traffic forecasting & network planning - lec 05

8/8/2019 Traffic Forecasting & Network Planning - Lec 05

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Traffic Forecasting & Network

Planning

Lec 05

Kamran Nadeem

[email protected]


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Trunking

A means for providing access to users a channel from the available pool

on demand

With this a technique a small number of channels can accommodate a

large number of random users

Telephone companies use trunking theory to calculate the number of

channels/circuits required for a population

Limited servers

Large population


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Terminologies

Traffic intensity is measured in Erlangs

1 Erlang = Traffic in a channel when it is completely occupied

0.5 Erlang = Channel is busy 30 minutes in a one hour period

Set-Up time: Time to allocate a channel to a user

Blocked Call: Call which cannot be completed at the time of request due

to congestion in the network

Lost Call: Same as blocked

Holding Time (H): Duration of a typical call

Load: Traffic intensity across the whole system

Request rate (): Average number of call requests in a unit time


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Quality of Service (1)

A network cannot be dimensioned for the worst case peaks. Then,occasionally the requested service is not available or the quality of theservice is reduced

The dimensioning has to made according to the stated (statistical) criteriafor the quality of service

grade of service (GoS): quality at the call level (e.g. telephone network) quality of service (QoS): quality during a connection or session (e.g. ATM

network)

In a telephone network a call that cannot be immediately carried may be blocked: loss system

may have to wait (ringing tone): waiting system

The GoS requirement loss system: P (call is blocked) < x %

waiting system: P (waiting time > z seconds) < x %


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Loss system

Call blocking may occur during the busy hour

Probabilistic, which depends on the traffic intensity during the busy hour and

the dimensioning of the network as described by Erlang's formula (so called

B formula)

Blocking probabilities in different parts of the network can summed to

approximately estimate the end-to-end blocking

Waiting system

If connecting the call is not immediately possible, the call may be put in a

waiting state

A small waiting time does not matter, a user may not notice it at all Long waiting times are unacceptable for the users

One sets an upper limit to the waiting time, after which the call is blocked

the behavior of a waiting system is described by so called Erlang's C formula

There may be reattempts after unsuccessful calls


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It is not reasonable to dimension the network for a very small blocking

probability, since the call may be unsuccessful due to other reasons witha much higher probability

User is busy

User is unavailable

Wrong number

The system is designed around a 1% blocking probability rate


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In networks other than the POTS, QoS may have different parameters

instead of or in addition to blocking probability

Internet traffic or ATM networks have

Packets or cell delays

Jitter

Lost packets

Erroneous packets

Throughput


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Erlangs B Formula (1)

Assumptions

A loss system: a blocked call is cleared (no reattempts) There are n trunks; any free trunk can be used

The arrivals constitute a Poisson process

The arrivals occur at average rate , otherwise, the arrivals are completely random

This is good model when the calls originate from a large population of independent

users

Traffic intensity is A = H


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The probability of blocked call is given by

Relates the system (n), the traffic (A) and quality of service (E)


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Example capacity table using Erlangs B system


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Grade of Service (1)


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In a blocked call cleared system, the required GOS is 0.5%. How many

users are there in the system with 5 channels if each user generates 0.1Erlangs of traffic?

You can refer to chart or use the formula

A = ~1.13

U = A/Au = ~11


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Time & Call Blocking

Time Blocking

Fraction of time when all resources are occupied

Call Blocking

Fraction of all calls that are blocked

Why do we take them as being equal?


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The End

traffic forecasting & network planning - lec 05

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