21-01-2013, 12:45 PM
CS65 COMPUTER NETWORKS NOTES
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Unit-1
ISO-OSI 7-Layer Network Architecture
This lecture notes 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
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.
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.
Network 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.