Bill's Blog on Industrial Ethernet

When asked if an Industrial Ethernet switch from Contemporary Controls is a wire-speed device, I reply that all of our switches are. But what does wire-speed mean?

Wire-speed operation means that a switching hub port will pass traffic at its specified rate regardless of how much activity may be occurring in the switch as a whole. A repeating hub cannot achieve wire-speed because it cannot provide the simultaneous data exchanges that occur within a switch.

To achieve wire-speed, the internal fabric of a switch must be capable of processing data at a rate no less than the specified data rate multiplied by the number of ports in the switch. For example, if a 100 Mbps switch has eight ports, its backplane circuitry must be able to process data at a rate of at least 800 Mbps. Some manufacturers imply this but state it in another way, claiming (for example) that their 12-port 100 Mbps switch has a 1.2 Gbps capability (12 ports x 100 Mbps per port).

Vendors typically claim their switches can sustain layer-2 wire-speed forwarding without frame loss. To properly comply with wire-speed switching, address lookup should be done while the IP packet is moved from the input port to the output port without buffering. To confirm that a switch is operating at true wire-speed, elaborate throughput testing is required. Typical tests disable features that transcend basic layer-2 functionality such as throughput limits, flood limits and VLAN port tagging.

Regardless of wire-speed assurances, switch performance in practical situations can depend less on data throughput and more on such features as QoS, IGMP snooping or VLANs. The need for true wire-speed operation is seldom an issue in building automation or industrial settings and is mainly of concern in applications that require intense packet-switching such as video security systems.