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The GSM System – Global System for Mobile Communications Magne Pettersen (acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut.

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Presentation on theme: "The GSM System – Global System for Mobile Communications Magne Pettersen (acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut."— Presentation transcript:

1 The GSM System – Global System for Mobile Communications Magne Pettersen (acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut Erik Walter, Anders Spilling)

2 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

3 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

4 GSM status (end 2006) 2.18 billion connections in 212 countries 82 % market share globally An incredible industry success!

5 But, let us take a few steps back…

6 GSM – The idea of a common European mobile communications system 1982: A Nordic group is considering the next generation of mobile telephone. – NMT (Nordisk Mobil Telefon), the analogue first generation system has only just been started These ideas are presented to CEPT (European Conference of Postal and Telecommunications Administrations) in June 1982 September 1982: The first meeting in CEPT GSM – Groupe Spécial Mobile In 1988 ETSI (European Telecommunications Standards Institute) is established and the work is continued under a new name: SMG – Special Mobile Group

7 GSM - Specifications Original specifications for the GSM system: –Good subjective voice quality –Low terminal and service cost –Support for international roaming –Support for handheld terminals –Support for new services –Spectrum efficient –Compatible with ISDN

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9 GSM - Growth 1991: First operational GSM network in Finland: Radiolinja 1993: Tele-mobil (later: Telenor Mobil) and NetCom GSM open their networks in Norway 1998: GSM 1800 is deployed to increase capacity in cities and other densely populated areas

10 GSM improvements – 2.5 G The need for data services increase: –In the HSCSD – High Speed Circuit Switched Data - is standardised. Introduced in Norway 1. July 2001 (Telenor) –I 1999 packet switching using GPRS (General Packet Radio Service) is standardised. Introduced in Norway 1. February 2001 (Telenor) Theoretical data rates up to 171 kbit/s "2.5 G" – EDGE – Enhanced Datarates for GSM Evolution –Standardised in –Introduced in September 2004 – deployment ongoing –Theoretical data rates up to 373 kbit/s

11 Some GSM terminals Development.. Sony Ericsson W950i ”the Walkman phone” HTC P4350 Pocket computer running Windows

12 Some more GSM terminals Samsung Blackjack Nokia N95 with ”everything”, e.g. GPS built in iPhone – Apple’s Mobile phone initiative

13 Competing standards The ”CDMA family” of standards is the second largest group of mobile communications systems –340 million connections (November 2006) Standard developed in USA Strongest standing in the Americas Also other

14 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

15 High level network architecture (1/2) SIM ME: Mobile equipment Services / Applications Core Network (CN) Ext. network UE: User equipment Access Network (AN)

16 High level network architecture (2/2) The network contains functionally of: User Equipment (UE), Access Network (AN), and Core Network (CN) –User equipment: Interfaces the user, handles radio functionality –Access network: Communication to and from the user equipment, handles all radio related functionality in the network –Core network: Communication between access network and external networks, handles all switching and routing Services and applications lie above the network

17 GSM user equipment User equipment: Mobile equipment (ME) + SIM card –Subscriber Identity Module (SIM) contains encryption key and personal data –The user is uniquely identified through ”International Mobile Subscriber Identity” (IMSI) –The mobile equipment is uniquely identified through ”International Mobile Equipment Identity” (IMEI) –Both equipment and user uniquely identified SIM ME SIM = Subscriber Identity Module

18 GSM Radio Access Network (GRAN) cell BTS BSC Packet domain Circuit domain BTS BSC Abis A Gb

19 Elements in GSM radio access network Base Transceiver Station (BTS): –The base station, radio access point. The coverage area of one BTS is a cell Base Station Controller (BSC) –Controls a number of BTSs. Owns and controls the radio resources within its domain GRAN must handle interfaces towards both a packet switched (packet domain) and a circuit switched (circuit domain) part of the core network

20 Some base station equipment

21 Some more base station equipment Typical macro cell Typical micro cell

22 Open interfaces access network The interfaces between network elements must be well defined to achieve open interfaces, i.e. different network elements can be delivered by different vendors Interfaces in GRAN: –Um: The air interface between the mobile equipment and the BTS –Abis: Interface between BTS and BSC –A: Interface between GRAN and circuit switched part of core network (CN). –Gb: Interface between GRAN and packet switched part of the core network (CN)

23 GSM core network MSCGMSC HLR GGSN GRAN External networks PSTN/ISDN IP network Service platforms A Gb SGSN

24 Elements in GSM core network MSC – Mobile Switching Centre –Switch in the circuit domain. Contains copy of service profile for all users currently in the MSC coverage area (Visiting Location Register –VLR, not shown explicitly in figure) GMSC – Gateway MSC –Handles all traffic to and from GSM and external circuit switched networks, such as PSTN, ISDN or other mobile networks HLR – Home Location Register –Database containing a master copy of all the mobile operator’s subscribers. There is only one logical HLR per GSM network. HLR contains information about e.g. permitted services and permitted roaming networks SGSN – Serving GPRS Support Node and GGSN – Gateway GPRS Support Node have similar functionality as MSC / GMSC, but for the packet switched part of the network. GGSN handles connections to external IP networks Also open interfaces between network elements. Not discussed here.

25 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

26 Fundamental functionality The following functions are described: –Circuit switched connectivity –Packet switched connectivity –Mobile messaging –Security –Roaming –Choice of network –Location update –Handover

27 Identification of users: ”Mobile number”: MS-ISDN number follows numbering plan for telephony/ISDN (max. 15 digits): Calling number associated with a subscription Used on interface towards users Identification of a mobile subscription: IMSI : International Mobile Subscription Identity [max 15 siffer] Identification of terminal: IMEI : International Mobile Equipment Identity. Not used in fundamental service handling, but to identify stolen or black listed equipment Identifying users and mobile terminals MCC [3] mobile country code Subscription Identification NC[2-3] network code Country codeNational mobile number (1- 3)

28 Circuit Switched connectivity ISDN Mobile network Fixed connection and reserved resources while the communication lasts. –(Mobile) telephony – Circuit switched data, e.g. WAP, mobile office solutions using data cards etc. Transparent channel with defined performance Billing typically per time unit and dependant on transport data rate Standard GSM: up to 14.4 kbit/s (more using HSCSD - High Speed Circuit Switched Data)

29 Packet Switched connectivity Resources allocated only when data is transferred Same ”path” through network can be maintained (but not necessarily) Billing typically dependant on amount of data transferred (or fixed tarrifs) GPRS: Theoretically up to 171 kbit/s, typically 40 – 50 kbit/s -4 different quality classes for packet ”bearer services”: Internet or different IP network Mobile network BackgroundTypically automatic download of , MMS InteractiveTypically web/WAP-browsing, MMS, games Streaming”Network radio”, video streaming, web TV ConversationalVoice, video conferencing

30 Mobile messaging formats SMS: Short Message Service –Text based service to transfer up to 160 characters per message (solutions exist to connect messages into longer messages, and also to carry other types of content – ring tones, logos…) MMS: Multimedia Messaging Service –A service for multimedia content, such as text, picture, sound, video Both SMS and MMS are ”store and forward” services, i.e. messages are intermediately stored in the network

31 Security functions The purpose of security functions is to protect users and network against improper and illegal use: –Verify that the user has a valid subscription –Protect the user’s identity against tracking –Protection against wiretapping on the radio connection The mechanisms in GSM are based on secure storage of information in the user’s SIM card

32 International network Roaming (1/2) Circuit switched call to a mobile in a visiting network Home network Visiting network ISDN (country A) ISDN (country B)

33 International network Roaming (2/2) Mobile to mobile call in a visiting network –Effect referred to as ”tromboning” Home network Visiting network ISDN (country A) ISDN (country B)

34 Choice of network In GSM the following procedure is followed: –The latest used network is stored on the SIM –As long as a cell that fulfils the criteria is available from this network, the mobile will not search for alternatives (the exception is national roaming, in which case the mobile will periodically search for the home network and connect when this becomes available) –If the previously used network is not available, the mobile searches for alternative networks –The mobiles can perform manually or automatic choice of network

35 Location Area / Routing Area (1/2) In GSM this is defined as follows: –Location area – LA is the area in which the network is ”searching” for a registered mobile (not currently active) – for circuit switched services –Routing area – RA: Similarly for packet switched service HLR..IMSI >LAI,RAI LA 1 LA 2 RA 1 RA 2

36 Location Area / Routing Area (2/2) The dependency between LA and RA is dependant on the practical realisation of the network. Normally they will be identical LA and RA contain a number of cells that can be reached from the MSC or SGSN LA and RA information for each mobile is stored in the HLR (in the home network) The mobile is responsible for updating the LA/RA information

37 Location update A location update is performed when: –The mobile is connecting to a cell and discovers that the LAI read is different than the one stored in the mobile –The mobile has been turned on, but not used, for a pre-defined period of time since the last location update (periodic location update) IMSI detach/attach: –An additional function where the mobile informs that it is turned on or off (in the same LA), saves resources on the radio interface and leads to fater response on incoming calls Periodic detach –A network functionality where the network assumes that the mobile has been turned off if periodic location update has not been performed and no other activity has been observed for a pre-defined amount of time

38 Handover To connect a call or communication session from one cell to another (or to a different channel in the same cell) Is normally performed because the signal level from the current cell is becoming to low, but can also be done for different reasons, such as too much traffic in a cell

39 Types of handover Intra cell (to another channel in the same cell) (1) Inter cell, intra BSC (2) Inter BSC, intra MSC (3) Inter MSC (4) In addition inter system handover can sometimes be performed, e.g. GSM to UMTS –Complicated, special rules apply Type of handover has network implications, but the algorithms to decide handover are the same

40 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

41 GSM radio interface – Main characteristics Frequency bands: –GSM 900: 890 – 915 MHz: Uplink (MS transmit) MHz: Downlink (MS receive) –GSM 1800: MHz: Uplink MHz: Downlink Carrier bandwidth: 200 kHz Channels / carrier:8 Multiple access: TDMA / FDMA Duplex:FDD Gross bit rate pr carrier: 270,833 kbit/s Modulation:GMSK Spectrum efficiency:1.35 bps/Hz

42 Radio parameters: MS: Sensitivity: -104 (-102) dBm Typical – 106 dBm Max. output power: 33 (30) dBm Numbers in parenthesis for GSM-1800 BTS: Sensitivity: -104 (-104) dBm Typical: – 107 dBm Max. output power: 43 dBm

43 Channels in GSM MHz 45 MHz 960 MHz 935 MHz 200 kHz MS transmit MS receive MHz

44 TDMA - principle GSM uses TDMA within each carrier Each user occupies the entire carrier one time slot pr. time frame –8 slots per frame

45 GSM Channel structure 25 MHz 124 carriers 577  s Burst period Time slot 1 Time slot 2 ….. Time slot 8 = ms =Physical channel TDMA frame Logical channels built up of physical channels –Control channels –Traffic channels Logical channels divided between: –Dedicated channels –Common channels

46 GSM traffic channels Traffic channels (TCH) are used to carry voice or data –Typically uses one time slot per frame –Gross data rate per TCH: 22 kbps Effective data rate lower because of forward error correction Training sequence BP0 BP1BP2 BP3 BP4 BP5BP6 BP frame length: 120 ms TDMA frame length: 4.6 ms Data bit Normal burst

47 Some GSM control channels BCCH Broadcast Control CHannel – Continuously transmitted from the BTS. Contains information about cell identity, frequency etc. FCCH SCH Frequency Correction CHannel / Synchronisation CHannel – Used to correct/synchronise the frequency (FCCH) + time synchronise to the frame structure. Each cell has a FCCH and a SCH RACH Random Access CHannel – Used by the mobile to send a request to the network for access. This is a slotted Aloha channel, no pre-allocation possible AGCH Access Grant CHannel – Used by the network to inform the mobile that access has been granted and information about which channel to use PCH Paging CHannel – Used by the network to notify users about incoming calls.

48 Error correction coding in GSM The different channels in GSM require different degree of protection, and therefore have different Forward Error Correction (FEC) schemes However, three types of techniques are often combined: –Block coding, well suited to detect and correct bursts of error –Convolutional coding, high performance but not optimal for bursts of errors –Interleaving, spreading neighbouring bits out, to decorrelate the relative position

49 Block coding GSM uses two types of block codes: –Fire code 224 / 184 (control channels only) k = 184 t = 20 –Parity codes (only error detection, e.g. RACH) No block codes used on data channels

50 Convolutional coding When choosing depth (register length) in a convolutional code there is a trade-off between complexity and performance –GSM uses a register length of 5 Example of GSM ½ rate convolutional code shown in figure (used e.g. on a number of traffic channels)

51 Interleaving “Whitening process", optimising the conditions for the convolutional coder Fundamentally important that the interleaving spreads the bit errors out Interleaving depth improves performance, but also increases delay GSM: Interleaving depth 4 – 19 Figure shows example with interleaving depth 4 –Write in vertically, read out horizontally –On reception, do the reverse process

52 Forward error correction - Overview

53 Modulation Assuming that everyone is familiar with digital modulation :-) Considerations upon choosing modulations scheme: –Spectrum efficiency –Out of band emission (rapid drop off desired to limit adjacent channel interference) –Constant envelope desired for low cost amplifiers, e.g. in handheld equipment Always a trade off In GSM: GMSK – Gaussian Minimum Shift Keying is used

54 GMSK (1/2) Leftmost figure show spectrum for MSK, QPSK and BPSK Rightmost figure shows envelope for different ”QPSK type” modulation schemes –MSK has constant envelope, relatively low sidelobes

55 GMSK (2/2) GMSK further reduces sidelobes by using a Gaussian filter –Cost: introduces inter-symbol-interference (ISI) Figures show time and frequency response –GSM uses BT = 0.3

56 Channel equaliser Because of reflections, diffractions etc. in the radio channel, time dispersion is often experienced –Transmitted signal arriving at the receiver from various directions over a multiplicity of paths –Broadening of transmitted pulse, inter symbol interference (ISI) –Frequency selective fading Must be counteracted by using some sort of equalisation

57 GSM uses a Maximum Likelihood Sequence estimator (MLSE) MLSE looks conceptually like shown in the figure below The impulse response of the radio channel is calculated A Viterbi algorithm is used to estimate the most likely (Maximum Likelihood - ML) symbol sequence MLSE is an optimal technique in terms of removing ISI, but the complexity increases exponentially with the length of the channel response GSM uses a MLSE which operates over 5 bit periods (approx. 16  s) Maximum likelihood sequence estimator

58 Power control GSM uses power control, adjusting transmit power level in accordance with path loss Advantages: –Reduces interference –Reduces power consumption Can also be used on downlink Manner of operation, GSM: –The system (BSC) measures bit error rate (BER) –Transmit power adjusted up or down according to target value –Step size 2 dB –Maximum update interval: 60 ms

59 Power control - Example

60 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

61 Fundamentals Planning and deploying a GSM network is from an operator’s point of view a question of: –Build as few sites as possible, while maintaining required coverage and capacity –Trade off

62 Coverage limited and capacity limited A network can be either –Coverage limited: The radio coverage decides the BTS density Typically rural areas, large cells, high masts Macrocells – Capacity limited: The traffic decides the BTS density Typically urban areas, small cells, low BTS position Microcells

63 Frequency reuse Frequencies can not be reused in every cell due to co-channel interference (CCI) A cell cluster uses all the operator’s frequencies (A, B, C, E, F, G, H in Figure) Co channel interference level decided by –Cell clustre size, and thereby Frequency reuse distance (D in Figure) –Propagation properties –Can be reduced by different techniques: Sectorisation Cell splitting Typical cell cluster size in GSM: 7

64 Coverage map example Unfortunately cell coverage is normally neither hexagonal or circular Figure shows coverage example from a city centre Complicates radio planning

65 Hierarchical cell structures In a GSM system it is common that cells of different sizes co-exist in that same area: –Picocells, microcells, macrocells This is called hierarchical cell structure Can make handover (cell change) complicated. Often different types of users are reserved for one cell type, e.g.: –Users walking indoors on picocell, users walking outdoor on microcell, users driving use macrocell

66 Radio planning tools Radio planning is most often performed assisted by an automated process using a computer Underlying functionality –Digital maps –Propagation modelling –System parameters and system performance –Traffic assumptions and theory Often theoretical computer based modelling can be tuned by real life data –Propagation measurements –Live network traffic data

67 Example – Astrix

68 Content Introduction Network architecture Fundamental functionality Physical layer / radio interface Radio planning GSM in the future

69 GSM development GPRS and EDGE has introduced packet data and support for higher data rates into GSM UMTS is a 3G technology building on GSM core network, which is “backwards compatible” with GSM –GSM-UMTS handover supported –Almost all UMTS terminals are also GSM terminals HSDPA / HSUPA (High Speed Downlink/Uplink Packet Access) supports real mobile broadband

70 Trends (1) – Convergence Mobile communications system become more broadband At the same time computer network solutions start to support mobility (e.g. WiFi, WiMAX) –Mobile goes broadband and broadband goes mobile? –Everything comes together?

71 Trends (2) – Horizontal integration The same services become available on different platforms and on different devices IP is the glue Will mobile circuit switch disappear? Fixed line Satellite GSM 3G WLAN WiMAX Service 1 Service 2 Service n IP Fixed line Satellite GSM 3G WLAN WiMAX Service 2 Service n IP

72

73 Thank you for your attention!


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