Public land mobile network
||This article needs attention from an expert in Telecommunications. (February 2009)|
A public land mobile network (PLMN) is a regulatory term in telecommunications. A PLMN is a network that is established and operated by an administration or by a recognized operating agency (ROA) for the specific purpose of providing land mobile telecommunications services to the public.
A PLMN is identified by the Mobile Country Code (MCC) and the Mobile Network Code (MNC). Each operator providing mobile services has its own PLMN. PLMNs interconnect with other PLMNs and Public switched telephone networks (PSTN) for telephone communications or with internet service providers for data and internet access of which links are defined as interconnect links between providers. These links mostly incorporate SDH digital transmission networks via fibre optic on land and digital microwave links.
Access to PLMN services is achieved by means of an air interface involving radio communications between mobile phones or other wireless enabled user equipment and land based radio transmitters or radio base stations or even fibre optic distributed SDH network between mobile base stations and central stations via SDH equipment (ADMs) with integrated IP network services.
Public switched telephone network 
Public Land Mobile Networks need to connect to the Public Switched Telephone Network (PSTN) in order to route calls.
The PSTN is the world's collection of interconnected voice-oriented public telephone networks, in much the same way that the Internet is the concentration of the world's public IP-based packet-switched networks. It is both commercially- and government-owned. This aggregation of circuit-switching telephone networks has evolved greatly from the days of Alexander Graham Bell, and in the late 20th century became almost entirely digital in nature — except for the final link from the central (local) telephone office to the user (the local loop). It also extends into mobile as well as fixed telephones.
The PSTN also furnishes much of the Internet's long-distance infrastructure and, for the majority of users, the access network as well. Because Internet Service Providers (ISPs) pay the long-distance carriers for access to their infrastructure, and share the circuits among many users through packet switching, the end Internet user avoids having to pay usage tolls to anyone other than their ISP.
Many observers[who?] believe that the long-term future of the PSTN is to be just one application of the Internet — however, the Internet has some way to go before this transition can be made. For example, the Quality of Service (QoS) guarantee is one aspect that needs to be improved for Voice over IP (VOIP) technology.
The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (usually called telephone numbers) for addressing. A number of large private telephone networks are not connected to the PSTN, and are used for military purposes (such as the Defense Switched Network). There are also private networks run by large companies which are linked to the PSTN, but only through controlled gateways such as private branch exchanges.
A GSM PLMN may be described by a limited set of access interfaces and a limited set of GSM PLMN connection types to support the telecommunication services described in the GSM 02-series of specifications.
PLMN is network that is established and operated by an administration or by a recognized operating agency (ROA) for the specific purpose of providing land mobile telecommunications services to the public. A PLMN may be considered as an extension of a fixed network, e.g. the Public Switched Telephone Network (PSTN) or as an integral part of the PSTN. This is just one view-point on PLMN. PLMN mostly refers to the whole system of networking hardware and software which enables wireless communication, irrespective of the service area or service provider (cf. Internet backbone). Sometimes separate PLMN is defined for each country or for each service provider. Its case is same as that of PSTN. Sometimes it refers to the whole circuit-switched system, or else specific to each country.
PLMN is not a term specific to GSM. In fact GSM can be treated as an example of a PLMN system. These days (as of January, 2006) much discussions are going on to form the structure of UMTS PLMN for the third generation systems. Access to PLMN services is achieved by means of an air interface involving radio communications between mobile phones or other wireless enabled user equipment and land based radio transmitters or radio base stations PLMNs interconnect with other PLMNs and PSTNs for telephone communications or with internet service providers for data and internet access.
A public land mobile network may be defined as a number of mobile services switching centre areas within a common numbering plan and a common routing plan. With respect to their functions, the PLMNs may be regarded as independent communications entities, even though different PLMNs may be interconnected through the PSTN/ISDN for the forwarding of calls or network information. The MSCs of a PLMN can be interconnected similarly to allow interaction. A PLMN may have several interfaces with the fixed network (e.g. one for each MSC). Inter-working between two PLMNs may be performed via an international switching centre. The PLMN is connected via an NCP to the PSTN/ISDN. If there are two mobile service suppliers in the same country, they can be connected through the same PSTN/ISDN.
Objectives of a GSM PLMN 
The general objective of a PLMN is to facilitate wireless communication and to interlink the wireless network with the fixed wired network. The PLMN was specified by the European Telecommunications Standard Institute (ETSI) following up with their GSM specification. Even as times changed, the GSM PLMN objectives conceptually remained the same.
- To give access to the GSM network for a mobile subscriber in a country that operates the GSM system.
- To provide facilities for automatic roaming, locating and updating of mobile subscribers.
- To provide the subscriber a wide range of services and facilities, both voice and non voice, that are compatible with those offered by existing networks like PSTN and ISDN.
- Bearer Services : These services give the subscriber the capacity required to transmit appropriate signals between certain access points (user-network interfaces).
- Tele Services : Provide subscriber with necessary capabilities including terminal equipment function to communicate with other subscribers.
- Supplementary Services : Modify or supplement basic telecommunication services and are offered together or in association with basic communication services.
GSM architecture is basically the PLMN architecture itself as the subject is GSM PLMN. Various interfaces between the GSM subsystems are to be considered, along with the signaling system and the various components (both hardware and software).
The GSM PLMN is divided into signaling network and mobile network. Each of these has various subsystems, which are grouped under three major systems: the Network and Switching Subsystem (NSS), the Base Station Subsystem (BSS), and the operation and support system (OSS).
Operation and Support System (OSS) 
The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.
Additional functional elements 
Other functional elements shown are as follows:
- message center (MXE)-The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, email, and notification.
- mobile service node (MSN)-The MSN is the node that handles the mobile intelligent network (IN) services.
- gateway mobile services switching center (GMSC)-A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.
- GSM interworking unit (GIWU)-The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.
There are three viewpoints of interoperability between PLMN and PSTN:
- PLMN can be treated as an integral part of PSTN, extending the services offered by PSTN to wireless networks.
- PSTN can be treated as an integral part of PLMN, through which it facilitates call routing.
- PLMN and PSTN can be treated as two separate systems, which depends on each other or connects to each other for the purpose of call routing.
A PLMN is essential for the effective working of any wireless network, just like the need for PSTN in wireline networks. PLMN facilitates interoperation with its own subsystems in order to perform operation of the GSM system in particular and any wired network in general.