Wireless Telephony in Germany Chapter 2 Technical Basics: Layer Methods for Medium Access: Layer 2 Chapter 3 Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN Mobile Telecommunication Networks: GSM, GPRS, UMTS Satellites and Broadcast Networks Chapter Mobility on the network layer: Mobile IP, Routing, Ad-Hoc Networks Mobility on the transport layer: reliable transmission, flow control, QoS Mobility support on the application layer Wireless Telecommunication Systems 2G GSM as basis of current systems 2.5G Enhancements for data communication: HSCSD, GPRS, EDGE 3G UMTS and enhancements A-Netz 958 introduced by Federal Post Office Analogous and connected by operator No handover between base stations 9 stopped B-Netz Introduced 92 Caller had to know in range of which base station the called resided (using a region dialing code!) Partly roaming agreements with Austria, The Netherlands, Luxemburg 99 stopped C-Netz No region dialing code necessary Cellular system with large number of base stations Also data and fax connections Stopped in 2000 2 Standardization of Networks GSM Basis of Current Mobile Systems In the 0 th and 80 th : analogous, cellular mobile systems in most European countries ( st generation networks) Incompatibility of the mobile systems 982: Foundation of Groupe Spéciale Mobile (GSM) for solving interoperability issues Goal: digital network (also called 2 nd generation, 2G because of change in technology - in Germany: D-Netz) 990: first specification of GSM: GSM900 (900 MHz) 99: specification of GSM800 (as E-Netz in Germany) 992: 3 networks in countries, D and D2 in Germany 99: E-Plus 995: GSM900 in the USA 998: E2-Netz, VIAG Interkom (o2) 2000: auctioning of UMTS licenses (Integration of voice and data: 3 rd generation, 3G) 200 Start of GPRS as enhancement to GSM for packet-oriented data transfer (also called 2.5G ) GSM today means Global System for Mobile Communications Introduction by the European telephone exchange offices (Germany: D and D2) seamless roaming within Europe possible Today many providers all over the world use GSM (more than 20 countries in in Asia, Africa, Europe, Australia, America) More than million subscribers in more than 00 networks More than 0 billion SMS per month in Germany, > 360 billion worldwide (more than 0% of the sales of the operators) Uses the frequency ranges of 900, 800, and 900 MHz Voice and data connections with up to 9.6 KBit/s (enhancement:. KBit/s) Access control by chip-cards Cell structure for a complete coverage of regions (00 500 m Ø per cell in cities, up to 35 km on country-side) 3
Performance Characteristics of GSM Bearer Services Most important technical aspects: Communication: mobile, wireless communication; support for voice and data services Total mobility: international access, chip-card enables use of base stations of different providers Worldwide connectivity: only one number, the network handles localization High capacity: good frequency efficiency; relatively small cells to allow for a high number of customers High transmission quality: high audio quality and reliability for uninterrupted wireless phone calls also at higher speeds (cars, trains, ) Security functions: access control and authorization via chip-card and PIN Basic telecommunication services to transfer data between access points Specification of services up to the terminal interface (corresponding to OSI layers 3) Different data rates for voice and data (original standard) Data service (circuit switched) synchronous: 2.,.8 or 9.6 KBit/s asynchronous: 300 200 Bit/s Data service (packet switched) synchronous: 2.,.8 or 9.6 KBit/s asynchronous: 300 9600 Bit/s Additionally: signaling channels for connection control (used by telematic services) GSM offers three types of services: Bearer Services Telematic Services Supplementary Services 5 6 Telematic Services Telematic Services Telecommunication services that enable voice communication via mobile phones All services have to obey cellular functions, security measurements, etc. Offered services: Mobile telephony Primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3. khz Emergency number Common number throughout Europe (2); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible) Multinumbering Several phone numbers per user possible 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 8
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 participants Locking of the mobile terminal (incoming or outgoing calls) Cellular Network Signal attenuation restricts distance between sender and receiver (~ d² in line of sight, d 5.5 within buildings) Frequency range very limited and not suited for high number of subscribers Frequency re-use by SDMA: divide the whole area in cells Intentionally restriction of a cell by lowering the transmission power Frequency ranges can be re-used in a larger distance without problems of interference Two subscribers in distant cells can use the same channel simultaneously Zelle technical possible transmission range Zelle intentionally restriction of transmission range 9 0 Cellular Network Cell Concept The size of a cell is determined by a maximum given transmission power and a minimum receiver signal strength for a good voice quality Hexagonal cell pattern is idealized (Cells overlap irregularly) No uniform cell size, size depends on attenuation as well as expected traffic amount (inner city vs. unpopulated regions) Cell change of mobile user during a phone call Passing the connection to the neighbor cell: handover Cluster: Area in which all frequencies are used. Each cell in the cluster at least is assigned one frequency, but also several frequencies per cell are possible More cells per cluster: Less channels per cell Lower system capacity Less co-channel interference (co-channel cells have larger distance in between) 3 5 2 6 3 3 5 2 5 2 6 6 Cluster Distance depends on remaining signal strength 6 Less cells per cluster: More channels per cell Higher system capacity More co-channel interference (co-channel cells are nearby) Cell planning: Optimize the luster size N in a way to maximize capacity and minimize interferences 2
Coverage of GSM Networks (www.gsmworld.com) T-Mobile (GSM-900/800) e-plus (GSM-800) Vodafone (GSM-900/800) O 2 (GSM-800) Architecture of the GSM System The GSM system is a so-called PLMNs (Public Land Mobile Network). Several providers setup mobile networks following the GSM standard within each country note: each provider has an own GSM network, but all are interconnected A GSM system consists of several components: MS (mobile station) BS (base station) (mobile switching center) LRs (location register) Different subsystems are defined: RSS (radio subsystem): covers all radio aspects NSS (network and switching subsystem): call forwarding, handover, switching OSS (operation subsystem): management of the network 3 GSM - Architecture GSM Architecture GSM Network Region with Mobile Switching Center () Location Area Base Station Subsystem Base Station Subsystem Cell Cell Base Station Subsystem Location Area Location Area Region Region GSM networks are hierarchical structured: At least one administrative region with Mobile Switching Center An administrative region consists of at least one location area A location area consists of several Base Station Subsystems A Base Station Subsystem consists of one Base Station Controller () and several Base Transceiver Stations (BTS, cells) 5 AUC: : EIR: G: HLR: Authentication Center Base Station Controller Equipment Identity Register Gateway Mobile Switching Center Home Location Register OMC ISC EIR AUC HLR VLR PLMN, international G PSTN ISDN OSS NSS RSS ISC: International Switching Center : Mobile Switching Center OMC: Operation and Maintenance Center PLMN: Public Land Mobile Network VLR: Visitor Location Register 6
Radio Subsystem Base Transceiver Station und Base Station Controller The radio subsystem is the cellular network up to the switching centers It comprises several components: Base Station Subsystem (BSS): Base Transceiver Station (BTS): radio components including sender, receiver, antenna. A BTS can serve one cell or, if directed antennas are used, several cells. Base Station Controller (): The performs the switching between BTSs and the control of BTSs. It manages the network resources, mapping of radio channels onto terestrial channels. The complexity of BTSs only is low by that separation. BSS = + Sum(BTS) + interconnection Mobile stations (MS) are seen as mobile network components. Functions BTS Management of radio channels Frequency hopping (FH) Management of terrestrial channels Mapping of terrestrial onto radio channels Channel coding and decoding Rate adaptation Encryption and decryption Paging Uplink signal measurements Traffic measurement Authentication Location registry, location update Handover management 8 Base Station Subsystem Mobile Station F, F6 F F2 F3 F5 F F8 F F6 F F9 F3 Base Station Controller Fx Frequency range of a cell Base Transceiver Station A BTS controls all transmission in a cell. Communication only is possible between a mobile station and its BTS Problems: Cell changes (Handover to another BTS), combined with a frequency change Location of a mobile station (HLR/VLR) Terminal for the use of GSM services; it comprises several functional groups: MT (Mobile Terminal): Offers common functions used by all services the MS offers Corresponds to the network termination (NT) of an ISDN access End-point of the radio interface TA (Terminal Adapter): Terminal adaptation, hides radio specific characteristics TE (Terminal Equipment): Peripheral device of the MS, offers services to a user Does not contain GSM specific functions SIM (Subscriber Identity Module): Personalization of the mobile terminal, stores user parameters 9 20
Network and Switching Subsystem Mobile Switching Center The network subsystem is the main component of the public mobile network GSM. It interconnects the BSSs with other networks and performs switching, mobility management, and system control Components are: Mobile Services Switching Center () Controls all connections via a separated network to/from a mobile terminal within the domain of the - several can belong to a Databases Home Location Register (HLR) Central master database containing user data, permanent and semipermanent 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 The exchange central of a GSM network is the Mobile Switching Center: path choice, signaling and processing of service features Administration of and access to radio resources Additional functions for location registration and handover when a cell change occurs (support of subscriber mobility) Certain gateways to other fixed or mobile telephony networks (Gateway-; G) Most important functions of a : Specific functions for paging and call forwarding 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 2 22 Home and Visitor Location Register Two types of databases are used for subscriber registration and location management: Home Location Register (HLR) Central location management, a subscriber can be searched for here, not the whole network has to be searched Contains all static subscriber data (number, access rights, subscribed services, service features) as well as a raw location information s use HLR to get information about rights, services and current (raw) location of subscribers Visitor Location Register (VLR) Locale database for a subset of subscriber data, most important the current (detailed) subscriber location; is assigned a Only stores information about subscribers which are in range of the corresponding Contains dynamic data which are updated by information exchange with HLR and the mobile stations Data from a VLR follow the subscriber when he comes into range of another VLR 23 Connection Establishment POTS 9 2 5 8 Gateway Destination 9 9 9 3 HLR VLR BSS 6 9 8 - Call for a mobile station 2 - POTS forwards call to the G connecting the GSM network 3 - G uses HLR to request currently responsible - Response with switching information to the current subscriber location 5 - Forwarding of the call to the destination 6 - requests exact position of the subscriber in its VLR - VLR checks service profile and availability of the MS and gives back the current BSS 8 - Paging of the mobile subscriber (broadcast in the whole BSS) 9 - MS answers, call can be established 2
Handover Handover Decision Automatic change of the responsible BTS without influence on the quality of a connection a caller should not be able to notice the change. Signal strength of signal A receiving power Signal strength of signal B Process:. Measurement During a transmission permanently measurements in the signaling channel are performed to detect the necessity of a handover (receiving power, bit error rates, distance to base station, participants in the cell, narrow-band interference) handover range 2. Initiation of handover Establishment of a connection from the responsible to the new base station Selection of a new channel with the new base station 3. Switching to new BTS Network-controlled handover (e.g. C-Netz), MS-supported handover (e.g. GSM) or MS-controlled handover (e.g. DECT) BTS A MS MS movement Last point of switching BTS B 25 26 Handover Procedure Operation Subsystem MS measurement report HO command BTS old measurement result old HO decision HO required HO command HO command HO access Link establishment clear command clear command clear complete clear complete HO request new BTS new resource allocation ch. activation HO request ack ch. activation ack HO complete HO complete The OSS performs some central tasks for the provision of the whole GSM network as well as maintenance of that network Components are: Authentication Center (AUC) Creates on demand of a VLR the access right parameters for a subscriber These parameters serve for security and protection of subscriber information in the GSM system Equipment Identity Register (EIR) Registers serial numbers of GSM mobile stations as well as the assigned usage right Devices which are registered in the AUC can be locked and maybe located if stolen Not a mandatory component in the GSM architecture Operation and Maintenance Center (OMC) Control centers for the maintenance of all other GSM architecture parts 2 28
GSM900 vs. GSM800 Frequency Assignment Criterion GSM900 GSM800 Frequency range (Uplink) 890 MHz - 95 MHz 0 MHz - 85 MHz Frequency range (Downlink) 935 MHz - 960 MHz 805 MHz - 880 MHz Duplexing distance 5 MHz 95 MHz Bandwidth Up- and Downlink 2 x 25 MHz 2 x 5 MHz Bandwidth of a channel 200 khz 200 khz Access method FDMA & TDMA FDMA & TDMA Number of carrier frequencies 2 32 Time per carrier frequency 8 8 Channels 992 296 Bit rate 20,833 KBit/s 20,833 KBit/s Net bit rate for voice 3 KBit/s 3 KBit/s Modulation method GMSK GMSK Cell size (radius) 2-35 km 0,2-8 km Transmission power of a MS max. 2 Watt max. Watt GSM900: T-Mobile Uplink: 892.6 899.8, 906.2 90., 9. 9.8 (62 channels) T-Mobile Downlink: 93.6 9.8, 95.2 955., 959. 959.8 Vodafone Uplink: 890.2 892., 900.0 906.0, 90.6 9.2 (62 channels) Vodafone Downlink: 935.2 93., 95.0 95.0, 955.6 959.2 GSM900 later addition: E-Plus Uplink: 880. 885. (25 channels) E-Plus Downlink: 925. 930. O2 Uplink: 885. 890. (25 channels) O2 Downlink: 930. 935. GSM800: T-Mobile: 25.2 30.0 (UL), 820.2 825.0 (DL) Vodafone: 52.8 58.0 (UL), 8.8 853.0 (DL) E-Plus: 58.2 80. (UL), 853.2 85. (DL) O2: 30.2 52. (UL), 825.0 8. (DL) 29 30 GSM Protocol (exemplarily for GSM900) TDMA Frames and Traffic Bursts Access method is combination of: Frequency multiplexing (FDMA/FDD) Sending on 2 channels of 200 KHz each between 890 and 95 MHz Receiving on 2 channels of 200 KHz each between 935 and 960 MHz Time multiplexing (TDMA) with a shift of 3 time between sending and receiving time by to avoid the need for duplex-enabled transceiver units 960 MHz 935.2 MHz 95 MHz 890.2 MHz f 2 23 22 2 23 22 20 MHz 200 khz t 3 890-95 MHz Frequency range 935-960 MHz GSM TDMA Frame GSM Timeslot 2 channels with 200 khz each Downstream 2 channels with 200 khz each Upstream Higher GSM Frame Structures 2 3 5 6 8 guard time,65 ms Time guard tail payload S training S payload tail time 3 5 26 5 3 bit 56,5 µs 5 µs GSM timeslot: Burst und guard times Tail (000): define start und end of a Bursts Training: synchronization sequence with well-known bit pattern for adapting the receiver to the current signal propagation characteristics, e.g. calculating the strongest signal part in case of multipath propagation S (Signaling): what is the content of the payload field: user or control data (optional: slow frequency hopping after each TDMA frame to avoid frequencydependent signal fading) 32
Logical Channels Using one timeslot per TDMA frame to send one burst of data defines a logical channel for a device. But, the are different types of channels (with different burst structures):. Traffic Channels (TCH) Full-Rate Traffic Channels (TCH/F), defined as before Half-Rate Traffic Channels (TCH/H), using only each second TDMA frame 2. Control Channels Standalone Dedicated Control Channel (SDCCH): for one traffic channel, e.g. for authentication, equipment validation, transfer of additional information as e.g. phone numbers,... (only established if necessary) Associated Control Channel (ACCH): for one traffic channel, to do synchronization, power regulation, handover initiation, (exists all the time the corresponding traffic channel is active) Common Control Channels (CCCH) for paging of mobile stations, CSMA/CDbased access procedure of mobile stations for joining a GSM network, 3. Broadcast Channels: allow the base station to send frequency correction bursts with full power (to allow stations to adapt transmit power), synchronization bursts, information about channel structure and hopping sequences, 33 Logical Channels in Communication A large number of channels is needed for a mobile station do a single phone call: Use RACH (random access channel, one of the CCCHs) for asking the base station to assign a TCH The base station answers on its AGCH (access grant channel, on of the CCCHs) and assigns a SDCCH exclusively to the mobile station The SDCCH is used for connection establishment, i.e. exchange control data between mobile station and base station A SACCH (slow ACCH) and a TCH are established The SDCCH can be terminated TCH is used for transmitting the voice data, the SACCH is used e.g. for transmit power adaptation If the mobile station moves to another cell, SACCH initiates a handover and becomes a FACCH (fast ACCH, by temporarily misuse other channels for having higher transmission capacity) which establishes a new SACCH an TCH with the new base station 3 Frame Hierarchy Data Services in GSM So we have a large number of logical channels: Half-rate and full-rate traffic channels Control channels for maintenance, requests of new stations to get assigned a channel, Broadcast channels for adaptation of all stations to the base station Result: complex frame hierarchy to come to a common time-structure in which all information is repeated Hyperframe 0 2... 205 206 20 Superframe 0 2... 8 9 50 0... 2 25 Multiframe 0... 2 25 0 2... 8 9 50 Frame 0... 6 slot burst 3 h 28 min 53,6 s 6,2 s 20 ms 235, ms,65 ms 5 µs Data transmission in GSM with only.8 resp. 9.6 kbit/s (depending on error protection) Advanced channel coding and reduced error-correction allows. kbit/s Still not enough for Internet access or even multimedia applications Thus: UMTS as 3G network : Integration of data and voice in one network But: new network infrastructure, new software, new devices, Development of other enhancements of GSM as interim solutions 2.5G networks as interim solution HSCSD as software solution GPRS as hardware solution EDGE as 3G solution in a 2G network 35 36
HSCSD HSCSD (High-Speed Circuit Switched Data) Put together several time for one AIUR (Air Interface User Rate, up to 5.6 kbit/s with Slots of. kbit/s) Symmetrical (2 time channels each for up- and downlink) and asymmetrical (3 + channels) communication are supported Mainly software update for the realization of the putting together Advantage: fast availability, continuous quality, simple Disadvantage: connection-oriented, channels are blocked the whole time, signaling for several channels necessary GPRS GPRS (General Packet Radio Service) Packet-oriented transmission, usable also for multicast Usage of up to 8 time of a TDMA frame on demand Usage of time only when data are available for sending (e.g. 50 kbit/s with short usage of ) Advantage: step towards UMTS, flexible Disadvantage: expensive because some new infrastructure is needed to handle the new transmission mechanism, wireless transmission becomes a bottleneck for high traffic amount AIUR [kbit/s] TCH/F.8 TCH/F9.6 TCH/F..8 9.6 2. 3 9.2 2 28.8 3 2 38. 3.2 3 5.6 Three possible data rates for a full channel depending on the used coding and error correction Needed infrastructure: GSN (GPRS Support Nodes) - GGSN and SGSN - GGSN (Gateway GSN): translation between GPRS und PDN (Packet Data Network) - SGSN (Serving GSN): support of the MS (location, accounting, security) - GR (GPRS Register): Management of user addresses 3 38 GPRS Infrastructure Components GPRS Data Rates [kbit/s] SGSN GGSN PDN (error-) coding CS- time slot 9,05 2 time 8,2 3 time 2,5 time 36,2 5 time 5,25 6 time 5,3 time 63,35 8 time 2, PCU G PSTN ISDN CS-2 CS-3 3, 5,6 26,8 3,2 0,2 6,8 53,6 62, 6 8 80, 93,6 93,8 09,2 0,2 2,8 CS- 2, 2,8 6,2 85,6 0 28, 9,8,2 EIR AUC HLR NSS GR VLR RSS AUC: Authentication Center : Base Station Controller GPRS OSS EIR: Equipment Identity Register G: Gateway Mobile Switching Center GGSN: Gateway GPRS Support Node HLR: Home Location Register GR: GPRS Register : Mobile Switching Center PCU: Packet Control Unit VLR: Visitor Location Register SGSN: Serving GPRS Support Node 39 Data rate CS- CS-3 CS-2 CS- Position CS- to CS-: decreasing error protection Dynamic choice of coding Basing on measurements of signal quality (and the needed QoS) The user is assigned the highest possible data rate 0
GPRS EDGE Unused channels can be used as GPRS channels; current allocation is announced on a CCCH so that all mobile station know about Introduction of new traffic and control channels for packet data Mobile stations can do reservations on demand Long duration for connection establishment are eliminated Accounting by data volume, not by connection duration Robust connection Coding of data bases on current signal quality Even the BSS checks the data correctness and initiates if necessary a re-transmission EDGE (Enhanced Data Rates for GSM Evolution) Up to 38 kbit/s by enhanced modulation (8-PSK instead of GMSK) Transmission repeat: Change of coding to adapt to the current channel quality Is build upon the existing GSM/GPRS system: New transceiver are needed (hardware upgrade in the BSS) Software-Upgrade BSS und New devices (8PSK) No changes in the core network! Also possible: modulation similar to 6-QAM for higher data rate Cheap alternative to UMTS? 2