communications1 communications c.s. french chapter 16

communicationscommunications 11

Communications

C.S. FrenchC.S. French

Chapter 16Chapter 16


Two possible communications setups.

Point to point transmissionPoint to point transmission– two computers connected togethertwo computers connected together– Not so common now.Not so common now.

Computer NetworkComputer Network– interconnection of many computers.interconnection of many computers.


Using the Telephone system. Circuit switchingCircuit switching

– When a caller dials a number the exchanges When a caller dials a number the exchanges involved make the connection between the two involved make the connection between the two points by means of switches.points by means of switches.

– The caller does not share the long distance lineThe caller does not share the long distance line

Sligo exchange

Dublin exchange


Data transmission Sound is carried along the wires as Sound is carried along the wires as an an analoganalog

signal signal – gradual gradual voltage variations of amplitude and frequency.voltage variations of amplitude and frequency.

Digital voltage variations must be converted to Digital voltage variations must be converted to analog for transmission over telephanalog for transmission over telephoone lines.ne lines.– This is done with a MODEMThis is done with a MODEM

MODulator/DEModulator MODulator/DEModulator

modem modem

High frequencyHigh frequencyLow frequencyLow frequency


Data Transmission rates. A data transmission link is normally called a ‘A data transmission link is normally called a ‘channelchannel’’ The capacity of a channel to carry data is related to the range of The capacity of a channel to carry data is related to the range of

frequencies with which it is possible to transmit data.frequencies with which it is possible to transmit data. Capacity is normally measured in ‘Capacity is normally measured in ‘bits per secondbits per second’ which is also ’ which is also

known as ‘known as ‘baudbaud’.’. Channels with higher bandwidth (range of frequencies) are called Channels with higher bandwidth (range of frequencies) are called

broadbandbroadband or or widebandwideband and have greater transmission capacities. and have greater transmission capacities. Approximate transmission baud rates:Approximate transmission baud rates:

– Twisted pair cables: Twisted pair cables: ~10K Baud~10K Baud

– Coaxial cables:Coaxial cables: ~100MBaud~100MBaud

– Optical fibre:Optical fibre: ~5GBaud~5GBaud optical fibre uses short pulses of light instead of voltages to transmit optical fibre uses short pulses of light instead of voltages to transmit

binary data.binary data.


Advantages of using DSL over a modem.

Faster data transfer ratesFaster data transfer rates– 10 times the capacity of a regular phone line.10 times the capacity of a regular phone line.

– Can use your phone at the same time as the Internet.Can use your phone at the same time as the Internet.

““Always on”Always on”– When your computer is on, you are connected to the When your computer is on, you are connected to the

Internet.Internet.

– No need to dial up every time you want access..No need to dial up every time you want access..


Multiplexers If a communications channel is being under-utilised it is If a communications channel is being under-utilised it is

possible to share the line using a device called a multiplexer.possible to share the line using a device called a multiplexer.– data being transmitted may be divided into packets.data being transmitted may be divided into packets.– each packet has the name of the sender and the destination added to it.each packet has the name of the sender and the destination added to it.– the multiplexer sends these packets seperately.the multiplexer sends these packets seperately.– The multiplxer may be transmitting several messages simultaneously The multiplxer may be transmitting several messages simultaneously

and can mix up the different packets.and can mix up the different packets.– the demultiplexer at the other end directs the packets to their correct the demultiplexer at the other end directs the packets to their correct

destinations.destinations.– this ‘packet switching’ technique is also used by telephone systems to this ‘packet switching’ technique is also used by telephone systems to

pack many phone conversations into a single wideband channel (eg an pack many phone conversations into a single wideband channel (eg an optical fibre cable or a satelite link)optical fibre cable or a satelite link)


Computer Networks Computer NetworkComputer Network

– an inter connected set of two or more computers.an inter connected set of two or more computers. Distributed systemDistributed system

– an computer network which appears to act as a single computer.an computer network which appears to act as a single computer. LAN - Local Area NetworkLAN - Local Area Network

– a network covering a single building or set of adjacent buildings.a network covering a single building or set of adjacent buildings.– computers are connected together using a wideband connection such as coaxial computers are connected together using a wideband connection such as coaxial

cable.cable.– some computers on the network may be dedicated to special tasks (servers)some computers on the network may be dedicated to special tasks (servers)

• storage of central data (file server)storage of central data (file server)• control of printed output (print server)control of printed output (print server)• communication with other systems (communications server)communication with other systems (communications server)

WAN - Wide Area NetworkWAN - Wide Area Network– a network covering more than one distant sites.a network covering more than one distant sites.– may consist of LANs interconnected by broadband channels.may consist of LANs interconnected by broadband channels.


Advantages of Networks Central StorageCentral Storage

– Sharing of resources, information and expertise. Sharing of resources, information and expertise. equipment (eg printers)equipment (eg printers) central data storagecentral data storage experts available on the network.experts available on the network.

– Personal data is available from any machine on the networkPersonal data is available from any machine on the network The provision of local facilities without the loss of central The provision of local facilities without the loss of central

control.control. Improved communication within an organisationImproved communication within an organisation

– shared data.shared data.– electronic mail.electronic mail.


Data transmission techniques.(not required for 1st years)

Different methods are used to send data Different methods are used to send data from one computer to another.from one computer to another.

Two techniques are:Two techniques are:– Collision detectionCollision detection– Token passingToken passing


Collision detection (not required for 1st years)

Each computer ‘listens’ to the network to see if it is free.Each computer ‘listens’ to the network to see if it is free. If it is, it transmits the data (a packet with the source and destination If it is, it transmits the data (a packet with the source and destination

addresses attached)addresses attached) It ‘listens’ again to see if it has clashed with any other data transmissions.It ‘listens’ again to see if it has clashed with any other data transmissions. If it has, it waits a random time and retransmits.If it has, it waits a random time and retransmits. Ethernet uses this technique.Ethernet uses this technique. Advantages:Advantages:

– computers can be removed from the network without effecting the operation.computers can be removed from the network without effecting the operation.– it is very fast at moderate traffic levels.it is very fast at moderate traffic levels.

Disadvantages:Disadvantages:– transmission rates deteriorate dramatically at high traffic levels because of transmission rates deteriorate dramatically at high traffic levels because of

the number of collisions and retransmissions.the number of collisions and retransmissions.


Token passing (not required for 1st years)

A ‘token’ (a certain binary code) is passed from one computer to the A ‘token’ (a certain binary code) is passed from one computer to the next.next.

If there is If there is nono data attached to the token a computer may attach some data attached to the token a computer may attach some data and send it on to the next computer.data and send it on to the next computer.

If there If there is is data attached the computer will check the destination address data attached the computer will check the destination address to see if it is for this computer:to see if it is for this computer:– if it is if it is notnot, it is just sent onto the next computer., it is just sent onto the next computer.

– if it if it isis the data is read, the token freed and then sent onto the next computer. the data is read, the token freed and then sent onto the next computer. Advantages:Advantages:

– transmission rates do not significantly deteriorate with high traffic.transmission rates do not significantly deteriorate with high traffic. Disadvantages:Disadvantages:

– if one computer breaks down the network will not operate.if one computer breaks down the network will not operate.


Network Topologies (not required for 1st years)

StarStar

LoopLoop

MultidropMultidrop


Star Networks (not required for 1st years)

Advantages:Advantages:– Messages do not need to wait for a free ‘token’.Messages do not need to wait for a free ‘token’.

– No ‘collision’ problem.No ‘collision’ problem.

Disadvantages:Disadvantages:– Requires a very high performance ‘hub’ for routing information.Requires a very high performance ‘hub’ for routing information.

– A separate cable must be laid down from the hub for each computer on the A separate cable must be laid down from the hub for each computer on the network.network.

Not currently very popular for LANs.Not currently very popular for LANs. Asynchronous Transfer Mode network (ATM)Asynchronous Transfer Mode network (ATM)

– high performance networkhigh performance network

– used for high trafficused for high traffic eg as a telephone exchangeeg as a telephone exchange hub is called an ATM switch.hub is called an ATM switch.


Ring Network (not required for 1st years)

Uses a ‘token’ system.Uses a ‘token’ system. eg IBM’s ‘token ring’ network.eg IBM’s ‘token ring’ network.


Multidrop Network (not required for 1st years)

Collision detection must be used for data Collision detection must be used for data transmission.transmission.

eg: Ethernet (as in Sligo RTC)eg: Ethernet (as in Sligo RTC)


The Internet A worldwide network of voluntary ‘service providers’ A worldwide network of voluntary ‘service providers’

located at points called ‘nodes’.located at points called ‘nodes’. Each node in the network may be connected permanently to Each node in the network may be connected permanently to

several other nodes with wideband connections.several other nodes with wideband connections. At the node is a device called a ‘router’ that is capable of At the node is a device called a ‘router’ that is capable of

receiving messages and sending them further if required.receiving messages and sending them further if required. This network of nodes is known as the ‘backbone’ of the This network of nodes is known as the ‘backbone’ of the

networknetwork (Physically it resembles a net) (Physically it resembles a net)

NodeNode

BroadbandBroadbandconnectionconnection

Athlone ITAthlone IT GMITGMIT

IT SligoIT Sligo


The World Wide Web The WWW is a huge set of documents stored on computers connected to the The WWW is a huge set of documents stored on computers connected to the

Internet interlinked by inserting phrases or images in documents that contain hidden Internet interlinked by inserting phrases or images in documents that contain hidden information specifying the location of other documents on the Internet. (Hyperlinks)information specifying the location of other documents on the Internet. (Hyperlinks)

These documents are described in a language called HyperText Markup Language These documents are described in a language called HyperText Markup Language (HTML) which describes the layout of text and images within the documents.(HTML) which describes the layout of text and images within the documents.

The documents may be viewed using programs called The documents may be viewed using programs called browsersbrowsers which are capable which are capable of interpreting HTML and displaying the documents.of interpreting HTML and displaying the documents.

HTML has become more advanced recently and now allows the use and submitting HTML has become more advanced recently and now allows the use and submitting of forms, embedding of sophisticated programs (Java or Javascript) and the of forms, embedding of sophisticated programs (Java or Javascript) and the definition of data structures (XML).definition of data structures (XML).

More sophisticated data (e.g. audio, video or multimedia) require extra programs More sophisticated data (e.g. audio, video or multimedia) require extra programs referred to as ‘plug-ins’.referred to as ‘plug-ins’.

The Internet servers that store pages can also create pages on the fly by using The Internet servers that store pages can also create pages on the fly by using ‘server side’ programs to generate the necessary HTML to describe the page. (eg. ‘server side’ programs to generate the necessary HTML to describe the page. (eg. Yahoo mail, Google etc)Yahoo mail, Google etc)


Service Providers Each service provider (node) has one or more Each service provider (node) has one or more

‘servers’.‘servers’. A server is a computer used for managing the A server is a computer used for managing the

connection to users:connection to users:– live transfer of data to and from the user to the network.live transfer of data to and from the user to the network.– holding of data until the user logs in.holding of data until the user logs in.

The user may connect to the server in the The user may connect to the server in the following ways:following ways:– ‘‘dial up’ connection (modem, ISDN e.g Ireland On Line)dial up’ connection (modem, ISDN e.g Ireland On Line)– LAN/WAN connection (if there is a node on your network e.g. LAN/WAN connection (if there is a node on your network e.g. IT IT

SligoSligo))


Services on the Internet Electronic MailElectronic Mail

– ability to send and receive text based messagesability to send and receive text based messages (but may also include other media) (but may also include other media)

File TransferFile Transfer– ability to see files on servers and download them to your own computer.ability to see files on servers and download them to your own computer.– Also as attachments to email messagesAlso as attachments to email messages

World Wide WebWorld Wide Web– use of special programs called browsers that can read specially prepared use of special programs called browsers that can read specially prepared

information files (‘rich’ text and graphics) on servers.information files (‘rich’ text and graphics) on servers.– these files can contain ‘hyperlinks’ that can automatically connect you to other these files can contain ‘hyperlinks’ that can automatically connect you to other

documents on other servers.documents on other servers.

Network News and NewsgroupsNetwork News and Newsgroups– ‘‘bulletin board’ where you can post notices and read others.bulletin board’ where you can post notices and read others.– Email user groups where you can post letters which will be distributed to all Email user groups where you can post letters which will be distributed to all

members.members.


Uses Customer Support Customer Support – helping a customer with a product.– helping a customer with a product. Electronic Shopping Electronic Shopping BankingBanking MarketingMarketing

– Information on Products and ServicesInformation on Products and Services– Selling the Organisation Selling the Organisation PRPR

Product Distribution Product Distribution – for software based products (programs and data)for software based products (programs and data)– Entertainment, programs, education etcEntertainment, programs, education etc

Information ServicesInformation Services Publishing - electronic books and journalsPublishing - electronic books and journals Virtual Corporations (Teleworking)Virtual Corporations (Teleworking)

– people working together on a single project at different locations.people working together on a single project at different locations. Special Interest GroupsSpecial Interest Groups


Teleworking Often known as Computer Supported Collaborative Working Often known as Computer Supported Collaborative Working

(CSCW)(CSCW)– people working together on a single project at different locationspeople working together on a single project at different locations

Working AidsWorking Aids– Email, file transferEmail, file transfer– live voice communicationlive voice communication– ‘‘video-conferencing’video-conferencing’

you can see the person you are talking to on your screen.you can see the person you are talking to on your screen.

– application sharingapplication sharing you can see the program the other person is working on (eg.AutoCAD) and also be allowed you can see the program the other person is working on (eg.AutoCAD) and also be allowed

control that application.control that application.

– ‘‘whiteboarding’whiteboarding’ a shared piece of computer screen on which both people can write or sketch.a shared piece of computer screen on which both people can write or sketch.

– Shared document editingShared document editing– Shared CalendarsShared Calendars– Web based project managementWeb based project management

communications1 communications c.s. french chapter 16

Documents

channel u

u capacity

u point

u channels

u circuit

data transmission link

u distributed system

u faster data transfer