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GSM: PLMN Not Found Issue Explained

gsm plmn mobile communication network failure signal condition

PLMN stands for Public Land Mobile Network. It’s comprised of various GSM network elements like the BSS (BSC, BTS, TRAU), MSC, VLR, HLR, and EIR. A GSM-PLMN can support a large number of mobile subscribers, each with different features and power classes. Every PLMN needs at least one HLR (Home Location Register). GSM mobile devices get PLMN information by decoding the GSM SCH (Synchronization Channel). The Network Color Code (NCC) identifies the PLMN. System information 5 and 6, carried on the BCCH (Broadcast Control Channel), indicates whether a PLMN is permitted or not.

There’s a field called CI (Cell Identity) that uniquely identifies a cell within a PLMN. It’s a 2-byte field. The MNC (Mobile Network Code), a two-digit identifier, along with the 3-bit NCC, uniquely identifies a PLMN.

A mobile device typically operates on its home PLMN (HPLMN) or an equivalent home PLMN (EHPLMN). However, a visited PLMN (VPLMN) may be selected if the mobile station loses radio signal coverage.

There are two modes for PLMN selection defined in GSM (or other cellular networks): automatic and manual.

  • Automatic Mode: The mobile uses a prioritized list of available PLMNs and selects the one with the highest priority.
  • Manual Mode: The mobile displays a list of available PLMNs to the user, who then manually selects one. After the user’s selection, the mobile attempts to obtain normal service on the V-PLMN.

PLMN not found errors usually occur in poor signal conditions. This can happen when the Carrier to Interference ratio (C/I) is below the limit required for the physical layer to decode the FCCH/SCH channel.

Causes of Poor C/I

Poor C/I can stem from various factors. If both the mobile and base station equipment are working correctly, the main cause is likely interference (co-channel and adjacent channel interference). Sometimes, a large frequency offset between the base station and the mobile can also lead to poor C/I. Another reason could be a lack of coverage, perhaps because the subscriber has moved to an area with no signal.

Network Failures and Congestion

The following cause values indicate PLMN-specific network failures and network congestion:

  • Cause value = 17 Network failure: This cause is sent to the Mobile Station if the MSC (Mobile Switching Center) can’t service a mobile-generated request due to a PLMN failure (or problems in MAP - Mobile Application Part).
  • Cause value = 22 Congestion: This cause is used and transmitted when the service request can’t be serviced by the network because of congestion (i.e., unavailability of a channel, the facility is busy, or the network is congested).

Data Stored on a SIM - Roaming Data

ParameterDescription
Network color codes (NCCs) of restricted PLMNs (mandatory/changeable)Maximum of 4 PLMNs can be entered on a SIM after an unsuccessful location update with the cause “PLMN not allowed.” The oldest entry is deleted when more than 4 restricted PLMNs are found.
NCCs of preferred PLMNs (optional/changeable)Decides which PLMN the mobile should select if there are multiple PLMNs and the home PLMN is not available.

Data Stored on a SIM - PLMN Data

ParameterDescription
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