CertiGuide to Network+  9  Chapter 0111: Wide Area Networking       9  VII  Wide Area Networking Technologies

X.25 1 Asynchronous Transfer Mode (ATM)

Frame Relay in the USA and Canada has replaced X.25 in all but the most rural areas. Frame Relay is digital and can exceed the 56Kb/sec limit of X.25.

When purchasing Frame Relay, you inform the carrier of the lowest acceptable bandwidth you can function with. This is known as the Committed Information Rate. This is the lowest performance you will get. At off peak times you may, and frequently do, get a higher information rate, yet your organization only pays for the bandwidth at the CIR.

While frame relay is distance insensitive, it is more variable in performance than a T-x or DS-x line. This is because it uses a route based on availability, and this route (and thus the connection speed) depends on how much capacity, on which lines, is being used by other subscribers.

Diagramming a frame relay connection always uses a ‘cloud’ because the exact nature is never the same. The good news about Frame Relay is when purchasing service; you select a CIR (Committed Information Rate).

This is the lowest through put you can expect. During lightly loaded time periods, a frame relay will perform at higher rates than you are paying for. A T-x is yours and yours alone. As far as usage, its use it, or lose it. Either way, you paid for it.

X.25 1 Asynchronous Transfer Mode (ATM)