### 11-3: Transmission Efficiency

```Chapter 11
The Transmission of Binary Data
in Communication Systems
11-1: Digital Codes
11-2: Principles of Digital Transmission
11-3: Transmission Efficiency
11-4: Basic Modem Concepts
11-5: Wideband Modulation
11-7: Error Detection and Correction
11-8: Protocols
11-1: Digital Codes
Need an efficient methods of transmission,
conversion, and reception of digital data.
Early Digital Code (the Morse code)
http://www.glassgiant.com/geek/morse/
Modern Binary Codes
 The most widely used data communication code is the 7-bit
binary code known as the American Standard Code for
Information Interchange (ASCII).
 ASCII code can represent 128 numbers, letters,
punctuation marks, and other symbols.
 ASCII code combinations are available to represent both
uppercase and lowercase letters of the alphabet.
http://www.asciitable.com/
11-2: Principles of Digital Transmission
 Data can be transmitted in two ways:
• Parallel
• Serial
 Data transfers in long-distance communication
 In a serial transmission, each bit of a word is
transmitted one after another.
 Parallel data transmission is not practical for
long-distance communication.
Figure 11-4: Serial transmission of the ASCII letter M
Serial Transmission: Expressing the Serial Data Rate
The speed of data transfer is usually indicated as
number of bits per second (bps or b/s).
Another term used to express the data speed in
digital communication systems is baud.
Baud rate is the number of signaling elements or
symbols that occur in a given unit of time.
A signaling element is simply some change in
the binary signal transmitted.
Bit Rate = Baud x N
where N is the number of bit in one symbol
Figure 11-6: Asynchronous transmission with start and stop bits.
Figure 11-8: Synchronous data transmission.
Example 11-1
A block of 256 sequential 12-bit data words is
transmitted serially in 0.016 s. Calculate
(a) the time duration of 1 word,
(b) the time duration of 1 bit, and
(c) the speed of transmission in bits per second.
Solution:
a. tword = 0.016 / 256 = 0.000625 = 625 (ms)
b. tbit = 625 ms / 12 bits = 52.0833 (ms)
c. bps = 1 / tbit = 19,200 (bps) or 19.2 (kbps)
Line-encoding formats: (a) UPNRZ; (b) BPNRZ; (c) UPRZ; (d) BPRZ; (e) BPRZ-AMI
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Line-Encoding Summary
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11-3: Transmission Efficiency
Hartley’s Law
 The amount of information that can be sent in a given
transmission is dependent on the bandwidth of the
communication channel and the duration of transmission.
 Mathematically, Hartley’s law is
C = 2B
where C is the channel capacity (bps) and B is the channel
bandwidth.
 Hartley’s law for multiple coding levels
C = 2B log2N
where N is the number of different encoding levels per time
interval.
Transmission Media and Bandwidth
The two types of wire cable:
 Coaxial cable has a center conductor surrounded by an
insulator over which is a braided shield. The entire cable is
covered with a plastic insulation.
 A twisted-pair cable is two insulated wires twisted together.
11-4: Basic Modem Concepts
Digital data are transmitted over the telephone
and cable television networks by using
modulation, which are implemented by a
modem, a device containing both a modulator
and a demodulator.
Modems convert binary signals to analog signals
capable of being transmitted over telephone and
cable TV lines and by radio, and then demodulate
such analog signals, reconstructing the
equivalent binary output.
 There are four widely used modem types:
1. Conventional analog dial-up modems.
2. Digital subscriber line (DSL) modems.
3. Cable TV modems.
4. Wireless modems.
Modulation for Data Communication
 The four main types of modulation used in
modern modems are:
1.
2.
3.
4.
Frequency-shift keying (FSK)
Phase-shift keying (PSK)
Orthogonal frequency division multiplexing (OFDM)
(a) Binary signal
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(b) FSK signal
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Noncoherent FSK demodulator
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Figure 11-18: Binary phase-shift keying
BPSK modulator: (a) truth table; (b) phasor diagram; (c) constellation diagram
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BPSK transmitter
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(a) Balanced ring modulator; (b) logic 1 input; (c) logic 0 input
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QPSK modulator: (a) truth table; (b) phasor diagram; (c) constellation diagram
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QPSK modulator
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Modulation for Data Communication: QAM
One of the most popular modulation techniques used
in modems for increasing the number of bits per baud
QAM uses both amplitude and phase modulation of a
carrier.
In 8-QAM, there are four possible phase shifts and
two different carrier amplitudes.
Eight different states can be transmitted.
With eight states, 3 bits can be encoded for each
baud or symbol transmitted.
Each 3-bit binary word transmitted uses a different
phase-amplitude combination.
Figure 11-29: A constellation diagram of a QAM signal.
Spectral Efficiency and Noise
Spectral efficiency is a measure of how fast data can
be transmitted in a given bandwidth (bps/Hz).
Different modulation methods give different
efficiencies.
Modulation
Spectral efficiency, bps/Hz
FSK
<1
BPSK
1
QPSK
2
8-PSK
3
16-QAM
4
Spectral Efficiency and Noise (cont.)
The signal-to-noise (S/N) ratio clearly influences the
spectral efficiency.
The greater the noise, the greater the number of bit
errors.
The number of errors that occur in a given time is
called the bit error rate (BER).
The BER is the ratio of the number of errors that
occur to the number of bits that occur in a one
second interval.
11-7: Error Detection and Correction
 When high-speed binary data is transmitted over a
 These errors are changes in the bit pattern caused by
interference, noise, or equipment malfunctions.
 The number of bit errors that occur for a given number
of bits transmitted is referred to as the bit error rate
(BER).
Example 11-3
Data is transmitted in 512-byte blocks or packets. Eight
sequential packets are transmitted. The system BER is 2:
10,000 or 2 X 10-4. On average, how many errors can
be expected in this transmission?
Solution:
 8 packets X 512 bytes = 4096 bytes
 4096 bytes X 8 bits = 32,768 bits
 Average number of errors = 32,768 x (2 x 10-4) = 6.5536
Error Detection and Correction (cont.)
 The process of error detection and correction involves
adding extra bits to the data characters to be transmitted.
This process is generally referred to as channel encoding.
 The data to be transmitted is processed in a way that
creates the extra bits and adds them to the original data.
At the receiver, these extra bits help in identifying any
errors that occur in transmission caused by noise or other
channel effects.
 Channel encoding methods fall into to two separate
categories, error detection codes and error correction
codes.
Error Detection
 One of the most widely used systems is known as parity, in
which each character transmitted contains one additional
bit, known as a parity bit.
 The cyclical redundancy check (CRC) is a mathematical
technique used in synchronous data transmission that
effectively catches 99.9 percent or more of transmission
errors.
Odd parity:
10110011
00101001
10101001
00110011
Even parity
Figure 11-51 A parity generator circuit
0
1
1
0
0
1
1
1
1
0
0
0
1
1
1
Error Correction: Block-Check Character
 The block check character (BCC) is also known as a
horizontal or longitudinal redundancy check (LRC).
 It is the process of logically adding, by exclusive-ORing,
all the characters in a specific block of transmitted data.
 The final bit value for each horizontal row becomes one
bit in a character known as the block-check character
(BCC).
Figure 11-54 Vertical and horizontal redundancy checks
(even)
0
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