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

8.2. Controlling Flow in Half Duplex Networks

If the ports connected to a switch are operating in half duplex (traditional, shared-LAN) mode, there are some tricks that a switch can play to try to improve performance both of the network and of the switch itself. These fall into two general classes of behaviors:

• Backpressure: To prevent buffer overflow from traffic arriving on its input ports, a switch can use the underlying access control method to throttle stations on the shared LAN and forestall incoming traffic.

• Aggressive transmission policy: On the output side, a switch can empty its transmit queue in an expedited manner by using an access control algorithm more aggressive than that permitted by the standard. This effectively gives the switch priority over other traffic sources on its output ports.

8.2.1. Backpressure

On a CSMA/CD LAN, two methods are available to prevent switch input buffer overflow by manipulating the behavior of the MAC algorithm itself:^[]

^[] The explanations in this section assume some familiarity with the Ethernet MAC algorithm. Readers unfamiliar with the terminology and behavior discussed here should see [SEIF98, Chapter 10], [SPUR97], [IEEE98e], or any good LAN reference text for a thorough discussion of the Ethernet half duplex MAC.

• Force collisions with incoming frames: On the surface, this appears to be a reasonable tactic. Like a horse flicking off an annoying fly with its tail, a forced collision will cause the sending station to reschedule ...