transmission media and data transmission

8/8/2019 Transmission Media and Data Transmission

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Transmission Media andData Transmission

2G1316 DataCommunications and

Computer Networks2E1623 Data Links and

Local Area Networks


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Transmission and Physical Infrastructure

Transmission Media

Attenuation and link budget

Signal distortion

Capacity limitations

Modulation and line coding

Synchronization and framing

Multiplexing

Capacity requirements ExamplesTDM, ADSL, SDH


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Transmission Media

Guided media

Electrical

o Twisted pair cable

o Coaxial cable

Optical

o Single-mode and multimode

Unguided media

Electromagnetic waves in air

o Radio

o Microwaves (terrestrial and

satellite)

T

R

Transmitter ReceiverAmplifier,Signal regenerator

Wave guideWave guide


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Attenuation

No link is perfect Attenuation

Power loss between sender and receiver

Relationship between incoming and outgoing power

Measured in decibel [dB]

o Example:

o Pin = 120 mW

o Pout= 30 mW

o Attenuation = 10 log10 4 6 dB

10 log10 Pin/Pout


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Power and Sensitivity

Measured in decibel wattdBW ordecibel milliwattdBm

PdbW= 10 log10 P

PdBm= 10 log10 P/110-3

For example, transmitter output powerand receiver input sensitivity

Note: absolute power measures!


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Transmission QualityDistortion

Signal changes form or shape

Each frequency component has itsown speed through the medium


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Transmission QualityNoise

Undesired signal added tothe transmitted signal

Thermal noise

o Random motion of

electronso Independent of frequency

(white noise) andamplitude

o Added to the signal

Signal-to-noise ratio, SNR

S/N, where S is signalpower, N is noise power


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Transmission QualityCounter Measures

Amplification

Compensates for attenuation and other losses

Adds noise

Regeneration (for digital signals)

Recreates the shape of the signal

Noise filters

Can attenuate the signal Protection against disturbances and crosstalk (verhrning)

For example shielding against electromagnetic fields

Protection against distortion

Equalizers

Dispersion compensation


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Bit Rate and Baud Rate

Link capacity: number of bits per second (bit rate)

Baud rate: number of signal elements per second

L = 2C = R

L = 4C = 2R

C = R log2L

C: capacity

L: number of levels

R: baud rate


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Nyquist Bit Rate

Harry Nyquist (1889 1976)

Also Nyquists/Hartleys Law

Nyquist bit rate Cmax is themaxiumum bit rate on an idealchannel

So maximum baud rate is 2B

Cmax = 2B log2L


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Channel CapacityShannons Formula

Claude Shannon (1916 2001)

Father of information theory

Highest possible bit rate in a channel withwhite noise

B is channel bandwidth S/N is signal to noise ratio

C = B log2 (1 + S/N)


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Shannons Formula

Example B = 3100 Hz

S/N = 20 dB = 100 times C = 3100 log2(1 + 100) = 20.6 kb/s

Telephone line B: 3100-3500 Hz S/N: 33-39 dB C 33-45 kb/s (What about ADSL and 56K modems?)


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Bandwidth for Different Media


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Guided Media

Wires, cables

Twisted pair cables

Coaxial cables

Optical fibers


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Twisted Pair Cable

Separately insulated

Pair of cables twisted together

Even out external disturbances

Receiver operates on signal differences


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Twisted Pair Cable

Several pairs bundled together Often with RJ-45 connector Often installed in building when built Shielded (STP) and unshielded (UTP)

Shielding protects from noise and crosstalk Bulkier and more expensive


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600 MHz

200 MHz

100 MHz

20 MHz

16 MHz

< 2

MHz

very low

Bandwidth

LANsDigital600 Mbps7 (draft)

LANsDigital200 Mbps6 (draft)

LANsDigital100 Mbps5

LANsDigital20 Mbps4

LANsDigital10 Mbps3

T-1 linesAnalog/digital2 Mbps2

TelephoneAnalog< 100 kbps1

UseDigital/AnalogData RateCategory

Cathegories of Unshielded Twisted Pair


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10Base-

T100

16 Mb/s16 MHzCAT 3

4 Mb/s

< 0.1 Mb/s

Bit rate

TelephoneDoorbell

Very lowCAT 1

T-1TokenRing


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Coaxial Cable

Solid inner connector Outer connector is braid or metal foil Separated by insulating material Higher bandwidth than twisted pair

But also higher attenuation


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Optical Fibre

Core of glass or plastic

Cladding with lower index of refraction

Light Emitting Diode (LED) or laser


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Fiber Transmission Modes


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Loss vs Wavelength

Wavelength = c/f

c is propagation speed,f is frequency


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Fiber Advantages and Disadvantages

Advantages

Very high capacity

Low attenuation

Low crosstalk: no interference between photons

Not sensitive to electromagnetic noise

Light weight

Disadvantages

Installation/maintenance

Unidirectional


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Propagation Methods for Unguided Signals


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Radio Waves

Radio, television,etc

Up to 1 GHz

Ground and skypropagation

Omnidirectional

antennas


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Microwaves

1-300 GHz

Cellular phones, satellite networks, wireless LANs

Line of sight propagation

Unidirectional antennas


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Infrared

300 GHz 400 THz

Line-of-sight propagation

Closed areas

Interference from sun rays

Short distances


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Analog and Digital Signals


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Analog and Digital Signals

Low-pass channel

Digital transmission

Dedicated mediumo One channel

Line coding(Basbandsmodulering)

Band-pass channel

Analog transmission

Multiple channels in thesame medium

Brvgsmodulering


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Line Coding

Turn binary data intodigital signal

Dedicated medium Full spectrum

Fourier transformation

of square wave isinfinite serie


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Unipolar Encoding

One signal level (and zero)

Contains DC component Distorted (blocked) in some components Extra energy

Lack of synchronization Long sequences of all ones or all zeros may cause receiver to loose synchronization


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Nonreturn to Zero (NRZ)

Polar signal (two levels)

NRZ-level (NRZ-L) and NRZ-invert (NRZ-I)

Average signal level reduced

Synchronization could still a problem


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Return to Zero (RZ) Encoding

Synchronization even for long strings of 1s or 0s

Two signal-changes per bitmore bandwidth

Differential RZ


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Manchester Encoding

Two signal levels

Higher pulse rate requires larger bandwidth


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Differential Manchester Encoding

Need only detect transition or no transition


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Block Coding

Bit stream is divided into m-bit groups

Groups are encoded as n-bit codes 4B/5B: 5-bit codes represent 4-bit groups 8B/10B: 10-bit codes represent 8-bit groups

01100110

00100010

11011101

01110111 4B/5B4B/5B 1101011010 0101001010 1001010010 0010100101


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Substitution in 4B/5B Block Coding

Chose codes in such a way that synchronization is ensured

In 4B/5B, there can never be more than three consecutive 0s

Error detection Control information Disadvantage: higher bandwidth


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4B/5B Control Characters

1000110001K (start delimiter)

0110101101T (end delimiter)

1100111001S (Set)

0011100111R (Reset)

1100011000J (start delimiter)

0010000100

1111111111

0000000000

Code

H (Halt)

I (Idle)

Q (Quiet)

Data


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8B/6T Encoding

Substitute an 8-bit group with a 6-symbolcode

Ternary symbols Limited bandwidth


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Transmission of Analog Signal


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Sampling

Coding of analog signals For example voice and video

Analog signal is measured at equal intervals Sampling Quantization in time PAMpulse amplitude modulation


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Sampling RateThe Nyquist Theorem

The sampling rate must be at least twice the highest frequency in the

analog signal This frequency is often called the Nyquist frequency, or Nyquist rate

Theoretically, no information is gained by sampling at a higher rate


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Aliasing

Sampling below the Nyquist rate

(undersampling) distorts the spectrum Vikningsdistortion

AmplitudeOriginal signal

Regenerated signal


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Amplitude Quantization

From analog to digital data

Binary coding

P l C d M d l i (PCM)


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Pulse Code Modulation (PCM)

Signal levels are represented by a fixednumber of bits

8 bit values: -127 to 127

Quantization noise introduced by roundingerrors

PCM E l


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PCM Examples

16, 20, 24

16

8

Sample size

(bits)

648Telephony

9600(max)

44.1, 48,88.2, 96,

176.4, 192

DVD Audio

705.644.1CD

Bit rate

(kb/s)

Sample rate

(kHz)

F A l t Di it l


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From Analog to Digital

M d l ti


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Modulation

Sine wave fully described by amplitude A,frequency fand phase

s(t) = A sin(2ft + )

Vary one (or more) to represent symbols

Amplit de Shift Ke ing (ASK)


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Amplitude Shift Keying (ASK)

Signal level is varied to represent symbols Amplitude sensitive to noise

Frequency Shift Keying (FSK)


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Frequency Shift Keying (FSK)

Signal frequency is varied to represent symbols Bandwidth limitations

Phase Shift Keying (PSK)


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Phase Shift Keying (PSK)

Signal phase is varied to represent symbols

Limited by receivers ability to detect phasechanges

PSK Constellation Diagram


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PSK Constellation Diagram

Phase-state diagram

4 PSK (Q PSK)


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4-PSK (Q-PSK)

Four different phases Each phase represents two bits

8-PSK Constellation Diagram


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8-PSK Constellation Diagram

Qadrature Amplitude Modulation (QAM)


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Qadrature Amplitude Modulation (QAM)

Combination of ASK and PSK Allows for more combinationsmore bits per baud

Maximum contrast between signal units

4-QAM and 8-QAM Constellations


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4 QAM and 8 QAM Constellations

Bit and Baud Rates


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Bit and Baud Rates

8N

7N

6N

5N

4N

3N

2N

N

Bit

Rate

N5Pentabit3232--QAMQAM

N6Hexabit6464--QAMQAM

N7Septabit128128--QAMQAM

N8Octabit256256--QAMQAM

N4Quadbit1616--QAMQAM

Tribit

Dibit

Bit

UnitsUnits

N388--PSK, 8PSK, 8--QAMQAM

N244--PSK, 4PSK, 4--QAMQAM

N1ASK, FSK, 2ASK, FSK, 2--PSKPSK

Baud rateBaud rateBits/BaudBits/BaudModulationModulation

Data Transmission Over Telephone Lines


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Data Transmission Over Telephone Lines

Modems


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Modems

Modulator/demodulator

V-series Modem Standards


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se es ode Sta da ds

V.32 9600 b/s: 32-QAM, baud rate 2400, 4+1 data bits/baud (trellis-coded)

V.32 bis

14400 b/s: 128-QAM, 6+1 data bits/baud

V.34 bis 28800 33600 b/s: 960 to 1664 points constellations

56K modems (PCM modems)


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( )

Quantization noise fromPCM sampling

ISPs have digital connection(no modem)

V.90 and V.92 standards

Asymmetric rate

56/33.6 Kb/s (V.90) 8000 samples per second

7 bits of data per sample

o 1 bit for control

Data Transmission Modes


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TransmissionTransmission

ParallelParallel SerialSerial

AsynchronousAsynchronous SynchronousSynchronous

Parallel Transmission


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High capacity But costly, if it requires multiple cables

Serial Transmission


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Need for synchronization at bit level

External clock, such as GPS

Separate link for clock signal

Line coding with embedded clock

o Manchester coding, for example

Receiver resynchronization

Asynchronous Transmission


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Asynchronous at the word (byte) level Start and stop bits mark the beginning and end of a byte

(Loose) synchronization at bit level

Receiver is resynchronized when start bit is detected

Clock frequencies sufficiently close to keep synchronization for the duration of a byte

Often combined with parity bit for error control (e.g. RS-232) Keyboard, serial port, etc

Synchronous Transmission


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Continuous stream of bits No extra bits or extra space between bytes

Special idle patterns to indicate absense of data

Bit stream can be divided into larger data units (frames)

Responsibility of the data link layer


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Multiplexing

Multiplexing


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Subdivision of a link into multiple channels

Multiple sender/receiver pairs can share the link

Resource sharing

Bandwidth divided into frequency channels Transmission time divided into time slots

Multiplexing


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Analog multiplexing

Frequency division multiplexing (FDM)

o Multiple frequency channels

o Band pass modulation

o TV and radio broadcast

Wavelength division multiplexing (WDM)

o Similar to FDM but for optical transmission

Digital multiplexing

Time division multiplexing (TDM)

o Access according to time slots

Synchronous TDM

o Statistical TDM

Time Division Multiplexing for Telephony


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Carries PCM voice channels T1 (North America, Japan)

o 24 channels, 1.544 Mb/s E1

o 30 channels, 2.048 Mb/s

Synchronous Time Division Multiplexing


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Access according to time slots

Time slots grouped into frames

Ifn is the number of inputs, the output link needs to be n times faster

than each input link

Frame duration is the same as the duration of a data unit on the input

Hierarchical Multiplexing


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E Line Rates


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19201920139.264139.264EE--44

34.36834.368

8.4488.448

2.0482.048

Rate

(Mbps)

480480EE--33

120120EE--22

3030EE--11

Voice

ChannelsE Line

Example: SDH/SONET


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ANSI: Synchronous Optical NETwork (SONET)

ITU-T: Synchronous Digital Hierarchy (SDH)

TDM system

Synchronous network

o A single, common clock allows channel multiplexing

Fiber-optic transmission system

Can carry tributaries

DS-0, DS-1, E1

SONET/SDH Equipment


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STS: Synchronous Transport Signal

Frame Format


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Organized as a matrix with 9 90 octets Three columns of administration overhead

Payload is called Synchronous Payload Envelope (SPE)

SONET/SDH Rates


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38486.01639813.120STM-256STS-768OC-768

9621.6049953.28STM-64STS-192OC-192

2405.3762488.32STM-16STS-48OC-48

601.344622.08STM-4STS-12OC-12

150.336155.52STM-1STS-3OC-3

50.11251.84-STS-1OC-1

Payloadrate(Mb/s)

Line rate(Mb/s)

SDHequivalent

Electricallevel

Opticallevel

Virtual Tributaries


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Carry lower rate data Partial payload

VT1.5 for DS-1 service (1.544 Mb/s), VT2for E1 service (2.048 Mb/s), etc

Example: Digital Subscriber Link (DSL)


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High-speed Digital Access to Internet

Exploit the actual bandwidth available

in twisted pair cables in local loop(subscriber access lines)

Up to 1.1 MHz

Subject to strict physical limitations

o Cable distance

o Size of cable

o Signalling

Asymmetrical DSL (ADSL)


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Adaptive Bandwidth and data rate depends on conditions

Lower rate in upstream direction (from subscriber)

For residential access

Upstream 64 kb/s to 1 Mb/s, Downstream 500 kb/s to 8 Mb/s

Bandwidth (typically) divided into 4 kHz channels

Discrete Multitone Technique (DMT)


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Combination of QAM and FDM

4 kHz channels and 15 bits/baud 60 kb/schannels

ADSL Modems and DSLAMs


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Digital Subscriber Line Access Multiplexer

ADSL2/ADSL2+


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ADSL2

Improved rate and reach

o Improvements in modulation, framing, coding, signal processing, etc.

o About 12 Mb/s downstream and 1 Mb/s upstream

o Slightly increased reach (+200 m) Higher rates on long lines

Higher capacity by bonding two or more phone lines

Channelized voice

o 64 kb/s DS-0 channels for TDM voice traffic

All-digital mode

o Use voice channel for data

ADSL2+

2.2 Mhz bandwidth

Up to 26 Mb/s downstream and 1.5 Mb/s upstream

Other DSL Technologies


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Symmetric DSL (SDSL)

Equally divided bandwidth

High-bit-rate DSL (HDSL)

Alternative to T1 access Up to 2 Mb/s

2B1Q encoding (four levels, two bits per baud)

Two twisted-pair wires for full-duplex

Very-high-bit-rate DSL (VDSL)

Similar to ADSL

DMT with up to 50-55 Mb/s downstream, 1.5-2.5 upstream

Short distances (300 to 1800 m) Fiber, coaxial cable, twisted-pair cable

Summary


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Transmission media Link budget

Capacity limitations

Transmission of digital information

Line coding

Digital modulation

Transmission of analog information

Conversion to digital signals

Sampling

Synchronization

Multiplexing

Examples

Modems

SDH/SONET

ADSL

Reading Instructions


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Behrouz A. Forouzan, Data Communications and Networking, third

edition

3 Signalso 3.4 Analog versus Digital

o 3.5 Data Rate Limitso 3.6 Transmission Impairment

4 Digital Transmissiono 4.1 Line Codingo 4.2 Block Codingo 4.4 Transmission mode

5 Analog Transmissiono 5.1 Modulation of Digital Datao 5.2 Telephone Modems

6 Multiplexing 7 Transmission Media

9 High-Speed Digital Accesso 9.1 DSL Technologyo 9.3 SONET

transmission media and data transmission

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