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

Asynchronous Transfer Mode (ATM) 1 VIII  Chapter 0111: Summary

Like frame relay, and unlike most local TCP/IP technologies, ATM is a circuit-based technology. That is, when a connection is established over ATM, a specific route, or virtual circuit, through the network is chosen, and that channel is maintained until the connection is closed (when the application ends). This provides for efficiency in many cases, but also potentially wastes network resources, since bandwidth is allocated to a connection that may never use it. Additionally, establishing the virtual circuit takes additional processing at connection-time, so short conversations may be slower over a circuit-switched network than a packet-switched network of the same speed.

In contrast, in the packet-switched gigabit Ethernet TCP/IP world, data packets may travel through different paths to get from point A to point B during a conversation. The paths they take are somewhat unpredictable, and depend on what other users are doing on the network at the time. Because the path is not reserved in advance for use by the entire conversation, a conversation may suddenly slow down in the middle, if network conditions degrade. If the TCP/IP network notices that one of the intermediate “hops” between routers along the path is excessively slow or down, it may be able to choose a different path to reach the destination, in hopes of avoiding the problematic area of the network. However, this is not guaranteed. Of course, if you know your network well, and have control over it (as you would in a campus network), the tradeoff of potential slowness if the network gets near capacity vs. quicker connection setup on packet-switched networks is likely to make packet-switching your best alternative.

Asynchronous Transfer Mode (ATM) 1 VIII  Chapter 0111: Summary