2.5G (GPRS) Architecture

The GPRS is an enhancement over the GSM and adds some nodes in the network to provide the packet switched services. These network nodes are called GSNs (GPRS Support Nodes) and are responsible for the routing and delivery of the data packets to and form the MS and external packet data networks (PDN).

The most important network nodes added to the existing GSM networks are:

-SGSN (Serving GPRS Support Node).
-GGSN (Gateway GPRS Support Node).

The serving GPRS support node (SGSN) is responsible for routing the packet switched data to and from the mobile stations (MS) within its area of responsibility. The main functions of SGSN are packet routing and transfer, mobile attach and detach procedure (Mobility Management (MM)), location management, assigning channels and time slots (Logical Link Management (LLM)), authentication and charging for calls. It stores the location information of the user (like the current location, current VLR) and user profile (like IMSI addresses used in packet data networks) of registered users in its location register.

The gateway GPRS support node (GGSN) acts as interface between the GPRS backbone and the external packet data network (PDN). It converts the GPRS packet coming from the SGSN into proper packet data protocol (PDP) format (i.e. X.25 or IP) before sending to the outside data network. Similarly it converts the external PDP addresses to the GSM address of the destination user. It sends these packets to proper SGSN. For this purpose the GGSN stores the current SGSN address of the user and his profile in its location register.The GGSN also performs the authentication and charging functions. In general there may be a many to many relationship between the SGSN and GGSN. However a service provider may have only one GGSN and few SGSNs due to cost constraints. A GGSN proved the interface to several SGSNs to the external PDN. For simplicity, we can consider SGSN as MSC but it works with packet switch and similarly, we can consider GGSN as GMSC.

2G architecture

I'm gonna talk about the architectures of 2G, 3G and 4G one by one. I skipped 1Gs architecture since it's already extinct so there is no reason to talk about it. 2G is called GSM which stands for Global System for Mobile Communication.

GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but don't address the hardware. The GSM network is divided into three major parts:

- Switching System (SS)

-Base Station System (BSS)

-Operation and Support System (OSS)

The basic GSM network elements are shown in the above figure. Now, I will talk briefly about each main part. The switching system is responsible for performing call processing and subscriber-related functions. It includes the following functional units:

- Mobile Services Switching Center (MSC):

The MSC performs the telephony switching functions of the system. It also performs such functions as toll ticketing, network interfacing, common channel signalling, and others.

- Gateway Mobile Switching Center (GMSC):

A gateway mobile services switching center (GMSC) is a node used to interconnect two networks.

- GSM Interworking Unit (GIWU):

The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GSM interworking unit (GIWU), users can alternate between speech and data during the same call.

- Message Center (MXE):

The MXE is a node that provides integrated voice, fax, and data messaging.

- Mobile Service Node (MSN):

The MSN is the node that handles the mobile intelligent network (IN) services.

- Home Location Register (HLR):

The HLR database is used for storage and management of subscriptions. The home location register stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status.

- Visitor Location Register (VLR):

The VLR database contains temporary information about subscribers that is needed by the mobile services switching center (MSC) in order to service visiting subscribers. When a mobile station roams into a new mobile services switching center (MSC) area, the visitor location register (VLR) connected to that MSC will request data about the mobile station from the HLR, reducing the need for interrogation of the home location register (HLR).

-Authentication Center (AUC):

The AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The authentication center (AUC) also protects network operators from fraud.

-Equipment Identity Register (EIR):

The EIR database contains information on the identity of mobile equipment to prevent calls from stolen, unauthorized or defective mobile stations.

The base station system is responsible for performing all radio-related functions. It includes the following functional units:

- Base Station Controller (BSC):

The BSC provides the control functions and physical links between the MSC and BTS. It provides functions such as handover, cell configuration data and control of RF power levels in base transceiver stations. A number of BSCs are served by a MSC.

- Base Tranceiver Station (BTS):

The base transceiver station (BTS) handles the radio interface to the mobile station. The base transceiver station is the radio equipment (transceivers and antennas)

The Operation and Support System (OSS) is is the functional entity from which the network operator monitors and controls the system. The purpose of operation and support system is to offer support for centralized, regional, and local operational and maintenance activities that are required for a GSM network.

The GSM mobile station (or mobile phone) communicates with other parts of the system through the base-station system.