Introduction to GSM: Mobile Phone Subscribers Worldwide 200 400 600 800 1000 1200 1996 1997 1998 1999 2000 2001 2002 year Subscribers [million] GSM total TDMA total CDMA total PDC total Analogue total Total wireless Prediction (1998)

Introduction to GSM: Development of Mobile Telecommunication Systems IS-95 cdmaOne IS-136 TDMA D-AMPS GSM PDC GPRS IMT-DS UTRA FDD / W-CDMA EDGE IMT-TC UTRA TDD / TD-CDMA cdma2000 1X 1X EV-DV (3X) AMPS NMT IMT-SC IS-136HS UWC-136 TD-SCDMA CT0/1 CT2 IMT-FT DECT CDMA FDMA IMT-MC cdma2000 1X EV-DO

GSM Overview Several first generation analog cellular systems in Europe but incompatible - limited roaming 1987-1989 ETSI standards for pan-European Global System for Mobile Communications (GSM, originally Group Spe’ciale Mobile 1982) at 900 MHz 1992 GSM is launched 1990-1993 Standards for Digital Cellular System at 1800 MHz (DCS 1800, recently renamed GSM 1800; US version is PCS 1900)

GSM: Overview Simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations  seamless roaming within Europe possible Today many providers all over the world use GSM (more than 184 countries in Asia, Africa, Europe, Australia, America) More than 747 million subscribers More than 70% of all digital mobile phones use GSM

GSM, cont.. Objectives: Broad offering of speech and data services Compatible with wireline networks Automatic roaming and handoff Highly efficient use of frequency spectrum Support for different types of mobile terminal equipment (e.g., cars, portable handsets) Digital signaling and transmission Low cost infrastructure and terminal equipment

GSM, cont.. 13 recommendations R.00: Preamble R.01: General structure of recommendations, GSM overview R.02: Service aspects: types of services R.03: Network aspects: architecture, call routing, performance objectives R.04: Mobile/base station interface and protocols

GSM, cont.. R.05: Physical layer on radio path: multiple access, channel coding, modulation, transmission R.06: Speech coding aspects R.07: Terminal adaptors for mobile stations R.08: Base station and mobile switching center (MSC) interface R.09: Interworking with PSTN and packet data networks R.10: Service interworking, short message service R.11: Equipment specification R.12: Operation and maintenance, tariffs, traffic administration

GSM, cont.. Summary of features Channel bandwidth 200 kHz Multiple access TDMA Users/carrier 8 Speech coding rate 13 kb/s FEC coded speech rate 22.8 kb/s

GSM, cont.. Summary of service quality requirements Speech intelligibility Summary of service quality requirements 90% Max one-way delay 90 ms Max handoff gap 150 ms if intercell Time to alert mobile of inbound cell 4 sec first attempt, 15 sec final attempt Release time to called network 2 sec Connect time to called network 4 sec

GSM: Performance Characteristics Communication Mobile, wireless communication; support for voice and data services Total mobility International access, chip-card enables use of access points for different providers Worldwide connectivity One number, the network handles localization High capacity Better frequency efficiency, smaller cells, more customers per cell High transmission quality High audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains) Security functions Access control, authentication via chip-card and PIN

GSM: Mobile Services GSM offers Several types of connections Voice connections, data connections, short message service Multi-service options (combination of basic services) Three service domains Bearer Services Telematic Services Supplementary Services

Bearer Services Telecommunication services to transfer data between access points 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) asynchronous: 300 - 9600 bit/s Today: data rates of approx. 50 kbit/s possible

Tele Services I Offered Services Mobile Telephony primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz Emergency Number common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)

Tele Services II Additional services Non-Voice-Teleservices Fax Voice mailbox (implemented in the fixed network supporting the mobile terminals) Electronic mail (MHS, Message Handling System, implemented in the fixed network) Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS

Supplementary Services 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

GSM: Architecture Overview OMC, EIR, AUC Fixed Network (PSTN) NSS with OSS HLR GMSC VLR MSC VLR MSC BSC BSC RSS

GSM: Reference Architecture Radio Subsystem (RSS)

GSM General Architecture PSTN GSM Public Land Mobile Network (PLMN) OMC VLR MSC HLR AUC NMC A EIR BSS BSS BSC Abis ADC BTS BTS OSS Um MS MS MT OSS: Operation Subsystem BSS: Base Station Subsystem MS: Mobile Station (Mobile User) TE

System Architecture: Radio Subsystem Network and Switching Subsystem Components MS (Mobile Station) BSS (Base Station Subsystem): consisting of BTS (Base Transceiver Station): sender and receiver BSC (Base Station Controller): controlling several transceivers Interfaces Um : radio interface Abis : standardized, open interface with 16 kbit/s user channels A: standardized, open interface with 64 kbit/s user channels MS MS Um Abis BTS BSC MSC BTS A BTS MSC BSC BTS BSS

GSM General Architecture, cont.. Mobile station (MS) communicates to base stations through radio interface Um Mobile termination (MT) supports physical channel between MS and base station (radio transmission, channel coding, speech coding)

GSM General Architecture, cont.. Terminal equipment (TE), e.g., telephone set. Contains terminal/user-specific data in form of smart card (subscriber identify module or SIM card), plugs into any GSM terminal like credit card and identifies user to network for personal mobility (in addition to terminal mobility) and security

GSM General Architecture, cont.. Base station Subsystem (BSS) communicates with mobile switching center through network interface A Base Transceiver Station (BTS) handles channel allocation, signaling, frequency hopping, handover initiation, etc. BTS communicates with BSC using Abis interface

GSM General Architecture, cont.. Base station controller (BSC) manages radio channels, paging, handoff for several BTSs BSC communicates with MSC using A interface Mobile switching center (MSC) is gateway to PSTN and packet data networks Performs switching, paging functions, MS location updating, handoff control, etc.

GSM General Architecture, cont.. Home location register (HLR) stores subscriber info and part of MS’s location info to route incoming calls to visitor location register (VLR) where mobile is roaming VLR registers users roaming in its area and assigns roaming numbers

GSM General Architecture, cont.. Authentication center (AUC) is accessed by HLR to authenticate a user for service. It contains authentication and encryption keys for subscribers Equipment identity register (EIR) allows stolen or fraudulent mobile stations to be identified

GSM General Architecture, cont.. Operation subsystem (OSS) contains: Operations and Maintenance Center (OMC), Network Management Center (NMC), and Administration Center (ADC). These network elements work together to monitor, control, maintain, and manage the network

System Architecture: Network and Switching Subsystem network subsystem fixed partner networks Components MSC (Mobile Services Switching Center): ISDN (Integrated Services Digital Network) PSTN (Public Switched Telephone Network) Databases HLR (Home Location Register) VLR (Visitor Location Register) EIR (Equipment Identity Register) ISDN PSTN MSC EIR SS7 HLR VLR ISDN PSTN MSC

Radio Subsystem The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers Components Mobile Stations (MS) 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 Station Terminal for the use of GSM services A mobile station (MS) comprises two elements: ME (Mobile Equipment): Peripheral device of the MS, offers services to a user Speaker, microphone, keypad, and radio modem Usually subsidized by the service provider to encourage new subscribers SIM (Subscriber Identity Module): Smart card issued at subscription time Personalization of the mobile terminal, stores user parameters such as address and type of service Calls are directed to the SIM rather than the terminal Stores short messages Carries a PIN number that needs to be verified to make the information on the card available to the user.

Base Station Subsystem Base Transceiver Station and Base Station Controller Tasks of a BSS are distributed over BSC and BTS BTS comprises radio specific functions BSC is the switching center for radio channels

GSM: Cellular Network segmentation of the area into cells possible radio coverage of the cell idealized shape of the cell use of several carrier frequencies not the same frequency in adjoining cells cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc. hexagonal shape of cells is idealized (cells overlap, shapes depend on geography) if a mobile user changes cells  handover of the connection to the neighbor cell

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

Network and Switching Subsystem 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 Equipment Identity Register (EIR): registers GSM mobile stations and user rights. Stolen or malfunctioning mobile stations can be locked and sometimes even localized.

Network and Switching Subsystem Authentication Center (AUC) Generates user specific authentication parameters on request of a VLR Authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system

Mobile Service Switching Center The MSC (mobile switching center) plays a central role in GSM switching functions additional functions for mobility support management of network resources interworking functions via Gateway MSC (GMSC) integration of several databases

Mobile Service Switching Center Functions of a MSC specific functions for paging and call forwarding termination of SS7 (signaling system no. 7) mobility specific signaling location registration and forwarding of location information provision of new services (fax, data calls) support of short message service (SMS) generation and forwarding of accounting and billing information

Mechanisms to Support Mobility Registration Performed as soon as the mobile unit is powered on Call establishment Performed when the user initiates or receives a call Handover Performed when the MS needs to change its connection point to the network Security Protects from fraud and eavesdropping

Registration Procedure As we turn on an MS, it passively synchronizes to the frequency, and frame timings of the closest BS to get ready for information exchange. The MS then listens to advertisements from the BS regarding system and cell identity to determine its location in the network. If the current location is not the same, the MS initiates a registration procedure.

Registration Procedure: MS turned on in a New MSC Area 1-4: A radio channel is established between the MS and BSS to process the registration 5-8: The NSS authenticates the MS 9-11: A TMSI (Temporary Mobile Subscriber Identity) is assigned and updates are made in the VLR and HLR 12: The temporary radio channel is released.

Call Establishment: Mobile Originated Call 1-5: Similar to registration procedure 6-7: Ciphering to protect against eavesdropping 8-15: Similar procedure as in POTS (Plain Old Telephone Service), except for the additional traffic channel assignment procedure (10-11)

Mobile Originated Call 1, 2: connection request 3, 4: security check 5-8: check resources (free circuit) 9-10: set up call VLR 3 4 PSTN 6 5 GMSC MSC 7 8 2 9 1 MS BSS 10

Call Establishment: Mobile Terminated Call After dialing, the PSTN directs the call to the MSC of the destination address The MSC requests routing information from the HLR Since the MS is in another area, the address of the new MSC is given to the original MSC. It contacts the new MSC. The VLR of the new MSC initiates a paging procedure in all BSS under the control of the new MSC After a reply from the MS, a link is established for communication.

Mobile Terminated Call 1: calling a GSM subscriber 2: forwarding call to GMSC 3: signal call setup to HLR 4, 5: request MSRN from VLR 6: forward responsible MSC to GMSC 7: forward call to current MSC 8, 9: get current status of MS 10, 11: paging of MS 12, 13: MS answers 14, 15: security checks 16, 17: set up connection 4 HLR VLR 5 8 9 3 6 14 15 PSTN 7 calling station GMSC MSC 1 2 10 13 10 10 16 BSS BSS BSS 11 11 11 11 12 17 MS

Handoff (Handover in Europe) Two types Internal: between BTSs that belong to the same BSSs External: between two different BSSs controlled by the same MSC Sometimes there are handoffs between BSSs that are controlled by two different MSCs (the old MSC continues to handle call management) Reasons to handoff: Signal strength deterioration at the edge of a cell Traffic balancing (to easy traffic congestion)

Handoff Decision receive level BTSold receive level BTSold Hysteresis MARGIN MS MS BTSold BTSnew

Handoff Procedure Mobile-assisted Handoff

Handoff Procedure HO access MS BTSold BSCold MSC BSCnew BTSnew measurement report measurement result HO decision HO required HO request resource allocation ch. activation ch. activation ack HO request ack HO command HO command HO command HO access Link establishment HO complete HO complete clear command clear command clear complete clear complete

Security in GSM Security Services (Features) Access Control/Authentication (Identity Authentication) User is assigned SIM (Subscriber Identity Module) at the subscription time which contains * IMSI: International Mobile Subscriber Identity * Individual Subscriber Authentication Key (Secret PIN) (Personal Identification Number) * and the Authentication Algorithm (Challenge response method) Identity Confidentiality voice and signaling encrypted on the wireless link (after successful authentication) User Data Confidentiality (Privacy/Anonymity) temporary identity TMSI (Temporary Mobile Subscriber Identity) newly assigned at each new location update (LUP) encrypted transmission

Security in GSM 3 algorithms specified in GSM A3 for authentication (“secret”) A5 for encryption (standardized) A8 for key generation (“secret”) A5 uses a key of 128 bits, and the “response to the challenge” is 32 bits long – not very secure Key information is not shared between systems: A triple consisting of the random number used in challenge, challenge response, and the data encryption key are exchanged between the VLR and the HLR. The VLR verifies if the response generated by the MS is the same.

Security in GSM Service Request authentication procedure includes: * Mobile transmitting its Temporary IMSI (TIMSI) * Network replying with a randomly generated number (RAND) * Mobile computes the Signed Response (SRES) using the authentiction algorithm, the key (which is a function of the frame number) and RAND, and sends the SRES back to the network * Network compares the SRES from the mobile with its computation of the SRES and authenticates a user or not. Privacy is provided by a Temporary Mobile Subscription Identity (TMSI) valid during its binding to a VLR and computed after the authentication procedure. IMSI (on the SIM CARD)  15 digits * MCC (Mobile Country Code) (3 digits) * MNC (Mobile Network Code) (2 digits) * MSIN (Mobile Subscriber Identifier) (2 digits) * NMSI (Network Mobil Subscriber Identifier) (2 digits)

The GSM Protocol Architecture

GSM Protocol Stack Layer 1: Physical layer A and A-bis interfaces follow the ISDN standard with 64 kbps digital data per voice user The new physical later in GSM is for the Um air interface Specifies how the voice and data are formatted into packets and sent through the radio channel Specifies radio modem details, structure of traffic and control packets in the air Modulation and coding techniques, power control methodology, and time synchronization approaches which enable establishment and maintenance of channels

Physical Packets Bursts Normal Burst (NB) 3 tail bits (TBs) at the beginning and end (gap time) 8.25 bits of gap period Two sets of 58 bits (116) encrypted bits: 114 bits of data and two flags to indicate user traffic or signaling and control 26-bit training sequence for the equalizers Frequency-correction burst (FB) Broadcasted by the BSs. The MS uses it to synchronize with the master clock in the system Synchronization burst (SB) Forwarded by the BTS. The MS uses it for training of the equalizer, learning of network identity and to synchronize the time slots Random access burst (RAB) Use by the MS to access the BS as it registers to the network

GSM – TDMA/FDMA (124 Frequencies; in each Freq there are 8 channels (time slots)) 935-960 MHz 124 channels (200 kHz) Downlink frequency 890-915 MHz 124 channels (200 kHz) Uplink time Each channel is separated in time via a frame GSM TDMA Frame 1 2 3 4 5 6 7 8 4.615 ms Each frame is subdivided into 8 GSM time-slots GSM Time-Slot (normal burst) Each slot represents a physical TDM channel and lasts for 577 µs guard space guard space tail user data S Training S user data tail 3 bits 57 bits 1 26 bits 1 57 bits 3 577 µs Each TDM channel occupies the 200 kHz carrier for 577 ms every 4.615 ms

GSM Physical Channels TDMA frame = 4.615 ms Timeslot 1 2 3 4 5 6 7 8 Frequency 1 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 Frequency 2 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 : : Frequency 124 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8

GSM-Channels The total RF Spectrum (~50 MHz) is located in 890-915 MHz (upstream) and 935 – 960MHz (downstream) This spectrum is divided into 124 RF carriers of 200KHz. The offset of the upstream/downstream pairing is fixed 45 MHz. Each RF carrier is further divided into 8 time slots (TDMA)-physical channels There is no pre-assignment of any channels to a specific and exclusive use.

GSM-Channels For a given channel the uplink F_u and downlink F_d frequency can be obtained from F_u = 890.2 + 0.2 (N-1) MHz F_d = 935.2 + 0.2 (N-1) MHz with N = 1,2,…, 124.

GSM-Channel & Frames When an MS is assigned to an information channel, a radio channel and a timeslot are also assigned. Radio channels are assigned in frequency pairs (one for the uplink and other for the downlink). Each pair of radio channels supports up to 8 simultaneous calls. Thus, the GSM can support up to 992 simultaneous users with full rate speech coder. This number can be doubled to 1984 with half rate speech coder.

GSM Hierarchy of Frame Structure Hyperframe (=2048 superframes=2,715,648 frames=21,725,184slots) 1 2 ... 2045 2046 2047 3 h 28 min 53.76 s Superframe (=51 multiframes)) 1 2 ... 48 49 50 6.12 s 1 ... 24 25 Multiframe (traffic or control multiframes) 1 ... 24 25 120 ms 1 2 ... 48 49 50 235.4 ms Frame: 8 burst of time slots 1 ... 6 7 4.615 ms slot burst 577 µs

GSM Logical Channel Structure TCH CCH CBCH TCH/F TCH/H BCH CCCH DCCH FCCH SCH BCCH PCH AGCH RACH ACCH SDCCH SACCH FACCH

GSM Channel Types 3 groups of logical (i.e., Virtual Circuits) channels, TCH, CCH and CBCH, which are realized on top of physical channels. TCH (Traffic Channel) To carry voice or data traffic of the users CCH (Control Channel) For control and signaling functions CBCH (Cell Broadcast Channel) For broadcast functions from a service center to a MS in a cell area.

GSM Logical Channels Traffic (TCH) Channels: Two-way, carrying voice and data Full-rate traffic channels (TCH/F) Half-rate traffic channels (TCH/H) Full rate channel may carry 13 kb/s speech or data at 12, 6, or 3.6 kb/s Half rate channel may carry 6.5 kb/s speech or data at 6 or 3.6 kb/s

GSM Logical Channels, cont.. CCH consists of 3 groups of logical control channels, BCH, CCCH and DCCH BCH (Broadcast Channel): Point-to-multipoint downlink only. Contains three sub-channels, BCCH, FCCH and SCH BCCH (Broadcast Control Channel): Used by the BTS to broadcast synchronization parameters, available services, and cell ID. I.o.w. sending cell identities, organization info about common control channels, cell service available, etc.

GSM Logical Channels FCCH (Frequency Correction Channel): An MS uses it to synchronize its carrier frequency and bit timing. SCH (Synchronization Channel): Used by the BTS to broadcast frame synchronization signals to all MSs; I.o,w., send TDMA frame number and base station identity code to synchronize MSs.

GSM Logical Channels, cont… CCCH (Common Control Channel) One way: Consists of three sub-channels, PCH, AGCH and RACH. These channels are used for paging and access - PCH (Paging Channel): Used by BTS to page the MS AGCH (Access Grant Channel): to assign MSs to stand-alone dedicated control channels for initial assignment; Used by the MS to access the BTS for call establishment RACH (Random Access Channel): for MS to send requests for dedicated connections; for the acknowledgement from the BTS to the MS after a successful attempt by MS using RCH.

GSM Logical Channels, cont… DCCH (Dedicated Control Channel): bi-directional point-to-point -- main signaling channels. Consist of two sub-channels, SDCCH and ACCH SDCCH (Stand-alone dedicated control channel): for service request, subscriber authentication, equipment validation, assignment to a traffic channel; Call establishment and mobility management

GSM Logical Channels, cont… ACCH consists of two sub-channels, SACCH and FACCH SACCH (slow associated control channel): for out-of-band signaling associated with a traffic channel, e.g., signal strength measurements; Assigned to each TCH and SDCCH. Used to exchange parameters between the BTS and the MS to maintain the link. FACCH (fast associated control channel): for preemptive signaling on a traffic channel, e.g., for handoff messages; Used to support fast transitions in the channel when SACCH is not adequate.