Published May 2002

ETHERNET TECHNOLOGY

Part 3: Network Building Blocks
by James Antonakos

Start • The NIC • Repeaters & Transceivers • Hubs • Bridges/Switches • Routers • Hubs versus Switches • Inside a Switch • Switching Methods • Switches Versus Routers • Layer 3 Switching • Inside an ISP • Bigger Faster More • Sources and PDF (part 1 • part 2)

SWITCHING METHODS

Initially, switches handled frames using a technique called store-and-forward. Using this technique, the entire frame is stored as it is received. If the FCS is valid, the destination MAC address is used to select an output port, and the frame is forwarded to the appropriate output port via the switching fabric. Because the entire frame is stored before any decisions are made, there is a delay (or latency) between the time the frame is received and the time it begins transmission on the appropriate output port. The latency varies depending on the length of the frame. The minimum latency is obtained with a minimum size frame. For 10 Mbps Ethernet, the minimum latency is 57.6 µs (576-bit times at 100 ns per bit, including the preamble). Maximum length frames have a latency of just greater than 1.2 ms. Some applications, such as streaming audio and video, are sensitive to latency.

A second technique of switching is cut-through switching. This method reduces the latency of a switch tremendously. As soon as the destination MAC address of an incoming frame is received, the forwarding process can begin (assuming there is a free output port and the switching fabric is available). This reduces the latency to just 11.2 µs plus any additional time for internal switch operations. In addition, the latency of the cut-through method is fixed, because forwarding can always begin as soon as the destination MAC address is received. Unfortunately, errors can be propagated using the cut-through method, because there is no way to know if a frame being forwarded is good until it has been completely received. Cut-through switches will revert to the store-and-forward method when multiple errors occur while using the cut-through method.

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