Specifications

The Open Systems Interconnect (OSI) Model

3-4 Ethernet LAN Standards

• LAYER TWO: Data Link Layer–The data link layer is involved with

transmission, error detection and ﬂow control of the data. The major

function of the data link layer is to act as a shield for the higher layers

of the network model, controlling the actual transmission and

reception process. Error detection and control of the physical layer are

the primary functions of this layer ensuring the upper layers that any

data received from the network is error free. The IEEE 802 model

divides the data link layer into two sub-layers: Logical Link Control

(LLC) and Media Access Control (MAC).

Figure 3-2. Data Link Layer of the OSI Model

- Logical Link Control–The LLC layer is responsible for shielding

the upper layers from any particular access method or media. The

upper layers need not worry about whether they are connected to a

Token Ring or Ethernet network because the logical link control

handles the interface. The LLC is a defined standard (IEEE 802.2)

that provides for a common interface of the layers above to any

physical network implementation.

- Media Access Control–The MAC layer is responsible for several

areas of operation. On the transmit side the MAC layer is

responsible for receiving data from the Logical Link Control layer

and encapsulating it into a packet ready for transmission. The MAC

layer is also responsible for determining if the communications

channel is available. If the channel is available, the MAC layer

transmits the data onto the cable through the physical layer and

monitors the physical layer status for an indication of a collision

(more than one station transmitting at the same time). If there is a

collision, the MAC layer also handles the backoff and

retransmission function.

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

LLC

MAC

1913-03