COMPUTER NETWORKS

INTRODUCTION

· A computer network consists of two or more computers that are linked in order to share resources, exchange data files or to allow electronic communication. The computers on a network may be linked through cables, telephone lines, radio waves, satellites or infrared light beans.

· There are two aspects of computer networks - hardware and software. Hardware includes physical connection between two machines by using adaptors, cables, routers, bridges, etc. software includes a set of protocols. Protocols define a formal language among various components. It makes hardware usable by applications.

Applications of Computer Networks

• Major application areas of computer networks are:

1) Business Applications

2) Home Applications

3) Mobile Computers

1. Business Application

• Now a days computers are being used in all business processes. For example, use of computers to monitor production, inventories, to make payments. Resource sharing is the important purpose of using computer networks. Resources like programs, equipments and data are required to share amongst various users.

a) Database resource :

• The database is required to access for decision making by various departments. The database is maintained by dedicated server and users (clients) can access the data. One server can provide service to many clients.
  
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• Client requests foe a service and server acknowledge the request. The server performs the requested work and sends back the result .the process of and reply for a client.

b) Communication medium:

Computer network is a powerful medium for communication. E-mail is very popularly used amongst company employees. Video-conference is other form of computer assisted communications.

c) Electronic commerce:

· Many companies doing business electronically with suppliers and customers. Customers can place order electronically; this assures fast delivery and efficient services

2. Home Applications

· Now a days, use of computers in home is wide spread. Popular uses of computers in home are as under.

1) Internet access 2) personal Communication

3) Entertainment 4) Electronic Commerce.

· Various magazines, scientific journals, e-books are available online. Many professional organizations also provide their journals, conference proceedings online.

· E-mail, instant messaging, chatting, Internet telephony, Video phone provides personal communication by using internet and WWW.

2. Mobile Computers

Many Professionals uses desktop computers at office and want to be connected to the office network while travelling and at home also. This is possible by wireless networks, hence use of Laptop, notebook computers and personal Digital Assistants (PDAs) is increased. With the help of wireless networks one can access Internet, read and e-mail. Wireless networks are used in :

1) Taxis, delivery vehicles and other mobile vehicles for keeping contacts with their office.

2) Geographical information system (GIS)

3) Military applications

4) Airports

5) Banking

6) Weather reporting

COMPUTER NETWORK

NETWORK SOFTWARE

Network Software



· Computer network is designed around the concept of layered protocols or functions.
For exchange of data between computers, terminals or other data processing devices, there is data path between two computers, either directly or via a communication network.

· Following factors should be considered.

1. The source system must either activate the direct data communication path or inform the communication network to identity the desired destination system.
2. Provide for standard interface between network functions.
3. Provide for symmetry in function performed at each node in the network. Each layer performs the same functions as its counterpart in the other node of network.

· The network software is now highly structured.

Protocol Hierarchies





· Most of all networks are organized as a series of layers, each one built upon the one below it. Because of layer, it reduces the design complexity.

· In layer protocols, a layer is a service provider and may consist of several service functions. Function is a sub-system of a layer.

· Each subsystem may also be made up of entities. An entity is a specialized module of a layer or subsystem.

· Name of the layer, total number so layer, function and content of each layer differ from network to network.

· Protocols are the rules that govern network communication.

· Layer n on one node carries on a conversation with layer n on other node.

· The entities comprising the corresponding layers on different machine are called peers.

· The actual data flow in from upper layer to its below layer and the from physical medium to destination layer.

· Between each pair of adjacent layers is called interface. The interface defines which primitive operations and services the lower layer offers to the upper one.
· A set of layers and protocols is called a network architecture.

DESIGN ISSUES OF LAYERS

Design issues for the Layers





· In the designing the layer, data transfer mode is considered for transmission.

The type of modes are –
A) Simplex communication
B) Half duplex communication
C) Full duplex communication.

· In simplex communication, data travel only in the direction (unidirectional).

· In half duplex mode, each node can both transmit and receive, but not at the same time.

· In full duplex mode, both nodes can transmit and receive simultaneously.

· The protocol also checks the logical channel for connection and the priority of the each logical channel.

· Many networks provide at least two logical channels per connections, one for normal data and one for urgent data.

· Actual data transfer takes place through physical channel. So error control is an important issue for error free data transfer. Many error detecting and error correcting codes are available, by both sides of the connection must agree correction codes are available, but both sides of the connection must agree on which one is being used.

· To receive the data in the same sequence, protocol must make provision for the receiver to allow the pieces to be put back together properly. For this, assign the number to each and every piece.

· While designing the layer, we also consider the loss of data because of slow receiver and fast sender.

· Various solutions are possible on this condition. One of the solutions is that sender agreed upon data transmission rate with receiver.

· If there is multiple path or channel in between sender and receiver, a route must be chosen.


Connection Oriented and Connectionless Services

· Connection oriented and connectionless are the two types of services that are offered by the layer.
· In connection oriented, direct path is established between source and destination. The telephone system is the example of the connection oriented service. This type of service provides a substantial amount of care for the user data.
· The connectionless (also called datagram) service goes directly from an idle condition into a data transfer mode, followed directly by the idle condition.
· The connectionless service is comparable to mailing a letter. Each message carries the full destination address, and each one is routed through the system independent of all the others.
· Each service can be characterized by a quality of service (QOS). Some service is reliable in the sense that they never lose data.
· Usually, a reliable service is implemented by having the receiver acknowledge the receipt of each message, so the sender is sure that it arrived. The acknowledgement process introduces overhead and delays, which are often worth it but are sometimes undesirable.

Service Primitives

· A service is formally specified by a set of primitives available to a user or other entity to access the service.

· These all primitives are implemented as software procedure calls.

· When user application invokes a service provider function by sending a request to the next lower layer, service provider confirm for request.

· If the service is going to provide a function for another user, the service provider must send identification to Y, after which Y is required to provide response.

· User X and Y are connected together by layer service access points (SAPs). If you consider service provider as a layer, the SAP contains the address of the specific function.

· Services can be either confirmed or unconfirmed.

· The request, an identification, a response, and a confirm are all in confirmed services.

· Connect and disconnect are the examples for confirmed and unconfirmed services respectively.

Relationship of Services to Protocols

· Service interface provides an entry point that users use to access the functionality exposed by the application.

· Service interface is usually network addressable.

· Service interface provides a much more coarse-grained interface while preserving the semantics and finer granularity of the application logic. It also provides a barrier that enables the application logic to change without affecting the users of the interface.

· The service interface should encapsulate all aspects of the network protocol used for communication between the user and service. For example, suppose that a service is exposed to consumers through HTTP over a TCP/IP network. User can implement the service interface as an ASP.NET component published to a well-known URL.

REFERENCE MODEL

Reference Model





ISO-OSI Reference Model

· The international organization for standardization developed the open System Interconnection (OSI) reference model. OSI model is the most widely used model for networking.

· OSI model is a seven layer standard.
· The OSI model does not specify the communication standard or protocols to be used to perform networking tasks.

OSI model provides following services.

1) Provides peer-toper logical services with layer physical implementation.

2) Provides standards for communication between systems.

3) Defines point of interconnection for the exchange of information between systems.

4) Each layer should perform a well defined function.

5) Narrows the options in order to increase the ability to communicate without expansive conversations and translations between products.



Principles in defining OSI layers


· Following principles are used in defining the OSI layers.

1. Do not create so many layers as to make the system engineering task of describing and integrating the layers more difficult than necessary.

2. Create a boundary at a point where the description of services can be small and the number of interrelations across the boundary is minimized.

3. Create separate layers to handle function that are manifestly different in the process performed.

4. Collect similar functions in to the same layer.

5. Select the boundaries at a point which past experience has demonstrated to be successful.

6. Create a layer of easily localized functions so that the layer could be totally redesigned and its protocols changed in a major way to take advantage of new advances in architecture, hardware or software technology without changing the services expected from and provided to the adjacent layers.

7. Create a boundary where it may be useful at some points in time to have the corresponding interface standardized.

8. Create a layer where where there is a need for a different level of abstraction in the handling of data.

9. Allow changes of functions or protocols to be made within a layer without affecting other layers.

10. Create for each layer boundaries with its upper and lower layer only.

LAYERS OF OSI MODEL

PHYSICAL LAYER

1) Physical layer is the lowest layer of the OSI model. This layer transmits raw bits over a communication channel. The design issue of physical layer considers four factors. They are electrical, mechanical, procedural attributes and functional attributes. The electrical attributes describe the voltage level or current level. The mechanical attributes describe the connectors and the wires of the interface. The functional attributes describe the function to be performed by the physical interface and the procedural attributes describe what connectors must do and the sequence of events required to effect data transfer across the interface.

DATA LINK LAYER

2) The data link layer is responsible for the transfer of data over the channel. It groups zeros and ones into frames. A frame is a series of bits that forms a unit of data.DLL detects and corrects the transmission errors using error correction method. It also provides data flow control to ensure that the DTE does
not become overburdened. It identifies the devices on the network.

NETWORK LAYER

3) The network layer specifies the intra-network operations and different types of addressing and rooting services. Logical and service addressing are provided from network layer. It also provides switching control and terminal connections. The X.25 specification is included in this layer.

TRANSPORT LAYER

4) The transport layer is responsible for reliable end to end data transfer. Hence optimum utilization of network resources. The transport layer performs the service of sequence numbering and message acknowledgement.

SESSION LAYER

5) The session layer adds mechanisms to establish, maintain, synchronize and manage communication between network entities. This layer has specific services, primitives, and protocol data unit which are defined in ISO.

PRESENTATION LAYER

6) The presentation layer is responsible for data compression, data expansion, data encryption and data decryption. It accepts data type i.e. int. char etc. from application layer and then negotiate with its peer layer as to the syntax
representation.

APPLICATION LAYER

7) The application layer supports end user functions like LOGIN, password file transfer. This layer contains service elements to support application process such as job management, financial data exchange and business data exchange. This layer also supports virtual terminal and virtual file concept.

NETWORK HARDWARE

Network Hardware

• Computer networks can transmit data by two methods-

A. Broadcast links
B. Point-to-point

A. Broadcast networks



Broadcast network uses single communication channel that is shared by many stations. The data to be transmitted is converted in small packet form. Each packet contains address field of the destination station. It is also possible to send same packets to all stations within a network, it is called as broadcasting. When data packets are sent to a specific group of stations it is called as multicasting. In multicasting data is sent to selected group of stations. Multicasting is a selective process.

B. Point –to- point networks

• Point-to-point networks provides a dedicated link between any two stations the data packets are sent from source station to the destination station. Such a transmission is called unicasting.
• Networks can also be categorised depending on their physical size. Major categories of computer networks are-

1) Local Area Networks (LAN)
2) Metropolitan Area Networks (MAN)
3) Wide Area Network (WAN)

MAN ,WAN NETWORKS

3) Metropolitan Area Networks (MAN)



• A MAN, while larger than LAN is limited to city or group of nearby corporate offices. It uses similar technology of LAN.

• The Metropolitan Area Network standards are sponsored by the IEEE, ANSI and the Regional Bell operating companies. The AMN standard is organised around a topology and technique called Distributed Queue Dual Bus (DQDB).



3) Wide Area Networks (WAN)




• A WAN provides long distance transmission of data

• A Network that covers a larger area such as a city, state, country or the world is called wide area network.

• Subnet consists of transmission lines and switching elements. The transmission line is used for data transfer between two machines. switching elements are used for connecting two transmission lines. Switching elements are specialized computers. It selects the proper outgoing line for incoming data and forward the data on that line.

• The switching elements are basically computers and they are called packet switching nodes, intermediate systems and data switching exchanges. These switching elements are also called routers.

• WAN uses hierarchical addressing because they facilitate routing. Addressing is required to identify which network input is to be connected to which network output.


• MAN provides the transfer rates from 34 to 150 bps.

• MAN is designed with two unidirectional buses. Each bus is independent of the other in the transfer of traffic. The topology can be designed as an open bus or a closed configuration.

• MAN's are based on fiber optic transmission technology and provide high speed interconnection between sites. It can support both data and voice.

• MAN as a special category is that a standard has been adopted for them and this standard is now being implemented. It is called IEEE 802.6.