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

8.3. MAC Control

Rather than define just a protocol for explicit flow control of full duplex Ethernet (that would have been too easy!), the IEEE 802.3x Task Force chose to specify a more generic architectural framework for control of the Ethernet MAC (MAC Control), within which full duplex flow control was the first (and currently, the only) operation defined. This allowed for future:

• Expansion of explicit flow control to half duplex networks

• Specification of alternative full duplex flow control mechanisms (besides the simple PAUSE function discussed later)

• Definition and specification of other functions (besides flow control)

While no such standard extensions of the MAC Control protocol have yet been developed, the architecture makes this a relatively easy task.

MAC Control is an optional capability in Ethernet. This avoided having to declare preexisting Ethernet-compliant devices to be noncompliant with a later revision of the specifications. Clearly, the use of flow control provides significant advantages in high-speed, full duplex-switched networks, but an implementor (and user) is allowed the choice of performance versus price. However, because the cost of implementation of the MAC Control protocol (specifically, the PAUSE function used for full duplex flow control) is extremely low (it can typically be implemented in hardware, in the Ethernet controller silicon itself), most vendors of full duplex Ethernet products implement this capability today, especially at 100 Mb/s and ...