Transcript Slaid_02 - narod.ru

Series of lectures
“Telecommunication networks”
Lecture#02
Telegraph
communications
Instructor: Prof. Nikolay Sokolov, e-mail: [email protected]
The Bonch-Bruevich Saint-Petersburg State
University of Telecommunications
A bit of history (1)
Chappe’s
telegraph signals
Part of Winter Palace’s facade
with the tower for optical
telegraph from the side of
Admiralty building.
Cabinet of the director of
tsar palace’s telegraph.
Picture by E. Hau.
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 ITU
ITU 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 5unit-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
Analog
Signal
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 message
The information about a specific object A in the instant t0 may be
represented as n-dimensional vector I A (t0 ) . Its coordinates bi (t0 ) ,
measured or obtained by some other method, reflect chosen attributes
of the object Ai (i  1, 2,..., n) . As a rule, there is a certain error
 i (t0 ) equal to a difference between true ai (t0 ) and measured bi (t0 )
values:  i (t0 )  ai (t0 )  bi (t0 ) . The value bi (t0 ) 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 B A (t0 ) about object A in addition to values bi (t0 )
containing useful information, must include:
 destination address – I1 ;
 data necessary for information delivery – I 2 ;
 subsidiary information – I 3 .
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
J (a )  log h
 log h p (a ) .
(2.1)
p(a)
The base of logarithm ( h ) serves as a measure of the information representation
method used for message exchange. It is presumed mostly that h  2 :
J (a)  log 2 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 J (a)  1. This means that information quantity equals to one bit. If p(a)  1,
then information quantity always equals to zero: J (a)  0 .
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:
n
1
 nlog 2 m  35 bit .
J (a )   log 2
(2.3)
m
j 1
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 j  1, m probabilities p(a j ) and corresponding estimates J (a j ) are
evaluated. The message source’s entropy H (a) is defined as an expected value of
the information quantity:
m
H (a )    p (a j )log 2 p (a j ) .
j 1
(2.4)
Transmission of message
Message
source
a (t )
Message –
signal
converter
s (t )
Telecommunications
network
s (t )
Signal –
message
converter
a (t )
Message
receiver
z (t )
Total effect on
signal
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  :
1
B .

Transmission in telegraph networks (1)
inking wheel
line
electric
relay
earth
battery
Transmission in telegraph networks (2)
S(t)
+
+
+
Time
-
-
-
Time
f1
f2
f1
f2
Switching in telegraph networks
Manual switching
Automatic switching
?
by cords
“transfer
of tape”
circuit
switching
message
switching
packet
switching
fax
Telegraphy
communication
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
LN 121
LN 111
Zone 1
RN 11 RN 12
TS
LN 112
TS
LN 123
Zone 3
PN 1
PN 3
LN 122
Zone 2
TS
PN 2
PN – Primary Node, RN – Regional Node, LN – Local Node.
Delivery time of message
Many 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 ( P1 ) and after ( P2 ) reception of information. It is
assumed, that P2  P1 . Then the information profit I , taking into
consideration formula (2.2), can be defined as follows:
I  log 2 P2  log 2 P1 .
(2.6)
Since P2  P1 , the information profit is nonnegative. Another
definition of information profit is based on two entropy values
H1 and H 2 , also defined before and after reception of
information. If there is a possibility of specifying corresponding
risk functions F ( H1 ) and F ( H 2 ) , then the information profit
will be evaluated according to the following formula:
I  F ( H1 )  F ( H 2 ) .
(2.7)
Information value
I (information value)
I1 (t )
I 3 (t )
I 4 (t )
t (time)
I 2 (t )
Disadvantages of telegraphy (1)
Communication effectiveness
2 people at
whiteboard
2 people
on phone
2 people
on mail
Videotape
Audiotape
Paper
Form of communication
Disadvantages of telegraphy (2)
S(t)
Transmitter
1
0
1
Time
undesired signal
Receiver
0
1
1
0
1
Time
Disadvantages of telegraphy (3)
Traffic
Year
1995
2009
The main factor is reduction of demand!
Conclusions
The
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
Telegraph communications
Questions?
Instructor: Prof. Nikolay Sokolov, e-mail: [email protected]