ecx4233 - session 11 - telecommunication switching principles - introduction to telephone traffic i...

ECX4233 - Session 11 - Telecommunication Switching Principles - Introduction to Telephone Traffic I

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Session 11 Telecommunication Switching Principles - Introduction to Telephone Traffic-I

Contents

2.1 Characteristics of Telephone Traffic

2.1.1 Busy Hour

2.1.2 Busiest Hour

2.1.3 Calling Rate

2.1.4 Traffic Intensity

2.1.5 Holding Time

2.1.6 Traffic Volume

Aim

To introduce the term "Telephone Traffic" and to discuss;

(i) Characteristics of telephone traffic.

(ii) Definitions of basic terms used in telephone traffic.

(iii) Simple calculations in telephone traffic.

Objectives

At the end of this Session, you will be able to:

(i) Describe what is meant by telephone traffic and the general characteristics

of telephone traffic.

(ii) Write down the definitions of busy hour, busiest hour, calling rate, traffic

intensity, traffic volume and holding time.

(iii) Perform simple calculations involving traffic intensity, calling rate, holding

time etc.

Introduction

As motor traffic relates to the number of motor vehicles passing on a network of

roads, 'Telephone Traffic' relates to the number of telephone calls passing in a

network of telephone circuits.

The traffic handled by a telephone network can be regarded as the aggregate of

telephone calls in progress in the network at a given duration of time.


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A study of telephone traffic is important as it assists in providing the optimum

amount of exchange equipment, at a given cost, for a particular level of satisfaction

of the telephone user.

2.1 Characteristics of Telephone Traffic

The need to originate a telephone call depends on a variety of factors. The need

may be of business nature, social nature or even it could be an emergency.

Therefore, calls may arrive at a switching system (telephone exchange);

(i) quite at random,

(ii) in a more or less a regular pattern or

(iii) in bursts, and

These could be separated by intervals during which no calls are made. However,

there are some common factors that affect human activities, and which in turn

cause certain predictable variations in telephone traffic. These variations can be

broadly classified as follows:

(a) Instantaneous variations:-

The traffic varies from instant to instant (from moment to moment) as calls are

originated or terminated.

(b) Hourly variations:-

In a local telephone network, the traffic is generally low during the night, but rises

rapidly in the morning when offices, shops and other business centres open for

daily activities. Traffic decreases during the lunchtime and increases again in the

afternoon. Towards the evening, traffic gradually diminishes with the closing of

offices, business centres etc. In exchanges serving mainly residential areas, an

increase in traffic in evening hours could be observed due to calls of social nature.

S.A.Q

Can you graphically represent the expected hourly variations of traffic on a normal

working day in a telephone exchange where approximately 50% are residential

subscribers?

Figure 2.1 graphically represents a possible pattern of hourly traffic variations in an

exchange where there are an appreciable number of residential subscribers.


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Figure 2.1

S.A.Q.

Could you expect same pattern of hourly variations on a Sunday where the

business or government office activities are a minimum, in an exchange that serves

mainly non-residential subscribers?

In this case, the most probable pattern would be an almost flat curve indicating a

very low value of traffic throughout the day.

(c) Daily Variations:-

Figure 2.2 illustrates possible daily variations of traffic within a normal week. A

falling of traffic can be observed during Saturday & Sunday due to less business

activities.

HOURS OF

THE DAY

0 2 4

TRAFFI

C

6 8 10 12 2 4 6 8 10 12

AM NOON MID NIGHT

PEAK DUE TO

RESIDENTAL TRAFFIC

PEAKS TO DAY TIME

BUSINESS ACTIVITES

PM


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Figure 2.2

(d) Seasonal Variations:-

During the New Year, Christmas or other festive seasons the traffic in a telephone

network is higher than the other times of the year.

(e) Long term Variations

A gradual growth of traffic can be observed over a period of years due to the

increase in population and the proportionate increase in other related activities.

State policies and world economic trends etc will affect the long-term variations,

eg: increase in the fuel prices.

The characteristics of telephone traffic discussed up to now are related to the

distribution of call arrival times.

Besides this, another important characteristic is the distribution of duration of these

calls. Duration of conversations can vary over a wide range from few seconds to

several minutes.

Now we will study the definitions of some commonly used terms in telephone

traffic.

2.1.1 Busy Hour

This is the 60-minute period in a day, during which the traffic is the highest in the

long run. This is also termed the "time-consistent busy hour".

2.1.2 Busiest Hour

This is the 60-minute period in a day during which the traffic is highest. The

busiest hour may occur at different times each day and may not coincide with the

"Busy Hour".

MON TUE WED THU FRI SAT SUN

TR

AF

FIC

BE

TW

EE

N

9 A

M T

O 1

0 A

M.


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S.A.Q,.

From Figure 2.1, roughly estimate the busiest hour and the busy hour.

From Figure 2.1 the busiest hour is roughly the 60-minute period from 9.30 AM to

10.30 AM.

Figure 2.1 illustrates only a particular day's variation of traffic. This alone, is not

sufficient to estimate the busy hour as it is the 60-minute period during which the

traffic is the highest in the long run. If the pattern of variation in Figure 2.1 is the

general pattern of hourly traffic variation in the long run too, the busy hour would

be roughly from 9.30 AM to 10.30 AM.

2.1.3 Calling Rate

This is defined as the average number of calls originated per line during a given

period. If the time period considered is the busy-hour, it is called the "busy-hour

calling rate". Therefore, if a subscriber, on average, originates 5 calls during the

busy-hour, the busy-hour calling rate would be 5 calls per hour. If the period

considered is one day then it is known as "day calling rate."

The calling rate varies significantly between residential and business users.

Therefore, the average calling rate for an exchange serving a high percentage of

business subscribers, will be substantially higher than for an exchange serving

mostly residential subscribers.

In certain Instances, a distinction is made between the "originating calling rate" &

the "terminating calling rate". Originating calling rate is same as the above defined

"calling rate" while the "terminating calling rate" is the average number of calls

terminated per line during a time unit.

2.1.4 Traffic Intensity

Traffic intensity is defined as the average number of simultaneous calls during a

given period of time.

If there are n simultaneous Calls during a time period T, and if t1, t2, t3, .........., tn

are the individual duration of these calls expressed in the same unit of time, then

the traffic intensity denoted by A will be :

T

t........tttA n321 ++++

=

A is dimensionless as the denominator and the numerator in the above equation

consist of dimensions of time only.

A is expressed in the unit of "Erlang".

If h is the average duration of a call,


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Then

Erlangs T

hnA

nht........tt ti.e.

n

t........ttth

n321

n321

×=∴

=++++

++++=

Usually, the time period T considered, is taken to be an hour and h, the average

duration of a call, is expressed in hours.

n = 1 Call

h = 1 hour, and

T = 1hour.

Then,

Erlang. 1

Hourper Hour Call 11

11A

=

=×

=

Therefore, if a circuit is occupied by a call lasting for an hour the circuit is said to

have carried a traffic intensity (or more commonly, traffic ) of 1 Erlang.

S.A.Q.

(a) In an exchange, averages of 15 calls per line are originated during the 24

hours of a day. Out of these 15 calls, 4 are originated during the busy hour.

What are the 'day calling rate' & 'the busy-hour calling rate'?

(b) 600 successful calls each having an average duration of 2 minutes are

carried over a group of circuits. How much is the traffic carried by the

group of circuits?

(a) Calling rate (or the originating calling rate) is the average number of calls

originated per line during a time unit. This time unit could be a day, an

hour a sec etc.

In the given question Day calling rate = 15 calls/ day/ line

Busy-hour calling rate = 4 calls/ busy hour/ line

(b) Traffic carrier by the group of circuits

= Traffic Intensity

Erlangs. 20

1

)60/2(600

hour) one as(taken period Time

hours(h)in call a ofduration Average(n) ofcalls .No

=

×=

×=


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Another unit of the traffic intensity is the "CCS-Century Call Second "which is

particularly used in the United States of America. In this case,

Hourper secods calls Hundred

1

1(100/100)Intensit Traffic

Hour, 1T

and sec, 1t

calls 100n

When

Hour) 1lly Hours(usuain considered periodT

secondsin call a ofduration averaget

calls of non

Where

T

ccst (n/100)intensity Traffic

=

×=

=

=

=

=

=

=

×=

ccs

Therefore, if an exchange device is occupied by 100 calls where the average

duration of a call is 1 sec, during a period of one hour, the device is said to have

carried a traffic of 1 CCS.

S.A.Q.

Can you find the relationship between the two units of traffic intensity we

discussed "Erlang" and "CCS"?

1 Erlang =1 call Hour/ Hour

=3600 call sec / Hour

=36 ×100 calls Seconds / Hour

=36 CCS (as i CCS = 100 calls seconds per Hour)

2.1.5 Holding Time

In determining to what extent, exchange equipment are utilised during a particular

period, the number of calls originated and / or terminated in a given time is not

sufficient. It is equally important to know the durations of these calls (or the

average duration of a call) and the durations of various switching equipment are

occupied during the period considered.

The total time during which the exchange equipment and circuits are held when a

call is made, includes the period the call is in process of being established the

conversation time, and the time taken to release the circuit from use after the call

has been terminated.

The length of this overall period is known as the "holding time" of a call. It

depends on the type of call (a call within the same exchange, a call to a distant


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exchange etc.) and the type of exchange equipment (Step by Step type exchange

equipment, Cross-Bar type equipment etc.) involved in the call.

On the other hand, the period a particular switch (a Register, a Sender etc) is

occupied, while carrying out its function in connection of a telephone call, is

known as the "holding time" of the switch concerned.

In the dimensioning of an exchanges, (estimating quantities of various types of

switching equipment and circuits to be provided in an exchange) the average

holding times of different types of calls and the average holding times of different

functional units (Registers, Senders etc.) are of utmost importance.

S.A.Q.

Table 1 shows the measured duration of 10 local calls (calls within the same local

exchange area) in an exchange. Calculate the average holding time of a local call,

from this data.

Table 1

Type of call Local Calls

Duration of each

call in Seconds

102 96 113 81 84 106 98 110 92 88

Average time of a local call

Sec 97

10

88)9211098106848111396(102

=

+++++++++=

In actual practice, the durations of large number of calls of similar type over a long

period are considered in arriving at the average holding time of that particular type

of call.

2.1.6 Traffic Volume

During a particular period, if there are n number of calls with average holding time

h,

then the Traffic volume is defined as;

Traffic volume = n× h call seconds (If h is expressed in seconds.)

Traffic volume (or volume of traffic) is a measure of total equipment usage in a

telephone exchange.

The 'traffic intensity" that we discussed previously in this session is, the "traffic

volume per unit time" as shown below.


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unit time,per volumeTraffic

h)n Volume Traffic (as

period

volumeTraffic

Period

(h) timeholding Average(n) calls of no. intensity Traffic

=

×=

=

×=

where the unit time is usually taken as an hour.

S.A.Q.

During the busy hour, a particular group of circuits in a telephone exchange carried

300 calls. If the average holding time of a call is 2 minutes, calculate

(a) the volume of traffic,

(b) the traffic intensity,

(c) the average number of calls, carried during the average holding time.

(d) the average number of calls in progress simultaneously.

(a) The volume of traffic ={no. of calls (n)}×{Average holding time (h)}

=300 × 2 call minutes

=(300 × 2)/60 call hours

=10 call hours

(b) The traffic intensity

=considered Period

call(h) a of time)hold average(or duration averagecalls(n) of no. ×

=hours 1

hours) 60/2300( ×

=10 Erlangs

We have already seen that,

traffic intensity = traffic volume per unit time

In part (a) above, the traffic volume of 10 call hours is during the busy hour, ie

during a unit time of 1 hour. Therefore, in this case, traffic volume (10 call hours)

is equal to the intensity (10 Erlangs).


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(c) During the busy hour(60min), no of calls carried = 300

(d)

∴Average no of calls carrier per minute = min/60

300calls

Average no of calls carried during the average holding time of 2 minutes

calls. 10

60

2300

=

×=

(e) Let Ns be the average no. of calls in progress simultaneously.

This means, Ns number of circuits will be fully occupied for one hour.

∴Traffic volume = Ns × 60 call minutes

∴But, during this one hour period, 300 calls, each of 2 minutes duration are

carried.

Traffic volume = minutes call 2300 ×

calls 10

60

2300N

230060N

s

s

=

×=∴

×=×∴

∴ Average number of calls in progress simultaneously = 10 calls

The (c) part of the above answer reveals an important relationship. Did you notice

that?

(i) In (c) part; average no of calls during the

average holding time

(10 calls)

= Traffic Intensity

(10 Erlangs)

This is an alternative way of expressing the traffic intensity.


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S.A.Q.

By observing a group of circuits at intervals of one minute over a period of 10

minutes, the number of simultaneous calls recorded were; 9, 7,7,8,6,4,7,9,5,7,8.

Find the traffic through the group of circuits.

Here, the traffic means the traffic intensity.

Erlangs 7

11

77

11

87597468779

recorded werensobservatio the timesof No.

calls ussimultaneo total theof Sum

calls usSimultaneo of no. Averageintensity Traffic

=

=

++++++++++=

=

=

In the next Session we will discuss the concept of Availability, Grade of Service

and the relationship between traffic offerred, traffic carried and traffic lost.

ecx4233 - session 11 - telecommunication switching principles - introduction to telephone traffic i...

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