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Z. Ghassemlooy

Mobile Communication Systems

Professor Z Ghassemlooy

Scholl of Engineering and TechnologyUniversity of Northumbria

U.K.http://soe.ac.uk/ocr

Professor Z Ghassemlooy

Scholl of Engineering and TechnologyUniversity of Northumbria

U.K.http://soe.ac.uk/ocr

Part III- Traffic Engineering


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Contents

Problems + Design Considerations

Grade of Services (GOS)

Traffic Intensity

Efficiency Measure Cellular Transcceiver

Propagation - See Part 4 Modulation - See Part 5

Performance- See Part 6


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Traffic Engineering

Necessary in telecommunications network planning to ensurethat network costs are minimised without compromising thequality of service delivered to the user of the network. It is based on probability theory and can be used to analyse mobile radio

networks as well as other telecommunications networks.

Mobile radio networks have traffic issues that do not arise in thefixed line PSTN. A mobile handset, moving in a cell, receives a

signal with varying strength. This signal strength is subject to: slow fading, fast fading

interference from other signals,

thus resulting in degradation of the carrier-to-interference (C/I)ratio. A high C/I ratio results in quality communication.

A good C/I ratio is achieved by using optimum power levelsthrough the power control of most links. When carrier power is too high, excessive interference is created, degrading the

C/Iratio for other traffic and reducing the traffic capacity of the radio subsystem.

When carrier power is too low, C/Iis too low and QoS targets are not met.


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Traffic Engineering

Traffic engineering balances the following factors basedon given amount of traffic

Grade of Service (GOS) Resources (e.g. trunk channels)

Two types of systems implemented to provide voice

communicationsBlocking

Voice or data is blocked (by a busy signal) if network

resource (e.g trunk channel) is not available. GOS = Blocking probability

Delay System

Voice or data is queued until network resource is available

GOS = Queueing Probability and average time in queue


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Blocking System - Example

PBX Trunking Resource: voice circuits between PBX and CO

Traffic: call routing attempts GOS: fraction of calls blocked

Network Trunking

Resource: voice circuits between CO and CO Traffic: call routing attempts

GOS: fraction of calls blocked

N-way conference circuits Resource: conference circuits available at CO

Traffic: call conferencing attempts

GOS: fraction of conferences denied


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Delay System - Example

Packet Switched Voice or Data

Resource: BW shared by multiple packets

Traffic: packets

GOS: fraction of packets buffered by router and averagequeueing time in router

Dial Tone delay

Resource: dial tone generator at Co

Traffic: call originations (off hook)

GOS: fraction of calls queued for dial tone and average dialtone delay

Automatic call Distributors (ACD)

Resource: customer service rep ready to handle calls

Traffic: customer calls

GOS: fraction of customer calls are put on hold (listen tobad music)


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A Switch - Blocking

E.g.: If input 1 is connected to output 2 and input 2 is connected to output4, then even if input 3 and output 4 are free, they cannot be connected

No path for 3


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A Switch Non-Blocking

By adding another line between the 3 x 2 and the 4 x 4 switches.


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Connecting Phones with Switches -Problems

Larger number of switches requires to connect n

phones together: No. of switches: s= (n-1)*n/2

Slow connection speeds

Too many regular faults

High cost (maintenance and switches)


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Design Considerations

Flexible design that takes into account low and high traffic

periods:

peak traffic period (mornings and afternoons)

low traffic (evening and weekends)

high traffic (usually 10 -20% of total capacity, all users need not

be directly connected)

cellular systems depend on trunking to connect a large number

of users


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Trunking

In a trunked radio system,

A large number of users share a small pool of channels

in a cell on a per call basis

A channel is allocated on a per call basis,

On termination of call, the previously occupied channel is

returned to the pool of available channels

When all channels are in use, access by a new user isblocked


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Traffic Engineering Traffic Intensity

Holding Time - the length of time that a resource is being held(e.g the duration of a phone call)

Traffic volume - for an interval is the sum of all the trafficholding times for that interval

Traffic intensity = traffic volume / time interval which is a

measure of demand

Erlangs - describe traffic intensity in terms of the number ofhours of resource time required per hour of elapsed time

CCS( Centum Call Seconds) measures the exact same trafficintensity as the Erlangs but expresses it as the number of 100second holding times required per hour. Traffic registers

sample stations every 100 seconds per hour to check forbusies. Since there are 36 sets of hundred seconds in an hour CCS = 36 x Erlangs


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Traffic Measurement Unites

Erlangs:

Traffic intensity (named after of a Danish mathematician) is the average

number of calls simultaneously in progress over a certain time. It is adimensionless unit.

Erlang

one hour of continuous use of one channel = 1 Erlang

1 Erlang = 1 hour (60 minutes) of traffic

In data communications, an 1 E = 64 kbps of data

In telephone, 1 Erlang = 60 mins = 1 x 3600 call seconds

% of Occupancy A.K. Erlang, 1878-1929


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Erlangs - Example

For example, if a group of user made 30 calls in onehour, and each call had an average call duration of 5

minutes, then the number of Erlangs this represents isworked out as follows:

Minutes of t raff ic in t he hour = number of calls x durat ionMinutes of traffic in the hour = 30 x 5

Minutes of traffic in the hour = 150

Hours of traffic in the hour = 150 / 60Hours of traffic in the hour = 2.5

Traf f ic f igure = 2.5 Er langs


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Traffic Capacity

How to compare the quality of services provides bydifferent service providers?

What is the probability of not being able to make acall?

What is the probability of waiting before a call isconnected?

All these can be explained by the Grade (Quality) of

Service (GOS):


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GOS

Is a measure of the call blocking

or

The ability to make call during the busiest time

It is typically given as the likelihood that a call isblocked or the likelihood of a call experiencing a delay

greater than a certain queuing time.

Is determined by the available number of channels andused to estimate the total number of users that anetwork can support.

For example, if GOS = 0.05, one call in 20 will be blocked during

the busiest hour because of insufficient capacity


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Factors Affecting QoS

The standard metrics used to measure the QoS: Coverage: the strength of the measured signal is used to estimate the

size of the cell.

accessibility (includes Grade of Service (GOS): is about determining theability of the network to handle successful calls from mobile-to-fixednetworks and from mobile-to-mobile networks.

audio quality: monitoring a successful call for a period of time for theclarity of the communication channel.

In general, GOSis measured by: looking at traffic carried, traffic offered

and calculating the traffic blocked and lost.

The proportion of lost calls is the measure of GOS. For cellular circuit groups an acceptable GOS = 0.02. This means that

two users of the circuit group out of a hundred will encounter a callrefusal during the busy hour at the end of the planning period.

GOS is calculated from the Erlang-B formula, as a function of thenumber of channels required for the offered traffic intensity.


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Traffic Intensity

The traffic intensity offered by each user is:

ErlangsHA =where

His the average holding time of a call

is the average number of call requested/hour

If there are Uusers and an unspecified number of channels.

The total offered traffic intensity is:

UAAT

= Erlangs

Busy hours traffic: Calls/busy hours *Mean call hold time


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Traffic Intensity - contd.

In a trunks system of Cchannels and equally distributedtraffic among the channels, the traffic intensity per

channel is:

CUAAc /= Erlangs/channels

The offered traffic: Volume of traffic offered toa switch that are all processed is defined as:

Offered traffic = carried traffic + overflow

The carried traffic: The actual traffic carried by a switch.Overflow (blocked) traffic: Portion of the traffic not processed.


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Example I

A call established at 1am between a mobile and MSC. Assuming acontinuous connection and data transfer rate at 30 kbit/s, determine

the traffic intensity if the call is terminated at 1.50am.

Solution:

Traffic intensity = (1 call)*(50 mins)*(1 hour/60 min)= 0.833

Note, traffic intensity has nothing to do with the data rate, only theholding time is taken into account.


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Example II

Consider a PSTN which receives 240 calls/hr. Each call lasts anaverage of 5 minutes. What is the outgoing traffic intensity to thepublic network.

Solution

A = *H = 240 calls/hr and H = 5 minutes

A = (240 calls /hr) x (5 min/call) = 1200 min/hr

Erlang cannot have any unit so

A= 1200 min/hr * (1 hour/60 minutes) = 20 Erlangs

So 20 hours of circuit talk time is required for every hour of elapsed time.An average of T1 voice circuits busy at any time is 20. (Or 20 hours of

continuous use of 20 channels.)


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Traffic Intensity contd.

Quality of service (QoS) is expressed in terms of blockingprobality as:

)( CABP =

Where B= Erlang B FormulaA = The traffic intensity

C= No of channels (lines)


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Traffic Intensity Models

Erlang B Formula: All blocked calls are cleared; The most common

Engset formula (probability of blocking in low density areas); used where

Erlang B model fails.

Extended Erlang B: Similar to Erlang B, but takes into account that a

percentage of calls are immediately represented to the system if they

encounter blocking (a busy signal). The retry percentage can be specified.

Erlang C Formula: Bblocked calls delayed or held in queue

indefinitely

Poisson Formula: Blocked calls held in queue for a limited time

only.

Binomial Formula: Lost calls held


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Erlang B Model - Characteristics

Provides the probability of blockage at the switch

due to congestion. Assumptions:

No waiting is allowed (lost calls are cleared) (I-e they disappear from the system.This assumption is valid for systems that can overflow blocked calls onto another trunk (e.g ahigh usage trunk)

Traffic originated from an infinite numbers of sources

Limited No. of trunk (or serving channels)

Memory-less, channel requests at any time

The probability of a user occupying a channel is based on exponentialdistribution

Calls arrival rate at the network = Poisson process (the holding time orduration of the call has exponentially distribution)


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Probability of Blocking P(B)

Equations for P(B), depend on assumption that we make aboutwhat happens to calls that are blocked.

Lost Calls Cleared

Assume that blocked calls are cleared (lost from the system.This assumption is valid for systems that can overflow blocked

calls onto another trunk (e.g a high usage trunk) Equation: Offered Traffic A = Carried Traffic AC/(1-P(B))

Lost Calls Returning

Assume that blocked calls are re-tried until they aresuccessfully carried. This assumption is valid for PBXs andcorporate tie lines.

Equation: Offered Traffic A > or = Carried Traffic AC

P b bili f Bl ki P(B)


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Probability of Blocking P(B)

Lost Calls Cleared Also known as the Erlang-B formula given by:

=

=C

k

k

C

k

AC

A

BP

0 !

!)(

whereA is the traffic intensity

Cis the number of channels

P b bilit f Bl ki P(B)


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Probability of Blocking P(B) - contd.

The carried traffic is )](1[ BpAAca =

The efficiency of the channel usage is

C

Aca=

* The start-up systems usually begins with a GOS of 0.02

(2% of the blocking probability) rising up to 0.5 as thesystem grows.

* If more subscribers are allowed in the system the

blocking probability may reach unacceptable values.

E l B T bl


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Erlang B Table

Number of

channels C

Traffic Intensity (Erlangs)

QoS=0.01 QoS= 0.005 QoS= 0.002 Qos= 0.001

2 0.153 0.105 0.065 0.046

4 0.869 0.701 0.535 0.439

5 1.36 1.13 0.9 0.762

10 4.46 3.96 3.43 3.09

20 12 11.1 10.1 9.41

24 15.3 14.2 13 12.2

40 29 27.3 25.7 24.5

70 56.1 53.7 51 49.2

100 84.1 80.9 77.4 75.2

E l B Ch t


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Erlang B Chart

E ample III


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Example III

A single GSM service provider support 10 digital speechchannels. Assume the probability of blocking is 1.0%. From the

Erlang B chart find the traffic intensity. How many 3 minutes ofcalls does this represent?

Solution:

From the Erlang B Chart the traffic intensity = ~5 Erlangs

AI

= H = AI/H= 5/(3 mins/60) = 100 calls

Example IV


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Example IV

A telephone switching board at the UNN can handle 120 phones.Assuming the followings, determine the outgoing traffic intensity andThe number of channels.- On average 5 calls/hour per phone,- Average call duration time = 4 minutes,- 60% of all calls made are external.

- QoS = 0.9%

Solution:

AT = U..H*U = (120 call*5 calls/hour)*60% =360 call/hourH = 4 mins/callTherefore AI =360 * 4 * (1 hour/60 mins) = 24 Erlangs.Thus 24 hours of circuit talk time is required for every hour of elapsed

time-No. of channels Cfrom Erlang B chart = ~ 34

Example V


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Example V

Consider a telephone switched board with 120 phones. Assuming thenumber of call is 3/hour/line, the average call duration is 4 minutes,and 55 % of all call are made external via a T-1 trunk (24

channels) to the PSTN. Determine carried traffic and channel usage.

Solution:

Offered traffic A = x H= (150 phones x 3 calls/hr x 58% ) x(4 mins./call) x (1 hour/60 mins.) = 17.4 Erlangs

Blocking Probability P(B), C= 24 and A = 17.4, therefore from the

Erlang B Chart or formula P(B) = 0.03

Carried Traffic, Aca= A (1- P(B))= 17.4 (1-.03)=16.9 Erlangs

Channel usage = Aca/C= 16.9/24 = 0.7 or 70%

Note: 16.9 Erlangs of traffic attempts to go across the T1 trunk and 0.5Erlang is blocked.


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Example V


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Example V

As a manger of a growing call center, you are looking atobtaining additional phones for the PBX since customers havecomplained about long hold times. On average, there are 4

incoming calls per hour on each phone. The traffic study yourequested from the Ameritech CO shows that on average, yourcompany receives 480 calls/hour. How many phones do youneed to order? Currently there are 100 phones connected to the

PBX for the customer service agents

Solution

is the average call arrival rate= 480calls/hour (from traffic study)

= phones x calls/hr 480 = N x 4 calls/hour N = 480/4 = 120 phones

So the manager needs to order 120-100 = 20 more phones and hirenew customer service reps as well

Efficiency Measures


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Efficiency Measures

1- Spectrum efficiency

It is a measure of how efficiently frequency, time and

space are used:

It depends on:

Number of required channels per cell

Cluster size of the interference group

)(AreaBandwidth

anneltraffic/chOfferedellchannels/cofNo.

AreaBandwidth

(Erlang)Traffic

2kmkHz

Erlang

se

=

=

Efficiency Measures


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Efficiency Measures

2- Trunking efficiency

Measures the number of subscribers that each channel inevery cell can accommodate

3- Economic efficiency

It measures how affordable is the mobile service to usersand the cellular operators.

No of Trunk Vs Utilization Efficiency


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No. of Trunk Vs. Utilization Efficiency


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Cellular Radio Transceiver


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Cellular Radio Transceiver

DiplexerDiplexer

IFIF

Frequency

synthesizer

Frequency

synthesizer

DemodulatorDemodulator

Power

amplifier

Power

amplifier ModulatorModulator

Controller

Voice out

Keyboard

& display

Voice in

Frequency

synthesizer

Frequency

synthesizer

Receiver

Transmitter

Received

RF signal

Transmitted

RF signal

Cellular Radio Transceiver - Receiving Path


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Ce u a ad o a sce e ece g at

Antenna

Diplexe Is a high performance selective filter for the receiving and the

transmitting signals.

Receiving and transmitting signals are in separate frequencybands.The pass-bands of the filters are designed to minimise thelevel of transmitting signal coupling into the receiver, see theFig.

IF and frequency synthesiser

To down convert the received signal. (Multi-stage IFs are alsoused).

Demodulator To recovers the original signal (data, voice etc.)

Cellular Radio Transceiver - Transmitting Path


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g

Modulator

To up convert the information to a much higher frequency band.

Power Amplifier To boost the signal strength

Antenna

Frequency synthesisers

Are used since transmitting and receiving paths are need

simultaneously. Single synthesiser may be used if the IF is

chosen to be the same as the spacing between the transmittingand receiving frequency bands (typically 45 MHz).

Questions and Answers


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Quest o s a d s e s

Next lecture: Propagation Characteristics

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