UPENA DALAL 002 Dalal PSN Chapter_2

7/24/2019 UPENA DALAL 002 Dalal PSN Chapter_2

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Oxford University Press 2014. All rights reserved.

Upena Dalal

Wireless

Communication

and Networks


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Cellular Theory

Chapter 2


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Why cellular technology

Cells, clusters, and cell splitting

Frequency reuse concept and reuse distance calculation

Cellular system components

Antennas for base station

Operations of cellular systems and handoff

Channel assignment fixed and dynamic

Cellular interferencesco-channel and adjacent channel

Sectorization Mobile traffic calculation

Spectrum efficiency of cellular systems

Location management

Key Topics


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The concept of trunking (resource sharing) between two

central telephone exchanges was used in conventionallandline telephone systems.

A trunked radio system is a complex, centrally controlled, full

duplex radio system that allows sharing of limited radio

frequency (RF) channels among a large group of users. It usescontrol or signalling channels as well.

Introduction


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The concept of cellular technology encompasses various

aspects of infrastructure, for instance, division of area,

frequency management or allocations, and call handling.

Following are the two major components of mobile

telecommunication system in general:

Central station-- This is common for many subscribers and

includes the switching equipment and an RF transmitter

and receiver.

Mobile telephone-- Every subscriber must have a mobile

telephone that includes a microphone, a speaker, dialling

facility, a radio transmitter, and a receiver.


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In modern cellular telephony, rural and urban regions are divided

into areas according to specific provisioning guidelines orstandard protocols.

Deployment parameters, such as amount of cell splitting and cell

sizes, are determined experimentally in the cellular system

architecture. Provisioning for each region is planned according to an

engineering plan, which includes cells, clusters, frequency reuse,

and handovers.

Today, software tools are also available to plan and designcellular architecture.

Cellular infrastructure


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A cell is the basic geographic unit of a cellular system.

The term cellular comes from the honeycomb (hexagonal)shape of the areas into which a coverage region is divided

theoretically.

Cellwise, one base station provides transmission over a small

geographic area. Cell sizes vary depending on the landscape (topographic

locations such as mountains, valleys, and plains).

Due to the constraints imposed by natural terrain and man-

made structures, practically, the cells are not perfecthexagons.

Cells



A group of cells form a cluster , the size of which is denoted as N.

Figure illustrates a seven-cell cluster. Each cluster is allocated a setof frequencies. N is restricted by the following equation:

Cluster



non-negative integers i

andj determine the relative locations of co-channel cells, as shown

in Figure.

i = vertical move, j = slant horizontal move

A relationship exists between the cluster

design equation and frequency reuse as well as

reuse distance.

Reaching to co-channel cell



The size of a cell depends on the density of subscribers in an

area.

The capacity of a network in a densely populated area can be

improved by reducing the size of the cells or by increasing the

number of cells along with installing low-power base stations.This will effectively increase the number of channels in that

area because of more frequency reuse.

On the basis of their size, cells may be categorized asmacrocells, microcells , picocells, and femtocells .

Cell Size



Cell Splittingfor maximizing number of users



A radio channel consists of a pair of frequencies for full duplex

operation. The concept of frequency reuse is based on assigning to each cluster

the same group of radio channels used within a small geographic area.

A set of N different frequency groups {f1, ..., fN} is used for each cluster

of N adjacent cells and shared among the cells almostequally.

The set of frequencies assigned to a cell is completely different from that

assigned to the neighbouring cells.

The coverage area of the cells is called the footprintsurrounded by outer

boundary of the cell.

If there are seven members in a cluster, number of available frequencysets is seven, the frequency reuse factor is 1/7. This means that each

cell uses one-seventh of the available cellular channel for reuse.

Cellular Frequency Reuse



Let L = total number of duplex channels available for reuse (i.e., frequencies per

cluster)

k = number of duplex channels allocated to each cell of a cluster (k < L)

N = cluster size (in which there are N cells)M = number of times the cluster is repeated

C = total effective number of duplex channels available in the area

Thus, L = k N

C = M L = MkN

Cluster of seven cells



For Example:

Reuse distance (D) calculation



Cellular planning can be done byusing various softwares and

planning tools

screenshot of Ericsson TEMS

Real world cells



There are three types of cellular systems

1. Analog Circuit-switched Cellular System

Mobile Unit or mobile subscriber unit (MSU)

Cell Site or base station Mobile Telecommunication Switching Office

(for connection management and billing)




2. Digital Circuit-switched Cellular System

Mobile Station

Base Station or Base Transceiver Station

Base Station Controller

Switching Subsystems 3. Packet-switched Cellular System

It has six elements: MS (user equipment), basestation, radio network controller(RNC), service

support node (SSN), gateway support node (GSN), andcharging gateway function (CGF).


E l f di it l i it it h d ll l t l



To be studied in Chapter 11

In detail.

Example of digital circuit switched cellular system example

(GSM system)


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Two main types of antennas are used in the

wireless industry for a BTS omnidirectional

and directional

Omnidirectional Directional with 3 sectors

Antennas for cellular systems


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Omnidirectional antennas are nothing but the dipoles and

they radiate in all lateral directions.

A directional antenna shapes and projects a beam of radioenergy in a specific direction and receives radio energy onlyfrom a specific direction, employing various horizontal

beamwidths.

Here, the term beamwidth refers to the conical size of theradiated beam.

Downtilting---The radiation pattern of a downtilt antenna iselectrically or mechanically tilted downwards at a specifiednumber of degrees to avoid shadowing under and near theantenna zone.



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A base station antenna is chosen according to the situation

depending upon many factors, such as the following: Size of the area to be covered

Configurations of the neighbouring cell sites

Type of antenna usedomnidirectional or directional

Antennas beamwidth in case of a directional antenna

Allotted RF spectrum the antenna can utilize



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The probable adverse effects of the radiation from the base stations are a

major concern, especially among residents in urban areas. Independent

monitoring of RF exposure levels around mobile phone base stations has been

conducted for many years under the control of government agencies.

Radiation pattern from base station (Lighter shades of grey indicate decreasing power

strength)

Adverse Effects of Base Stations


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Currently, PIFA (planar inverted-F antennaa shorted patch)wideband antennas are used widely and sometimes fractalantennas are also used. PIFA is the modified version of thequarter-wavelength patch antenna.

As the patch is shorted at the end, the current at the end of the

patch antenna is no longer forced to be zero. This results in thesame currentvoltage distribution as that of a half-wave patchantenna. The antenna is resonant at a quarter-wavelength, thusreducing the space needed on the telephone; moreover, ittypically has good specific absorption rate (SAR) properties. Ithas a low profile and an omnidirectional pattern.

Antennas for Mobile Radio Frequency Front End


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Stages for Mobile-originated Call

There are two possibilities in a mobile-originated call:

mobile-to-mobile call and mobile-to-landline call .

As an example, the call set-up sequences for mobile-originated calls in a GSM

system are given in Fig. 11.3 in Chapter 11

1. Mobile-originated Call

Operations of cellular systems


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There are two possibilities:

mobile-to-mobile call and landline-to-mobile call

2. Mobile-Terminated Call

Stages for Mobile-terminated Call

Operations of cellular systems


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Mobile-originated calls are routed to the BTS first and then to the BSC.

The BSC forwards this call to the MSC. The MSC authenticates and routes the call to the called subscriber as

per the dialled digits.

If the called subscriber is in mobility, the process to receive the call on

his mobile device is described in Section 2.5.2

Mobile-terminated calls come to the MSC first, where the HLR/VLR

enquiry is carried out, and as per the information, the MS is paged in

the suitable BSC.

The BSC forwards this page to all BTSs where the actual paging is done.

After a BTS gets a response from the mobile, it allocates a channel forthis call.

On ending the call, the BTS informs the BSC and MSC.

Steps


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A feature called registration is used for landline-originated calls.

A landline user dials a mobile unit number.

The telephone company zone office recognizes that the call is for

a mobile number and forwards it to the MTSO/MSC.

The MTSO/MSC sends a paging message to certain cell sites

based on the mobile unit number and the search algorithm. Eachcell site transmits the page on its own set-up channel.

If the mobile unit is registered, the registered site pages the

mobile. The mobile unit recognizes its own identification

on a strong set-up channel, locks onto it, and responds to the cellsite.

The mobile unit also follows the instruction to tune to an assigned

voice channel and initiates user alert.

Network-originated or Landline-originated Call

ll


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When a mobile user turns off the transmitter, a particular

signalling tone is transmitted to the cell site and the voicechannel is freed by both the sides.

The mobile unit resumes monitoring pages through the strongest

set-up channel.

Call Termination

d ff d


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Handoff is also knownas handover.

It can be categorized

as:

hard handover

soft handoversofter handover

Hard Handover scenario

Handoff Procedure


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Handoff probability It is the probability that a handoff is executedbefore call termination.

Rate of handover It is the number of handovers per unit time.

Interruption duration It is the duration of time during a handoverprocedure in which a mobile is not connected to any base station.

Handoff delay It is the distance between the point at which the handoffshould occur and the point at which it does occur.

Probability of unsuccessful handoff It is the probability that a handoffis executed while the reception conditions are inadequate.

Handoff blocking probability It is the probability that a handovercannot be completed successfully.

Few terms associated with the handoff procedure

H d i


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The handoff occurs only if the new base station is sufficiently stronger than the

current one by handover margin .

Hard handover scheme prevents theping-pong effect.

Hard handover procedure exhibits hysteresis .

Handover strategies The following are the two different strategies for

handover:

(a) Centralized methods (used in GSM)

(b) Decentralized methods [used in DECT (WLL)]

Handover margin

M h d f h d ff


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Mobile-assisted handoff The mobile measures the power receivedfrom the surrounding base stations and continuously reports theresults of the measurements to the serving base station. A handoff isinitiated when the power received from the base station of aneighbouring cell begins to exceed that from the current base stationby a certain level or a certain period of time. This method is currentlybeing used in the mobile systems.

Base station-assisted handoff In the first-generation systems, thestrength measurements are made by the base stations and supervisedby the MTSO. Here, the base station measures the signals from themobiles served by it, as well as from the mobiles in the neighbouringcells and reports to the MTSO. The MTSO decides whether a handoff is

necessary as well as who needs it. Here, the load of the MTSO is morecompared to the mobile-assisted method, and hence handoffs areslower.

Methods of hand-off

S ft H d


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It allows the MSC to make a soft decision regarding the version of

the users signal to pass.

The ability to select between the instantaneous received signals

from a variety of base stations is called soft handover. The

technique is discussed in detail in Chapter 11.

More suitable for CDMA based systems.

Soft Handover


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Figure for Example 2.3

Ch l A i t


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Fixed Channel AssignmentIn the fixed channel assignment method, each cell is allocated apredetermined set of voice channels. Only the unused channels in a particular cell canserve any call attempt within that cell. If all the channels are occupied, then the call will be

blocked and the subscribers will not get any service.

Borrowing StrategyIn the borrowing strategy method, a cell is allowed to borrow achannel from a neighbouring cell, if all of its own channels are occupied. The MSCsupervises the borrowing procedure and ensures that the borrowing of the channel doesnot disrupt or interfere with any of the calls in progress in the donor cell.

Dynamic Channel AssignmentIn the dynamic channel assignment method, voice channelsare not allocated permanently in any of the cell. When a call request is made, the servingbase station requests a channel from the MSC, which then allocates a channel to therequested cell.

Advantages of the dynamic channel assignment --the likelihood of call blocking reduces, co-channel

interference reduces and channel utilization increases.

Disadvantage is that the MSC must be fast and capable of collecting real-time data on channel occupancy,traffic distribution, and radio signal strength indications of all channels on a continuous basis.

Channel Assignment

C ll l i t f


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Two major types of cellular

interferences are co-channel

interference and adjacent

channel

interference.

Cochannel Interference

due to this scenario

Cellular interferences

C h l i t f l l ti


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The co-channel cells surrounding a particular cell exist in a circular (or rather

hexagonal) pattern. There will be maximum interference from the nearest circle of

cells, which are also called first-tier cells and are always six in number. Based on

thatthe signal-to-interference ratio (SIR) for a mobile receiver

where S is the desired signal power from the desired base station and Ii is the

interference power of the base station of the ith interfering co-channel cell

(neglecting noise). n is the path loss exponent. The empirical value ofn ranges from

two to five.

Based on the distance of the interfering base stations to the desired mobile receiver

Di, D/R ratio and cluster size N

Co-channel interference calculations

C h l i t f l l ti


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where k is the proportionality constant, which depends on the power

radiated by the base station at the centre. I represents the power radiated by

the other base stations in the first tier, which are six in number.

Hence,

From above two equations

From the equation it is clear that N should be designed critically for the

desired SIR.

Co-channel interference calculations


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SINR f dj t h l i t f


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Adjacent channel

interference can beminimized through

careful filtering at IF

stages and careful

channel assignments.

SINR for adjacent channel interference

S t i ti


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Sectorized base stations are created by subdividing an omnicell into sectors that

are covered using directional antennas mounted in the same base station

location. Following is the example of 3 sector division and each sector is treatedas different cell.

To sectorize a cell, a

horizontal, equilateral

platform resembling a

triangle is deployed on a

tower. Each side of the

platform is called a face.

Three, four, or six

directional antennas are

installed on the platform,depending on the number

of sectors.

Sectorization

Why Sectorization


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Sectorization facilitates wireless engineering and operations in

the following ways: It minimizes or eliminates co-channel interference.

It optimizes the frequency reuse plan. This is facilitated

through another concept known as thefront-to-back ratio .

At a minimum, it triples the capacity of any given coveragearea when compared to the capacity offered by deploying

omni-antennas.

Why Sectorization

Mobile traffic calculation


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Traffic calculation is essential to both mobile phone and landline

systems. On the basis of the traffic condition, the telephone system must

be designed such that only a minimum number of calls are

blocked and uninterrupted service is provided to the subscribers.

The traffic varies considerably throughout a day, but mostsystems are designed to handle the traffic during the peak busy

hour in a day.

Mobile traffic calculation


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Traffic variation in a small business firm as per normal activities in a day

from 6 a.m. to 10 p.m. is shown in figure by average traffic measured every

hour

In teletraffic engineering, trunk describes any entity that will carry one call. The

number of trunks connecting one MSC with another is the number of voice pairsused in the connection; hence, it is important to determine the number of trunks

required between the MSCs.

Trunking efficiency refers to the decrease in call blocking and increase in coverage

without call drops, at the cost of management overhead.

Some definitions


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Call blocking It is the non-availability of any trunk or call propagation path.

Busy hour or peak busy hour It is the 60-min interval in which the traffic or the number

of call attempts is the highest in a day. It usually varies from day to day or over anumber of days.

Time-consistent busy hourIt is the one hour period starting at the same time each day

for which the average traffic volume or the number of call attempts is the highest over

the days under consideration.

Call completion rate It is the ratio of the number of successful calls to the number of callattempts. It can be represented in percentage.

Busy hour call attempts It is the number of call attempts in the busy hour and it decides

the network capacity.

Average busy hour calls (completed) = Busy hour call attempts call completion rate

Busy hour calling rate It is the average number of calls originated by a subscriber during

the busy hour or the call intensity per traffic path during the busy hour.

Day to busy hour traffic ratio It is the ratio of busy hour calling rate to the average

calling rate for the day. It indicates how much of the days total traffic is carried during

the busy hour.

Some definitions

Contd

Some definitions


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Cont d

Centum call second (CCS) It represents the calltime product and is a traffic measurement

unit. 1 CCS is one call for a duration of 100 s or 100 calls for 1 s duration each or any other

combination. Other measures are call seconds (CS) and call minutes (CM).

Average call arrival rate It is the average number of calls that arrive during the specified

time duration.

Average call holding time It is the average duration of calls that arise within the specified

time duration or the average duration of occupancy of a traffic path by a call.

Erlang It is the unit of traffic that represents the total use of one channel or one call perhour that lasts for one hour.

Set-up time It is the time required to allocate a trunk (or trunked radio) channel.

Blocked call or lost call It is the call that cannot be completed due to lack of channels.

Some definitions

Traffic Characterization


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Traffic intensity, more commonly called the traffic, is defined as the average

number ofcalls in progress. Although this is a dimensionless quantity, the unit of

traffic has been given a nameerlang (abbreviated as E).

In a group of channels, the average number of calls in progress depends on both

the number of calls that arrive and their duration. The duration of a call is often

called its holding time .

The traffic carried by a group of trunks is given by

Where,A is the traffic in erlangs, c is the average number of call arrivals during

time T , and h is the average call holding time.

Traffic Characterization


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Call Handling and Grade of Service


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When all the channels become occupied, the system cannot accept further calls.

This state is known as congestion .

The arrival of a new call can then be handled by either of the following two

methods:

(a) Blocked (for a lost call system based on circuit switching)

(b) Queued (for a delayed system based on message or packet switching)

Traffic carried = Traffic offered - Traffic lost

Call Handling and Grade of Service

The proportion of calls lost or delayed due to congestion is used to measure


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The proportion of calls lost or delayed due to congestion is used to measure

the service quality, called the grade of service (GOS). The GOS B for a lost call

system may be defined as

B = Number of calls lost/Number of calls offered

B = Traffic lost/Traffic offered

= Probability that a call will be lost due to congestion

Thus, if a traffic ofA erlangs is offered to a group of trunks or channels having

a GOS B , the traffic lost is AB and the traffic carried is

A (1 B ) erlangs.

Larger the GOS value, worse will be the service provided.

Erlang B and C tables are readily available in Appendix F

Mathematical Modelling of Traffic


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A simple mathematical model is based on the following assumptions:

Pure chance traffic--If the traffic is pure chance, all call arrivalsand call terminations are independent random events,

therefore, sometimes called memoryless traffic . A commonly

used model for random, mutually independent message (here,

call) arrivals is the Poisson process .

Statistical equilibrium--According to the statistical equilibrium

assumption, the generation of traffic is a stationary random

process; that is, the probabilities do not change during the

period being considered. Consequently, the mean number of

calls in progress remains constant.

Mathematical Modelling of Traffic

Poisson Process


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The probability of two or more arrivals happening during is

negligible compared to the probability of zero or one arrival. The distribution of the number of arrivals in a time interval of t to

t + T is independent of the starting time t . T is the interval between

call arrivals or the interval between two random events.

The probability of the number of call arrivals in a given time has aPoisson distribution given by

Poisson Process

Markov model for a number of occupied channels in a


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P(1), P (2), , P ( N ) are the state probabilities .

P0,1, P1,2,and so on are the probabilities of a state increment.

network wit N channels

Erlang B Formula


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Erlang determined the GOS (i.e., the loss

probability) of a lost call system having N trunkswhen the offered traffic isA . Thesolution/formula was obtained on the basis of the

following assumptions.: Pure chance traffic

Statistical equilibrium

Full availability

Loss of calls encountering congestion

The formula is

Erlang B Formula

Erlang C Formula


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In an Erlang C telephone system, N channels areavailable. New calls are assigned a channeluntilall channels are full. When all the channels areoccupied, a new call is queued until it can beserved. This is in contrast to an Erlang B system in

which new calls are blocked. The assumptions made here are the same as in

an Erlang B system, except the fourth one.

The formula is

Erlang C Formula



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Let

k = number of channels per cell

N = cluster size

Wchannel = total bandwidth for the cellular net

Wsignal = occupied bandwidth per channel

Then Wchannel = kNWsignal

The spectrum efficiency SE of a cellular net can be defined as the carried traffic per

cell Ac, expressed in erlangs, divided by the bandwidth of the total systemWchannel and the area of the cell Su. Ac is mostly computed from the Erlang B

formula, with Ac equal to the attempted traffic multiplied by the probability of

success (= 1 blocking probability). Spectrum efficiency is expressed in

erlang/MHz/


Location management


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The study of location management aims at tracking a subscriber

(with an active mobile unit) on move and at reducing the

overhead incurred in locating that subscriber in a cellular

environment, though he/she moves continuously from cell to cell.

Actually, mobility management in cellular systems consists of two

components:

handover management

location management.

Location management

Location management


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In literature, location update schemes are classified into two main groups:

Static or global scheme Location update is triggered based on thetopology of the network.

Dynamic or local scheme A mobile sends a location update messageaccording to the time elapsed (time-based method), the number of

cells visited (movement-based method), or the distance in terms ofcells travelled (distance-based method).

The following are a few terms related to location management:

Centre cell It is the cell where the last location update occurred.

Residing area It is the area in which the mobile unit can be located. Polling cycle It is the process performed by the network when a call

arrives at a mobile terminal. The network sends a polling signal to thetarget cell in the residing area and waits for the response.

Location management

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