FWSC UNIT-IV.pptx

UNIT-IV
Cellular Telephone Concepts
Introduction
Mobile Telephone Service
Evolution of Cellular Telephone
Cellular Telephone
Frequency Reuse
Interference
Cell Splitting, Sectoring
Cellular System Topology
Roaming and Handoffs

INTRODUCTION
&
MOBILE TELEPHONE SERVICE

Cellular telephone evolved from two-way mobile FM radio.
Cellular services include standard cellular telephone service (CTS),
personal communications systems (PCS), and personal communications
satellite systems (PCSS).
Mobile telephone services began in the 1940s and were called MTSs
(mobile telephone systems or sometimes manual telephone systems, as
all calls were handled by an operator).
MTS systems utilized frequency modulation and were generally
assigned a single carrier frequency in the 35-MHz to 45-MHz range that
was used by both the mobile unit and the base station.
The mobile unit used a push-to-talk (PTT) switch to activate the
transceiver. Depressing the PTT button turned the transmitter on and
the receiver off, whereas releasing the PTT turned the receiver on and
the transmitter off.

 In 1964, the Improved Mobile Telephone System (IMTS) was
introduced, which used several carrier frequencies and could, therefore,
handle several simultaneous mobile conversations at the same time.
 IMTS subscribers were assigned a regular PSTN telephone number so
that callers could reach an IMTS mobile phone by dialling the PSTN
directly, eliminating the need for an operator.
 IMTS and MTS base station transmitters outputted powers in the 100-
W to 200-W range, and mobile units transmitted between 5 W & 25 W.
Therefore, IMTS and MTS mobile telephone systems typically covered
a wide area using only one base station transmitter.
Two-way mobile radio systems operate half duplex and use PTT
transceivers. With PTT transceivers, depressing the PTT button turns on
the transmitter and turns off the receiver, whereas releasing the PTT
button turns on the receiver and turns off the transmitter.
Two-way mobile radio is a one-to-many radio communications system.
Examples of two-way mobile radio are citizens band (CB), which is an
AM system, and public land mobile radio, which is a two way FM system
such as those used by police and fire departments.

 Cellular telephone offers full-duplex transmissions and operates
much the same way as the standard Wireline telephone service provided to
homes and businesses by local telephone companies.
 Mobile telephone is a one-to-one system that permits two-way
simultaneous transmissions and, for privacy, each cellular telephone is
assigned a unique telephone number.
Coded transmissions from base stations activate only the intended
receiver.
 With mobile telephone, a person can virtually call anyone with a
telephone number, whether it be through a cellular or a Wireline service.
Cellular telephone systems offer a relatively high user capacity within a
limited frequency spectrum providing a significant innovation in solving
inherent mobile telephone communications problems, such as spectral
congestion and user capacity.

Evolution of Cellular Telephone

In the July 28, 1945, Saturday Evening Post, E. K. Jett, then the
commissioner of the FCC, hinted of a cellular telephone scheme that he
referred to as simply a small-zone radiotelephone system.
 On June 17, 1946, in St. Louis, Missouri, AT&T and South western Bell
introduced the first American commercial mobile radio-telephone
service to private customers.
In 1947,AT&T introduced a radio-telephone service they called
highway service between New York and Boston.
 The system operated in the 35-MHz to 45-MHz band. The first half-
duplex, PTT FM mobile telephone systems introduced in the 1940s
operated in the 35-MHz to 45-MHz band and required 120-kHz
bandwidth per channel.
 In the early 1950s, the FCC doubled the number of mobile telephone
channels by reducing the bandwidth to 60 kHz per channel.

In 1960, AT&T introduced direct-dialling, full-duplex mobile telephone
service with other performance enhancements, and in 1968, AT&T
proposed the concept of a cellular mobile system to the FCC with the
intent of alleviating the problem of spectrum congestion in the existing
mobile telephone systems.
Cellular mobile telephone systems, such as the Improved Mobile
Telephone System (IMTS), were developed, and recently developed
miniature integrated circuits enabled management of the necessarily
complex algorithms needed to control network switching and control
operations.
Channel bandwidth was again halved to 30 kHz, increasing the
number of mobile telephone channels by twofold.
In 1983, the FCC allocated 666 30-kHz half-duplex mobile telephone
channels to AT&T to form the first U.S. cellular telephone system called
Advanced Mobile Phone System (AMPS).

In 1991, the first digital cellular services were introduced in several
major U.S. cities, enabling a more efficient utilization of the available
bandwidth using voice compression.
The calling capacity specified in the U.S. Digital Cellular (USDC)
standard (EIA IS-54) accommodates three times the user capacity of
AMPS, which used conventional frequency modulation (FM) and
frequency-division multiple accessing (FDMA).
The USDC standard specifies digital modulation, speech coding, and
time-division multiple accessing (TDMA).
Qualcomm developed the first cellular telephone system based on
code-division multiple accessing (CDMA).
The Telecommunications Industry Association (TIA) standardized
Qualcomm’s system as Interim Standard 95 (IS-95).
On November 17, 1998, a subsidiary of Motorola Corporation
implemented Iridium, a satellite-based wireless personal
communications satellite system (PCSS).

cellular telephone systems allow a large number of users to share the
limited number of common-usage radio channels available in a region.
Integrated-circuit technology, microprocessors and microcontroller
chips, and the implementation of Signalling System No. 7 (SS7) have
recently enabled complex radio and logic circuits to be used in
electronic switching machines to store programs that provide faster and
more efficient call processing.
Fundamental Concepts of Cellular Telephone
The FCC originally defined geographic cellular radio coverage areas on
the basis of modified 1980 census figures.
With the cellular concept, each area is further divided into hexagonal-
shaped cells that fit together to form a honeycomb pattern as shown in
Figure 1a..

The hexagon shape was chosen because it provides the most effective
transmission by approximating a circular pattern while eliminating gaps
inherently present between adjacent circles.
A cell is defined by its physical size and, more importantly, by the size
of its population and traffic patterns.
The physical size of a cell varies, depending on user density and calling
patterns.
For example, large cells (called macro cells) typically have a radius
between 1 mile and 15 miles with base station transmit powers
between 1 W and 6 W. The smallest cells (called microcells) typically
have a radius of 1500 feet or less with base station transmit powers
between 0.1 W and 1 W.
Microcells are used most often in high-density areas such as found in
large cities and inside buildings.

FIGURE 2: Hexagonal cell grid
Superimposed over a metropolitan
area
FIGURE 3 (a) Centre excited cell
(b) edge excited cell
(c) corner excited cell

Occasionally, cellular radio signals are too weak to provide reliable
communications indoors.
This is especially true in well-shielded areas or areas with high levels
of interference.
In these circumstances, very small cells, called picocells, are used.
Indoor picocells can use the same frequencies as regular cells in the
same areas if the surrounding infrastructure is conducive, such as in
underground malls.
When designing a system using hexagonal-shaped cells, base station
transmitters can be located in the centre of a cell (centre-excited cell
shown in Figure 3a), or on three of the cells’ six vertices (edge- or
corner-excited cells shown in Figures 3b and c).
Omni directional antennas are normally used in centre-excited cells,
and sectored directional antennas are used in edge- and corner-excited
cells (Omni-directional antennas radiate and receive signals equally well
in all directions).

FREQUENCY REUSE
FIGURE 4 . Cellular frequency reuse concept

A cell: basic geographical unit of a cellular
network; is the area around an antenna where a
specific frequency range is used.
when a subscriber moves to another cell, the
antenna of the new cell takes over the signal
transmission
a cluster: is a group of adjacent cells, usually 7
cells; no frequency reuse is done within a cluster
the frequency spectrum is divided into sub-bands
and each sub-band is used within one cell of the
cluster
in heavy traffic zones cells are smaller, while in
isolated zones cells are larger

A geographic cellular radio coverage area containing three groups of
cells called clusters.
Each cluster has seven cells in it, and all cells are assigned the same
number of full-duplex cellular telephone channels.
Cells with the same letter use the same set of channel frequencies. As
the figure shows, the same sets of frequencies are used in all three
clusters, which essentially increases the number of usable cellular
channels available threefold.
The letters A, B, C, D, E, F, and G denote the seven sets of frequencies.
The total number of cellular channels available in a cluster can be
expressed mathematically as

When a cluster is duplicated m times within a given service area, the
total number of full-duplex channels can be expressed mathematically
as

The number of users is called the frequency reuse factor (FRF). The
frequency reuse factor is defined mathematically as

Cells use a hexagonal shape, which provides exactly six equidistant
neighbouring cells, and the lines joining the centres of any cell with its
neighbouring cell are separated by multiples of 60.
Therefore, a limited number of cluster sizes and cell layouts is
possible.
To connect cells without gaps in between (tessellate), the geometry
of a hexagon is such that the number of cells per cluster can have only
values that satisfy the equation
where N = number of cells per cluster
i and j = nonnegative integer values

The process of finding the tier with the nearest co-channel cells
(called the first tier) is as follows and shown in Figure 5:
1. Move i cells through the centre of successive cells.
2. Turn 60° in a counter clockwise direction.
3. Move j cells forward through the centre of successive cells.
FIGURE 5. Locating first tier co-channel cells

Example 2:
Determine the number of cells in a cluster and locate the first-tier co-
channel cells values are: j = 2 and I = 3.
FIGURE 6 Determining first tier co-channel cells for Example 2

Co-channel Interference:
When frequency reuse is implemented, several cells within a given
coverage area use the same set of frequencies.
Two cells using the same set of frequencies are called co-channel cells,
and the interference between them is called co-channel interference.
Unlike thermal noise, co-channel interference cannot be reduced by
simply increasing transmit powers because increasing the transmit
power in one cell increases the likelihood of that cell’s transmissions
interfering with another cell’s transmission.
To reduce co-channel interference, a certain minimum distance must
separate co-channels.
Interference between cells is proportional not to the distance
between the two cells but rather to the ratio of the distance to the cell’s
radius.

FIGURE 7. Co-channel interference
Figure 7 shows co-channel interference. The base station in cell A of cluster 1 is
transmitting on frequency f1, and at the same time, the base station in cell A of
cluster 2 is transmitting on the same frequency.
Although the two cells are in different clusters, they both use the A-group of
frequencies.
The mobile unit in cluster 2 is receiving the same frequency from two different
base stations.
Although the mobile unit is under the control of the base station in cluster 2, the
signal from cluster 1 is received at a lower power level as co-channel interference.

A cell’s radius is proportional to transmit power, more radio channels
can be added to a system by either
(1) decreasing the transmit power per cell,
(2) making cells smaller or
(3) filling vacated coverage areas with new cells.
Increasing the D/R ratio (sometimes called co-channel reuse ratio)
increases the spatial separation between co-channel cells relative to the
coverage distance. Therefore, increasing the co-channel reuse ratio (Q)
can reduce co-channel interference.
For a hexagonal geometry,

FIGURE 8. Co-channel reuse ratio
The smaller the value of Q, the larger the channel capacity since the
cluster size is also smaller. However, a large value of Q improves the co-
channel interference and, thus, the overall transmission quality.

Signal to Interference ratio S/I
 The Signal-to-Interference (S/I) for a mobile is
 S is desired signal power ,Ii : interference power from
i th co-channel cell
 The average received power at distance d is
Pr=Po (d/do)-n
 The RSS decays as a power law of the distance of
separation between transmitter and receiver
 Where Po is received power at reference distance do
and n is the path loss exponent and ranges between 2-4
 If Di is the distance of ith interferer, the received power
is proportional to (Di)-n

 The S/I for mobile is given by
With only the first tier(layer of) equidistant
interferers.
For a hexagonal cluster size, which always have
6 CC cell in first tier

 The MS is at cell
boundary
 The approximate S/I is
given by, both in terms
of R and D, along with
channel reuse ratio Q

Adjacent-Channel Interference
Adjacent-channel interference occurs when transmissions from
adjacent channels (channels next to one another in the frequency
domain) interfere with each other.
Adjacent-channel interference results from imperfect filters in
receivers that allow nearby frequencies to enter the receiver.
Adjacent-channel interference is most prevalent when an adjacent
channel is transmitting very close to a mobile unit’s receiver at the same
time the mobile unit is trying to receive transmissions from the base
station on an adjacent frequency.
This is called the near-far effect and is most prevalent when a mobile
unit is receiving a weak signal from the base station.

FIGURE 9. Adjacent-channel interference

Adjacent-channel interference is depicted in Figure 9. Mobile unit 1 is
receiving frequency f1 from base station A.
At the same time, base station A is transmitting frequency f2 to mobile unit
2. Because mobile unit 2 is much farther from the base station than mobile
unit 1, f2 is transmitted at a much higher power level than f1.
Mobile unit 1 is located very close to the base station, and f2 is located next
to f1 in the frequency spectrum (i.e., the adjacent channel), therefore, mobile
unit 1 is receiving f2 at a much higher power level than f1.
Because of the high power level, the filters in mobile unit 1 cannot block all
the energy from f2, and the signal intended for mobile unit 2 interferes with
mobile unit 1’s reception of f1.
f1 does not interfere with mobile unit 2’s reception because f1 is received at
a much lower power level than f2.
Using precise filtering and making careful channel assignments can minimize
adjacent channel interference in receivers.
Maintaining a reasonable frequency separation between channels in a given
cell can also reduce adjacent-channel interference.
However, if the reuse factor is small, the separation between adjacent
channels may not be sufficient to maintain an adequate adjacent-channel
interference level.

Adjacent Channel Interference
 Results from imperfect receiver filters, allowing nearby
frequencies to leak into pass-band.
 Can be minimized by careful filtering and channel
assignments.
 Channels are assigned such that frequency separations
between channels are maximized.
 For example, by sequentially assigning adjacent bands to
different cells
 Total 832 channels, divided into two groups with 416 channels
each.
 Out of 416, 395 are voice and 21 are control channels.
 395 channels are divided into 21 subsets, each containing
almost 19 channels, with closet channel 21 channels away
 If N=7 is used, each cell uses 3 subsets, assigned in such a way
that each channel in a cell is 7 channels away.

Cell Splitting
Cell splitting is when the area of a cell, or independent component
coverage areas of a cellular system, is further divided, thus creating more
cell areas.
The purpose of cell splitting is to increase the channel capacity and
improve the availability and reliability of a cellular telephone network.
The point when a cell reaches maximum capacity occurs when the
number of subscribers wishing to place a call at any given time equals the
number of channels in the cell, this is called the maximum traffic load of
the cell.
Splitting cell areas creates new cells, providing an increase in the degree
of frequency reuse, thus increasing the channel capacity of a cellular
network. Cell splitting provides for orderly growth in a cellular system.
The major drawback of cell splitting is that it results in more base station
transfers (handoffs) per call and a higher processing load per subscriber.

FIGURE 10. Cell splitting
 It has been proven that a reduction of a cell radius by a factor of
4 produces a 10-fold increase in the handoff rate per subscriber.

Figure 10 illustrates the concept of cell splitting. Macro cells are
divided into mini cells, which are then further divided into microcells as
traffic density increases.
Each time a cell is split, its transmit power is reduced.
As Figure 10 shows, cell splitting increases the channel capacity of a
cellular telephone system by rescaling the system and increasing the
number of channels per unit area (channel density).
Cell splitting decreases the cell radius while maintaining the same co-
channel reuse ratio (D/R).

Sectoring
In a cellular telephone system, co-channel
interference can be decreased by replacing a single
Omni-directional antenna with several directional
antennas, each radiating within a smaller area.
These smaller areas are called sectors, and decreasing
co-channel interference while increasing capacity by
using directional antennas is called sectoring.
The degree in which co-channel interference is
reduced is dependent on the amount of sectoring used.
A cell is normally partitioned either into six 60° or three
120° sectors as shown in Figure 11.

Sectoring
 In the three-sector configuration shown in Figure 11a,
three antennas would be placed in each 120° sector—
one transmit antenna and two receive antennas.
 Placing two receive antennas (one above the other) is
called space diversity. Space diversity improves
reception by effectively providing a larger target for
signals radiated from mobile units.
 The separation between the two receive antennas
depends on the height of the antennas above the
ground. This height is generally taken to be the height
of the tower holding the antenna.

FIGURE 11. Sectoring: (a) 120-degree sectors (b) 60-degree sectors

antennas located 30 meters above the ground require a separation of
eight wavelengths, and antennas located 50 meters above the ground
require a separation of 11 wavelengths.
When sectoring is used, the channels utilized in a particular sector
are broken down into sectored groups that are used only within a
particular sector.
With seven-cell reuse and 120° sectors, the number of interfering
cells in the closest tier is reduced from six to two.
Sectoring improves the signal-to-interference ratio, thus increasing
the system’s capacity.

Problems with Sectoring
Increases the number of antennas at each BS
Decrease in Trunking efficiency due to
sectoring(dividing the bigger pool of channels
into smaller groups)
Increase number of handoffs(sector-to sector)
Good news:Many modern BS support
sectoring and related handoff without help of
MSC

A simplified cellular telephone system that includes all the basic
components necessary for cellular telephone communications.
The figure shows a wireless radio network covering a set of geographical
areas (cells) inside of which mobile two-way radio units, such as cellular or
PCS telephones, can communicate.
The radio network is defined by a set of radio-frequency transceivers
located within each of the cells.
The locations of these radio-frequency transceivers are called base
stations.
A base station serves as central control for all users within that cell.
Mobile units (such as automobiles and pedestrians) communicate
directly with the base stations, and the base stations communicate with a
Mobile Telephone Switching Office (MTSO).
An MTSO controls channel assignment, call processing, call setup, and
call termination, which includes signalling, switching, supervision, and
allocating radio-frequency channels.
The MTSO provides a centralized administration and maintenance point for
the entire network and interfaces with the public telephone network over
wireline voice trunks and data links.

FIGURE 12 Simplified cellular telephone system

MTSOs are equivalent to class 4 toll offices, except smaller. Local loops
(or the cellular equivalent) do not terminate in MTSOs.
The only facilities that connect to an MTSO are trunk circuits. Most
MTSOs are connected to the SS7 signaling network, which allows cellular
telephones to operate outside their service area.
Base stations can improve the transmission quality, but they cannot
increase the channel capacity within the fixed bandwidth of the network.
Base stations are distributed over the area of system coverage and are
managed and controlled by an on-site computerized cell-site controller that
handles all cell-site control and switching functions.
Base stations communicate not only directly with mobile units through
the airways using control channels but also directly with the MTSO over
dedicated data control links (usually four wire, full duplex).

The base station consists of a low-power radio transceiver, power
amplifiers, a control unit (computer), and other hardware, depending on
the system configuration.
Cellular and PCS telephones use several moderately powered
transceivers over a relatively wide service area.
The function of the base station is to provide an interface between
mobile telephone sets and the MTSO.
Base stations communicate with the MTSO over dedicated data links,
both metallic and non-metallic facilities, and with mobile units over the
airwaves using control channels.
The MTSO provides a centralized administration and maintenance
point for the entire network, and it interfaces with the PSTN over
wireline voice trunks to honor services from conventional wireline
telephone subscribers.

Cell-Site Controllers
Each cell contains one cell-site controller (sometimes called base
station controller) that operates under the direction of the
switching center (MTSO).
Cell-site controllers manage each of the radio channels at each
site, supervises calls, turns the radio transmitter and receiver on
and off, injects data onto the control and voice channels, and
performs diagnostic tests on the cell-site equipment.
Base station controllers make up one part of the base station
subsystem. The second part is the base transceiver station (BTS).

Roaming is when a mobile unit moves from one cell to another—possibly
from one company’s service area into another company’s service area
(requiring roaming agreements).
As a mobile unit (car or pedestrian) moves away from the base station
transceiver it is communicating with, the signal strength begins to decrease.
The output power of the mobile unit is controlled by the base station
through the transmission of up/down commands, which depends on the signal
strength the base station is currently receiving from the mobile unit.
One of the most important features of a cellular system is its ability to
transfer calls that are already in progress from one cell-site controller to
another as the mobile unit moves from cell to cell within the cellular network.
The base station transfer includes converting the call to an available channel
within the new cell’s allocated frequency subset.
The transfer of a mobile unit from one base station’s control to another base
station’s control is called a handoff (or handover).
Handoffs should be performed as infrequently as possible and be completely
transparent (seamless) to the subscriber (i.e., the subscribers cannot perceive
that their facility has been switched).

Hand-off
 Mobile moves into a different cell during a conversation,
MSC transfers the call to new channel belonging to new
BS
 Handoff operation involves identifying the new BS and
allocation of voice and control signal to channels
associated with new BS
 Must be performed successfully, infrequently and
imperceptible to user
 To meet these requirements an optimum signal level
must be defined to initiate a handoff.
 Min useable signal for acceptable voice qualtiy -90 to -
100 dBm
 A slight higher value is used as threshold

Hand-off strategies
 Handoff is made when received signal at the BS falls below a
certain threshold
 During handoff: to avoid call termination, safety margin should
exist and should not be too large or small
 Large ∆ results in unnecessary handoff and for small insufficient
time to complete handoff, so carefully chosen to meet the
requirements.
 In Fig , handoff not made and signal falls below min acceptable
level to keep the channel active.
 Can happen due to excessive delay by MSC in assigning handoff,
or when threshold is set to small.

Hand-off strategies
Excessive delay may occur during high traffic
conditions due to computational loading or non
availability of channels in nearby cells
In deciding when to handoff , it is important to
ensure that the drop in signal level is not due to
momentary fading.
In order to ensure this the BS monitors the signal
for a certain period of time before initiating a
handoff
The length of time needed to decide if handoff is
necessary depends on the speed at which the
mobile is moving

Umbrella cell approach to accommodate wide
range of velocities

The handoff process involves four basic steps:
Initiation: Either the mobile unit or the network determines the
need for a handoff and initiates the necessary network procedures.
Resource reservation: Appropriate network procedures reserve the
resources needed to support the handoff (i.e., a voice and a control
channel).
Execution: The actual transfer of control from one base station to
another base station takes place.
Completion: Unnecessary network resources are relinquished and
made available to other mobile units.

Channel Assignment Strategies
 A scheme for increasing capacity and minimizing
interference is required.
 CAS can be classified as either fixed or dynamic
 Choice of CAS impacts the performance of system.
 In Fixed CA each cell is assigned a predetermined set of
voice channels
 Any call attempt within the cell can only be served by
the unused channel in that particular cell
 If all the channels in the cell are occupied, the call is
blocked. The user does not get service.
 In variation of FCA, a cell can borrow channels from its
neighboring cell if its own channels are full.

Dynamic Channel Assignment
 Voice channels are not allocated to different cells permanently.
 Each time a call request is made, the BS request a channel from the MSC.
 MSC allocates a channel to the requesting cell using an algorithm that
takes into account
 likelihood of future blocking
 The reuse distance of the channel ( should not cause interference)
 Other parameters like cost
 To ensure min QoS, MSC only allocates a given frequency if that frequency
is not currently in use in the cell or any other cell which falls within the
limiting reuse distance.
 DCA reduce the likelihood of blocking and increases capacity
 Requires the MSC to collect real-time data on channel occupancy and
traffic distribution on continuous basis.

Trunking & Grade of Service
 Cellular radio systems rely on trunking to
accommodate a large number of users in a limited
radio spectrum.
 Trunking allows a large no of users to share a
relatively small number of channels in a cell by
providing access to each user, on demand, from a
pool of available channels.
 In a trunked radio system (TRS) each user is allocated
a channel on a per call basis, upon termination of the
call, the previously occupied channel is immediately
returned to the pool of available channels.

Key Definitions
 Setup Time: Time required to allocate a radio channel
to a requesting user
 Blocked Call: Call which cannot be completed at the
time of request, due to congestion(lost call)
 Holding Time: Average duration of a typical call.
Denoted by H(in seconds)
 Request Rate: The average number of calls requests
per unit time( λ)
 Traffic Intensity: Measure of channel time utilization or
the average channel occupancy measured in Erlangs.
Dimensionless quantity. Denoted by A
 Load: Traffic intensity across the entire TRS (Erlangs)

Erlang-a unit of traffic
 The fundamentals of trunking theory were developed by
Erlang, a Danish mathematician, the unit bears his name.
 An Erlang is a unit of telecommunications traffic
measurement.
 Erlang represents the continuous use of one voice path.
 It is used to describe the total traffic volume of one hour
 A channel kept busy for one hour is defined as having a
load of one Erlang
 For example, a radio channel that is occupied for thirty
minutes during an hour carries 0.5 Erlangs of traffic
 For 1 channel
 Min load=0 Erlang (0% time utilization)
 Max load=1 Erlang (100% time utilization)

Erlang-a unit of traffic
 For example, if a group of 100 users made 30 calls in
one hour, and each call had an average call
duration(holding time) of 5 minutes, then the number
of Erlangs this represents is worked out as follows:
 Minutes of traffic in the hour = number of calls x
duration
 Minutes of traffic in the hour = 30 x 5 = 150
 Hours of traffic in the hour = 150 / 60 = 2.5
 Traffic Intensity= 2.5 Erlangs

Traffic Concepts
 Traffic Intensity offered by each user(Au): Equals
average call arrival rate multiplied by the holding
time(service time)
Au=λH(Erlangs)
 Total Offered Traffic Intensity for a system of U users
(A):
A =U*Au(Erlangs)
 Traffic Intensity per channel, in a C channel trunked
system
Ac=U*Au/C(Erlangs)

 In a TRS, when a particular user requests service and all
the available radio channels are already in use , the
user is blocked or denied access to the system. In some
systems a queue may be used to hold the requesting
users until a channel becomes available.
 Trunking systems must be designed carefully in order
to ensure that there is a low likelihood that a user will
be blocked or denied access.
 The likelihood that a call is blocked, or the likelihood
that a call experiences a delay greater than a certain
queuing time is called “Grade of Service” (GOS)’’.

Grade of Service (GOS): Measure of ability of a
user to access a trunked system during the busiest
hour. Measure of the congestion which is specified
as a probability.
The probability of a call being blocked
Blocked calls cleared(BCC) or Lost Call
Cleared(LCC) or Erlang B systems
The probability of a call being delayed beyond a
certain amount of time before being granted access
Blocked call delayed or Lost Call Delayed(LCD) or
Erlang C systems

Blocked Call Cleared Systems
When a user requests service, there is a minimal
call set-up time and the user is given immediate
access to a channel if one is available
If channels are already in use and no new channels
are available, call is blocked without access to the
system
The user does not receive service, but is free to try
again later
All blocked calls are instantly returned to the user
pool

Modeling of BCC Systems
 The Erlang B model is based on following assumptions :
 Calls are assumed to arrive with a Poisson distribution
 There are nearly an infinite number of users
 Call requests are memory less ,implying that all users, including
blocked users, may request a channel at any time
 All free channels are fully available for servicing calls until all
channels are occupied
 The probability of a user occupying a channel(called service time)
is exponentially distributed. Longer calls are less likely to happen
 There are a finite number of channels available in the trunking
pool.
 Inter-arrival times of call requests are independent of each other

Modeling of BCC Systems
Erlang B formula is given by
where C is the number of trunked channels offered by a
trunked radio system and A is the total offered traffic.

BCC System Example
 Assuming that each user in a system generates a traffic
intensity of 0.2 Erlangs, how many users can been
supported for 0.1% probability of blocking in an Erlang
B system for a number of trunked channels equal to 60.
Solution 1:
System is an Erlang B
Au = 0.2 Erlangs
Pr [Blocking] = 0.001
C = 60 Channels
From the Erlang B figure, we see that
A ≈ 40 Erlangs
Therefore U=A/Au=40/0.02=2000users.

Blocked Call Delayed(BCD) Systems
 Queues are used to hold call requests that are initially
blocked
 When a user attempts a call and a channel is not
immediately available, the call request may be delayed
until a channel becomes available
 Mathematical modeling of such systems is done by
Erlang C formula
 The Erlang C model is based on following assumptions :
 Similar to those of Erlang B
 Additionally, if offered call cannot be assigned a
channel, it is placed in a queue of infinite length
 Each call is then serviced in the order of its arrival

Blocked Call Delayed Systems
Erlang C formula which gives likelihood of a
call not having immediate access to a channel
(all channels are already in use)

Modeling of BCD Systems
 Probability that any caller is delayed in queue for a wait time
greater than t seconds is given as GOS of a BCD System
 The probability of a call getting delayed for any period of time
greater than zero is
P[delayed call is forced to wait > t sec]=P[delayed] x
Conditional P[delay is >t sec]
 Mathematically
Pr[delay>t] = Pr [delay>0] Pr [delay>t| delay>0]
Where P[delay>t| delay>0]= e(-(C-A)t/H)
Pr[delay>t] = Pr [delay>0] e(-(C-A)t/H)
 where C = total number of channels, t =delay time of
interest, H=average duration of call

FWSC UNIT-IV.pptx

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