Iii Data Transmission Fundamentals

Data Transmission Fundamentals 1

DATA TRANSMISSION FUNDAMENTALS TRANSMISSION MODES F Simplex Transmission

Allows data to flow in one direction

only (unidirectional). 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 change direction).

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.


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).

b0

b1

b2

b3

b4

b5

b6

b7

strobe

ground

b0

b1

b2

b3

b4

b5

b6

b7

strobe

ground

transmitter receiver


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 signal ground.

F All communication between chips and components

inside a computer system (internal computer data transfer) unit takes place in parallel through the system unit bus.

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

ground

data

ground

transmitter receiver

1 1 0 0 0 1 1 0


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.


PARALLEL-TO-SERIAL AND SERIAL-TO-PARALLEL CONVERSION F Transmitter Part (Parallel-to-Serial)

F Receiver Part (Serial-to-Parallel)

F The transmit and receive registers are simply shift

registers.

b7 b6 b5 b4 b3 b2 b1 b0

TransmittedData

TransmitBuffer

TransmitRegister

From CPU

b7 b6 b5 b4 b3 b2 b1 b0

ReceivedData

ReceiveBuffer

ReceiveRegister

To CPU


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 computed as:

Tx = N / R where: 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 as:

Tp = S / V where: S = distance to be travelled V = velocity of propagation


Example:

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 200,000 Km/second.

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.


SIGNAL MODULATION 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 information).

F A modulator is an electronic device that varies the

signal carrier to reflect or represent the information in the original signal.


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 allowed)

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.

Modulation F At the receiving end, the tones are converted back to

digital signals or pulses.

Demodulation 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.


Example of a typical computer-to-computer

communication using modems and the public telephone system:

ComputerModem

TelephoneSystem

ComputerModem


F In modems, a sine wave is used as a carrier.

τ = period or length of one cycle in terms of time (seconds).

f = frequency of signal in cycles per sec or Hz. = 1/τ A = amplitude or magnitude of the signal in volts

(signal strength).

amplitude

onecycle


The phase angle of a signal is the number of degrees

in which the signal or sine wave differs a reference sine wave.

The phase angle of this signal is 90 degrees. Take note that one complete cycle is equivalent to 360

degrees.

360o

90o


F Modulation is therefore the process of changing the

amplitude, frequency, or phase of a carrier sine wave signal to represent information.

Amplitude, frequency, and phase modulation are also

known as amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK).

carriersignal

informationsignal

amplitudemodulation

frequencymodulation

phasemodulation

0 1 0 0 1 1


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 factors.

F A wire channel requires a proper termination to

prevent signal reflection from further distorting the signal.


time

original digital signal

time

digital signal after travelling 100 feet

time

digital signal after travelling 500 feet


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 also amplified.

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.

RegenerativeRepeater

RegeneratedDigitalSignal

DistortedDigitalSignal


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 another.

F For the receiving device to decode and interpret the

incoming bit pattern correctly, it must be able to determine:

1. the start of each bit cell. This is known as bit or clock synchronization.

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 synchronization.


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 exchange.

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).


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 bits.

Start Bit - logic 0 Stop Bit - logic 1

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 bits.

01 0 1 0 0 1 0

8-bit characterstart bit 1, 1.5, or 2 stop bits to

ensure a negativetransition at the start ofeach new character

line idleline idle


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).


Input bit = 1 ?

Input bit = 0 ?

Wait 1/2 Bit Delay

Input bit = 0 ?

Bit Counter = 8

Wait 1 Bit Delay

Read Incoming Bit

Decrement BitCounter

Counter = 0 ?

Wait 1 Bit Delay

Input Bit = 1 ?

Store the Byte

Framing Error

yes

yes

yes

no

no

no

no

no

yesyes

Flowchart of theProcess Required

to RecoverAsynchronous

Serial Data


star

t

1 2 3 4 5 6 7 8

receiveddata

samplestrobe

output 0 1 1 0 1 0 0 1 0 1

Ideal Sampling at Midpoint of Each Bitst

art

stop

1 2 3 4 5 6 7 8

receiveddata

samplestrobe

output 0 1 1 0 1 0 0 1 0 0

Sampling When Receiver Clock is Slightly Fast

star

t

stop

1 2 3 4 5 6 7 8

receiveddata

samplestrobe

output 0 1 1 0 1 0 1 0 1

Sampling When Receiver Clock is Too Slow

stop


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.

Example: 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% 11 72.73% of what is transmitted actually contain

data. The remaining 27.27% contain control bits.


If the data rate of the transmission is 9,600 bps,

then the effective data rate will be: Effective = 0.7273 x 9600 Data rate = 6,982.08 bps F The overhead problem becomes more apparent for

data transmission involving large quantities of data. Example: 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 11,534,336 bits


SYNCHRONOUS SIGNAL SYNCHRONIZATION F Synchronous signal modulation and demodulation

require precise clocks at both ends of the communications link.

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

synchronous transmission: 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.


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.

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,

1 0 0 1 1 1 0 1bit steam to be

transmitted

Manchesterencoded

waveform

extractedclock

decodedsignal


the receiver can easily read the incoming bit stream.


Option 2: Data Encoding and Clock

Synchronization 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 change.

1 0 0 1 1 1 0 1bit steam to betransmitted

NRZIwaveform


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 stuffing technique.

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 bit.

Example: 1011111110010111101011111001101111111 1011111011001011110101111100011011111011 stuffed zeros 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.


F Sample Synchronous Frame Formats: 1. Binary Synchronous Control (BSC)

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.

SYN SYN STX ETX BCC BCC

DATA BYTES


2. Synchronous Data Link Control (SDLC)

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 the frame.

C - Control. This indicates if the frame is an

information frame or supervisory frame. FCS - Frame Check Sequence. This is for error

handling 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.

SF SSA C INFORMATION FCS EF


F Comparison of Synchronous and Asynchronous Points Regarding Synchronous Transmission 1. Low overhead.

2. Ideal for high-volume, high-speed data transfer.

3. Very complicated to implement. Points Regarding Asynchronous Transmission 1. High overhead.

2. Ideal for low-volume, low-speed data transfer.

3. Very easy to implement. However, most networks use asynchronous

transmission even for high-volume file transfer because of its simplicity.


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 information.

Education

Iii Data Transmission Fundamentals