08-02-2013, 11:00 AM
Global System for Mobile Communication ( GSM)
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GSM: Overview
GSM
formerly: Groupe Spéciale Mobile (founded 1982)
now: Global System for Mobile Communication
Pan-European standard (ETSI, European Telecommunications Standardisation Institute)
simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations (Germany: D1 and D2) seamless roaming within Europe possible
today many providers all over the world use GSM (more than 200 countries in Asia, Africa, Europe, Australia, America)
more than 1.2 billion subscribers in more than 630 networks
more than 75% of all digital mobile phones use GSM (74% total)
over 200 million SMS per month in Germany, > 550 billion/year worldwide(> 10% of the revenues for many operators)[be aware: these are only rough numbers…]
Bearer Services
Telecommunication services to transfer data between access points
Specification of services up to the terminal interface (OSI layers 1-3)
Different data rates for voice and data (original standard)
data service (circuit switched)
synchronous: 2.4, 4.8 or 9.6 kbit/s
asynchronous: 300 - 1200 bit/s
data service (packet switched)
synchronous: 2.4, 4.8 or 9.6 kbit/s
asynchronous: 300 - 9600 bit/s
Today: data rates of approx. 50 kbit/s possible – will be covered later!
Supplementary services
Services in addition to the basic services, cannot be offered stand-alone
Similar to ISDN services besides lower bandwidth due to the radio link
May differ between different service providers, countries and protocol versions
Important services
identification: forwarding of caller number
suppression of number forwarding
automatic call-back
conferencing with up to 7 participants
locking of the mobile terminal (incoming or outgoing calls)
Architecture of the GSM system
GSM is a PLMN (Public Land Mobile Network)
several providers setup mobile networks following the GSM standard within each country
components
MS (mobile station)
BS (base station)
MSC (mobile switching center)
LR (location register)
subsystems
RSS (radio subsystem): covers all radio aspects
NSS (network and switching subsystem): call forwarding, handover, switching
OSS (operation subsystem): management of the network
Radio subsystem
The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers
Components
Base Station Subsystem (BSS):
Base Transceiver Station (BTS): radio components including sender, receiver, antenna - if directed antennas are used one BTS can cover several cells
Base Station Controller (BSC): switching between BTSs, controlling BTSs, managing of network resources, mapping of radio channels (Um) onto terrestrial channels (A interface)
BSS = BSC + sum(BTS) + interconnection
Mobile Stations (MS)
Network and switching subsystem
NSS is the main component of the public mobile network GSM
switching, mobility management, interconnection to other networks, system control
Components
Mobile Services Switching Center (MSC)controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC
Databases (important: scalability, high capacity, low delay)
Home Location Register (HLR)central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)
Visitor Location Register (VLR)local database for a subset of user data, including data about all user currently in the domain of the VLR