GSM • Example of a PLMN (Public Land Mobile Network) • At present most successful cellular mobile system (over 200 million subscribers worldwide) • Digital (2 nd Generation) cellular mobile system operating in several frequency bands (GSM 900, GSM 1800 = DCS 1800, GSM 1900 = PCS 900) • ETSI Specifications ( www.etsi.org ) • Future evolution ?
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gsm - TKK Tietoli · PDF fileGSM/GPRS channel allocation example time (observation intervals) Allocation schemes are network operator dependent Dynamical allocation (circuit switched
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GSM• Example of a PLMN (Public Land Mobile Network)• At present most successful cellular mobile system
(over 200 million subscribers worldwide)• Digital (2nd Generation) cellular mobile system
operating in several frequency bands (GSM 900,GSM 1800 = DCS 1800, GSM 1900 = PCS 900)
• ETSI Specifications ( www.etsi.org )• Future evolution ?
GSM
Course requirements: ”Understanding Telecommunications” book by Ericsson(Part D – PLMN) + supporting material (= these slides)
GPRS
Course requirements: ”GPRS: Architecture, Protocols, and Air Interface” articleavailable at
(constructive use of the multipath channel)• Diversity techniques• Circuit vs. packet switched access• Protocols: random access, power control, handover
(with associated measurement procedures)
Radio interface – access techniques
Frequency divisionCode
division
Time divisiontime
code nr.
frequency
Radio interface - physical channels
Frame of length 8 time slots
T S S T T T T T T S S T
T T T T T T T T T T T T
T T T T T T T T T T T T
T T T T T T T T T T T T
Carrier 0
Carrier 1
Carrier 2
Carrier 3
TS2 TS2
TS0 TS1Typically used for signaling
Radio interface – logical channels
Traffic channels Control channels (for signaling)
TCH/F
TCH/H
Broadcast Common control Dedicated
SCH
FCCH
BCCH
PCH
AGCH
SDCCH
SACCH
FACCHRACH
bidirectionaldownlinkuplink
GPRS channel structure
PDCH:s for GPRS use (for example)
T S S T T T T T T S S T
T T T T T T T
T T T T T T T
Carrier 0
Carrier 1
Carrier 2
TS0 TS1Used for GSM signaling
PBCCH – Packet Broadcast Control Channel (optional)PCCCH – Packet Common Control Channel (optional)
PDTCH – Packet Data Traffic Channel
Also, PACCH and PTCCH possible
GSM/GPRS channel allocation example
time (observation intervals)
Allocation schemes are network operator dependent
Dynamicalallocation
(circuit switchedGSM channelsare given higherpriority)
Voice coding: 260 bits in 20 ms blocks (13 kbit/s) MS - BSC
Channel coding: 456 coded bits (22.8 kbit/s) MS - BTS
Interleaving: 8 x 57 bits (22.8 kbit/s)
260 bits
57 bits
260 bits
456 bits
57 bits 57 bits
bits 4, 12, 20, 28,36, 44, etc. fromthe 456 bit frame
GSM signaling message encoding
Signalling message in (split into) block(s) of 184 bits:
Each block is coded into 456 bits (22.8 kbit/s)
Interleaving: 8 x 57 bits (22.8 kbit/s)
184 bits
57 bits
456 bits
57 bits 57 bits
bits 4, 12, 20, 28,36, 44, etc. fromthe 456 bit frame
GPRS packet encoding
Interleaving: 8 x 57 bits (22.8 kbit/s)
LLC frameFH
BH
57 bits
Convolutional encoding into 456 bits
57 bits 57 bits
bits 4, 12, 20, 28,36, 44, etc. fromthe 456 bit frame
Info BCS T
Segment Segment Segment Segment ….
For instance, segment of IP packet FCS
RLC block
GSM protocols (MS Ø MSC)
CM
MM
RR
LAPDm
Radio
RR
LAPDm
Radio
BTSM
LAPD
64 kb/s
RR
BTSM
LAPD
64 kb/s
BSSAP
SCCPMTP
64 kb/s
CM
MM
BSSAP
SCCPMTP
64 kb/s
MS BTS BSC MSC
Um Abis A
BSSMAPDTAP
GPRS protocols (user plane)
IP/X.25 IP/X.25
SNDCP GTP
LLC TCP
SNDCP
LLC
RLC
MAC
Radio
RLC
MAC
Radio
BSSGP
NS
L1bis
BSSGP
NS
L1bis
IP
L2
L1
GTP
TCP
IP
L2
L1
MS BSS SGSN GGSN
Um Gb Gn
GSM signaling at ”layer 3”
RR (Radio Resource management)• Access and initial assignment (reserving a SDCCH)• Handover management
MM (Mobility Management)• Registration (MS power switch on)• Location updating (MS moves to another location area)• Authentication
CM (Call control Management)• Signaling for setting up and releasing circuit switched
connections (very similar to DSS 1 in N-ISDN)
Connectivity ”modes” in GSM and GPRS
GSM:
Disconnected MS is switched off (circuit mode)Idle location updates are performedConnected handovers in c.s. connection
GPRS:
Idle MS is switched off (packet mode)Standby location updates on a larger basisReady location updates on cell-by-cell basis
Random access in GSM / GPRS
No communication between MS and network can be startedwithout first using the random access procedure in
• network originated activity (paging, e.g. for MTC)• MS originated activity (MOC, location updating,
registration, de-registration at power switch-off)
1) MS sends a short access burst over the RACH (uplink), (Slotted Aloha, collision possibility Ø retransmission)2) Network (BSC) returns ”permission” message including:
- allocated channel (frequency, time slot)- timing advance for correct time slot alignment
Important identifiers in GSM
IMSI – International Mobile Subscriber Identity (global)TMSI – Temporary Mobile Subscriber Identity (local and
temporary)LAI – Location Area Identity (global)MSISDN – Mobile Subscriber ISDN number (address of
subscriber HLR database)PIN – Personal Identification Number (only within MS)IMEI – International Mobile Equipment Identity (global)Temporary, local numbers for routing (MSRN, HON …)
Case study: location updating (1)
Last LAI and TMSI stored in SIM.MS monitors broadcast LAI.LAI matching => everything ok !
VLR 1
VLR 2
HLRLAITMSI Ø IMSI
IMSILAI => VLR 1
MS
LAITMSI
SIM
LAI
Case study: location updating (2)
LAI different => location update needed !
VLR 1
VLR 2
HLRLAITMSI Ø IMSI
IMSILAI => VLR 1
MS
LAITMSI
SIM
LAI
Case study: location updating (3)
SIM sends old LAI and TMSI to VLR 2But, VLR 2 does not know IMSI!
VLR 1
VLR 2
HLRLAITMSI Ø IMSI
IMSILAI => VLR 1
MS
LAITMSI
SIM
LAILAITMSI
Case study: location updating (4)
VLR 2 sends new LAI to VLR 1which in turn updates the HLR
VLR 1
VLR 2
HLRLAITMSI Ø IMSILAI => VLR 2
IMSILAI => VLR 1LAI => VLR 2TMSI
MS
LAITMSI
SIM
LAILAITMSI
Case study: location updating (5)
HLR sends IMSI to VLR 2 andcancels subscriber data in VLR 1
VLR 1
VLR 2
HLRLAI
IMSILAI => VLR 2
MS
LAITMSI
SIM
LAILAITMSI Ø IMSI
Case study: location updating (6)
VLR 2 sends new TMSI to MS (SIM).MS also updates LAI. Compare withslide (1). Location update successful!
VLR 1
VLR 2
HLRLAIMS
LAITMSI
SIM
LAITMSI Ø IMSI
Important information for MTC (see below)
IMSILAI – VLR 2
Case study: mobile terminated call (1)(mobile terminated call = MTC)
MTC directed through PSTN to GMSC using MSISDN.GMSC contacts HLR (MSISDN is in fact the address ofthe register location of the subscriber with given IMSI)
GMSC
HLRLAITMSI => IMSI
MSISDN => IMSILAI => VLR 2
MS
LAITMSI
SIM MSISDNMSC
VLR 2
Case study: mobile terminated call (2)
HLR returns to GMSC the current LAI of called mobilesubscriber. The GMSC can now route the call to the MSCserving the subscriber
GMSC
HLRLAITMSI => IMSI
MSISDN => IMSILAI => VLR 2
MS
LAITMSI
SIM MSISDNLAI => VLR 2
MSC
VLR 2
Case study: mobile terminated call (3)
The MSC broadcasts a paging message (including TMSI)within the location area defined by LAI
GMSC
HLRLAITMSI => IMSI
IMSILAI => VLR 2
MS
LAITMSI
SIM
MSC
VLR 2
Case study: mobile terminated call (4)
Only the mobile subscriber with the correct TMSI reactsto the paging. A connection between MS and MSC isestablished and the call set-up is completed.