When a number of computers and terminal equipment are to be connected together to form an integrated system, a well understood standard method of communication and physical inter connection should be established. This becomes particularly critical when equipment supplied by different vendors are to be connected since each vendor would have his own standards.

If computers in different countries are to be connected together, yet another problem arises due to the need to use communication system belonging to different nations which have their own telecommunication regulations. Common agreed rules followed to interconnect and communicate between computers are known as protocols. A protocol is a standard set of rules to be followed in order to communicate over the internet.

Electronic industries association (USA) has proposed a standard method of interconnecting user terminals to computer. This comletely specifies the interface between data communication devices. Computers and terminals the RS 232-C interface proposed by E.I.A., consists of 25 connection points which specify the physical pin connections, voltage levels and single transmission rate, timing information and control information such as ready and send.

The interconnection protocol for computer to computer defines the following:

• Beginning and termination of session between two computers.
• Framing of messages in a session.
• Detection of errors in transmission.
• Finding out which message box is sent by which terminal/ user to whom.
• How the dialogue on communication process proceeds.

The most common method of sharing communication lines in a network is for a central communication controller to allocate unique addresses to computers and terminals in the network and allocates resources by polling. In polling, the communications controller asks a terminal or computer, using its address, whether a message block is sent if the answer is ‘yes’, it accepts the message and routes it to the computer or terminal specified, if it is free to receive it. several protocols were developed by different manufacturers viz-system data link control (SDLC) by IBM. Bisync by National Cash Register’s(NCR).

An interconnection protocol developed and recommended by international standard organization is gaining wide acceptance. This mode is called as ISO model for open system which has certain set of layers for interlinking itself. The ISO model is made up of seven layers as shown

Physical link layer:
This layer defines the electrical and mechanical aspects of interfacing to a physical medium for transmitting data. It also defines how physical links are set up – maintained and disconnected.

Data link layer:
This layer establishes an error free communications path between computers over the physical channel. It gives the standard for framing messages, checking integrity of received message, accessing and using channels and sequencing of transmitted data.

Network control layer:
This determines the setting up of a logical path between computers in a network, message addressing to computers and controlling message flow between computer nodes.

Transport layer:
Once a path is established between computers, it provides control standards for a communication session for enabling processes to exchange data reliably and sequentially, independent of which system are communicating or their location in the network.

Session control layer:
This establishes and controls system dependent aspects of communication between specific computers in the network and bridges the gap between the service provided by the transport layer and logical functions running under the operation system of a particular computer in the network.

Presentation control layer:
This layer provides facilities to convert encoded, transmitted data into forms which can be displayed on a video terminal or printer.

Application / user layer:
This provides services that directly support users such as file transfers, remote file access, database management etc.

The main advantage of the layered approach is that each one can be improved and modified independent of other layers. With changes in communication technology and standards, easy adaptation is important. One standard which defines the first three layers of ISO is the CCITTX.25 protocol. This standard has been integrated in the network architecture of many vendors.

In an ideal computer network, a user working at any of the computers at any location in the network should be able to utilize the special facilities, languages etc. available in any other computer in the network without having to know the detailed filing method, the type of operating system commands,etc.of the machine.In other words, there should be a standard user interface at all locations. The network operations system should be able to decide and provide the optimal computing facility to a user to fulfill his requirements wherever he may be in the network without his having to even know from where and how his requirements are met.

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