Transcript Wireless Communications and Networks

Teletraffic Engineering
and
Cellular Concepts
Chapter 2 & 5 (Vijoy Garg)
Teletraffic Engineering
Traffic engineering uses statistical techniques such as queuing
theory to predict and engineer the behavior of
telecommunications networks such as telephone networks
or the Internet.
 One of the important steps of teletraffic engineering
determines number trunks required on a route or a connection
between two MSCs.
 Another important steps of teletraffic engineering is to ensure
the desired service level.
2
Teletraffic Engineering
Service Level
Service Level can be divided into two main areas:
1.
Dial tone delay: The maximum waiting time to hear a dial tone
after removing the hand-set from the hook.
2.
Service blocking probability:
 the probability that service delay will exceed some specific
value or
 The probability that the call will be denied or blocked
Teletraffic Engineering
Service or call blocking probability is known as the grade of service (GoS)
GoS 
Blocked Call
Total Call
Serviced Calls = 380, blocked calls = 10, GoS?
GoS 
Blocked calls
10
1


Serviced calls  Blocked calls 380  10 39
Traffic Usage and Measurements Units
Traffic goes through a traffic path
A communication channel, time slot, frequency
band, line, trunk, switch or circuit over which
individual communications take place.
Carried Traffic: volume of traffic carried by a switch.
Offered Traffic: volume of traffic offered to a switch.
Offered traffic = carried traffic + overflow
Traffic Usage and Measurements Units
Traffic Usage is defined by two parameters:
1. Calling rate or call intensity: The number of times a route or
traffic path is used per unit time.
2. Call holding time: the average duration of occupancy of a traffic
path by a call.
Some Terminologies:
Busy Hour (BH): Span of time (not necessarily clock hour) that has the
highest average traffic load for the business day throughout the busy
season.
Peak Hour: the clock hour with highest traffic load for a single day.
Average Busy Season (ABS): the three months (not necessarily
consecutive) with the highest average BH traffic load per access line.
Traffic Usage and Measurements Units
Traffic is measured by traffic intensity.
Traffic Intensity: the average number of calls simultaneously in progress
during a particular period of time.
(the sum of circuit holding time)
T raffic intensity (I) 
(the duration of monitoringperiod)
Nc

 ti
i 1
T

Nc t
 nc t
T
nc = number of calls per unit time.
Traffic Usage and Measurements Units
Traffic is measured using one of the following units1.
Erlangs: An average of one call in progress during an hour represents a
traffic intensity of 1 Erlang.
2.
Centrum call seconds (CCS): Centrum means 100.
1 Erlangs = 1×3600 call seconds = 36 CCS.
Percentage of occupancy: percentage of time a server is busy.
Peg Count: The number of attempts to use a piece of equipment.
The relationship between the usage (U), peg count (PC), overflow per period
(O), and average holding time ( t ) is as follows:
U  PC  O.t
Traffic Usage and Measurements Units
Average holding time = 5 seconds,
peg count = 450 for a one
hour period and there is no overflow.
traffic usage = ?
T raffic Usage (U)  PC-O.t
5
 ( 450  0 ) 
 0.625 Erlangs
3600
 0.625 3600 call seconds  2250 call seconds
 22.50 CCS
Definitions of Terms
Number of calls attempted (number of bids):
Best measure of unconstrained customer demand.
Number of calls attempted (number of bids):
 Calls reaching ringing tones or being answered.
 When compared with calls attempted, provides a measure of the state of
network congestion.
Grade of Service (GoS):
GoS 
Nnumber of busy hour call attempts - Number of busy hour call completed
Number of busy hour call attempts
Answer Seizure Ratio (ASR):
ASR 
Number of calls answered
Number of calls attempted
Answer Busy Ratio (ABR):
Number of calls answered
ABR 
Number of busy calls
Definitions of Terms
Significance of ASR and ABR:
 ASR and ABR is measured over relatively short period of time (5 to 15
minutes)
 Both ASR and ABR are good indicators of instantaneous network
congestion.
 Lower value of ASR and ABR indicates higher value of
congestions.
 Higher value of ASR or ABR does not indicate lower value of
congestion since call may remain unanswered due to other
reasons.
Definitions of Terms
Quality of Service (QoS): Factors affecting QoS
 Transmission quality (level, crosstalk, echo, etc)
 Dial-tone delay and post dial delay
 Grade of service
 Fault incidence and service deficiency
 Adaptation of the system to the subscribers
Cellular Network Organization


Use multiple low-power transmitters (100 W or
less)
Areas divided into cells




Each served by its own antenna
Served by base station consisting of transmitter,
receiver, and control unit
Band of frequencies allocated
Cells set up such that antennas of all neighbors are
equidistant (hexagonal pattern)
Cell Structure
F1
F2
F3
F1
F2
F3
F7
F1
F2
F3
(a) Line
Structure
Note:
F6
F2
F3
F1
F4
F5
F4
(b) Plan Structure
Fx is a set of frequencies i.e., frequency group.
Signal Strength
Signal strength
(in dB)
Cell i
Cell j
-60
-70
-80
-90
-60
-70
-80
-90
-100
-100
Select cell i on left of boundary
Select cell j on right of boundary
Ideal boundary
Actual Signal Strength
Signal strength
(in dB)
Cell i
Cell j
-60
-60
-70
-80
-90
-100
-70
-80
-90
-100
Signal strength contours indicating actual cell
tiling. This happens because of terrain, presence of
obstacles and signal attenuation in the atmosphere.
Frequency Reuse


Adjacent cells assigned different frequencies to
avoid interference or crosstalk
Objective is to reuse frequency in nearby cells



10 to 50 frequencies assigned to each cell
Transmission power controlled to limit power at that
frequency escaping to adjacent cells
The issue is to determine how many cells must
intervene between two cells using the same frequency
Frequency Reuse
F7
F7
F6
F2
F3
F1
F1
F5
F6
F4
F2
F5
F7
F6
F4
F2
F7
F2
F1
F1
F6
F3
F1
F1
F5
F3
F1
F1
F5
F3
F4
F4
Fx: Set of frequencies
7 cell reuse cluster
Reuse Distance
Cluster
R
F7
• For hexagonal cells, the
reuse distance is given by
F2
D  3N R
F6
F3
F1
F1
F5
F4
F7
F6
F2
F3
F1
F5
where R is cell radius and N is the
reuse pattern (the cluster size or
the number of cells per cluster).
F4
• Reuse factor is
D
q   3N
R
Reuse Distance (Cont’d)
 The cluster size or
the number of cells
per cluster is given by
N  i 2  ij  j 2
where i and j are nonnegative integers
N = 1, 3, 4, 7, 9, 12,
13, 16, 19, 21, 28, …,
etc.
The popular value
of N being 4 and 7
Co-channel Interference
Six effective interfering cells in tier 1 of cell 1.
Cellular Systems: Example 1
We consider a cellular system in which total available voice channels to handle the traffic are
960. The area of each cell is 6 km2 and the total coverage area of the system is 2000 km2.
Calculate: (a) the system capacity if the cluster size, N (reuse factor), is 4 and
(b) the system capacity if the cluster size is 7. How many times would a cluster of
size 4 have to be replicated to cover the entire cellular area? Does decreasing the reuse
factor N increase the system capacity? Explain.
It is evident when we decrease the value of N from 7 to 4, we increase the system
capacity from 46,080 to 79,680 channels. Thus, decreasing the reuse factor (N) increases
the system capacity.
Cellular Systems: Example 2
Approaches to Cope with
Increasing Capacity



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
Adding new channels
Frequency borrowing – frequencies are taken from
adjacent cells by congested cells
Cell splitting – cells in areas of high usage can be
split into smaller cells
Cell sectoring – cells are divided into a number of
wedge-shaped sectors, each with their own set of
channels
Microcells – antennas move to buildings, hills,
and lamp posts
Cellular System Overview
Cellular Systems Terms



Base Station (BS) – includes an antenna, a
controller, and a number of receivers
Mobile telecommunications switching office
(MTSO) – connects calls between mobile units
Two types of channels available between mobile
unit and BS


Control channels – used to exchange information
having to do with setting up and maintaining calls
Traffic channels – carry voice or data connection
between users
Steps in an MTSO Controlled
Call between Mobile Users
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Mobile unit initialization
Mobile-originated call
Paging
Call accepted
Ongoing call
Handoff
Additional Functions in an
MTSO Controlled Call
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Call blocking
Call termination
Call drop
Calls to/from fixed and remote mobile
subscriber
Mobile Radio Propagation
Effects

Signal strength



Must be strong enough between base station and mobile
unit to maintain signal quality at the receiver
Must not be so strong as to create too much cochannel
interference with channels in another cell using the
same frequency band
Fading

Signal propagation effects may disrupt the signal and
cause errors
Handoff Region
Signal strength
due to BSj
Signal strength
due to BSi
Pj(x)
Pi(x)
E
Pmin
BSi
X1
X3
MS
X5
Xth
X4
BSj
X2
By looking at the variation of signal strength from either base station it is
possible to decide on the optimum area where handoff can take place
Handoff Performance Metrics




Call blocking probability – probability of a new
call being blocked
Call dropping probability – probability that a call
is terminated due to a handoff
Call completion probability – probability that an
admitted call is not dropped before it terminates
Probability of unsuccessful handoff – probability
that a handoff is executed while the reception
conditions are inadequate
Handoff Performance Metrics





Handoff blocking probability – probability that a
handoff cannot be successfully completed
Handoff probability – probability that a handoff
occurs before call termination
Rate of handoff – number of handoffs per unit
time
Interruption duration – duration of time during a
handoff in which a mobile is not connected to
either base station
Handoff delay – distance the mobile moves from
the point at which the handoff should occur to the
point at which it does occur
Handoff Strategies Used to
Determine Instant of Handoff

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Relative signal strength
Relative signal strength
with threshold
Relative signal strength
with hysteresis
Relative signal strength
with hysteresis and
threshold
Prediction techniques
Back to Traffic Engineering

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
Ideally, available channels would equal
number of subscribers active at one time
In practice, not feasible to have capacity
handle all possible load
For N simultaneous user capacity and L
subscribers

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L < N – nonblocking system
L > N – blocking system
Blocking System Performance
Questions
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Probability that call request is blocked?
What capacity is needed to achieve a certain
upper bound on probability of blocking?
What is the average delay?
What capacity is needed to achieve a certain
average delay?
Traffic Intensity

Load presented to a system:
A  h



 = mean rate of calls attempted per unit time
h = mean holding time per successful call
A = average number of calls arriving during average
holding period, for normalized 
Factors that Determine the Nature
of the Traffic Model

Manner in which blocked calls are handled


Lost calls delayed (LCD) – blocked calls put in a queue
awaiting a free channel
Blocked calls rejected and dropped



Lost calls cleared (LCC) – user waits before another attempt
Lost calls held (LCH) – user repeatedly attempts calling
Number of traffic sources

Whether number of users is assumed to be finite or
infinite
Cell Capacity



Average number of MSs requesting service (Average
arrival rate): 
Average length of time MS requires service (Average
holding time): T
Offered load: a = T where a is in Erlangs
e.g., in a cell with 100 MSs, on an average 30 requests are generated
during an hour, with average holding time T=360 seconds
Then, arrival rate =30/3600 requests/sec
A completely occupied channel (1 call-hour per hour) is
defined as a load of one Erlang, i.e.,
30 calls 360 sec
a

 3 Erlangs
3600sec call
Capacity of a Cell

The probability P(S) of an arriving call being blocked is the
probability that all S channels are busy
aS
P ( S )  S S! i
a

i  0 i!


which is also defines the
Grade of Service (GOS)
This is Erlang B formula B(S, a)
In the previous example, if S = 2 and a = 3, the blocking
probability B(2, 3) is
32
B ( 2,3) 

So, the number of calls
blocked 30x0.529 = 15.87
2!
 0 .5 2 9
k
2
3

k  0 k!
Cell Splitting
Large cell (low
density)
Small cell (high
density)
Smaller cell (higher
density)
Depending on traffic patterns the smaller
cells may be activated/deactivated in
order to efficiently use cell resources.
Cell Sectoring by Antenna Design
c
c
120o
120o
a
b
b
(b). 120o sector
(a). Omni
(c). 120o sector (alternate)
d
f
90o
a
c
e
60o
d
b
(d). 90o sector
a
a
b
c
(e). 60o sector