Public land mobile network

In telecommunication, a public land mobile network (PLMN) is a combination of wireless communication services offered by a specific operator in a specific country.[1][2] A PLMN typically consists of several cellular technologies like GSM/2G, UMTS/3G, LTE/4G, NR/5G, offered by a single operator within a given country, often referred to as a cellular network.

PLMN code

A PLMN is identified by a globally unique PLMN code, which consists of a MCC (Mobile Country Code) and MNC (Mobile Network Code). Hence, it is a five- to six-digit number identifying a country, and a mobile network operator in that country, usually represented in the form 001-01 or 001–001.

A PLMN is part of a:

• Location Area Identity (LAI) (PLMN and Location Area Code)
• Cell Global Identity (CGI) (LAI and Cell Identifier)
• IMSI (see PLMN code and IMSI)

Leading zeros in PLMN codes

Note that an MNC can be of two-digit form and three-digit form with leading zeros. It is administered by the respective national numbering plan administrator.[3] From PLMN assignments, it is apparent that such dualities of two-digit and three-digit MNCs with the same number value are avoided (see the list of mobile country codes and mobile network codes). An example for an actual three-digit/two-digit MNC with leading zeros is in Bermuda MCC, 350-007 and 350-00, 350-01.

PLMN code and IMSI

The IMSI, which identifies a SIM or USIM for one subscriber, typically starts with the PLMN code. For example, an IMSI belonging to the PLMN 262-33 would look like 262330000000001. Mobile phones use this to detect roaming, so that a mobile phone subscribed on a network with a PLMN code that mismatches the start of the USIM's IMSI will typically display an "R" on the icon that indicates connection strength.

PLMN services

A PLMN typically offers the following services to a mobile subscriber:

• Emergency calls to local Fire/Ambulance/Police stations.
• Voice calls to/from any other PLMN ("cellular network") or PSTN ("landline"/VoIP).
• Short message service (SMS) services to/from any other PLMN or SIP service (the original form of texting on a mobile phone, now often replaced by Messaging apps).
• Multimedia Messaging Service (MMS) services to/from any other PLMN or SIP service.
• Unstructured Supplementary Service Data (USSD) for operator specific interactions (e.g. dialing "*#100#" to indicate the current balance).
• Internet data connectivity for arbitrary services, e.g. via GPRS in GSM, IuPS in UMTS, or LTE.

The availability, quality and bandwidth of these services strongly depends on the particular technology used to implement a PLMN.

References

1. 3GPP TS 22.101 https://www.etsi.org/deliver/etsi_ts/122100_122199/122101/15.06.00_60/ts_122101v150600p.pdf
2. 3GPP TS 21.905 https://www.etsi.org/deliver/etsi_tr/121900_121999/121905/15.00.00_60/tr_121905v150000p.pdf
3. ITU-T Rec. E.212 (09/2016), section 3.4 mobile network code (MNC)