Lecture 2 (Communication Channel)

8/8/2019 Lecture 2 (Communication Channel)

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CommunicationChannel

Facilitated by: Baguma innocent

Prepared by: Baguma Innocent


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Definition

p A path through which information aretransmitted from one place to another iscalled communication channel.

p It is also referred to as communicationmedium or link.

p In network communication, thecommunication media are wires, cablesand other means through whichinformation travels from its source to itsdestination.



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Characteristics of a communication channel

Transmission speed (measured in bps) Band width (capacity of communication

channel, measure is Hertz or bps) Transmission mode

Asynchronous transmissionSynchronous transmission

Transmission direction

Simplex

Half duplex

Full-duplex

p



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Contp In communication channel, data is transmitted in the

form of signals (analog signal).p The data transmission is measured in bandwidth. The

bandwidth will be higher if more signals can betransmitted. Actually, the bandwidth measures the

amount of information that can be transmittedthrough the media within the given period of time.p For analog signals bandwidth is represented in hertz

(Hz). It means number of signals transmitted persecond. For digital signals, it is represented in bitsper second (bps). Different transmission media

have different bandwidth.p The higher the bandwidth of the transmission media,

the more information can be transmitted.



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Transmission media are

divided into:p Physical Transmission Median In Physical Transmission Media, communication

devices are directly linked with each other viacables or other physical materials for data

communication. These are also referred to aswired channels. The physical transmissionmedia are usually used in LAN.

p Wireless Transmission Media

n In wireless transmission media, communicationdevices communicate with each other anddata is communicated through the air orspace



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Physical Transmission Media

p Twisted-Pair Cable

p Coaxial Cable

p Fiber-Optic Cable

p



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Twisted-Pair Cable

p Twisted-pair cable is one of the mostcommonly used communication mediafor network cabling.

p It consists of two separate copper wiresthat are covered by insulating materialsuch as plastic and are twisted togetherto form a cable. One of the wires is used

to carry signals to the receiver, andother is used only as ground reference.



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Contp The wires are twisted

together to reducethe noise. The noiseis created due tohigh-voltage

equipment or due tolight energy of sun.

p The noise createsdisturbance in thesignal when it is

received. It meansthat noise createsproblem incommunication.



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Coaxial Cable

p Coaxial cable is also referred to as Coax. Itcarries signals of higher frequency rangesthan twisted-pair cable.

p Coaxial cable can be used for telephone linesfor voice and data transmission with veryhigh frequency. The bandwidth of coaxialcable is 80 times greater than twisted pair

media, Coaxial cable is also widely used inlocal area network (LAN). It is moreexpensive than twisted-pair wire.



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Cont

The structure of coaxial cableis shown in figure below

p Coaxial cable consists of a single solid copperwire, which is called the inner conductor.Usually, the inner conductor is surroundedby three layers:

n An insulting material, which covers theinner conductor.

n An outer conductor of solid metal foil(shield). The outer conductor (shield)

serves both as a shield against noiseand as a second conductor, whichcompletes the circuit.

n An insulting material, which covers theouter conductor.

p The whole cable is protected by an externalplastic cover.

p



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The main advantages of coaxialcable are:

p It creates less noise and protects the cable frominterference of external electromagnetic waves.Therefore coaxial cable has low transmissionerror rates.

p These cables can transmit data over longerdistances than that of twisted pair.

The data transmission speed of coax is from 10Mbps to 200 Mbps. Now-a-days, this transmissionmedium is not used in computer network because

other transmission media such as fiber-optic cableshave higher transmission speed.

p



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Fiber-Optic Cablep In twisted-pair cable and coaxial cable, data is

transmitted in the form of electric frequencies. Socopper wire has to be protected from water andelectromagnetic waves.

p In fiber optic cable, these types of problems are not

present. A fiber optic cable is made of glass orplastic and transmits signals in the form of light.p A fiber optic cable consists of tubes of glass (or thin

glass fibers) through which data is transmitted aspulses of light. A typical optical fiber consists of avery narrow strand or fiber of glass called the core

(It is as thin as a human hair). The core issurrounded by a concentric layer of glass calledCladding.



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ContThe data transmission in fiberoptic cable is shown below.p The diameter of a core

is 62.5 microns. (1micron l0-6meters). Thediameter of claddingis about 125microns.

p Optic-fiber cable uses

reflectionphenomena to guidelight through achannel



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Advantages and

DisadvantagesThe major advantages of fiber-optical media are:p The data transmission speed is very high because fiber-opticcable uses light to transmit data. The data transmissionspeed is up to billions bits per second.

p It has high level of security.p

It is not affected by electromagnetic waves.p It is more reliable and has lower data transmission errors.

Disadvantagesp The disadvantages of fiber optic cable are:p It is more costly.

p It is difficult to install and modify.p

p



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Wireless Transmission

Mediap This transmission medium is used when itis impossible to install the cables. Thedata can be transmitted all over the

world through this medium. Thecommonly used wireless transmissionmedia are discussed below.

n Broadcast Radio

n Cellular Radion Microwaves

n Communication Satellite



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Broadcast Radiop It is a wireless transmission medium that is used to

communicate information through radio signals inair, over long distance such as between cities andcountries.

p In this medium, a transmitter is required to sendmessages (signals) and receiver is required toreceive them. To receive the broadcast radiosignal, the receiver has an antenna that is locatedin the range of signal.

p Some networks use a special device called transceiver used tosend and to receive messages in the form of radio signals.The data transmission speed of broadcast radio is up to54Mbps.



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Cellular Radiop Cellular radio is a form of

broadcast radio that is usedfor mobile communicationssuch as cellular telephonesand wireless modems.

p A cellular telephone is a

telephone device that useshigh frequency radio waves totransmit voice and digitalmessages.

p Some mobile users connect theirlaptop computer or other

mobile device to a cellulartelephone to access the Web,send and receive e-mail etc.while away from a standardtelephone line.



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Microwavesp Microwaves are radio waves that provide a high speed

transmission. In Microwaves transmission, data istransmitted from one station to another.

p Microwave station contains an antenna, transceiver(transmitter & receiver) and other equipments that arerequired for microwave communication.

p Microwave uses the line-of-sight transmission, whichmeans that in microwave transmission system the datasignals travel in a straight path and cannot bend.Microwave stations or antennas are usually installed onthe high towers or buildings. Thus microwave stationsneed to be placed within 20 to 30 miles of each other.

p Each microwave station receives signals from the previousstations. In this way, data is transmitted from one placeto another.



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Contp The data transmission

speed of microwavetransmission is up to 150Mbps.

p Microwave transmission isused in environments

where installing physicaltransmission media isimpossible and whereline-of-sighttransmission is available.It is used in wide-openareas.

p Today, it is used bytelephone companies,cable televisionproviders, universitiesetc.



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Communication Satellitep A communication satellite is a space station

that receives microwave signals from earthmicrowave station (earth based station).

p

The earth based stations often are microwavestations. Other devices, such as PDAs andOPS receivers, also functions as earthbased stations. Transmitting a signal from

ground or earth station to a satellite stationin space is called up-linking and the reverseis called the down- linking.



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Contp Now-a-days, television & radio broadcast,

global positioning systems and Internetalso use the communication satellites.

p The communication satellite is launchedabout 22300 miles (or more) above theearth into space. The communication

satellite consists of solar powered,transceiver that receives and sendssignals.

p The signals are transmitted from one earthstation to the satellite. The satellitereceives and amplifies the signals and

sends them to another earth station. Thisentire process takes only a few seconds.To communicate information from onecountry to another country more than onesatellites are required.

p The data transmission speed of communication satellite isvery high such as up to 1 Gbps.



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Summaryp Technology has happened at a fast pace. Since modern

communications begun, early forms of media quicklybecome outdated and are replaced with newer moremodern methods.

p Each media is suited to different purposes and each hastheir place. In the early days of telephones, the

telephone companies wired each persons home with atwisted pair cable. What limitations do you think such aconnection now poses? In addition, as people becomemore and more involved in the technological revolution,what additional services might they want to access fromtheir home (one such example is the Internet) and how

do you think the existing telephone connection limits theprovision of those additional services?p Make a list of some additional services you think people

might be interested in and the benefits of having accessto such services from their home might bring



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Clarifications, additions,comments, questions?


Documents

Lecture 2 (Communication Channel)