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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