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Florida Institute of technologies ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 17: Traffic planning Spring 2011
Florida Institute of technologies
ECE 5221 Personal Communication Systems Prepared by:
Dr. Ivica Kostanic
Lecture 17: Traffic planning
Spring 2011
Florida Institute of technologies
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Traffic in communication networksCircuit switched versus packet switched trafficQueuing systemElements of queuing systemTraffic in erlangs
Outline
Important note: Slides present summary of the results. Detailed derivations are given in notes.
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Traffic in communication networks
Traffic - flow of information messages through a communication network
Generated as a result ofo phone conversationso data exchangeo audio, video deliveryo signaling
Communication networks are designed to provide service to many users
At any instant of time not all users are active
o network resources are shared o resource sharing may result in
temporary service unavailability Traffic planning allows sharing of
resources with minimum performance degradation
Com m unication N etwork
Modern communication networks carry mixture of voice and data traffic
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Outline of a cellular network
Cellular network consists of many connected elements
Analysis of the entire network is complicated
o Common practice - analyze each link individually
M SC M SCB SC BS CB TS
BTS
B TS
B TS
B TS
BTS
BTS - B ase S ta tionBSC - B ase S ta tion C ontro le rMSC - M obile S w itchn ing C en ter
Traffic dimensioning has two aspectso Dimensioning the network elements to
have enough processing powero Dimensioning the connecting lines to
have sufficient capacity
Traditionally, traffic bottleneck - Air interface
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Circuit switched communication services First and second generation provides connection oriented services to the users A dedicated channel is allocated over the entire duration of the call In the case of voice communication this is “only” 50% wasteful This mode of communication is called “circuit-switching” Circuit switching is very inefficient for data communication (major driver of 3G
cellular systems) Circuit switching is abandoned in 4G
Interpretation of term circuit for various cellular technologies
Technology Circuit resourceFDMA/TDMA Pair of frequencies and associated time slot
FDMA/CDMA Pair of frequencies + associated codes
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Packet switched communication services
Virtual path packet switchingo Virtual path (sequence of network
nodes) is established through the network
o Implemented within ATM networks
Datagram packet switchingo Every packet travels independentlyo Implemented within IP based networkso Transport layer has to assure the proper
order of the packets
Virtual Path
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Virtual path switching Datagram switching
Note: Modern packet data networks are using datagram switching
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Types of traffic in cellular networks
Cellular networks supporto circuit switched (CS) voiceo dispatch voice (push to talk)o circuit switched datao packet data (PD)
Communication resources may beo Shared between CS and PSo Separated resources may be set for
CS and PS First and second generation -
dominated with circuit switched voice Third generation and beyond -
dominated by data
ITU vision for cellular services
Traffic planning in heterogeneous cellular networks of the future takes central stage
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Description of queuing systems Queuing systems
o Mathematical abstraction o Used to develop the traffic
analysis and planning methodology
Elements of a queuing system
o source populationo queueo serverso distributions of interarrival
times, service times, queuing discipline, etc.
S 1
S 2
S c
SourcePopulation
G eneratedT raffic
Q ueue Servers
M eanArriva lRatel
InterarrivalT im et
Num ber ofU sers in the
Q ueueN q
Q ueuingT im eq
Outline of a queuing system
Queuing system – cell site Servers – channel resources – trunks Population – users connecting to cellular network
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Source population
Consists of all users that are eligible for service The most important property - size
o infinite population - arrival rate does not depend on the number of users in the system
o finite population - arrival rate depends on the number of users in the system
o if the population is large relative to the number of servers we routinely assume that its is infinite
In cellular systems population are all eligible users within the coverage area of the cell
It is assumed that the number of eligible users is much greater than the number of the users using the system at any given moment
Over a course of day, the size of population changes
Traditionally cellular systems are dimensioned for a good performance during the busiest hour
Example of a call stats benchmarking map
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Arrival rate and interarrival times Arrival rate - number of service
requests per unit time The ability of the queuing system to
provide effective service depends on distribution of arrival rates
Standard way of specifying arrival rate is through probability density function of interarrival times
Example:
The average number of call arrivals in two figures is the same: 20 arrivals per minute.
The traffic pattern in second figure requires more resources to accommodate for higher demand peaks.
0 10 20 30 40 50 600
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20
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time [min]
num
ber o
f cal
l atte
mpt
s
0 10 20 30 40 50 600
10
20
30
40
50
time [min]
num
ber o
f cal
l atte
mpt
s
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Service time (call holding time-CHT) Service time-period of
time that the resource is allocated to individual user
Usually specified through its distribution
Most commonly, CHT is exponentially distributed
0 100 200 300 400 500 6000
0.005
0.01
0.015
c all duration [sec ]
relat
ive fr
eque
ncy o
f occ
uran
ce
H is togram of c all holding tim e (C HT) , m ean = 91.6s, s td = 95.08sec , 498 m easurem ents
Exponential P DF, mean = 92 sec
Example: Duration of CHT at a cell
0,exp1exp
xTx
Txpdf
Exponential distribution
T – average call holding time
Note: Exponential distribution is a good model for demand generated by humans (voice, SMS, email,..)
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Average resource occupancy - traffic in erlangs
Erlang - unit for measuring of traffic intensity
Defined as a fraction of time that the resource is occupied
Occupancy does not have to continuous
Specified relative to some averaging time
Maximum traffic carried by a single resource - 1 erlang
Total traffic carried by service facility cannot exceed number of servers T
3t2t1t
E5635.085.4
8125.1321
++
++
Tttt
A
Average tra ffic
Example
Tta
Definition
tT
Resource occupancy time
Averaging time
[erlang]
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Alternative interpretation of erlang traffic Traffic in erlangs = average number of
simultaneously occupied servers Can be measured easily
o regular poling of service facility and logging the number of occupied resources
+++C
n
nc
Ttn
TtC
Tt
TtA
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Traffic in erlangs for multi-server system
nt Sum of times during exactly n out of C servers are held simultaneously
C Number of servers
T Averaging time
0 10 20 30 40 50 600
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time [min]
num
ber o
f occ
upie
d ch
anne
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Average traffic
Example of traffic measurements.Averaging time is 60 min.Poling time is 1 min.
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Offered, carried and lost traffic
Offered traffic - traffic that would be served if the number of resources is unlimited
Lost traffic - traffic that could not be served due to finite resources
Served traffic - difference between offered and lost traffic
Attempt to serve all offered traffic results in allocation of large number of resources
Relation between offered, carried and lost traffic
0 10 20 30 40 50 602
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time [min]
numb
er of
occu
pied c
hann
els
Offered traffic
Lost traffic Number of channels
Note : Communication systems are frequently designed to operate with a certain percentage of lost traffic
