GSM as a Modular Network
GSM separates the radio access functions from switching, subscriber data, and network management. The separation gives each part a defined responsibility and makes roaming possible between networks that implement the same reference model. A physical product may combine several functions, but the logical entities and reference interfaces remain useful when analyzing a call. The same model let operators evolve radio sites, switching centers, and databases on different hardware cycles.
A common functional view groups the network into three areas:
- Base Station Subsystem (BSS): The radio access part. It contains the BTS and BSC and provides the radio path between the mobile station and the core network.
- Network and Switching Subsystem (NSS): The circuit-switched core. It handles call control, mobility, interworking, subscriber data, and security functions.
- Operations and Support Subsystem (OSS): The operational layer for configuration, alarms, performance measurements, and maintenance of network elements.
High-Level GSM Reference Model
The reaches the BSS over the Um radio interface. Um carries the radio channels, Abis carries traffic between BTS and BSC, and A carries traffic and signaling between the BSS and MSC. Inside the BSS, BTS radio equipment connects to a BSC over Abis. The BSS presents the access network to the MSC through the A interface. The MSC then connects calls to another mobile subscriber or to an external network such as the PSTN. HLR, VLR, AuC, and EIR exchange signaling with the switching layer, while the OSS supervises operation and maintenance.
Part 1: Base Station Subsystem (BSS)
The BSS controls the radio resources of one area and carries traffic and signaling toward the MSC. In the classic GSM reference model, a BSS contains one BSC and one or more BTSs. The MS is the endpoint served by the BSS, not a component of the BSS itself.
The MS consists of Mobile Equipment (ME) and a Subscriber Identity Module (SIM). The ME carries the equipment identity, the . The SIM contains the subscriber identity, the , and the subscriber's secret authentication material. The public directory number, the , belongs to the subscription data held by the operator. A SIM may store related service data, but MSISDN is not the physical identity of the handset.
- Mobile Equipment (ME): The handset and radio hardware have an equipment identity, normally represented as a 15-digit IMEI.
- SIM: The card stores the IMSI and secret authentication material. Moving the SIM changes the subscription identity used by the network.
The BTS serves one or a defined sector of a cell. It contains the radio transceivers and antennas that send and receive GSM bursts, perform the radio modulation and demodulation functions, and apply the radio parameters commanded by the BSC. It also participates in radio-link ciphering after the cipher mode procedure. The BTS forwards traffic and signaling to its BSC over Abis.
- Transmit and receive radio bursts from the MS.
- Perform radio modulation and demodulation.
- Apply channel parameters and ciphering after network commands.
- Forward traffic and signaling to the BSC over Abis.
The BSC controls one or more BTSs. It manages radio channels and time slots, radio measurements, power-control commands, timing-related parameters, and many intra-BSS . The BSS handles the initial random access and Immediate Assignment autonomously. For a speech call that needs a traffic channel, the MSC sends an ASSIGNMENT REQUEST over BSSMAP on the A interface, and the BSS selects and establishes the radio resource.
- Channels: Assigns and releases frequencies and time slots.
- Measurements: Collects level and quality reports and controls transmit power.
- Mobility: Coordinates intra-BSS handovers and prepares resources for the MSC.
The main boundaries are therefore explicit. Um connects MS and BSS, Abis connects BSC and BTS, and A connects the BSS with the MSC. These interfaces carry different layers of radio, access, and call-control information.
Part 2: Network and Switching Subsystem (NSS)
The NSS provides the circuit-switched core and the databases needed for mobility. The MSC is the switching entity visible to the BSS. The registers and security functions supply the subscriber and equipment information required during call setup, location update, and roaming. MAP signaling and the logical register interfaces exchange this information, while the MSC establishes the speech path.
The MSC performs call control, mobility procedures, charging-related processing, and interworking with external networks. It manages the A-interface connection to the BSS and can coordinate handover between BSS areas. A Gateway MSC (GMSC) is an MSC function used as the entry point for calls arriving from an external network. It interrogates the HLR and routes a mobile-terminated call to the serving MSC.
The HLR is the permanent subscription database of the home network. It stores the IMSI, the MSISDN, subscribed services, and the address of the VLR currently serving the subscription. It also receives or retains authentication data supplied by the AuC. During a terminating call, the HLR provides routing information based on the serving VLR and a temporary MSRN.
- The IMSI and MSISDN of the subscription.
- Services allowed for the subscriber, such as forwarding and international calls.
- Authentication data supplied by the AuC.
- The VLR address needed for terminating-call routing.
The VLR is a temporary location and service database associated with one or more MSCs. It stores the data needed to serve subscribers currently located in its area, including a temporary identity and authentication parameters. The VLR exchanges location and subscription information with the HLR, which avoids a full HLR transaction for every local call.
The AuC protects the subscriber's long-term secret key, , and generates authentication data for the network. In GSM authentication, the network sends a challenge such as RAND to the SIM. The SIM calculates a response, while the AuC has calculated the expected response with the same . A matching response authenticates the subscription. The key itself remains in the SIM and the AuC; it is not sent over the radio interface.
The EIR stores equipment identities such as IMEI values. The MSC can query it after obtaining an equipment identity from the MS. An operator may use separate white, grey, and black lists to allow, observe, or bar equipment. The exact policy is an operator decision, while the EIR interface supplies the equipment-status check.
- White list: Equipment allowed to operate.
- Grey list: Equipment monitored by the operator.
- Black list: Equipment barred, such as a reported stolen device.
GSM names several logical NSS interfaces. B connects the MSC with its VLR, C connects the GMSC or MSC with the HLR, and D connects the HLR with the VLR. E connects MSCs for inter-MSC procedures, F connects the MSC with the EIR, G connects VLRs, and H connects the HLR with the AuC. MAP signaling commonly carries the subscriber and mobility operations over the GSM signaling network. For an incoming call, the HLR and VLR identify the serving MSC before the BSS pages the MS.
Part 3: Operations and Support Subsystem (OSS)
The OSS is the operational layer around the service network. It configures network elements, collects alarms and performance measurements, supports diagnostics, and coordinates maintenance. It can also support subscriber and service provisioning through operator systems. This scope is often described as operations, administration, maintenance, and provisioning. The OSS supervises the call path, but it is not a forwarding element in the user-plane connection.
- Operations: Alarm handling and supervision of network status.
- Administration: Configuration of network resources and service parameters.
- Maintenance: Diagnostics, software management, and restoration activities.
- Provisioning: Activation of subscribers, services, and network equipment.
Why the Separation Matters
GSM architecture assigns radio-resource work to the BSS, call and mobility control to the NSS, and operational work to the OSS. The explicit interfaces make the signal path traceable from MS through Um, Abis, and A to the MSC, while MAP and the remaining NSS interfaces exchange subscriber and routing data. This modular reference model supported equipment interoperability, roaming, and later evolution of individual network functions. The boundary also helps identify whether a fault belongs to radio access, switching, a subscriber database, or operations.
During a mobile-originated call, the MS uses Um, while BTS and BSC carry the request across Abis and A to the MSC, which selects the route and speech resource. NSS signaling obtains the data needed for authentication and routing. For a mobile-terminated call, the GMSC queries the HLR, the VLR allocates an MSRN, and the serving MSC asks the BSS to page the MS.