Telecom Acronyms and Infrastructure Glossary
Posted 16th April 2021 in Guides
We love a good acronym in the telecoms world. Unfortunately, this makes a lot of technology difficult to follow, so we have provided a shortlist of telecoms acronyms you might see when researching HLR lookups.
For an understanding of what the infrastructure looks like when you use a mobile phone, it’s good to have a high-level understanding of how a call (or text message) can get from your mobile phone to a friend’s mobile phone when you are on different telephone networks.
In the diagram below your mobile phone is on the left, and the connection to other telephone networks (typically via SS7) is on the right-hand side.
A high-level overview of what a Mobile Network Operators’ infrastructure looks like
Below is our Telecom Acronyms Glossary.
Table of Contents – Telecom Acronyms Glossary
API – Application Programming Interface
An Application Programming Interface (API) allows servers to talk to each other without manual intervention. Using an API, a programmer can tell one server how to query or integrate to another server. API documentation explains to the programmer how the API works and what they need to do to program their server to use the API from another server.
AuC – Authentication Centre
The Authentication Centre (AuC) provides the security mechanism to allow a SIM card to authenticate itself on the mobile network.
ETSI – European Telecommunications Standards Institute
The European Telecommunications Standards Institute (ETSI) is an independent, nonprofit standardization organisation that develops and distributes globally applicable standards for information and communication technologies, including mobile, fixed, radio, broadcast and Internet technologies.
GSM – Global System for Mobile Communications
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for digital cellular networks used by mobile devices such as mobile phones and tablets. HLRLookup works with GSM-based mobile phone networks, which covers 90%+ of all mobile handsets.
GSM Core Network (also referred to as the Network Switching Subsystem (NSS))
The GSM core network is the internal network of a Mobile Network Operator. The core network contains components that allow GSM services to be used and allows mobile phone handsets to communicate to each other on the same network, and to communicate with external networks.
The GSM core network includes services that allow voice calls, SMS messages and data to be exchanged with mobile phone handsets. One feature of the core network is the HLR database.
HLR – Home Location Register
The Home Location Register (HLR) is a database that is maintained by each mobile network operator to store details of which users are allowed to use their mobile phone network. The HLR database includes a unique identifier (IMSI) for each SIM card, the telephone number (MSISDN) currently assigned to that IMSI, details about which services the SIM card is allowed to access and which Mobile Switching Centre and Visitor Location Register is currently servicing the SIM card (if any).
See our blog post “What is a HLR Lookup?” for more details of the Home Location Register.
IMEI – International Mobile Equipment Identity
The International Mobile Equipment Identity (IMEI) is a number that uniquely identifies a device used on a mobile phone network. The IMEI number is unique and typically 15 or 17 digits made up of an 8 digit prefix (the Type Allocation Code / TAC) which is the same prefix for each model of the device. The remainder of the IMEI is unique to each device and randomly generated by the manufacturer of the device.
You are able to swap your SIM card between different devices and therefore the IMEI is not fixed to any particular SIM card’s IMSI as this often changes, for example when you upgrade your phone to a different model.
HLR Lookup does not query a handset’s IMEI and this service is not available from us.
IMSI – International Mobile Subscriber Identity
An International Mobile Subscriber Identity (IMSI) is a unique number, usually fifteen digits, that is issued by a Mobile Network Operator and stored within a SIM card. The IMSI uniquely identifies the SIM card and this allows a Mobile Network Operator to decide if a SIM card is allowed to connect to the mobile network and then to route telephone calls, SMS messages and data to and from a SIM card.
The IMSI number is composed of the Mobile Country Code (MCC) followed by the Mobile Network Code (MNC) followed by a Mobile Subscriber Identification Number (MSIN). Because the MCCMNC defines the Country and Mobile Network Operator, all IMSI’s generated by one Mobile Network Operator will begin with the same MCCMNC prefix. Because different Mobile Network Operators have different MCCMNCs allocated to them, no two IMSI’s will be the same between different Mobile Network Operators even if two different Mobile Network Operators generate matching MSINs.
ITU – International Telecommunication Union
The International Telecommunication Union (ITU) is a specialised agency of the United Nations responsible for all matters related to information and communication technologies. The ITU promotes the shared global use of the radio spectrum, facilitates international cooperation in assigning satellite orbits, assists in developing and coordinating worldwide technical standards, and works to improve telecommunication infrastructure in the developing world. It is also active in the areas of broadband Internet, wireless technologies, aeronautical and maritime navigation, radio astronomy, satellite-based meteorology, TV broadcasting, and next-generation networks.
MCC – Mobile Country Code
A 3 digit number which uniquely identifies a country. The MCC is not the same as the telephone dialling prefix, it’s a uniquely generated number that is defined in E.212 from the ITU-T.
MCCMNC – Mobile Country Code, Mobile Network Code
If we push the MCC and MNC together we get the MCCMNC. The MCCMNC provides a number that uniquely identifies a country and Mobile Network Operator. Eg. MCCMNC 23410 is for the UK (234) mobile network operator Telefónica (10).
MNC – Mobile Network Code
A 2 to 3 digit number that uniquely identifies a Mobile Network Operator within a country. Most of the world uses 2 digit MNCs, but North America sometimes use 3 because they like to feel special and they have a lot of different network operators who all need their own unique number. The ITU has a list of MNCs in their E.212 document.
MNO – Mobile Network Operator
A telephone network company that provides services to end-users via mobile phones (cellphone) and, crucially, the company owns or controls access to a radio spectrum license.
MVNO – Mobile Virtual Network Operator
A telephone company that provides services to end-users via mobile phones (cellphone) but do not own the radio infrastructure that is used to communicate with mobile devices. An MVNO uses the radio network of an MNO to deliver communications to and from the MVNO’s customers.
MSC – Mobile Switching Centre
A Mobile Switching Centre (MSC) acts as a control centre for many mobile phone towers (cell towers/base stations) within one geographical area. They are a core part of GSM networks. When a SIM card roams into a cell tower’s area, the cell tower reports the signal strength to the MSC. Because multiple towers can usually see the same SIM card, the MSC decides which tower should currently be used for communicating with the SIM card. The MSC receives regular updates from all towers it controls as to which SIM cards can be seen and the radio signal strength for each SIM card.
If a call is in progress and the SIM moves into a new tower coverage area, then the call is handed off by the MSC to a new tower (hopefully seamlessly without dropping the call!). The MSC keeps the VLR up to date with details about where a SIM card can currently be reached if a new communication arrives for that SIM. Based on the size of the mobile operator, multiple MSCscan be implemented.
MSIN – Mobile Subscriber Identification Number
The Mobile Subscriber Identification Number (MSIN) is the final part of the IMSI number which uniquely identifies a SIM card. A Mobile Network Operator generates a unique MSIN for each SIM card. The MSIN is 10 or 9 digits long, depending on if the MCCMNC was 5 or 6 digits.
MSISDN (pronounced MizDen) – Mobile Station International Subscriber Directory Number
A telephone number used to uniquely identify a user’s subscription within a mobile phone network. The MSISDN is a maximum of 15 digits and is defined by the ITU’s E.164 recommendation. The number includes the internationally recognised country calling code as a prefix and then it’s a free-for-all as to whether that country has implemented a logical national numbering plan or not (we’re looking at you, Mexico!)
The acronym MSISDN was never formally acknowledged and so some people sometimes refer to it as the Mobile Subscriber International Subscriber Directory Number or even sometimes the Mobile Subscriber Integrated Services Digital Network number. In any case, the MSISDN is the telephone number that is assigned by a mobile network operator to a user’s mobile subscription.
PSTN – Public Switched Telephone Network
The Public Switched Telephone Network (PSTN) provides infrastructure and services for public telecommunication. Each time you use a telephone which is able to call other public telephones then the service you are using is part of the PSTN. Each telephone company maintains their own network infrastructure and has links to other telephone networks. Because each telephone network is linked to another telephone network then you are able to make a telephone call from one network to another via these links.
SIM – Subscriber Identity Module
A SIM card is a portable memory chip used in GSM phones. It is a crucial component in mobile telecommunications as it identifies and stores a unique identifier called an IMSI, which allows that SIM card to authenticate with a mobile phone network. The primary use of a SIM card is to securely store the IMSI and an encryption key (Ki ).
Some SIM cards are able to store additional information such as a user’s phone book contacts and text messages.
A common misconception is that the user’s telephone number is stored within the SIM. This is incorrect as the telephone number is assigned from the mobile network operator when the SIM card registers with the telephone network.
SRIforSM – Send Routing Information for Short Message
A type of SS7 message which is sent from one SS7 network operator to another, requesting details from the receiving network about an MSISDN. The request is sent so that the originating network can know if the telephone number is available to receive a communication and where to send that communication.
SS7 – Signalling System Number 7
A data communication network that telephone networks connect to in order to transmit signalling messages between each other (and internally to different parts of their own network). The SS7 network is highly reliable, fault-tolerant and links all the world’s telephone networks together so that customers from one telephone network can send and receive communications with customers in a different telephone network.
A signalling message sent over the SS7 network can be used to query other telephone networks, update telephone switching servers with information, or request the setup of a telephone call, SMS message, data connection or other communication. SS7 is the backbone network that provides seamless roaming of mobile phone handsets.
To send and receive messages on the SS7 network, a company must implement a set of signalling protocols that were developed originally in 1975 and have been standardised and built upon since then. The ITU has the recommended specifications for how to implement the SS7 protocols, but there are a few variants of this, most notably the ANSI SS7 standard (used in North America because North America likes to do its own thing).
UMTS / 3G – Universal Mobile Telecommunications System
An umbrella term that encompasses the third generation (3G) radio technologies developed by the 3GPP (3rd Generation Partnership Project). In general 3G mobile handsets provide significantly faster data transmission, but still rely on 2G (GSM) for voice calls.
VLR – Visitor Location Register
The Visitor Location Register (VLR) is a database that contains a subset of information about subscribers who are currently roaming within a Mobile Switching Centre’s coverage area. The VLR temporarily includes some of the details of the home network’s own subscribers as well as subscribers visiting from other mobile phone networks. The primary role of the VLR is to minimise the number of queries that Mobile Switching Centres have to make to the Home Location Register (HLR).
The VLR obtains details about the IMSI front on the HLR and whilst the IMSI is within the serving area it stores the SIM card’s IMSI, the MSISDN, a Temporary Mobile Subscriber Identity (TMSI) and a temporary roaming telephone number where the SIM card may be reached whilst in the area covered by the VLR (MRSN).
The VLR is also the database that stores the cell site information where the SIM card is geographically located. Because the Visitor Location Register informs the Home Location Register that it is the current VLR servicing the SIM card but it does not pass the cell tower details to the HLR, then the location data is stored in the VLR and not the HLR. The cell site location is not available from a HLR Lookup and we do not provide any facility to remotely check the location of a SIM card.
VoIP – Voice over Internet Protocol
Voice over Internet Protocol (VoIP) is a technology used for delivering different kinds of data from a source to a destination using IP (Internet Protocol). The data may be in many forms, including files, voice communication, pictures, fax or multimedia messages. VoIP is most often used for telephone calls, which are almost free of charge.
Written by David MorrisChief Executive Officer & Founder at HLR Lookup
The centralised HLR lookup service. We provide dependable status lookup for mobile telephone numbers. We welcome any suggestions for blog posts and are happy to share our insights. If you’d like the team to write up an article about a specific part of HLR Lookup please email us at email@example.com.
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