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
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 CableHybrid CableWireless CardNetwork 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 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 and 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 netopologies that we decide to use may be ring, bus, star or a hybrid of these topologies depending on our requirements.
Data Link Layer
This layer provides reliable transmission of a packet by using the services of the physical layer which transmits bits over the medium in an unreliable fashion. This layer is concerned with: 1.Framing: Breaking input data into frames (typically a few hundred bytes) and caring about the frame boundaries and the size of each frame. 2.Acknowledgment: Sent by the receiving end to inform the source that the frame was received without any error. 3.Sequence Numbering: To acknowledge which frame was received. 4.Error Detection: The frames may be damaged, lost or duplicated leading to errors. The error control is on link to link basis. 5.Retransmission: The packet is retransmitted if the source fails to receive acknowledgment. 6.Flow Control: Necessary for a fast transmitter to keep pace with a slow receiver.
Data Link Layer
Its basic functions are routing and congestion control.
Routing: This deals with determining how packets will be routed (transferred) from source to destination. It can be of three types: •Static: Routes are based on static tables that are "wired into" the network and are rarely changed. •Dynamic: All packets of one application can follow different routes depending upon the topology of the network, the shortest path and the current network load. •Semi-Dynamic: A route is chosen at the start of each conversation and then all the packets of the application follow the same route.
The services provided by the network can...