basic gsm_chennal concept

7/28/2019 BASIC GSM_Chennal Concept

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TACS GSM 900 DCS 1800

Uplink 890 - 905 MHz 890 - 915 MHz 1710 - 1785 MHz

Downlink 935 - 950 MHz 935 - 960 MHz 1805 - 1880 MHz

Duplex Distance 45 MHz 45 MHz 95 MHz

Carrier Separation 25 kHz 200 kHz 200 kHz

Number of Channels 15 Mhz / 25 kHz = 600 25 MHz / 200 kHz = 124 75 MHz / 200 kHz = 374

Channels defined in Switch 1, 2, .599, 600 1, 2, .123, 124 512 , 513 .884, 885

Access Method FDMA TDMA TDMA

COMPARISON


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TACS GSM 900/DCS 1800


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3 Broadcast

Channels

1) FCCH2) SCH

3) BCCH

3 Common Control

Channels1) PCH

2) RACH

3) AGCH

3 Dedicated Control

Channels1) SDCCH

2) SACCH

3) FACCH

TCH

CBCH

LOGICAL CHANNELS


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Full rate => Used for speech at 13 Kbits/s

or sending data at 9.6 Kbits/s

Half rate => Used for speech at 6.5 Kbits/s

or sending data at 4.8 Kbits/s

Enhanced Full rate => Used for speech at 13 Kbits/s

or sending data at 9.6 Kbits/s but

with almost Land line quality

TCH = TRAFFIC CHANNEL


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FCCH = FREQUENCY CORRECTION CHANNEL

=> To tell the Mobile that this is the BCCH carrier

=> To able the Mobile to synchronize to the frequency

(Downlink only)

SCH = SYNCHRONISATION CHANNEL

=> Used for sending BSIC (Base station Identity Code)

=> Give TDMA frame number to the Mobile.

(Downlink only)

BCCH = BROADCAST CONTROL CHANNEL

=> Used for sending information to the mobile likeCGI (Cell Global identity), LAI (Location Area Identity),

BCCH carriers of the neighboring cells,

maximum output power allowed in the cell and other

broadcast messages like barred cell. (Downlink only)

BROADCAST CHANNELS


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PCH = PAGING CHANNEL

=> Used for paging the Mobile. (Downlink only)

Reason could be an incoming call or an incoming Short Message.

RACH = RANDOM ACCESS CHANNEL

=> Used for responding to the paging (terminating), Location updating

or to make call access (originating) by asking for a signaling channel.

(Uplink only)

AGCH = ACCESS GRANT CHANNEL=> Used to allocate SDCCH to the mobile.

(Downlink only)

COMMON CONTROL CHANNELS


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SDCCH = STAND ALONE DEDICATED CONTROL CHANNEL

=> Used for allocating voice channel (TCH) to the mobile (call setup) and

Location updating.

=> Send Short Text message to Idle Mobile

(Uplink & Downlink)

SACCH = SLOW ASSOCIATED CONTROL CHANNEL=> Used for sending information to the mobile like

CGI (Cell Global identity), LAI (Location Area Identity),

BCCH of all the neighbors and TA (Timing Advance)

=> Send Short Text message to Busy Mobile

(Downlink

=> Used for sending signal strength & bit error rate measurement of theserving cell and signal strength of the BCCHs of the neighboring cells.

(Uplink)

FACCH = FAST ASSOCIATED CONTROL CHANNEL

=> Used for handover. (Uplink & Downlink)

DEDICATED CONTROL CHANNELS


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=> SMS messages are short TEXT messages up to 160 characters in length that you

can send or receive. The messages are not sent straight to the other mobile but is

sent to message centre operated by the Network provider.

=> If the mobile was switched off or is at outside of the coverage area,the message is stored in the Message Service Center. The message

will be offered to the subscriber when the mobile is switched on again

or has reentered the coverage area again.

=> If the mobile is in the Idle mode the short message will be send through

the SDCCH. If the mobile is Busy the short message will send through

the SACCH.

CBCH = CELL BROADCAST CHANNEL

=> Used for sending short messages to all the mobiles within a geographic area.Typical example is Traffic congestion in a major road or a major accident

in an area. Up to 93 characters can be sent.

=> If the mobile is in the Idle mode then the short message will be send through

the CBCH. If the mobile is Busy, it will not be sent.

NOT TO BE CONFUSED WITH SMS !!!!!!!!(SHORT MESSAGE SERVICE)


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TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7

200 kHz

4.615 ms

0.577 ms

FCCH,

SCH,BCCH

PCH,

RACH,

AGCH

SDCCH,

SACCHCBCH

TCH,

SACCHFACCH

TCH,

SACCHFACCH

TCH,

SACCHFACCH

TCH,

SACCHFACCH

TCH,

SACCHFACCH

TCH,

SACCHFACCH

1 TDMA FRAME


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F D T T T T T TS D T T T T T T

B D T T T T T TB D T T T T T T

B D T T T T T T

B D T T T T T T

C D T T T T T T

C D T T T T T T

C D T T T T T T

C D T T T T T T

F D T T T T T T

S D T T T T T T

C D A T A T A T

C D T T T T T T

C D T T T T T T

C D T T T T T T

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

4.615 ms

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1 Frame

(Downlink - BTS transmit)

1 carrier = 200 kHz

F = FCCH

S = SCHB = BCCH

C = PCH or

AGCH

D = SDCCH

A = SACCH

T = TCH

R D T T T T T TR D T T T T T T

R D T T T T T TR D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D A I A I A I

R D T T T T T T

R D T T T T T T

R D T T T T T T

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

4.615 ms

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1 Frame

(Uplink - Mobile transmit)

1 carrier = 200 kHz

R = RACH

A = SACCHT = TCH


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3 TIMESLOT

3 * 0.577ms = 1.73ms

Downlink

Uplink

TCH UP-DOWNLINK OFFSET

3 TIMESLOT

3 * 0.577ms = 1.73ms

3 TIMESLOT

3 * 0.577ms = 1.73ms

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 75 6 7

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..

...

..

This means that the mobile does not transmit and receive at the same time.

Also note that :

in TS 0 : All the Logical Channels will repeat itself after 51 framesin TS 1 : All the Logical Channels will repeat itself after 102 frames

in TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames


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MOBILE STATIONS ISDN NUMBER (MSISDN)

=> Is the mobile number used in a GSM PLMN (Public Land Mobile Network)

MSISDN = Country Code + National Destination Code + Subscriber numbere.x. 63 + 0918 + 8889999

Maximum length is 15 digits.

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)

=> Is the subscriber number used over radio path for all signaling in the GSM PLMN.

This number is stored in SIM (Subscriber Identity Module), HLR (Home Location Register,

and VLR (Visitor Location Register).

IMSI = MCC + MNC + MSIN

= Mobile Country Code + Mobile Network Code + Mobile Identification Number

[ 3 digit ] [ 2 digit ] [ 11 digit ]

e.x. 502 + 19 + 2345451

TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)=> Is used for the subscriber's confidentiality. Since the TMSI has only local significance

(within MSC/VLR) the structure of the TMSI can be chosen by the Vendor.

But the size must be 1/2 of the size of IMSI. Each time a mobile request for location

updating or call setup, MSC/VLR allocates to the IMSI a new TMSI, so the TMSI

is used on the signaling path, protecting the IMSI identity. Plus since the TMSI is half

the size of IMSI, we can page twice the amount compared to IMSI.


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LOCATION AREA IDENTITY (LAI)

=> Is used to uniquely identify each location area in the GSM PLMN. When the systemreceives an incoming call it knows in which location area it should page the mobile

and does not page the entire network.

LAI = MCC + MNC + LAC

Mobile Country Code + Mobile Network Code + Location Area Code

[ 3 digit ] [ 2 digit ] [ 1 to 65 536 ]

e.x. = 502 + 20 + 60001

CELL GLOBAL IDENTITY (CGI)

=> Is used for cell identification within the GSM network.

CGI = MCC + MNC + LAC + CI

Mobile Country Code + Mobile Network Code + Location Area Code + Cell Identity

[ 3 digit ] [ 2 digit ] [ 1 to 65 536 ] [ 1 to 65 536 ]

e.x. = 502 + 20 + 60001 + 50001

BASE STATION IDENTITY CODE (BSIC)

=> Is used to distinguish co channel Frequency used in the neighboring cell.

BSIC = NCC + BCC

Network Color Code + Base Station Color Code

[ 1 to 7 ] [ 1 to 7 ]


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SUBSCRIBER IDENTITY MODULE (SIM)

SIM is used to provide storage on subscriber related information as following :

IMSI (International Mobile Subscriber Identity). Temporary network data like TMSI, LAI, Location update status.

Subscriber Authentication Key (Ki) and Ciphering Key (Kc) which are used for security purposes.

BCCH information : List of carrier frequencies to be used for cell selection.

Forbidden PLMN.

Language preference.

PIN number (Personal Identification Number) and PIN error counter.

PUK number (Personal Unlock Key) and PUK error counter.

PIN management

The PIN number consist of 4 to 8 digit and it is loaded by the service activator an subscription time. Afterwards

the PIN number can be changed as many times an user wishes including the length of the PIN number.

The user can disable the PIN function but again can be inhibited at subscription time by a authorized

person. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive entries

the SIM is blocked, even if if the SIM is removed or the mobile is switch off and on.

If the SIM card is blocked the user cannot access the network. The unblocking of the SIM card can only be

done by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is given to the user at subscription

time. If an incorrect PUK is entered more than 10 times then the PUK will not work anymore and the

SIM card will continue to be blocked until taken to the mobile vendor service center.

Two physical types of SIM are specified :

ID - 1 SIM - Looks like a Credit card

Plug in SIM - Look like a small chip is installed semi permanent in the mobile equipment.


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PLMN SELECTION

CELL SELECTION

CELL RESELECTION

LOCATION UPDATING

AN IDLE MOBILE DOES 4 TASKS:

When the mobile is in idle mode it must always

be camped to a BCCH carrier. Why ???

There are 3 reasons:

1) For the PLMN to know in which location area the mobile is so that it can

page the mobile when an incoming call or Short Text Message is received.

2) The Mobile can initiate a call by accessing the network on Random Access

Channel (RACH) of the cell which it camped on.

3) To receive system information from the PLMN like Traffic congestion andmajor Accidents.


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When the mobile is switched on it will select the registered PLMN in the mobile if there exist one. If

there is no registered PLMN orthe registered PLMN is not available (no coverage) then the mobilewill try to select another PLMN eitherautomaticallyormanuallydepending on it mode.

AUTOMATIC MODE

The automatic mode uses a list of PLMNS in an order of priority. The priority will be :

1)The last network on which you were registered.

2) Home PLMN.3) Each PLMN stored in the SIM card in priority order.

4) Other PLMN with signal level above -85 dBm in random order.

5) All other PLMN in decreasing signal strength.

MANUAL MODE

In the manual mode the mobile will try to connect to the Home PLMN first. If it is unsuccessful then it

will provide a list of available PLMN and ask the user to choose one. If the second chosen PLMN is notsuccessful then the mobile will make an indication to the user to select another PLMN. Until the users

selects another network a message No access will be displayed. If there is no GSM or DCS coverage at

all then a message No Network will be displayed.

PLMN SELECTION


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Once the mobile is switched on and the registered home PLMN was selected (e.x. SMART), it will next

search for a BCCH frequency list, stored in its memory or in its SIM card. The list can have up to

32 BCCH frequencies for the mobile to scan. This reduces the time of cell selection, compared to

scanning the whole frequency band. If this feature is turned off at the switch then the mobile has to scan

the entire frequency band for the strongest BCCH carrier.

The BCCH frequency list is called BA (BCCH Allocation) list and there are 2 types, Active and Idle.

Idle is a list of BCCH used for scanning when the mobile is in an idle mode and Active is a list of BCCH

used during mobile busy mode. Why 2 List ???

When the mobile is in idle mode it may want to scan a longer list of BCCH and tune to the strongest

whereas when in Active mode the list of BCCH should be shorter (correspond to defined neighbors) sothat the mobile will scan the short list and get a more accurate signal strength measurements to achieve

better handover performance. It is also to reduce the time spend by the mobile to decode the BSIC.

Recommendation : ACTIVE MODE LIST SHOULD NOT BE MORE THAN 15 BCCH

FREQUENCIES.

If there is no BA list stored in the Mobile or SIM card then the mobile will scan all the 124 GSM

channels and 374 DCS channel and arrange the frequencies in a DESCENDING order of signal

strength. It will take the mobile 3 to 5 seconds to scan the whole band. After which it will tune to the

strongest frequency. The mobile will check if this is a BCCH carrier by looking out for the frequency

correction burst send by the FCCH (Frequency Correction Channel). If it is the BCCH carrier than

mobile tunes to this carrier to read the SCH (Synchronization Channel) for the BSIC parameter.

Next it will read the BCCH for system information like CGI (Cell Global identity), LAI (Location

Area Identity), BCCH carriers of the neighboring cells (BA List), maximum output power allowed

in the cell and other broadcast messages like barred cell.

[Continues ]

CELL SELECTION


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Next the mobile will compare if the selected cell belongs to a forbidden PLMN stored in its SIM card. It

will look at the 2 digit Network Mobile Code transmitted by the BCCH on the LAI (Location AreaIdentity). If those 2 digits ware registered as forbidden in the SIM card then the mobile will not select

this cell. The mobile then will tune to the second strongest BCCH carrier and subsequently does the same

process over and over again until it finds the right cell. Once it finds the right cell it will start using the

BA (BCCH allocation) list transmitted by the BCCH carrier for cell reselection, will be discussed later.

Lets say the chosen PLMN is correct, able to read the FCH, SCH and BCCH and the chosen Cell is

accessible (no cell barring), DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ON

THIS SITE ??? NO !!!!!!!!!!!THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTION

CRITERION WHICH MUST BE CALCULATED BY THE MOBILE AND

IF THE C1 VALUE IS GREATER THAN 0 THEN THE MOBILE

CAN CAMP ON THIS CELL OR ELSE THE NEXT CELL WITH C1 > 0WILL BE SELECTED

CELL SELECTION


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C1 = A - Max (B,0)

and C1 > 0 for the mobile to camp on this BTS

where:

A = RxLev - RxLevAccMin

B = MsTxPwrMaxCCH - P

C1-CELL SELECTION CRITERION

RxLev = Signal strength received by the mobile

RxLevAccMin = Minimum Signal level to be received by the mobile

from BTS (BCCH) before it could access the BTS

MsTxPwrMaxCCH = Maximum Transmit Power allowed to access the

BTS (using RACH)P = Mobile Class power

C1 = (RxLev - RxLevAccMin) - Max(MsTxPwrMaxCCH - P, 0)

Ex : C1 = ( -80 - (-100) ) - Max (33 - 33, 0)

= -80 + 100 - Max (0, 0)

= 20

=> C1 > 0 so mobile will camp on this site

ShortCut : If RxLevel > RxLevAccMin then Mobile can camp

on this site


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Minimum Signal level that must be received by the mobile

from BTS (BCCH) before it could access the BTSRxLevAccessMin (Nokia)

ACCMIN (Ericsson)

SSACC (TACS) (Uplink)

= -102 (GSM900) = -100 (DCS1800)

General rule : The signal received by the mobile should be2 dB higher than the mobile sensitivity


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What is the accurate way of setting the RxLevAccessMin parameter ?

RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference Margin

Mobile Sensitivity = -104 for GSM900 and -102 for DCS1800

Body loss = 3 dB recommended by ETSI and 5 dB recommended by Ericsson for GSM 900

= 3 dB recommended by ETSI and 3 dB recommended by Ericsson for DCS 1800

Multipath loss = Signal loss from base station due to reflection by buildings, etc before reaching mobile.

Normally the Multipath loss is around 3 dB but can be overcome by Antenna Diversity which has gain

around 3 dB too. (Space diversity = 3 dB, 90 degrees polarized diversity = 3 dB,

45 degrees slant polarized diversity = 4.5 dB)

Interference Margin = Margin allocated to overcome C/I and C/N, the recommended value is 2 dB

RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference MarginRxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 with ETSI standard)

= - 99 dBm

RxLevAccessMin = -104 + 5 + 0 + 2 (for GSM 900 with Ericsson standard)

= - 97 dBm

RxLevAccessMin = -104 + 3 + 0 + 2 (for DCS 1800)

= - 99 dBm


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Class 1 10 Watt (40dBm) 20 Watt (43dBm) 1 Watt (30dBm)

Class 2 4 Watt (36dBm) 8 Watt (39dBm) 0.25 Watt (24dBm)

Class 3 1 Watt (30dBm) 5 Watt (37dBm) 4 Watt (36dBm)

Class 4 0.6 Watt (28dBm) 2 Watt (33dBm) *

Class 5 * 0.8 Watt (29dBm) *


Mobile Sensitivity -113 dBm -104 dBm -102 dBm

BTS Sensitivity -116 dBm -107 dBm -106 dBm


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Maximum Transmit Power allowed to access the

BTS (using RACH) - Mobile is IdleMsTxPwrMaxCCH(Nokia)

CCHPWR (Ericsson)

PLC (TACS)

= 30 dBm (DCS1800) = 0 (28 dBm) (TACS)= 33 dBm (GSM900)

Maximum Transmit Power allowed to use in a BTS

during busy status (Using TCH) - Mobile is BusyMsTxPwrMax(Nokia)

MsTxPwr (Ericsson)

PLVM (TACS)

= 30 dBm (DCS1800) = 0 (28 dBm) (TACS)= 33 dBm (GSM900)

Minimum Transmit Power allowed to use in a BTSduring busy status (Using TCH) - Mobile is Busy

MsTxPwrMin(Nokia)

= 13 dBm (DCS1800)= 13 dBm (GSM900)


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Switch on the Mobile

For 2 to 3 seconds the Mobile will scan all the 124

channels in GSM900 and 374 channels in DCS1800

The mobile will synchronize to this carrierand read the BCCH info like LAI, CGI

Tune to the second

strongest channel

Mobile will compare the signal strengthof the 124 channels and tune to the strongest

Mobile will check if it is a BCCH carrier ?

Does the BCCH belong to the wanted PLMN,

E.x: Smart, Globe, Islacom ?

Is the Cell Barred from accessing ?

Is C1 >0 ?

Camp on this site !!!

Yes

Yes

Yes

No

Yes

No

No

No


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CELL RESELECTION

After the cell has been successfully selected, the mobile now will start reselection tasks. It will continuouslymake measurements on its neighboring cells (as indicated by the BA list) to initiate cell reselection if

necessary. At least 5 measurement sample per neighboring cell is needed. A running average of the

received signal level will be maintained for each carrier in the BA list.

All system information messages sent on the current BCCH on the serving cell must be read by the mobile

every 30 seconds to monitor changes in cell parameters (ex: MsTxPwrMax). The mobile also has to read the 6

strongest BCCH every 5 minutes to receive its cell parameters (ex: MsTxPwrMax). The 6 strongest can beseen from the BA list which has the updated measurement of the 32 BCCH carrier. The neighboring list for the

best 6 neighbors is updated every 60 seconds, which means the mobile has to measure each neighbor by 10

seconds. The mobile also has to read the BSIC of the 6 strongest BCCH every 30 seconds to confirm that it is

still monitoring the same cells. If a new BSIC is detected, then the BCCH of this BSIC will be read to receive

the cell parameters.

-

Every 30 secs Every 5 minute

Every 30 secs

BSIC BCCH

Serving cell

Six neighbors

[Continues ]

1) Perform Cell reselection measurement first


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The mobile will reselect and camp on another cell if any of the following criteria is satisfied :

a) The serving cell is barred.

b) C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss is high and the

mobile needs to change cell.

c) The Mobile has unsuccessfully tried to access the network as defined by the MAXRET (Ericsson) parameter

or MaxNumberRetransmissions (Nokia).

MAXRET is the maximum number of retransmission a mobile can do when it is accessing the system

It is defined per cell.

CELL RESELECTION

2) Cell reselection Criteria :

Assuming that one of the criteria above was satisfied then the mobile

will select a cell with a better C1.

However if the cell belongs to a different location area then the C1 for

that cell has to exceed a reselection hysterisis parameter called CRH

(Ericsson) or CellReselectHyseteris (Nokia) for the reselection to happen !!

[Continues ]


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If the mobile is moving in a border area between location areas, it might repeatedly change between

location areas. Each change requires location updating and cause heavy signaling load and risk

paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used.The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 of

the current serving cell by the value of the Reselect Hysteresis.

Since the Value of CRH maybe different for each cell, the CRH used for comparison will be the one

broadcast by the serving cell. If the value is set very low then the mobile will Ping-Pong between

location areas which will increase signaling load. If the value is set very high the mobile may camp in

the wrong cell too long.

Recommendations : Set the value to 6

C1 = 1

CRH = 4C1 = 6

Serving cell Target cell

LA 1 LA 2

CELL RESELECTION


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MOBILE IDLEWhen the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell by itself

without the help of BSC. This is done by comparing signal strength of each BCCH frequency and if

found the strongest then it will camp once the C1 > 0 (Cell Selection Criterion). If after camping to this

cell, it finds that a neighboring cell is much better then it will change to that neighboring cell. If the new

cell is in same location area the mobile does not have to inform the BSC about its new cell but if the new

cell is from a different different location area then the mobile will perform a location updating to inform

the BSC.

BASIC COMPARISON

MOBILE BUSYA mobile is considered busy when there is a call going on (speech, data or fax) or it is in the middle of

a call setup. At this stage the mobile cannot decide by itself whether it is necessary for the mobile to

handover to a better cell. Only BSC can determine if a mobile has to change to another cell other than the

serving cell. BSC makes the decision based on measurement reports sent by both Mobile and BTS.

This decision making is called locating. In a busy state, mobiles can receive Short Text Message (SMS)but cannot receive Cell Broadcast Messages.

basic gsm_chennal concept

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