Optical & Wireless Commu n - module-5 MODULE 5 GLOBAL SYSTEM FOR MOBILE GSM Network Architecture, GSM Signalling protocol architecture, Identifiers used in GSM system, GSM Channels, Frame structure for GSM, GSM Call procedures, GSM hand-off Procedures, GSM Services and features Global System for Mobile Communications (GSM) services are a standard collection of applications and features available to mobile phone subscribers all overthe world, The GSM standards are defined by the 3GPP collaboration and implemented in, hardware and software by equipment manufacturers and mobile phone operators. The common standard)makes it possible to use the same phones with different companies’ services, or even)roam into different countries. GSM is the world’s most dominant mobile phone standard. The design of the service is moderately complex because it must be able to locate a moving phone anywhere in the world, and accommodate the relatively smalll battery capacity, limited input/output capabilities, and weak radio transmitters on mobile devices In order to gain access to GSM servies, a user needs three things: + A billing relationship with a mobile Phone operator, This is usually either where services are paid for in advance of them being consumed (prepaid), or where bills are issued and settled after the service has been consumed (postpaid) + A mobile phone that is GSM compliant and operates at the same frequency as the operator. Most phone companies sel! phones from third-party manufacturers. + A Subscriber Identity Module (SIM) card, which is activated by the operator once the billing relationship, is established. After activation the card is then programmed with the subscriber's Mobile Subscriber Integrated Services Digital Network Number (MSISDN) (the telephone number). Personal information such as contact numbers of frierids and family tan also be stored on the SIM by the subscriber. After subscribers sign up, information about their identity (telephone number) and what services they ate allowed to access are stored in a "SIM record" in the Home Location Register (HLR). Once the SIM eard is loaded into the phone and the phone is powered on, it will search for the nearest mobile phone mast (also called a Base Transceiver Station/BTS) with the strongest signal in the operator's frequency band. Ifa mast can be successfully contacted, then there is said to be coverage in the area. The phone then identifies itself to the network through the control channel. Once this is successfully completed, the phone is said to be attached to the network. The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available, This is generally called roaming from a customer perspective, but also called visiting when describing the underlying technical process. Each geographic area has a database called the Visitor Location Register (VLR), which contains details of all the mobiles Shruthi.N, Assistant prof, Dept of ECE, SVIT Page 1Optical & Wireless Communication - module-5 currently in that area. Whenever a phone attaches, or visits, a new area, the Visitor Location Register must contact the Home Location Register to obtain the details for that phone. The Current cellular location of the phone (i.e., which BTS it is at) is entered into the VLR record and will be used during a process called paging when the GSM network wishes to locate the mobile phone. GSM Band Uplink Frequency Downlink Frequency P-GSM 900 890 -915 MHz 935 - 960 MHz ARFON= 1.124 XARFCN-1) 0.2 MHz + 890,2 MHz Uplink frequency +45 MHz E-GSM 900 880 - 890 MHz (ARFCN = 0= 890 MHz) | 925 -935 MHz ARFCN=975...1023 | (ARFCN-975) x 0.2 MHz +890 MHz Uplink frequency +45 MHz R-GSM 900 876 - 890 MHz 921-935 MHz ARFCN=955..1023 | (ARFCN-1023) x 0.2 MHz+890MHz | Uplink frequency +45 MHz GSM 1800 1710-1785 MHz 1805 - 1880 MHz ARFCN=512...885 | (ARFCN-512) x 0.2 MHz+1710.2MHz | Uplink frequency +95 MHz GSM 1900 1850 - 1910 MHz 1930-1990 MHz ARFCN=512...810 | (ARFCN-512) x 0.2 MHz-+ 1850.2MHz | Uplink frequency +90 MHz GsM4s0 450.4 - 457.6 MHz 460.4 - 467.6 MHz ARFCN=259...293 | (ARFCN-259) x 0.2 MHz+450.6MHz | Uplink frequency + 10 MHz GSM 480 478.8 - 486 MHz 488.8 - 496 MHz ARFCN=306...340 | (ARFCN-306) x 0.2 MHz-+478.8MHz | Uplink frequency +10 MHz Every SIM card contains a secret key, ealled'the'Ki, which is used to provide authentication and encryption services. This is usefill to, prevent theft of service, and also to prevent "over the air” snooping of a user's activity. The network does this by utilizing the Authentication Center and is accomplished without transmitting the key directly. Every GSM phone, contains a unique identifier (different from the phone number), called the International Mobile Equipment Identity (IMEI). This can be found by dialing *#06#. When a phone contaets the network, its IMEI may be checked against the Equipment Identity Register to locate stolen phones and facilitate monitoring. ‘Time Division Multiple Access (TDMA): It can be easily adapted to the transmission of data and voice communication, TDMA offers the ability to carry data rates of 64 kbps to 120 Mbps (expandable in multiples of 64 kbps). This enables operators to offer personal communication-like services including fax, voice band data, and short message services (SMSs) as well as bandwidth-intensive applications such as multimedia and videoconferencing. It will not experience interference from other simultaneous transmissions Unlike spread-spectrum techniques which can suffer from interference among the users all of whom are on the same frequency band and transmitting at the same time, TDMA’s technology, which separates users in time, ensures that they will not TDMA is the only Shruthi.N, Assistant prof, Dept of ECE, SVIT Page 2Optical & Wireless Communication - module-5 technology that offers an efficient utilization of hierarchical cell structures (HCSs) offering pico, micro, and macro cells. HCSs allow coverage for the system to be tailored to support specific traffic and service needs. By using this approach, system capacities of more than 40- times AMPS can be achieved in a cost-efficient way. TDMA allows service compatibility with the use of dual-mode handsets because of its inherent compatibility with FDMA analog systems. 2.2 GSM Network and System Architecture Mobile Station (MS) ‘Mobile Equipment (ME) Subscriber Identity Module (SIM) Base Station Subsystem (BSS) Base Transceiver Station (BTS) Base Station Controller (BSC) Network Switching Subsystem (NSS) Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) ‘Authentication Center (AUC) Equipment Identity Register(EIR) Mor tntelligent| Serres ‘Oreer Gateway Gateway. Operation and Support system (OSS) System Station (HBS) Controfler (HSC) Fig 4.1 components of GSM network a. Mobile Station (MS) ‘The Mobile Station is made up of two entities: ‘Mobile Equipment (ME): + Portable, vehicle mounted, hand held device Shruthi.N, Assistant prof, Dept of ECE, SVIT Page 3Optical & Wireless Commu + Uniquely identified =~ by an IMEI_ (International Mobile Equipment Identity) + Voice and data transmission + Monitoring power and signal quality of surrounding cells for optimum handover + Power level : 0.8W- 20 W + 160 character long SMS. Subscriber Identity Module (SIM) + Smart card contains the International Mobile Subseriber Identity (IMST) + Allows user to send and receive calls and receive other subscribed services *+ Protected by a password or PIN + Can be moved from phone to phone — contains key information to activate the phone b. Base Station Subsystem (BSS): Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers Base Transceiver Station (BTS): It is responsible for following fiietions + Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antemma, + Communicates with Mobile station andBSC + Consists of Transceivers (TRX) units Base Station Controller (BSC): It is responsible for following functions + Manages Radio resources for BTS + Assigns Frequency and time slots for all!MS's in its area + Handles call set up + Handover for each MS + Itcommunicates with MSC and BTS. ¢. Network Switching Subsystem(NSS) Mobile Switching Center (MSC) : * Heart of the network + Manages comiuitication between GSM and other networks ‘+ Billing information and collection + Mobility management > Registration + Location Updating Inter BSS and inter MSC all handoff Home Location Register (HLR): + Stores information about each subscriber that belongs to it MSC in permanent and, temporary fashion. + As soon as mobile subscriber leaves its current local area, the information in the HLR is updated. + Database contains IMSI, MSISDN, prepaid/ postpaid, roaming restrictions, supplementary servi Shruthi.N, Assistant prof, Dept of ECE, SVIT Page 4