ENVS 1200 Lecture Notes - Lecture 40: Carrier-Sense Multiple Access, Media Access Control, Data Link Layer
13 May 2018
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ENVS 1200 Lecture 40 Notes – The data link layer
Introduction
• The task of the data link layer is to transmit the packet from the current node to the
next node.
• We presented a brief overview of the Ethernet protocols.
• We take a closer look at the medium access protocol for wired Ethernet.
• Node access for a particular network is defined by the medium access control protocol.
• The purposes of a medium access protocol are to steer data to its destination, to detect
errors, and to prevent multiple nodes from accessing the network simultaneously in
such a way that their messages become mixed together and garbled.
• Such an event is called a collision.
• As you already know, the predominant medium-access protocol for local area networks
is Ethernet.
• MAC protocols are implemented primarily in hardware in the NIC.
• Local area networks are defined generally in the IEEE Standard 802 suite.
• Ethernet is defined in Standard 802.3.
• We introduced two different forms of wired Ethernet, based on hubs and switches.
• Technically, Ethernet is called the Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) protocol.
• Ethernet is a trade name for this protocol.
• The trade name belongs to Xerox Corporation, who did the original development on the
protocol.
• Ethernet was originally based on bus topology.
• The name CSMA/CD reflects that fact. Switched Ethernet, which is defined in the same
specification, does not actually implement the CSMA/CD protocol, because connections
are point-to-point and collisions are not possible.
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