MAC - Getting data to the Media

Media Access Control (MAC) is the lower Ethernet sublayer of the Data Link layer. Media Access Control is implemented by hardware, typically in the computer Network Interface Card (NIC).

The Ethernet MAC sublayer has two primary responsibilities:
Data Encapsulation
Media Access Control

Data Encapsulation

Data encapsulation provides three primary functions:
Frame delimiting
Addressing
Error detection

The data encapsulation process includes frame assembly before transmission and frame parsing upon reception of a frame. In forming the frame, the MAC layer adds a header and trailer to the Layer 3 PDU. The use of frames aids in the transmission of bits as they are placed on the media and in the grouping of bits at the receiving node.

The framing process provides important delimiters that are used to identify a group of bits that make up a frame. This process provides synchronization between the transmitting and receiving nodes.

The encapsulation process also provides for Data Link layer addressing. Each Ethernet header added in the frame contains the physical address (MAC address) that enables a frame to be delivered to a destination node.

An additional function of data encapsulation is error detection. Each Ethernet frame contains a trailer with a cyclic redundancy check (CRC) of the frame contents. After reception of a frame, the receiving node creates a CRC to compare to the one in the frame. If these two CRC calculations match, the frame can be trusted to have been received without error.

Media Access Control

The MAC sublayer controls the placement of frames on the media and the removal of frames from the media. As its name implies, it manages the media access control. This includes the initiation of frame transmission and recovery from transmission failure due to collisions.

Logical Topology

The underlying logical topology of Ethernet is a multi-access bus. This means that all the nodes (devices) in that network segment share the medium. This further means that all the nodes in that segment receive all the frames transmitted by any node on that segment.

Because all the nodes receive all the frames, each node needs to determine if a frame is to be accepted and processed by that node. This requires examining the addressing in the frame provided by the MAC address.
Ethernet provides a method for determining how the nodes share access to the media. The media access control method for classic Ethernet is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This method is described later in the chapter.

http://standards.ieee.org/regauth/groupmac/tutorial.html