How a LAN Switch Works

Chapter Description

This chapter explains how frame switching works, and what precisely happens while a frame is switching.

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Frame Size

Frame size is measured in bytes and has a minimum and maximum length, depending on the implemented technology. For example, the minimum frame size for an Ethernet LAN is 64 bytes with a 4-byte cyclic redundancy check (CRC), and the maximum frame size is 1518 bytes. The minimum/maximum for a Token Ring LAN is 32 bytes/16 kilobytes (KB), respectively.

Why is it important to know the minimum and maximum frame sizes your network can support? Knowing the sizes enables you to ensure that your users' message traffic gets to where it needs to go quickly and accurately.

Suppose your corporate mailroom is equipped only to handle letter- and business-sized envelopes and is not equipped to handle postcards or larger legal-sized envelopes. The letter-sized envelope is the minimum size, and the business-sized envelope is the maximum sized "frame" allowed by your mailroom. Anything smaller than the letter-sized envelope, such as a postcard, might be considered a runt, and anything larger than the business-sized envelope might be considered a giant.

Figure 6-6 illustrates the concept of a minimum and maximum frame size, and the result, in a corporate mailroom. (Let's hope this doesn't really happen, although it might explain a few missing pieces of mail.)

Figure 6Figure 6-6 Mailroom in Action

In this mailroom (switch) scenario, both the postcards (runts) and legal-sized envelopes (giants) would not be accepted by the mailroom (the switch) and therefore would be dropped into the trash.

NOTE

The maximum frame size is also known as the maximum transmission unit, or MTU. When a frame is larger than the MTU, it is broken down, or fragmented, into smaller pieces by the Layer 3 protocol to accommodate the MTU of the network.

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