GSM Network architecture continued [Under
GSM]
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GSM Network architecture - 23.002 «
Mobile-services Switching Center (MSC) is an entry point into network. Similar to telephone exchange, its important function is to take care of establishing the calls. MSC also drives many tasks not present in traditional (wireline) telephone network like authentication, location update, encryption etc. (with the help of VLR, HLR/AuC, EIR).
Our voice is in the range 0 to 4kHz, digitally this rate maps to 64 kbps (Nyquist theorem). For efficient utilisation of limited air interface, GSM recommends voice transmission data rates much lower than 64 kbps. There are some digital voice encoding techniques using which voice can be encoded at much lower rate (almost till 8/6.5 kbps). Though of poor quality, this speech can still be considered as sufficient enough for practical purposes. Using reduced rate, more number of users can be accommodated.
On Network side though, 64 kbps data rate will be utilised. Transcoding and Rate Adaptation Unit (TRAU) bridges the gap between these two rates in transmission path. Best place for TRAU is at MSC to take advantage of reduced number of resources required between BSS and MSC. 16/13 kbps is known as full rate channel and 8/6.5 known as half rate channel.
Visitor Location Register (VLR) stores a temporary subscriber data required for serving the mobile user like its current location, supplementary services status (e.g. call waiting), part of authentication/ciphering data. Some of the data are specific to VLR for its operation like location data and rest are as present in HLR like supplementary service status. In practice, MSC and VLR functions are implemented in one system (avoiding interface overhead).
Home Location Register (HLR) contain (more of) static subscriber data - shared among number of MSC/VLRs. The data include Authentication/Ciphering keys, VLR to which MS is currently present etc. Authentication Center (AuC), normally part of HLR, is required for authentication related calculations (generally done offline and kept ready for use).
Gateway MSC (GMSC) bridges the gap - both in terms of signaling and transmission (IWF) - between GSM network and other network (for example PSTN network). Note that MSC is a SS7 SSP supporting ISUP protocol. PSTN network most likely to already have support for SS7 due to which MSC can actually act as GMSC while interacting with PSTN network. Most of the non-GSM networks will be connected to PSTN network; it is normal practice to route the call to non-GSM network through PSTN network avoiding interworking directly with non-GSM network. Thus in practice, MSC and GMSC functions are implemented in one system.
The authentication that was mentioned above is subscriber or SIM authentication - the data of which is stored in HLR/AuC. For ME authentication, Equipment Identity Register (EIR) is used. ME authentication is optional in GSM specification. EIR is either not implemented or implemented only within PLMN and not shared with other PLMNs. But nowadays in many countries, rules are being set to avoid ME thefts and illegal/non-authorised ME sale - these rules will make mandatory to use EIR and related authentication signaling.
We will continue with GSM interfaces and protocol layers in next article.
References: 23.002 360 (GSM Network architecture), GSM Networks book by Heine, GSM book by Mouly and Pautet
© Copyright Samir Amberkar 2010
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