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DataLink Layer :

This layer is charged to give a significance to the bits of the physical layer and to transfer the DATA of an end FROM a wire the other has. The chart network assembles a screen of DATA then adds information of addressing to him. After which, it controls the displacement of the DATA on the wire. The protocols which function has this level deliver DATA of chart has chart. They provide limited diagrams of addressing but are unable to GROUP logical addresses in partition (or networks).selon the protocol of connection in service, it is possible to detect a possible corruption and to correct the errors.

Examples WAN of these types of protocols include :

• HDLC (high Level Datalink Control), used by networks X25.
• Frame Relay.
• ATM (Asynchronous Transfert Mode)

In field LAN, there are several protocols. Most significant are those of series 802 which describe the access methods to the cables, the formats of screens and physical addressing. These protocols are as follows :

• Ethernet (IEEE 802.3)
• Token Bus (IEEE 802.4)
• Token Ring (IEEE 802.5)

The protocol IEEE 802.2 functions in the higher part of this layer and gets a coherent interface with the protocols of networks functioning with the top. It identifies the protocols of the network layer of source and destination (also called Access points of service). Standard 802.2 provides also the detection of errors, the recovery on incidents and the control of flow.

Ethernet_II is very close to standard 802.3 as for its function and has its access methods to the cable. But because of subtle differences in the formatting of the screens, the two protocols are incompatible. Ethernet_II preserved until A there is little, a strong adhesion of the users, because of many systems TCP/IP functioning on Ethernet.

The DATA can be filtered has this level when two similar networks are connected between them by bridges. Bridging is the term used for the switching unit which functions has this level. With the bridges, the administrator has the possibility of using various access methods of each dimensioned, provided that the networks layer is the same one. These units must also include/understand the physical level; this is why a component repeater should be included.

This layer has the characteristic to be divided INTO 2 underlayers: LLC and MAC

• The layer 802.2 LLC (Logical Link Control) which deals with the transfer of the screens between the stations of the network. This layer is completely independent of the type of access method used.
  In the case of wide area network, this layer is replaced by a layer HDLC (High level DATA Link Control).
• The layer MAC (Medium Access Control) which has as a role, amongst other things, to manage the physical addressing of the charts networks. This layer is completely independent of the type of media used: 802.3 (= Ethernet), 802.5 (Token Ring), FDDI... The MAC address is made of 48 bits, it is single in the world. It is generally made of a part addresses of the manufacturer and a n° of series of chart in this manufacturer.

Easy : The DATA link layer is responsible for the transfer of DATA between peripherals, it organizes the bits coming FROM the physical layer in a structured unit called frame. Conversely, this layer encapsulates the packages coming FROM the network layer in a screen adapted has the transmission on the network.