Group11_dnaa:Protocol Stack and Addressing

International Standards Organization (ISO) developed OSI model as a first step towards international standardization of the protocols used in the various layers. The model is known as the ISO OSI Reference Model because it is related with connecting open system i.e. systems that are open for communication with other systems.

The OSI model includes seven layers:

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

Different layers of ISO OSI model are as follows:

1) Physical Layer describes the physical & electrical properties of the communications media.
e.g., voltage levels, data rates, max distances.

2) Data Link layer describes the logical organization of data bits transmitted on a particular
medium.
e.g., frame sequencing, error notification

3) Network Layer describes how a series of exchanges over various data links can deliver data across a network.

e.g., addressing and routing

4) Transport Layer describes the quality and nature of data delivery
e.g., how retransmissions are used to ensure delivery

5) Session layer describes the organization of large data sequences & manages
communication session.
e.g., coordinates requests/responses

6) Presentation Layer describes the form of data being transferred & ensures that it will be
readable by receiver.
e.g., floating point formats, data compression, encryption

7) Application Layer describes how applications will communicate
e.g., HTTP, FTP, Telnet, SMTP

Physical layer:

The physical layer defines the hardware implementation, mechanical, electrical and procedural interfaces and the physical communication channel which lies below the physical layer. At this layer information is placed on the physical medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it. It is related with delivering raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received as a 1 bit by the other side but not as a 0 bit. The physical layer performs services requested by the Data Link Layer.

The major functions and services performed by the Physical Layer:

1. Definition of Hardware Specifications: The details of operation of cables, connectors wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer.
2. Physical topology: establishment and termination of a connection to a communications media. It defines how devices are connected to devices to media. Devices can be connected by using mesh topology, star topology, ring topology or bus topology.
3. Participation in the process whereby the communication resources are effectively shared among multiple users.
4. Representation of bits: Conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. To be transmitted bits must be encoded into signals electrical or optical.
5. Encoding and Signaling: The physical layer data consists of a stream of bits with no interpretation thus it is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
6. Date transmission rate: The transmission rate the no of bits sent each second is also defined by the physical layer.
7. Transmission mode: The physical layer also defines the direction of transmission between two devices:simplex,half dupolex,full duplex.