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Low cut-through latency gives Madge a performance edge in the lab.
Bandwidth-starved token-ring users, take heart: Another batch of tokenring switches has hit the market since our review of Bay Networks, Inc.'s Centillion 100 (NW Sept. 4, 1995, page 43). We tested three switches that deliver the low latency and high throughput you need to ease your network performance burden.
We tested Madge Networks, Inc.'s Smart Ringswitch; Nashoba Networks, Inc.'s Concord Token-Ring Switch; and 3Com Corp.'s LANplex 6000. All three enable you to set each port to operate independently at 4M or 16M bit/sec. Likewise, every one of the tested units can switch any combination of 4M or 16M bit/sec rings.
Each unit offers different levels of latency and throughput performance based on the method of switching and bridging support offered. Of the three switches, Madge has the only one with a cutthrough switching design. The other two use a storeand-forward technique. In cut-through switching, packets can start moving from one port to another as soon as the switch determines the destination address. Store-and-forward switching requires full packets to be buffered before they can be switched.
Each vendor also supports different bridging methodologies and options. There are two major bridging methodologies in use today: transparent bridging and source routing. Both operate at the Data Link Control (DLC) layer. The Madge product supports source routing only. The 3Com and Nashoba switches support both modes, but this did not result in any appreciable difference in performance throughout our tests.
Transparent bridging, made popular by Ethernet bridge vendors and the first to be standardized as IEEE 802.1 d, uses the 48-bit DLC destination and source addresses to make a packet-forward or drop decision. These addresses are often referred to as the burned-in address or the media access control (MAC) address of an adapter.
In source route bridging, the decision to forward is based on ring and bridge numbers contained in the routing information field (RIF), which immediately follows the source address of a packet. A packet that has RIF information has a broadcast bit that is set to a binary 1 in the source address field.
The bridging mechanism a switch supports can be a key factor in the one you select. After all, a switch is...