GSM Architecture

GSM ARCHITECTURE

  •  It provides an overview of GSM network architecture.
  • Que. What is GSM network architecture?                                                               Ans. GSM (Global System for mobile communication), is a cellular expertise accessed for diffusing mobile voice and data facilities.
  • Now days over 90% market share with mobile communication and is available in over 219 countries and territories.
  • It is replacement for 1G.
  • Circuit -switched network optimized for full duplex voice telephony.
  • Basically GSM network architecture gives the knowledge about Call connections , mobile network , SMS connectivity , data connection etc.
  • GSM network architecture defines in four stages.
  • MS (mobile equipment + SIM operator )
  • BSS (base station subsystem)
  • NSS (network switching subsystem)




1. MS (mobile equipment + SIM operator = Mobile Station)
  •  The MS (mobile station) is the combination of terminal equipment and subscriber data. The terminal equipment as such is called ME (mobile equipment).
  • The subscriber's data is stored in a separate module called SIM (subscriber identity module).
  •   Therefore ME + SIM = MS.
  •  Every ME having their own identity named IMEI 
  • (International mobile equipment Identity), it is only use for identify the device and has no permanent gives the detail of subscriber.
  • Similarly , every SIM has their own identity.
  •  SIM (subscriber identity module), which gives the subscriber identity.
  •   It gives the HLR (home location register).
  •    HLR – 1.It is database used for storage and management.
  •                 2. It store permanent data about subscribers.





2. BSS (BASE STATION SUBSYSTEM)

                   
  •   The Base Station Subsystem is composed of two parts, the Base Transceiver Station Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.
  •  BSS consists of four components-

    1.  Abis interface
    2.  BTS (base transceiver station)
    3.  BSC (base station controller)
    4. Transcoder

   (1). Abis interface

     The interface between the BTS and BSC . Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM sub channels for traffic (TCH), LAPD protocol for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS.

   (2). BTS (base transceiver station)
  • The BTS is network element maintaining the air interface. It takes care of air     interface signaling.
  • In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost
  • Some basic points of BTS-:

  1. Handles the radio interface to the mobile station.
  2. Each Radio terminal represents an RF Channel
  3. TRX and MS communicates over Um interface
  4.  Received data transcoding                                         
  5.  Signal processing functions of the radio interface
  6.  Uplink Radio channel power measurements

(3). BSC (base station controller)



 The BSC is the central network element of BSS and it controls the radio network. This means the main responsibility of BSC is -: Connection establishment between MS and   

                                                                                  
NSS, mobility management, statistical raw data collection as well as Air and A interface signaling.                                      
                Some basic points of BSC are -:
  1. Controls one or more Base Transceiver Stations
  2. Radio network management (such as radio frequency control)
  3. BTS Handover management and call setup
  4. A BSC works with a mobile switching center (MSC) component that is external to the BTS, enabling it to provide full mobile telephony and fulfill capacity requirements. 

  Base stations must communicate with the MSC and data must be managed as information overflow, impacting MSC efficiency.     


     A BSC eliminates MSC base station activity management requirements, allowing the MSC to handle critical tasks, such as traffic balancing and database management.

(4). Transcoder (TC)

       The TC is a BSS element taking care of speech transcoding , it is capable of         converting of speech from one digital coding format to another and vice versa.

3. NSS (network switching subsystem)


  • The Network switching subsystem (NSS), the main part of which is the Mobile Switching Center (MSC), performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication.
  • The Network switching subsystem (NSS), contains five components -:

                                               
(1)  MSC(mobile services switching center)
(2)  VLR (visitor Location Register)
(3)  HLR (Home Location Register)
(4)  AuC (Authentication Center)
(5)  EIR (Equipment Identity register)
(6)  LAI(Location Area Identity)




(1). MSC (mobile services switching center), 

  •   The MSC is responsible for controlling calls in the mobile network. Its identifies the origin and destination of the call as well as type of call.
  •   An MSC acting as a bridge between a mobile network and a fixed network is called a gateway MSC.
  •  Some points which define the task of MSC are -:

  1.  Delivering calls to subscribers as they arrive based on information from the VLR.
  2. Connecting outgoing calls to other mobile subscribers or the PSTN.
  3. Delivering SMSs from subscribers to the short message service center (SMSC) and vice versa.
  4. Arranging handovers from BSC to BSC.
  5.  Carrying out handovers from this MSC to another.
  6.  Supporting supplementary services such as conference calls or call hold.
  7. Generating billing information.

(2). VLR (visitor Location Register),
  • Database that contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR .
  • Identity of Mobile Subscriber.
  • Copy of subscriber data from HLR.
  • Generates and allocates a Temporary Mobile Subscriber Identity (TMSI).
  • Location Area Code.
  •  Provides necessary data when mobile originates call.
(3). HLR (Home Location Register),
  • HLR is centralized network database that stores all mobile subscription                
  • Stores user data of all Subscribers related to the GMSC
  •  International Mobile Subscriber Identity(IMSI)                              
  • Users telephone number (MS ISDN)
  • Subscription information and services
  • VLR address
  • Subscriber Authentication center for key (Ki).
  •  Referred when call comes from public land network.

  (4). AuC (Authentication Center),
  • Cipher produces on the radio link.
  • Stores Subscriber authentication data called Ki, a copy of which is also stored in the SIM card
  •  Generates security related parameters to authorize a subscriber (SRES-Signed Response)
  • Generates unique data pattern called Cipher key (Kc) for user data encryption


  (5). EIR (Equipment Identity Register),
  •  EIR is a database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI).
  • EIR has three databases.,

  1. White list - For all known, good IMEI’s
  2. Black list - For all bad or stolen handsets
  3. Grey list - For handsets/IMEI’s that are on observation


(6).LAI (Location Area Identity) ,
  •  LAI identifies a location area which is a group of cells..
  • It is transmitted in the BCCH.
  • When the MS moves into another LA (Detected by monitoring LAI transmitted on the  BCCH ) it must perform a LU.
  • LAI = MCC + MNC + LAC

  1. MCC= Mobile Country Code(3 digits), identifies the country
  2. MNC= Mobile Network Code(1-2 digits), identifies the GSM-PLMN
  3. LAC= Location Area Code, identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits, enabling 65536 different location areas to be defined in one GSM PLMN.




4.BSS INTERFACES -:
Um
The air interface between the mobile station (MS) and the BTS. This interface uses LAPDm protocol for signaling, to conduct call control, measurement reporting, handover, power control, authentication, authorization, location update and so on. Traffic and signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.
Abis
The interface between the BTS and BSC. Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM sub channels for traffic (TCH), LAPD protocol for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS.
A
The interface between the BSC and MSC. It is used for carrying traffic 
channels and the BSSAP user part of the SS7 stack. Although there are usually transcoding units between BSC and MSC, the signaling communication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.

Ater
The interface between the BSC and transcoder. It is a proprietary interface whose name depends on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC leaving it untouched.
GB
Connects the BSS to the SGSN in the GPRS core network.





   6. GPRS / Packet Data Separation


GPRS network elements:
  •  GSN (GPRS Support Nodes): GGSN and SGSN
  •  GGSN (Gateway GSN)
  • Interworking unit between GPRS and PDN (Packet Data Network)
  • Acts as an interface and a router to external networks. The GGSN contains routing information for GPRS mobiles, which is used to tunnel packets through the IP based internal backbone to the correct Serving GPRS Support Node. The GGSN also collects charging information connected to the use of the external data networks and can act as a packet filter for incoming traffic.
  • SGSN (Serving GSN)

  1. Responsible for authentication of GPRS mobiles, registration of mobiles in the network, mobility management, and collecting information for charging for the use of the air interface.  
  2. user addresses     

                                                                         
     GPRS modifications on GSM networks:

    Mobile Stations: New Mobile Station is required to access GPRS services. These new terminals will be backward compatible with GSM for voice calls.

    BTS: A software upgrade is required in the existing base transceiver site

    BSC: The base station controller (BSC) requires a software upgrade and the installation of new hardware called the packet control unit (PCU). The PCU directs the data traffic to the GPRS network and can be a separate hardware element associated with the BSC.

    GPRS support nodes (GSN):  The deployment of GPRS requires the installation of new core network elements called the serving GPRS support node (SGSN) and gateway GPRS support node (GGSN).

    Databases (HLR, VLR etc.):  All the databases involved in the network will require software upgrades to handle the new call models and functions introduced by GPRS. 




    * This info was gathered from Internet and Telecom Sources


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