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- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
Data and the
Binary Code System
‘Data’, a plural noun, the term used to describe information which is storedand processed by
is in
computers. It is essential to know how such data are represented electronically before we can
begin to understand how it can be communicated between computers, communication devices
or
(e.g. facsimile machines) other data storage devices. As a necessary introduction to the concept
of ‘digital’ transmission, this chapter is devoted to a description of tha method of representing
textual and numeric information which is called the ‘binary code’.
4.1 THE BINARY CODE
Binary code is a means of representing numbers. Normally, numbers are quoted in
decimal (or ten-state) code. A single digit in decimal code may represent any of ten
different unit values, from nought to nine, and is written as one of the figures 0, 1 ,2, 3,
4, 5, 6, 7, 8, 9. Numbers greater than nine are represented by two or more digits: twenty
for example is represented by two digits, 20, the first ‘2’ indicating the number of ‘tens’,
so that ‘twice ten’ must be added to ‘0’ units, making twenty in all. In a three digit
decimal number, such as 235, the first digit indicates the number of ‘hundreds’ (or ‘ten
tens’), the second digit the number of ‘tens’ and the third digit the number of ‘units’.
The principle extends to numbers of greater value, comprising four or indeed many
more digits.
Consider now another means of representingnumbersusingonlya two-state or
binary code system. In such a system a single digit is restricted to one of two values,
either zero o r one. How then are values of two or more to be represented? The answer,
as in the decimal case, is to use more digits. ‘Two’ itself is represented as the two digits,
one-zero or 10. In the binary code scheme, therefore, 10 does not mean ‘ten’ but ‘two’.
The rationale for this is similar to the rationale of the decimal number system with
which we are all familiar.
43
- 44 DATA AND THE BINARY CODE SYSTEM
In decimal the number one-thousand three-hundred and forty-five is written ‘1345’.
The rationale is
(1 X 103) + (3 X 10’) + (4 X 10) +5
The same number in binary requires many more digits, as follows.
1345(decimal) = 10101000001(binary)
(binary) (decimal)
= 1X 210 1024
+ox 29 + O
+l X28 + 256
+o X 27 + O
+l X26 + 64
+ o x 25 C O
+ o X 24 + O
+o X 23 $ 0
+O X 22 + o
+ox2 + o
+l + l
= 1345
Any number may be represented in the binary code system, just as any number can be
represented in decimal.
All numbers when expressed in binary consist only of O and Is, arranged as a series
s
of binary digits a term which is usually shortened to the jargon
bits. The string of bits of
abinarynumberare usually suffixed witha ‘B’, to denote abinarynumber.This
prevents any confusion that the number might be a decimal one. Thus 41 is written
‘101001B’.
4.2 ELECTRICAL REPRESENTATION AND STORAGE OF
BINARY CODENUMBERS
The advantage of the binary code system is the ease with which binary numbers can be
represented electrically. As each digit, or bit, of a binary number may only be either 0
or 1, the entire number can easily be transmitted as a series of ‘off’ or ‘on’ (some-
times also called space and mark) pulses of electricity. Thus forty one (101001B) could
berepresented as on-off-on-off-off-on, or mark-space-mark-space-space-mark. The
number could be conveyed between two people on opposite sides of a valley, by flashing
a torch, either on or off, say every half second. Figure 4.1 illustrates this simple binary
- USING THE BINARY
CODE TO REPRESENT
TEXTUAL
INFORMATION 45
( Transmitter )
flashing
torch
Figure 4.1 A simple binary communication system
communication system in which two binary digits (or bits) are conveyed every second.
The speed at which the binary code number, or other information can be conveyed is
called the information conveyance rate (or more briefly the information rate). In this
example the rate is two bits per second, which can be expressed also as 2 bit/s.
Figure 4.1 illustrates a means of transmitting numbers, or other binary coded data by
a series of ‘on’ or ‘off electrical states. Transmission of data, however, is not in itself
sufficient to permit proper exchange of information between the computers or other
equipment located at either end of the line; some method of data storage is needed as
well. At the sending end the data have to be stored prior to transmission, and at the
receiving end a storage medium is needed not only for the incoming data, but also for
the computer programmes required to interpret it.
4.3 USINGTHE BINARY CODETO REPRESENT
TEXTUAL INFORMATION
The letters of thealphabetcan be stored transmitted binary
and over coded
communication systems inthesame way as numbers, provided that they have first
been binary-encoded. There are four notable binary codingsystems for alphabetic text.
In chronological order these are the Morse code, the Baudot code (used in Telex, and
also known as international alphabet number 2 IA2), EBCDIC (extended binary coded
- 46 THE DATA CODE
SYSTEM AND BINARY
A N 0
B
C
D
. 0
P
Q
1
2
3
E R 4
F S 5
G T 6
H U 7
I v 8
J W 9
K X
L Y
M 2 ?
Figure 4.2 The Morse code
decimal
interchangecode), and ASCII (American(national) standard
code for
information
interchange, also
known international
as alphabet IA5). Thesefour
coding schemes are now described briefly.
4.4 MORSE CODE
The Morse code system of dots and dashes was for use over key and lamp telegraph
systems. It wasalso used for signalling by heliographand by flag. Itstwobinary
elements are dit and da (dot and dash). Thirty-nine characters were coded, as shown in
Figure 4.2. When transmitting, a short pause is inserted to mark the beginning and end
of each character; and between words there is a longer pause.
As an example of morsecode, we see fromthe figure thatthe word Morse is
transmitted as ‘da da’ (pause) ‘da da da’ (pause) ’dit da dit’ (pause) ‘dit dit dit’ (pause)
‘dit’ (which would be written as --/---l. ./. . .l.).
-
4.5 BAUDOTCODE (ALPHABET IA2)
When the telex system was introduced, the Baudot Code (now called the international
alphabet I A 2 ) was developed,with significant advantagesovertheMorsecodefor
automatic use. Each character is represented by five binaryelements(usually called
mark and space), but seven elements are transmitted in total, because start (space) and
stop (mark)bits are also used. Fixing the number of elements cuts out the need for gaps
or pauses between alphabetic characters, and separate words are delimited without a
break by introducing the space (SP) character (00100). The regular flow of these signals
suits automatic transmitting and receiving devices, and makes them easier to design.
Figure 4.3 illustrates the Baudot code. Thus thesequence of seven bits sent to represent
the letter A are ‘space(start)-mark-mark-space-space-space-mark(stop)’.
- ASCII 41
Character Pattern Character Pattern
Case (figures)
(letters) 5 4 3 2 7 Case
(letters)
(figures) 5 4 3 2 7
A 0 0 0 1 1 Q 1 1 0 1 1 1
B ? 1 1 0 0 1 R 4 0 1 0 1 0
C 0 1 1 1 0 S 0 0 1 0 1
D f 0 1 0 0 1 T 5 1 0 0 0 0
E 3 0 0 0 0 1 U 7 0 0 1 1 1
F ! 0 1 1 0 1 v 1 1 1 1 0
G 84 1 1 0 1 0 W 2 1 0 0 1 1
H 1 0 1 0 0 X l 1 1 1 0 1
I 8 0 0 1 1 0 Y 6 1 0 1 0 1
J (Bell) 0 1 0 1 1 2 1 0 0 0 1
K ( 0 1 1 1 1 Shift (figures to letters) 1 1 1 1 1
L ) 1 0 0 1 0 Shift (letters to figures) 1 1 0 1 1
M 1 1 1 0 0 Space (SP) 0 0 1 0 0
N 0 1 1 0 0 Carriage Return< 0 1 0 0 0
0 9 1 1 0 0 0 LineFeed 0 0 0 1 0
P 0 1 0 1 1 0 Blank 0 0 0 0 0
1 = Mark (Punch hole on paper tape)
0 = Space (No hole)
Figure 4.3 Baudot code (International Alphabet IA2)
The word Baudot would thus be transmitted in Baudot code as:
A
order of B U D 0 T
transmit 10011
11000
11100 10010 00011 00001
In passing it is also worth mentioning that the term Baud is commonly used in data
communications as the unit of rate of signal change on the line transmission medium
(the so-called Baud rate). Telex networks usually operate at a rate of 50 Baud (50 signal
changes per second) and they use the Baudot code. As 5 line state changes (from mark-
to-space, space-to-mark, space-to-space or mark-to-mark) are required to convey each
character, this produces an informationrate of 50 divided by 5, that is to say 10
alphabetic characters per second, which incidentally corresponds roughly to ordinary
human speech, when we are speaking or reading deliberately.
4.6 ASCII
With the advent of semi-conductors and the first computers, 1963 saw the development
of a new seven-bit binary code for computer characters. This code encompassed a wider
character range, including not only the alphabetic and numeric characters but also a
range of new control characters which are needed to govern the flow of data in and
around the computers. The code, named ASCII (pronounced ‘Askey’) is now common
in computersystems. The letters stand for American (National) Standard Code for
Information Interchange. It is also known as International Alphabet number 5 (IA5)
and is defined by ITU-T recommendation T.50. Figure 4.4 illustrates it.
- DATA AND THE BINARY CODE SYSTEM
of
Note that thebit numbers 1-7 (top left-hand corner the table) represent the least to
themostsignijicantbits, respectively. Eachletter,however, is usually writtenmost
significant bit (i.e. bit number 7) first. Thus the letter C is written ‘1000011’. However,
to confuse matters further, the least significant bit is transmitted first. Thus the orderof
transmission for the word ‘ASCII’ is
(1) (1) (C> (S) (A) order
1001001 1001001 1000011 1010011
1000001 of
last
Figure 4.4 The ASCII code (International Alphabet IA5)
- The characters, may need not be transmitted directly in the formof the 35 bits shown,but
are usually separated by other control characters. In particular delimiting bits, so-called
start and stop bitsmay be used to separatethe strings representing individual characters.
We shall return this subject in Chapter 9 when discussing asynchronous and synchronous
to
transmission methods. Other control characters are also used by modern computer soft-
ware to control the formatting of text (e.g. in Microsoft’s Word format).
4.7 EBCDIC
EBCDIC or extended binary coded decimal interchange code is an extension of ASCII,
giving more control characters. It uses an 8-bit representation for each character, as
shown in Figure 4.5, and is widely used in IBM computers and compatible machines.
4.8 USE OF THE BINARY CODE TO CONVEY GRAPHICAL IMAGES
Besides representing numerical and alphabetical (or textual) characters, the binary code
can also be used to transmit pictorial and graphical images as well as complex computer
information and formatting.
Pictures are sentbinary
as information by sending (typically) S-bit numbers
(representing a value between 1 and 256) to represent the particular colour and shade of
a miniscule dot, making up a part of the picture. Put all the coloured dots together
again in the right pattern (like an impressionist painting) and the picture reappears.
This is the principle by which computer images are communicated.
Send a series of pictures, one after the other at a rate of 25 Hz (25 picture frames per
second) and you have a television or video signal. Alternatively, if you are willing to
tradesome of thedynamicquality of thepicture thus
(and cost), there
then is
videorelephony and videoconferencing, a television-like signal sent over telephone-type
connections. ITU-T recommendation H.261 lays down a standard for conversion of a
video signal to binary code. To illustrate the principles of graphic image transfer using
the binary code, we next take the example of facsimile.
4.9 FACSIMILE
Facsimile machines work in pairs, separated by some form of transmission link. At the
transmitting end of the link, one facsimile machine scans a piece of paper, and converts
the black-and-white image which it sees into a binary-coded stream of data. This data is
then transmitted to the receiving facsimile machine, where it is used to produce a black-
and-white facsimile reproduction of the original paper image. The working principle of
these machines is simple enough, as we may now see.
The image on the original is assumed to be composed of a very large number of tiny
dots, arranged in a grid pattern on the paper. Figure 4.6, for example, shows how one
word on the paper may be broken down into a grid of dots.
- 50 SYSTEM CODE BINARY AND THE
DATA
I
IX
z
>
f
c
I
n
o r -
c r -
0 0
c c
- FACSIMILE 51
Figure 4.6 Facsimilescanninggrid
The image is reproduced by making a copy of that same grid pattern of dots at the
receiving end. The procedure is as follows. Starting at the top left-hand corner, the
transmitting facsimile machine scans the original paper document from left to right,
following each line of the grid in turn. At the end of each line, the machine returns to
the left-hand side of the grid, and moves down to the line below. Each line scanned is
transposed by the machine into a string of binary coded data, comprising a series of
variable length codewords. code
Each word representsnumber
a of consecutive
squares, or ‘runs’, along the horizontal row of the grid, either an all-black run or an
all-white one.
White runs and black runs necessarily alternate, as these are the only two colours
distinguishable by the scanning device. A small section of the grid is shown in Figure 4.6.
For A4 paper, 1728 small picture elements represent one scan a horizontal rowof the
of
grid, some 21 5 mm in length. (In other words, there are around64 dots, termed picture
elements (pixels),per square millimetre). The datasent to represent each line of the grid
are thus in the form ‘two white, three black, ten white, two black, etc., etc.’, describing
the colours of each consecutive picture element along the row. The end of the row is
indicated in the data stream by a terminating code word. Each string of data, corres-
ponding to one horizontal scan of the grid, starts with the assumption that the first
colour on the left-hand sideis going to be ‘white’ by indicating the white run length. This
allows the receiver always to be in the correct colour synchronization at the beginning
of the line. If, as frequently, the new line starts with a black picture element, then the
initial signal will be ‘white run length of zero elements’. Figure 4.7 shows a small section
of two consecutive runs, as a way of explaining the coding method.
Starting on line 1 of Figure 4.7, the scanning and transmitting facsimile machine
sends a string of data saying ‘white-run length, one; black-run length, one; white, one;
black,four; white, four;black, two; white. . .end of line’. For thesecond line, the
transmitting facsimile machine carries on ‘white-run length, zero; black, two; white,
one; back, six, etc., etc.). At the receiving end, the second facsimile machine slavishly
prints out a corresponding series of black and white picture elements, which reproduce
Figure 4.7 Facsimilescanningandcoding
- 52 CODE
AND BINARY
DATA THE SYSTEM
Figure 4.8 Facsimile terminal. A group 3 facsimile terminal receiving an incoming document.
Typically around 25-60 seconds is required to transmit one page, though darker documentsmay
take longer (Courtesy of British Telecom)
the original image. Returning to Figure4.6, we see how the image of the word ‘paper’
has been coded for transmission and subsequent reproduction with the aid of the
scanning grid.
Notsurprisingly, facsimile machinesactually slightly sophisticated
use more
techniquesthanthosedescribed,buttheprinciplesarethesame.Thepurpose of
these enhancements to the basic technique to improve the accuracy and overall speed
is
of transmission and so reduce the time reluired for conveying each paper sheet.
Any type of image can conveyed using facsimile machines: typed text, manuscript,
be
pictures and diagrams. The scanning and image reproducing machinery works in the
same way for all of them.
Since 1968, when recommendations for CCITT’s first Group I standard apparatus
were published, various generations of facsimile machines have been developed. The
latest Group 4 facsimile machines produce extremely high quality pictures, and can
transmit a page of A4 in a few seconds, as compared with the minutes that group 1
six
apparatus took over the same job.
4.10 DIGITAL TRANSMISSION
Nowadays most data and much other information are communicated in one or other of
the binary coded forms and the ability to sendall sorts of information simultaneously
- DIGITAL TRANSMISSION 53
over a single network has led to multimedia communication and computing. This is the
term applied to sound, video and data signals transferred simultaneously. As binary
coded data are transmitted as a sequence of ‘on’ or ‘off’ states, with each ‘on’ or ‘off’
representing the value ‘1’ or ‘0’ of consecutive binary digits or bits, all information is
conveyed essentially as a string of digits, and so the process has acquired the name
digitaltransmission. We goon now to assessitsconsiderableadvantagesoverthe
analogue technique, and how it can be extended to speech and other analogue signals.
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