Transcript Transmission of Binary Data

EE302 Lesson 20:
Transmission of Binary
Data in Communication
Systems
Topics Covered in Chapter 11
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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
11-1: Digital Codes
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The proliferation of applications that send digital
data over communication channels has resulted
in the need for efficient methods of transmission,
conversion, and reception of digital data.
Digital codes have evolved as technology has
advanced.
11-1: Digital Codes
Early Digital Codes
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The first digital code was
developed by Samual
Morse.
The Morse code was
originally designed for
wired telegraph, but was
later adapted for radio
communication.
The Morse code consists of
a series of “dots” and
“dashes” that represent
letters of the alphabet,
numbers, and punctuation
marks.
Figure 11-1 The Morse Code
11-1: Digital Codes
Baudot Code
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The Baudot (baw dough) code was one
of the first alphanumeric codes
developed in the early days of teletype
machines.
The Baudot code is a 5-bit code giving
it 25 or 32 possible values (it actually
had 52 symbols using a control
character).
It is obsolete and of historical interest
only.
11-1: Digital Codes
Baudot Code
11-1: Digital Codes
ASCII
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Binary representation of alphanumeric symbols (letters,
numbers, punctuation, etc.) are given by American
Standard Code of Information Interchange (ASCII) code.
Each ASCII codeword is 7-bits long yielding 27 or 128
possible characters.
ASCII has remained the international standard in data
communications.
ASCII
Figure 11-3 The ASCII Code
11-1: Digital Codes
Modern Binary Codes: Extended Binary Coded
Decimal Interchange Code
 The
Extended Binary Coded Decimal Interchange
Code (EBCDIC) was developed by IBM.
 The EBDIC is an 8-bit code allowing a maximum of
256 characters to be represented.
 The EBCDIC is used primarily in IBM and IBMcompatible computing systems and is not widely used
as ASCII.
11-2: Principles of Digital Transmission
Serial Transmission
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As discussed earlier, data can be transmitted in two
ways:
1.
2.
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Parallel: all bits transmitted simultaneously
Serial: all bits transmitted one after another
Data transfers in long-distance communication
systems are made serially. Parallel data
transmission is not practical.
The LSB is transmitted first and the MSB is
transmitted last.
Each bit is transmitted for a fixed interval of time, t.
11-2: Principles of Digital Transmission
Figure 11-4: Serial transmission of the ASCII letter M.
11-2: Principles of Digital Transmission
Serial Transmission: Expressing the Serial Data Rate
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The speed of data transfer is usually indicated as number of bits
per second (bps or b/s).
The speed in bps is the reciprocal of the bit time, t.
bps = 1/t.
Example: if bit time is 104.17 µs, bps=1/104.17µs = 9600 bps
Another term used to express the data speed in digital
communication systems is baud rate.
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. In many cases it is a binary logic voltage level
change, either a 1 or a 0.
11-2: Principles of Digital Transmission
Serial Transmission: Expressing the Serial Data
Rate
 With
the new modulation schemes (discussed later),
multiple bits can be transmitted with one symbol.
Now,
Bit rate = baud rate x bits per symbol
or
Bit rate = baud rate x log2S,
where S = number of states per symbol.
 These modulation schemes were developed to
improve transmission rates over bandwidth-limited
communication channels, such as the telephone
lines.
11-2: Principles of Digital Transmission
Asynchronous Transmission
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In asynchronous transmission each data word is
accompanied by start and stop bits that indicate the beginning
and ending of the word.
When no information is being transmitted, the communication
line is usually high, or binary 1.
In data communication terminology, this high level is referred to
as a mark.
To signal the beginning of a word, a start bit, a binary 0 or space
is transmitted.
The change from ‘mark’ to ‘space’ indicates the beginning of a
word.
11-2: Principles of Digital Transmission
Figure 11-6: Asynchronous transmission with start and stop bits.
11-2: Principles of Digital Transmission
Asynchronous Transmission
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Asynchronous transmissions are extremely reliable.
Most low-speed digital transmission (the 1200- to
56,000-bps range) is asynchronous.
The primary disadvantage of asynchronous
communication is that the extra start and stop bits
effectively slow down data transmission.
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The extra start and stop bits are called ‘overhead’ and
reduce efficiency
11-2: Principles of Digital Transmission
Synchronous Transmission
 The
technique of transmitting each data word one
after another without start and stop bits, usually in
multiword blocks, is referred to as synchronous data
transmission.
 To maintain synchronization between transmitter and
receiver, a group of synchronization bits is placed at
the beginning and at the end of the block.
 Each block of data can represent hundreds or even
thousands of 1-byte characters.
11-2: Principles of Digital Transmission
Synchronous Transmission
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The special synchronization codes at the beginning and end of a
block represent a very small percentage of the total number of
bits being transmitted, especially in relation to the number of
start and stop bits used in asynchronous transmission.
Synchronous transmission is therefore much faster than
asynchronous transmission because of the lower overhead.
An error detection code usually appears at the end of the
transmission (discussed later).
Synchronous transmission uses a precise clock to track the
individual bits.
11-2: Principles of Digital Transmission
Figure 11-8: Synchronous data transmission.
11-2: Principles of Digital Transmission
Encoding Methods
 Whether
digital signals are being transmitted by
baseband methods or broadband methods, before the
data is put on the medium, it is usually encoded in
some way to make it compatible with the medium.
11-2: Principles of Digital Transmission
Encoding Methods
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In the nonreturn to zero (NRZ) method of encoding, the signal
remains at the binary level assigned to it for the entire bit time.
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Normally used at slow speeds, when asynchronous transmission is
being used.
Since there is no voltage change when there are long strings of 1’s
and 0’s transmitted, it is difficult for the receiver to determine where
one bit begins and ends.
In return to zero (RZ) encoding the voltage level assigned to a
binary 1 level returns to zero during the bit period.
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Because there is clearly one discernible pulse per bit, it is extremely
easy to derive the clock from the transmitted data.
11-2: Principles of Digital Transmission
Encoding Methods
 Manchester
encoding, also referred to as biphase
encoding, is widely used in LANs.
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In this system a binary 1 is transmitted first as a positive
pulse, for one half of the bit interval, and then as a negative
pulse for the remaining part of the bit interval.
A binary 0 is transmitted first as a negative pulse, for one half
of the bit interval, and then as a positive pulse for the
remaining part of the bit interval.
 The
choice of an encoding method depends on the
application
11-2: Principles of Digital Transmission
Figure 11-9
Serial binary
coding methods
Unipolar NRZ
Bipolar NRZ
Unipolar RZ
Bipolar RZ
Bipolar RZ-AMI
Manchester
11-3: Transmission Efficiency
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Transmission efficiency is the accuracy and
speed with which information, whether it is voice
or video, analog or digital, is sent and received
over communication media.
It is the basic subject matter of the field of
information theory.
11-3: Transmission Efficiency
Transmission Media and Bandwidth
 The
two most common types of media used in data
communication are wire cable and radio.
 The two types of wire cable used:
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Coaxial cable: usable bandwidth 200 MHz-3 GHz depending
on the size. Bandwidth decreases with length.
Twisted-pair cable: usable bandwidth 2 KHz-100 MHz.
 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.
11-3: Transmission Efficiency
Coaxial Cable
Twisted Pair
Figure 11-10 Types of cable used for digital data transmission
11-3: Transmission Efficiency
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The radio channel bandwidth must be wide enough to
pass all harmonics and preserve the waveshape.
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If the higher harmonics are filtered out, the signal will be
distorted.
Hartley’s Law
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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
(Hz). Assuming there is no noise in the system.
11-3: Transmission Efficiency
Hartley’s Law
 The
greater the number of bits transmitted in a given
time, the greater the amount of information that is
conveyed.
 The higher the bit rate, the wider the bandwidth
needed to pass the signal with minimum distortion.
Example: The maximum theoretical bit capacity for a 10
kHz bandwidth channel is
C = 2B = 2(10,000 Hz) = 20,000 bps
11-3: Transmission Efficiency
 The
encoding method used also effects the required
bandwidth for a given signal.
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The bandwidth requirement for an RZ scheme is twice that
for an NRZ scheme.
Multiple Coding Levels
 Channel
capacity can be increased by using multiplelevel encoding schemes that permit more bits per
symbol to be transmitted (Section 11-4).
11-3: Transmission Efficiency
Impact of Noise in the Channel
 Increasing
bandwidth increases the rate of
transmission but also allows more noise to pass.
 Shannon-Hartley Theorem determines channel
capacity in the presence of noise.
Shannon-Hartley Theorem
C = B log2(1 + S/N)
C = Channel capacity, bps
B = bandwidth, Hz
S/N = signal-to-noise ratio (power)
11-3: Transmission Efficiency
Example Problem 1
Find the channel capacity for a voice grade
telephone line with a bandwidth of 3100 Hz and
a S/N ratio of 30 dB (dB = 10 log P)?
11-3: Transmission Efficiency
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This answer conflicts with Hartley’s Law
C = 2B = 2(3100 Hz) = 6200 bps
Shannon-Hartley Theorem determines what is
theoretically possible. But, multilevel coding is
required to achieve these higher rates.