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GSM Issue-PLMN not found due to weak signal condition

PLMN stands for public land mobile network. It is the composition of various GSM network elements such as BSS(BSC,BTS,TRAU),MSC,VLR,HLR and EIR. GSM-PLMN can consist of as many mobile subscribers as possible having different features and power classes. Every PLMN requires at least one HLR.

GSM mobile gets the PLMN information by decoding the GSM SCH channel. Network color code(NCC) identifies the PLMN. System information 5 and 6 carried on BCCH tells whether PLMN is permitted or not. There is a field called CI(Cell identity) which uniquely identifies a cell within a PLMN. It is 2 byte long field. MNC(Mobile Network Code) which is two digit identifier used like the 3-bit-long NCC is used to uniquely identify a PLMN.

The Mobile normally operates on its home PLMN referred as HPLMN or equivalent home PLMN referred as EHPLMN. However a visited PLMN i.e. VPLMN may also be selected in the condition when mobile station loses radio signal coverage. There are two modes for PLMN selection defined in the GSM or in any other cellular network. They are automatic and manual.

In the automatic mode mobile utilizes a list of available PLMNs as per priority. Based on this mobile selects the PLMN with the highest priority.

In the manual mode the mobile indicates to the user which PLMNs are available based on manual search. After the manual selction of the user in the mobile, Mobile will try to obtain normal service on the V-PLMN.

PLMN not found can happen only in poor signal condition. This can happen when carrier to interference ratio (C/I) is below the limit required for physical layer to decode the FCCH/SCH channel. Poor C/I can be due to many reasons. If the equipments (Mobile and base station) is fine inspite of this, if C/I is poor than the cause will be due to interference (co-channel and adjacent channel). Sometimes higher frequency offset between base station and mobile can also lead to poor C/I.

The other reason for the PLMN not found might be because of no coverage where subscriber might have moved while user is travelling. Following are causes which indicates PLMN specific network failures and network congestion.

Cause value = 17 Network failure
This cause is sent to the Mobile station if the MSC can not service an Mobile generated request due to failure of PLMN (or problems in MAP).

Cause value = 22 Congestion
This cause is used and transmitted when the service request can not be serviced by the network because of congestion (i.e. unavailability of channel, facility is busy or congested etc.)

Data Stored on a SIM -Roaming Data

Parameter Description
Network color codes (NCCs) of restricted PLMNs (mandatory/changeable) Maximum of 4 PLMNs can be entered on a SIM after unsuccessful location update; cause "PLMN not allowed." Oldest entry deleted when more than 4 restricted PLMNs are found.
NCCs of preferred PLMNs (optional/changeable) Decides what PLMN mobile should select, if there are more than one PLMNs and home PLMN is not available

Data Stored on a SIM -PLMN Data

Parameter Description
NCC, mobile country code (MCC), and mobile network code (MNC) of the home PLMN (mandatory/fixed) Network Identifier
Absolute radio frequency channel numbers (ARFCNs) of home PLMN (mandatory/fixed) Frequencies for which the home PLMN is licensed.

Other LAU Reject Causes

PLMN Not Found
PLMN Not Allowed
Location Area(LA) Not Allowed
Roaming Not Allowed
No Suitable Cells in this LA

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