Layer 2: The Data-Link Layer

The Data-Link layer of the OSI model has three primary responsibilities:

• Establishing and maintaining the communication channel

• Identifying computers on the network by physical address

• Organizing the data into a logical group called a frame

Establishing and maintaining the communication channel is low-level work. At the Data-Link layer, it just means confirming that a data channel exists and that it is open. At the Session layer, there is higher-level communication management.

Each computer on the network is identified with a unique physical address, which is called the MAC address. On Ethernet and Token Ring cards, this is a 6-byte hexadecimal address that is burned into an EPROM chip on the network card. This address uniquely identifies computers when you send data from a source to a destination.

Data is logically grouped into a frame. A frame logically organizes the bits from the Physical layer. Here is an example of a frame:

Frame Description

In the preceding frame:

• The Start field identifies the beginning of the frame. This might be 10101010.

• The destination address is the MAC address of the computer to which the frame is being sent.

• The source address is the MAC address of the computer from which the frame is being sent.

• The Logical Link Control (LLC) header manages the LLC connection.

• The Data field contains the data from the upper-layer protocols.

• The cyclic redundancy check (CRC) is used for low-level error control. For example, is the connection valid?

• The End field indicates that this is the last field of the current frame.

Logical Link Control (LLC)

A Data-Link sublayer that establishes whether communication with another device is going to be con-nectionless or connection oriented.

cyclic redundancy check (CRC)

A form of error detection that performs a mathematical calculation on data at both the sender's end and the receiver's end to ensure that the data is received reliably.

managed hubs

Similar to hubs but with the exception that the device can be managed using software to monitor and control network communication.

switch

The modern name used for a multiport bridge. Like a regular bridge, each port on a switch represents a separate network. Traffic on each port is kept isolated except when the packet is destined for another device on a different port or if the packet is a broadcast. Broadcasts must be sent out all ports.

Sublayers of Data-Link

The Data-Link layer has two sublayers:

• The LLC sublayer

• The MAC sublayer

The LLC sublayer provides the upper layers with access to the physical hardware at the lower layers, and the MAC sublayer is used for physical addressing.

Note 

Data is referred to as a frame at the Data-Link layer.

Data-Link Layer Devices

Examples of Data-Link layer devices are:

• Bridges

• Managed hubs

• Switch

• Network interface cards