repeater and bridge functionality. Whereas STP-based Ethernet bridges are strictly store-and-forward engines, Ethernet switches can also operate at near wire-speed by forwarding Ethernet frames as soon as the destination MAC address is received and verified. The difference in latency between these two techniques can be quite meaningful: whereas the store-and forward of a large Ethernet frame can take up to 1.2 milliseconds, a similar frame can be fast-forwarded through the Desktop Switch and Switch 1000 in 40 microseconds.

The Desktop Switch and Switch 1000 support two additional forwarding modes: Fragment Free is a cut-through technique where the initial 64 bytes of every MAC frame received are buffered prior to forwarding to eliminate runts (collision fragments). Intelligent forwarding monitors error traffic and dynamically selects either store-and-forwarding or fast forwarding based on the observed error rate.

Unlike a bridge, which discard frames when its buffers are depleted, an Ethernet switch can actively engage in congestion management. The Desktop Switch can detect congestion at a port, and will suppress additional traffic to that port until congestion is abated. The Switch 1000 will do this for both dedicated and repeated segment ports, and will additionally suppress traffic forwarding among the end stations on the segment. Suppression of additional traffic during (brief) periods of congestion prevents the switch from dropping packets. When Ethernet ports are dedicated to end stations, packet loss is virtually eliminated by jamming the transmitter on end station until congestion is abated. This can significantly improve performance, especially with TCP-based Internet applications, where packet loss will incur a longer delay than the jamming period ( a consequence of how TCP adjusts its retransmission timers when it detects segment loss).

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