DATA TRANSMISSION FUNDAMENTALS
F Simplex Transmission
Allows data to flow in one direction
F Half-duplex Transmission
Allows data to flow in both directions
but only one at a time.
There is a problem with
turnaround time (the
time it takes for the
transmission circuits to
F Full-duplex Transmission
Allows data to flow in both directions
simultaneously. This usually requires
one set of transmission circuits each for
transmission and reception.
Data Transmission Fundamentals 1
PARALLEL VS. SERIAL TRANSMISSION
F Parallel transmission is the sending of several bits at
the same time. One line or wire is needed for each bit
(plus one line or wire for the signal ground and another
for the timing or strobe).
Data Transmission Fundamentals 2
F Serial transmission is when bits are transmitted one at
a time. Two lines are needed in the implementation of
serial transmission, one for the signal and one for the
1 1 0 0 0 1 1 0
F All communication between chips and components
inside a computer system (internal computer data
transfer) unit takes place in parallel through the system
F The type of communication between a computer and
an external device (external computer data transfer)
depends on the distance between them.
F Parallel transmission is common for distances less than
10 feet. Serial transmission is ideal for distances
greater than 10 feet.
Data Transmission Fundamentals 3
F The reasons why parallel transmission is not suitable
for long distance communication are:
1. cost (parallel transmission uses more lines)
2. varying delays among the different bits or
signals (bus skew). In other words, bits may
arrive at the receiver at different times
F For long distance communication, it would be more
cost-effective to transmit data using serial
transmission. The telephone lines can be readily used
for serial transmission.
F Since data inside a computer system move in parallel,
it is necessary to convert them to serial before external
communication can take place.
Data Transmission Fundamentals 4
PARALLEL-TO-SERIAL AND SERIAL-TO-PARALLEL
F Transmitter Part (Parallel-to-Serial)
b7 b6 b5 b4 b3 b2 b1 b0
F Receiver Part (Serial-to-Parallel)
b7 b6 b5 b4 b3 b2 b1 b0
F The transmit and receive registers are simply shift
Data Transmission Fundamentals 5
SIGNAL PROPAGATION DELAY
F The transmission delay (Tx ) of a signal is the time
taken to transmit binary data at a given data rate. It is
Tx = N / R
N = number of bits to be transmitted
R = data rate (bps)
F There is always a short but finite time delay for a
signal to propagate or travel from one end of a
transmission medium to the other. This is the
propagation delay (Tp) of the channel and is computed
Tp = S / V
S = distance to be travelled
V = velocity of propagation
Data Transmission Fundamentals 6
A 1 Mbyte file is to be transmitted between two
machines. Determine the propagation and
transmission delays if the distance between the
two is 10 Km and the data rate is 19.2 Kbps.
Assume that the velocity of propagation is
S = 10,000 m
V = 200,000 x 103 m/s
R = 19,200 bps
N = 1 x (1,048,576) x 8
= 8,388,608 bits
Tp = 10000 / 200,000 x 103
= 0.00005 sec
Tx = 8388608 / 19200
= 436.91 sec
Total Transmission Time
= Tp + Tx
= 0.00005 + 436.91
= 436.91005 sec.
Data Transmission Fundamentals 7
F When moving a voice or data signal through a
communications channel, it is necessary to vary
electrical energy in the channel so that the information
moves from one point in the media to another.
F Modulation of the process of varying the electrical
energy in the channel.
F A signal carrier is the electrical energy that flows in
the channel (the one that is varied to transmit
F A modulator is an electronic device that varies the
signal carrier to reflect or represent the information in
the original signal.
Data Transmission Fundamentals 8
CASE STUDY : MODEMS
F Digital signals cannot be transmitted directly over
telephone lines which are basically analog lines.
Limited bandwidth of telephone lines
(300 to 3,400 Hz)
Internal capacitance of telephone lines
(sudden changes in voltages are not
F Modems (modulator-demodulator) convert digital
signals (1’s and 0’s) to analog signals (tones) having
frequencies within the 300 to 3,400 Hz range.
F At the receiving end, the tones are converted back to
digital signals or pulses.
F The frequency used is approximately 1,700 to 1,800
Hz since transmission is best at frequencies at the
center of the 300 to 3,400 Hz passband.
Data Transmission Fundamentals 9
Example of a typical computer-to-computer
communication using modems and the public
Data Transmission Fundamentals 10
F In modems, a sine wave is used as a carrier.
τ = period or length of one cycle in terms of
f = frequency of signal in cycles per sec or Hz.
A = amplitude or magnitude of the signal in volts
Data Transmission Fundamentals 11
The phase angle of a signal is the number of degrees
in which the signal or sine wave differs a reference
The phase angle of this signal is 90 degrees.
Take note that one complete cycle is equivalent to 360
Data Transmission Fundamentals 12
F Modulation is therefore the process of changing the
amplitude, frequency, or phase of a carrier sine wave
signal to represent information.
0 1 0 0 1 1
Amplitude, frequency, and phase modulation are also
known as amplitude shift keying (ASK), frequency
shift keying (FSK), and phase shift keying (PSK).
Data Transmission Fundamentals 13
DIGITAL SIGNAL MODULATION
F Analog modulation techniques do not apply to digital
communications. Digital modulation does not require
the presence of an analog carrier.
F The digital signal remains at a given voltage for a
specified period to signal a binary or digital value. The
signal modulates from one discrete value to another
only when the information changes value.
F Several factors combine to limit the channel length a
digital signal can traverse without revitalization:
1. Electronic Noise
2. Signal Attenuation
3. Signal Reflection
F The farther the signal travels through a medium, the
more the signal becomes distorted because of the three
F A wire channel requires a proper termination to
prevent signal reflection from further distorting the
Data Transmission Fundamentals 14
original digital signal
digital signal after travelling 100 feet
digital signal after travelling 500 feet
Data Transmission Fundamentals 15
F A digital signal cannot be amplified to increase its
distance range in a channel. If a digital signal is
amplified, the noise that has contaminated the signal is
F In the case of signal distortion, repeaters are placed
along the digital channel to regenerate a digital signal.
Regenerating a signal means that the signal is received
and rebuilt to its original strength and shape.
F Repeaters remove the noise from a signal while it is
regenerating the signal.
Data Transmission Fundamentals 16
SYNCHRONIZATION OF DIGITAL MODULATION
F Digital Communications depend upon exact timing of
signal generation and reception to be successful.
F If the transmitter sends a signal and the receiver starts
to examine the signal at the wrong time, the receiver
will get meaningless information.
F Synchronization is the process in which the receiver
looks at the digital signal at the appropriate times to
detect the proper transition from one energy level to
F For the receiving device to decode and interpret the
incoming bit pattern correctly, it must be able to
1. the start of each bit cell. This is known as bit or
2. the start and end of each character or byte. This
is known as character or byte synchronization.
3. the start and end of each complete message block
or frame. This is known as block or frame
Data Transmission Fundamentals 17
F Synchronization between a sending and receiving
device requires an agreement on bit period or bit time
between the two devices.
F There are two types of synchronization techniques:
1. Asynchronous. The transmitter and receiver
work independently of each other and exchange a
specified signal pattern at the start of each signal
In asynchronous communication, each character
or byte is treated independently for clock (bit)
and character (byte) synchronization purposes.
2. Synchronous. The transmitter and the receiver
exchange initial synchronizing information, then
continuously exchanges a digital stream that
keeps them in lock step.
In synchronous transmission, the complete frame
(block) of characters is transmitted as a
contiguous string of bits and the receiver
endeavors to keep in synchronism with the
incoming bit stream for the duration of the
complete frame (block).
Data Transmission Fundamentals 18
ASYNCHRONOUS SIGNAL SYNCHRONIZATION
F The clocks of the transmitter and the receiver are not
continually synchronized. But the receiver needs to
know when the character begins and ends.
F Each transmitted character is encapsulated or framed
between an additional start bit and one or more stop
Start Bit - logic 0
Stop Bit - logic 1
line idle line idle
1 0 0 1 0 0 1 0
start bit 1, 1.5, or 2 stop bits to
ensure a negative
transition at the start of
each new character
F The start bit resets the receiver’s clock so that it
matches the transmitter’s. The clock needs to be
accurate enough to stay in synch for the next 8 to 11
Data Transmission Fundamentals 19
F The receiving device can determine the state of each
transmitted bit in the character by sampling or reading
the received signal approximately at the center of each
bit cell period.
F In order to receive the incomiong bits correctly, the
receiving device performs the following operations:
1. Wait for the line to become a logic 0 (start bit of
the incoming character).
2. Once the line becomes a logic 0, the receiving
device should wait for ½ of the bit period. At this
point the receiving device is approximately at the
center of the start bit.
3. The receiving device should then sample or read
the bit (which is still the start bit) to ensure that it
is not a false start bit (voltage fluctuation). If the
bit read is a logic 1, then it is assumed that it was
a false start bit (go back to step 1).
4. The receiving device should then wait for a
period of time equal to 1 bit period. This would
take the receiving device to the center of the first
data bit. Then the device should sample this bit.
This step is repeated 8 times (since there are 8
data bits per character).
Data Transmission Fundamentals 20
Input bit = 1 ?
Input bit = 0 ?
yes Flowchart of the
Wait 1/2 Bit Delay
no Serial Data
Input bit = 0 ?
Bit Counter = 8
Wait 1 Bit Delay
Read Incoming Bit Wait 1 Bit Delay
Decrement Bit no
Counter Input Bit = 1 ? Framing Error
Counter = 0 ? Store the Byte
Data Transmission Fundamentals 21
1 2 3 4 5 6 7 8
output 0 1 1 0 1 0 0 1 0 1
Ideal Sampling at Midpoint of Each Bit
1 2 3 4 5 6 7 8
output 0 1 1 0 1 0 0 1 0 0
Sampling When Receiver Clock is Slightly Fast
1 2 3 4 5 6 7 8
output 0 1 1 0 1 0 1 0 1
Sampling When Receiver Clock is Too Slow
Data Transmission Fundamentals 22
F Asynchronous transmission is often used in situations
when characters may be generated at random
intervals, such as when a user types at a terminal.
F The main problem with asynchronous transmission is
its high overhead primarily due to the additional start
and stop bits for every byte.
1 start bit and 2 stop bits
To transmit 1 byte (8 bits), a total
of 11 bits are needed.
8 bits for data plus
3 bits for control
% Overhead = 3 x 100 = 27.27%
72.73% of what is transmitted actually contain
data. The remaining 27.27% contain control bits.
Data Transmission Fundamentals 23
If the data rate of the transmission is 9,600 bps,
then the effective data rate will be:
Effective = 0.7273 x 9600
= 6,982.08 bps
F The overhead problem becomes more apparent for
data transmission involving large quantities of data.
1 MB file
1 MB = 1,048,576 bytes
Total Data Bits = 8 x 1,048,576 = 8,388,608 bits
Total Control Bits = 3 x 1,048,576 = 3,145,728 bits
Data Transmission Fundamentals 24
SYNCHRONOUS SIGNAL SYNCHRONIZATION
F Synchronous signal modulation and demodulation
require precise clocks at both ends of the
F The sender provides the clock signal to generate the
transmission frames. The receiver provides a clock to
decipher the transmission when it arrives.
F There are two techniques in implementing
1. Clock Encoding and Extraction
The clock (timing) information is embedded
into the transmitted signal and subsequently
extracted by the receiver.
2. Data Encoding and Clock Synchronization
This technique utilizes a stable clock source
at the receiver which is kept in synchronism
with the incoming bit stream. However, as
there are no start and stop bits with a
synchronous transmission scheme, it is
necessary to encode the information in such
a way that are always sufficient bit
transitions (1→0 or 0→1) in the transmitted
waveform to enable the receiver clock to be
resynchronized at frequent intervals.
Data Transmission Fundamentals 25
Option 1: Clock Encoding and Extraction
This uses the Manchester encoding scheme (also
known as Biphase-Level) in encoding the bit
stream to be transmitted.
1 0 0 1 1 1 0 1
bit steam to be
The presence of a positive or negative transition
at the center of each bit cell period in the
Machester encoded waveform is used by the
clock extraction circuit at the receiving side to
produce a clock pulse at approximately the center
of the bit.
The Manchester encoded waveform is then
decoded into the conventional encoding form
(Non-Return-to-Zero Level or NRZ-L). With
the extracted clock and the decoded waveform,
Data Transmission Fundamentals 26
the receiver can easily read the incoming bit
Data Transmission Fundamentals 27
Option 2: Data Encoding and Clock
This technique uses bit transitions (1→0 or
0→1) in the transmitted waveform to enable
the receiver clock to be resynchronized at
frequent intervals. However, there has to be
sufficient bit transitions in order for this to
be accomplished. A contiguous stream of 1s
or 0s will prevent the resynchronization of
the receiver clock.
This technique therefore uses the Non-
Return-to-Zero Space (NRZ-S) scheme in
encoding the bit stream to be transmitted.
With NRZ-S encoding, the signal level (1 or
0) does not change for the transmission of a
binary 1 whereas a binary 0 does cause a
bit steam to be 1 0 0 1 1 1 0 1
Data Transmission Fundamentals 28
This means that there will be bit transitions in this
incoming signal of the an NRZ-S waveform,
provided there are no contiguous streams of
binary 1’s. To solve the problem of continuous
streams of 1’s, use the zero bit insertion or bit
In the zero-bit insertion technique, if there is a
sequence of five contiguous binary 1 digits, a zero
is automatically inserted after the fifth binary 1
Consequently, the resulting waveform will
contain a guaranteed number of transitions, since
0’s cause a transition in a bit cell, and this enables
the receiver to adjust its clock so that it is in
synchronism with the incoming bit stream.
Data Transmission Fundamentals 29
F Sample Synchronous Frame Formats:
1. Binary Synchronous Control (BSC)
SYN SYN STX ETX BCC BCC
SYN (00010110) - Synchronizing Character. It
main function is to enable the receiver to achieve
character synchronization (reading each character
on the correct bit boundary).
STX (00000010) - Start of Text Character. It
indicates the start of a frame.
ETX (00000011) - End of Text Character. It
indicates the end of a frame.
BCC - Block Check Character. This allows the
receiver to identify errors in the frame and
request a retransmission of the frame.
BSC is a character-oriented synchronous
transmission control scheme.
Data Transmission Fundamentals 30
2. Synchronous Data Link Control (SDLC)
SF SSA C INFORMATION FCS EF
SF (01111110) - Opening Flag. This signals the
start of a frame.
SSA - Secondary Station Address. This contains
the unique address of the intended recipient of
C - Control. This indicates if the frame is an
information frame or supervisory frame.
FCS - Frame Check Sequence. This is for error
EF (01111110). Ending Flag. This signals the
end of a frame.
The SDLC is a bit-oriented protocol. The frame
contents need not necessarily comprise multiples
of eight bits.
Data Transmission Fundamentals 31
F Comparison of Synchronous and Asynchronous
Points Regarding Synchronous Transmission
1. Low overhead.
2. Ideal for high-volume, high-speed data
3. Very complicated to implement.
Points Regarding Asynchronous Transmission
1. High overhead.
2. Ideal for low-volume, low-speed data
3. Very easy to implement.
However, most networks use asynchronous
transmission even for high-volume file transfer
because of its simplicity.
Data Transmission Fundamentals 32
DIGITAL SIGNAL ADVANTAGES
F It takes more electrical noise to corrupt a digital signal
than it does to contaminate an analog signal.
If the voltage levels that represent each
digital value are far apart, it will take a
large amount of noise to get the signal
to move from one digital value to
another to cause an error.
F Most digital communications systems also send
specific and separate data, along with the information
they convey, that allows the receiver to detect errors.
The receiver can request a
retransmission of the erroneous
Data Transmission Fundamentals 33