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31.10.2012 12:55 1
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
Digital Transmission
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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
Digital Transmission
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Transmission Modes : Serialone bit follows anotheronly one communication channel required
Digital Transmission
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
Digital Transmission
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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
Digital Transmission
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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
Digital Transmission
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Transmission Modes : SerialAsynchronous transmission
Digital 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
Digital Transmission
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Transmission Modes : SerialSynchronous transmission
Digital 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
Digital Transmission
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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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Example UTP - performance BF4e7.6.11
From the UTP cable performance chart,
@ 1 KHz, the attenuation is 3 dB
@ 10 KHz, the attenuation is 5 dB
@ 100 KHz, the attenuation is 7 dB
Transmission Media
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Example UTP - performance BF4e7.6.11
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