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Giuseppe Bianchi Wireless Cellular Networks (basics) Part 3 – GSM networks.

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1 Giuseppe Bianchi Wireless Cellular Networks (basics) Part 3 – GSM networks

2 Giuseppe Bianchi History of Cellular systems  1960's: Bell Labs developed cellular concept  1974-1978: First field Trial for Cellular System  AMPS (Advanced Mobile Phone System), Chicago  1981, Sweden, first European Systems  NMT-450 (Nordic Mobile Telephone)  1985, first italian cellular system  RTMS (Radio Telefono Mobile di Seconda Generazione), 450 MhZ  1990, TACS, first italian widespread systems  Total Access Communication System (TACS, 900 MHZ)  Second generation system:  GSM in europe, D-AMPS & IS95 (CDMA) in USA, PDC Japan  Digital, versus fist generation analog (frequency modulation)  GSM:  Specification started in 1982; EU deployment since 1992; DCS-1800 since 1994  Generation 2 ½ : GPRS, EDGE (8PSK), HSCSD  Generation 3: UMTS, HSDPA  HSPA,  In ITA since 2004  Generation 4: LTE (2011/2012?)

3 Giuseppe Bianchi GSM essential components BTS BSC MSC VLRHLRAUCEIR GMSC To fixed network (PSTN, ISDN, PDN) OMC MSMobile Station BTSBase Transceiver Station BSC Base Station Controller MSC Mobile Switching Center GMSCGateway MSC OMCOperation and Maintenance Center EIREquipment Identity Register AUCAuthentication Center HLRHome Location Register VLRVisitor Location Register MS

4 Giuseppe Bianchi GSM system hierarchy BTS BSC LOCATION AREA MSC MSC region Hierarchy: MSC region  n x Location Areas  m x BSC  k x BTS MSC: Mobile Switching Center LA: Location Area BSC: Base Station Controller BTS: Base Transceiver Station

5 Giuseppe Bianchi Mobile Station (MS)  Key fact:  GSM separates user mobility from equipment mobility, by defining two distinct components  Mobile Equipment  The cellular telephone itself (or the vehicular telephone)  Address / identifier:  IMEI (International Mobile Equipment Identity)  Control: Equipment Identity Register (EIR):  White list, black list, gray list  Stolen terminals, malfunctioning terminale  Subscriber Identity Module (SIM)  Fixed installed chip (plug-in SIM) or exchangeable card (SIM card)  Addresses / identifiers:  IMSI (International Mobile Subscriber Identity)  MSISDN (Mobile Subscriber ISDN number – the phone number)

6 Giuseppe Bianchi Base Station Sub-System BTS BSC A-bis Interface Um - Radio Interface BSS A Interface OSS  Base Transceiver Station (BTS)  Transmitter and receiver devices, voice coding & decoding, rate adaptation for data  Provides signaling channels on the radio interface  Limited signal and protocol processing (error protection coding, link layer LAPDm)  Base Station Controller (BSC)  performs most important radio interface management functions:  Radio channels allocation and deallocation; handover management; …

7 Giuseppe Bianchi Base Transceiver Station - BTS Output filter Input Filter HF Transmitter HF Receiver Slow freq. Hopping TRX Digital Signal Processing Transmission System Operation and Maintenance Functionality/clock distribution Abis Interface (to BSC) U m Interface (to MS) TRX radio interface functions: - GMSK modulation-demodulation - channel coding - encryption/decryption - burst formatting, interleaving - signal strength measurements - interference measurements In essence, BTS is a complex modem!

8 Giuseppe Bianchi Base Station Controller - BSC FUNCTIONS:  switch calls from MSC to correct BTS  and conversely  Protocol and coding conversion  for traffic (voice) & signaling (GSM-specific to ISDN-specific)  Manage MS mobility  Enforce power control X switch matrix BTS-1 BTS-2 BTS-K 1 BSC may control up to 40 BTS DB From/to MSC DB contains - state information for all BSS - BTS software

9 Giuseppe Bianchi Transcoding and Rate Adaptation BTS: -collects speech traffic -Deciphers and removes error protection -Result: -13 kbps air-interface GSM speech-coded signal MSC: -A modified ISDN switch -Needs to receive ISDN-coded speech -64 kbps PCM format (A-law) Transcoding and Rate Adaptation Unit (TRAU) needed! Rationale: re-use existing ISDN switches & protocols

10 Giuseppe Bianchi TRAU possible placements BTS BSC 64 kbit/s 16 kbit/s 13 kbit/s On BSC TRAU MSC BTS BSC 64 kbit/s (4x16 sub-mux) 16 kbit/s 13 kbit/s On MSC TRAU MSC Why 16 kbps instead of 13? Inband signalling needed for BTS control of TRAU (TRAU needs to receive synchro & decoding information from BTS) BTS TRAU BSC MSC 64 kbit/s On BTS 13 kbit/s

11 Giuseppe Bianchi Network Switching Sub-System  Elements:  Mobile Switching Center (MSC) / Gateway MSC (GMSC)  Enhanced telephone switching centers (digital, ISDN)  With support for user mobility, registration, handover  Home Location Register (HLR ) / Authentication Center (AuC)  Visitor Location Register (VLR)  Equipment Identity Register (EIR)  Functions:  Call control  User management  Inter-component communication  Via SS7 signalling network with suitable extensions  (e.g. MAP – Mobile Application Part)

12 Giuseppe Bianchi Location LA-4…LA-n LA-1LA-2LA-3 MSC VLR  1 MSC  1 VLR  Several Location Areas

13 Giuseppe Bianchi Location Registration (Update) (very) basic idea MSC VLR BTS BSC MS HLR 1 1)MS switches ON; detects cell through BCCH carrier; Obtain Location Area Identifier (LAI) from BCCH 2 2) Register MS ID (IMSI) into local VLR; Authenticate; receive TMSI for local paging purposes 3 3) Update pointer at HLR, which now knows which LAI/VLR the user is located

14 Giuseppe Bianchi Location Registration - details MS VLRHLRAUC BSS/MSC Loc. Upd. Request IMSI, LAI Update Loc. Area IMSI, LAI Auth. Param. Req. IMSI Auth. Info. Req. IMSI Auth. Info (Auth. Parameters) Auth. Info (Auth. Parameters) authentication Activate ciphering Update Location IMSI, MSRN Insert Subscrib. Data IMSI, additional data Insert Subscrib. Data ACK Locat. Upd. Accept IMSI Start Ciphering Kc Locat. Upd. Accept Forward new TMSI TMSI TMSI Realloc Cmd Locat. Upd. Accept TMSI Realloc ACK TMSI ACK

15 Giuseppe Bianchi Changing MSC/VLR Base Station MSC Public switched telephone network PSTN Public switched telephone network PSTN Base Station MSC VLR HLR An MS always has a dedicated entry in the HLR Plus one entry in JUST 1 VLR (related to the MSC the user is connected to )

16 Giuseppe Bianchi Location Update: different VLR MS VLR-newHLRVLR-old BSS/MSC Loc. Upd. Request TMSI(+ old LAI), LAI Update Loc. Area TMSI(+ old LAI), LAI authentication Activate ciphering Update Location IMSI, MSRN Insert Subscrib. Data IMSI, additional data Ins. subs. data ACK Locat. Upd. Accept IMSI Start Ciphering Kc … Forward new TMSI Generate New TMSI Send parameters (TMSI, old LAI) IMSI response (IMSI,RAND,SRES,Kc) Cancel Location IMSI Cancel Locat. ACK determine VLR-old From old LAI

17 Giuseppe Bianchi Call switching Gateway MSC – GMSC XXX XX MSC PLMN Public Land Mobile Network MSC GMSC Needed, as fixed network switches are not mobile capable!! GMSC task: query HLR for current MS location (if fixed network switches were able to query HLR, direct connection with local MSC would be available) HLR

18 Giuseppe Bianchi Notation  A call involves two “Parties”  Calling Party (caller)  user generating the call  Called Party (callee)  user receiving the call  Mobile Originating Call (MOC)  Call originated by an MS  Mobile Terminating Call (MTC)  Call directed to an MS

19 Giuseppe Bianchi Call establishment basics MSMSC Fixed party setup MSMSC Fixed party setup Call confirmed alerting connect DATA setup Mobile Terminated Call Mobile Originated Call Call proceeding alerting Connect Connect Ack DATA In ISDN ISUP: - setup= IAM (Initial Address Message); - Alerting= ACM (Address Complete Message); - Connect= ANS (Answer)

20 Giuseppe Bianchi Call establishment steps Channel request Paging request Paging Response Immediate Assignment Authentication Response Authentication Request Ciphering Mode Complete Ciphering mode command Call Confirmed Setup Assignment Complete Assignment Command Alerting Connect Connect Acknowledge Mobile Terminated Call MSnetwork Channel request Service Request Immediate Assignment Authentication Response Authentication Request Ciphering Mode Complete Ciphering mode command Call proceeding Setup Alerting Connect Connect Acknowledge Mobile Originated Call MSnetwork Assignment Complete Assignment Command

21 Giuseppe Bianchi MSC A HLR MSC C MSC B PLMN ISDN GMSC VLR B Routing an MTC 1: MSISDN 4: MSRN 2: MSISDN 3: MSRN 5: MSRN 6: TMSI 7: paging

22 Giuseppe Bianchi PLMN 1 (ITA) MSC GMSC 1 HLR PLMN 2 (UK) MSC ISDN (ita) Transit Exchange Local Exchange International Switching Center MSISDN +39.335.1234567 335.1234567 International Switching Center ISDN (UK) MSRN +44.NDC.8877665 Routing calls to Roaming MS

23 Giuseppe Bianchi “tromboning” PLMN 1 (ITA) MSC GMSC 1 HLR PLMN 2 (UK) MSC ISC (UK) MSISDN +39.335.1234567 MSRN +44.NDC.9876543 Call to MSISDN +39.335.1234567 ISC (ITA) Is the PRICE (!) to pay for simple routing and billing Call to MSISDN +39.335.3043125

24 Giuseppe Bianchi Number portability  Subscriber may switch operator without changing his number  First implemented in fixed network  may 2002: extended to mobile networks  Essential for fair competition among network operators  UK 2002 survey: 42% of corporate subscribers were willing to change mobile operator; but 96% were, if number could be ported  Resistence from leading operators  Number portability helps newer operators to compete with traditional ones

25 Giuseppe Bianchi Notation  Donor switch  The switch corresponding to a “ported” telephone number  Recipient switch  The switch to which the ported number is attached

26 Giuseppe Bianchi Technical solutions a) call forwarding switch Originating networkDonor network Recipient network Originating switch sets-up trunk to donor switch Donor switch sets-up trunk to recipient switch Simplest solution, as call forwarding is a feature available in virtually all switches But extremely inefficient routing and trunking resource consumption!

27 Giuseppe Bianchi Technical solutions b) query on release switch Originating networkDonor network Recipient network Donor switch “blocks” incoming call with a release message (REL) REL carries a QoR cause value, stating that called party number is ported Originating switch then queries Number Portability database SS7 ISUP IAM SS7 ISUP REL Number Portability DataBase

28 Giuseppe Bianchi Technical solutions c) all-call query switch Originating networkDonor network Recipient network Originating switch queries Number Portability database for every call!! - best solution if majority of numbers are ported (no interaction with donor) - but very high DB load, as EVERY number must be looked-up! Number Portability DataBase

29 Giuseppe Bianchi Mobile Number Portability  Same ideas as fixed number portability  The donor switch is the GMSC of the donor network  Donor GMSC Call forwarding (if more efficient fixed number portability not supported)  While porting number, may also get MSRN! GMSC Incoming call Donor network HLR Signaling relay function GMSC Recipient network HLR MSC Note: If path must cross GMSC: Use Intermediate Routing Number MSRN (or IRN) MSRNIRN Clearly, still suffers of tromboning!

30 Giuseppe Bianchi Return IRN Mobile Number Portability (with all call query approach) switch Incoming call GMSC Recipient network HLR MSC IRN Number Portability DataBase Query IRN Return MSRN Query MSRN

31 Giuseppe Bianchi Mobile Number Portability improved – (with all call query approach) Return MSRN switch Incoming call GMSC Recipient network HLR MSC MSRN Number Portability DataBase Query MSRN Signaling relay function

32 Giuseppe Bianchi Wireless Cellular Networks (basics) Part 4 – GSM radio interface

33 Giuseppe Bianchi GSM Radio Spectrum  2 x 25 Mhz band  Duplex spacing: 45 MHz  124 carriers x band  200 KHz channels  Suggested use: only 122  Use top & bottom as additional guard  8 TDMA slots x carrier  full rate calls – 13 Kbps  If half-rate used, 16 calls at 6.5 kbps Frequency [MHz] 890 915 935 960 UPLINK MS  BS DOWNLINK BS  MS 890.2 890.4 “guard band” 12345678

34 Giuseppe Bianchi Adjacent channels (due to GMSK) 35dB 60dB Specification: 9dB In practice, due to power control and shadowing, adjacent channels Cannot be used within the same cell…

35 Giuseppe Bianchi Physical channel  200 KHz bandwidth + GMSK modulation  1625/6 kbps gross channel rate (270.8333 kbps)  1 time slot = 625/4 bits  156.25 bits  15/26 ms = 576.9  s time time slot 0 577  s time slot 7 1 frame = 60/13 ms = 4.615 ms 26 frames = 120 ms (this is the key number)

36 Giuseppe Bianchi Hybrid FDMA-TDMA physical channel = (time slot, frequency) time 577us frequency 200 KHz slot Total n. of channels: 992

37 Giuseppe Bianchi Slow Frequency hopping (optional procedure within individual cell) f1 f2 f3 f4 f5 f6 f7 Hopping sequence (example): …  f1  f2  f5  f6  f3  f7  f4  f1… Slow = on a per-frame basis - 1 hop per frame (4.615 ms) = 217 hops/second Physical motivation: - combat frequency-selective fading - combat Co-Channel Interference next slot may not interferere with adjacent cell slot (different hopping sequence) - improvements: acceptable quality with 9 dB SNR versus 11 dB

38 Giuseppe Bianchi Duplexing 0 123 4 567 UPLINK 0 123 4 567 DOWNLINK - MS uses SAME slot number on uplink and downlink - Uplink and downlink carriers always have a 45 MHz separation -I.e. if uplink carrier is 894.2  downlink is 919.2 -3 slot delay shift!! MS: no need to transmit and receive in the same time on two different frequencies!

39 Giuseppe Bianchi GP 8.25 Structure of a TDMA slot  Symmetric structure  DATA: 2 x 57 data bits  114 data bits per burst  “gross” bits (error-protected; channel coded)  “gross” rate: 24 traffic burst every 26 frames (120 ms)  22.8 kbps gross rate  13 kbps net rate!  S: 2 x 1 stealing bit  Also called stealing flags, toggle bits  Needed to grab slot for FACCH (other signalling possible) TB 3 DATA 57 S1S1 S1S1 Training sequence 26 Data 57 TB 3 148 bit burst 156.25 bit (15/26 ms = 0.577 ms) Normal burst

40 Giuseppe Bianchi Guard Period rationale  Assume the following synchro mechanism:  BTS transmits at time 0  MS receives at time d/c  MS transmits at time 3+d/c  BTS receives at time 3+ 2d/c  Offset depending on d! BTS d 1234 BTS downlink tx MS downlink rx 1234 1 1 MS uplink rx BTS uplink rx 1 Expected RX time!

41 Giuseppe Bianchi Guard period sizing BTS time MS time dwlink slot 1dwlink slot 4 … dwlink slot 1dwlink slot 4 … uplink slot 1 … … Maximum cell radius: Is there something wrong? (GSM says that cells go up to 35 km)

42 Giuseppe Bianchi Frame synchronization  Timing Advance (TA)  Parameter periodically transmitted by BTS during MS activity  6 bits = 0-63  Meaning: anticipate transmission of TA bits  TA=0: no advance  I.e. transmit after 468.75 bits after downlink slot  TA=63:  Transmit after 405.75 bits time BTS TA (transmitted in the SACCH) dwlink slot 4 uplink slot 1 MS time TA dwlink slot 4 uplink slot 1 BTS time TA avoids collision!

43 Giuseppe Bianchi Timing Advance analysis  Downlink propagation delay:  d/c  Uplink propagation delay:  d/c  Uplink delay with TA:  d/c-TA  Perfect resynchronization occurs when  TA = 2d/c  Maximum cell size for perfect resync: 8.25 bits Guard time additionally available for imperfect sync (+/- error)

44 Giuseppe Bianchi And when the user is not connected? But wants to connect… TB 8 Training sequence 41 Data 36 TB 3 88 bit burst 156.25 bit (0.577 ms) Access burst Solution: USE A DIFFERENT BURST FORMAT Access Burst: much longer Guard Period available drawback: much less space for useful information GP 68.25 No collision with subsequent slot for distances up to 37.8 km

45 Giuseppe Bianchi Logical vs Physical channels  Physical channels  Time slots @ given frequencies  Issues: modulation, slot synchronization, multiple access techniques, duplexing, frequency hopping, etc  Logical channels  Built on top of phy channels  Issue: which information is exchanged between MS and BSS Physical channels (FDMA/TDMA) Logical channels (traffic channels, signalling (=control) channels)

46 Giuseppe Bianchi Logical – physical mapping frequency Physical Channel: data rate r, time slot i frequency Logical Channel Mapping: Different channels may share a same physical channel Logical channel A: data rate r/3, time slot i, frame 3k Logical channel B: data rate 2r/3, time slot i, frame 3k+1, 3k+2 Frame 8Frame 9Frame 10Frame 11Frame 12

47 Giuseppe Bianchi GSM logical channels Traffic channel (TCH)TCH/F TCH full rate MS  BSS TCH/H TCH half Rate MS  BSS Broadcast channelBCCH Broadcast control BSS  MS (same information to all MS in a cell) FCCH Frequency Correction BSS  MS SCH Synchronization BSS  MS Common Control channel (CCCH)RACH Random Access MS  BSS (point to multipoint channels) AGCH Access Grant BSS  MS (used for access management) PCH Paging BSS  MS Dedicated Control channel (DCCH)SDCCH Stand-alone Dedicated control MS  BSS (point-to-point signalling channels) SACCH Slow associated control MS  BSS (dedicated to a specific MS) FACCH Fast associated control MS  BSS Additional logical channels available for special purposes (SMS, group calls, …)

48 Giuseppe Bianchi Traffic channels (TCH/F) 123456789101112131415161718192021222324 Periodic pattern of 26 frames (120 ms = 15/26 ms/TS * 8 TS/frame* 26 frame ) 24 TCH frames over 26 20134567201345672013456720134567 Same TS in every frame GP 8.25 TB 3 DATA 57 S1S1 S1S1 Training Seq. (26) Data 57 TB 3 148 bit burst 156.25 bit (0.577 ms) Theoretical rate: 1/8 channel rate: r=33.85 kbps 2 signalling frames: r  31.25 kbps Burst overhead (114 bits over 156.25): r  22.8 kbps

49 Giuseppe Bianchi Slow Associated Control Channel 123456789101112131415161718192021222324 TCH/F(0…7) SACCH(0…7)IDLE frame SACCH-0SACCH-1SACCH-2SACCH-3SACCH-4SACCH-5SACCH-6SACCH-7 1 SACCH burst (per TCH) every 26 frames (120 ms)  Always associated to instaurated call on TCH (TCH + SACCH = TACH)  On the same Time Slot  Periodic (order of ½ second) time-scale information for radio link control

50 Giuseppe Bianchi SFBBBBPPPPSFPPPPPPPPSFPPPPPPPPSFPPPPPPPPSFPPPPPPPP Broadcast Channel (usual) organization  51 frame structure vs 26  235.38 ms period (vs 120 ms)  Sub-blocks with 10 frames  Starting with Frequency Correction Channel (FCCH)  Immediately followed by Synchronization Channel (SCH)  Other frames (numbered from #0 to #50) :  #50 idle  #2,3,4,5 BCCH  Remaining: Paging (PCH) / Access Grant (AGCH) [=PAGCH] 51 frame structure - downlink 10 frame sub-block

51 Giuseppe Bianchi BCCH contents  184 bits  Coded in 456 bits and interleafed in 4 bursts  same coding and interleaving as SACCH  BCCH capacity  184 bits / (51*8*15/26 ms) ~ 782 bps  Information provided  Details of the control channel configuration  Parameters to be used in the cell  Random access backoff values  Maximum power an MS may access (MS_TXPWR_MAX_CCCH)  Minimum received power at MS (RXLEV_ACCESS_MIN)  Is cell allowed? (CELL_BAR_ACCESS)  Etc.  List of carriers used in the cell  Needed if frequency hopping is applied  List of BCCH carriers and BSIC of neighboring cells

52 Giuseppe Bianchi BCCH carrier placement  On Downlink  Corresponding uplink dedicated to Random Access Channel RRRRRRRRRRR 51 frame structure - uplink  On one frequency per cell (beacon)  MUST BE on Time Slot #0  Other Time slots may be used by TCH Provided that: All empty slots are filled with DUMMY bursts Downlink power control must be disabled RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR

53 Giuseppe Bianchi Fast Associated Control Channel  FACCH: urgent signalling  Used when several signalling information needs to be transmitted  Call setup  Handover  FACCH block = 184  456 after coding  Interleaved as voice block  Spreaded on 8 bursts  Replaces a voice block (20 ms) on the TCH  Via stealing bits  Voice block(s) discarded  Maximum FACCH bit rate  184*6/120 [bits/ms] = 9.2 kbps (vs 383 bps of SACCH!)

54 Giuseppe Bianchi FACCH insertion in TCH Via Stealing bits - upper bit = odd bits stolen - lower bit = even bits stolen - both bits = all burst stolen time Figure: shows example of 2 FACCH blocks stealing a TCH (note begin and end behavior due to interleaving)

55 Giuseppe Bianchi Why paging  Channel assignment:  only upon explicit request from MS  Paging  needed to “wake-up” MS from IDLE state when incoming call arrives to MS  MS accesses on RACH to ask for a channel  Generally SDCCH (but immediate TCH assignment is possible) BSS/MSC MS 1) paging 3) Channel assignment 2) Random access Paging channel:PCH Access Grant Channel:AGCH Random Access Channel:RACH PAGCH CCCH Common Control CHannel

56 Giuseppe Bianchi Paging  Paging message generated by MSC  Which receives incoming call  Transferred to subset of BSC  Paging limited to user’s location area  Paging message contains:  List of cells where paging should be performed  Identity of paged user (IMSI or TMSI)  Paging message coded in 4 consecutive bursts over the air interface  Same coding/interleaving structure of SACCH (184  456 bits)  Paging for more MSs may be joined in one unique paging message

57 Giuseppe Bianchi An example procedure involving signalling Setup for an incoming call (call arriving from fixed network part - MS responds to a call) Steps: - paging for MS - MS responds on RACH - MS granted an SDCCH - authentication & ciphering on SDCCH - MS granted a TS (TCH/FACCH) - connection completed on FACCH - Data transmitted on TCH

58 Giuseppe Bianchi Radio Resource allocation three standardized solutions  Non-Off Air Call Set-Up (Non-OACSU)  Normally used (previous description)  Off Air Call Set-Up (OACSU)  TCH assigned only when the called party actually responds!  Best utilization of radio resource (avoids allocation if callee not available)  Call drop if no TCH is available at this point  Very Early Assignment (VEA)  Immediate assignment of TCH  Fastest signalling process  Waste of resources RACH VEA TCH (FACCH) Non-OACSU OACSU SDCCH TCH (DATA) SDCCH TCH (DATA) Connection establishedCallee responds

59 Giuseppe Bianchi handover  Procedure in which an MS releases a connection with a BTS, and establishes a connection with a new BTS, while ensuring that the ongoing call is maintained  The MS remains in dedicated state (unlike cell reselection, where MS is in idle state)  Handoff: synonymous of handover  Needs two mechanisms  Handover preparation: detection of cell-border crossing  Based on radio link quality measurements  Handover execution: setup of a new channel in a cell, and tear-down of a previous channel  Improved handover mechanisms:  Seamless handover: when active call performance is not impaired  Not possible in GSM: for about 100-200ms, communication is interrupted  Soft Handover: when two channels are simultaneously set-up (old and new)  Not possible in GSM; possible in UMTS

60 Giuseppe Bianchi Hard, Seamless, Soft handover MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f1f1 MSC BSS 1BSS 2 MS f2f2 MSC BSS 1BSS 2 MS MSC BSS 1BSS 2 MS f2f2 f1f1 f2f2 f1f1 f1f1 beforeduringafter Hard handover (GSM) Seamless (DECT) Soft handover (UMTS)

61 Giuseppe Bianchi Handover classification  Rescue handover (mandatory handover)  Driven by radio channel quality degradation  Confinement handover (network-directed handover)  Target: minimize radio interference  Assign new channel when old channel results critical for total interference  Traffic handover (network-directed handover)  Driven by traffic congestion conditions  Also called load-balancing  Internal handover  Intra-BTS  New radio channel in the same cell  Not termed as “handover” but as“subsequent assignment”  Inter-BTS (Intra-BSC)  Under control of same BSC  External handover  Inter-BSC (Intra-MSC)  Change reference BSC; may imply a location area update  Inter-MSC  Most complex: need to change MSC Classification by motivationClassification by typology

62 Giuseppe Bianchi Types of handover A-MSC BSC BTS BSC R-MSC A A-bis radio interface Anchor MSC: the MSC that first managed the current call Relay MSC: the MSC that currently manages the call Switching point for internal handover Switching point for all inter-MSC handover Switching point for inter-BSC handover

63 Giuseppe Bianchi Handover taxonomy  BCHO: Base station Controlled Handover  Handover detection: BS  Handover Execution: BS  MCHO: Mobile Controlled Handover  Handover detection: MS  Handover Execution: MS  MAHO: Mobile Assisted Handover  Handover detection: MS  Handover Execution: BS  GSM: somehow a BCHO with a flavor of MAHO  Handover decision always taken by BSC  Based on measures taken at both BTS and MS  New channel selection decision taken at BSC or R-MSC or A-MSC (depending on handover type) based on traffic consideration

64 Giuseppe Bianchi Handover preparation  Measurements performed at BTS  Up-link signal level received from MS lower than threshold  RXLEV_UL < L_RXLEV_UL_H  Up-link signal quality (BER) received from MS  RXQUAL_UL < L_RXQUAL_UL_H  Distance between MS and BTS  adaptive timing advance parameter > MAX_MS_RANGE  Interference level in unallocated time slots.  Measurements performed at MS.  Down-link signal level received from serving cell  RXLEV_DL < L_RXLEV_DL_H  Down-link signal quality (BER) received from serving cell  RXQUAL_DL < L_RXQUAL_DL_H  Down-link signal level received from n- th neighbor cell  RXLEV_NCELL(n) > RXLEV_MIN(n) RX signal level From (dBm) To (dBm) RXLEV_0--110 RXLEV_1-110-109 RXLEV_2-109-108 RXLEV_3-108-107 ……… ……… RXLEV_62-49-48 RXLEV_63-48- Bit error Ratio From (%) To (%) RXQUAL_0-0.2 RXQUAL_10.20.4 RXQUAL_20.40.8 RXQUAL_30.81.6 RXQUAL_41.63.2 RXQUAL_53.26.4 RXQUAL_66.412.8 RXQUAL_712.8-

65 Giuseppe Bianchi Handover preparation – additional metrics  Transmission power  Maximum MS transmission power  Maximum serving BTS transmission power  Maximum neighboring BTSs transmission power  congestion status  of serving BTS  of neighboring BTSs  provided they can support the MS.  Handover Margin  To avoid ping-pong handover effect  5-10 dB in normal operation; up to 30dB in urban operation (to fight shadowing) RXLEV (cell A) RXLEV (cell B) Handover RXLEV (cell A) RXLEV (cell B) Handover hysteresis HANDOVER ALGORITHM: operator-dependent! GSM standard SUGGESTS a simple reference algorithm, but implementation left to operator

66 Giuseppe Bianchi handover procedure skeleton 2) Switching point prepares new path on fixed net 2 1) Handover request goes up to switching point 1 MSC BTS BSC 3) Switching point sends HO command to MS 3 4) MS accesses new channel 4 5) Old channel/path torn down 5

67 Giuseppe Bianchi Inter-MSC handover  More complex, as an ISDN circuit must be set between MSCs  We’ll not enter into details (just the basic ideas)  Two cases MSC-AMSC-R1 First MSC change (basic handover) MSC-AMSC-R1 Second MSC change (subsequent handover) MSC-R2 X X X Note the role of the Anchor MSC!

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