IEEE P802.3as/Draft 0.1/March 2005
4. Media Access Control
Editors’ Notes: To be removed prior to final publication. Revision History: Draft 0.1, March 2005 Initial draft for FE Task Force review.
4.1 Functional model of the MAC method
4.1.1 Overview The architectural model described in Clause 1 is used in this clause to provide a functional description of the LAN CSMA/CD MAC sublayer. The MAC sublayer defines a medium-independent facility, built on the medium-dependent physical facility provided by the Physical Layer, and under the access-layer-independent LAN LLC sublayer (or other MAC client). It is applicable to a general class of local area broadcast media suitable for use with the media access discipline known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). The LLC sublayer and the MAC sublayer together are intended to have the same function as that described in the OSI model for the Data Link Layer alone. In a broadcast network, the notion of a data link between two network entities does not correspond directly to a distinct physical connection. Nevertheless, the partitioning of functions presented in this standard requires two main functions generally associated with a data link control procedure to be performed in the MAC sublayer. They are as follows: a) Data encapsulation (transmit and receive) 1) Framing (frame boundary delimitation, frame synchronization) 2) Addressing (handling of source and destination addresses) 3) Error detection (detection of physical medium transmission errors) Media Access Management 1) Medium allocation (collision avoidance) 2) Contention resolution (collision handling)
An optional MAC control sublayer, architecturally positioned between LLC (or other MAC client) and the MAC, is specified in Clause 31. This MAC Control sublayer is transparent to both the underlying MAC and its client (typically LLC). The MAC sublayer operates independently of its client; i.e., it is unaware whether the client is LLC or the MAC Control sublayer. This allows the MAC to be specified and implemented in one manner, whether or not the MAC Control sublayer is implemented. References to LLC as the MAC client in text and figures apply equally to the MAC Control sublayer, if implemented. This standard provides for two modes of operation of the MAC sublayer: a) In half duplex mode, stations contend for the use of the physical medium, using the CSMA/CD algorithms specified. Bidirectional communication is accomplished by rapid exchange of frames, rather than full duplex operation. Half duplex operation is possible on all supported media; it is required on those media that are incapable of supporting simultaneous transmission and reception without interference, for example, 10BASE2 and 100BASE-T4. The full duplex mode of operation can be used when all of the following are true: 1) The physical medium is capable of supporting simultaneous transmission and reception without interference (e.g., 10BASE-T, 10BASE-FL, and 100BASE-TX/FX). 2) There are exactly two stations on the LAN. This allows the physical medium to be treated as a full duplex point-to-point link between the stations. Since there is no contention for use of a shared medium, the multiple access (i.e., CSMA/CD) algorithms are unnecessary. 57
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IEEE P802.3as/Draft 0.1/March 2005
DRAFT AMMENDMENT TO IEEE STD 802.3-2005
Both stations on the LAN are capable of and have been configured to use full duplex operation.
The most common configuration envisioned for full duplex operation consists of a central bridge (also known as a switch) with a dedicated LAN connecting each bridge port to a single device. The formal specification of the MAC in 4.2 comprises both the half duplex and full duplex modes of operation. The remainder of this clause provides a functional model of...