Post on 16-Dec-2015
Giuseppe Bianchi
Wireless Cellular NetworksWireless Cellular Networks(basics) (basics)
Part 3 – GSM networks
Giuseppe Bianchi
History of Cellular systemsHistory 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 JapanDigital, 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?)
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GSM essential componentsGSM essential components
BTS
BTS
BTS
BTS
BTS
BSC
BSC
MSC
VLRHLRAUCEIRGMSC
To fixed network (PSTN, ISDN, PDN)
OMC
MS Mobile StationBTS Base Transceiver StationBSC Base Station ControllerMSC Mobile Switching Center GMSC Gateway MSCOMC Operation and Maintenance CenterEIR Equipment Identity RegisterAUC Authentication CenterHLR Home Location RegisterVLR Visitor Location Register
MS
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GSM system hierarchyGSM system hierarchy
BTS
BSCLOCATION
AREA
MSC MSC region
Hierarchy: MSC region n x Location Areas m x BSC k x BTS
MSC: Mobile Switching CenterLA: Location AreaBSC: Base Station ControllerBTS: Base Transceiver Station
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Mobile Station (MS)Mobile Station (MS) Key fact:
GSM separates user mobility from equipment mobility, by defining two distinct components
Mobile EquipmentThe cellular telephone itself (or the vehicular telephone)
Address / identifier: IMEI (International Mobile Equipment Identity)
Control: Equipment Identity Register (EIR): White list, black list, gray listStolen 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)
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Base Station Sub-SystemBase Station Sub-System
BTS
BTS
BTS
BSC
A-bisInterface
Um - RadioInterface
BSS
AInterface
OSS
Base Transceiver Station (BTS)Transmitter and receiver devices, voice coding & decoding, rate adaptation for dataProvides signaling channels on the radio interfaceLimited 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; …
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Base Transceiver Station - Base Transceiver Station - BTSBTS
Outputfilter
InputFilter
HFTransmitter
HFReceiver
Slo
w f
req
.H
op
pin
g TRXDigital Signal
Processing
Tra
nsm
iss
ion
Sys
tem
Operation and Maintenance Functionality/clock distribution
Abi
s In
terf
ace
(to
BS
C)
Um 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!
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Base Station Controller - Base Station Controller - BSCBSC
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
Xswitchmatrix
BTS-1
BTS-2
BTS-K
1 BSC may controlup to 40 BTS
DB
From/to MSC
DB contains - state information for all BSS- BTS software
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Transcoding and Rate Transcoding and Rate AdaptationAdaptation
BTS: -collects speech traffic-Deciphers and removes error protection-Result:
-13 kbps air-interface GSM speech-coded signalMSC:-A modified ISDN switch-Needs to receive ISDN-coded speech
-64 kbps PCM format (A-law)
Transcoding andRate Adaptation Unit (TRAU)needed!
Rationale: re-use existing ISDN switches & protocols
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TRAU possible placementsTRAU possible placements
BTS BSC
64 kbit/s16 kbit/s13 kbit/sOn BSCTRAU MSC
BTS BSC
64 kbit/s(4x16 sub-mux)
16 kbit/s13 kbit/sOn MSCTRAU 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/s64 kbit/sOn BTS
13 kbit/s
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Network Switching Sub-Network Switching Sub-SystemSystem
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 controlUser management
Inter-component communicationVia SS7 signalling network with suitable extensions
(e.g. MAP – Mobile Application Part)
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LocationLocation
LA-4 … LA-n
LA-1 LA-2 LA-3
MSC VLR
1 MSC 1 VLR Several Location Areas
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Location Registration Location Registration (Update)(Update)
(very) basic idea(very) basic idea
MSC VLR
BTS
BTS
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
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Location Registration - Location Registration - detailsdetailsMS VLR HLR AUCBSS/MSC
Loc. Upd. RequestIMSI, LAI
Update Loc. AreaIMSI, LAI
Auth. Param. Req.IMSI
Auth. Info. Req.IMSI
Auth. Info(Auth. Parameters)
Auth. Info(Auth. Parameters)
authentication
Activateciphering
Update LocationIMSI, MSRN
Insert Subscrib. Data IMSI, additional data
Insert Subscrib. DataACK
Locat. Upd. AcceptIMSI
Start CipheringKc
Locat. Upd. Accept
Forward new TMSI TMSI
TMSI Realloc Cmd
Locat. Upd. Accept
TMSI Realloc ACKTMSI ACK
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Changing MSC/VLRChanging MSC/VLR
Base Station
MSC Public switched telephone network
PSTN
Public switched telephone network
PSTN
Base Station
MSC
VLR
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)
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Location Update: different Location Update: different VLRVLR
MS VLR-new HLR VLR-oldBSS/MSCLoc. Upd. RequestTMSI(+ old LAI), LAI
Update Loc. AreaTMSI(+ old LAI), LAI
authentication
Activateciphering
Update LocationIMSI, MSRN
Insert Subscrib. Data IMSI, additional data Ins. subs. data ACK
Locat. Upd. AcceptIMSI
Start CipheringKc
… …
Forward new TMSI
GenerateNew TMSI
Send parameters (TMSI, old LAI)
IMSI response (IMSI,RAND,SRES,Kc)
Cancel LocationIMSI
Cancel Locat. ACK
determineVLR-old
From old LAI
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Call switchingCall switchingGateway MSC – GMSCGateway MSC – GMSC
X XX
XX
MSC
PLMNPublic Land
Mobile Network
MSCMSC
GMSC
Needed, as fixed networkswitches are not mobilecapable!!
GMSC task: query HLR forcurrent MS location
(if fixed network switcheswere able to query HLR,direct connection with local MSC would be available)
HLR
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NotationNotation
A call involves two “Parties”Calling Party (caller)
user generating the callCalled Party (callee)
user receiving the callMobile Originating Call (MOC)
Call originated by an MSMobile Terminating Call (MTC)
Call directed to an MS
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Call establishment basicsCall establishment basics
MS MSCFixedparty
setup
MS MSCFixedparty
setup
setup
Call confirmed
alertingalerting
connectconnect
DATA
setup
Mobile Terminated Call Mobile Originated Call
Call proceeding
alertingalerting
ConnectConnect
Connect Ack
DATA
In ISDN ISUP: - setup = IAM (Initial Address Message); - Alerting = ACM (Address Complete Message); - Connect = ANS (Answer)
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Call establishment stepsCall 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
MobileTerminated CallMS network
Channel request
Service Request
Immediate Assignment
Authentication Response
Authentication Request
Ciphering Mode Complete
Ciphering mode command
Call proceeding
Setup
Alerting
Connect
Connect Acknowledge
MobileOriginated CallMS network
Assignment Complete
Assignment Command
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MSCAHLR
MSCC
MSCB
PLMN
ISDN
GMSC
VLRB
Routing an MTCRouting an MTC
1: M
SISDN
4: M
SRN
2: MSISDN
3: MSRN
5: MSRN
6: TMSI 7: paging
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PLMN 1(ITA)
MSC
GMSC 1
HLR
PLMN 2(UK)
MSC
ISDN(ita)
TransitExchange
LocalExchange
InternationalSwitching
Center
MSISDN+39.335.1234567
335.1234567
InternationalSwitching
Center
ISDN(UK)
MSRN+44.NDC.8877665
Routing calls to Roaming MSRouting calls to Roaming MS
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““tromboning”tromboning”
PLMN 1(ITA)
MSC
GMSC 1
HLR
PLMN 2(UK)
MSCISC(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
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Number portabilityNumber portability
Subscriber may switch operator without changing his number
First implemented in fixed networkmay 2002: extended to mobile networks
Essential for fair competition among network operatorsUK 2002 survey: 42% of corporate subscribers were willing
to change mobile operator; but 96% were, if number could be ported
Resistence from leading operatorsNumber portability helps newer operators to compete with
traditional ones
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NotationNotation
Donor switchThe switch corresponding to a “ported” telephone
numberRecipient switch
The switch to which the ported number is attached
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Technical solutionsTechnical solutionsa) call forwardinga) call forwarding
switch switch
switch
Originating network Donor network
Recipient network
Originating switch sets-up trunk to donor switchDonor switch sets-up trunk to recipient switchSimplest solution, as call forwarding is a feature available in virtually all switches
But extremely inefficient routing and trunking resource consumption!
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Technical solutionsTechnical solutionsb) query on releaseb) query on release
switch switch
switch
Originating network Donor 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
NumberPortabilityDataBase
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Technical solutionsTechnical solutionsc) all-call queryc) all-call query
switch switch
switch
Originating network Donor 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!
NumberPortabilityDataBase
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Mobile Number PortabilityMobile 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
HLRSignaling relay
function
GMSC
Recipient network
HLRMSC Note: If path must cross GMSC:Use Intermediate Routing Number
MSRN(or IRN)
MSRN IRN
Clearly, still suffers of tromboning!
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Return IRN
Mobile Number PortabilityMobile Number Portability(with all call query approach)(with all call query approach)
switch
Incoming call
GMSC
Recipient network
HLRMSC
IRN
NumberPortabilityDataBase
Query IRN
Return MSRN
Query MSRN
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Mobile Number PortabilityMobile Number Portabilityimproved – (with all call query approach)improved – (with all call query approach)
Return MSRNswitch
Incoming call
GMSC
Recipient network
HLRMSC
MSRN
NumberPortabilityDataBase
Query MSRN
Signaling relayfunction
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Wireless Cellular NetworksWireless Cellular Networks(basics) (basics)
Part 4 – GSM radio interface
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GSM Radio SpectrumGSM Radio Spectrum
2 x 25 Mhz bandDuplex spacing: 45 MHz
124 carriers x band200 KHz channelsSuggested 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
UPLINKMS BS
DOWNLINKBS MS
890.2
890.4
“guard band”
1 2 3 4 5 6 7 8
MHz12.02.935
MHz12.02.890
nnF
nnF
dwlink
uplink
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Adjacent channelsAdjacent channels(due to GMSK)(due to GMSK)
35dB60dB
Specification: 9dBIn practice, due to power control and shadowing, adjacent channelsCannot be used within the same cell…
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Physical channel 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
timeslot0
577 s
timeslot7
1 frame = 60/13 ms = 4.615 ms26 frames = 120 ms (this is the key number)
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Hybrid FDMA-TDMAHybrid FDMA-TDMAphysical channel = (time slot, frequency)physical channel = (time slot, frequency)
time
577us 577us 577us 577us 577us 577us 577us 577us
frequency
200 KHz
200 KHz
200 KHz
200 KHz
200 KHz
200 KHz
200 KHz
200 KHz
200 KHz
slot
Total n. of channels: 992
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Slow Frequency hoppingSlow Frequency hopping(optional procedure within (optional procedure within individualindividual cell) cell)
f1f2f3f4f5f6f7
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
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DuplexingDuplexing
0 1 2 3 4 5 6 7UPLINK
0 1 2 3 4 5 6 7 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!
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GP8.25
Structure of a TDMA slotStructure 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 rate13 kbps net rate!
S: 2 x 1 stealing bit Also called stealing flags, toggle bitsNeeded to grab slot for FACCH (other signalling possible)
TB3
DATA57
S1
S1
Trainingsequence
26
Data57
TB3
148 bit burst
156.25 bit (15/26 ms = 0.577 ms)
Normal burst
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Guard Period rationaleGuard Period rationale
Assume the following synchro mechanism:BTS transmits at time 0MS receives at time d/cMS transmits at time 3+d/cBTS receives at time 3+ 2d/c
Offset depending on d!
BTS
d
1 2 3 4BTS downlink tx
MS downlink rx 1 2 3 4
1
1
MS uplink rx
BTS uplink rx1
Expected RX time!
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Guard period sizingGuard period sizing
BTS timeMS time
dwlink slot 1 dwlink slot 4…
dwlink slot 1 dwlink slot 4…
uplink slot 1
uplink slot 1
…
…
Maximum cell radius:
KmC
GTcd
c
d
C
GT
rate
bits
rate
bits 5.42708332
25.8300000
2
2
Is there something wrong? (GSM says that cells go up to 35 km)
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Frame synchronizationFrame 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 bitsafter downlink slot
TA=63: Transmit after 405.75 bits time
BTS
TA (transmitted in the SACCH)
dwlink slot 4
uplink slot 1MS timeTA
dwlink slot 4
uplink slot 1uplink slot 1
BTS time
TA avoids collision!
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Timing Advance analysisTiming Advance analysis
Downlink propagation delay:d/c
Uplink propagation delay:d/c
Uplink delay with TA:d/c-TA
Perfect resynchronization occurs whenTA = 2d/c
Maximum cell size for perfect resync:
kmskm
sbits
bitsc
TAd 89.34/300000
/270833
5.31
2
8.25 bits Guard time additionally available for imperfect sync (+/- error)
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And when the user is not And when the user is not connected?connected?
But wants to connect…But wants to connect…
TB8
Trainingsequence
41
Data36
TB3
88 bit burst
156.25 bit (0.577 ms)
Access burst
Solution: USE A DIFFERENT BURST FORMATAccess Burst: much longer Guard Period availabledrawback: much less space for useful information
GP68.25
No collision with subsequent slot for distances up to 37.8 km
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Logical vs Physical channelsLogical vs Physical channels
Physical channelsTime 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)
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Logical – physical mappingLogical – 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 3kLogical channel B: data rate 2r/3, time slot i, frame 3k+1, 3k+2
Frame 8 Frame 9 Frame 10 Frame 11 Frame 12
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GSM logical channelsGSM logical channels
Traffic channel (TCH) TCH/F TCH full rate MSBSS
TCH/H TCH half Rate MSBSS
Broadcast channel BCCH Broadcast control BSSMS
(same information to all MS in a cell) FCCH Frequency Correction BSSMS
SCH Synchronization BSSMS
Common Control channel (CCCH) RACH Random Access MSBSS
(point to multipoint channels) AGCH Access Grant BSSMS
(used for access management) PCH Paging BSSMS
Dedicated Control channel (DCCH) SDCCH Stand-alone Dedicated control
MSBSS
(point-to-point signalling channels) SACCH Slow associated control MSBSS
(dedicated to a specific MS) FACCH Fast associated control MSBSS
Additional logical channels available for special purposes(SMS, group calls, …)
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Traffic channels (TCH/F)Traffic channels (TCH/F)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Periodic pattern of 26 frames (120 ms = 15/26 ms/TS * 8 TS/frame* 26 frame)
24 TCH frames over 26
20 1 3 4 5 6 7 20 1 3 4 5 6 7 20 1 3 4 5 6 7 20 1 3 4 5 6 7Same TS in every frame
GP8.25
TB3
DATA57
S1
S1
TrainingSeq. (26)
Data57
TB3
148 bit burst156.25 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
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Slow Associated Control Slow Associated Control ChannelChannel
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
TCH/F(0…7) TCH/F(0…7)
SACCH(0…7) IDLE frame
SACCH-0 SACCH-1 SACCH-2 SACCH-3 SACCH-4 SACCH-5 SACCH-6 SACCH-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
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SF BB BB PP PP SF PP PPPP PP SF PP PPPP PP SF PP PPPP PP SF PP PPPP PP
Broadcast Channel (usual) Broadcast Channel (usual) organizationorganization
51 frame structure vs 26235.38 ms period (vs 120 ms)
Sub-blocks with 10 framesStarting with Frequency Correction Channel (FCCH) Immediately followed by Synchronization Channel (SCH)
Other frames (numbered from #0 to #50):#50 idle#2,3,4,5 BCCHRemaining: Paging (PCH) / Access Grant (AGCH) [=PAGCH]
51 frame structure - downlink
10 frame sub-block
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BCCH contentsBCCH contents 184 bits
Coded in 456 bits and interleafed in 4 bursts same coding and interleaving as SACCHBCCH capacity
184 bits / (51*8*15/26 ms) ~ 782 bps Information provided
Details of the control channel configurationParameters 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 cellNeeded if frequency hopping is applied
List of BCCH carriers and BSIC of neighboring cells
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BCCH carrier placementBCCH carrier placement
On DownlinkCorresponding uplink dedicated to Random Access Channel
RR RR RR RR RR R
51 frame structure - uplink
On one frequency per cell (beacon)MUST BE on Time Slot #0Other Time slots may be used by TCH
Provided that:• All empty slots are filled with DUMMY bursts
• Downlink power control must be disabled
RR RR RR RR RR RR RR RR RR RR RR RR RR RR RRRR RR RR RR RR
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Fast Associated Control Fast Associated Control ChannelChannel
FACCH: urgent signallingUsed when several signalling information needs to be transmitted
Call setupHandover
FACCH block = 184456 after coding
Interleaved as voice blockSpreaded on 8 bursts
Replaces a voice block (20 ms) on the TCHVia stealing bitsVoice block(s) discarded
Maximum FACCH bit rate184*6/120 [bits/ms] = 9.2 kbps (vs 383 bps of SACCH!)
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FACCH insertion in TCHFACCH 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)
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Why pagingWhy pagingChannel assignment:
only upon explicit request from MSPaging
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/MSCMS
1) paging
3) Channel assignment
2) Random access
Paging channel: PCHAccess Grant Channel: AGCHRandom Access Channel: RACH
PAGCHCCCHCommon ControlCHannel
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PagingPaging
Paging message generated by MSCWhich receives incoming call
Transferred to subset of BSCPaging limited to user’s location areaPaging message contains:
List of cells where paging should be performedIdentity of paged user (IMSI or TMSI)
Paging message coded in 4 consecutive bursts over the air interfaceSame coding/interleaving structure of SACCH (184456 bits)
Paging for more MSs may be joined in one unique paging message
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An example procedure involving signallingAn 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
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Radio Resource allocationRadio Resource allocationthree standardized solutionsthree 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 processWaste of resources
RACH
RACH
RACH
VEA TCH (FACCH)
TCH (FACCH)Non-OACSU
OACSU
SDCCH
TCH (DATA)
SDCCH
TCH (DATA)
TCH (DATA)
Connection established Callee responds
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handoverhandover 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 maintainedThe 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 crossingBased 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 interruptedSoft Handover: when two channels are simultaneously set-up (old and new)
Not possible in GSM; possible in UMTS
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Hard, Seamless, Soft Hard, Seamless, Soft handoverhandover
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf1
MSC
BSS 1 BSS 2
MSf2
MSC
BSS 1 BSS 2
MS
MSC
BSS 1 BSS 2
MS
f2
f1
f2
f1f1
before during after
Hardhandover
(GSM)
Seamless(DECT)
Softhandover(UMTS)
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Handover classificationHandover classification
Rescue handover (mandatory handover)Driven by radio channel quality
degradation Confinement handover
(network-directed handover)Target: minimize radio interferenceAssign new channel when old
channel results critical for total interference
Traffic handover (network-directed handover)Driven by traffic congestion
conditionsAlso 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-MSCMost complex: need to change
MSC
Classification by motivation Classification by typology
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Types of handoverTypes of handover
A-MSC
BSC
BTS BTS BTS BTS
BSC BSC
R-MSC
A
A-bis
radiointerface
Anchor MSC: the MSC that first
managed the current call
Relay MSC: the MSC that currently
manages the call
Switchingpoint forinternal
handover
Switchingpoint for
all inter-MSChandover
Switchingpoint for
inter-BSChandover
Giuseppe Bianchi
Handover taxonomyHandover taxonomy BCHO: Base station Controlled Handover
Handover detection: BSHandover Execution: BS
MCHO: Mobile Controlled HandoverHandover detection: MSHandover Execution: MS
MAHO: Mobile Assisted HandoverHandover detection: MSHandover Execution: BS
GSM: somehow a BCHO with a flavor of MAHOHandover decision always taken by BSCBased on measures taken at both BTS and MSNew channel selection decision taken at BSC or R-MSC or A-MSC
(depending on handover type) based on traffic consideration
Giuseppe Bianchi
Handover preparationHandover preparation Measurements performed at BTS
Up-link signal level received from MS lower than thresholdRXLEV_UL < L_RXLEV_UL_H
Up-link signal quality (BER) received from MSRXQUAL_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 cellRXLEV_DL < L_RXLEV_DL_H
Down-link signal quality (BER) received from serving cellRXQUAL_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_1 0.2 0.4
RXQUAL_2 0.4 0.8
RXQUAL_3 0.8 1.6
RXQUAL_4 1.6 3.2
RXQUAL_5 3.2 6.4
RXQUAL_6 6.4 12.8
RXQUAL_7 12.8 -
Giuseppe Bianchi
Handover preparation – Handover preparation – additional metricsadditional 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 referencealgorithm, but implementation left to operator
Giuseppe Bianchi
handover procedure handover procedure skeletonskeleton
2) Switching point prepares new path on fixed net
2
1) Handover request goes up to switching point
1
MSC
BTSBTS
BSCBSC
3) Switching point sends HO command to MS
3
4) MS accesses new channel
4
5) Old channel/path torn down
5
Giuseppe Bianchi
Inter-MSC handoverInter-MSC handover More complex, as an ISDN circuit must be set
between MSCsWe’ll not enter into details (just the basic ideas)
Two cases
MSC-A MSC-R1
First MSC change (basic handover)
MSC-A MSC-R1
Second MSC change (subsequent handover)
MSC-R2
X
X
XNote the role of the
Anchor MSC!