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DATA LINK LAYER: Manages framing, error detection, and retransmission of message. Access to and control of transmission medium.

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PHYSICAL LAYER: Medium and signal formed of raw bit information. Electrical interface (type of signal), Mechanical interface (type of connector), converts electrical signal to bits, transmits and receives electrical signals.

The seven layers of OSI are grouped into three layers. The layer 1, 2 and 3 are called network support layers. The layers 5, 6 and 7 are called support layers and layer 4 is transport layer. Fig. 11.11 shows the OSI network architecture.

Let computer A sends a data stream of bits to computer B. Communication must move from higher layer down through the lower layers on computer A. Each layer in sending machine adds its own information to the message. In receiving computer B, communication must move from lower layer up through the higher layers. At the receiving machine, the message is unwrapped layer by layer.

Computer A Computer B

Computer A Computer B

Physical channel

Fig. 11.11. OSI network architecture.

Physical channel

Fig. 11.11. OSI network architecture.

The information added to the message in layers of the sending computer is in the form of headers and trailers. Headers are added to the message at layers 6, 5, 4, 3 and 2. A trailer is added at layer 2. In receiving computer, each layer removes the data meant for it and original message is recovered by the receiving computer B.

Between each layers, there is an interface (not shown in Fig. 11.11). The passing of the data and network information down through the layers of the sending computer and back up through the layers of the receiving computer is made possible by this interface. Each interface defines what information and services a layer must provide.

Between computers, layer x on one computer communicates with layer x on another machine. This communication is governed by certain protocols. The process on each machine at a given layer are called pear to pear processes.

In the following sections, all the layers are explained under three headings as network support layers, transport layer and support layers.

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