Global System for Mobile Communications

Definition of terms Base Transceiver Station - Equipment that facilitates wireless communications between the user equipment and a network .

Definition of terms Base Station Controller - A critical mobile network component t hat controls one or more base transceiver stations. - Also known as base stations or cell sites.

Definition of terms Mobile Services Switching Center - A 2g core network element which controls the network switching subsystem elements.

Mobile services Switching Center The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (either a mobile station or a fixed telephone in both cases), as well as the type of a call.

Mobile services Switching Center An MSC acting as a bridge between a mobile network and a fixed network is called a Gateway MSC. An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registrations and updates. The MSC associated with it initiates the paging process

Definition of terms Home Location Register The main database of permanent subscriber information for a mobile network. the HLR contains pertinent user information, including address, account status, and preferences. The HLR interacts with the Mobile Switching Center (MSC), which is a switch used for call control and processing

Definition of terms Visitor Location Register A database that contains information about the subscribers roaming within a mobile switching center’s location area. The primary role of the VLR is to minimize the number of queries that MSCs have to make to the home location register, which holds permanent data regarding the cellular network’s subscribers.

Visitor Location Register A VLR database is always temporary (in the sense that the data is held as long as the subscriber is within its service area), whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its costumers. This data is required for routing calls.

Other network elements Transcoder (TC) – Equipment belonging to the Base Station Subsystem (BSS) Equipment Identity Register (EIR) – Equipment belonging to the Network Switching Subsystem (NSS). Authentication Center (AC) Equipment belonging to the Network Switching Subsystem. The EIR and AC are usually implemented as part of HLR and they deal with security functions

Another Look @ GSM System Hierarchy Paging – the search for the mobile-terminated call from among the sets of base stations, if each cell is an Location Area then paging cost is minimal. Registration – at each LA boundary mobile stations register their new locations in order to update the location management data base, if LA size = MSC paging cost is maximum but there is no registration cost.

Transcoder In the case of the air interface, the media carrying the traffic is a radio frequency, but as we saw in the example of a Public Switched Telephone Network originated call, the traffic signal is also carried through fixed networks. To enable the efficient transmission of the digital speech information over the radio Air Interface, the digital speech signal is compressed.

Transcoder For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 5.6Kbits/s (Half Rate). This compression algorithm is known as “Regular Pulse Excitation with Long Term Prediction” (RPE-LTP). However the standard bit rate for speech in the PSTN is 64Kbits/s Therefore, a converter has to be provided in the network to change the bit rate from one to another. This is called the Transcoder (TC).

Network Management subsystem Has three main functions Fault Management Configuration Management Performance Management

Functions of BTS/Node B and BSC/RNC Tasks of a BTS Maintain synch with MSC Gaussian Minimum Shift Keying modulation RF signal processing Diversity reception Radio interference timing Detect access attempts De-/encryption on radio path Channel de-/coding and interleaving on radio path Timing advances Perform frequency hopping Forward measurements to BSC. Tasks of a BSC Configure and manage Radio Resources Handover management: signal quality, signal level, interference, power budget and distance Frequency hopping management Measurements and observations: traffic, signalling and mobile tracing Transcoding. Cellular Geographic Service Area