8.6 Data Support

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Multiplexers support only a limited selection of data rates because vendors typically required a different card to support each transmission rate. This practice not only inflated the initial hardware investment but compounded the cost and logistics of maintenance. Today’s multiplexers are more efficient, using only one type of data channel card to support a variety of data rates: 75 bps to 19.2 Kbps for asynchronous transmission and 75 bps to 1.544 or 2.048 Mbps for synchronous transmission. Under software control, multiple data types and a broad range of data rates are available to suit different applications.

To minimize disruption due to maintenance or failure, each channel card should have only one port. Multiport cards can leave large numbers of users stranded in case of failure. Each channel provided by a four-port card, for example, may be configured to provide up to 32 terminals with “logical” access to a cluster controller. This may appear to be very efficient, but when one channel fails, three other channels must be taken out of service until a card swap can be made.

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