Computer Networks
Information Technology / Technology

Computer Networks

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Introduction

The purpose of this post is to explain the basics of computer network. A computer network can be simply defined as the interconnection of two or more independent computers. Applications of computer networks are found everywhere. They are used in our homes, schools, colleges, railway stations, offices and business. They help us to send an email, to watch a live sports event at our computer, to book rail/air tickets, to do chatting with our friends. The post explains uses of computer networks, a client/server network, peer-­to-­peer network architecture, network topology, OSI & TCP/IP models and some useful application of computer networks.

 

Why Network?

We use a Computer Network for the following reasons:

Resource Sharing: A network is needed because of the desire to make all programs, data, and equipment available to anyone on the network without regard to the physical location of the resource and the user. Load sharing is another aspect of resource sharing.

High Reliability: A network may have alternative sources of supply (e.g., replicated files, multiple CPUs, etc.). In case of one resource failure, the others could be used and the system continues to operate at reduced performance. This is a very important property for military, banking, air traffic control, and many other applications.

Saving Money: A network may consist of many powerful small computers, one per user, with data kept on one or more shared file server machines, which offers a much better price/performance ratio than mainframes. In this model, the users are called clients, and the whole arrangement is called the client­-server model.

Scalability: The ability to increase system performance gradually by adding more processors (incremental upgrade).

Powerful Communication Medium: Networks make cooperation among far-flung groups of people easy where it previously had been impossible. In the long run, the use of networks to enhance human­ to ­human communication may prove more important than technical goals such as improved reliability.

 

Network Architecture

Whenever you have more than one computer being used at the same location, networking them together makes a lot of sense. You transfer files between them quickly and easily, you can also share expensive resources like laser printers, hard disc arrays, backup tape drives, scanners, internet connections, operating system, multimedia a S/W and so on.

A network may comprise of multiple computers connected through interface, as a Network Interface Card (NIC) and appropriate networking software that provides the server or client functionality. The third component besides NIC and network is communication media for carrying data from one machine to another machine. Usually networks are categorized in two main categories which are: [1] Client/Server based network. [2] Peer-to-peer network.

In a server based network, there are computers which work as providers of services such as file service or mail service. The computers providing the service are called servers machine and the computers that request and use the service are called client machine. The server kind of machine is supposed to be most powerful than the client machine.

In a peer-­to-­peer network, various computers on the network can act both as clients and servers. For instance, many Microsoft Windows based computers will allow file and print sharing. These computers can act both as a client and a server and are also referred to as peers. Many networks are combination of peer-­to-­peer and server based networks. The network operating system uses a network protocol to communicate on the network to other computers. A Network Operating System (NOS) includes Windows NT, Novell Netware, Unix, Linux and others.

 

Topologies

In computer networking, topology refers to the layout of connected devices. You can think of a topology as a structure of a network. This shape does not necessarily correspond to the actual physical layout of the devices on the network.

Network Topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network interconnection between the nodes. It also determines the strategy for physically expanding the network, in future Topologies can be physical or logical. Physical Topology means the physical design of a network including the devices, location and cable installation. Logical Topology refers to the fact that how data actually transfers in a network as opposed to its design.

Following are the considerations for choosing a Topology:

    • Costing: Some kind of topology may be less expensive compared to others in terms of installation (for example Bus topology).
    • Length Of Cable: It is needed to connect machines in a network.
    • Network Scalability: The topology should support future growth of network without much expenditure.
    • Cable Types: There are difference types of cable media: unshielded twisted pair, coaxial cable, Fiber optics. The most common cable is unshielded twisted
      pair, which is most often used with star topology.

Now we shall learn about different types of the topologies like bus, ring, tree, mesh in detail.

 

Bus Topology

Bus topology is a single common communication to which all the computers are connected. It has a single length of cable with a terminator at each end. It is a passive topology which means only one computer at a time can send a message. Hence, the number of computers attached to a bus network can significantly affect the speed of the network. A computer must wait until the bus is free before it can transmit. Each node is connected to others nodes. The network operating system keeps track of a unique address of each node and manages the flow of data between machines.

The bus topology is the simplest and mostly widely used with local area network design. A bus topology is considered to be a passive topology. The computers on the bus keep on listening. When they hear data that belongs to them, they receive. When one device on the network wants to send a broadcast message to another device on the network, it first makes sure no one else on the bus is transmitting, and then it sends information out on the media. All other devices on the network see it, but only the intended recipient accepts and processes it. This is accomplished by using data frames which contain source and destination addresses.

Advantages

    • It is simple, reliable, and easy to be used in a small sized local area network.
    • It requires least amount of cable to connect computers together and is therefore less expensive than other cabling arrangements.
    • It is easy to implement and extend using connectors.
    • If one computer on the bus fails, it does not affect the rest of the traffic on the
      bus.

Disadvantages

    • In this topology, no two computers can transmit data at the same time.
    • It does not cope well with heavy load which can slow down a bus considerably.
    • Performance degrades as additional computers are added.
    • Terminators are required at both ends of the cable.
    • It is hard to reconfigure once it’s set up.

 

Ring Topology

Ring topology is also known as circular topology. This layout is similar to the linear bus, except that the nodes are connected in a circle. In this topology, each node is connected to two and only two neighboring nodes. The ring does not have an end. It is made of short segments that connect one PC to the next PC and so on Data is accepted from one of the neighboring nodes and is transmitted onwards to another node. Therefore data travels in only direction from node to node around the rings. Since, each computer retransmits what it receives, a ring is an active network and is not subject to the signal loss problems. There is no termination because there is no end to the ring.

This type of topology can be found in peer-­to-­peer networks, in which each machine manages both information processing and the distribution of data files. Examples of such topology: [1] IBM Token Ring. [2] Fiber Distributed Data Interface (FDD).

Advantages

    • It is an orderly network where every device has access to the token (control signal) and the opportunity to transmit – because every computer is given equal access to the token, no computer can monopolize the network.
    • It performs better than a star topology under heavy network load.
    • It can create much larger network using Token Ring.
    • It does not require network server to manage the connectivity between the
      computers.

Disadvantages

    • Network adapter cards and Multi Access Units used in this topology are much more expensive than Ethernet cards and hubs used in bus topology.
    • It is much slower than an Ethernet network under normal load.
    • It is difficult to troubleshoot.
    • One malfunctioning node or bad port in the Multi Access Units can create problems for the entire network.

 

Star Topology

In star topology each computer on a network communicates with a central hub (also called as a concentrator) that re­sends the message either to all the computers or only to the destination computer. A hub expands one network connection into many. For example a four-­port hub connects up to four machines. A single hub is sufficient for a small network, however large networks require multiple hubs. But, it increases hardware and cabling costs.

Advantages

    • It is more reliable (if one connection fails, it doesn’t affect others) – The center of a star network is a good place to diagnose network faults and if one computer fails whole network is not disturbed. Hub detects the fault and isolates the faulty computer.
    • It is easy to replace, install or remove hosts or other devices, problem can be easily detected. ­It is easier to modify or add a new computer without disturbing the rest of the network by simply running a new line from the computer to the central location and plugging it to the hub.
    • Use of multiple cables types in a same network with a hub.
    • It has good performance.

Disadvantages

    • It is expensive to install as it require more cable, it costs more to cable a star
      network because all network cables must be pulled to one central point, requiring more cable length than other networking topologies.
    • Central node dependency, if central hub fails, the whole network fails to operate.
    • Many star networks require a device at the central point to rebroadcast or switch the network traffic.

 

Tree Topology

It is one of the most common types of network setups that is similar to a bus topology and a star topology. Tree topology integrates multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus and each hub functions as the “root” of a tree of devices.

Advantages

    • It required point-­to-­point wiring for individual segments.
    • It is easily supported by several hardware and software vendors.

Disadvantages

    • In this topology, the overall length of each segment is limited by the type of cabling used.
    • If the backbone line breaks, the entire segment goes down.
    • It is more difficult to configure and wire than other topologies.

 

Mesh Topology

A type of network setup where each of the computers and network devices are interconnected with one another allowing for most transmissions to be distributed, even if one of the connections goes down, the network remains intact. While the data is traveling on the Mesh Network it is automatically configured to reach the destination by taking the shortest route which means the least number of hops.

Advantages

    • It provides redundant paths between devices and hence it is more reliable and stable.
    • The network can be expanded without disruption to current users.

Disadvantages

    • It requires more cabling than the other LAN topologies.
    • The implementation of this topology is quite complicated.

 

Hybrid Topology

With the hybrid topology, two or more topologies are combined to form a complete network. For example, a hybrid topology could be the combination of a star and bus topology. These are also the most common in use.

In a star-­bus topology, several star topology networks are linked to a bus connection. In this topology, if a computer fails, it will not affect the rest of the network. However, if the central component, or hub, that attaches all computers in a star, fails, then you have big problems since no computer will be able to communicate.

Advantages

    • Network expansion is very simple in case of hybrid technology.
    • If one client fails, the entire network does not fail.

Disadvantages

    • If one hub fails, all connections to that hub fail, although other hubs continue to function.

 

Classification Of Networks

There are mainly three types of networks:

    1. LAN [ Local Area Network ]
    2. MAN [ Metropolitan Area Network ]
    3. WAN [ Wide Area Network ]

 

Local Area Network [ LAN ]

LAN is a group of computers located in the same room, on the same floor or in the same building that are connected to form a single network as to share resources such as disk drives, printers, data, CPU, fax/modem, application, etc.

LAN is generally limited to specific geographical area less than 2 K.M., supporting high speed networks. A wide variety of LANs have been built and installed, but a few types have more recently become dominant. The most widely used LAN system is the Ethernet system based on the bus topology.

Intermediate nodes (i.e., repeaters, bridges and switches) allow to be connected together to from larger LANs. A LAN may also be connected to another LAN or to WANs and MANs using a “router” device.

 

Components Of A LAN

    1. Network devices such as workstations, printers, file servers which are normally accessed by all other computers.
    2. Network Communication Devices i.e., devices such as hubs, routers, switches etc. used for network connectivity.
    3. Network Interface Cards (NICs) for each network device required to access the network. It is the interface between the machine and the physical network.
    4. Cable as a physical transmission medium.
    5. Network Operating System –software applications required to control the use of network operation and administration.

Characteristics Of LAN

    • It connects computers in a single building, block or campus, i.e. they work in a restricted geographical area.
    • LAN’s are private networks, not subject to tariffs or other regulatory controls.
    • LAN’s operate at relatively high speed when compared to the typical WAN (.2 to 100 MB/sec).
    • There are different types of Media Access Control methods in a LAN, the prominent ones are Ethernet, Token ring.
    • The networking is done through Buses or Rings.

Advantages Of LAN

    • It allows sharing of expensive resources such as Laser printers, software and mass storage devices among a number of computers.
    • LAN allows for high­speed exchange of essential information.
    • It contributes to increased productivity. A LAN installation should be studied closely in the context of its proposed contribution to the long range interest of the organization.

Disadvantages Of LAN

    • The financial cost of LAN is still high in comparison with many other alternatives.
    • It requires memory space in each of the computers used on the network. This reduces the memory space available for the user’s programs.
    • Some type of security system must be implemented if it is important to
    • protect confidential data.
    • Some control on the part of the user is lost. You may have to share a printer with other users. You may face a situation like, for example, the entire network suddenly locking up because one user has made a mistake.

 

Metropolitan Area Network [ MAN ]

Metropolitan area networks, or MANs, are large computer network usually spanning a city. They typically use wireless infrastructure or Optical fiber connections to link their sites.

A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderate­-to-­high data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as public utilities or privately owned. They will often provide means for internetworking of local networks. Metropolitan area networks can span up to 50km, devices used are modem and wire/cable.

A Metropolitan Area Network is a large computer network that spans a metropolitan area or campus. Its geographic scope falls between a WAN and LAN. MANs provide Internet connectivity for LANs in a metropolitan region, and connect them to wider area networks like the Internet.

  • The network size falls intermediate between LAN and WAN. A MAN typically covers an area of between 5 and 50 km diameter. Many MANs cover an area the size of a city, although in some cases MANs may be as small as a group of buildings or as large as the North of Scotland.
  • A MAN often acts as a high speed network to allow sharing of regional resources. It is also frequently used to provide a shared connection to other networks using a link to a WAN.

Characteristics Of MAN

    • It generally covers towns and cities (50 km).
    • It is developed in 1980s.
    • Communication medium used for MAN are optical fibers, cables etc.
    • Data rates adequate for distributed computing applications.

 

Wide Area Network [ WAN ]

Wide Area Network is a network system connecting cities, countries or continents, a network that uses routers and public communications links. The largest and most well-­known example of a WAN is the Internet.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization’s LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built around public network or Internet.

Characteristics Of WAN

    • It generally covers large distances (states, countries, continents).
    • Communication medium used are satellite, public telephone networks which are connected by routers.
    • Routers forward packets from one to another a route from the sender to the receiver.

 

Reference Models

We will discuss two reference models:

 

OSI [ Open System Interconnection ] Model

The OSI model is an abstract description for layered communications and computer network protocol design open system means that it can communicate with any other system that follows the specified standards, formats and semantics. PROTOCOLS give the rules that specify how the different parties may communicate.

In its most basic form, it divides network architecture into seven layers which from top to bottom are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error­ free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

OSI Model – Layers

        • [7] Application
        • [6] Presentation
        • [5] Session
        • [4] Transport
        • [3] Network
        • [2] Data Link
        • [1] Physical

In transmission side data flows from layer 7 to layer 1, then to cabling or suitable medium. When data reaches the reception side it flows from layer 1 to layer 7.

    • Application Layer: This layer is the layer for user interaction. We must have an application software for dealing with the data.
    • Presentation Layer: It converts the data into suitable format. It does tasks like compression, decompression, encryption and decryption.
    • Session Layer: This layer manages connections between different application layers.
    • Transport Layer: This layer converts data into segments and re­assembles the data stream. TCP and UDP are the protocols used in this layer. In this layer, data is converted into so called segments.
    • Network Layer: This layer translates logical address into physical address. This layer also fixes the route for data path. Router works in this layer. In this layer data is to called a packet.
    • Data Link Layer: This layer provides physical identification of a device using MAC Address. It adds source and destination address to packets and convert them into frames.
    • Physical Layer: This layer provides the functional requirements for activating a physical link. In this layer data is carried from one device to another.

This layer provides the functional requirements for activating a physical link. In this layer data is carried from one device to another.

Now, we can better understand the OSI layer with an example. Consider that we have to send a word document to a different network or through internet. The following are the process that will take place:

1– In the APPLICATION LAYER, the user can edit the file by using application software like Microsoft word or Word Viewer etc.

2– In the PRESENTATION LAYER, user can compress the word file by using WINRAR or WINZIP and convert the data into different format i.e.­– .zip or .rar. He can also convert the word document into different formats.

3– In the SESSION LAYER, the particular file has to be integrated with browser for attaching it to email or likewise clients.

4– In the TRANSPORT LAYER, data is converted to segments. Source IP and destination IP are added to each packets. Frame checks and parity bits are also added in this layer.

5– In the NETWORK LAYER, the data is handed over to a router. The router calculates the best path for data transmission.

6– In the DATA­-LINK LAYER, a transmission errors are handled and also flow of data is regulated so that receivers are not swamped by fast senders.

7– In the PHYSICAL LAYER, frames are transmitted as bits through media such as network cable, Optic fiber etc.

 

TCP/IP Model [ Transmission Control Protocol/Internet Protocol ]

Just like the OSI model, the TCP/IP model has many layers which are described below:

Host To Host Network: In fact TCP/IP model does not specify this layer. But it basically combines functionally of physical and data link layers. Starting at the bottom, the Physical layer is what deals with hardware (wires, cables, satellite links, NICs, etc.). Utilizing the existing Physical layer, TCP/IP does not define its own, thus letting the layer be compatible with all network suites. This layer also encodes and transmits data over network communications media in the form of bits which are received by the Physical layer of the destination device. Often combined with this layer is the Data link layer which is responsible for moving packets from the network layer onto different hosts. Depending on the connection type, IP packets are transmitted using various methods. Dial-­up modems transmit IP packets using PPP while broadband users transmit using PPoE.

Internet Layer: This layer routes and delivers data across similar networks or completely different networks. The Network layer is responsible for end to end packet delivery while maintaining routing, flow control, and error control functions. An example of this layer is the actual Internet Protocol (IP) or the Internet Protocol Security (IPSec).

Transport Layer: The Network layer can be thought of the actual vehicle which transports information. This layer categorizes end to end message transmissions or connecting applications as either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). TCP is a connection­-oriented protocol which is considered to provide a reliable byte stream. Some characteristics of TCP are traffic congestion control, data arrives in order, data has minimal error, and duplicate data is discarded.

The top layer of the TCP/IP model is the Application layer which is used for network communication. Ports are used by programs to transfer data through this layer. For example, the File Transfer Protocol uses port 21 while the Hypertext Transfer Protocol uses port 80.

TCP/IP has many benefits. TCP/IP enables cross­-platform networking which is useful in this day­-in-­age. This suite also has superior failure recovery and the ability to add networks without interrupting existing services. The reliability of TCP/IP is also a huge benefit to using this protocol. The fact that if one part of the network goes down, other parts are still able to function is what has set TCP/IP above other networking protocols. TCP/IP is also easily expandable which allows for the unprecedented rate of growth which the Internet possesses.

 

Network Applications

There are wide varieties of computer application in today’s world, without computer networking it is difficult to imagine the world. Few of these applications are described below:

Person-­To-­Person Communication: Real­time systems, such as videoconferencing and virtual meeting environments over the network or internet, allow remote users to communicate with no delay, possibly seeing and hearing each others as well.

Email: Email is simply the shortened form of electronic mail, a protocol for receiving, sending and storing electronic messages. Email has gained popularity with the spread of the computer network. In many cases, email has become the preferred method of communication. Emails are accessed using through email client software like Outlook, In addition to email client software, Webmail has also become very popular. Most email service providers offer this as an additional benefit, where the user can access their mailbox over the Internet. Some email services are specifically designed using the World Wide Web as its primary interface. Sites like Yahoo! Mail, Google Mail and Hotmail have become very popular, as they offer individuals a chance to open an email account at no charge.

Interactive Entertainment: Computer network is extensively being used in entertainment industry. Services like Video on demand in which the user can select any movie or TV program ever made, in any country, and have it displayed on his screen instantly. Other applications include Live and interactive TV in which audience may participate in quiz shows, multiplayer real­-time games flight simulators, etc.

FTP: File Transfer Protocol (FTP) is a network protocol used to exchange and manipulate files over a computer network using Transmission Control Protocol Network. FTP is commonly used to transfer Web page files from their creator to the computer that acts as their server for everyone on the Internet. It’s also commonly used to download programs and other files to your computer from other servers.

Telnet: A terminal emulation program for TCP/IP networks such as the Internet. Telnet is a protocol that allows you to connect to remote computers (called hosts) over a computer network using TCP/IP. You can use software called telnet client on the computer to make a connection to a telnet server (i.e., the remote host). Once your telnet client establishes a connection to the remote host, your client becomes a virtual terminal, allowing you to communicate with the remote host from your computer. In most cases, you’ll need to log into the remote host, which requires that you have an account on that system. Occasionally, you can log in as guest or public without having an account.

Marketing And Sales: Computer networks are used extensively in both marketing and sales organizations. Marketing professionals use them to collect, exchange, and analyze data related to customer needs and product development cycles. Sales application includes teleshopping, and online­-reservation services for hotels, airlines and so on.

Financial Services: Today’s financial services are totally depended on computer networks. Application includes credit history searches, foreign exchange and investment services, and electronic fund transfer, which allow user to transfer money without going into a bank (an automated teller machine is an example of electronic fund transfer, automatic pay­check is another).

 

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