GSM (Global System for Mobile Communications) is a digital communication standard developed by the European Telecommunications Standards Institute (ETSI) and maintained by 3rd Generation Partnership Project (3GPP). The stndard describes the protocols for second-generation (2G) digital cellular networks.
Key Facts:
The GSM network is divided into three main subsystems, i.e Base Station Subsystem, Network Switching Subsystem and Operation Support Subsystem.
Consists of:
Includes:
| Component | Function |
|---|---|
| Base Transceiver Station (BTS) | Handles radio transmission/reception with mobile stations |
| Base Station Controller (BSC) | Manages radio resources and handovers for multiple BTSs |
| Transcoder and Rate Adaptation Unit (TRAU) | Converts between different speech coding formats |
The Network switching Subsystem (NSS) is responsible for performing call processing and subscriber-related functions. NSS includes the following functional units.
Mobile services switching center (MSC) performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
Home location register (HLR): A database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
Visitor location register (VLR) is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
Authentication center (AUC) provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.
Equipment identity register (EIR) is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes
| Component | Function |
|---|---|
| Mobile Switching Center (MSC) | Central switch for call processing and subscriber services |
| Home Location Register (HLR) | Database of permanent subscriber information |
| Visitor Location Register (VLR) | Database of subscribers currently in the MSC service area |
| Authentication Center (AUC) | Provides authentication and encryption parameters |
| Equipment Identity Register (EIR) | Database of valid mobile equipment |
GSM uses a combination of:
Key Parameters:
GSM defines several logical channel types:
| Channel Type | Purpose | Examples |
|---|---|---|
| Traffic Channels (TCH) | Carry voice or user data | TCH/F (full rate), TCH/H (half rate) |
| Broadcast Channels (BCH) | Downlink information to all MS | BCCH, FCCH, SCH |
| Common Control Channels (CCCH) | Control signaling | PCH, RACH, AGCH |
| Dedicated Control Channels (DCCH) | Point-to-point control signaling | SDCCH, SACCH, FACCH |
| Interface | Connects | Protocol |
|---|---|---|
| Um | MS to BTS | LAPDm |
| Abis | BTS to BSC | LAPD |
| A | BSC to MSC | BSSAP |
| MAP | Between MSCs and HLR | Mobile Application Part |
[MS] - RR (Radio Resource) - MM (Mobility Management) - CM (Connection Management)
[BTS] - RR
[BSC] - RR
[MSC] - MM - CM
GSM Security Triad:
Security Elements:
Wideband CDMA technology with backward compatibility to GSM
Answer: Mobile Station (MS), Base Station Subsystem (BSS), Network Switching Subsystem (NSS)
Answer: 8 time slots