dtm

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COMPUTER NETWORKS - I

( 10CS52 )

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Transmission Modesused for sending streams of data containing 0s and 1s from one device to another over wire or wires

Digital Transmission

multiple bits aresent with each

clock tick

one bit issent with each

clock tick

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Transmission Modes : Parallelbinary data is organized into groups of n bits each and sent at a timerequires use of n wires


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Transmission Modes : Parallel

advantage :speed … transfer speed can be increased by a factor of n over the serial method

disadvantage : cost of communication lines / wires

usually limited to short distances


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Transmission Modes : Serialone bit follows anotheronly one communication channel required


additional← →

requirement

costreduction

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Transmission Modes : SerialAsynchronous transmissionthe timing of a signal is not importantinformation is received and translated by agreed upon patternsby following this pattern, the receiver can retrieve the informationpatterns are based on grouping the bit stream into byteseach group, usually 8 bits, is sent along the link as a unit and handled independently


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Transmission Modes : SerialAsynchronous transmissionstart bit :usually a 0used to alert the receiver to the arrival of a new group of bits, usually 8 bits (a byte)

stop bits :usually a 11 or more additional bits are appended to the end of the byte (total, say 10 bits)a gap of varying duration may be inserted in between these 10 bytes


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Transmission Modes : SerialAsynchronous transmissionthe start & stop bits and the gap alert the receiver to the beginning and end of each byte

receiving device synchronizes at the onset of each new bytewhen the receiver detects a start bit, it sets a timer and begins counting bits as they come inafter n (say, 8) bits, the receiver looks for the stop bit


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Transmission Modes : SerialAsynchronous transmission


asynchronous at the byte levelbut, within each byte, the receiver must be synchronized with the incoming bit stream

advantage : easy to implement, inexpensive

disadvantage : slower, due to overheads

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Transmission Modes : SerialSynchronous transmissionbit stream is combined into longer “frames”, containing multiple bytestransmitted as unbroken stringsreceiver needs to separate the bit stream into bytes for decoding by counting bits as they arrive and groups them into 8-bit unitstiming is very important : accuracy of the received information is completely dependent on the ability of the receiver to keep an accurate count of bits as they are received


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Transmission Modes : SerialSynchronous transmission


advantage : speedissue : byte synchronization

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Transmission Modes : SerialIsochronous transmissionin real-time audio / video, uneven delays between frames is unacceptablesynchronization between characters is inadequatesynchronization of the entire stream is necessaryisochronous transmission ensures that the data arrive at a fixed rate


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transmission media are located below the physical layer and …..are directly controlled by the physical layer

Transmission Media

Transmission medium and Physical layer

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transmission medium :is defined as anything that can carry information from source to destinationis usually free space or metallic cable or fibre-optic cableinformation (in data communications) :defined as a signal that is the result of conversion of data from another formsignals are transmitted from one device to another in the form of electric energy, which is propagated thro’ transmission media

Transmission Media

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Taxonomy of transmission media

Transmission Media

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Guided media

• twisted - pair cable : electric current• coaxial cable : electric current• fibre - optic cable : light

signal travelling is directed and contained by the physical limits of the medium

Transmission Media

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Guided media : twisted - pair cable

Transmission Media

copperplastic

unshielded

UTP STPreduction in noise / cross talk;

but bulkier and more expensive

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Guided media : UTP - categories

Transmission Media

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Guided media : UTP - performance

Transmission Media

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Example UTP - performance BF4e7.6.11

The power at the beginning of a 1 Km 18 - gauge UTP link is 200 mw, …..

what is the power at the end of the link for frequencies 1 KHz, 10 KHz and 100 KHz ?

Transmission Media

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From the UTP cable performance chart,

@ 1 KHz, the attenuation is 3 dB



Transmission Media

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Power @ 1 KHz = 200 x 10-0.3 ≈ 100 mW

Power @ 10 KHz = 200 x 10-0.5 ≈ 63 mW

Power @ 100 KHz = 200 x 10-0.7 ≈ 40 mW

Transmission Media

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Guided media : UTP - applications• telephone lines for voice and data channels through local loop or last mile

• DSL lines for providing Internet access• LAN connectionsUTP connector : RJ - 45 (registered jack)

Transmission Media

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Guided media : Coaxial cablecapable of carrying higher frequency range signals compared to twisted pair cable

Transmission Media

acts as shield against noiseand the second conductor to complete the circuit

copper

protection

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Guided media : Coaxial cable - standardscategorised by radio government (RG) ratings : physical specifications :• wire gauge of inner conductor• thickness & type of inner insulator• construction of shield• size and type of outer casing

Transmission Media

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Guided media : Coaxial cable - connectors

Bayone-Neill-Concelman (BNC) connectors• connector

• T connector• terminator

Transmission Media

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Guided media : Coaxial cable - performance

Transmission Media

higher bandwidth than UTPhigher attenuation than UTP; requires repeaters

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Guided media : Coaxial cable - applications

used earlier (now replaced by OFCs) in : • analog telephone networks (10,000 voice channels in one cable)

• digital telephone networks (600 Mbps on one cable)

• cable TV networks (RG - 59)• traditional Ethernet LAN : thin and thick : RG - 58 : 10 Mbps with a range of 185 metres

RG - 11 : 10 Mbps with a range of 500 metres

Transmission Media

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Guided media : Fibre - Optic cablemade of glass or plastictransmits signals in the form of lightprinciple : bending of light ray, depending

upon angle of incidence and critical angle,a property of the substance of the medium

Transmission Media

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Guided media : Fibre - Optic cable

Transmission Media

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Guided media : Optical fibre

Transmission Media

(less dense than core)Glass or Plastic

Propagation Modes

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Guided media : Fibre - Optic cableMultimode fibre - step index

density of core remains constant from centre to edgesat the interface between core and cladding

there is an abrupt change in density →

angle of beam’s motion is altered →distortion of signal passing through fibre

Transmission Media

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Guided media : Fibre - Optic cableMultimode fibre - graded index

has a refractive index that decreases with increasing radial distance from the axis →light rays follow sinusoidal paths down the fibre →considerable decrease in distortion

Transmission Media

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Guided media : Fibre - Optic cableSingle-mode fibre - step index

source of light is highly focusedfibre diameter much smaller than multi-mode

substantially lower density / refractive index → critical angle close to 90

o

propagation of beams is almost horizontaldistortion is minimum

Transmission Media

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Guided media : Fibre - Optic cableTypes / Sizes :

Transmission Media

Applications :back-bone networks, SONET, LANs can work at >1600 Gbps

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Guided media : Fibre - Optic cableComposition :

Transmission Media

PVC or Teflon

cushion

strands

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Guided media : Fibre - Optic cableConnectors :

Transmission Media

SC → subscriber channel;used for cable TV

ST → straight - tip;used in networks;more reliable than SC

same size as RJ - 45

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Guided media : Optical fibreadvantages over metallic cable :• higher bandwidth• less signal attenuation• immunity to EMI (electro - magnetic interference)

• resistance to corrosiondisadvantages :

• installation and maintenance• unidirectional light propagation - two fibres are required for bi - directional

• higher cost

Transmission Media

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Unguided media : Wireless• does not use physical conductor• known as wireless communication• signals are broadcast through free space• can be received by anyone who has a suitable device

electromagnetic spectrum :

Transmission Media

3 KHz to 900 THz

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Unguided media : WirelessMethods of propagation of signals

Transmission Media

radio waves travel

following the curvature of / hugging the earth

range depends on Tx power

radio waves radiate

upwards into the ionosphere where they are reflected back to the earth

longer range

radio waves travel in

straight lines - from one directional

antenna to the other

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Unguided media : WirelessFrequency bands : regulated by govt.

Transmission Media

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Unguided media : Wireless - transmission waves

Transmission Media

≈ 3 KHz to 1 GHz ≈ 1 GHz to 300 GHz ≈ 300 GHz to 400 THzλ ≈ 1 mm to 770 nm

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Unguided media : WirelessRadio Waves• spectrum regulated in India by • Wireless Planning & Coordination (WPC)• radio waves are mostly omni - directional

• susceptible to interference by other antennas using the same frequency / band

• can travel long distances - good for AM radio• can penetrate walls - good for indoor use also

• band and sub-bands are narrow →low data rate for digital communications

Transmission Media

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Unguided media : WirelessRadio Wavesuse omni - directional antennas

Transmission Media

Applications :multicasting →• AM / FM radio• television• maritime radio• cordless phones• pagers

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Unguided media : WirelessMicrowaves• use of certain portions of the band is regulated

• unidirectional• propagation is line - of - sight → sending

and receiving antennas need to be aligned• towers need to be tall• repeaters may be required • interference can be minimized• VHF cannot penetrate walls• wide band → sub-bands can be wide →high data rate is possible

Transmission Media

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Unguided media : WirelessMicrowavesuse unidirectional antennas

Transmission Media

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Unguided media : Wireless

Microwaves : applications

• used in one-to-one (unicast) communication between sender and receiver

• used in cellular phones, satellite networks and wireless LANs

Transmission Media

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Unguided media : WirelessInfrared• used for short - range communications• cannot penetrate walls - no interference• not suitable for long-range communications• cannot be used outdoors

Applications :• high data rate communications• communication between PC, keyboard, mouse, printer etc.

• IrDA port can support upto 4 Mbps

Transmission Media

dtm

Documents

additional bits

groups of n bits

new group of bits

accurate count of bits

clock tick multiple

new byte

serial method disadvantage

stream advantage