Media Access Control and Collision Domains PDF

2.1.3 Media Access Control and Collision Domains Ethernet is a multiple access area network, which means that the available communications capacity is shared between the nodes that are connected to the same media. Media access control (MAC) refers to the methods a network technology uses to determin...

2.1.3 Media Access Control and Collision Domains Ethernet is a multiple access area network, which means that the available communications capacity is shared between the nodes that are connected to the same media. Media access control (MAC) refers to the methods a network technology uses to determine when nodes can communicate on shared media and to deal with possible problems, such as two devices attempting to communicate simultaneously. Ethernet uses a contention-based MAC system. Each network node connected to the same media is in the same collision domain. When two nodes transmit at the same time, the signals are said to collide, and neither signal can reach its destination. This means that they must be resent, reducing available bandwidth. The collisions become more frequent as more nodes are added, and consequently the effective data rate is reduced. The Ethernet protocol governing contention and media access is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). A collision is the state when a signal is present on an interface's transmit and receives lines simultaneously. On detecting a collision, the node broadcasts a jam signal. Each node that was attempting to use the media then waits for a random period (backoff) before attempting to transmit again.  Description The steps of C S M A or C D media access method are as follows: 1. Data. 2. Check. 3. Transmit data. 4. Collision. 5. Wait. 6. Retransmit data. The CSMA/CD media access method. (Images © 123RF.com.) The collision detection mechanism means that only half-duplex transmission is possible. This means that a node can transmit or receive, but it cannot do both at the same time.  In the 10BASE-T star wiring physical topology, each node is cabled to an Ethernet hub. The hub repeats incoming signals to each connected node. Consequently, every host connected to the same hub is within the same collision domain. However, this 10BASE-T physical topology dates from 1990. You are very unlikely to find it deployed in a modern network. Copyright © The Computing Technology Industry Association, Inc. All rights reserved.