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

13.2. LAN Priority Mechanisms

Many LAN technologies provide internal mechanisms that support one or more forms of Priority operation. These priority mechanisms fall into two classes:

• Access priority: In a shared-bandwidth LAN, multiple stations are capable of requesting use of the underlying physical medium at the same time. Shared LANs therefore impose some form of Medium Access Control (MAC) protocol to allow the stations to arbitrate for the use of the physical channel when there are multiple simultaneous access requests.

  Access priority mechanisms allow one station to obtain preferred access to the LAN over others. Thus, the MAC algorithm is unfair to the extent that some stations have priority over others. Access priority may be static (i.e., priority is granted to a station for all time, regardless of the nature of its traffic flow) or dynamic (a station's priority changes over time, possibly on a frame-by-frame basis, as a function of the needs of the applications running on the device).

  In a full-duplex environment, access priority is meaningless because each device has unfettered access to the channel at all times.

• User priority: User priority is unfortunately a misnomer. It doesn't refer to the priority of one (human) user of the network over another, but to the priority assigned to a given frame (or stream of frames) by the application sourcing those frames within a station.

Thus, access priority indicates the preferential treatment given to one station over another to ...