Layer 2 - Data Link Layer
The Data Link layer is the very powerful and complete set of functions for message transfer between hosts.
Protocols of this layer provide the interface between a physical network and protocols of the top layers. The main protocol consists of a frame of a special format that encapsulates the data of the network layer protocol and mechanism, thus, regulating access to a shared medium.
The physical medium can be inaccessible to view if it is busy when a large number of computers transfer information simultaneously. In this situation, the second layers laws and definitions determine the action entered for the solution to the problem. Data Link checks the availability of the data medium, also known as the flow control. Another task of the second layer is to utilize its mechanisms of detection to find and correct mistakes, otherwise known as error notification. The bits are then grouped in sets called frames. This layer provides an accurate transfer of each frame, placing a special sequence bit in the beginning and at the end of the frame determining its allocation, then calculates the control sum (usually called a checksum), and then, processes all bits of the frame in the same specific way and adds the control sum to the frame. At reception of the frame, the control sum of the received data is again calculated and the result is compared to the result of the control sum from the frame. By making and exact match, the frame is considered correct and is accepted. If they do not coincide, the mistake is fixed. With retransmission of the damaged frame, the Data Link layer can correct the mistakes. However, this function of correcting mistakes is absent in some protocols of the second layer, such as Ethernet and Frame Relay.
A method of addressing between hosts is incorporated in protocols of the second layer for a LAN medium. The Data Link layer provides delivery of the frames between any two LAN units if the network topology is typical. Typical topology consists of bus, ring and star, and their hybrid versions. Examples of the LAN protocols served are Ethernet, Token Ring, FDDI and 100VG Any-LAN. In LANs, the protocols of the second layer are used by computers with the help of NICs, bridges, switches and routers.
In WAN networks, which usually do not have the regular topology, the second layer provides an exchange of the messages between only two neighboring hosts. Such an example is the protocol PPP (point-to-point protocol). In this case, delivery of the messages through the entire network uses the top layers facilities. Sometimes functions of the second layer are indistinct, as in one protocol, they are incorporated with functions of a network layer, an example is the ATM (Asynchronous Transfer Mode)177
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