Network Support Layers

The physical layer, data link layer and network layer are referred as network support layers. All these layers are explained below.

PHYSICAL LAYER. The physical layer is the lowest layer of the OSI model. It defines the mechanical, electrical, functional and procedural aspects of the physical link between networks. Physical layer standards have been widely used for years in a point-to-point wide area network applications. (CCITT/ITU has established X-21-X-24 to specify the functions at the physical level of the based circuits. Numerous other standards such as EIA-232 and V-21-V-34 are widely used for various purpose at the physical layers.

The physical layer implements an unreliable bit link. A link consists of a transmitter, a receiver and a medium over which signals are propagated. The physical layer data consists of stream bits. The physical layer defines the type of encoding to convert the bit stream into electrical or optical signal to transmit in the medium. At receiver the physical layer converts back into bit stream. The receiver must be in synchronism with transmitter to receive the specific bit pattern. To assist synchronization, the physical layer adds a specific bit pattern called preamble at the beginning of the packet.

DATA LINK LAYER. The data link layer defines the frame format such as start of frame, end of frame, size of frame and type of transmission. The principal service provided by the data link layer to higher layer is that of error detection and control. This layer is the first software protocol layer of the OSI model. It specifies the data format, sequence, acknowledgement process and error detection methods.

The data link layer accepts information from the network layer and breaks the information into frames. It then adds the destination address, source address, frame check sequence (FCS) field and length field to each frame and passes each frame to the physical layer for transmission on the receiving side, the data link layer accepts the bits from the physical layer and froms them into frame, performing error detection. If the frame is free of error, the data link layer passes the frame up to the network layer. It perform frame synchronization, that is, it identifies the beginning and end of each frame.

Existing protocols for the data link layer are :

1. Synchronous Data Link Control (SDLC). Developed by IBM as link access for System Network Architecture.

2. High Level Data Link Control (HDLC). It is a version of SDLC modified by the ISO for use in the OSI model.

3. Link Access Procedure Balanced (LAPB). The modified version of HDLC is LAPB.

The data link layer also performs the flow control and access control. By flow control, if the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver. By access control, when two or more devices are connected to the same link, the protocol of data link layer determines which device has control over the link at any given time.

Two sub layers defined in the data link are the media access control (MAC) and the logical link control (LLC) layer. MAC performs address management function. LLC manages flow and error control, automatic requests for retransmission (APQ) and handshake processes.

NETWORK LAYER. If two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source to destination delivery. Thus the function of the network layer is to perform routing. The network layer checks the logical address of each frame and forwards the frame to the next router based on a look up table. The network layer is responsible for translating each logic address (name address) to a physical address (MAC address).

There are two types of virtual circuits used in the network layer. Connectionless and connection oriented. In connection oriented service, the network layer makes a connection between source and destination, then the transmission starts. Connectionless circuits are also known as "bandwidth on demand'' circuits, which establish a connection when they are needed. The source transmits information regardless of whether the destination is ready or not. A common example of this type of service is e-mail.

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