IMS Glossary (Useful terms)

AMR (Adaptive Multi-rate)

Adaptive Multi-Rate (AMR) is an audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now widely used in GSM. It uses link adaptation to select from one of eight different bit rates based on link conditions.

AMR is also a file format for storing spoken audio using the AMR codec. Many modern mobile telephone handsets will allow you to store short recordings in the AMR format, and some commercial programs exist to convert between this and other formats such as MP3, although it should be remembered that AMR is a speech format and is unlikely to give ideal results for other audio. The common filename extension is .amr.

API (Application Program Interface)

An interface for letting a program communicate with another program. In web terms: An interface for letting web browsers or web servers.

AS (Application Server)

An Application Server (AS) i.e., SIP Application Server, OSA Application Server, or CAMEL IM-SSF, offers value added IM services and resides either in the user’s home network or in a third party location. The third party could be a network or simply a stand-alone AS.

BCSM (Basic Call State Model)

In Intelligent Network and CAMEL switching, a BCSM is a Basic Call State Model

BG (Border Gateway)

The Border Gateway (BG) is a gateway between a PLMN supporting GPRS and an external inter-PLMN backbone network used to interconnect with other PLMNs also supporting GPRS. The role of the BG is to provide the appropriate level of security to protect the PLMN and its subscribers. The BG is only needed in PLMNs supporting GPRS.

BGCF (Breakout Gateway Control Function)

The Breakout Gateway control function (BGCF) selects the network in which PSTN breakout is to occur and – within the network where the breakout is to occur – selects the MGCF.

CAMEL (Customised Application Mobile Enhanced Logic)

The Customized Applications for Mobile network Enhanced Logic (CAMEL) provides the mechanisms to support services of operators, which are not covered by standardized GSM services even when roaming outside the HPLMN (Home Public Land Mobile Network).

CDR (Charging Data Record)

The row records into which IMS components capture IMS traffic usage, which then helps to generate final consolidated billing record for the subscriber.

CN (Core Network)

This is the part where the key IMS components reside. There is one unified CN for IMS, while there could be multiple different Access Networks (e.g. GPRS, CDMA 1X, WiFi, Cable Modem).

CSCF (Call Session Control Function)

The CSCF can act as Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) or Interrogating CSCF (I-CSCF). The P-CSCF is the first contact point for the UE within the IM subsystem (IMS); the S-CSCF actually handles the session states in the network; the I-CSCF is mainly the contact point within an operator’s network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator’s service area.

GGSN (Gateway GPRS Support Node)

The location register function in the GGSN stores subscriber data received from the HLR and the SGSN. There are two types of subscriber data needed to handle originating and terminating packet data transfer:
– subscription information:
– the IMSI;
– zero or more PDP addresses.
– location information:
– the SGSN address for the SGSN where the MS is registered.

GLMS (Group and List Management Server)

The Group and List Management Server manages contact lists, groups and access in 3GPP IMS based PoC (Push To Talk Over Cellular) network. It works together with the Presence Server, which manages the presence information of the PoC Clients.

HSS (Home Subscriber Server)

The HSS is the master database for a given user. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions.
A Home Network may contain one or several HSSs: it depends on the number of mobile subscribers, on the capacity of the equipment and on the organisation of the network.
As an example, the HSS provides support to the call control servers in order to complete the routing/roaming procedures by solving authentication, authorisation, naming/addressing resolution, location dependencies, etc…
The HSS is responsible for holding the following user related information:
• User Identification, Numbering and addressing information.
• User Security information: Network access control information for authentication and authorization
• User Location information at inter-system level: the HSS supports the user registration, and stores inter-system location information, etc.
• User profile information
The HSS also generates User Security information for mutual authentication, communication integrity check and ciphering. Based on this information, the HSS also is responsible to support the call control and session management entities of the different Domains and Subsystems of the operator.

I-CSCF (Interrogating-CSCF)

Interrogating-CSCF (I-CSCF) is the contact point within an operator’s network for all connections destined to a user of that network operator, or a roaming user currently located within that network operator’s service area. There may be multiple I-CSCFs within an operator’s network.

IMS (IP Multimedia Core Network Subsystem)
IMS or IP Multimedia Subsystem represents a 3GPP and 3GPP2 effort to define an all IP based wireless network as compared to the historically disparate voice, data, signaling, and control network elements.

IPv4 (Internet Protocol version 4)

The practical IP version, which IMS systems use today.

IPv6 (Internet Protocol version 6)

The IP version, which IMS systems were originally planned to use (according the 3GPP standard).


An IP Multimedia Services Identity Module (ISIM) is an application running on a UICC smart card in a 3G mobile telephone in the IP Multimedia Subsystem (IMS). It contains parameters for identifying and authenticating the user to the IMS. The ISIM application can co-exist with SIM and USIM on the same UICC making it possible to use the same smartcard in both GSM networks and earlier releases of UMTS. The ISIM contains a private user identity (a NAI address,, one or more public user identities (a SIP,, or TEL, tel:+1-212-555-12345, address) and a long-term secret used to authenticate and calculate cipher keys.

MAP (Mobile Application Part)

The MAP (Mobile Application Part) protocol typically runs on top of the Signaling System 7 (SS7) protocol. MAP is a non call-associated signaling protocol. It provides the support for interactive mobile applications ( cellular, paging, voice messaging, etc.) in a distributed environment. MAP defines the end-to-end protocol between applications which may be located in an SS7 network, and/or other networks supporting the MAP protocol.

MGCF (Media Gateway Control Function)

– Controls the parts of the call state that pertain to connection control for media channels in an IMS-MGW.
– Communicates with CSCF.
– Selects the CSCF depending on the routing number for incoming calls from legacy networks.
– Performs protocol conversion between ISUP and the IM subsystem call control protocols.
– Out of band information assumed to be received in MGCF and may be forwarded to CSCF/IMS-MGW.

MGW (Media Gateway)

A IMS-MGW may terminate bearer channels from a switched circuit network and media streams from a packet network (e.g., RTP streams in an IP network). The IMS-MGW may support media conversion, bearer control and payload processing (e.g. codec, echo canceller, conference bridge), it:
• Interacts with the MGCF for resource control.
• Owns and handles resources such as echo cancellers etc.
• May need to have codecs.

MRFC (Multimedia Resource Function Controller)

– Controls the media stream resources in the MRFP.
– Interprets information coming from an AS and S-CSCF (e.g session identifier) and control MRFP accordingly.
– Generates CDRs .

MRFP (Multimedia Resource Function Processor)

– Controls bearers on the Mb reference point.
– Provides resources to be controlled by the MRFC.
– Mixes incoming media streams (e.g. for multiple parties).
– Sources media streams (for multimedia announcements).
– Processes media streams (e.g. audio transcoding, media analysis).


The Proxy-CSCF (P-CSCF) is the first contact point within the IM CN subsystem. Its address is discovered by UEs using the mechanism described in section “Procedures related to Local CSCF Discovery”. The P-CSCF behaves like a Proxy (as defined in RFC 3261 or subsequent versions), i.e. it accepts requests and services them internally or forwards them on.

PDF (Policy Decision Function)

The Policy Decision Function (PDF) acts as a policy decision point for service based local policy control of IP bearer resources. The PDF makes decisions about IP bearer resource allocation requests. The PDF may be a logical entity of the P-CSCF, or a separate physical node.

PDN (Packet Data Network)

That’s the way 3GPP IMS specification normally refers to the IP Network.

PEF (Policy Enforcement Function)

The IP-Connectivity Access Network contains a Policy Enforcement Function (PEF) that has the capability of policing packet flow into the IP network, and restricting the set of IP destinations that may be reached from/through an IP-CAN bearer according to a packet classifier. This service-based policy ‘gate’ function has an external control interface that allows it to be selectively ‘opened’ or ‘closed’ on the basis of IP destination address and port. When open, the gate allows packets to pass through (to the destination specified in the classifier) and when closed, no packets are allowed to pass through. The control is performed by a PDF, which maybe a logical entity of the P-CSCF, or a separate physical node.

PSI (Public Service Identity)

With the introduction of standardized presence, messaging, conferencing, and group service capabilities in IM CN subsystem, there is a need for Public Service Identities (PSIs). These identities are different from the Public User Identities in the respect that they identify services, which are hosted by Application Servers. In particular, Public Service Identities are used to identify groups. For example a chat-type service may use a Public Service Identity (e.g. to which the users establish a session to be able to send and receive messages from other session participants. As another example, local service may be identified by a globally routable Public Service Identity.

Security Gateway (SEG)
The UMTS network domain shall be logically and physically divided into security domains in order to protect IP based control plane signalling. These security domains typically coincide with operator borders. The interface between different security domains is protected by Security Gateways (SEGs)The SEGs are responsible for enforcing the security policy of a IP security domain towards other SEGs in the destination IP security domain.

SGW (Signalling Gateway Function)
The SGW performs the signalling conversion (both ways) at transport level between the SS7 based transport of signaling used in pre-Rel 4 networks, and the IP based transport of signalling possibly used in post-R99 networks (i.e. between Sigtran SCTP/IP and SS7 MTP). The SGW does not interpret the application layer (e.g. MAP, CAP, BICC, ISUP) messages but may have to interpret the underlying SCCP or SCTP layer to ensure proper routing of the signaling.

S-CSCF (Serving-CSCF)

The Serving-CSCF (S-CSCF) performs the session control services for the UE. It maintains a session state as needed by the network operator for support of the services. Within an operator’s network, different S-CSCFs may have different functionalities.

SGSN (Serving GPRS Support Node)

The location register function in the SGSN stores two types of subscriber data needed to handle originating and terminating packet data transfer:
– subscription information:
– the IMSI;
– one or more temporary identities;
– zero or more PDP addresses.
– location information:
– depending on the operating mode of the MS, the cell or the routeing area where the MS is registered;
– the VLR number of the associated VLR (if the Gs interface is implemented);
– the GGSN address of each GGSN for which an active PDP context exists.

SLF (Subscription Locator Function)

The SLF:
– Is queried by the I-CSCF during the Registration and Session Setup to get the name of the HSS containing the required subscriber specific data. Furthermore the SLF is also queried by the S-CSCF during the Registration.
– Is queried by the AS in conjunction with the Sh interface operation to get the name of the HSS containing the required subscriber specific data.

SIM (Subscriber Identity Module) card

A SIM card, also known as a subscriber identity module, is a smart card that stores data for GSM cellular telephone subscribers. Such data includes user identity, location and phone number, network authorization data, personal security keys, contact lists and stored text messages. Security features include authentication and encryption to protect data and prevent eavesdropping. A SIM card and can be switched easily from one phone set to another.

SGW (Signalling Gateway)

A Signalling Gateway (SGW) interfaces with the signalling plane of the CS (Circuit Switched domain). It transforms lower layer protocols as Stream Control Transmission Protocol (SCTP, an Internet Protocol (IP) protocol) into Message Transfer Part (MTP, an Signalling System 7 (SS7) protocol), to pass ISDN User Part (ISUP) from the MGCF to the CS network.

THIG (Topology Hiding Inter-network Gateway)

When an IMS operator wishes to hide the internal structure of his IMS network, he would add a special Topology hiding function to the I-CSCF. The I-CSCF then would perform the required SIP header manipulation, which scramble all internal sensitive information. In particular it would scramble the information about the IMS servers, which may have served the SIP request inside the operator’s network.

UE (User Equipment)

In seven words – the 3GPP way to say IMS/SIP UA (User Agent). Formally put – Allows a user access to network services. For the purpose of 3GPP specifications the interface between the UE and the network is the radio interface. A User Equipment can be subdivided into a number of domains, the domains being separated by reference points. Currently the User Equipment is subdivided into the UICC domain and the ME Domain. The ME Domain can further be subdivided into one or more Mobile Termination (MT) and Terminal Equipment (TE) components showing the connectivity between multiple functional groups.

UICC (Universal ICC)

a physically secure device, an IC card (or ‘smart card’), that can be inserted and removed from the terminal equipment. It may contain one or more applications. One of the applications may be a

USIM (Universal Subscriber Identity Module)

An application residing on the UICC used for accessing services provided by mobile networks, which the application is able to register on with the appropriate security.

UMTS (Universal Mobile Telecommunications System)

UMTS stands for Universal Mobile Telecommunications System. UMTS is one of the emerging mobile phone technologies known as third-generation, or 3G. Third-generation systems are designed to include such traditional phone tasks as calls, voice mail, and paging, but also new technology tasks such as Internet access, video, and SMS, or text messaging. UMTS is the original access network over which IMS was designed to work.