Digital Communications

• Is the transmission of digital pulses

between two or more points in a

communication system.

Binary Transmission

BIT• Is a contraction of the term “binary digit”

• A unit of information represented by either a „1‟ or „0‟

BIT RATE / DATA RATE• The number of 0‟s and 1‟s that pass on a

communication channel each second

• Transmission rate in bits per second

ERROR RATE• A measure of data connectivity‟s performance

• Often the unit of measurement is bit error rate (BER),which is probability of error

• A BER of 1 in 105 means that there is a probability ofone error will occur for every 100,000 bits

Binary Code Formats

NON-RETURN TO ZERO (NRZ)

A “1” represents the active state and a “0” the

passive state

A change of state occurs when there is 1-to-0

or 0-to-1 transition

A string of 1‟s is a continuous pulse or “ON”

condition and a string of 0‟s is a continuous

“OFF” condition

Information is extracted from transitions or lack

of transitions in synchronous format, and a

single pulse completely occupies the

designated bit interval

NON-RETURN TO ZERO (NRZ)

Binary Code Formats

RETURN TO ZERO (RZ)

There is a transition for every bit transmitted,

whether a 1 or 0, and as a result, a pulse width

is less than the bit interval to permit the NZR

condition

Binary Code Formats

BIPOLAR NRZ

This is similar to NRZ except that binary 1‟s

alternate in polarity.

Binary Code Formats

BIPOLAR RZ

The same as bipolar NRZ, but there is a return-

to-zero condition for each signal element, and

again, the pulse width is always less than the bit

interval.

Binary Code Formats

MANCHESTER CODE

Is commonly used in digital fiber optic systems.

The binary information is carried in the transition

which occurs at midpulse.

By convention, logic “0” is defined as a positive

going transition and logic “1” as a negative going

transmission. This signal can either be unipolar

or bipolar.

MANCHESTER CODE

Binary Information Codes

Types of Characters

Data Link Control Characters-used to facilitate the orderly flow of data

from a source to a destination

Graphic Control Characters

-involve the syntax or presentation of thedata at the receive terminal

Alphanumeric Characters

-used to represent the various symbols usedfor letters, numbers, and punctuation in theEnglish language


Data Communication Codes

-character sets or character languages


MORSE CODE

the first fixed-length character code. It is a 5

bit character code, with a start bit and a 1.5

stop bitsTable of Morse Code


BAUDOT CODE

the first datacom code with

three unequal length symbols

Jean-Maurice-Emile Baudot


AMERICAN STANDARD CODE FOR

INFORMATION INTERCHANGE (ASCII)

it uses 7 bits of

information, an 8th bit

for use as parity, a start

bit and two stop bits


EXTENDED BINARY CODE DECIMAL

INTERCHANGE CODE (EBCDIC)

the true 8 level code


CCITT ALPHABET NO. 5 CODE

a 7 bit character set which has 27 or 128

codes

also with a parity bit

CCITT ALPHABET NO. 2 CODE

character length similar to Baudot code but

without start and stop bits


HOLLERITH CODE

consist of 12 bits information with the 12th bit

for parity.

Herman Hollerith

Binary Transmission Conventions

• PARALLEL TRANSMISSION

bits are transmitted character at a time i.e.

eight bits are transmitted simultaneously over

eight wires.

an additional wire called strobe or clock lead

notifies the receiver unit that all the bits are

present on their respective wires so that the

voltages on the wires can be sampled.

Binary Transmission Conventions

• SERIAL TRANSMISSION

bits are transmitted over a single line one bit

at a time

used when the cost of the communication

medium is high

PARALLEL AND SERIAL TRANSMISSION

Synchronization on Digital Signals

• SYNCHRONOUS

transmission in which data are sent at a fixed

rate, with the receiver and transmitter

synchronized

synchronized transmission eliminates the

need for start and stop bits


• ASYNCHRONOUS

transmission in which time intervals between

transmitted characters may be of unequal

length

transmission is controlled by start and stop

bits at the beginning and end of each

character


• ISOCHRONOUS

a combination of asynchronous and

synchronous transmission

the data are clocked by a common timing

base, and bytes are also framed with start

and stop bits

Digital Transmission Modes

• SIMPLEX

data transmission is unidirectional

information can be sent only in one direction

simplex lines are also called receive-only,

transmit-only, or one-way only lines


• HALF DUPLEX (HDX)

data transmission is possible in both

directions but not at the same time

HDX lines are also called two-way alternate or

either way lines


• FULL-DUPLEX (FDX)

data transmission is possible in both

directions at the same time but they must be

between the same two stations

FDX lines are also called two-way

simultaneous, duplex, or both-way lines


• FULL/FULL DUPLEX (F/FDX)

data transmission is in both directions at the

same time but not the same two stations

F/FDX is possible only on multipoint circuits

Types of Data Circuits

• TWO-WIRE CIRCUIT

it involves a transmission medium that either

uses two wires (a signal and a reference lead

or a configuration that is equivalent to having

only two wires

simplex, HDX, or FDX transmission is

possible


• FOUR-WIRE CIRCUIT

it involves a transmission medium that uses

four wires (two are used as signals that are

propagating in opposite direction and two are

used for reference leads) or a configuration

that is equivalent to having four wires


• SWITCHED CIRCUIT

in telephone network, a call is automatically

switched through its destination after dialing

has been completed.


• LEASED CIRCUIT

a permanent circuit used for private use within

a communication network with the line directly

between two locations or routed through a

central office

TRANSMISSION LEVEL POINT

• Is the ratio (in dB) of the power of a signal at that

point to the power of the same signal at the

reference point

• CCITT recommends -13 dBrn0 at 0 TLP

TLPdB + dBm0 = Psignal(dB)

Where: TLP = transmission level point in dB

dBm0 = signal magnitude referred to

0 TLP in dBm

Psignal = signal magnitude at a point

Voice Channel Transmission Impairments

AMPLITUDE DISTORTION

distortion caused by the variation of

transmission loss with frequency


PHASE DISTORTION

is the resultant of different velocities of

propagation at different frequencies across

the voice channel.


NOISE

it is a spurious or extraneous signal that

interferes with the wanted signal.


CROSSTALK

it is defined as the undesired energy

appearing in one signal path as a result of

coupling from another signal path


ECHO

it is the return of talker‟s voice or other end-

user signal

SINGING

it is the result of sustained oscillations due to

positive feedback in amplifying circuits

Digital Modulation Techniques


1. AMPLITUDE MODULATION

-two (2) power levels (high and low) are to

represent 1 or 0 based on amplitude (1-high; 0-

low)


1.a QUADRATURE AMPLITUDE MODULATION

(QAM)

-four (4) power levels are used to represent

4 pairs of bits


2. PHASE SHIFT KEYING

-there is a phase reversal of 180̊ to indicate

change in state from 0 to 1 or vice versa


2.a BINARY PHASE SHIFT KEYING (BPSK)

-two output phases are possible for a single

carrier frequency (1 & 0)

-as the input signal changes state, the phase

of the output carrier shifts between 2 angles that

are 180 ̊


2.b BPSK-PHASE REVERSAL KEYING (PRK)

- a biphase modulation

- a form of suppressed carrier, square-wave

modulation of a continuous wave signal

- depending on the logic condition on the

digital input, the carrier is transferred to the

output either in phase or 180 ̊ out of phase with

the reference carrier oscillator


2.c QUADRATURE PHASE SHIFT KEYING

(QPSK)

- four (4) possible pair of bits are

represented by 4 different phases of the carrier


3. FREQUENCY SHIFT KEYING (FSK)

-binary states are represented by two

different frequencies

-a simple, low performance digital

modulation


3.a BINARY FSK

- a form of constant envelope angle

modulation similar to conventional FM except

that the modulating signal is a binary pulse

stream that varies between two discrete voltage

levels rather than a continuously changing

analog waveform

- has a poorer performance than PSK or

QAM

- it is seldom used for high-performance

digital radio system


- its use is restricted to low-performance,

low-cost, asynchronous data modems for data

communications over analog, voice band

telephone lines


3.b FSK TRANSMITTER-BINARY FSK

-use voltage controlled oscillators (VCO):

MARK -logic “1” frequency

SPACE -logic “0” frequency

A Voltage Controlled Oscillator


3.c MINIMUM SHIFT KEYING

- a form of continuous phase shift keying

(CPFSK) with mark and space frequencies

synchronized with input binary rate separated by

½ of bit rate

- it requires synchronizing circuits and is

more expensive

PHASE-LOCKED LOOP (PLL)

MODULATOR

• the most common circuit used for demodulating

binary FSK signal

PLL NATURAL FREQUENCY

• Center frequency of FSK modulator

Error Detection and Correction Techniques

• ERROR DETECTION

The process of monitoring the received data

and determining when a transmission error has

occurred.

Error Detection

1. REDUNDANCY

-involves transmitting each character twice. If

the same character is not received 2x in

succession, an error has occurred

-retransmission of the entire message is very

inefficient, because second transmission of a

message is 100% redundant

Error Detection

2. PARITY-CHECK (50% detection)

-the simplest error detection scheme

-used for data communications systems and

with both vertical and horizontal redundancy

checking

-with parity, a single bit is added to each

character to force a total number of 1‟s in the

character, including the parity bit, to be either

an odd (odd parity) or even number (even

parity)

Error Detection

• EVEN PARITY

Bias bit = logic “0”

> a “1” indicates an error, “0” means no error

Error Detection

• ODD PARITY

Bias bit = logic “1”

> a “1” indicates no error, “0” means there is no

error

Error Detection

• VERTICAL & HORIZONTAL REDUNDANCY

CHECK (95-98% detection)

-a parity bit is added to each character to

force the total number of 1‟s in the character

including the parity bit, to be either an odd

number (odd parity) or an even number (even

parity)

Error Detection

2.a VERTICAL REDUNDANCY CHECKING

(VRC)

-an error detection scheme that uses parity

to determine if a transmission error has occurred

within a character

-VRC is XORing of the bits within a single

character

CHARACTER PARITY

-each character has a parity added to it prior

to transmission

Error Detection

2.b HORIZONTAL / LONGITUDINAL

REDUNDANCY CHECKING

(HRC OR LRC)– an error-detection scheme that uses parity to

determine if an error has occurred in amessage (message parity)

– with LRC, each bit position has a parity bit

– LRC is the result of XORing the charactersthat make up a message and only even parityis used. The bit sequence of an LRC is oftencalled Block Check Sequence (BCS)

– VRC bit for each character is computed in thevertical direction, LRC bit is computed inhorizontal direction

Error Detection

3. EXACT COUNT ENCODING

- the number of 1‟s in each character is the

same

4. ECHOBACK / ECHOPLEX

- a character is sent back to the operator for

the operator to check errors

- mode of transmission that achieves less than

full-duplex but more than half-duplex

- achieved by having the answer DTE

retransmit (echo) the received message back to

the originating DTE for decoding and display

Error Detection

5. CYCLIC REDUNDANCY CHECK (CRC)

- most reliable scheme for error detection;99.95 % of errors are detected

- it is generally used with 8 bit codes such asEBCDIC or 7 bit codes without parity

- the CRC character is the remainder of adivision process using an XOR operation. If notransmission occurred, the remainder will bezero.

- the number of bits in CRC code is equal tothe highest exponent o the generatingpolynomial. The exponent identifies the bitpositions that contain a 1.

Error Correction

1. SYMBOL SUBSTITUTION

-designed to be used in a humanenvironment at the receiver

-a reverse question mark is substitutedfor “bad character”

2. RETRANSMISSION / AUTOMATIC

REQUEST FOR RETRANSMISSION (ARQ)

-resending a message when it is receivedin error. The received terminal automaticallycalls for retransmission of the entiremessage

-optimal ARQ message blocks = 256 to512 characters

Error Correction

3. FORWARD ERROR CORRECTION

- the only error correction scheme that

actually detects and corrects transmission

errors at the receive end without calling for

retransmission of the entire message

- bits are added to the message prior to

transmission

Error Correction

R. W. HAMMING CODE

• The most popular correcting code

• Developed by R. W. Hamming at Bell Labs

• The number of bits in a Hamming code is

dependent on the number of bits in the data

character,

2n ≥ m + n +1

Where: m = no. of bits in the data

character

n = no. of Hamming bits

Levels of Synchronization

1. BIT OR CLOCK SYNCHRONIZATION

- identifies the start / beginning and stop /

end of each bit transmitted

- it ensures that the transmitter and receiver

agree on a precise time slot for the occurrence

of the bit

- it allows the receive DTE to know when to

sample the incoming bit stream

- Fast or slow bit sampling rate result to

errors

Levels of Synchronization

2. CHARACTER SYNCHRONIZATION

- identifies start and stop of each individual

character transmitted

3. BLOCK OR MESSAGE SYNCHRONIZATION

- addresses the start and stop of large

amounts of data

4. MODEM OR CARRIER SYNCHRONIZATION

- performed between modems so that the

received signals may be properly demodulated

EIA Standards for Digital Interfacing

RS 232C

• It is an interface between the DTE (Data

Terminal Equipment) and DCE (Data

Communications Equipment) employing serial

binary data interchange

• It is a first level protocol standard as well as an

electrical standard specifying handshaking and

functions between the DTE and DCE

• Transmission rate is 20 kbps for a distance not

more than 50 ft.; load impedance at terminator

side is between 3000 to 7000 ohms


RS 422A

• It defines electrical characteristics of balanced-

voltage digital interface circuits.

• It is a differential balanced voltage interface

standard capable of significantly higher data

rates over long distances.

• It can accommodate 100 kbps over a distance of

4000 ft (1200 m) or rates up to 10 Mbps over a

maximum distance of 40 ft (12 m)


RS 423A

• It defines electrical characteristics of

unbalanced-voltage digital interface circuits

• Single-ended, bipolar and unterminated voltage

circuit like RS 232C

• It extends the distance and data rate capabilities

to distances up to 4000 ft (1200 m) at a data rate

of 3 kbps or at higher data rates of up to 300

kbps over a maximum distance of 40 ft (12 m)


RS 357

• It defines interface between Facsimile Terminal

Equipment and VF Data Terminal Equipment

RS 366

• It defines interface between DTE and Automatic

Calling Equipment for Data Communications


RS 408

• It recommends the standardization of the two

interfaces between the numerical control

equipment (such as tape reader) and the serial-

to-parallel converter with less than 40 ft (12 m)

distance.


RS 449

• It is general-purpose 37-position and 9-position

interface for DTE and DCE employing serial

binary data interchange.

• It offers greater immunity to noise and increase

the data signaling rate to 2 Mbps and permits an

increase up to 200 m in the length of the

interconnecting cable.

CCITT V-Series for Digital Interfacing

CCITT Rec. V.10 / X.26

• It defines electrical characteristics of Unbalanced

Double Current Interchange Circuits for General

Use with IC Equipment in the field of Data

Communications.

CCITT Rec. V.11 / X.27

• It defines electrical characteristics of Unbalanced

Double Current Interchange Circuits for General

Use with IC Equipment in the field of Data

Communications.


CCITT Rec. V.24

• It gives the list of definitions for interfacing

circuits between DTE and DCE for transfer of

binary data and control and timing signals.

• The definitions are applicable to synchronous

and asynchronous data communications.


CCITT Rec. V.28

• It defines the electrical characteristics for

Unbalanced Double-Current Interchange Circuits

.

• Electrical characteristics specified are applicable

to interchange circuits operating with data

signaling rates below 20 kbps.


CCITT Rec. V.35

• Defines interface circuits similar to RS 232C and

Rec. V.24 with balanced line on Transmit Data,

Receive Data, Transmit Clock, and Receive

Clock.

CCITT Rec. V.57

• Comprehensive Data Test Set for High Data

Signaling Rates.


CCITT Rec. V.36

• It covers the synchronous data transmission

modems using 60-108 kHz group band circuits

and is applicable to the extension of a PCM

channel at 64 kbps, extension of the Single

Channel Per Carrier, SCPC circuit from a

satellite earth station, and the transmission of a

multiplex aggregate bit stream for telegraph and

data signals.

CCITT Signaling System (SS)

CCITT SS 4

• System in Europe only for operator-controlled

and full automatic international services on

unidirectional circuits.

CCITT SS 5

• Uses two in band frequencies for line and

supervisory signals (2400-2600 Hz).

CCITT Signaling System (SS)

CCITT SS 6

• International specification for common channel

signaling

CCITT SS 7

• Common channel signaling system use between

SPC exchanges

• Designed for use in a digital environment

CCITT X-Series for Digital Interfacing

X.21

• Interface between DTE and Data Terminating

Equipment for Synchronous operation on Public

Data Networks

X.24

• List of Definitions for Interchange Circuits

between Data Terminal equipment and Data

Terminating Equipment on Public Data Networks


X.25

• Interface between DTE and DCE for Terminals

Operating in the Packet Mode on Public data

Networks.

• It is a standard protocol for interfacing a terminal

to packet network.

• Defines the architecture of three levels of

protocols existing in the serial interface cable

between a packet mode terminal and give away

to a packet network.


X.26

• Electrical characteristics for Unbalanced Double

Current Interchange Circuits for General Use

with Integrated Circuit equipment in the field of

Data Communications

X.27

• Electrical characteristics for Unbalanced Double

Current Interchange Circuits for General Use

with Integrated Circuit equipment in the field of

Data Communications

1. The internet was started in

a. 1969

b. 1988

c. 1980

d. 1996

2. Data rate is

a. a measure of how quickly data is transmitted

( bits per seconds)

b. the number of bits transferred per unit of time

c. the time it takes to transmit a frame

d. the time it takes for the signal to travel from

one end of a transmission medium to other

3. Internet address is

a. 32 bits

b. 48 bits

c. 64 bits

d. 16 bits

4. When an end station receives a 100

bytes datagram it knows it is complete if

in the IP header

a. Flag = last, offset = 0

b. Flag = don‟t, offset = 0

c. Flag = 0, offset = 100

d. Flag = may, offset = 0

5. The difference between circuit switching

and packet switching is

a. Only in circuit switching can be active on a

single physical link

b. The circuit switching allows higher line

utilization than packet switching

c. Only packet switching, transmission

resources are wasted when traffic is bursty

d. Circuit switching uses physical links where

as packet switching uses virtual circuit

6. What is RFP?

a. Radio Frequency Program

b. Radio Frequency Proposal

c. Request For Proposal

d. Request For Projection

7. Error checking is accomplished by using

______ in a computer telephone

communications.

a. CRC

b. LRC

c. VRC

d. Parity

8. _____ codes are very popular for use in

optical systems and satellite telemetry

links.

a. Bi-phase

b. NRZ

c. RZ

d. ASCII

9. The largest amount of data that can be

sent across a given network in a single

packet

a. Byte

b. Nibble

c. MTU

d. MB

10. When a signal band limited to fm is

sampled at a rate less than 2fm, the

reconstructive signal will

a. Be smaller in amplitude

b. Be distorted

c. Be magnified

d. Have higher frequencies suppressed

11. The number of changes in a signal

per second

a. Bit rate

b. Baud rate

c. Period

d. Frequency

12. A modulation process in which a train

of fixed-width pulses is transmitted

whose polarity indicates whether the

demodulator output should rise or fall at

each pulse

a. PAM (Pulse Amplitude Modulation)

b. PWM (Pulse Width Modulation)

c. DM (Delta Modulation)

d. PPM (pulse Position Modulation)

13. It is an ANSI version of bit-oriented

data link layer protocol

a. ACK

b. ADCCP

c. ASK

d. ISDN

14. Determine the dynamic range

capability of an 8-bit linear

(noncompounded) PCM system

a. 48 dB

b. 18 dB

c. 8 dB

d. 40 dB

SolutionD.R. (dB) = 6n

= 6 (8)=48 dB

15. A connection-oriented technology that

sends all data in fixed cells with 48

octets per data cell

a. Frame relay

b. HLDC

c. ATM

d. SDH

16. An FSK system must maintained less

than 10-3 BER. What is the

maximum allowable received Eb /

N0?

a. 6

b. 18

c. 12.43

d. 15.29

Digital Communications

Documents

bits of information

bits information

transmission rate

term binary

binary transmissionbit

nrz binary code formats

condition information

synchronous transmission