what is computer network

what is computer network

A computer network is an interconnected collection of autonomous computers. An autonomous computer is one whose start, stop and control do not depend on any other computer.

Two computers are said to be interconnected if they are able to exchange information. This connection between two computers may be using copper wire, fiber optics, microwaves, and communication satellites.

Computer Networks and Distributed Systems There is a considerable confusion between a computer network and a distributed system. In a computer network, users explicitly log onto one computer, explicitly submit jobs remotely, explicitly move files around and generally handle all the network management personally.

In a distributed system, the existence of multiple autonomous computers is not visible to the user. A user can type a command to run a program, and it runs. It is up to the operating system to select best processor, find and transport all the input files to that processor, and put the results in the appropriate place. In a distributed system, nothing has to be done explicitly, it is all automatically done by the system without the user’s knowledge. In effect, a distributed system is a software system built on top of a computer network.

Goals of Computer Networks

Computer networks have the following goals :

(i) Resource sharing : Programs, equipment, and data available to anyone on the network can be shared without regard to the physical location of resource and the user.

(ii) High reliability : Alternative sources of resource supply provide high reliability. For example, files could be placed on two or three computers, so if one of them is unavailable due to a hardware failure, the other copies could be used. In addition, for presence of multiple CPUs, if one goes down the other may be able to take over its work.

(iii) Saving money : Resource sharing provides considerable saving of money.

(iv) Salability : Computer network provides ability to increase system performance gradually as the workload grows just by adding more processor in the network.

(v) Communication medium : A computer network provides a powerful communication medium among widely separatedpeople. For example, it is easy for two or more people who live far apart to write a report using a computer network.

Message Transmission over a Computer Network

A message in a computer network is a single unit of communication. For example, in an e-mail system, a message would consist of a document sent from one user to another. A message in an image transmission system could be a single figure, image, or diagram.

To transmit a message over a computer network, it is usually represented as a string of bits. Transmitting long message as one complete unit is generally not done for various reasons. Long messages are normally broken up into shorter bit strings called packets. These packets are then sent through the network as individual units and are reassembled into complete message at the destination computer.

Types Of Computer Networks

Types of Computer Networks Based on Transmission

Technology There are two types of computer networks based on transmission technology :

(i) Broadcast Networks : Broadcast networks have a single communication channel that is shared by all the computers on the network. Packets sent by any computer are received by all the others. An address field within the packet specifies for whom it is intended. Upon receiving a packet, a computer checks the address field. If the packet is intended for itself, it process the packet; if the packet is intended for some other computer, it is just ignored.

(ii) Point-to-point Networks : Point-to-point networks consist of many connections between individual pairs of computers. To go from source to the destination, a packet on this type of network may have to first visit one or more intermediate computers.

Types of Computer Networks Based on Their Scale

There are three types of computer networks based on their scale :
(i) Local Area Networks : Local area networks, generally called LANs, are privately owned networks within a single building or campus of up to a few kilometers in size. They are widely used to connect personal computers and workstations in company offices and factories to share resources and exchange information.

(ii) Metropolitan Area Networks : A metropolitan area network, or MAN, is basically a bigger version of a LAN and normally uses similar technologies. It might cover a group of nearby corporate offices or a city and might be either private or public.

(iii) Wide Area Networks : A wide are network, or WAN, spans a large geographical area, often a country or continent. It contains a collection of computers intended for running user programs called hosts or end systems. The hosts are connected by a communication subnet, or just subnet for short.

Inter networks

Many computer networks exist in the world, often with different hardware and software. People connected to one network often want to communicate with people attached to a different one. This desire requires connecting together different and frequently incompatible networks. The connection is made by using computers called gateways to provide the necessary translation both in terms of hardware and software. A collection of interconnected networks is called an internetwork or just internet.

A common form of internet is a collection of LANs connected by a WAN.

The Internet is a specific world wide internet that is widely used to connect universities, government offices, companies, and private individuals.

Network Topology

The manner in which computers in a network are geometrically arranged and connected is known as the topology of the network.

Topologies used in LANs

Broadcast networks are commonly used in LANs.

Bus Topology : In a bus topology, all the computers are connected to a common transmission medium. As a result of this, only one pair of computers on the network can communicate at the same time. Each computer has a unique address which is used when information is transmitted. When a data packet is sent out, it propagates throughout the medium and is received by all computers. To receive messages, each computer continuously monitors the medium and copies those messages that are addressed to itself as the data packets go by. Since the transmission medium in a bus is generally time-shared, there must be some type of control mechanism to prevent several stations from transmitting simultaneously.

Ring Topology : In a ring topology, consecutive computers are connected by point-to-point links which are arranged to form a single closed path. Data are transmitted from node to node around the ring. The interface at each computer has the ability to recognize packets destined to it.

Star Topology : In a star topology, all computers are joined at a single point called the hub.

Topologies Used In WANs

A computer connected to the WAN is called host or end system. The hosts are connected by a communication subnet or subnet for short. The job of the subnet is to carry messages from host to host.

In most WANs, the subnet consists of two distinct components:

transmission lines (also called circuits, channels, or trunks) and routers (also called nodes or switching elements).

The transmission lines move packets between computers. The routers connect two or more transmission lines. Routers are specialized computers.

this case, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and then forwarded. A subnet using this principle is called a point-topoint (also called store-and-forward or packet switching) subnet.

Computer Networks Architecture

Network hardware is reasonably standard and generally presents few problems. However, when communication is desired among heterogeneous (different vendors, different models of same vendor) computers or hosts, the software development effort can be a nightmare. A one-at-a-time special-purpose approach to network software development is too costly to be acceptable. The only alternative is for computer vendors to adopt and implement a common set of conventions. This set of convention is referred to as protocol. The task of communication in a truly cooperative way between applications on different computers is too complex to be handled as a unit. The problem must be decomposed into manageable parts. Hence before one can develop standards, there should be a structure or architecture that defines the communications task.

Protocols

Characteristics of Protocols

In computer networks, entities in different hosts need tocommunicate. Examples of entities are user application programs, file transfer packages, data base management systems, electronic mail facilities, and terminals. In general, an entity is anything capable of sending or receiving information.

For two entities to successfully communicate they must “speak the same language”. What is communicated, how it is communicated, and when it is communicated must conform to some mutually accepted set of conventions between the entities evolved. The set of conventions is referred to as protocol, which may be defined as set of rules governing the exchange of data between two entities. The key elements of a protocol are :

♦ Syntax : includes such things as data format, coding, and signal levels.
♦ Semantics : includes control information for coordination and error handling.
♦ Timing : includes speed matching and sequencing.

Some important characteristics of a protocol are :
♦ Direct/indirect
♦ Monolithic/structured
♦ Symmetric/asymmetric
♦ Standard/nonstandard.

Direct/indirect : Communication between two entities may be direct or indirect. If two hosts share a point-to-point link, the entities in these hosts may communicate directly; that is, data and control information pass directly between entities with no intervening active agent. If hosts connect through a switched communication networks, the two entities must depend on the functioning of other entities to exchange data.

Monolithic/structured : A protocol may be monolithic or structured. The task of communication between entities on different hosts is too complex to be handled as a unit. An alternative is to use structured design and implementation techniques. Instead of a single protocol, there is a set of protocols that exhibit a hierarchical or layered structure. More primitive functions are implemented in lower-level entities that provide service to higher-level entities. When structured protocol design is used, we refer to the hardware and software used to implement the communications functions as a network architecture.

Symmetric/asymmetric : Protocol may be symmetric or asymmetric. The symmetric protocols involve communication between peer entities. Asymmetric may be dictated by the logic of an exchange, or by the desire to keep one of the entities as simple as possible.

Standard/nonstandard : A protocol may be either standard or nonstandard. A nonstandard protocol is one built for a specific communication situation. The increasing use of distributed processing dictate that all vendors implement protocols that conform to an agreed upon standard.

Functions of Protocols

We can group protocol functions into the following categories:
♦ Segmentation and reassembly
♦ Encapsulation
♦ Connection control
♦ Ordered delivery
♦ Flow control
♦ Synchronization
♦ Addressing
♦ Multiplexing
♦ Transmission services.

Segmentation and Reassembly : A protocol is concerned with exchanging streams of data between two entities. Usually, the transfer can be characterized as consisting of a sequence of blocks of data of some bounded size. Lower level protocols may need to break the data up into blocks of some smaller bounded size. This process is called segmentation, or fragmentation. A block of data exchanged between two entities via a protocol is referred to as a protocol data unit (PDU). The counterpart of segmentation is reassembled.

Encapsulation : Each PDU contains not only data but control information. Indeed, some PDUs consists solely of control information and no data. The control information falls into three general categories :

♦ Address : The address of the sender and/or receiver may be indicated.
♦ Error-detecting code : Some sort of frame check sequence is often included for error detection.
♦ Protocol control : Additional information is included to implement the protocol function.

The addition of control information to data is referred to as encapsulation.

Connection Control : An entity may transmit data to another entity in an unplanned fashion and without prior coordination. This is known as connectionless data transfer. If stations anticipate a lengthy exchange of data and/or certain details of their protocol must be worked out dynamically, a logical connection is established between the entities. This is known as connection-oriented data transfer. In connection-oriented data transfer, three phases occur :

♦ Connection establishment.
♦ Data transfer.
♦ Connection termination.

Ordered Delivery : If two communicating entities are in different hosts connected by a network, there is a risk that PDUs will not arrive in the order in which they are sent, because they may traverse different paths through the network. In connectionoriented protocols, it is generally required that PDU order be maintained.

Flow Control : Flow control is a function performed by a receiving entity to limit the amount or rate of data that is sent by a transmitting entity. Flow control is a function that must be implemented in several protocols.

Error Control : Techniques are needed to guard against loss or damage of data and control information. Most techniques involve error detection and PDU retransmission. Error control is a function that must be performed at various levels of protocols. Synchronization : It is occasionally important that two communicating protocol entities simultaneously in a well-defined state, for example at initialization, check pointing, and termination. This is termed synchronization.

Addressing : For two entities to communicate, other than over a point-to-point link, they must somehow be able to identify each other.

Multiplexing : Multiplexing permits multiple simultaneous connection.

Transmission Services : A protocol may provide a variety of additional services to the entities that use it. Three common examples of services are :

♦ Priority : Certain message, such as control messages, may need to get through to the destination entity with minimum delay. Thus priority may be assigned on a message basis.
♦ Grade of Service : Certain classes of data may require a minimum throughput or a maximum delay threshold.
♦ Security : Security mechanism may be involved.

what is computer network in summary:

Computer Networks

A computer network is a set of communicating devices (for example, computers, printers, tablets) interconnected by some communication media (such as coaxial cables, wireless) and connecting devices (such as hubs, switches) to enable the transmission of data or sharing of data among the devices. Depending on the size of the network it can be broadly classified as follows:

• Local Area Network (LAN) is confined in a small area (such as home, office) and it directly connects devices by hub or switches.

• Wide Area Network (WAN) spans over a large geographical area and typically interconnect multiple LANs by routers.

• Metropolitan Area Network (MAN) refers to a network that covers a single city or metropolitan area.

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