Computer Network
- Computer network is a large number of separate computers that are interconnected to exchange data and information.
- In computer network, users directly interact with the actual machine to invoke the data exchange and the system do not attempt to make the computers or machines to act coherently.
Uses of computer network
1. Business Applications
Resource sharing (Availability of programs, equipments and data to anyone on the network).
E-commerce
2. Home Applications
Access to remote information
Communication
Entertainment
Networking Models - Client-Server Model
- Data are stored in powerful computers known as server.
- Server is managed by a system administrator.
- Users have simple computers called clients.
- Client and server computers are connected by a network.
- Communication occurs as follows:-
1. Client sends request to server over the network.
2. Client waits for a reply.
3. Server gets the request, performs the requested tasks and sends back a reply.
- It is a centralized communication model.
- A single server can have multiple clients associated to it.
Networking Models - Peer-to-peer Model
- Peer-to-peer model is a decentralized communication model.
- Every member of a network have equal capacity. It means there is no any distinction of client and server computers.
- For communication, any member is able to initiate the session.
- It is difficult to filter network traffic that which files are being shared.
- It is impossible to administrate the network.
- A failure in the single peer does not fails the whole system.
- It is very much scalable and robust.
Networking Models - Active Network Model
- Active network model is a communication model in which packets flowing through a network can - dynamically change or modify the operation of the network.
- The packets used are known as active packets.
- Active networking places computation within packets traveling through the network.
- It allows sending code along with packets of information allowing the data to change its form (code) to match the channel characteristics.
- One of the challenges of active networking has been the inability of information theory to mathematically model the active network paradigm and enable active network engineering.
- Every network is built as a stack of layers, each layer offering some services to the layer above it but do not show the actual implementation of those services.
- When layer n of one computer communicates with layer n of another computer within a network, the set of rules specified for this communication is known as layer n protocol.
- Protocol is a special set of rules that should be followed to communicate within a network.
- When some layer n communicates with layer n of another machine, actual data is not transferred between these layers directly. Such communication is called virtual communication.
- Each layer passes data and information to the layer below it until it reaches lowest layer (physical layer). The actual communication occurs in physical layer via a physically connected mediums.
- Between each pair of adjacent layers, there is an interface that defines the set of services offered by the lower layer to the upper layer.
OSI Reference Model
- The model is called OSI (Open Systems Interconnection) Reference Model because it deals with connecting systems which are open for connection with other systems.
- The OSI model has seven layers: physical, data link, network, transport, session, presentation and application layers.
- The principles involved in formation of these layers are:-
1. A layer should be created where different abstraction is needed.
2. Each layer should perform a well defined functions.
3. The function of each layer should be chosen following the international standard.
4. The layer boundary should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be chosen such that distinct function should be in distinct layers.
- OSI layer does not specify the exact services and protocols to be used by each layer but only defines what each layer must performs.
TCP/IP Reference Model
- It consists of four layers. They are link, internet, transport and application layers.
- It provides a packet-switching network based on connectionless layer that runs across different networks.
Comparison of OSI and TCP/IP Model
OSI Model
- It is a generic, protocol independent standard, acting as a communication gateway between network and end user.
- Transport layer guarantees the delivery of packets.
- It follows vertical approach.
- It has separate presentation and session layers.
- Network layer provides connection oriented and connectionless services.
- Protocols are hidden and are easily replaced as technology changes.
- The services, interfaces and protocols defined are clearly separated.
- It has seven layers.
TCP/IP Model
- It is based on standard protocol that allows connection of hosts over the network.
- Transport layer does not guarantee packet delivery.
- It follows horizontal approach.
- It does not have separate presentation and session layers.
- Network layer provides connectionless services.
- Protocol replacing is difficult.
- The services, interfaces and protocols defined are not clearly separated.
- It has four layers.
X.25
- X.25 is a standard protocol for packet switched WAN communication.
- It contains packet switching exchange (PSE) nodes as networking hardware and plain old telephone service connection or ISDN connection as physical links.
- It contains physical, data link and packet layer.
- It is based on traditional telephony concept of establishing reliable circuits through a shared network.
Frame Relay
- Frame relay is a packet switching telecommunication service designed for cost-efficient data transmission for intermittent traffic between LANs and between endpoints in WAN.
- It puts data in variable sized unit called frame along with necessary error correction.
- It provides a permanent virtual circuit as well as switched virtual circuits.
- DTE and DCE are used to transmit data.
- DTE are located at user premise.
- DCE are managed by carriers and provide switching service.
- It is based on X.25 technology.
- It transmits packets at data link layer.
Q) Though we have MAC address, why do we use IP address to represent host in network?
- MAC address is a low level component of LAN which allow a device to communicate with a machine on the LAN but can not be used to route across the Internet.
IP address is a network layer address which can be used by the routers to figure out where to send data even if it needs multiple hops to reach the destination.
- MAC address works at layer 2 and permits two machines to send messages that are physically connected to each other.
Q) Write the OSI Layer in which following tasks are done:
Error detection and correction = Transport layer
Encryption and decryption of data = Presentation layer
Logical identification of computer = Network layer
Physical identification of computer = Data link layer
Point-to-point socket connection = Transport layer
Dialogue control = Session layer
Data framing = Data link layer
Timing and voltage of received signals = Physical layer
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