Computer Networks(Introduction)Networks(Introduction)

Debasis Das

computer networks

The old model of a single computer serving all of the organizations computational needs has been replaced by one in which a large number of separate but interconnected computers do of separate but interconnected computers do the job. These systems are called computer networks.

Computer Networks and Internet

we will use the term computer network tomean a collection of autonomous computersinterconnected by a single technology.

Two computers are said to be interconnected ifthey are able to exchange information. Thethey are able to exchange information. Theconnection need not be via a copper wire; fiberoptics, microwaves, infrared, and communicationsatellites can also be used.

They are usually connected together to makelarger networks, with the Internet being the mostwell-known example of a network of networks.

Computer Networks and Internet

Distributed Systems and WWW

There is considerable confusion in the literaturebetween a computer network and a distributedsystem. The key distinction is that in a distributedsystem, a collection of independent computerssystem, a collection of independent computersappears to its users as a single coherent system.

A well-known example of a distributed system isthe World Wide Web. It runs on top of theInternet and presents a model in whicheverything looks like a document (Web page).

Distributed Systems and WWW

NETWORK HARDWARE

There are two types of transmissiontechnology that are in wide spread use:broadcast links and point-to-point links.

Point-to-Point Point-to-point links connect individual pairs of

machines. To go from the source to thedestination on a network made up of point-to-point links, short messages, called packets incertain contexts, may have to first visit one ormore intermediate machines.more intermediate machines.

Often multiple routes, of different lengths, arepossible, so finding good ones is important inpoint-to-point networks.

Point-to-point transmission with exactly onesender and exactly one receiver is sometimescalled unicasting.

Point-to-point

Broadcasting

In contrast, on a broadcast network, thecommunication channel is shared by all themachines on the network; packets sent by anymachine are received by all the others.

A wireless network is a common example of abroadcast link, with communication sharedover a coverage region that depends on thewireless channel and the transmittingmachine.

Broadcasting

Broadcasting

Broadcast systems usually also allow thepossibility of addressing a packet to alldestinations by using a special code in theaddress field. When a packet with this code isaddress field. When a packet with this code istransmitted, it is received and processed byevery machine on the network. This mode ofoperation is called broadcasting.

Multicasting

Some broadcast systems also supporttransmission to a subset of the machines,which known as multicasting.

Multicasting is a Transmission method in Multicasting is a Transmission method inwhich one device communicates with severaldevices with a single transmission.

Introduction to network hardware by scale

An alternative criterion for classifyingnetworks is by scale. Distance is important asa classification metric because differenttechnologies are used at different scales.

In Fig. 1-6 we classify multiple processor In Fig. 1-6 we classify multiple processorsystems by their rough physical size.

At the top are the personal area networks,networks that are meant for one person.Beyond these come longer-range networks.

Introduction to network hardware by scale

These can be divided into local, metropolitan,and wide area networks, each with increasingscale.

Finally, the connection of two or morenetworks is called an internetwork. The worldnetworks is called an internetwork. The worldwide Internet is certainly the best-known (butnot the only) example of an internetwork.

Soon we will have even larger internetworkswith the Interplanetary Internet that connectsnetworks across space (Burleigh et al., 2003).

Introduction to network hardware by scale

Personal Area Networks PANs(Personal Area Networks) let devices

communicate over the range of a person. Acommon example is a wireless network thatconnects a computer with its peripherals. Almostevery computer has an attached monitor,every computer has an attached monitor,keyboard, mouse, and printer. Without usingwireless, this connection must be done withcables.

Bluetooth

To help these users, some companies gottogether to design a short-range wirelessnetwork called Bluetooth to connect thesecomponents without wires.

The idea is that if your devices haveBluetooth, then you need no cables. You justput them down, turn them on, and they worktogether. For many people, this ease ofoperation is a big plus.

Bluetooth

In the simplest form, Bluetooth networks usethe master-slave paradigm of Fig. 1-7. Thesystem unit (the PC) is normally the master,talking to the mouse, keyboard, etc., as slaves.talking to the mouse, keyboard, etc., as slaves.The master tells the slaves what addresses touse, when they can broadcast, how long theycan transmit, what frequencies they can use,and so on.

Bluetooth PAN configuration

Local Area Networks(LAN)

A LAN is a privately owned network thatoperates within and nearby a single buildinglike a home, office or factory. LANs are widelyused to connect personal computers andconsumer electronics to let them shareconsumer electronics to let them shareresources (e.g., printers) and exchangeinformation. When LANs are used bycompanies, they are called enterprisenetworks.

Wireless LANs Wireless LANs are very popular these days,

especially in homes, older office buildings,cafeterias, and other places where it is toomuch trouble to install cables.

In these systems, every computer has a radio In these systems, every computer has a radiomodem and an antenna that it uses tocommunicate with other computers.

In most cases, each computer talks to a devicein the ceiling as shown in Fig. 1-8(a).

Wireless LANs

This device, called an AP (Access Point),wireless router, or base station, relayspackets between the wireless computers andalso between them and the Internet.

There is a standard for wireless LANs calledIEEE 802.11, popularly known As WiFi, whichhas become very widespread.

It runs at speeds anywhere from 11 tohundreds of Mbps.

Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet.

Wired LANs Wired LANs use a range of different transmission

technologies. Most of them use copper wires, butsome use optical fiber.

LANs are restricted in size, which means that theworst-case transmission time is bounded andknown in advance. Knowing these bounds helpsworst-case transmission time is bounded andknown in advance. Knowing these bounds helpswith the task of designing network protocols.

Typically, wired LANs run at speeds of 100 Mbpsto 1 Gbps, have low delay (microseconds ornanoseconds), and make very few errors. NewerLANs can operate at up to 10 Gbps.

Wired LAN and Wireless LAN

Compared to wireless networks, wired LANsexceed them in all dimensions ofperformance. It is just easier to send signalsover a wire or through a fiber than throughover a wire or through a fiber than throughthe air.

Wired LAN The topology of many wired LANs is built from

point-to-point links. IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN.

Fig. 1-8(b) shows a sample topology of Fig. 1-8(b) shows a sample topology ofswitched Ethernet. Each computer speaks theEthernet protocol and connects to a boxcalled a switch with a point-to-point link.Hence the name. A switch has multiple ports,each of which can connect to one computer.

MAN(Metropolitan Area Network)

A MAN(Metropolitan Area Network) covers acity. The best-known examples of MANs are thecable television networks available in many cities.

These systems grew from earlier community These systems grew from earlier communityantenna systems used in areas with poor over-the-air television reception. In those earlysystems, a large antenna was placed on top of anearby hill and a signal was then piped to thesubscribers houses.

MAN(Metropolitan Area Network)

To a first approximation, a MAN might looksomething like the system shown in Fig. 1-9. Inthis figure we see both television signals andInternet being fed into the centralized cable headend for subsequent distribution to peopleshomes.end for subsequent distribution to peopleshomes.

Cable television is not the only MAN, though.Recent developments in high speed wirelessInternet access have resulted in another MAN,which has been standardized as IEEE 802.16 andis popularly known as WiMAX.

MAN(Metropolitan Area Network)

Wide Area Network A WAN(Wide Area Network) spans a large geographical area, often

a country or continent. We will begin our discussion with wiredWANs, using the example of a company with branch offices indifferent cities.

The WAN in Fig. 1-10 is a network that connects offices in Perth,Melbourne, and Brisbane. Each of these offices contains computersMelbourne, and Brisbane. Each of these offices contains computersintended for running user(i.e., application) programs. We will followtraditional usage and call these machines hosts. The rest of thenetwork that connects these hosts is then called thecommunication subnet, or just subnet for short.

This means that many WANs will in fact be internetworks, orcomposite networks that are made up of more than one network.

WAN

WAN The subnet consists of two distinct components:

transmission lines and switching elements.Transmission lines move bits between machines. Theycan be made of copper wire, optical fiber, or even radiolinks. Most companies do not have transmission lineslying about, so instead they lease the lines from atelecommunications company.telecommunications company.

Switching elements, or just switches, are specializedcomputers that connect two or more transmissionlines. When data arrive on an incoming line, theswitching element must choose an outgoing line onwhich to forward them. These switching computershave been called by various names in the past; thename router is now most commonly used.

VPN

First, rather than lease dedicated transmission lines, acompany might connect its offices to the Internet. Thisallows connections to be made between the offices asvirtual links .This arrangement, shown in Fig. 1-11, is calleda VPN(Virtual Private Network).

Compared to the dedicated arrangement, a VPN has theusual advantage of virtualization, which is that it providesflexible reuse of a resource (Internet connectivity).Consider how easy it is to add a fourth office to see this.

A VPN also has the usual disadvantage of virtualization,which is a lack of control over the underlying resources.

WAN

Subnet and ISP

The subnet operator is known as a network serviceprovider and the offices are its customers. Thisstructure is shown in Fig. 1-12. The subnet operatorwill connect to other customers too, as long as theycan pay and it can provide service. Since it would be adisappointing network service if the customers coulddisappointing network service if the customers couldonly send packets to each other, the subnet operatorwill also connect to other networks that are part of theInternet. Such a subnet operator is called an ISP(Internet Service Provider) and the subnet is an ISPnetwork. Its customers who connect to the ISP receiveInternet service.

ISP

Internetworks

A collection of interconnected networks iscalled an internetwork or internet. Theseterms will be used in a generic sense, incontrast to the worldwide Internet (which iscontrast to the worldwide Internet (which isone specific internet), which we will alwayscapitalize. The Internet uses ISP networks toconnect enterprise networks, homenetworks, and many other networks.

Connection-Oriented and Connectionless Service

to use a connection-oriented network service, the service user first establishes a connection, uses the connection, and then releases the connection.connection.

Connectionless communication is just packet switching where no call establishment and release occur.

Connection-Oriented and Connectionless Services

Six different types of service.

Service Primitives

Five service primitives for implementing a simple connection-oriented service.

Service Primitives (2)

Packets sent in a simple client-server interaction on a connection-oriented network.

Services to Protocols Relationship

The relationship between a service and a protocol.

OSI and TCP/IP Model