Medium Access Control (MAC)

The Medium Access Control (MAC) protocol is used to provide the data link layer of the Ethernet LAN system. The MAC protocol encapsulates a SDU (payload data) by adding a 14 byte header (Protocol Control Information (PCI)) before the data and appending a 4-byte (32-bit) Cyclic Redundancy Check (CRC) after the data. The entire frame is preceded by a small idle period (the minimum inter-frame gap, 9.6 microsecond (µS)) and a 8 byte preamble.

Header

MAC encapsulation of a packet of data

The header consists of three parts:

A 6-byte destination address, which specifies either a single recipient node (unicast mode), a group of recipient nodes (multicast mode), or the set of all recipient nodes (broadcast mode).
A 6-byte source address, which is set to the sender's globally unique node address. This may be used by the network layer protocol to identify the sender, but usually other mechanisms are used (e.g. arp). Its main function is to allow address learning which may be used to configure the filter tables in a bridge.
A 2-byte type field, which provides a Service Access Point (SAP) to identify the type of protocol being carried (e.g. the values 0x0800 is used to identify the IP network protocol, other values are used to indicate other network layer protocols). In the case of IEEE 802.3 LLC, this may also be used to indicate the length of the data part.

CRC

The 32-bit CRC added at the end of the frame provides error detection in the case where line errors (or transmission collisions in Ethernet) result in corruption of the MAC frame. Any frame with an invalid CRC is discarded by the MAC receiver without further processing. The MAC protocol does not provide any indication that a frame has been discarded due to an invalid CRC.

Inter Frame Gap

After transmission of each frame, a transmitter must wait for a period of 9.6 microseconds (at 10 Mbps) to allow the signal to propagate through the receiver electronics at the destination. This period of time is known as the Inter-Frame Gap (IFG). While every transmitter must wait for this time between sending frames, receivers do not necessarily see a "silent" period of 9.6 microseconds. The way in which repeaters operate is such that they may reduce the IFG between the frames which they regenerate.

Other Issues

The Ethernet standard dictates a minimum size of frame, which requires at least 46 bytes of data to be present in every MAC frame. If the network layer wishes to send less than 46 bytes of data the MAC protocol adds sufficient number of zero bytes (0x00, is also known as null padding characters) to satisfy this requirement. The maximum size of data which may be carried in a MAC frame using Ethernet is 1500 bytes (this is known as the MTU in IP).

A protocol known as the "Address Resolution Protocol" (arp) is used to identify the MAC source address of remote computers when IP is used over an Ethernet LAN.

See also

Ethernet

MAC Address Assignment

Network Interface Cards

Gorry Fairhurst - Department of Engineering, University of Aberdeen, Scotland, UK.

http://www.erg.abdn.ac.uk/users/gorry Date: 10/11/1995 Revised: 7/10/2003