BSS and NSS in GSM networks are subsystems that consist of various network components to support radio and core network functions. BSS deals with the mobile radio network whereas NSS deals with the mobile core network.
BSS in GSM networks refers to Base Station Subsystem which is essentially the radio access part of a GSM network and consists of BTS and BSC. NSS in GSM stands for Network Switching Centre which mainly represents the circuit-switched mobile core network and consists of MSC, HLR, VLR, AuC and EIR.
Base Station Subsystem – BSS
In second-generation (2G) GSM networks, BSS refers to Base Station Subsystem, which is responsible for managing the radio network resources within a GSM mobile network. It consists of Base Transceiver Stations (BTS) and Base Station Controllers (BSC). BSS manages the radio propagation path, signalling, mobility management and synchronisation.
Base Station Controller (BSC) is a vital part of the BSS, which controls a number of radio base stations (BTSs). The BSC within a BSS, for example, decides which frequency should be allocated to a mobile phone to successfully continue a phone call.
When a mobile user is on the move, for example, in a car, the BSC keeps handing over the call from one base transceiver station (BTS) to the next to make sure that the call continues in an acceptable quality. When the user moves from the location covered by the current BSC to the location covered by another BSC, the call is handed over to a new base station (BTS) within the new BSC with better signal strength.
Before the arrival of the General Packet Radio Service (GPRS), the GSM networks only had the circuit-switching capability which on the core network side was enabled by MSC (Mobile Switching Centre). MSC is an essential core network component in GSM and is responsible for controlling multiple Base Station Subsystems (BSSs).
When GPRS was introduced, the existing BSS in GSM networks was upgraded to support the GPRS protocols. The new core network entity, Serving GPRS Support Node (SGSN), was linked to the BSS through the Gb interface. The BSS upgrade also introduced support for enhanced Layer 2 (L2) protocols in the air interface for Radio Link Control (RLC) and Medium Access Control (MAC).
While the most important entities for packet-based GPRS services are the SGSN and GGSN nodes, BSS is responsible for allocating radio network resources to a user when any data-related network activity is detected. An example of this activity is when mobile data is sent or received by the mobile phone.
The Gb interface between BSS and SGSN is for signalling and data transmission. The radio or air interface (Um) and the Gb interface allow multiple users to be multiplexed over the same network resources to efficiently support more users. BSS GPRS Protocol (BSSGP) provides Quality of Service (QoS) for the radio access interface and the routing of the data packets between the BSS and SGSN.
Network Switching Subsystem – NSS
Network Switching Subsystem or Network and Switching Subsystem (NSS) represents the mobile core network in 2G GSM responsible for the switching of calls and mobility management functions. The original NSS architecture only included the circuit-switched components for voice calls and SMS. The current GSM core network also includes packet-switched components for data services.
The original Network Switching Subsystem architecture in GSM networks mainly consists of Mobile Switching Centre (MSC), Visitor Location Register (VLR), Home Location Register (HLR), Authentication Centre (AuC) and Equipment ID Register (EIR). These network entities enable circuit-switched services like voice calls and short message service (SMS).
The introduction of General Packet Radio Service (GPRS) in 2G GSM networks in the late 1990s extended the GSM core network by adding two additional nodes SGSN (Serving GPRS Support Node) and Gateway GPRS Support Node (GGSN). SGSN and GGSN enable packet-switched mobile data services (mobile internet) in GSM networks.
MSC – Mobile Switching Centre
In the original NSS design, the most critical network entity is the Mobile Switching Centre or MSC which facilitates the circuit-switched voice calls and SMS in GSM networks. MSC is the interface between a mobile network and external phone networks such as PSTN (Public Switched Telephone Network) and ISDN (Integrated Services Digital Network). MSC connects to the external networks through another entity called Gateway MSC (GMSC).
The MSC is also responsible for various other functions including mobility management and authentication. It works with a number of key databases or registers including HLR, VLR, AuC and EIR to allow mobile phone users to connect to the mobile network.
AuC and EIR
For authenticating a mobile phone to connect to the mobile network, the MSC works with a network entity Authentication Centre (AuC). AuC is a database that validates the SIM and uses ciphering to securely allow mobile phones to connect to the network.
MSC also works with another database called Equipment Identity Register or EIR which is the entity that contains the IMEI numbers (International Mobile Equipment Identity) for all mobile phones. MSC works with EIR for registering a mobile phone onto the mobile network before every phone call.
HLR and VLR
Home Location Register and Visitor Location Register are two essential databases in NSS that the MSC works very closely with. VLR is an integral part of the MSC whereas HLR is a separate database.
HLR contains information about the subscription status of mobile phone users and their geographical locations. The information about a user’s location and their service status allows the mobile network to dispatch the right services to them while allowing them to connect to the network through the closest base station (BTS).
VLR, on the other hand, is a distributed location register, unlike HLR, which is a central database. VLR is integrated with specific MSCs and works closely with the HLR to keep it updated on the location and service status of a mobile phone user. As a result, if a mobile phone user X wants to call another phone user Y, the network is able to find the location and account status of both users to facilitate the call.
BSS also means Business Support Systems
In wider telecom networks, including mobile and fixed, BSS also refers to Business Support Systems. The Business Support System is responsible for managing the customer-facing functions within a telecom network. It includes tasks like billing, charging, service fulfilment, revenue management, customer management, order management, product catalogues, and so on.
Interestingly both Business Support Systems (BSS) and Base Station Subsystem (BSS) work closely with another network entity Operations Support Systems (OSS). However, Base Station Subsystem is limited to GSM and GPRS networks only, whereas BSS is a much broader entity used in most telecom networks.
