digital-communications.ppt

• 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
BIT 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 of
one error will occur for every 100,000 bits

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 the
data at the received terminal
 Alphanumeric Characters
-used to represent the various symbols used
for letters, numbers, and punctuation in the
English language

Data Communication Codes
-character sets or character languages

 MORSE CODE
the first data communication code with three
unequal length symbols.
Table of Morse Code

 BAUDOT CODE
 the first fixed-length character code. It is a 5 bit
character code, with a start bit and a 1.5 stop
bits
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
• 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

• 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

• 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

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 SHIFT KEYING
-two (2) power levels (high and low) are to
represent 1 or 0 based on amplitude (1-high; 0-
low)

2. FREQUENCY SHIFT KEYING (FSK)
-binary states are represented by two
different frequencies
-a simple, low performance digital
modulation

3. PHASE SHIFT KEYING
-there is a phase reversal of 180̊ to indicate
change in state from 0 to 1 or vice versa

3.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 180̊

3.b QUADRATURE PHASE SHIFT KEYING
(QPSK)
- four (4) possible pair of bits are
represented by 4 different phases of the carrier

QUADRATURE AMPLITUDE MODULATION
(QAM)
-digital information is contained in both the
amplitude and phase of the transmitted carrier.

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 a
message (message parity)
– with LRC, each bit position has a parity bit
– LRC is the result of XORing the characters
that make up a message and only even parity
is used. The bit sequence of an LRC is often
called Block Check Sequence (BCS)
– VRC bit for each character is computed in the
vertical direction, LRC bit is computed in
horizontal 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
as EBCDIC or 7 bit codes without parity
- the CRC character is the remainder of a
division process using an XOR operation. If
no transmission occurred, the remainder will
be zero.
- the number of bits in CRC code is equal
to the highest exponent o the generating
polynomial. The exponent identifies the bit
positions that contain a 1.

Error Correction
1. SYMBOL SUBSTITUTION
-designed to be used in a human
environment at the receiver
-a reverse question mark is substituted
for “bad character”
2. RETRANSMISSION / AUTOMATIC
REQUEST FOR RETRANSMISSION (ARQ)
-resending a message when it is received
in error. The received terminal automatically
calls for retransmission of the entire
message
-optimal ARQ message blocks = 256 to
512 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 (V.24 – CCITT Equivalent)
• 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 A (V.25 – CCITT Equivalent)
• 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 (V.35 – CCITT Equivalent)
• 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 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

CCITT X-Series for Digital Interfacing
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.

Digital Interfacing
Centronics Parallel Interface
• Designed by Centronics Computer data
Corporation for use with their line of printers
• 36 pins
IEEE 488 Bus
• General Purpose Interface Bus
• Hewlett Packard Interface Bus
• Used in connecting printers to mini and micro-
computer systems
• Interface for the remote control of and data
acquisition from test instruments (24 pins)

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