PROTOCOLS

A computer network consists of communication media, devices, and software needed to connect two or more computer systems or devices. It is the communication software that holds all telecommunications systems together. It instructs computer systems and devices how data is to be transferred from one place to another. These procedures embedded in software are called a protocol. It is a set of rules that dictates the methods and procedures used in transporting data.

 Communication protocols make communication possible. They are the hardware and software standards for communications. Both computers must have the same settings, called Parameters, and must follow table lists the most common parameters.

Communication Protocols  

Most computers use full-duplex, enabling simultaneous two-way communication. Half-duplex enables data to flow in only one direction at a time.

A number of protocols, or standards, are used to varying degrees by companies of all sizes. Many protocols have layers of standards of procedures. Open Systems Interconnection (OSI) is a protocol that was endorsed by the International Standards Committee. Like most of the protocols, OSI uses layers to describe various aspects of communications standards having each layer relates to a different aspect of communications. They are described below.

OSI Reference Model

ISO (International Standards Organization) has defined a 7-layer model to clarify various tasks in communications systems. The main idea is to have independent standards for the different layers so that a change in a layer would not cause changes in other layers. In the layered approach it is possible to use different network hardware without changing the existing application programs, for example.
 

Figure OSI model: ISO/OSI network architecture reference model

Each layer communicates with its peer using services that the layer below provides. As can be seen from the following picture, the transport layer and layers above it are end-to-end layers and they do not know anything about the network below them.

Layer 1: Physical layer:

It defines electrical signaling on the transmission channel; how bits are converted into electrical current, light pulses or any other physical form. Physical-layer specifications provide information about four different aspects of physical-layer interfaces: mechanical, electrical, functional, and procedural. Mechanical aspects of physical-layer interfaces include connector specifics, circuit-to-pin assignments, and connector-latching arrangements. Electrical aspects include voltage levels that represent binary values and electrical impedance. Functional aspects of physical-layer interface specifications assign functions (control, data, and ground) to particular circuits. Finally, the procedural aspects define those procedures associated with various data exchange operations. Serial line is an example of the physical layer. A network device functioning at this layer only is called a repeater. This part is used to attach various telecommunication devices, including modems.

Layer 2: The data link layer:

It defines how the network layer packets are transmitted as bits. This layer also checks the data flowing into and out of each device on the network for errors to make sure no data loss occurs between two points in a network. Examples of data link layer protocols are PPP (Point to Point Protocol) and Ethernet framing protocol. Bridges work at the data link layer only.

Layer 3: The network layer:

It defines how information from the transport layer is sent over networks from one place to another. Since there are many communication lines to send a message from one location to another, the network layer can determine the best path through the network. An example of a network layer protocol is the Internet Protocol. A device that takes care of the network level functions is called a router or sometimes a gateway in the Internet terminology.

Layer 4: The transport layer:

This layer isolates some of the physical and functional aspects of a packet network from its other (upper) three layers. It takes care of data transfer, ensuring the integrity of data if desired by the upper layers. TCP and UDP are operating at this layer. This layer also provides data security features.

Layer 5: The session layer:

It establishes and terminates connections and arranges sessions to logical parts. It also sets priorities for communications and helps reestablish the connection if there is a failure. Some of the functions that the session layer provides for are flow control, establishment of synchronization points for duplex dialogs between processes, and aborts and restarts. The session layer and the remaining layers have not been defined or standardized to the same extent as the above layers. TCP and RPC provide some functions at this layer.

Layer 6: The presentation layer:

It takes care of data type conversion. This layer assumes that an end-to-end path or connection already exists across the network between the two communicating parties. In the OSI model, the AppleTalk Filing Protocol (AFP) spans the presentation and application layers. AFP provides an interface between an application and a file server. An example of protocol residing at this layer is XDR (External Data Representation), which is used by RPC applications to provide inter operability between heterogeneous computer systems.

Layer 7: The application layer:

It is the highest layer of the OSI model and defines the protocols to be used between the application programs. Examples of protocols at this layer are protocols for electronic mail (e.g. SMTP,) file transfer (e.g. FTP) and remote login.

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