Ch 2(Data Communication Principles)

8/6/2019 Ch 2(Data Communication Principles)

1/25

Data Communication and Computer Networks

Chapter 2: Data Communication Principles

Er. Nabendra Shrestha College of Applied Business

BIM II Semester

The successful transmission of data depends principally on two factors: the quality of the signal being

transmitted and the characteristics of the transmission medium.

Data transmission occurs between transmitter and receiver over some transmission medium.

Transmission media may be classified as guided or unguided. In both cases, communication is in the

form of electromagnetic waves. With guided media, the waves are guided along a physical path;

examples of guided media are twisted pair, coaxial cable, and optical fiber. Unguided media, also called

wireless, provide a means for transmitting electromagnetic waves but do not guide them; examples are

propagation through air, vacuum, and seawater.

TransmissionTerminology:

simplex

one direction

Signals are transmitted in only one direction; one station is transmitter and the other isreceiver.

e.g. television

half duplex

either direction, but only one way at a time

both stations may transmit, but only one at a time

e.g. police radio (Walky-talky)

full duplex

both directions at the same time

both stations may transmit simultaneously, and the medium is carrying signals in both

directions at the same time e.g. telephone

Signal: A signal can be defined as a function of one or more variables in time or frequency, which

conveys information on the nature generally about the state or behavior of a physical phenomenon. The

signal is the response of a system. E.g. Speech signal as a function of time

The signal is a function of time, but it can also be expressed as a function of frequency; that is, the signal

consists of components of different frequencies. It turns out that the frequency domain view of a signal

is more important to an understanding of data transmission than a time domain view.


2/25




BIM II Semester

Time domain concepts

Analog signal

the signal intensity varies in a smooth fashion over time

Continuous time signals

Examples like (naturally occurring) music and voice

Digital signal

the signal intensity maintains a constant level for some period of time and then

abruptly changes to another constant level

Discrete time signals that are in the form of either 1 or 0

Common Format

High immunity to interference

Increased functional bandwidth

Easier and efficient to multiplex several digital signals

Storage relatively easier and inexpensive

Increased system complexity

Periodic signal

pattern repeated over time (Above Digital signal is periodic)

Aperiodic signal

pattern not repeated over time (Above analog signal is Aperiodic)


3/25




BIM II Semester

The sine wave is the fundamental periodic signal. The general sine wave can be written as:

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

A general sine wave can be represented by three parameters:

Peak amplitude (A)

the maximum value or strength of the signal over time; typically measured in volts

is the height of the wave above or below a given reference point

Frequency (f)

The rate [in cycles per second, or Hertz (Hz)] at which the signal repeats. An equivalent

parameter is the period (T) of a signal, so T= 1/f


4/25




BIM II Semester

phase (J)

measure of relative position in time within a single period of a signal

A change in phase can be any number of angles between 0 and 360 degrees

Frequency Domain Concepts:

In practice, an electromagnetic signal will be made up of many frequencies. It can be shown, using a

discipline known as Fourier analysis, that any signal is made up of components at various frequencies, in

which each component is a sinusoid. By adding together enough sinusoidal signals, each with the

appropriate amplitude, frequency, and phase, any electromagnetic signal can be constructed.

Addition of Frequency Components (T= 1/f) in next page

Fig a. fty T2sin! having f frequency

Fig b. tfy )3(2sin)3/1(1 T! having 3f frequency

Fig c. y2 = y + y1

= ])3(2sin)3/1(2)[sin/4( tfft TTT

Fig c. gives the sum of two frequencies f and 3f and the combination of these sine waves gives

Distorted Square wave


5/25




BIM II Semester

Frequency domain representation of Fig c. signal is shown below:


6/25




BIM II Semester

Spectrum: The range of frequencies that a signal spans from minimum to maximum. In previous page, it

extends from f to 3f.

Bandwidth The absolute value of the difference between the lowest and highest frequencies of a

signal or width of a spectrum. For fig. c, Bandwidth is 3f f = 2f

If a signal includes a component ofzero frequency, it is a directcurrent (dc)orconstantcomponent.

Relationship between Data Rate and Bandwidth: any transmission system has a limited band of frequencies

this limits the data rate that can be carried

A square wave has an infinite number of frequency components and hence an infinite

bandwidth but most energy in first few components

For any given medium, the greater the bandwidth transmitted, the greater the cost. The more

limited the bandwidth, the greater the distortion, and the greater the potential for error by the

receiver

If a data rate of a digital signal is R bps, then the good representation of a signal can be achieved

with a bandwidth of 2R Hz.

a direct relationship between data rate and bandwidth: the higher the data rate of a signal, thegreater is its required effective bandwidth

Analog and Digital Data Transmission:

Analog Data:

1. Audio Signals: (Human Speech)

freq range 20Hz-20kHz (speech 100Hz-7kHz)

easily converted into electromagnetic signals

Varying volume converted to varying voltage. The telephone handset contains a simple

mechanism for making such a conversion. can limit frequency range for voice channel to 300-3400Hz

2. Video Signals: (produced by Video Camera)

Frequency range 0 to 6 MHz

DC component shows average brightness that is used for background image

Low frequency components show outer borders

High frequency components show fine details

0 0In time domain (t)

y(t)

5V

In frequency domain (f)

y(f)

5V


7/25




BIM II Semester

Digital Data:

as generated by terminals, computers, and other data processing equipment and then

converted into digital voltage pulses for transmission

uses two constant (dc) voltage levels, one level for binary 1 and one level for binary 0

bandwidth depends on data rate (The greater the bandwidth of the signal, the more

faithfully it gets a digital pulse stream at the receiver)

In a communications system, data are propagated from one point to another by means of

electromagnetic signals. Both analog and digital signals may be transmitted on suitable transmission

media.

In Analog transmission:

the signals that are in either analog or digital form are propagated by means of continuously

varying electromagnetic wave i.e. analog signal over a variety of media, depending on

spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber optic cable;

and unguided media, such as atmosphere or space propagation.

doesnt concerned with the content of the signal

for a long distance transmission, amplifiers are used to boost the energy in signal but it also

boost the noise components that distorts the desired signal

Analog Voice signals are converted into analog electromagnetic signals by telephone.

Digital data are converted into analog using a modem (modulator/demodulator) by modulating the

digital data on some carrier frequency

In digital transmission:

Sequence of voltage pulses i.e. Digital signal is transmitted over a wire medium;

Both analog signals and digital data from source are converted into digital form. Analog data can converted to digital using a codec (coder-decoder), which takes an analog signal

that directly represents the voice data and approximates that signal by a bit stream.

Digital data can be directly represented by digital signals.

A digital signal can be transmitted only a limited distance before attenuation, noise, and other

impairments endanger the integrity of the data. To achieve greater distances, repeaters are

used. A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a

new signal. Thus the attenuation is overcome.


8/25




BIM II Semester

Digital Transmission over Analog Transmission

Digital technology

o Continuous drop in cost and size of digital circuits in VLSI , ULSI form as compared to

Analog equipments

Data Integrity

o Digital signals are less susceptible to noise and repeaters are used instead of amplifiers,

so maintains data integrity

Capacity Utilization

o Easier to multiplex several digital signals

Security and Privacy

o Encryption technique can be used

Integration

o All signals in different format (audio, video or text) are integrated into a common format

of 1 and 0.

Transmission Impairments:

In any communication system, the signal that is transmitted from the transmitter is different to the

signal that is received by the receiver through a medium. This is because of Transmission Impairments.

For Analog signals, these impairments can degrade the signal quality.

For Digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or

vice versa.

Most significant impairments are

Attenuation and

Attenuation distortion

For any long distance transmission medium, the strength of a signal falls off with

increase in distance

Three considerations for receiver to detect signal with less attenuation

The signal must have sufficient strength so that the electronic circuitry in the

receiver to detect the signal


9/25




BIM II Semester

The signal power must maintain a level sufficiently higher than noise to be

received without error.

Third, attenuation varies increasingly with frequency.

The first and second problems are dealt with by increasing signal strength and the use of

amplifiers or repeaters.

The third problem is particularly noticeable for analog signals. To overcome this

problem, equalizing attenuation techniques are available across a band of frequencies

and to use amplifiers that amplify high frequencies more than lower frequencies.

For digital signals, the content of the signal is concentrated near fundamental

frequency, so can be detected at the receiver

Distortion (delay)

Certain change or scaling of amplitude and phase of different frequency components of

input signals is called distortion

Linear Distortion- no new frequency components are produced at the output

Non-Linear Distortion- new frequency components are produced at the output

Delay distortion only occurs in guided media

As the velocity of propagation of a signal through a guided medium varies with

frequency, for a band limited signal, the velocity tends to be highest near the center

frequency and fall off toward the two edges of the band. Thus various frequency

components of a signal will arrive at the receiver at different times, resulting in phase

shifts between the different frequencies. This effect is called delay distortion

Delay distortion is particularly critical for digital data, because some of the signal

components of one bit position will spill over into other bit positions, causing

Intersymbol Interference (ISI). This is a major limitation to maximum bit rate over a

transmission channel. ISI is produced within the system not from the external source

Equalizing technique can be used to eliminate delay distortion

Noise:

Unwanted signal that adds up with the message signal (desired signal) and degrade the

signal

Major factor to degrade the performance of communication system

Noise can be further divided as:

Thermal Noise:

Due to random motion of charge carriers in electronic devices

Is uniformly distributed across the frequency spectrum, so referred to as

white noise

Is a function of temperature

In any device or conductor, The amount of thermal noise:

N = kTB (Watt)

k = Boltzmanns Constant = 1.3803 x 10-23

J/0K T = Temperature (

0K ) and B = Bandwidth (Hz)


10/25




BIM II Semester

Noise Power Density (Watt/Hz) : for 1 Hz Bandwidth is

N0 = Kt

Do: For B = 1Hz, T = 170C i.e. (17+273)

0K

In Watt/Hz, N0 = kT

In dBW/Hz, N0 = 10 Log(kT)

Intermodulation Noise:

When signals at different frequencies share the common transmission

medium, noise signals are produced at a frequency i.e. either the sum

or difference of those frequencies or multiple of those frequencies and

the result is Intermodulation Noise.

Produced because of non linearity in Transmitter and receiver

(component malfunction) and in transmission system by using excessive

signal strength

Crosstalk:

unwanted coupling between signal paths

the phenomenon in which signal transmitted on one circuit or a channel

of transmission system creates an undesirable effect in other circuit or

channel

E.g.

Electrical coupling between nearby twisted pairs

Receiving of unwanted signals by microwave antennas

Cross conversation in telephone

Impulse noise:

Non-continuous

Sudden scaling of amplitude i.e. noise spikes for short duration and of

relatively high amplitude.

Mainly generated due to external electromagnetic disturbances, such as

lightning, and faults and flaws in the communications system

the primary source of error in digital data communication

Channel Capacity:

The maximum rate at which data can be transmitted over a communication medium is called

Channel Capacity. It depends upon data rate, bandwidth, noise and error rate. For a reliable

communication system design, for limited bandwidth, data rate should be high and error rate should be

low.


11/25




BIM II Semester

Nyquist Bandwidth:

For a noise free channel, data rate is proportional to the bandwidth of the signal.

Nyquist states that if the rate of signal transmission is 2B, then a signal with frequencies no

greater than B is sufficient to carry the signal rate.

Conversely given a bandwidth ofB, the highest signal rate that can be carried is 2B.

i.e. C = 2B C in bps and B in Hz

E.g. for Voice Channel, B = 4000 Hz, so C = 8000 bps

With multilevel signaling,

the Nyquist formulation becomes: C= 2B log2M,

where M is the number of discrete signal or voltage levels.

So, for a given bandwidth, the data rate can be increased by increasing the number of

different signal levels at cost of receiver complexity and limited by noise & other

impairments

Shannon Capacity Formula:

For a channel with additive Gaussian white noise, the relationship between channel capacity, channel

bandwidth and the received signal to noise ratio is given by

Capacity C=B Log2(1+SNR)

C = Channel Capacity in bps

B = Bandwidth in Hz

SNR = Signal to Noise Ratio (Ratio of signal

power to Noise power) = PS/PN In dB, 10 Log10(PS/PN)

SNR detects the output quality of signal

Implication of this theorem is

a)

designer can estimate C for required SNR and B for reliable communicationb) for limited channel capacity, designer can trade off between B and SNR: for limited

Bandwidth, SNR can be increase by increasing signal power or if SNR is less, then by

increasing bandwidth, desired channel capacity can be met

For SNR tends to Infinity, Channel capacity becomes infinity. So, this type of channel is referred to as

ideal channel.

With Bandwidth increased, noise power also increased, so SNR decreases.

For voice channel, B= 4KHz, SNR = 104

then Using above formula, C = 4000 Log2(1+104)

= 4000 (Log(1+104) / Log 2)

= 53.15 Kbps

Let Spectrum of Channel is between 3 MHz to 4 MHz, SNR = 24 dB, C = ?

B = 4 3 = 1MHz and SNR(dB) = 10 Log SNR

So, 24 = 10 Log SNR ------ Log SNR =2.4 ------ SNR = 102.4

So, C = 1000000 x (Log (1+102.4

)/ Log 2) = 7.97 Mbps

We can also find signalling levels M, using Nyquist theorem, C = 2B Log2M

Log2M = C/2B = 7.97/2 ----- M = 2(7.97/2) = 15.83 =16 Levels


12/25




BIM II Semester

Do: Channel Capacity = 10000 bps, B = 3000 Hz, SNR =?

And if C = 10000 bps, B = 10000 Hz , SNR = ? Ans: (9 and 1)

Find in dB as well (9.5 dB and 0 dB)

Expression (Eb/No) is used to determine digital data rates and error rates

= Signal Energy per bit / Noise Power Density (Power = Energy / time)

= Signal Power (Ps) x Tb / kT 1/Tb = data rate (R)

= Ps / kTR

Synchronous and Asynchronous Transmission:

The transmission of a stream of bits from one device to another across a transmission link

involves a great deal of synchronization.

The receiver must know the rate at which bits are being received so that it can sample the line

at appropriate intervals and to determine the value of each received bit.

Two techniques are usedasynchronous and synchronous transmission

Asynchronous Transmission:

Each character of data is treated independently.

avoids the timing problem by not sending long, uninterrupted streams of bits

data are transmitted one character (5 to 8 bits) at a time

The bits of the character are transmitted beginning with the least significant bit

Each character begins with a start bit with a value of binary 0 that alerts the receiver that a

character is arriving.

data bits are usually followed by a parity bit (even, odd or unused)

The final element is a stopelement, which is a binary 1 usually 1 to 2 times the duration of an

ordinary bit.

The receiver samples each bit in the character and then looks for the beginning of the next

character.

When no character is being transmitted, the line between transmitter and receiver is in an idle

state (binary 1 level) same as stopping element and the transmitter will continue to transmit the

stop element until it is ready to send the next character.

Time interval between characters cant be predicted

Asynchronous transmission is simple and cheap

But requires an overhead of two to three bits per character.

Character with start and stop element is 1 frame and when receiver is faster or slower than

transmitter, sampling will be displaced and data incorrectly received or bit out of alignment, theerror is called framing error.

For larger blocks of data, the clock synchronization between transmitter and receiver will

eventually drift out, so framing error can be more severe for large blocks of data. Also more

unnecessary overheads are required. So, to achieve greater efficiency, a different form of

synchronization, known as synchronous transmission, is used.

Figure is for NRZ L signaling :


13/25




BIM II Semester

Synchronous Transmission:

Block of data (containing many bits) is formatted as a frame that includes a starting and an

ending flag, and is transmitted in a steady stream without start and stop codes.

To prevent timing drift between transmitter and receiver, their clocks must be synchronized.

o By providing a separate clock line between transmitter and receiver.

o By embedding the clocking information in the data signal.

For digital signals, this can be accomplished with Manchester or differential

Manchester encoding.

For analog signals, the carrier frequency itself can be used to synchronize the

receiver based on the phase of the carrier.

To allow the receiver to determine the beginning and end of a block of data, each block begins

with a preamble bit pattern (8 bits) and generally ends with a postamble bit pattern (8 bits). The

data plus preamble, postamble, and control fields (containing data link control protocol

information) are called a frame.

Far more efficient than asynchronous

Requires less overhead than asynchronous

Preamble Bit

Pattern

(8 bit Flag)

Control Fields Data Fields Control Fields

Postamble Bit

Pattern

(8 bit Flag)

1 Frame


14/25




BIM II Semester

Data Encoding:

Both analog and digital information can be encoded as either analog or digital signals.

Depending upon specific requirements and communication facilities available, encoding is chosen.

j Digital data, digital signals: simplest form of digital encoding of digital data

Equipment for encoding digital data into digital signal is less expensive, less complex as

compared to others.

Assigning voltage level to binary 1 and 0. Other complex encoding techniques can also be

used to improve performance by altering the spectrum of signal and providing

synchronization capability

Before discussing this further, we need to define some terms:

j Unipolar - All signal elements have the same sign (Pulses of only one polarity either

+ve or -ve)

j Polar - One logic state represented by positive voltage the other by negative voltagej Data rate - Rate of data (R) transmission in bits per second

j Duration or length of a bit - Time taken for transmitter to emit the bit (1/R)

j Modulation rate -Rate at which the signal level changes, measured in baud = signal

elements per second. Depends on type of digital encoding used.

j Mark and Space - Binary 1 and Binary 0 respectively

There are numerous techniques available to convert digital data into digital signals.

1. Non return to Zero-Level (NRZ-L)

2. Non return to Zero Inverted (NRZI)

3. Bipolar -AMI

4. Pseudoternary

5. Manchester

6. Differential Manchester

These encoding techniques can be evaluated or compared in following ways:j Signal Spectrum - Lack of high frequencies reduces required bandwidth, lack of dc component

allows ac coupling via transformer, providing isolation, should concentrate power in the middle of

the bandwidth

j Clocking - need for synchronizing transmitter and receiver either with an external clock or with async mechanism based on signal

j Error detection - useful if can be built in to signal encoding

j Signal interference and noise immunity - some codes are better than others

j Cost and complexity - Higher signal rate (& thus data rate) lead to higher costs, some codesrequire signal rate greater than data rate

1. NRZ-L: 0 High Level and 1 - Low Level voltage constant during bit interval

no transition i.e. no return to zero voltage

2. NRZI: Non return to zero inverted on ones

0 - no transition at beginning of interval 1 transition at the beginning of the interval

constant voltage pulse for duration of bit

more reliable detection of transition rather than level


15/25




BIM II Semester

NRZPros & Cons:

Pros

simple

make good use of bandwidth (Low frequency response)

Cons

dc component

lack of synchronization capability

used for magnetic recording

not often used for signal transmission

3. Bipolar AMI: Use more than two levels

0 - no line signal

1 - positive or negative pulse, alternating for successive ones

no loss of sync if a long string of ones

long runs of zeros still a problem

no net dc component

lower bandwidth

easy error detection

4. Pseudoternary: 0 positive or negative level, alternating for successive zeros

1 no line signal

5. Manchester: 0 transition from high to low in the middle of the interval

1 transition from low to high in the middle of the interval

used by IEEE 802.3

6. Differential Manchester: Always a transition in the middle of the interval

0 transition at the beginning of the interval

1 no transition at the beginning of the interval

used by IEEE 802.5

Biphase Pros and Cons:

Pros

synchronization on mid bit transition (self clocking)

has no dc component

has error detection

Cons at least one transition per bit time and possibly two

maximum modulation rate is twice NRZ

requires more bandwidth


16/25




BIM II Semester

j Digital data, analog signals: main use is public telephone system

j has freq range of 300Hz to 3400Hz use modem (modulator-demodulator)

A modem converts digital data to an analog signal so that it can be transmitted over an

analog.

Techniques used are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase

Shift Keying (PSK).

Transmission media such as optical fiber, unguided media are used for the propagation of

Analog signals.

Modulation: Process of encoding a message from a message source in a manner suitable for

transmission. (encoding Low frequency message signal with high frequency carrier signal)

Modulation involves operation on one or more of the three characteristics of a carrier

signal: amplitude, frequency, and phase.

Modulation In Digital Shift Keying


17/25




BIM II Semester

Amplitude Shift Keying (ASK):

The two binary values (1 and 0) are represented by two different amplitudes of the carrier

frequency. usually have one amplitude zero

Binary 1 high frequency carrier wave of fixed Amplitude Ac and fixed frequency fc for a bitduration Tb

Binary 0 no amplitude for Tb sec

So amplitude of the carrier signal is varied

Mathematically:

!

!!

0)(0

1)(2cos)(

tmfor

tmforftAtu

c T

susceptible to sudden gain changes

inefficient modulation technique

used for digital data transfer

up to 1200bps on voice grade lines

very high speeds over optical fiber

Some systems use multiple amplitudes

ASK is generated by applying the incoming binary data represented in Unipolar form and the

sinusoidal carrier to a product modulator.

Binary ASK waveBinary wave in

unipolar form

m(t)Carrier wave

ft

c T2cos

Product

Modulator


18/25




BIM II Semester

Binary FSK wave

Control line

Binary data Input

Multiplexer

Frequency Shift Keying (FSK):

most common is binary FSK (BFSK)

two binary values represented by two sinusoidal waves of same amplitudes but different

frequencies f1 and f2

Mathematically:

!!!0)(2cos1)(2cos)(

2

1

tmfortfAtmfortfAtu

c

c

TT

less susceptible to error than ASK

used for

up to 1200bps on voice grade lines

high frequency radio

FSK can be generated by:

tf

C 12cos T tf

C 22cos T

Multiple FSK

each signaling element represents more than one bit

more than two frequencies used more bandwidth efficient

more prone to error

Phase Shift Keying:

Binary PSK (BPSK)

Binary data are represented by two sinusoidal signals of fixed amplitude and frequency

but with different phase.

Generally phase are 0 and

Mathematically:

!

!!

0)()2cos(

1)(2cos)(

tmfortfA

tmfortfAtu

c

c

TT

T


19/25




BIM II Semester

2 t*

0

S2

1 t*

E

1U

11S101

S2

00

S2

10

Z1

Differential PSK

phase shifted relative to previous transmission rather than some reference signal

Quadrature PSK (QPSK):

get more efficient use if each

signal element represents more

than one bit

To increase the bandwidth

efficiency

Four different phase angles used

45 degrees (/4)

135 degrees (3/4)

225 degrees (-3/4)

315 degrees (-/4)

In QPSK system, data bits are

divided into group of two bits,

called dibits. There are four

possible dibits 00,01,10,11. Each ofthe four QPSK signals is used to

represent one of them. The signal

constellation uses gray coding.

Mathematically:

4

)12(,4,3,2,1

0)2cos()(

TU

UT

!

ee!

iwherei

Tttf

tu

i

ic

QPSK Signal Constellation:


20/25




BIM II Semester

j Analog data, digital signals: Analog data, such as voice and video, are often digitized using PCM (Pulse Code

Modulation) (Sampling, Quantization, and Coding) to be able to use digital transmission

facilities.

In digital transmission, modern digital transmission and switching equipment are used.

j Analog data, analog signals: Analog data are modulated and converted into Analog Signals using modulation techniques

Amplitude Modulation(AM), Frequency Modulation (FM), Phase Modulation (PM) on an

analog transmission system

Multiplexing Techniques:

Multiplexing is technique whereby a number of independent signals can be combined into a composite

signal suitable for transmission over a common channel.

y Under the simplest conditions, a medium can carry only one signal at any moment in time

y For multiple signals to share a medium, the medium must somehow be divided, giving each signal a

portion of the total bandwidth

y The current techniques include frequency division multiplexing, time division multiplexing, and code

division multiplexing

Frequency DivisionMultiplexing

y Assignment of non-overlapping frequency ranges to each user or signal on a medium. Thus, all

signals are transmitted at the same time, each using different frequencies

y A multiplexor accepts inputs and assigns frequencies to each device

y The multiplexor is attached to a high-speed communications line

y A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates

the multiplexed signals

y Analog signaling is used to transmit the data


21/25




BIM II Semester

y Broadcast radio and television, cable television, and cellular telephone systems use frequency

division multiplexing

y This technique is the oldest multiplexing technique

y Since it involves analog signaling, it is more susceptible to noise

Time Division Multiplexing:

y Sharing of the signal is accomplished by dividing available transmission time on a medium

among users

y Digital signaling is used exclusively

y Time division multiplexing comes in two basic forms:

1. Synchronous time division multiplexing

2. Statistical time division multiplexing

Synchronous Time Division Multiplexing:

y The original time division multiplexing

y The multiplexor accepts input from attached devices in a round-robin fashion and transmits the

data in a never ending pattern

y T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing


22/25




BIM II Semester

y If one device generates data at a faster rate than other devices, then the multiplexor must

either sample the incoming data stream from that device more often than it samples the other

devices, or buffer the faster incoming stream

y If a device has nothing to transmit, the multiplexor must still insert something into the

multiplexed stream

y So that the receiver may stay synchronized with the incoming data stream, the transmitting

multiplexor can insert alternating 1s and 0s into the data stream


23/25




BIM II Semester

y The T-1 multiplexor stream is a continuous series of frames

Statistical Time DivisionMultiplexing:

y A statistical multiplexor transmits the data from active workstations only

y If a workstation is not active, no space is wasted in the multiplexed stream

y A statistical multiplexor accepts the incoming data streams and creates a frame containing the

data to be transmitted

y To identify each piece of data, an address is included


24/25




BIM II Semester

y If the data is of variable size, a length is also included

y More precisely, the transmitted frame contains a collection of data groups

Wavelength DivisionMultiplexing:

y Wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line

y Different wavelength lasers (called lambdas) transmit the multiple signals

y Each signal carried on the fiber can be transmitted at a different rate from the other signals

y Dense wavelength division multiplexing combines many (30, 40, 50 or more) onto one fiber

y Coarse wavelength divisionmultiplexing combines only a few lambdas


25/25




BIM II S t

y FDM with multiple beams of light at different frequency

y carried over optical fiber links

1. commercial systems with 160 channels of 10 Gbps

2. lab demo of 256 channels 39.8 Gbps

y architecture similar to other FDM systems

1. multiplexer consolidates laser sources (1550nm) for transmission over single fiber

2. Optical amplifiers amplify all wavelengths

3. Demux separates channels at the destination

Advantages and disadvantages of multiplexing techniques:

Multiplexing Techniques Advantages Disadvantages

Frequency Division Multiplexing Simple

Popular with radio, TV, cable TV,

All the receivers, such as cellular

telephones, do not need to be at

the same location

Noise problems due to analog

signals

Wastes bandwidth

Limited by frequency ranges

Synchronous Time Division

Multiplexing

Digital signals

Relatively simple

Commonly used with T-1, ISDN

Wastes bandwidth

Statistical Time Division

Multiplexing

More efficient use of bandwidth

frame can contain control and

error information

Packets can be of varying size

More complex than synchronous

time division multiplexing

Wavelength Division

Multiplexing

Very high capacities over fiber

signals can have varying speeds

scalable

Cost

Complexity

Documents

Ch 2(Data Communication Principles)