Iso-Osi 7-Layer Network Architecture

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ISO-OSI 7-Layer Network Architecture
This lecture introduces 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. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
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 fibers, 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.

Coaxial Cable Hybrid Cable Wireless Card Network Card

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.

Binary Phase Shift Keying

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 there is no error correction in this layer.
4. Topology and Network Design: The network design is



Links: Of the remaining 16 codes, 7 are invalid and others are used to send some control information like line idle (11111), line dead (00000), Halt (00100) etc. There are other variants for this scheme viz

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