Computer and Data Communication Networks These are the telecommunication networks used for information and data exchange in the form of text, multimedia. These networks interconnect intelligent machines like computers, teletype-writers over physical media like twisted wire pair, coaxial cable, optical fiber, microwave links etc. Each system is equipment with a trans-receiver for transmitting and receiving of information. The complete system at each site is called a station or node. In the network each station is assigned a unique address for accessing. The distance among stations ranges from few meters to thousands of kilometers.

Fig 1: Data and Computer Communication Network

Depending on the geographical distance the networks are classified as (1) Local area networks: Maximum distance is few kilometers.

(2) Metropolitan networks: The distance ranges from few kilometers to few hundred kilometers. (3) Wide Area Networks: The distance is in thousands of kilometers and the network is spread over number of cities and even countries. Applications of Data Communication Networks: These networks have numerous applications. Some of them are listed below: (1) Distributed Computing (2) e-mail (3) e-governance

     Physical Network 

   S   SS

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(4) e-commerce (5) Net banking (6) On-line education (7) Tele-Education (8) Tele-medicine (9) on-line shairing of information and multi media contant. (10) Video on demand, Voice over Internet (VOI) (11) Telecommuting which allows the employees to perform office work at home by remote login. (12) On-line collaboration on projects. (13) Teleconferencing which allows people to share information without meeting at common place. (14) Disaster warning and management. (15) Data base management system.

Protocol Architecture of Data communication networks To facilate the efficient data and information exchange the communicating stations have to follow a well defined set of rules, which constitute a protocols. It is a layered structure of hardware and software that supports the exchange of data. At each layer a set of rules are provided for exchange of data among stations. Typical tasks to be performed by a protocol include: (i) The source station must activate the direct communication path or it must inform the network of the address of the destination station. (ii) Before commencing the transfer of data the source station must ascertaion that the destination is ready to accept the information. (iii) The file transfer application on the source station must see that the destination station is ready to accept and store the file. (iv) Formatn translation has to be done either by source or destination if the same is not compatible. The exchange of information between source and destination occurs by means of formatted data blocks. The a protocol must have following features

Syntax: which takes care of formatting of data blocks Semantics: It takes care of coordination and error handling Timing and Synchronization: It takes care of speed matching and data Sequencing.

OS (Open System Interconnection) Refrence model

To remote the operational compatibility among the vendor equipment and to encourage the competition well defined standards are needed. For communication task no single standard can work efficiently as it is a complex task. The entire task is divided into sub tasks which are more manageable and together organize a communication-architecture. International Organization of Standardization (ISO) proposed an OSI standard model. In this model the communication functions are partitioned into a hierarchical set of layers.Each layer performs a subset of functions needed for communication with other stations. The upper layer depends on lower layer to perform more primitive functions and conceals the details of these functions. The OSI model has seven layers. These are Application, Presentation, Session, Transport, network, Data link and physical layers.Each station incorporates all the seven layers. Communication between two stations takes place at application layer. If a station A wishes to send file to station B, it invokes application layer of station A. However the application layer of station a requires the services of the all the sixes layers from presentation layer down to physical layer. The physical layer transmits bits over the transmission medium. No direct transmission between the corresponding layers of the two stations takes place except at physical layers. At station A application layers appends a header to the file as per the set of rules and functions and passes it on to next layer. The next layer does the same and forwards it to next lower layer. In this way the file reaches to physical layer where it is transmitter to the physical layer of the station B. After receiving the entire file the physical layer of B passes the file to its upper layer and so on. Each layer at B removes the overhead information before sendin it to next higher layer.

                  Application Layer

                 Presentation Layer   

                    Session Layer   

                   Transport Layer   

                    Network Layer   

                    Data link Layer   

                     Physical Layer   

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Station A Station B

Fig. 3: Information Flow Path between Layers

       Application Layer 

         Physical  Layer 

      Presentation Layer              

          Session Layer 

        Transport Layer 

          Network Layer 

          Data link Layer 

       Presentation Layer   

       Application Layer 

            Session Layer 

           Transport Layer 

           Network Layer 

          Data link Layer 

         Physical  Layer 

Physical Layer

The physical layer is concerned with the transmission and reception of unstructured binary bit stream over physical medium. It covers the physical interface between the devices and rules by which data are passed from one station to another. The layer has following four important properties:

• Mechanical: It specifies the physical properties of the interface i.e. the pluggable connector to a transmission medium. The connector joins number of signals wires, called circuits.

• Electrical: It specifies the electrical characterstics by which data bits are represented such as voltage levels, circuit impedances, signal format and also the transmission bit rate.

• Functional: It specifies the different functions which are performed by individual circuits of the physical interface between a system and the transmission medium.

• Procedural: It elaborates sequence of events by which data bits are exchanged between two communicating stations over the physical medium. Examples of physical standards at this layer are EIA-232F, wireless standards and LAN standards.

A basic data communication link between two stations is shown in following fig.

Data Transmission Channel

Station A Station B

Data Terminal Equipment (DTE) is the user equipment which is connected to data communication network. It may be a simple PC to a large computer system.

Data Communication Equipment (DCE) is the interface that provides a network attachment point for user equipment. DCE provides the functions required to establish, maintain and terminate a connection. It also performs the signal conversion and coding required for the interface.

Data Terminal Equipment 

Data Communicating Equipment 

     Transmission Medium 

Data Communicating Equipment 

Data Terminal Equipment 

Data Link Layer

In physical layer the main concern is about transmission of raw data bits on a physical link. In actual systems much more is needed to control and manage the information exchange over the physical medium. A layer of logic is required over the physical interface to achieve the necessary control. This logic has been given the name Data link control protocol and the transmission medium between systems is called as Data Link. Various requirements for effective data communication between two stations are; Frame Synchronization: A station sends data in block or frames. The frame boundries must be marked for proper identification. Flow Control: The sending station must transmit frames at the rate which is either equal or less than the rate the receiving station can sink them. Error Control: When data bit stream are transmitted over a physical medium, discrepancies occur in the data bits called errors. These errors have to be controlled or corrected. Addresssing: On a multipoint link the identity of the two communication stations involved on information exchange must be specified. Control and Data on Same Link: control information and data is sent on the same physical link. Accordingly the receiver must be able to distinguish between the two types of informations. Link management: The initiation, maintenance and termination of a connection requires a good amount of co-ordination and co-operation among communication stations. Well defined procedures for management of this are required. The above requirements are not met by physical layer. All these requirements are incorporated in the data link layer.

Network Layer: The basic function of network layer is to provide the transfer of information between the stations across some type of communication network such as circuit switched, packet switched network. The higher layers are not involved in this task. This layer specifies the destination address of the other system and also requests for certain network facilities like priority. There are three possible connections Case I: Two stations are connected by direct point-to point communication link. The network layer has minimal job in this case. Case II: Two communicating stations are connected to same network.The lower three layers are responsible for communicating with the network. The messages or packets may pass through number of intermediate stations called network nodes before being delivered to the destination station. The network nodes implement the lower three layers

of the architecture for the purpose. At each intermediate node there are two data link layers and two physical layers. Each data link and Physical layer operates independently to provise service to network layer.

A B

Case III: The two stations may be connected o different networks, which are connected directly or indirectly. In this case network layer has to implement internetworking facilities to establish connection and reliable information exchange.

Application Layer 

Presentation Layer 

Session Layer 

Transport Layer 

Network Layer 

Physical Layer 

Data Link Layer 

Physical Layer 

Data Link Layer 

Network Layer 

Transport Layer 

Session Layer 

Presentation Layer 

Application Layer 

Physical       Layer 

Network                  Layer 

Data Link      Layer 

Transport Layer:

• The transport layer provides a mechanism for the exchange of data between two end users.

• The transport layer is also concerned with optimizing the use of network resources and services.

• The objective of the transport layer is to provide all the functiond and protocols needed to satisfy a quality of service requested by user application. The parameters used to express quality of service are throughput, delay, residual error rate, delay time in establishing a connection, cost, security and priority.

• Following five protocol options exist at the transport layer (i) Routing information protocol:

This is used to update the routing information at each station. This information is needed to facilitate the communication with other stations.

(ii) Error protocol: This protocol is used to report the errors by sending standard error messages to the information source.

(iii) Echo protocol: Special packets are transmitted over the network, which are echoed by other stations. These verify that the other nodes are operating correctly. This feature is useful for performing network diagnostics.

(iv) Packet exchange protocol: This protocol is used for a simple request/reply transactions.

(v) Sequenced-packet protocol: This protocol allows the users to transmit many packets in sequential order before requesting for a response. This facilities high speed data transfer.

Session Layer: The lowest four layers of OSI model provide the means for reliable communication and also provide various options for quality of service. The session layer is the user interface with the network. The session layer allows the two presentation entities to establish, use and terminate the connection, which is called session. It includes following functions (i) Manages the dialog in an orderly manner and decides whose turn it is to transmit and for how much time a user can have line. (ii) Determines whether the communication in full duplex, half-duplex or simplex. (iii) It establishes data synchronization checkpoints in the data flow to permit backup and recovery from errors and failures. When ever a transport layer connection is broken, the session layer automatically sets up a new

connection, resynchronizes both end of the connection and resumes dialog from the last valid check point. (iv) It provides functions for address mapping. This helps the remote users to identify themselves. (v) The session layer can also provide for ordering of message packets when the transport layer does not.

Presentation Layer:

This layer performs functions of general nature which give the user a variety of services, User has the choice of either using these services or not. These functions are performed by user by calling library routines. Some important functions of presentation layer are

(i) Data encryption for information security. (ii) Data compression fo conserving bandwidth. (iii) Terminal handling which facilitates the communication between

incompatible terminals. (iv) File transfer from one user to another.

Application Layer:

This is the top layer which contains application protocols with which user gains access to the network.

An application is a set of information processing requirements desired by the user e.g. air line control, e-mail processing, credit card checking and train reservation etc. An application process is a logical element with in a system that carries out the information processing required for a specific application. Sometimes an application is distributed over several systems and in such cases each distributed application portion is an application process. To exchange meaningful information, the two application processes must agree on the semantics of all aspects concerned with the intended exchange.

Some of the tasks performed by the application processes are

(i) Allow users to acess the network resources and associate them with the right application.

(ii) Transfer files, messages and documents electronically. (iii) Standardize database and operating system command language. (iv) Ensures that users transmit functions like carriage return, line feeds, tabs

and subscripts in a commonly understood format.

 

Application                                                       Application  

process 1                                                           process 2