GSM & GPRS. Antti Siitonen. Head of Department, MSc (EE) T Telecommunications architectures

Transcription

1 GSM & GPRS Antti Siitonen Head of Department, MSc (EE) T Telecommunications architectures Lectures on Introduction to GSM & GPRS 1

2 Contents 1. Short introduction to mobile telephony / mobile telecommunication 2. GSM 1.1. Radio interface 1.2. Mobility management 1.3. Technology generations 2.1. Architecture overview BSS+NSS 2.2. Mobile handset and SIM 2.3. GSM services 2.4. GSM data enhancements 3. GPRS 3.1. Architecture overview 3.2. GPRS terminals 3.3. GPRS services 2

3 Advantages of wireless access Speed of deployment and easiness of reconfiguration In certain cases cost efficiency Independence of wired infrastructure etc.. BUT THE MAIN ADVANTAGE IS 3

4 Frequecy spectrum and spectrum allocation Who? What band and what bandwidth? What technology? Globally CEPT, Europe ERC» World Radio Conference (WRC) every fourth year Nationally local regulators (Ficora in Finland) Licenced frequencies Requires licence, typically for operator use» Co-ordination, frequency planning, interference regulation» Mobile networks, LMDS, Broadcasting TV and radio, links» T.ex. 3G spectrum allocation 155 MHz (2*60+35) in 2 GHz band Unlicenced frequencies Free for all, but within regulated boundaries (maximum power, technology)» WLANs, cordeless phones, DECT,..» 2,4 GHz, 5 GHz, 17 GHz,.. 4

5 Wireless environment Path loss Radio signals decrease exponentially with distance The wide range of path loss associated with different radio links results in the Near-Far problems Shadow Fading and Multipath Fading Blockage of radio paths by buildings and terrain causes shadow fading. Signals may drop 20 to 30 db by turning a building corner Radio signal received from different paths may destructively combine and result in multipath fading In broadband systems the delay spread caused by multipath propagation may result in inter symbol interference Traffic loading Mobile speed and users density are elusive in nature 5

6 Multiple access methods FDMA Frequency Division Multiple Access (NMT, TACS, AMPS) One frequency per user Filtering! TDMA Time Division Multiple Access (GSM, D-AMPS, PDC) Same frequency, different timeslots Timing advance! CDMA Code Division Multiple Access (IS-95, UMTS) Same frequency, same timeslots, but different codes Power Control! Picture: Ericsson 6

7 Mobility management Status of the mobile (on/off) - pageing Location of the mobile - Location update Moving mobile Change the active cell - handover (handoff)» Network optimization» Movement prediction Change the active operator - roaming» Typically when crossing the country border 7

8 Handover cell 1 cell 2 BSC 1. Call in progress cell 1 2. Signal cell 2 for handover 3. Switch to cell 2 = handover complete 8

9 Soft handover cell 1 cell 2 1. Call in progress cell 1 2. Cell 2 has good signal strength 3. Call in progress cell 1 and cell 2 = soft handover 9

10 Wireless systems in general kbit/s F U T U R E B L U E T O O T H S Y S T E M S ( >2003) Wireless Local Area Networks = WLANs (Available) Broadband Fixed Wireless Access (Available) Universal Mobile Telecommunication Service = UMTS (> 2002) SCOPE GSM + Evolution - GPRS (2000) (General Packet Radio Service) - EDGE (2002) (Enhanced Development for GSM Evolution) Satellite Room House Campus City Country Continent Coverage area 10

11 1G, 2G, 3G G Analog Voice services ARP, NMT, AMPS, TACS Digital Voice, messaging and low speed data services GSM (TDMA), cdma one (IS-41), D-AMPS (TDMA), PDC (TDMA) 2 G 3 G Advanced digital Beyond 3 G (4G) Voice, medium speed data, multimedia messaging High speed data, real-time Voice over IP Convergence of multimedia services wireless and internet IMT 2000 = UMTS, CDMA

12 GSM 1982 Group Special Mobile was established by CEPT 1987 Air interface TDMA was chosen as access method and Initial Memorandum of Understanding was signed by telecom operators (from 12 countries) 1989 GSM specification responsibility is passed to ETSI 1990 GSM First Phase specification 1991 First commercial GSM service (Global System for Mobile Communications) 1992 More operators signed the MoU 1995 GSM Second Phase specification check 12

13 GSM radio technology basics Frequency bands GSM400 GSM900 GSM1800 GSM1900» MHz paired with MHz or MHz paired with MHz» MHz paired with MHz» MHz paired with MHz» MHz paired with MHz Multiple access method TDMA Modulation GMSK Radio transmitted power (Mobile Station) 1 W Handset on GSM1800, 2 W Handset on GSM900, 8W Fixed installation (Car) Channels per carrier 8 timeslots Channel bit rate 22,8 kbit/s 13

14 PLMN Architecture overview HLR MSC BSS BTS BSC TRAU BTS BSS BTS VLR MS + SIM Customer care & Billing system BSS MSC area MSC area HLR MSC area EIR MSC area HLR 14

15 Base station subsystem BTS = Base Transceiver Station Radio-related tasks, Air interface Standard configuration (cell size 300 m - 35 km) Standard Umbrella configuration Sectorized configuration BSC = Base Station Controller Sectorized Controls several BTSs and takes care of all central functions of BSS TRAU = Transcoding Rate and Adaptation unit Bitrate adaptation and voice coding/decoding OSS = Operation and support subsystem (also for NSS) Umbrella 15

16 Network switching subsystem MSC = Mobile-Services Switching Center Basically ISDN telephone exchange + mobility modifications» Modifications: BSS channel assignments and inter-msc handover Gateway MSCs interface other networks HLR = Home Location Register / Authentication Center (AuC) Database for subscriber identity - Home AuC provides triplets for authentication and ciphering VLR = Visitor Location Register Database for temporary subscriber identity - Visited Integrated into each MSC EIR = Equipment Identity Register GSM phone IMEI register (Lists: White, Black, Gray) 16

17 GSM Protocols GMSC ISUP ISDN (PSTN) ISUP CM MM MS BTS BSC MSC VLR RIL3-CC RIL3-MM MAP/D HLR RIL3-protocols Radio management MAP-protocols Mobilty management for switching SS7-protocols Call management RR RIL3-RR RSM BSSMAP DTAP TCAP SCCP LAPDm LAPD MTP SCCP MTP RIL3 = Radio Interface Layer 3 MAP = Mobility/Management Application Part ISUP = ISDN User Part CM = Call Management TCAP = Transaction Capabilities Application Part GMSC = Gateway MSC CC = Call Control SCCP = Signalling Connection Control Part DTAP = Direct Transfer Application Part for GSM MM = Mobility Management LAPD = Link Access Protocol for D-channels RR = Radio Resource MTP = Message Transfer Part 17

18 Simplfied connection establishment example 1. The subscriber in the fixed network dials the B-subscriber's mobile number. The PSTN identifies the number and sets up a connection to the called network Default signalling protocol is the signalling protocol between the GMSC and the PSTN is ISUP, but TUP(Telephone User Part) or a channel-associated protocol could also be used. ISUP and TUP use MTP as a signalling bearer. 2. The GMSC does not know through which MSC the mobile can be reached; nor does it know whether the mobile is free, busy, turned on or turned off. To be able to continue, the GMSC must therefore request a routing number from the HLR The GMSC uses the MAP protocol for this request. MAP uses the transaction capabilities application part (TCAP), which in turn uses the SCCP as a bearer. 3. The registration function continuously updates the HLR on the location of the mobile (MSC service area). Provided the mobile is turned on and free, the HLR will call that service area's VLR to request a free routing number. The mobile's subscriber data is sent along with this request. The MAP protocol is used for this communication 4. The GMSC receives the routing number and uses it to select a route in the mobile network. GMSC forwards the call to MSC ISUP is used for signalling between the GMSC and the MSC 5. MSC consults its VLR to find out in which group of cells (location area) the mobile is at the moment. (Keeping the VLR informed of the mobile's location is also part of the registration function.) MSC then orders BSC to find the mobile. Communication at this stage is BSSMAP. These signals are carried between the MSC and the BSC by the connectionless SCCP service. 6. BSC sends a paging call to all cells in its service area that can be visited by the mobile at the moment. The paging procedure is described in the GSM standard. The paging call is carried by LAPD between the BSC and the base station, and on control channel across the air interface. 7. Next, the call is answered by the mobile. The answering procedure is described in the GSM standard. The answer is carried by control channel across the air interface and then sent on to the BSC on LAPD. 8. BSC allocates the mobile a control channel for signalling with MSC. This information is carried by LAPD between the BSC and the base station, and by control channel across the air interface. A traffic channel over radio access and between MSC and BSC is also reserved for the call. 9. Now the mobile communicates directly with the MSC This communication is in accordance with the DTAP protocol. The signals are carried by SCCP/MTP between the MSC and the BSC, relayed through the BSC, and carried by LAPD between the BSC and the base station The DTAP signalling concludes with the set-up of a traffic channel through the switches in BSC and MSC. An acknowledgement of this through-connection is sent backwards to the PSTN 18

19 Mobile Station Voice encoding Voice decoding Channel encoding Channel decoding Interleaving De-interleaving Burst generation Re-formatting Ciphering Deciphering Modulation Demodulation Amplifier Receiver filter + Antenna Burned in IMEI = Individual Moblie Station Equipment Identity Central processor, clock and tone, internal bus system, keyboard SIM interface 19

20 SIM Subscriber Identity Module Data storage and algorithm executions Memory 8 kb / 16kB / 32 kb /64 kb Administrative data: PIN/PUK, SIM service table.. Security related data: Algorithms A3 and A8, Ki, Kc, CKSN Subscriber data: IMSI (International mobile subscriber identity = code for VLR and HLR), MSISDN, Access control classes Roaming data: TMSI (Temporary MSI), NCCs (Network Color Codes) PLMN data: Network identifier and home network frequencies 20

21 GSM authentication and ciphering - Algorithms A3, A5/X ja A8. Algorithms are secret. - SIM parameters: Parameter Algorithms A3 ja A8 (mandatory, fixed) Key Ki (individual) (mandatory, fixed) Key Kc (cipher) (mandatory, changeable) CKSN (mandatory, changeable) Need: Authentication, Key Kc calculation Unique key, stored in SIM+HLR Result of A8 algorithm, calculated from Ki and RAND Ciphering key sequence number, referred to Kc - Ciphering:MS has at least A5/1 and A5/2 (There are seven different ciphering algorithms, MS informs the network of it s capabilities). Cipher is only for air interface. - Note! Ciphering is optional! - Network parameters: Every subscription has unique Ki stored in HLR AuC provides authentication triples. For one user five triplets may be generated at one time. HLR forwards triplets to VLR. VLR negotiates authentication and ciphering with MS Triplet:» Signed Response (SRES), Random Number (RAND) and Kc 21

22 GSM authentication process Network challenges the user. User must prove to have correct identity. MSC AuC/HLR 1. Triplet to VLR - Key Kc - RAND - A3(Ki&RAND) = SRES BTS BSC VLR 2. Terminal (SIM) - Receives RAND - A3(Ki&RAND) = SRES - Returns SRES to VLR 3. VLR compares SRES 22

23 GSM ciphering process MSC AuC/HLR BTS BSC 1. Cipher key A8(Ki&RAND) =Kc 2. Cipher key A8(Ki&RAND)=Kc VLR 3. VLR provides BTS information of chosen A5/x algorithm and key Kc 4. BTS transfers the information to MS 5. Traffic is ciphered and deciphered in 114 bit sequences 23

24 GSM Services Bearer services: Bit transmission, 3,1 khz voice, sync./async.data Voice Full Rate codec 13 kbit/s (+ 9,8 kbit/s for error correction) Enhanced Full Rate codec 13 kbit/s (+ 9,8 kbit/s for error correction) Half Rate codec 6,5 kbit/s (+ 4,9 kbit/s for error correction) Emergency call Suplementary services: Call forwarding, Calling line identity.. Fax 9,6 kbit/s Fax Data 9,6 kbit/s transparent data (no error correction) 9,6 kbit/s non-transparent data (with error correction) 24

25 GSM Services (cont.) Voice mail SMS Answering machine type of service. Colocated with MSC The initial SMS Center may be simply a voice mail platform module or alternatively a standalone SMS Center. It is not possible to make the Short Message Service available without an SMS Center since all short messages pass through the SMS Center. SMS Mobile terminate Often SMS Mobile Terminate Services are offered along with voice mail notifications. SMS Mobile originate True two-way SMS capability. 25

26 Value added services (examples) Mainly SMS based services Edited content for SMS Ringtones Logos and picture messages CHAT Payment methods Menu driven SMS applications Sim Application Toolkit (STK) - set of tools possible to implement applications and menus executed on the SIM card Wireless Application Protocol WAP Attemp to standardize mobile applications Optimized for wireless networks (not only for GSM) 26

27 Operator service offering Operators Traditionally Network and Service operators are same corporation Pure service operators have entered during last years Subscriptions for different needs Business use, Private use, Pure data Basic services without extra subscription Call waiting, hold, group call, SMS, Calling line identity, Roaming, data Premium services Voice mail, SMS-service packets, Billing limits, Mobile , MMS.. Mobile number portability Possibility to change service operator and keep the old mobile number 27

28 Data in GSM-network - features and restrictions Connection time 5-30 s Data rate 9.6 kbit/s Poor user interface for data applications Internet ISP (access server) PSTN MSC GSM-network BS Time based billing Circuit switched connection 28

29 GSM data development Basic GSM data: Connections to any modem service 9,6 Kbit/s data rate Enhancements: ISDN type connection - faster connection time V.42bis compression - 4:1 basic text compression (limited) HSCSD - High Speed Circuit Switched Data» Non transparent: 1-4 time slot per MS - up to 38,4 kbit/s - dynamic allocation of channels» Transparent: Up to 8 time slots per MS - static allocation of channels 14,4 kbit/s channel coding» Less bits for error correction if signal is good enough» Can be combined to HSCSD and/or V.42bis 29

30 New service requirements Location Related Services Can be implemented to the current architecture Different methods give different accuracy Need for packet based mobile data? Asymmetric traffic» WAP-services» Internet services: Web browsing Multiple Services with Variable Bit Rates Multi-Session and Always On Features Variable QoS Requirements 30

31 Traffic in mobile network traffic GSMnetwork is planned for circuit switched voice services Rush hour is the capacity planning key 08:00 17:00 time 31

32 WWW browsing traffic - Measured usage rate on a circuit switched* data connection** Rate (kbit/s) (kbit/s) uplink downlink % 19.5% % 20% % 11.7% Typical downlink traffic GPRS uses channel only when there is traffic» Resources are availbe for other users when there is no traffic aika Home page... Reading... WWW download... Reading... * GSM and HSCSD are circuit switched * measurements are made during six days on may 1999 from the Kolumbus ISP s access server Data traffic No traffic 32

33 GPRS Data rates - Link layer CS-1 9,05 kbps CS-2 CS-3 13,4 kbps 15,6 kbps 1-8 channel 171,2 kbps in theory CS-4 21,4 kbps Practical data rate is kbit/s 33

34 GPRS Reference network architecture MSC/VLR HLR D A Gs Gc Gb Gr Gi MS BSS SGSN GGSN PDN TE Gn Um Gp Gf EIR GGSN Other GPRS Nw Signalling Interface Signalling and Data Transfer Interface BSS=Base Station Subsystem, GGSN=Gateway GPRS Support Node, HLR=Home Location Register, MS=Mobile Station, MSC=Mobile Switching Centre, SGSN=Serving GPRS Support Node, PDN=Packet Data Network, VLR=Visitor Location Register 34

35 GPRS Network architecture BTS BSC MSC/VLR HLR IP Network MS SGSN GGSN Backbone Network IP X.25 Network 35

36 GPRS updates to GSM network Element Software Hardware MS Upgrade required Upgrade required BTS Upgrade required No change BSC Upgrade required PCU Interface TRAU No change No change MSC/VLR Upgrade required No change HLR Upgrade required No change SGSN New New GGSN New New 36

37 SGSN = Serving GPRS Support Node Authentication, Authorization GTP tunneling to GGSN Ciphering and compression Mobility management Session management Interaction with HLR, MSC/VLR Collects charging and statistics information about data network usage Interfaces towards OSS (Operation and Management) 37

38 GGSN = Gateway GPRS Support Node Interfaces external data networks Internet, Intranets, Operator ISP network Encapsulates end user data in GTP packets Routes mobile originated packets to right destination Filters end user traffic Collects charging and statistics information about data network usage 38

39 GPRS- connections SGSN APN= internet GPRS GGSN RL ISP Internet External WAP GW or ISP GSM APN= wap WAP GW soittosarja RL service network HLR (GSM&GPRS) APN = Access Point Name (Logical Name) APN= yritys.fi WAP GW Corporate network 39

40 Connection protocol stack Application IP / X.25 SNDCP Relay SNDCP GTP IP / X.25 GTP LLC RLC MAC GSM RF Relay RLC BSSGP MAC GSM RF Network Service L1bis LLC BSSGP Network Service L1bis UDP / TCP IP L2 L1 UDP / TCP Um Gb Gn Gi MS BSS SGSN GGSN IP L2 L1 BSSGP=BSS GPRS Protocol, GTP=GPRS Tunneling Protocol, LLC=Logical Link Protocol, MAC=Medium Access Control, GSMRF=GSM Radio Physical Layer, SNDCP=Subnetwork Dependent Convergence, UDP=User Datagram Protocol, TCP=Transmission Control Protocol 40

41 GPRS phases Phase 1 Point-to-Point packet service CS1 and CS2 channel coding Internal interfaces Flexible radio resource allocation, I.e. Multiple users per timeslot and multiple timeslots per user Phase 2 Enhanced QoS support Access to ISP and Intranets GPRS prepaid Group call Point to multipoint services Support for Class B and C Mobiles GPRS Charging (packet based billing) IP and X.25 packet data networks Static and dynamic IP address allocation Authentication and Ciphering 41

42 GPRS roaming International Roaming Experts Group (IREG) Proposal Roaming traffic will be carried over central managed hierarchical Roaming Network where commonly agreed policies are followed GPRS Roaming Exchange (GRX) at least in every continent; some cases thre could be many GRX s in one country Every Roaming Operator will have connection to GRX using some of the following methods» Layer 1 connection (Leased Line, fibre, etc.)» Layer 2 logical connection (ATM/Frame Relay, etc)» Layer 3 IP VPN connection over public IP Network (IPsec tunnel) Connection will carry BGP routing (Border Gateway) Public addressing in GPRS backbone networks 42

43 Roaming network BTS BSC Border Gateway Inter-operator Backbone Visited network SGSN Visited Network BB BG DNS GGSN BTS Inter-operator BB FW Internet BSC Home network SGSN DNS Home BB BG GGSN FW 43

44 Three categories of terminals Class A simultaneous circuit switched and packet Class B connection both circuit and packed switched connections possible but not at the same time Class C only packet switched or circuit switched connection 44

45 Different capabilities Terminals are typically asymmetric Receiving is more simple than sending Full duplex radio Tx Rx Ch Ch Ch Ch 45

46 GPRS-terminal types and service usage Card phone+ Notebook Smart phone WAP PDA 46

47 EDGE Enhanced Data rates for Global (GSM) Evolution Modulation update: from GMSK to 8-PSK In theory modulation efficiency will be tripled In practice the maximum bit rate increases from GPRS s 171,2 kbit/s up to 384 kbit/s Needs new radio interface & terminals Transport network needs to be also upgraded Currently EDGE deployments are mainly going on in USA During 2003 several European operators have announced EDGE deployments 47

48 Summary and conclusion Functionality & bitrate IMT-2000 capable systems GSM capable systems GPRS EDGE WCDMA HSCSD Circuit data Speech Time 48

49 GSM-based network evolution HSCSD GSM GSM EDGE EDGE UMTS UMTS GPRS GPRS Non-UMTS 49

50 For those who are interested to know more Literature beyond course material: Yi-Bing Lin, Imric Chlamtac: Wireless and Mobile Network Architectures Christoffer Andersson: GPRS and 3G Wireless Applications: Professional Developer s Guide Timo Halonen, Javier Romero, Juan Melero: GSM, GPRS and EDGE Performance: Evolution Toward 3G/UMTS Internet: