GSM(2G-2.5G) Structure (NSS)

GSM Structure 

Network Switching Subsystem (NSS)
NSS is core part of GSM system. NSS handle switching functions, mobility management function and manages the communications between mobile phone other telephone network. NSS includes with MSC, VLR, HLR, AUC and EIR that all are described in more detail below.

Mobile Service Switching Center (MSC)

MSC is a one type of telephone exchange which provides circuit-switched calling (Ex. Call set-up, Routing control and Call Terminate) , mobility management (Hand over), GSM supplementary services (Ex. Call forwarding, Call Waiting, CLIP) and SMS terminating to the mobile phones roaming within the area that it serves. And some MSC has Gateway function and Charging and Accounting function. This means that it connects a call to other telephone network and generates a call detail record for charging and accounting.

Visitor Location Register (VLR)VLR is a temporary database of the subscribers who have roamed into the particular area which it serves. The data stored in the VLR has either been received from the HLR, or collected from the MS. These temporary data contain with IMSI, MSISDN, Subscriber services, Category, Triplets and LAI.In practice, for performance reasons, most vendors integrate the VLR directly to the MSC.

Home Location Register (HLR)
HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. Examples of data stored in the HLR are Mobile Station ISDN Number (MSISDN), International Mobile Subscriber Identity (IMSI), Subscriber location (MSC/VLR, SGSN address), services (Ex. supplementary service, GPRS, SMS) and triplets for AUC. HLR will provide subscriber data to MSC/VLR/SMSC When MSC/VLR/SMSC have requested data of visited subscribers to HLR. MSISDN = CC + NDC + SN
CC: Country Code, NDC: National Destination Code, SN: Subscriber Number

Cell Global Identity (CGI) = LAI + CI, LAI :Location Area Identity, CI : Cell Identity

Authentication Center (AUC)

AUC is a function to authenticate (A3 Algorithm) each SIM card that attempts to connect to the GSM core network (when mobile do mobile originated call, Location update Active or Deactivate services). If the authentication is successful, the HLR shall allow SIM to Location Update and use GSM services. Beyond above, AUC is a function to Ciphering (A5 and A8 Algorithm) for protecting voice interception

The AUC generates data known as triplets for the MSC to use during this procedure. The key of this process is Ki. Ki is shared secret between the AUC and the SIM. The Ki is securely burned into the SIM during manufacture and is also securely replicated onto the AUC.
When the MSC asks the AUC for a new set of triplets for a particular IMSI, the AUC first generates a random number called as RAND. This RAND is combined with the Ki in A3 algorithm Signed RESponse or SRES. The Ki and RAND are fed into the A8 algorithm and a session key called Kc is calculated for ciphering process.
The Triplet (RAND, SRES, KC) sent back to the MSC. When a particular IMSI access to the GSM core network, the authentication process is started. The MSC sends the RAND value of the

triplet to the SIM. The SIM use this number and the Ki into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet, the mobile is allowed to access GSM network.

After finished authentication, the MSC sends Kc to the Base Station Controller (BSC) so that all communications can be encrypted and decrypted. When mobile phone has a call, the mobile phone can generate the Kc itself by use the same RAND supplied during authentication and the Ki into the A8 algorithm for ciphering process.
For performance reasons, most vendors integrate the AUC directly to the HLR.

Equipment Identity Register (EIR)

Mobile operator uses EIR to keeps a list of mobile phones IMSI (IMEI: International Mobile Equipment Identity), IMEI is burnt to memory of mobile phone by factory, and it is identity of each mobile phone terminal.
If mobile operators have active EIR function, only valid mobile phone is allowed to access their network.

Operation Support System (OSS)

OSS are computer systems used for operation and maintenance telecommunications system such as provisioning services, configuring network components, managing faults and traffic managing.

Introduction to GSM(2G-2.5G)

Introduction to GSM(2G-2.5G) 
The Global System for Mobile communications is the most popular standard for mobile phone in the world. GSM service is used by over 2 billion people across more than 212 countries and territories.The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system.


GSM networks operate in different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including the United States and Canada) use the 850 MHz and 1900 MHz bands.
In the 900 MHz band the uplink frequency band is 890-915 MHz, and the downlink frequency band is 935-960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radiotimeslots grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1kHz audio into between 6 and 13kbps. Originally, two codecs, named after the types of data channel they were allocated, were used, called "Full Rate" (13kbps) and "Half Rate" (6kbps). GSM was further enhanced in 1997 with the GSM-EFR codec, a 12.2kbps codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
There are four different cell sizes in a GSM network - macro, micro, pico and umbrella cells. The longest distance the GSM specification supports in practical use is 35 km. There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

GSM(2G-2.5G) Structure (BSS)

GSM Structure 

Base Station System : BSS

- Base Station Controller (BSC)
- Base Transceiver Station (BTS)
- Transcoding and Rate Adaption Unit (TRAU) orTranscoding and Sub-Multiplexing (TCSM)

Base Station Controller (BSC) The Base Station Control Function provides common control functions within a BTS , for example frequency hopping, handover, performs radio signal management functions for base transceiver stations, frequency assignment and handoff ,traffic measurement.

Base Transceiver Station (BTS)BTS contains the equipment for transmitting and receiving of radio signals (between BSC and MS), antennas, equipment for modulation, encrypting and decrypting communications with the BSC. BTS can do Local Maintenance and Self Test by non connecting to BSC.
Transcoding and Rate Adaption Unit (TRAU) TRAU, inserted between the BSC and MSC, enables speech compression and data rate adaptation within the radio cellular network. The TRAU is designed to reduce transmission costs by minimizing transmission resources between the BSC and MSC. This is achieved by reducing the number of PCM links going to the BSC, since four traffic channels can be handled by one PCM time slot.

GSM(2G-2.5G) Structure (MS)

GSM Structure

Mobile Station (MS)

MS of GSM shall include 2 parts. First part is Mobile Equipment (ME) and second part is SIM (Subscriber Identity Module).

Each ME is identity by IMEI (International Mobile Equipment Identity ). The IMEI number is a 15 digit unique code that is used to identify a mobile phone to a GSM network. The IMEI number can be retrieved by keying in *#06# and the 15 digit unique code will be displayed on the screen. In the unfortunate event that your mobile is misplaced, provision of this IMEI number to the police and telco service provider would be useful.

The Subscriber Identity Module (SIM) is a small smart card which contains both programming and information.The A3 and A8 algorithms are implemented in the Subscriber Identity Module (SIM). Subscriber information, such as the IMSI (International Mobile Subscriber Identity), is stored in the Subscriber Identity Module (SIM).The Subscriber Identity Module (SIM) can be used to store user-defined information such as phonebook entries, Ki(Subscriber Authentication key), TMSI, LAI, Kc (Ciphering key), PIN, Language and Frequency Range.

IMSI = MCC + MNC + MSIN
MCC:Mobile Country Code 3 digits, MNC:Mobile Network Code 2 digits, MSIN: Mobile Subscriber Identification Number 10 digits