OSI LAYERS

To reduce their design complexity, most networks art: organized as a series of levels called layers each one built upon its predecessor. Purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.

Communication functions are divided into a hierarchy of seven layers. Transmission medium is not included in the seven layers therefore it can be regarded as 0th layer.

Subnetwork has at most three layers as shown. These three layers interface with the corresponding pear layers of the end system to carry on the functions relating to the transport of messages from one end system to the other.

Physical Layer:-

Concerned with transmission of bits across the inter connection media, carries out following functions:-

• Conversion of bits into electrical signals having characteristics suitable for transmission over media.

• Signal encoding if required.

• Relaying of digital signals using m. It does not have capability to date it and correct errors which are introduced due to noise and distortion of electrical signals during transmission. It makes sure that when one side sends a 1 bit, it is received by the other side as a 1 bit and not as a zero bit. The design issues here largely deal with mechanical, electrical and procedural interfaces and the physical transmission medium that lies between the physical layers.

Data Link Layer:-

Its primary function is to improve the quality of service provided by physical layer by correcting the errors which are introduced during transmission of electrical signals. It adds error detection bits to a block of data (frame) before handing it over to physical layer for transmission. These bits are used for detecting the errors if any in the data blocks received by the Data Link layer at the other end. If an error is detected transmission mechanisms are employed to correct the errors.

Receiving end is provided with some control to regulate the flow of incoming frames. Flow Control Mechanisms are also an integral part of the error mechanism in the Data Link layer.

Network Layer:-

It provides the means to access the Subnetwork for routing the messages to the destination and system. It is concerned with controlling the operator of the subnet. It determines how packets (i.e. frames) are routed from source lo destination. If too many frames are present in a subnet at the same lime, they will in each other's way, forming bottlenecks. Control of such congestion also belongs to the network layer. Accounting function built into the network layer must count how many packets or characters or bits are sent by each customer, to produce filling information.

When a frame of data crosses a national border, with different rates on each side, the accounting can become complicated. When a packet has to travel from one network to another to get its destination, many problems arise. Addressing techniques, protects etc. of the time networks may differ. The second one may not accept the packet because it is too large. The network layer overcomes all these problems to allow heterogeneous networks to be interconnected.

Transport Layer:-

Basic function of this layer is to accept data from session layer, split it up into smaller units, pass these to network layer and ensure that the pieces all arrive correctly at the other end. Quality of Transport connection is specified in terms of residual error rate, delay, throughput and other quality determining parameters. For achieving the quality of service the transport layer may do multiplexing, splitting and blocking a segmenting. Transport layers provide end-to-end connectivity. In other words a program on the source machine carries on a conversation with a similar program on destination machine, using message headers and control messages.

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