|  | 6.2 |
| Ethernet Operation
|  | 6.2.1 |
| Media Access Control (MAC)
MAC refers to protocols that determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data. MAC, with LLC, comprises the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served). Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, individual hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment as only one host is able to transmit at any given time. Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for an absence of a signal on the media and starts transmitting. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit. Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows: Ethernet – logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star) Token Ring – logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star) FDDI – logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring)
Web Links
Media Access Control http://howto.lycos.com/lycos/step/ 1,,pop-26166+ 25845+18046,00.html