The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

Chapter 8. LAN and Switch Flow Control

In this chapter, we look at issues relating to frame loss, and the prevention of frame loss, within switches. Much of this discussion also is applicable to issues of frame loss within end stations, as the same solutions often can be applied to both problems.

The discussion is somewhat Ethernet-centric, for three reasons:

• While a Token Ring LAN is itself connectionless, many Token Ring environments use a Logical Link Control protocol (i.e., LLC-2) that directly addresses the problem of frame loss within the Data Link by forcing a connection-oriented approach. LLC-2 sees much less use on Ethernet systems; the connectionless nature of Ethernet systems therefore makes frame loss more problematic.

• Ethernet can address receiver overload on half duplex networks by applying backpressure (see section 8.2.1). While theoretically a Token Ring station can backpressure the network by artificially delaying token release, such behavior is severely constrained.

• The explicit flow control protocol discussed here was designed for, and is implemented solely on, Ethernet LANs.

The problems (and solutions) for switch flow control evolved directly from the development of Ethernet switches with full duplex interfaces. As such, the solutions tend to be Ethernet-specific.