PLMN: Public Land Mobile Network

A Public Land Mobile Network (PLMN) is established and operated by an administration or Recognized Private Operating Agency (RPOA) for the specific purpose of providing land mobile telecommunications service services to the public. A PLMN may be regarded as an extension of networks (e.g. ISDN, corporate and public PDNs, etc);
It is a collection of MSCs areas in CS domain and SGSN areas for GPRS and SGSN or MME areas for EPC in PS domain within a common numbering plan (e.g. same National Destination Code) and a common routing plan.

The MSCs are the functional interfaces between the fixed networks and a PLMN for call set-up in CS domain. The GGSN and the SGSN are the functional interfaces between the fixed networks and a PLMN for packet transmission in GPRS PS domain. In case of EPC PS Domain, the PDN-GW, Serving-GW and the SGSN and the MME are the functional interfaces between the fixed networks and a PLMN for packet transmission.

Functionally the PLMNs may be regarded as independent telecommunications entities even though different PLMNs may be interconnected through the ISDN/PSTN and PDNs for forwarding of calls or network information. A similar type of interconnection may exist for the interaction between the MSCs/SGSNs/MMEs of one PLMN.



fig a- overall Idle mode process. (release - 1999 )

fig b - overall Idle mode process (Release -09)





General description in idle mode.
When MS is switched on, it attempts to make contacts with PLMN, the perticular PLMN to be contacted may be automatically or manually.
The MS looks for a suitable cell of chosen PLMN and chooses that cell to provide available services, and tunes to it's control panel. the choosing is known as "camping on cell". the MS then register it's presense in the registration area of choosen cell if necessary by means of LR (Location registrar), GPRS attach or IMSI attach procedure.
If the MS looses coverage of the cell, or find a more suitable cell, it reselects on to most suitable cell of selected PLMN and camps on that cell. if the new cell is in new registration area, an LR request is performed. If MS looses a coverage of PLMN, either a new PLMN is selected or availabe PLMN information is given to user, so, that manual selection can be made.

Idle mode tasks can be sub divided into 4 processes as per rel - 1999 and 5 processes as per rel 09.
1) PLMN selection
2) CSG selection (UTRAN and E-UTRAN selection)
3) Cell selection and reselection
4) Location registration
5) CTS fix part selection (A/Gb mode only)

References:
TS 23.122 NAS function related to MS (Mobile station) in idle mode
TS 23.002 v9.2.0