OSI Model - Data Link Layer

Layer 2 - Data Link Layer
Data Link layer (DLL) provides the physical transmission of data and handles error notification, network topology and flow control. Here, source and destination address is added to the data. This data frames are then checked for errors with the help of Frames Check Sequence ( FCS ), which are placed at the end of the frame. If errors are found, data link layer rejects the frame, if not the data will be passed to the Network layer.

Data Link Layer is also concerned with unique identification of each device and provides physical means of data transfer.

Some of the responsibility of data link layer includes

• Sharing the media among the devices.
• Data flow control between the receiver and the transmitter.
• Packaging the data into frames
• Detecting ans correcting errors from the physical layer on the receiving end.

The important functions of Data Link Layer is

• Data framing : It is concerned about encapsulation of higher level messages into frames which are sent across the network at the physical layer.
• Addressing : It is concerned with labeling the data with a particular destination locations. Every device on a network has unique hardware address or MAC address which is used by the data link protocol to ensure that data intended for a particular device gets it properly.

Error detection and recovery
Data Link Layer check the errors which may occur in a data. For example : cycle Redundancy Check (CRC) field is used to allow the receiving device to detect if data received properly.

The Institute of Electronics Engineers has divided data link layer into two sublayer

1. Media Access Layer
2. Logical Link Layer

Media Access Layer ( MAC) :
The lower sublayer of data link layer, Media Access Layer performs functions such as controlling access and encoding data into a valid format. MAC sublayer is concerned with the physical address of device which is usually given by the hardware manufacturer. Every device in a network should have unique MAC address which ensures proper transmission of data. MAC layer is concerned with adding source and destination address. Bridges and Switches which operate at the data link layer filters and forwards the data based on the MAC address.

The important functions of MAC are

• Physical addressing
• Error notification and ordered delivery of frames
• Examples : CSMA/CD (used in Ethernet and IEEE 802.3), Token bus (IEEE 802.4), Token Ring(802.5) and Token passing (used in FDDI)

MAC Sublayer Frames

Preamble	Destination Address	Source Address	Type	Data	Frame Check Sequence
8 Byte	6 Byte	6 Byte	2 Byte	Variable	4 Byte

Definitions of MAC sub-layer frames are

1. Preamble : A sequence of 8 bytes having alternating 1 and 0 values that are used for synchronized.
2. Destination Address and Source Address : These addresses are referred as MAC layer address and are unique to each device.MAC address is 6 byte (48 bit) address. The destination MAC address field identifies which station should receive the frame. A Destination MAC address may specify either an "individual address" or a "multi-cast address". The source MAC Address identifies the station that has intended the frame. It always specifies individual address.
3. Type/Length : The length field identifies the length of MAC frame in bytes.
4. Data : This field contains the data transferred from the source station to destination. The data field contains LLC information and information from the user.
5. Frame Check Sequence (FRC) : This field contains a 4 byte (32 bit) Cyclic Redundancy Check (CRC) value which is created by the sending device and is recalculated by the receiving device to check for damaged frames. ( CRC contains block of data called as frames which are used to detect errors.)

Logical Link Control (LLC) Layer:
LLC Layer, which is concerned with managing traffic over the physical medium. LLC protects the type of higher level layer from concern with the specific LAN implementation. It is concerned with managing traffic over physical medium. It establishes and controls a logical link between devices on a network. The LLC Layer controls the flow of date and checks errors.

LLC Sublayer frames :
LLC frame type is divided into SAP (Service Access Point ) and SNAP ( Subnetwork Access Protocol ). The frame type depends upon the type of application which runs on a computer. SAP is the logical point at which services are provided. Typically, the protocols in the network layer (like IP) bind at specific SAP in the LLC layer for accessing the services provided by it.

Devices used in DLL : Switches and Bridges operate at this layer