IEEE 802.1D. There is a standard for bridges that is available from the IEEE as standard 802.1D. This standard is entitled, "Information technology—Telecommunications and information exchange between systems-Local and metropolitan area networks—Common specifications-Part 3: Media Access Control (MAC) Bridges." This standard addresses the uses of bridges and, therefore, switches. There are some interesting parameters in the standard that impact the operation of real-time control networks. Aging Aging is the amount of time the bridge waits until it removes a source address from the table due to the fact that the source address has not initiated a transmission within the aging time. The standard allows for an extremely wide range of values from 10 seconds to 1,000,000 seconds. The default, however, is 300 seconds or five minutes, which is what most bridges use. Bridge Transit Delay The maximum amount of data latency introduced by a switch is specified. Although the recommended maximum is one second, up to four seconds is allowed. This amount of time seems long. Couple this time with the maximum allowable number of switches that can be cascaded (seven), the theoretical delay could be as much as 28 seconds! This is an eternity for an industrial control system. Fortunately, modern switches operate much faster than the standard requires. FCS Checking A switch is required to perform a frame check sequence test on incoming frames and discard defective ones. To do this, the switch must receive the complete frame before forwarding. This means that the standard does not allow cut-through or modified cut-through operation. Bridge Addressing The standard requires that not only must the bridge have a MAC address, each port must have a MAC address. This is unnecessary for normal switch operation. Many commercial switches do not support this requirement. SUMMARY Switches are classified as bridges and operate at the data link layer. They can create a much larger network diameter by segmenting the network into separate collision domains. Switches can learn from their environment and then restrict traffic only to necessary ports. This frees up other ports to initiate their own independent transmissions; thereby, increasing the performance over a shared Ethernet network. Repeating hubs have their place but depending upon the application, switches could provide a better solution.