Network switching subsystem ( NSS ) is the component of a GSM system that carries out switching functions and manages the communications between mobile phones and the Public Switched Telephone Network (PSTN). It is owned and deployed by mobile phone operators and allows mobile phones to communicate with each other and telephones in the wider telecommunications network. The architecture closely resembles a telephone exchange, but there are additional functions which are needed because the phones are not fixed in one location. Each of these functions handle different aspects of mobility management and are described in more detail below.
The Network Switching Subsystem, also referred to as the GSM core network, usually refers to the circuit-switched core network, used for traditional GSM services such as voice calls, SMS, and circuit switched data calls.
There is also an overlay architecture on the GSM core network to provide packet-switched data services and is known as the GPRS core network. This allows mobile phones to have access to services such as WAP, MMS, and Internet access.
All mobile phones manufactured today have both circuit and packet based services, so most operators have a GPRS network in addition to the standard GSM core network.
Mobile switching center (MSC)
Description
The mobile switching center (MSC) is the primary service delivery node for GSM, responsible for handling voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data). The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.
In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in America).
There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.
The gateway MSC (G-MSC) is the MSC that determines which visited MSC the subscriber who is being called is currently located. It also interfaces with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed through a G-MSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSSs connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.
The visited MSC (V-MSC) is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.
The anchor MSC is the MSC from which a handover has been initiated. The target MSC is the MSC toward which a Handover should take place. A mobile switching centre server is a part of the redesigned MSC concept starting from 3GPP Release 5.
Mobile switching centre server (MSCS)
Main article: Mobile switching centre serverThe mobile switching centre server is a soft-switch variant of the mobile switching centre, which provides circuit-switched calling, mobility management, and GSM services to the mobile phones roaming within the area that it serves. MSS functionality enables split between control (signalling) and user plane (bearer in network element called as media gateway/MG), which guarantees more optimal placement of network elements within the network.
MSS and MGW media gateway makes it possible to cross-connect circuit switched calls switched by using IP, ATM AAL2 as well as TDM. More information is available in 3GPP TS 23.205.
Other GSM core network elements connected to the MSC
The MSC connects to the following elements:
- The home location register (HLR) for obtaining data about the SIM and mobile services ISDN number (MSISDN; i.e., the telephone number).
- The base station subsystem which handles the radio communication with 2G and 2.5G mobile phones.
- The UMTS terrestrial radio access network (UTRAN) which handles the radio communication with 3G mobile phones.
- The visitor location register (VLR) for determining where other mobile subscribers are located.
- Other MSCs for procedures such as handover.
Procedures implemented
Tasks of the MSC include:
- Delivering calls to subscribers as they arrive based on information from the VLR.
- Connecting outgoing calls to other mobile subscribers or the PSTN.
- Delivering SMSs from subscribers to the short message service centre (SMSC) and vice versa.
- Arranging handovers from BSC to BSC.
- Carrying out handovers from this MSC to another.
- Supporting supplementary services such as conference calls or call hold.
- Generating billing information.
Home location register (HLR)
The home location register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs per public land mobile network (PLMN), though one international mobile subscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time.
The HLR stores details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record.
The next important items of data associated with the SIM are the MSISDNs, which are the telephone numbers used by mobile phones to make and receive calls. The primary MSISDN is the number used for making and receiving voice calls and SMS, but it is possible for a SIM to have other secondary MSISDNs associated with it for fax and data calls. Each MSISDN is also a primary key to the HLR record. The HLR data is stored for as long as a subscriber remains with the mobile phone operator.
Examples of other data stored in the HLR against an IMSI record is:
- GSM services that the subscriber has requested or been given.
- GPRS settings to allow the subscriber to access packet services.
- Current location of subscriber (VLR and serving GPRS support node/SGSN).
- Call divert settings applicable for each associated MSISDN.
The HLR is a system which directly receives and processes MAP transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around.
Other GSM core network elements connected to the HLR
The HLR connects to the following elements:
- The G-MSC for handling incoming calls
- The VLR for handling requests from mobile phones to attach to the network
- The SMSC for handling incoming SMS
- The voice mail system for delivering notifications to the mobile phone that a message is waiting
- The AUC for authentication and ciphering and exchange of data (triplets)
Procedures implemented
The main function of the HLR is to manage the fact that SIMs and phones move around a lot. The following procedures are implemented to deal with this:
- Manage the mobility of subscribers by means of updating their position in administrative areas called 'location areas', which are identified with a LAC. The action of a user of moving from one LA to another is followed by the HLR with a Location area update procedure.
- Send the subscriber data to a VLR or SGSN when a subscriber first roams there.
- Broker between the G-MSC or SMSC and the subscriber's current VLR in order to allow incoming calls or text messages to be delivered.
- Remove subscriber data from the previous VLR when a subscriber has roamed away from it.
Authentication centre (AUC)
Description
The authentication centre (AUC) is a function to authenticate each SIM card that attempts to connect to the GSM core network (typically when the phone is powered on). Once the authentication is successful, the HLR is allowed to manage the SIM and services described above. An encryption key is also generated that is subsequently used to encrypt all wireless communications (voice, SMS, etc.) between the mobile phone and the GSM core network.
If the authentication fails, then no services are possible from that particular combination of SIM card and mobile phone operator attempted. There is an additional form of identification check performed on the serial number of the mobile phone described in the EIR section below, but this is not relevant to the AUC processing.
Proper implementation of security in and around the AUC is a key part of an operator's strategy to avoid SIM cloning.
The AUC does not engage directly in the authentication process, but instead generates data known as triplets for the MSC to use during the procedure. The security of the process depends upon a shared secret between the AUC and the SIM called the K i . The K i is securel
Home Location Register | starsolutions.com
Overview. The Sonata Home Location Register (HLR) is a subscriber service database platform designed for CDMA2000 and 3G GSM/GPRS wireless networks.
Network switching subsystem - Wikipedia, the free encyclopedia
The home location register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network.
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Acronym Finder: SHLR stands for Smart Home Location Register
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