ISO-OSI 7 Layer Network Architecture ( Physical Layer )

ISO-OSI 7-Layer Network Architecture

 The ISO-OSI layered architecture of Networks. According to the ISO standards, networks have been divided into 7 layers depending on the complexity of the functionality each of these layers provide. The detailed description of each of these layers is given in the notes below. We will first list the layers as defined by the standard in the increasing order of function complexity:

1 - Physical Layer
2 - Datalink Layer
3 - Network Layer
4 - Transport Layer
5 - Session Layer
6 - Presentation Layer
7 - Application Layer

1 - PHYSICAL LAYER

This layer is the lowest layer in the OSI model. It helps in the transmission of data between two machines that are communicating through a physical medium, which can be optical fibres,copper wire or wireless etc. The following are the main functions of the physical layer:

1. Hardware Specification: The details of the physical cables, network interface cards, wireless radios, etc are a part of this layer.

 2.   Encoding and Signalling: How are the bits encoded in the medium is also decided by this layer.        For example, on the copper wire medium, we can use different voltage levels for a certain time            interval to represent '0' and '1'. We may use +5mV for 1nsec to represent '1' and -5mV for 1nsec          to represent '0'. All the issues of modulation is dealt with in this layer. eg, we may use Binary              phase shift keying for the representation of '1' and '0' rather than using different voltage levels if          we have to transfer in RF waves.

3. Data Transmission and Reception: The transfer of each bit of data is the responsibility of this layer. This layer assures the transmission of each bit with a high probability. The transmission of the bits is not completely reliable as their is no error correction in this layer.

4. Topology and Network Design: The network design is the integral part of the physical layer. Which part of the network is the router going to be placed, where the switches will be used, where we will put the hubs, how many machines is each switch going to handle, what server is going to be placed where, and many such concerns are to be taken care of by the physical layer. The various kinds of topologies that we decide to use may be ring, bus, star or a hybrid of these topologies depending on our requirements.