The GSM system is divided into three major subsystems to provide efficient mobile communication: Radio Subsystem (RSS), Network Switching Subsystem (NSS), and Operation Support-Subsystem
In the GSM architecture, interfaces are defined paths with specific rules (protocols) that allow different network components to exchange data and signaling information. They ensure that equipment from different vendors can work together seamlessly within the same network.
Key GSM Interfaces The primary interfaces connect the three main subsystems: the Radio Subsystem (RSS), the Network and Switching Subsystem (NSS), and the Operation Subsystem (OSS).
• Um Interface (Air Interface):
◦ Connection: Connects the Mobile Station (MS) to the Base Transceiver Station (BTS).
◦ Use: It is the wireless radio link used for transmitting voice, data, and signaling over the air.
• Abis Interface:
◦ Connection: Connects the BTS to the Base Station Controller (BSC).
◦ Use: This is a standardized, open wired interface (typically $16 \text{ kbit/s}$ for user channels) used for transmitting traffic and signaling between the station and its controller. It is often the first physical connection for a call.
• A Interface:
◦ Connection: Connects the BSC to the Mobile Switching Center (MSC).
◦ Use: It carries user traffic (at $64 \text{ kbit/s}$) and signaling information between the radio network and the core switching network.
• B Interface:
◦ Connection: Connects the MSC to the Visitor Location Register (VLR).
◦ Use: Used to exchange data when the MSC needs to check a subscriber's local status or location.
• C Interface:
◦ Connection: Connects the MSC to the Home Location Register (HLR).
◦ Use: Essential for call routing, allowing the MSC to find where a subscriber is currently located.
• D Interface:
◦ Connection: Connects the VLR to the HLR.
◦ Use: Used to exchange location information and subscriber profiles as users move (roam) between different VLR areas.
• E Interface:
◦ Connection: Connects one MSC to another MSC.
◦ Use: Facilitates inter-MSC handovers, allowing a call to continue uninterrupted as a user moves from the domain of one switching center to another.
In a GSM network, the HLR (Home Location Register) and EIR (Equipment Identity Register) serve as critical databases within the Network and Switching Subsystem (NSS) and the Operation Subsystem (OSS), respectively. While the HLR identifies the subscriber (the person), the EIR identifies the physical device (the hardware).
1. Home Location Register (HLR) The HLR is the central master database of a mobile network provider. It is the "permanent home" for all user data.
Key Roles: • Subscriber Data Storage: It contains permanent and semi-permanent data for every subscriber assigned to that specific HLR.
• Location Management: It stores the current "location area" of the user, essentially tracking which VLR (Visitor Location Register) area the user is currently visiting.
• Call Routing: When someone dials a mobile number, the GMSC (Gateway MSC) queries the HLR to determine the subscriber's current status and where to route the call.
• Service Profiles: It stores information about the services a user is allowed to access, such as SMS, data roaming, or call forwarding.
2. Equipment Identity Register (EIR) The EIR is a database used to track the physical mobile stations (phones) themselves, regardless of the SIM card being used.
Key Roles: • Hardware Identification: It maintains a list of IMEI (International Mobile Equipment Identity) numbers, which are unique to each physical device.
• Device Security & Locking: It allows the network to lock stolen or malfunctioning mobile stations so they cannot access the network.
• List Management: Devices are categorized into three lists:
◦ White List: Valid and authorized devices.
◦ Gray List: Devices that are observed for possible malfunctions or non-standard behavior.
◦ Black List: Stolen or barred devices that are prohibited from connecting to the network.
• Theft Prevention: By checking the IMEI during connection setup, the EIR helps deter mobile phone theft, as a blacklisted device becomes useless on all participating GSM networks.
In the Global System for Mobile Communication (GSM), services are categorized into three distinct domains based on their technical implementation and the utility they provide to the user.
1. Bearer Services Bearer services provide the technical "path" or transmission medium required to move data between access points without the network interpreting the content of that data.
• Functionality: They act as a transport mechanism, similar to a road carrying various types of vehicles.
• Data Transmission Modes:
◦ Circuit Switched: A dedicated communication path is reserved for the entire duration of the call.
◦ Packet Switched: Data is broken into packets and sent across the network without reserving a fixed path.
• Data Rates: Original standards supported synchronous rates of up to 9.6 kbit/s, though classical GSM eventually reached speeds of approximately 50 kbit/s.
2. Tele (Telematic) Services Telematic services are the end-user applications that utilize bearer services to enable actual communication.
• Mobile Telephony: The primary goal of GSM was to provide mobile voice calls with a bandwidth of 3.1 kHz.
• Emergency Services: A mandatory service (e.g., dialing 112) that is free of charge and given the highest network priority, capable of preempting other connections.
• Short Message Service (SMS): This allows for the transmission of alphanumeric messages up to 160 characters. It uses the signaling channel, enabling users to send or receive messages even while a voice call is active.
• Non-Voice Services: Includes group 3 fax, voice mailboxes (implemented in the fixed network), and electronic mail.
3. Supplementary Services These are additional features that enhance basic telecommunication services but cannot be offered as standalone products.
• Identification Services: Includes Caller ID (forwarding the caller's number) and Call Hiding (suppression of the number).
• Call Handling: Features such as Automatic Call-back, which reconnects users when a line becomes available, and Call Forwarding.
• Multi-party Services: Enables conferencing for up to 7 participants simultaneously.
• Security Locking: Allows for the locking of a mobile terminal for specific incoming or outgoing call types.
When a Mobile Node (MN)—such as a mobile phone or smartphone—shifts from one region to another while switched on, the network performs a series of automated procedures to ensure the device remains reachable and the user's connection is not dropped.
In the GSM (Global System for Mobile Communication) architecture, this movement is managed through two primary processes: Localization (Location Updating) and Handover
Handover (or handoff) in GSM refers to the process of transferring an ongoing call or data session from one cell (base station) to another without disconnecting the user.
It is essential to maintain call continuity when a mobile user moves from one cell to another.