GSM Idle Mode Behavior

GSM Idle Mode Behavior
Md Mustafizur Rahman
RNPO Engineer

2
Introduction
Idle mode- Powered on but not allocated a dedicated channel
• It enables the MS to receive system information from the network
• the MS can initiate a call by accessing the network on the Random
Access Channel (RACH)
• the network will know the location area of the cell in which the MS is
camped (unless the MS has entered a limited service state) and can
therefore page the MS
While the MS is in idle mode it will continuously make measurements on the BCCH-
carriers of serving and neighboring cells to decide on which cell to camp It will also, if
necessary, register its presence in the location area of the chosen cell by performing a
location updating. The purpose of camping on a cell is: Enables the MS to receive system
information from the network, MS can initiate a call by accessing the network on the
Random Access Channel (RACH) of the cell on which it is camped, Network will know the
location area of the cell in which the MS is camped and can therefore page the MS when
an incoming call is received.

3
Idle Mode Tasks
The idle mode task can be subdivided into four
processes:
•PLMN selection
•Cell selection
•Cell reselection
•Location updating.

5
•Powered on
•Requested by user
•Recovery from lack of coverage
•To return to home PLMN (periodic attempt)
PLMN Selection
*The periodic attempt is controlled by a timer stored in the SIM and only the service provider is able to
set the timer value. A value of T minutes may be stored in the SIM. T is either 6 minutes to 8 hours (in 6
minute steps) or indicates that no periodic attempts shall be made. If no values is stored in the SIM, a
default value of 30 minutes is used.
PLMN SELECTION
There are two modes of PLMN selection; these are automatic and manual. The power on the MS selects the
registered PLMN and attempts to perform location updating regardless of selection mode. The registered PLMN is
the network used at power off and is given in the MCC+MNC part of LAI, which is stored on the SIM card at the
last successful location updating.

6
PLMN Selection: Automatic/Manual Mode
Automatic mode: if no last registered PLMN exists or is
available
•home PLMN
•each PLMN that has been stored in the Subscriber Identity
Module (SIM) in priority order
•other PLMNs with received signal level above -85 dBm in
random order
•all other PLMNs in order of decreasing signal strength.
Manual mode:
User choice

7
PLMN Selection: Normal/Stored List
Cell selection process
no
Camp on it and
register
yes
124 GSM 900 carriers
Sample the signal
strength on...
PLMN selection
>30/40 attempts
Use the rejected
cell’s BA-list and...
Suitable
cell?
Check for BCCH carriers
of the selected PLMN
in order of decreasing SS

Cell Selection
After selecting PLMN, MS will search for a suitable cell.
For Cell selection, MS does have a BA list available so it will scan all RF signals in it.
At least five samples per frequency will be taken and among them, MS will check whether the
RF frequency is BCCH or not. It will also try to decode BSIC.
If not, it will go for another frequency.
The cell selection criterion is C1 which is as follows:
C1 = RXLEV - RXLEV_ACCESS_MIN - MAX((MS_TXPWR_MAX_CCH - P), 0)
Cell Selection
When an MS is powered on or enters the network coverage area, it scans all the frequencies allowed by
the PLMN and selects a suitable cell to camp on. This procedure is called cell selection.

Switch on the Mobile
For 3 to 5 seconds the Mobile will scan all the 124
channels in GSM900 and 374 channels in DCS1800
The mobile will synchronize to this carrier
and read the BCCH info like LAI, CGI
Tune to the second
strongest channel
Tune to the RF channel with the highest received average signal strength
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
Normal Cell Selection

10
Cell Selection: Suitable Cell Criteria
it belongs to the selected PLMN,
it is not barred (when a cell is barred it will not be camped on by
an MS in idle mode but a MS in dedicated mode can perform
handover to it),
it does not belong to a location area included in the list of
"forbidden location areas for roaming",
the cell selection criterion (C1>0) is fulfilled.

11
Cell Selection: C1 Criterion
ACCMIN- is the cell parameter that indicates the minimum
received signal level at the MS required for accessing the system.
CCHPWR - is the cell parameter that indicates the maximum
transmitting power that an MS is allowed to use when accessing the
system.
P - is the maximum power output of the MS according to its class.
C1 = (received signal level - ACCMIN) – max (CCHPWR
- P, 0)
Parameter(HUAW
EI)
Parameter
Value(HUAWEI)
Parameter(Ericss
on)
Ericsson Value Consequence Short Des.
RXLEV_ACCES_
MIN
0 ACCMIN -110
If set to higher
value, performance
will improve but
Traffic will decrease
Min Access level

Cell selection…Cell Bar and Access
HUAWEI topology
Ericsson
topology
Map:
In Ericsson CBQ=HIGH, In HUAWEI CBQ=0
In Ericsson CB=No, In HUAWEI CELL BAR ACCESS= 0

14
•The serving cell becomes barred;
•The MS has unsuccessfully tried to access the network the allowed
number of times, as defined by the MAXRET parameter;
•The MS detects a downlink signaling failure (fails to decode CCCH)
•C1 < 0 for 5 seconds
•C2 of serving cell < C2 of neighbor cell for 5 seconds.
Cell Reselection
Cell Reselection
After a cell has been successfully selected, the MS will start the cell reselection tasks. It will continuously make measurements
on its current serving cell and neighboring cells, in order to initiate cell reselection if necessary. For multiband MSs the strongest
non serving carriers may belong to different frequency bands.
The MS continuously monitors all neighboring BCCH carriers, as indicated by the BA list, in addition to the BCCH carrier of the
serving cell, to detect if it is more suitable to camp on another cell. At least five received signal level measurement samples are
required for each defined neighboring cell. A running average of the received signal level will be maintained for each carrier in the
BA list.

15
Cell Reselection: C2 Criterion
CRO applies an offset to the C2 reselection quantity for the cell.
TO applies a temporary negative offset to C2 for the duration of PT . This
prevents fast moving MSs from selecting the cell.
PT = 31is reserved to change the sign of CRO

16
LAC border: Cell Reselect Hysteresis
9702915
CRH=4 dB
nominal LA border
camp on
this cell
actual LA border
C2=5 dBm
C2=7 dBm
current LA
different LA
C2=4 dBm
LA = Location Area
[C2 of serving cell < C2 of neighbor cell + CRH] for
5 seconds

17
• PLMN not allowed
• No SIM card inserted
• Illegal MS
• IMSI unknown in HLR
• .....
• Emergency calls onl
• Cell reselection as n
but CRH is ignored
• No updating
Limited service state
Cell selec
Limited Service State

Location Updating
 Location updating is done by MS in order to save unnecessary paging .
 Location updating is done in three ways
1. Normal Updating: When MS enters a new LA. It compares the value wit the stored LAI and the report
to system
2. Periodic registration: After a fixed time period, MS informs the system to indicate whether it is active
or not. Defined by T3212 parameter
3. IMSI attach/detach: MS will do IMSI attach/detach during switch on/off state. System can also
implicitly detach MS if no successful communication takes place during BDTM+GDTM.
Value of BDTM+GTDM must be greater than T3212.
BTDM is the base time duration of implicit detach of a mobile subscriber by the network. It is an MSC parameter (Ericsson MSC only).
GTDM is the guard time for implicit detach. It is an MSC parameter (Ericsson MSC only).
Location Updating
To make it possible for the mobile subscriber to receive a call, the network must know where the MS is located. To keep the
network updated on the location of the MS, the system is informed by the MS on a regular basis. This is called Location
Updating. There are three different types of location updating defined; normal, periodic registration and IMSI attach. The
MS may also inform the network when it enters an inactive state, IMSI detach.

19
Location Updating
•Normal- changing location area
•Periodic- based on timer T3212
•IMSI Attach/Detach
When T3212 is changed, the mobile’s timer is reloaded:
New_timer = Old_timer.modulo.New_T3212 This prevent updating peaks.
The timer is reset whenever a mobile goes from dedicated mode to idle mode.
IMSI attach/detach is possible to turn off. This is done by the parameter ATT broadcasted on SI3
Implicit detach
MSC Supervision time = BTDM + GTDM
where BTDM = T3212

Locating Update Flow diagram
1. When the MS initiates a location area update request, an
immediate assignment procedure is started. The BTS sends the
BSC an Establish Indication message, which contains the
content of the Location Updating Request message.
2. The BSC establishes an SCCP link over the A interface.
Then, the BSC sends the MSC a Complete Layer3 Information
message, which contains the cell global identity (CGI) of the
current cell.
3. The MSC responds to the BSC with a Connection Confirm
message.
4. The MSC sends the MS a Location Updating Accepted
message, indicating that the location area update is successful.
5. If the network rejects the location area update request, it
sends a Location Updating Rejected message to the MS.
6. If the TMSI allocation in the case of location area update is
enabled on the MSC side, a TMSI Reallocation Complete
message is sent to the MSC in the location area update
procedure.
7. The MSC sends the BSC a Clear Command message to
start the resource release procedure on the network side.

Paging…..
Number of Paging Groups=(9-BSAGBLKSRES)*BSPAMFRAMS
The paging group belongs to a MS= ((IMSI mod 100)mod(Number of paging group))div BSPAMFRAMS
BSC calculated the paging sub group within the determined paging group based on IMSI and then pages in that group. The
IMSI/TMSI mapping is there in the VLR.
The smaller the value of BSPAMFRAMS, the less paging groups will be generated and less time delay occurs between two
consecutive time delay
Service performance will increase but battery backup of MS will be less as it will have less time to remain in DRX mode.
While configuring the parameter, we also should look about the system overload and optimize where needed.
Default value of BSPAMFRAMS is 2.
Paging is done in three different modes.
Common paging mode: Paging is done in the defined paging group
Complete paging mode: Paging is done in all the paging groups of the BTS
Spacing paging mode: Paging is done in N or N+2 paging group.
After an MS tunes to the BCCH carrier and decodes the system information data, it performs an evaluation, taking into account the IMSI
number, which determines to which paging group it belongs. The particular method, by which an MS determines to which paging group it
belongs, and hence, which particular CCCH block of the available blocks on the paging channel that is to be monitored, is defined in 3GPP
TS 45.002. When there are no paging messages to be transmitted to MSs in a certain paging group, dummy pagings will be sent instead. The
MS stays in sleep mode to minimize power consumption in the time gap between when its own paging group occurs. However, the MS must
still read the BCCH data sent by the serving cell at least every 30 seconds.

Paging Function
 Paging Queue function is implemented by BTS.
 Paging messages can only be transferred in the channel defined for the paging group.
 But the paging message has to be sent before PAGINGLIFETIME is expired.
 This timer has to be shorter than T3113 by 1or 2 sec.
 If Paging is unsuccessful, then it will be transmitted after PAGTIMES period. If set higher, system
performance will degrade.
 Paging can also be done in local/Global end.
 Paging can be done both by IMSI/TMSI. TMSI handles larger number but it declined performance
to some extent.
 Combination of IMSI/TMSI is preferable for paging.

Important Parameters…
Parameter(HUAWEI)
Parameter
Value(HUAWEI)
Parameter(Ericss
on)
Ericsson Value Consequence Short Des.
ATT YES ATT YES
If set to no, no IMSI
Attach/Detach
message will be sent
If IMSI Attach/Detach
message is not sent
unnecessary paging will
take place
T3212 50*6min T3212 50 deci hours
If set lower, System
load will be higher
Periodic Registration time
PI YES PI YES
If set to No, the cell
reselection will be done
by C1 value
Whether C1/C2 will be used
during cell reselection
BSAGBLKSRES 1 AGBLK 1
If set to higher, paging
capacity reduce
The setting of this
parameter affects the
MS paging response and
performance of the
system. A large value
decreases the number
of paging blocks, paging
capacity, and paging
success rate.
BSPAMFRAMS. 2 MFRMS 2
If set to higher, paging
group will increase but
signaling failure
criterion will decrease
Determines Paging group
number and determines
downlink signaling failure
criterion

System Information
All GSM base stations forward continuously informations about their current system configuration and other informations needed by
the mobile phones before they are allowed to access the network. These informations are organized in six different SYSTEM
INFORMATION words containing specific parameters. Type 1 to 4 are transmitted within the BCCH (broadcast control channel).
Type 5 and 6 are only transmitted during an established individual radio link in downlink direction in a multiplexed service channel
called SACCH (slow associated control channel). To transfer one system information word four bursts are needed. In a BCCH these
four bursts are available within one 51-multiframe, in a SACCH one system information word is spread over four 26-multiframes.
GSM System Information message type SI Description
System information-1(SI1)
Cell ARFCN, RACH parameters required to access the system by MS and hopping related
information are sent in this SI message.
System information-2(SI2)
Neighbour BCCH frequencies and PLMN information are sent in this SI. MS uses these frequencies
for signal strength measurements required for handover.
System information-2bis(SI-2bis) RACH control and BCCH extension on neighbor cells
System information-2ter(SI-2ter)
Information of BCCH extended allocated on which neighbor cells are provided in this SI.
Broadcasted optionally on BCCH by the network to all the MSs.
System information-2 Quarter (SI-2Quater) 3G neighbor cell related information
System information-3 (SI3)
Carry following : 1. LAI of the current location area, 2. Cell identity, 3. Control channel information
required to calculate paging group, 4. Cell options to achieve good performance in the cell, 5. cell
selection parameters required by MS.
CBCH and CBCH related frequency information, LAI, Cell selection parameters and RACH control
information are carried by this SI4 message.
It carries neighbor cell informations. In active mode, MS sends measurement reports in the uplink
and output power/timing advance information in the downlink (on SACCH). Also gets BCCH carrier
related information of the neighbor cells.
System information-6 (SI6) Information on LAI, cell options, Cell identity and PLMN permitted or not is transmitted on this SI.
System information-7 (SI7) Cell re-selection parameters needed by MS are sent on this SI.
System information-8 (SI8) Cell re-selection parameters needed by MS are sent on this SI.
System information-9 (SI9) Scheduling related some of the informations are sent on this SI.
System information-13 (SI13) Carry GPRS related information needed for PS call.

System Information
System Information 1: Sent on BCCH
√ BA list
√ MSMAXRETRAN-maximum retransmission of MS
√ CELLBARACCESS2- Cell Bar access
√ COMMACC and SPECACC-Whether access class is supported or not
√ CALLRESTABDIS-Reestablishment allowed or not
√ ERGCALLDIS-Whether Emergency call is allowed
√ Neighbor cell information(BA list)-SI 2bis,2ter,2querter
√ NCC permitted data
√ RACH control parameter
√ MBR- indicating dual band MS to report about dual band neighbor. Default value of MBR is 0.

System Information
√ LAI
√ CI
√ ATT
√ AGBLK and MFRMS
√ RLT
√ FMSMAXOPCC- Maximum transmit power of MS
√ HRATESPT- NECI data indicating whether MS is GSM phase 1 or phase 2.
√ Cell selection and reselection parameters( PI,CBQ,CB,TO,PT, Send 2 TER/2 quarter etc)
√ LAI and CI
√ Cell selection and reselection parameter
√ CBCH

System Information
System Information 8: Sent on BCCH about cell reselection
√ RAC
√ NCO- Network Control Mode
√ In addition, System Information Type 13 also contains parameters such as GPRS Mobile
Allocation, GPRS Cell Options, GPRS Power Control Parameters , and PBCCH Description
.

MEASUREMENTS in IDLE and Active
mode

29
MEASUREMENTS Contents
1. Coding Level and Quality
2. MS Measurements in Idle Mode
3. MS Measurements in Dedicated Mode

MEASUREMENTS
Coding of Level and Quality
LEVEL QUALITY
P (dBm) FS (dBuV/m) RXLEV
-110 27 0
-109 28 1
-108 29 2
. . .
. . .
. . .
-49 88 61
-48 89 62
-47 90 63
BER (%) BER (%) RXQUAL
RANGE MEAN
< 0.2 0.14 0
0.2-0.4 0.28 1
0.4-0.8 0.57 2
0.8-1.6 1.13 3
1.6-3.2 2.26 4
3.2-6.4 4.53 5
6.4-12.8 9.05 6
> 12.8 18.1 7
P=Power
FS= Field Strength
LEV= Level
BSC BSC
The relationship between Field Stregth (measured in dBuV/m) and Received Power (measured in dBm) is
FS (dBuV/m) = RxLev (dBm) +77,2+20Log[freq(MHz)]
Based on the formula above for GSM 1800 "P" corresponds to "FS" values different from GSM 900 case.

31
MS Measurements in IDLE Mode
(GSM Specs)
• MS decodes BCCH of serving (camped)cell every 30 s (read system
information messages to get changes in cell parameters)
• Decodes BCCH of six strongest neighbours at least every 5 min
(parameters affecting cell re-selection)
• Pre-synchronization and BSIC decoding of neighbours Once in 30 s (to
confirm that it is still monitoring the same cell)
• New neighbour within six strongest- BCCH decoding in 30 s

32
MS Measurements in DEDICATED Mode (1)
• Measures the RxLev and RxQual of the Server
• Measures the RxLev of all defined neighbours (BA list
from System Info 5)
• BSIC-decoding of adjacent cells Once in 10 s
• Measurement results of 6 best neighbours will be sent to
BSC Every SACCH period 480 msec.
• Detects whether DTX is used

33
The MS activities marked 1-4 in Figure 5-43 are:
1. MS receives and measures signal strength on serving cell, TS 2 downlink.
2. MS transmits, TS 2 uplink.
3. MS measures signal strength for at least one of the surrounding cells.
4. MS tries to read BSIC on SCH (TS 0, c0) for one of the six strongest
surrounding cells.

34
Idle mode Active mode
BSIC BCCH BSIC BCCH
Serving cell - every 30 s - -
Six neighbors every 30s every 5 min every 10 s -
Summary

GSM Idle Mode Behavior

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