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Global System for Mobile communication (GSM) protocol family

GSM is a technology for digital wireless telecommunications, represented by a decent number of specifications. Parts of GSM are based on the fixed-line ISDN technology.

The original "air interface" for GSM handsets, for second-generation (2G) wireless telephony, was a TDMA interface; the third-generation interface, W-CDMA, is a CDMA interface. GSM, however, refers to more than just the "air interface"; it refers to the complete set of protocols.

The 3rd Generation Partnership Project (3GPP) maintains the GSM standards; most of the specifications for GSM can now be found at the 3GPP Web site.


Incidentally, the initial abbreviation of GSM was "Groupe Sp├ęcial Mobile" (Special Mobile Group). The acronym was preserved but a new, English meaning was given to it later, once the potential of the technology was understood.


The GSM protocol family consists of many protocols, and other protocols are conveyed on top of these.

Network Elements and Interfaces


Network and Switching Subsystem

Circuit Switching Control Nodes ("Core Circuit")

MSC Mobile Switching Center

TSC Transit Switching Center


VLR Visitor Location Registry

HLR Home Location Registry

EIR Equipment Information Registry

AuC Authentication Centre

FNR Flexible Number Registry

GSM Radio Access Network (GERAN)

BSC Base Station Controller

BTS Base Tranceiver Station

MS Mobile Set

UMTS Radio Access Network (UTRAN)

RNC Radio Network Controller

Node-B (RBS) Radio Base Station

UE User Equipment

GPRS Nodes (Packet Core)

SGSN Serving GPRS Support Node

GGSN Gateway GPRS Support Node

GSM Interfaces

A Interface

Interface between the MSC and the BSC. Control Plane Protocols:

Abis (Ab) Interface

Interface between the BTS and the BSC:

B Interface

Interface between an MSC and a VLR.

C Interface

Interface between an MSC and a HLR.

D Interface

Interface between a VLR and a HLR.

E Interface

Interface between two MSCs or TSCs.

F Interface

Interface between an MSC and an EIR.

G Interface

Interface between VLRs.

H Interface

Interface Between an HLR and the AuC

Um Interface

Interface between an MS and a BTS.

UMTS Interfaces

UMTS uses packet networks, ATM and/or IP, instead of TDM to transport user data (Voice, Video, etc.) So two sets of protocols are used - Control Plane Protocols (that control the calls) and user plane protocols (that carry the user's data).


Interface between two MSCs or TSCs.


Interface between an MSC or TSC and its controlled MGws.


Interface between two Media Gateways.


Interface between the Circuit Switched Core Network (via MGw) and the UMTS Radio Network (UTRAN, via Anchor RNC).


Interface between the two RNCs (used for switching calls that had been handed over from one RNC to the other).


Interface between an RNC and a Node-B.



Interface between the UTRAN (RNC) and the Packet Switched Core Network (SGSN) .


Radio Resource Control (RRC) 3G TS 25.331

GPRS Interfaces


The Gb interface is the name given to the logical connection between a SGSN and a BSS (also referred to as a PCUSN or PCU). (The Um interface applies between the BSS and MS.) Even though the physical Gb interface is between the SGSN and the BSS, it includes the LLC and SNDCP protocol layers, which are used for logical communication directly between the SGSN and MS.


Used to carry signalling and data traffic between GSNs using GTP protocol.


The interface between GGSN and external packet data networks such as the Internet.



This is an optional interface for PS/CS interoperability. Using the Gs interface it is possible to perform combined GPRS/IMSI attaches, combined location updates and paging the subscriber using PS facilities. The protocol is BSSAP+ specified in 3GPP TS 29.016 ( supported by Wireshark on ssn 98)


GSMMAP MAP-based interface between SGSN and HLR.



Same as Gn, but hadles GTP traffic between GSNs in different PLMNs.


The Ge is an interface between gprsSSF entity in SGSN and gsmSCF entity in SCP. It is used to handle CAMEL dialogues using the CAP protocol (supported by Wireshark).


GsmProtocolFamily (last edited 2013-11-25 12:02:50 by glcomm)