Lecture#02

Telegraph communications

The Bonch-Bruevich Saint-Petersburg State University of Telecommunications

Series of lectures “Telecommunication networks”

Instructor: Prof. Nikolay Sokolov, e-mail: sokolov@niits.ru

A bit of history (1)

Cabinet of the director of tsar palace’s telegraph.

Picture by E. Hau.

Part of Winter Palace’s facade with the tower for optical telegraph from the side of Admiralty building.

Chappe’s telegraph signals

A bit of history (2)

1890-th years.

Cossack’s squad is guarding the telegraph line (from “Illustrated history of equitation", published in Paris in 1893)Picture by Louis Valle (1856–1940)

The main element of the telegraph set

Telegraphy and ITUITU was International Telegraph Union (in 17/05/1865)

World Telecommunication Day has been celebrated annually on 17 May since 1969, marking the foundation of ITU and the signing of the first International Telegraph Convention in 1865. It was instituted by the Plenipotentiary Conference in Malaga-Torremolinos, Spain, in 1973.

In November 2006, the ITU Plenipotentiary Conference in Antalya, Turkey, decided to celebrate  both events on 17 May as World Telecommunication and Information Society Day.

Telegraph set (1860)

Telegraph set (1900)

Telegraph set (1960)

This set was installed for hot line “Moscow – Washington” (1960)

Message on the paper tape

Paper tape with holes representing the "Baudot Code"

International telegraph alphabet №2 Source: ITU-T Recommendation S.1

The coded character set of ITA2 is based on a 5-unit-structure.

Condition A corresponds to start polarity, no perforation in paper tape and symbol 0 of the binary notation.Condition Z corresponds to stop polarity, perforation in paper tape and symbol 1 in the binary notation.

Teleprinter

Keyboard of a teleprinter using the Baudot code

Telegraph switching

Telegraph lines

Chain: information – message – signal

Information Message Signal

Analog Discrete

Analogue signal can be represented by continuous functions. A typical example is voice transmission in the period, when a subscriber is talking. A discrete signal is a group of elements, belonging to the finite set. A typical example is a telegram, which consists of letters, digits and ordering information.

Information and messageThe information about a specific object A in the instant 0t may be represented as n-dimensional vector 0( )AI t . Its coordinates 0( )ib t , measured or obtained by some other method, reflect chosen attributes of the object ( 1,2,..., )iA i n . As a rule, there is a certain error

0( )i t equal to a difference between true 0( )ia t and measured 0( )ib t values: 0 0 0( ) ( ) ( )i i it a t b t . The value 0( )ib t may be represented by a number (e.g. 17), by a range of the investigated value changes (e.g. from 14 to 19), and also by words. Message 0( )AB t about object A in addition to values 0( )ib t containing useful information, must include:

destination address – 1I ; data necessary for information delivery – 2I ; subsidiary information – 3I .

Information quantity (1)

Information quantity a contained in message is evaluated by the probability of its appearance – ( )p a . In that case, a high-probability message contains insignificant quantity of new information. Significant quantity of new information resides in low-probability messages. Generally, the information quantity – ( )J a is estimated by logarithm of the value inverse to probability ( )p a :

1( ) ( )( )h hJ a log log p a

p a . (2.1)

The base of logarithm (h ) serves as a measure of the information representation method used for message exchange. It is presumed mostly that 2h :

2( ) ( )J a log p a . (2.2) The binary unit of information which can take only two values (e.g. zero or one) is called bit. If the probabilities of appearance of those two values are 0.5 and 0.5 then ( ) 1J a . This means that information quantity equals to one bit. If ( ) 1p a , then information quantity always equals to zero: ( ) 0J a .

Information quantity (2)

As an example, we will estimate information quantity in a word of seven letters on conditions that the alphabet contains 32 letters. We shall suppose, that all the probabilities of every letter's appearance are the same. Then:

2 21

1( ) 35n

jJ a log nlog m bit

m . (2.3)

It is essential to estimate the message source’s informational characteristics as a whole for solving the number of problems. An average value of information quantity associated with one message is used as such an estimate. This value is called the entropy of message source. In a text frequency of appearance of the different letter varies considerably. So, for all values 1,j m probabilities ( )jp a and corresponding estimates ( )jJ a are evaluated. The message source’s entropy ( )H a is defined as an expected value of the information quantity:

21

( ) ( ) ( )m

j jj

H a p a log p a

. (2.4)

Transmission of message

A signal – message converter allows receiving of the letters and digits combination, which is reproduced on a printing device or on a graphic display. The speed of telegraphy is inversely proportional to the impulse duration :

1B

.

Message source

Message – signal

converter

Telecommu- nications network

Signal – message converter

Message receiver

Total effect on signal

( )a t ( )s t ( )s t ( )a t

( )z t

Transmission in telegraph networks (1)

earth

battery

lineinking wheel

electric relay

Transmission in telegraph networks (2)

+ +

- - -

+

S(t)

Time

Time

f1 f1 f2f2

Switching in telegraph networks

Manual switching Automatic switching

by cords “transferof tape”

circuit switching

message switching

packet switching

?

Telegraphy communication

fax

data

SMS

Telecommunication system

Customer Premises Network

Access Network Core Network Service Nodes

Responsibility of the Telecom Operator

This model is proposed by ITU-T for the GII (Global Information Infrastructure). On the other hand, this model is useful for any telecommunication network.

Telegraph network structure

Zone 1

Zone 2Zone 3

PN 2PN 3

PN 1

RN 11 RN 12

LN 121

LN 122

LN 123

LN 111

LN 112

TS

TS

TS

PN – Primary Node, RN – Regional Node, LN – Local Node.

Delivery time of messageMany factors, related to technical and economical reasons, are accounted in the selection of messages delivery time. The profit made on information also has an important role. The term "Information profit" isn’t easy to define from a formal point of view. The following approach seems appropriate. Two probabilities of the necessary goal achievement are defined: before ( 1P ) and after ( 2P ) reception of information. It is assumed, that 2 1P P . Then the information profit I , taking into consideration formula (2.2), can be defined as follows:

2 2 2 1I log P log P . (2.6) Since 2 1P P , the information profit is nonnegative. Another definition of information profit is based on two entropy values

1H and 2H , also defined before and after reception of information. If there is a possibility of specifying corresponding risk functions 1( )F H and 2( )F H , then the information profit will be evaluated according to the following formula:

1 2( ) ( )I F H F H . (2.7)

Information value

I (information value)

t (time)

1( )I t

2 ( )I t

3( )I t 4 ( )I t

Disadvantages of telegraphy (1)

Form of communication

Com

mun

icat

ion

effe

ctiv

enes

s

2 people at whiteboard

2 people on phone

2 people on mail

Videotape

AudiotapePaper

Disadvantages of telegraphy (2)

Time

Time

S(t)

1 10

Transmitter

Receiver

undesired signal

1 1

0

0 1

Disadvantages of telegraphy (3)

The main factor is reduction of demand!

Year

Traffic

1995 2009

ConclusionsThe decrease of demand for telegraph communications services and the growth of modern services demand is a worldwide tendency. In many countries the problem of supporting public telegraph network services is solved in a variety of ways. For example, in the Holland telegraph communication has ceased functioning in 2004. In January, 2006 the oldest American national operator Western Union has announced a total closure of the telegraph service. At the same time in Canada, Belgium, Germany, Japan some companies still support services on sending and delivering traditional telegraph messages.

History

Instructor: Prof. Nikolay Sokolov, e-mail: sokolov@niits.ru

Questions?

Telegraph communications