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1
Agenda, GSM & MPA Training course
• Agenda:
» Definition and History.
» GSM Services.
» GSM System architecture.
» GSM Functional model.
» GSM Radio (Um) Interface.
» GSM A-bis.
» GSM A-Interface.
» GSM A-Interface BSSAP.
» GSM A-Interface DTAP.
» GSM Inter-MSC Signalling MAP.
» GSM Signalling procedures
» Evaluation.
2
Global System
for
Mobile Communication
(Groupe Spéciale Mobile)
GSM
BTS
Definition and History
• Does mobile mean that you have
to be driving in a car ?
• Does it work in aeroplanes ?
• ………onboard ships in the ocean ?
• …….inside buildings ??
• ….. everywhere in the nature ?
?
?
?
?
? SONOFON
M N
SONOFON
M N
SONOFON
M N
SONOFON
M N
SONOFON
M N
What is Mobile telephony ??
Local
Exchange
Mobile
Switching
Centre
fixed
medium
Mobile
Station
fixed
location
Variable
Location
Diffuse
Medium
Telephone
SONOFON
M N
Basic concept - 1 GSM vs. fixed network Telephony
NMT 450/900
C-Netz
RTMS
AMPS
Nordic Countries, France, Belgium, Netherland, Switzerland,
Austria, Turkey, Yugoslavia, Thailand, Malaysia, North Africa
West Germany, Portugal
UK, Ireland, Italy, Spain, Austria, Greece, Hong Kong, China,
Malaysia, Thailand, Sri Lanka
USA, Canada, Australia, New Zealand, Malaysia, Pakistan
Singapore, Hong Kong
Analog Mobile Systems
RC 2000
Italy
France
TACS / ETACS
PCS 1900 GSM at
1900 MHz
C-NET
Analog
450 MHz
UMTS / IMT-2000 (FPLMTS)
CT2
DCS 1800 GSM at
1800 MHz
Cordless
Trunked
mobile radio
(TETRA)
Satellite
(IRIDIUM)
GSM Digital
900 MHz
Mobile
Other systems CT0,CT1
DECT
GSM Digital
900 MHz
NMT
Analog
450/900 MHz
TACS/ETACS
Analog
900 MHz
AMPS
Analog
800 MHz
GSM Digital
900 MHz
DAMPS (TDMA)
GSM Digital
900 MHz
CDMA
The evolution
8
• The GSM Standard is divided into phases (phase 1, phase 2 and phase 2+) all the phases has been finalized by ETSI.
• Many of the GSM networks in operation today are currently using the phase 2. However many of the GSM network operators are starting to implement phase 2+.
• The ETSI GSM standard specification is around 5500 pages, and are divided into12 series.
GSM Standard part 1
• Series 00 Preamble
• Series 01 General
• Series 02 Service aspects
• Series 03 Network Aspects
• Series 04 BS-MS interface / protocols
• Series 05 Physical layer of radio path
• Series 06 Speech Encoding
• Series 07 Adaptation techniques
• Series 08 BS-MSC interface
• Series 09 Interworking procedures (network)
• Series 10 Interworking between services
• Series 11 Equipment and type approval specifications
• Series 12 Operation and maintenance procedures
GSM Standard part 2
• GSM 900
» The original system
» Widely applied in EUR
• DCS 1800
» Typical expansion path when running out of capacity with GSM 900
• PCS 1900
» Widely used in the United States
GSM systems today
• Specification start-up: 1980
• First network in operation: Jan. 1992 (Radiolinja, Finland)
• Forecast in 1995:
» At the ITU's Telecom '95 event, were stated that we will reach 100 million
subscribers Worldwide before the year 2000.
• September 1997: ~55 million subscribers.
~1 new subscriber each second.
~250 networks in 110 countries.
• July 1998:
» More than15 months early then year 2000 the magic figure of 100 million
subscribers was reached.
• Today : Over 200 million subscribers.
369 networks in 137 countries.
Status
BTS
GSM Services
BTS
• Telephony
• Data services (up to 9600 b/s)
• Fax group 3 (special modem)
• Short Message Service (SMS)
• Supplementary services, e.g.
» Call Forwarding
» Call Barring
» Call Waiting
» Three Party Service
» Advice of Charge
Services
BTS
• Integrated voice/data (ISDN)
• Improved performance
• Improved security
» Digital encryption
» Authentication (IMSI)
» TMSI assignment
• All types of Mobile Stations
• Automatic roaming
• Sophisticated radio functions
» Discontinuous transmission - DTX
» Frequency hopping
GSM Features
BTS
• Half-rate and enhanced full-rate speech
• New supplementary services:
» Display of called and calling user's number
» Multi-party conversations (up to 6 parties)
» Closed user groups / virtual private networks
» Call completion services (busy, no answer etc.)
» Intelligent network services (CAMEL)
» Roaming between GSM and DCS 1800 (PCS 1900)
• High speed data services:
» High Speed Circuit Switched Data (HSCSD)
» General Packet Radio Service (GPRS)
Services, phase 2 and 2+
BTS
System architecture.
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
System Overview
Home
Location
Register
Visitor
Location
Register
Mobile
Switching
Centre
Base
Station
Controller
Base
Transceiver
Station
MSC BTS
BSC
VLR
HLR
SONOFON
M N
System Building Blocks
• “Home Base” of information regarding customers subscribing to
a particular operators GSM network
• Keeps track of subscriber profile, conditions and whereabouts
MSC BTS
BSC
VLR
HLR
SONOFON
M N
HLR (Home Location register)
BTS
• Subscriber information:
» IMSI (International Mobile Subscriber Identity)
» MSISDN (International Mobile Station ISDN Number)
» MS Category (e.g. payphone)
» Authentication vectors (RAND, SRES and Kc: AUC and
SIM)
» Allowed services (subscription data)
• Mobile location information:
» VLR number
» (MSRN - Mobile Station Roaming Number)
HLR contains
• Database with information about mobile users
present/active in the network segment served by the MSC
• Handles true visitors as well as subscribers of the operator himself
MSC BTS
BSC
VLR
HLR
SONOFON
M N
VLR (Visitor Location register)
BTS
• Subscriber information:
» IMSI
» TMSI - Temporary Mobile Subscriber Identity
» MS category
» Authentication vectors
» Allowed services
• Mobile location information:
» MSRN - Mobile Station Roaming Number
» LAI - Location Area Identity
VLR contains.
• Contains the radio transmitters and receivers (transceivers)
covering a certain geographical area of the GSM network
MSC BTS
BSC
VLR
HLR
SONOFON
M N
BTS (Base Transceiver Station)
• Controls a group of BTS’s in relation to power control and
handover.
• The combination of a BSC and its BTS’s is called a Base Station
Subsystem (BSS).
• The interface between the BTS and the is called the A-bis interface.
MSC BTS
BSC
VLR
HLR
SONOFON
M N
BSC (Base Station Controller)
• Serves a number of BSS’s (Base Station Subsystem) via the A-interface.
• Responsible for call control (set-up, routing, control and termination of the calls)
• Management of inter-MSC handover and supplementary services, and
for collecting charging/accounting information.
• Gateway to other to other GSM networks and public-switched networks)
BSC (Base Station Controller)
MSC BTS
BSC
VLR
HLR
SONOFON
M N
26
• Contains the individual subscriber-identification key (also contained in the SIM), and provides the subscriber data to the HLR and VLR used for authentication and encryption of calls.
AUC
HLR
AUC - Authentication Centre
BTS
27
BTS
• Stores information about mobile stations in use
and may block calls from a MS if the MS is stolen,
not type-approved or has faults which may disturb
the network.
• Each MS is identified by a unique International
Mobile Station Equipment Identity (IMEI)
MSC
EIR
EIR - Equipment Identity Registration.
• Power:
» - Class 1: 20 W Vehicle/
» - Class 2: 8 W portable
» - Class 3: 5 W Hand-held
» - Class 4: 2 W Hand-held
» - Class 5: 0.8 W Hand-held
(MS) Mobile Station
BTS
Functional model.
30
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
The overview of the System.
• Call Management (CM)
» Call Control (CC)
» SMS
» Non Call-related SS
• Mobility Management
(MM)
• Radio Resource
Management (RR) CM
MM
RR
BTS BSC MSC
VLR
HLR SONOFON
M N
A functional model
32
• DTAP - Direct Transfer Application Part
• BSSMAP - BSS Management Application Part
• CM - Call Management
• MM - Mobile Management
• RR - Radio Resource Management
• BTSM - BTS Management
• SCCP - Signalling Connection Control Part
• MAP - Mobile Application Part
• TCAP - Transaction Capability Application Part
• ISUP - ISDN User Part
• MTP - Message Transfer Part
DTAP CM
MM
RR
Sig. layer 2
Layer 1 (air)
Sig. layer 2
Sig. layer 1
MTP
SCCP
BSSMAP RR
(CM+MM)
MS BSC
MTP MTP
SCCP SCCP
CM
MM
BSSMAP TCAP
MAP
I
S
U
P
MSC
Sig. layer 2
Layer 1 (air)
BTS
(CM)
(MM)
(RR)
(CM)
(MM)
(RR)
(CM)
(MM)
(LAPDm) (LAPDm) (LAPD)
Sig. layer 2
Sig. layer 1
(LAPD)
BTSM BTSM RR'
Um Interface Abis Interface A Interface Inter-MSC
Layer 1
Layer 2
Layer 3
GSM Protocol Architecture
33
• The Network layer contains the signalling procedures
and is divided into:
» CC - Call Management.
» MM - Mobility Management.
» RR - Radio Resource Management.
Layer 3 Network Layer
BTS
34
BTS
• Call Management takes care of the ordinary
call-control procedure:
» Establishment and release of calls, as well as access
to services and facilities.
• CM is divided into:
» Call Control (CC), short messages services (SMS).
» Non-call-related supplementary services (SS).
Layer 3 Call Management (CM)
35
• Mobility Management handles roaming and
authentication procedure.
Layer 3 Mobility Management (MM)
BTS
36
• Radio Resource Management comprise:
» Paging.
» Radio-channel access.
» Ciphering.
» Handover.
» Radio-signal control
» Radio-signal measurement
BTS
Layer 3 Radio Resource Management (RR)
37
BTS
• The Data Link Protocol is used at the Um and A-bis interface, the Data Link Protocol is based on LAPD (ISDN D-channel layer 2 protocol).
• On the A-Interface MTP and SCCP are used as signaling-transport function.
• On the inter-MSC interface, MTP is used for ISUP, TUP and MTP + SCCP + TCAP is used for MAP.
Layer 2 Data Link Protocol.
38
• Physical Link of the signaling is time slots in
the radio carriers and digital PCM lines.
BTS
Layer 1 Physical Link
Um
BTS
Radio (Um) Interface
The System
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base
Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
41
• DTAP - Direct Transfer Application Part
• BSSMAP - BSS Management Application Part
• CM - Call Management
• MM - Mobile Management
• RR - Radio Resource Management
• BTSM - BTS Management
• SCCP - Signalling Connection Control Part
• MAP - Mobile Application Part
• TCAP - Transaction Capability Application Part
• ISUP - ISDN User Part
• MTP - Message Transfer Part
DTAP CM
MM
RR
Sig. layer 2
Layer 1 (air)
Sig. layer 2
Sig. layer 1
MTP
SCCP
BSSMAP RR
(CM+MM)
MS BSC
MTP MTP
SCCP SCCP
CM
MM
BSSMAP TCAP
MAP
I
S
U
P
MSC
Sig. layer 2
Layer 1 (air)
BTS
(CM)
(MM)
(RR)
(CM)
(MM)
(RR)
(CM)
(MM)
(LAPDm) (LAPDm) (LAPD)
Sig. layer 2
Sig. layer 1
(LAPD)
BTSM BTSM RR
Um Interface Abis Interface A Interface Inter-MSC
Protocol Architecture
• GSM has been assigned 1000 radio channels in the 900
MHz band. More precisely:
» 890 - 915 MHz “Uplink”
» 935 - 960 MHz “Downlink”
• A combination of frequency and time division is used.
» 124 carriers
» Carrier spacing is 200 kHz
» 8 timeslots per carrier
BTS
SONOFON
M N
GSM 900 Radio (Um) Interface Physical Channels
• GSM has been assigned 2992 radio channels in the 1800
MHz band. More precisely:
» 1710 - 1785 MHz “Uplink”
» 1805 - 1880 MHz “Downlink”
• A combination of frequency and time division is used.
» 374 carriers
» Carrier spacing is 200 kHz
» 8 timeslots per carrier
BTS
SONOFON
M N
DCS-1800 Radio (Um) Interface Physical Channels
• Except for the difference in power level range and
frequency, PCS-1900 are identical to DCS-1800.
• The frequency shift is required in US because of presence
of some point to point radio links on the 1800 MHz band.
• A combination of frequency and time division is used.
» 299 carriers
» Carrier spacing is 200 kHz
» 8 timeslots per carrier
PCS-1900 Radio (Um) Interface Physical Channels
BTS
SONOFON
M N
BTS
TDMA Frame
Time Slot
4.615 msec
3 57 1 26 1 57 3
TB Coded Data C TS C Coded Data TB
8.25
GP
0.577 msec
Duration of 1 bit: 3.692 usec
0 1 2 3 4 5 6 7
TS : Tail bit
TS : Training Sequence (setting up the receiver equaliser)
GP : Guard Period
C : Control bit
13 kbit/s user data
TDMA Frame Structure
46
BTS
Physical Channels
TS 5
TS 4
TS 3
TS2
TS
1 TS0
TS 4
TS 3
TS2
TS
0 TS7 TS6
TS 5
TS7 TS6
Control Channels
Control Channels
Traffic Channels
Traffic channels
showing three timeslot
delays between the
down and up links.
Eight TS, or eight physical
channels compromise a
FRAME
TS 2
TS1
TS 0
TS7
TS
6 TS5
TS 0
TS 7
TS6
TS
5 TS4 TS3
TS 1
TS3 TS2
Downlink Uplink
47
FCCH
burst 3 142 3
8.25
Guard Tail Tail Information
Tail Information Training
3 57+1 (TCH/FACCH) 26 57+ 1 (TCH/FACCH) 3 Normal
burst
8.25
Guard Tail Information
Access
burst 7 41 36 3
Guard Tail Tail
68,25
Information Training
SCH
burst 3 39 64 39 2
8.25
Guard Tail Tail Information Information Training
» Not illustrated is the “dummy” burst which has the same structure as
the “Normal” burst. The dummy burst is sent when no information is
transmitted on a TCH
Radio (Um) Interface burst modulation structure
48
• For the “Normal” burst, one of the 58 information bits on
each side of the training sequence is a flag bit indicating
whether the burst is a TCH - traffic channel (0) or for a
FACCH - fast associated control channel (1).
• The burst is converted to FACCH when signalling is
required after a TCH has been allocated.
» Note: Each “Normal” burst (TDMA) time slot period consist of
156.25 bits (equal to 33.9 kbit/s per time slot or 270.8 kbit/s
per frame carrier), of which 144 (2*57) bits are coded data
including forward error correction. All information is transferred
in blocks of 456bits divided into four time slot periods (456 =
4*2*57). The maximum net bit rate is 13 kbit/s (Excluding the
error correction)
Tail Information Training
3 57+1 (TCH/FACCH) 26 57+ 1 (TCH/FACCH) 3 Normal
burst
8.25
Guard Tail Information
Radio (Um) Interface Normal burst
49
• The “access” burst is a shortened burst used by the mobile station
when it first access a cell.
• Its short length guarantees it will arrive within the correct time slot
at the BTS receiver if the mobile station is no greater than 35km
from the BTS.
» 68,25 bits * 3,7gs (1 bit) = 251gs ~ (75Km / 2) = 37,5Km
Access
burst 7 41 36 3
Guard Tail Tail
68,25
Information Training
Radio (Um) Interface Access burst
• Downstream:
» A series of bits intended for different
users, who must select only the one
intended for him and filter out the rest
• Upstream:
» Individual bits from each of the users
arrive at the BTS
» Strict timing of when the MS should
transmit is required to avoid collisions at
the BTS
Time Division Multiple Access burst
Wrong Uplink Timing
BTS
SONOFON
M N
SONOFON
M N
SONOFON
M N
SONOFON
M N
51
15 Km 30 Km 1 Km
BTS
SONOFON
M N
SONOFON
M N SONOFON
M N
TS TS TS TS TS TS
TS
Timing
advance
Access
burst • The transmitted radio burst from BTS must travel whatever the
distance is to the mobile station , and then transmitted burst from the mobile station (three burst later) must travel back the same distance. By measuring the time between the last bit in the access burst and the last bit in the TS the mobile then know the distance to the BTS and will adjust its Timing advance to compensate for the distance.
Timing advance (Access burst)
52
Mobile Station Timing advance Measurement Report
Message Type : 3Fh = Immediate assignment
--- Channel description ---
Time slot number : .....001
Channel type and TDMA offset : 01011... = SDCCH/8 + SACCH/C8 or CBCH
(SACCH/8)
Training Sequence Code : 011.....
Hopping channel : ...0.... = Single RF channel
spare : ....00..
Absolute RF Channel Number : 720
--- Request reference ---
Random access information : 4
T1 : 7
T3 : 19
T2 : 7
--- Timing advance ---
Timing advance value : ..000010
Spare : 00......
Timing advance = 2
The mobile station is 1km
from the BTS.
53
• The SCH burst is the synchronization channel burst which carries
the the BSIC - Base Station Identity Code and the FN - Frame
Number.
• As this is the first burst decoded by the mobile station it has an
extended training sequence.
SCH
burst 3 39 64 39 2
8.25
Guard Tail Tail Information Information Training
SCH burst
54
• The FCCH burst is the frequency correction channel burst which
is modulated with zero
FCCH
burst 3 142 3
8.25
Guard Tail Tail Information
FCCH burst
BTS
• Traffic channels (TCH):
» Carrying Voice/data
» Bm: 13 kbit/s user data
» Lm: Half rate (6,5 kbit/s)
• Common control channels (CCCH):
» Channels that all Mobile Stations can share
• Dedicated control channels (DCCH):
» Control channels for individual Mobile Stations
Radio (Um) Interface Logical Channels
BTS
• Broadcast: BCCH
» Carry system info intended for everybody, e.g.
Location Area Identity
• Paging: PCH
» To request a specific Mobile User to react/reply, e.g.
when there is a call for him
• Random Access: RACH
» Used by the Mobile Station to initiate contact with the
network, e.g. when trying to start a call
• Access Granted: AGCH
» Used to respond to the RACH to inform that the
Mobile is now being allowed to access the network
Radio (Um) Interface Common Control Channels
BTS
• Stand-alone Dedicated : SDCCH
» Used for settling practicalities such as roaming,
authentication, encryption and call control before
allocating the traffic channel
• Slow Associated: SACCH
» Associated to a TCH
» Used together with the Traffic Channel to deal with
control and measurement of radio signals
• Fast Associated: FACCH
» Large bandwidth version of the SACCH
» Used for sudden control action such as handovers
» Implemented a robbed bits in a TCH
Radio (Um) Interface Dedicated Control Channels
1 hyperframe = 2048 superframes (3h 28min 53s 760ms)
0 1 2 3 4 5 6 7
1 TDMA frame = 8 timeslots (4.615
ms)
0 1 2 3 4 5 6 2042 2043 2044 2045 2046 2047
0 1 2 3 4 5 6 7 18 19 20 21 22 23 24 25
1 superframe = 26 (51-frames) or 51 (26-frames) multiframes
(6.12s)
0 1 2 3 4 5 45 46 47 48 49 50 6
0 1 2 3 4 5 6 7 18 19 20 21 22 23 24 25
1 (26-frame) multiframe = 26 TDMA frames (120ms)
1 (51-frame) multiframe = 51 TDMA frames (235.38 ms)
0 1 2 3 4 5 45 46 47 48 49 50 6
0 1 2 3 4 5 6 7
1 TDMA frame = 8 timeslots (4.615
ms)
Hyper-, Super- and Multiframes
Downlink and Uplink
Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm SA Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm Bm -
0 1 2 3 4 5 6 7 TDMA Frame (8 timeslots)
F S BCCH PCH/AGCH F S F S SDCCH/1 SDCCH/2 F S SDCCH/3 SDCCH/4 F S SACCH/1 SACCH/2 - PCH/AGCH PCH/AGCH
F S BCCH PCH/AGCH F S F S SDCCH/1 SDCCH/2 F S SDCCH/3 SDCCH/4 F S SACCH/3 SACCH/4 - PCH/AGCH PCH/AGCH
R R SACCH/1 SACCH/2 R R R R R R SDCCH/1 SDCCH/2 R R SDCCH/3 SDCCH/4
R R SACCH/3 SACCH/4 R R R R R R SDCCH/1 SDCCH/2 R R SDCCH/3 SDCCH/4
R R R R R R R R
R R R R R R R R
R R R R R R R R
R R R R R R R R
Cyklus: 1 TCH multiframe = 26 TDMA frames = 120 ms
4.615 ms
Downlink: Cyklus: 1 CCH multiframe = 51 TDMA frames = 235.38 ms
F = Frequency correction burst S = Synchronisation burst
R = RACH
R
R
Uplink:
TDMA Frame with 1 combined CCH and 7 TCH
R 5xR
F
1
F
2
F3 F4
F
5 F6
F4
Omni-directional
BTS
3-directional BTS
Safety
distance BTS
BTS
BTS
BTS • To avoid interference between two cells using the
same frequency, a safety distance of about 5 times
the cell radius is required.
• A BTS may cover one cell (Omni-directional) or
several cells (typical three directional cells).
• Each cell may be served by on or more TRXs
depending on the required capacity.
• Note: each TRX controls one carrier with eight TS.
Cell Structure
61
BTS
• Mobile station in IDLE mode
» Besides listening to the BCCH and the PCH the mobile
station is measuring for neighbour cells.
• Mobile station in active mode
» In active mode the mobile station is using the time
between the down and uplink TS (three TS 2ms) to do
neighbour cell measuring.
• The mobile station can measure up to 31
neighbour cells.
» In practice the mobile station measures up to 12
neighbour cells.
» Very often only three or four cells are measured.
Radio (Um) Interface Neighbour Cells
62
Mobile Station Neighbour Cells Measurement Report
--- MEAS REP ---
--- MEAS RES ---
NO NCELL M : 100b = 4 neighbour cell measurement result
RXL NCEL 1 : 36 = minimum received signal level = -75 dBm to -74 dBm
BCCH NCEL1 : 1
BSIC NCEL1 : 57
RXL NCEL 2 : 24 = minimum received signal level = -87 dBm to -86 dBm
BCCH NCEL2 : 12
BSIC NCEL2 : 63
RXL NCEL 3 : 23 = minimum received signal level = -88 dBm to -87 dBm
BCCH NCEL3 : 7
BSIC NCEL3 : 59
RXL NCEL 4 : 16 = minimum received signal level = -95 dBm to -94 dBm
BCCH NCEL4 : 2
BSIC NCEL4 : 56
RXL NCEL 5 : 0 = minimum received signal level less than -110 dBm
BCCH NCEL5 : 0
BSIC NCEL5 : 0
RXL NCEL 6 : 0 = minimum received signal level less than -110 dBm
BCCH NCEL6 : 0
BSIC NCEL6 : 0
63
• IMSI
» - International Mobile subscriber Number
• MSISDN
» - Mobile Station ISDN Number
• Latest BCCH List
» The latest BCCH used last time the mobile station was
connected to the network.
• Preferred Network List.
• Forbidden Network List.
• KI
» The Key identifier refers to an authentication key for the
mobile subscriber.
(MS) Mobile Station SIM Card
BTS
A-bis
The System.
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
66
• DTAP - Direct Transfer Application Part
• BSSMAP - BSS Management Application Part
• CM - Call Management
• MM - Mobile Management
• RR - Radio Resource Management
• BTSM - BTS Management
• SCCP - Signalling Connection Control Part
• MAP - Mobile Application Part
• TCAP - Transaction Capability Application Part
• ISUP - ISDN User Part
• MTP - Message Transfer Part
DTAP CM
MM
RR
Sig. layer 2
Layer 1 (air)
Sig. layer 2
Sig. layer 1
MTP
SCCP
BSSMAP RR
(CM+MM)
MS BSC
MTP MTP
SCCP SCCP
CM
MM
BSSMAP TCAP
MAP
I
S
U
P
MSC
Sig. layer 2
Layer 1 (air)
BTS
(CM)
(MM)
(RR)
(CM)
(MM)
(RR)
(CM)
(MM)
(LAPDm) (LAPDm) (LAPD)
Sig. layer 2
Sig. layer 1
(LAPD)
BTSM BTSM RR'
Um Interface Abis Interface A Interface Inter-MSC
Protocol Architecture
TS 1 TS 31 TS 5 - - - -
16 kbit/s traffic channels
64 kbit/s signalling channels
TS = 64 kbit/s timeslot
TS 2 TS 0 TS 3 TS 4
• One 2Mbit/s line may cover several BTSs. This means that
normally several time slots in the same PCM frame are used as
signalling channels.
» Three time slots divided into one 64Kbit/s signalling channel and eight
16Kbit/s traffic channels are sufficient to cover one TRX, giving up to 10 TRXs
and 10 signaling channels per 2 Mbit/s.
» In practice , the configuration of the transmission lines depends on the actual
network structure and the GSM equipment used.
A-bis (A) Layer 1 Structures
68
Flag
01111110
Flag
01111110 16 Bits
CRC Information
N - Bits
Control Address
16 Bits 8 or 16 Bits
SAPI
TEI
C/R EA 0
EA 1
SAPI value
0
1
16
62
63
Related entity
Radio signalling
Reserved for packet mode /Q.931
Reserved for packet mode /X.25
Operation and maintenance
Layer 2 management
TEI value
0-63
64-126
For fixed TRX addresses
For additional TRX addresses
User type
All others Reserved for future standardisation
Not used in GSM
Vendor-specific
• SAPI -Service Access Point Identifier
• TEI - Terminal End Point Identifier
• C/R -Command / Response bit
• EA -Address Extension bit
» 0 = Extend 1 = Final
A-bis Layer 2 Structure
Flag
01111110
Flag
01111110 16 Bits
CRC Information
N - Bits
Control Address
16 Bits 8 or 16 Bits
8 7 6 5 4 3 2 1
0
1
1
0
1
S S
M M
OCTET 1 OCTET 2 + 3 OCTET 4 ( + 5 ) OCTET N
OCTET 4
OCTET 4
OCTET 4 M M M
P
P/F
P/F
N ( R )
N ( R )
N ( S )
Control field bits
( modulo 8 )
I format
S format
U format
8 7 6 5 4 3 2 1
0
1
1
0
1
S S
M M
OCTET 4
OCTET 4
OCTET 4 M M M P/F
N ( R )
N ( R )
N ( S )
Control field bits
( modulo 128 )
I format
S format
U format
P 5
X X X X
P/F 5
• N(S) - Transmitter send sequence number
• N(R) - Transmitter receive sequence number
• S -Supervisory function bit
• M -Modifier function bit
• P/F - Poll bit when issued as a command Poll
bit when issued as a command Final bit when
issued as a response
• X - Reserved and set to 0
• I forma - Information transfer format
» Used for information transfer
between layer 3 entities
• S format - Supervisory format
» Used for control functions
• U format - Unnumbered format
» Used for additional control
functions and information
transfer
A-bis Layer 2 Control Field
Flag
01111110
Flag
01111110 16 Bits
CRC Information
N - Bits
Control Address
16 Bits 8 or 16 Bits
Message Discriminator
1 2 3 4 5 6 7 8
Octet 1
Octet 2
Octet n
EM Message Type
Information Elements
8 7 6 5 4 3 1 2
Fixed Length Info. Element Format
Information Element Identifier
Length of Information Elements
Content of Information Elements
Variable Length Info. Element Format
T
Information Element Identifier
Content of Information Elements
T:
EM
=
=
0: Non-transparent message
Extension bit (future use)
Message Discriminator
0
1
4
6
8
Other
Reserved
Radio Link Management
Dedicated Channel Management
Common Channel Management
Transceiver Management
Reserved for Future Use
A-bis Layer 3 Structure I
Radio Link Layer Management messages
DATA REQuest (Transfer of transparent messages in layer 2
DATA INDication I-frames on radio interface)
ERROR INDication (Indicates protocol error on radio link layer)
ESTablish REQuest (Establishment of layer 2 link on radio
interface)
ESTablish CONFirm
ESTablish INDication
RELease REQuest (Release of layer 2 link on radio interface)
RELease CONFirm
RELease INDication
UNIT DATA REQuest (Transfer of transparent messages in layer 2 UI-
UNIT DATA INDication frames on radio interface)
0000- - - -
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
Message type Code
A-bis Messages, MD=1
Dedicated Channel Management messages:
CHANnel ACTIVation (Activation of a radio channel)
CHANnel ACTIVation ACKnowledge
CHANnel ACTIVation Negative ACKnowledge
CONNection FAILure INDication (Failure on radio connection)
DEACTIVATE SACCH
ENCRyption CoMmanD (Start of ciphering on radio interface)
HANDOver DETection (MS handover to new BTS detected)
MEASurement RESult (Radio signal measurement data from BTS/MS)
MODE MODIFY REQuest(Change of channel mode, e.g. speech to data)
MODE MODIFY ACKnowledge
MODE MODIFY Negative ACKnowledge
A-bis Messages, MD=4, part 1
Message type Code
001- - - - -
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
Dedicated Channel Management messages:
PHYsical CONTEXT REQuest(Physical context is not specified by ETSI)
PHYsical CONTEXT CONFirm
RF CHANnel RELease (Release of radio channel)
MS POWER CONTROL (Change of MS power level or control limits)
BS POWER CONTROL (Change of TRX power level or control limits)
PREPROCess CONFIGure(Conveys pre-processing parameters to BTS)
PREPROCessed MEASurement RESult (From BTS)
RF CHANnel RELease ACKnowledge
001- - - - -
01100
01101
01110
01111
10000
10001
10010
10011
A-bis Messages, MD=4, part 2
Message type Code
Common Channel Management messages:
BCCH INFOrmation (Indicates new information to be sent on BCCH)
CCCH LOAD INDication (Indicates load on RACH and PCH)
CHANnel REQuired (Reception of RR Channel Request message)
DELETE INDication (Deletion of RR Immediate Assign message due
to overload on AGCH)
PAGING CoMmanD (Requests paging of MS)
IMMediate ASSign CoMmanD (Setup of DCCH, answer to CHAN REQ)
SMS BroadCast REQuest (Broadcast of SMS-message in cell)
A-bis Messages, MD = 6
Message type Code
00010 - - - 001
010
011
100
101
110
111
TRX Management messages:
RF RESource INDication (Interference level on idle radio channels)
SACCH FILLing (New filling information to be used on SACCH)
OVERLOAD (Control channel or TRX processor overload)
ERROR REPORT (Detection of errored message)
00011 - - -
001
010
011
100
A-bis Messages, MD=8
Message type Code
- Channel number (Indicates channel on radio interface)
- Link identifier (Signalling link and SAPI used on radio interface)
- Activation type (Intra-cell, inter-cell or additional assignment CHAN ACTIV)
- BS power (BTS/TRX power level)
- Channel identification (Description of channels allocated to MS)
- Channel mode (Indicates discontinuous transmission and channel type, e.g.
speech)
- Encryption information (Encryption algorithm and key)
- Frame number (On radio interface, modulo 42432)
- Handover reference (Identical to handover reference in RR information elements)
- L1 information (MS power level and timing advance)
- L3 information (Contains transparent RR, MM or CM message)
- MS identity (IMSI or TMSI)
- MS power (MS power level)
- Paging group (Identifies MS paging group)
- Paging load (Load on paging channel, PCH)
- Physical context (Not specified)
- Access delay (Delay of MS access burst at random access or handover)
- RACH load (Load of random access channel, RACH)
- Request reference (Random ref. in RR Channel Request message)
A-bis Information Elements, part 1
- Release mode (Normal release or local end release)
- Resource information (Interference level for idle TRX channels)
- RLM cause (Indicates protocol error on radio link layer)
- Starting time (Expressed as Frame Number modulo 42432)
- Timing advance (To be used by MS in subsequent communications)
- Uplink measurements (Radio signal measurement results from TRX)
- Cause (Reason for event/failure)
- Measurement result num (For a radio channel; set to 0 at activation)
- Message identifier (In ERROR REPORT message: Message type of errored message)
- Message indicator (In ERROR REPORT message: Copy of errored message follows)
- System info type (Type of RR System Information message)
- MS power parameters (Limits set by BSC for BTS control of MS power)
- BS power parameters (Limits set by BSC for BTS control of TRX power)
- Preprocessing param. (For preproc. of radio measurement data in BTS)
- Preprocessed
measurements (Preprocessed radio measurement data)
- Immediate assign info (Conveys complete RR Immediate Assign msg.)
- SMSCB information (SMS-message to be broadcasted in a radio cell)
A-bis Information Elements, part 2
Bm + ACCHs
Lm + ACCHs
SDCCH/4 + ACCH
SDCCH/8 + ACCH
BCCH
Uplink CCCH (RACH)
Downlink CCCH (PCH + AGCH)
TDMA timeslot number
C5 C4 C3 C2 C1
0 0 0 0 1
0 0 0 1 T
0 0 1 T T
0 1 T T T
1 0 0 0 0
1 0 0 0 1
1 0 0 1 0
TN = 0 - 7
Element identifier
C5 C4 C3 C2 C1 TN
8 7 6 5 4 3 2 1 • C5 - C1 (Channel Number )
describes the types of radio
channel used
• TN is the physical TDMA
time slot number that the
radio channel is using. It is
coded 0-7 in binary
representation
(There are 8 timeslots per
TRX)
Channel number information element
Flag
01111110
Flag
01111110 16 Bits
CRC Information
N - Bits
Control Address
16 Bits 8 or 16 Bits
Message Discriminator
1 2 3 4 5 6 7 8
Octet 1
Octet 2
Octet n
EM Message Type
Channel Number
T
Message Discriminator := 1 (Radio Link Management)
L3 Information
Protocol
Discriminator
0 Message Type
Information Elements
Protocol Discriminator
3
5
6
9
11
15
Call control, packet, suppl. service
Mobility management
Radio resources management
Short message services
Non call related suppl. services
Reserved for test procedures
Other Reserved for future use
T := 1 (transparent message)
TI
Flag
TI
Value
Transaction Identifier
TI-flag := 0 Message is sent from the TI-originating side
TI-flag := 1 Message is sent to the TI-originating side
TI-value := 0-7 Reference Information Elements
• All CM and MM messages as well as most of the RR messages are transferred
across the A-bis interface inside a L3 information element in A-bis layer 3 Radio
Link management messages.
A-bis Layer 3 Structure II
80
BTS
BSC BTS
ENCRyption CoMmanD (Ciphering Mode Command [RR])
ESTablish INDication {MS establishes layer 2 on TCH}
DEACTIVATE SACCH {on BTS}
RF CHANnnel RELease {release of TCH}
DATA REQuest (Release [CM])
DATA INDication (Release Complete [CM])
RF CHANnel RELease {release of SDCCH}
DATA INDication (Assign Complete [RR]) {MS now ready on TCH}
DATA REQuest (Assign Command [RR]) {assigns TCH to MS}
CHANnel ACTIVation ACKnowledge {TCH activ}
DATA INDication (Ciphering Mode Complete [RR])
CHANnel ACTIVation {activation of TCH}
CHANnel REQuired {MS requests DCCH}
CHANnel ACTIVation {activation of SDCCH}
CHANnel ACTIVation ACKnowledge {SDCCH activ}
IMMediate ASSign CoMmanD {assigns SDCCH to MS}
ESTablish INDication (CM Service Request [MM]) {L2 up on SDCCH}
DATA INDication (Setup [CM])
DATA REQuest (Call Proceeding [CM])
DATA REQuest (Alerting [CM]) {call setup continues on TCH}
RELease INDication {MS releases layer 2 on SDCCH}
RF CHANnel RELease ACKnowledge {SDCCH released}
.
RELease INDicaton {MS releases layer 2 on TCH}
DATA INDication (Disconnect [CM]) {MS disconnects call}
DATA REQuest (Channel Release [RR]) {to MS, deactivation of TCH}
RF CHANnel RELease ACKnowledge {TCH released}
Active Call
DATA REQuest (Connect [CM])
DATA INDication (Connect Acknowledge [CM])
DATA REQuest (Authentication Request (RAND) [MM])
DATA INDication (Authentication Response (SRES) [MM])
A-bis signalling example
BTS
A-Interface.
The System.
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
83
• DTAP - Direct Transfer Application Part
• BSSMAP - BSS Management Application Part
• CM - Call Management
• MM - Mobile Management
• RR - Radio Resource Management
• BTSM - BTS Management
• SCCP - Signalling Connection Control Part
• MAP - Mobile Application Part
• TCAP - Transaction Capability Application Part
• ISUP - ISDN User Part
• MTP - Message Transfer Part
DTAP CM
MM
RR
Sig. layer 2
Layer 1 (air)
Sig. layer 2
Sig. layer 1
MTP
SCCP
BSSMAP RR
(CM+MM)
MS BSC
MTP MTP
SCCP SCCP
CM
MM
BSSMAP TCAP
MAP
I
S
U
P
MSC
Sig. layer 2
Layer 1 (air)
BTS
(CM)
(MM)
(RR)
(CM)
(MM)
(RR)
(CM)
(MM)
(LAPDm) (LAPDm) (LAPD)
Sig. layer 2
Sig. layer 1
(LAPD)
BTSM BTSM RR'
Um Interface Abis Interface A Interface Inter-MSC
Protocol Architecture
84
BTS
• Based on System 7 MTP and SCCP
• Uses Base Station Subsystem Application Part
(BSSAP)
» BSS Management Application Part (BSSMAP)
• Radio Resource (RR) and BSC management
• Uses SCCP connectionless service
» Direct Transfer Application Part (DTAP)
• Transfer of Call Control (CM) messages
• Transfer of Mobility Management (MM) messages
• Uses SCCP connection-oriented service
A-Interface
85
Signalling System Number 7
MTP Level 3
Physical
Data Link
Network
Transport
Session
Presentation
Application
OSI Layer SS7 Levels
7
6
5
4
3
2
1 MTP Level 1
MTP Level 2
Physical MTP Level 1
Data Link 2 MTP Level 2
MTP Level 3 Network 3
I
S
U
P
SCCP
TCAP
T
U
P
IN, MAP
I
S
U
P
Transport
Session
Presentation
Application
SCCP
• Level 4/User Parts
• SCCP
Basic format of MSU - SCCP message
Label SIF F CK F I B
FSN F B I B
BSN LI > 2 SIO
User Data MTC SLS Originating
Point Code
Destination
Point Code
N x 8 bits 8 bits 14 bits 14 bits 4 bits
Mandatory fixed part
Mandatory variable part
Optional part
Service Indicator
0 0 1 1 Sub-service
Field
=== MTP ===
BSN : 66
BIB : 0.......
FSN : 4
FIB : 1.......
LI : 28 = MSU
SPARE : 00......
SIO : 03h =
SCCP
87
SCCP message format
• An SCCP Messages contains the
following information.
» Routing label.
» Messages type.
» Mandatory fixed part.
» Mandatory variable part.
» Optional part.
Pointer to parameter P
Length Indicator of parameter M
Parameter M
Length Indicator of parameter P
Parameter P
Parameter name = X
Length Indicator of parameter X
Parameter X
Routing Label
Message Type Code
Mandatory parameter A
Pointer to start of optional part
Mandatory parameter F
Pointer to parameter M
End of optional parameters
Parameter name = Z
Length Indicator of parameter Z
Parameter Z
User Data MTC SLS Originating
Point Code
Destination
Point Code
N x 8 bits 8 bits 14 bits 14 bits 4 bits
Mandatory fixed part
Mandatory variable part
Optional part
Signalling Connection Control Part (SCCP)
• The SCCP itself has users called Subsystems (SS).
• The SCCP provides additional functions to the MTP for an OSI
network service.
» In particular, the non circuit related data transfer between signalling
end points is supported by the SCCP.
• Special protocol functions are provided by SCCP.
» Segmentation.
• Allows messages of any great length to be transmitted.
» Addressing and Routing.
89
SCCP four classes of service
• Basic connectionless Class (Class 0).
» Data are transparent independently of each other and may therefore be
delivered out of sequence. This corresponds to a pure connectionless
network service.
• Sequenced connectionless Class (Class 1).
» In protocol class 1 the features of class 0 are complemented by a sequence
control.
• Basic Connection-oriented Class (Class 2).
» Bi-directional transfer of NSDUs is done by setting up a temporary or
permanent signalling connection. This corresponds to a simple connection-
oriented network service.
• Flow control connection-oriented Class (Class 3).
» In protocol class 3 the features of class 2 are complemented by the
inclusion of flow control.
90
Connection-oriented Data Transfer
Data Transfer Data Transfer
= Release Resource = Reserved Resource
CR
CC CC
CR
RLC
RLSD RLSD
RLC
91
Connectionless Data Transfer
UDT
UDT
UDT
UDT
BTS
• UDT (Unitdata) Class 0
» Used by a SCCP wanting to send data in a
connectionless mode.
• DT1 (Data Form 1) Class 2
» A Data Form 1 message is sent by either end of a
signalling connection to pass transparently SCCP user
data between two SCCP nodes.
• Note: Only point to point signalling route is used in at the A-
inter phase, meaning that the MTP does not have to
contain the functions related to the signalling transfer point
(STP) and multiple signalling-route management.
SCCP Message Types, for A-Interface Class 0 and 2
CR Connection Request
CC Connection Confirm
CREF Connection Refused
RLSD Released
RLC Release Complete
DT1 Data Form 1
DT2 Data Form 2
AK Data Acknowledgement
UDT Unitdata
UDTS Unitdata Service
ED Expedited Data
EA Expedited Data Acknowledgement
RSR Reset Request
RSC Reset Confirm
ERR Protocol Data Unit Error
IT Inactivity Test
Protocol class
0
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Message type Code
0000 0001
0000 0010
0000 0011
0000 0100
0000 0101
0000 0110
0000 0111
0000 1000
0000 1001
0000 1010
0000 1011
0000 1100
0000 1101
0000 1110
0000 1111
0001 0000
1 2 3
X
X
SCCP Message Types
94
• Message type code
• Destination local reference
• Source local reference
• Called party address
• Calling party address
• Protocol class
• Segmenting/reassembling
• Release cause
• Return cause
• Error cause
• Refusal cause
• Data
• End of optional parameters
BTS
SCCP Information elements
95
Short descriptions SCCP Message Types, Class 2 and 3. Part 1
•Connection Request (CR ).
» A connection Request message is
sent by a calling SCCP to a called
SCCP to request the setting up of
a signalling connection between
the two entities. The required
characteristics of the signalling
connection are carried in various
parameter fields. On reception of
a Connection Request message,
the called SCCP initiates the
setup of the signalling connection
if possible.
•Connection Confirm (CC )
» A connection confirm message is sent by the called SCCP to indicate to the calling SCCP that it has performed the setup of the signalling connection. On reception of a Connection confirm message, the calling SCCP completes the setup of the signalling connection if possible.
•Connection Refused (CREF)
» A Connection Refused message is sent by the called SCCP or an intermediate node to indicate to the calling SCCP that the setup of the signalling connection has been refused.
96
Short descriptions SCCP Message Types, Class 2 and 3. Part 2
•Released (RLSD).
» A released message is sent, in
the forward or backward direction,
to indicate that the sending SCCP
wants to release a signalling
connection and the associated
resources at the sending SCCP
have been brought into the
disconnect pending condition. It
also indicates that the receiving
node should release the
connection and any other
associated resources as well.
•Release Complete (RLC).
» A Release Complete message is sent
in response to the Released message
indicating that the Released message
has been received, and the appropriate
procedures have been completed.
•Data Form1 (DT1).
» A Data Form 1 message is sent by
either end of a signalling connection to
pass transparently SCCP user data
between two SCCP nodes.
97
Short descriptions SCCP Message Types, Class 2 and 3. Part 3
•Data Form 2 (DT2).
» A Data Form 2 message is sent
by either end of a signalling
connection to pass transparently
SCCP user data between two
SCCP nodes and to acknowledge
message flowing in the other
direction.
•data acknowledgement (AK).
» A Data Acknowledgement
message is used to control the
window flow control mechanism,
which has been selected for the
data transfer phase.
•Expedited Data (ED).
» An Expedited Data message functions
as a Data Form 2 message but
includes the ability to bypass the flow
control mechanism which has been
selected for the data transfer phase. It
may be sent by either end of the
signalling connection.
•Expedited Data acknowledgement (EA).
» An Expedited Data Acknowledgement
message is used to acknowledge an
Expedited Data message.Every ED
message has to be acknowledged by
an EA message before another ED
message may be sent.
98
Short descriptions SCCP Message Types, Class 2 and 3. Part 4 •Reset Request (RSR).
» A Reset Request message is sent
to indicate that the sending SCCP
wants to initiate a reset procedure
(re-initialization of sequence
numbers) with the receiving
SCCP.
•Reset Confirm (RSC).
» A Reset Confirm message is sent
in response to a Reset Request
message to indicate that Reset
Request has been received and
the appropriate procedure has
been completed.
•Protocol Data Unit Error (ERR).
» A Protocol Data Unit Error message is sent on detection of any protocol errors.
•Inactivity Test (IT).
» An Inactivity Test message may be sent periodically by either end of a signalling connection to check if this signalling connection is active at both ends, and to audit the consistency of connection data at both ends.
•Extended Unitdata (XUDT).
» An Extended Unitdata message is used by the SCCP wanting to send data along with optional parameters in a connectionless mode. It can also be used by a SCCP to send data without optional parameters.
99
Short descriptions SCCP Message Types, Class 2 and 3. Part 5
•Extended Unitdata Service (XUDTS).
» An Extended Unitdata Service
message is used to indicate to the
originating SCCP that a XUDT with
optional parameters cannot be
delivered to its destination. A
XUDTS message is sent only when
the option field in the XUDT
message is set to "return on error".
•Long Unitdata (LUDT).
» A Long Unitdata message is used by
the SCCP to send data (along with
optional parameters) in a connection
mode, when MTP-3b capabilities are
present. It allows sending of NSDU
sizes up to 3952 octets without
segmentation.
•Long Unitdata Service (LUDTS).
» A long Unitdata Service message is
used to indicate to the originating
SCCP that a LUDT cannot be delivered
to its destination. A LUDTS message is
sent only when the return option in the
LUDT is set.
BTS
A-Interface BSSAP
101
• ETSI has specified an SS7 Base Station
Subsystem Application Part (BSSAP) as the user
of the SCCP/MTP transport service.
» SCCP subsystem number for BSSAP is FEh.
BTS
Base Station Subsystem Application Part (BSSAP)
SCCP Information elements
SCCP Header
SSN FEh: BSSAP
Discriminator
0 0 0 0 0 0 0 0
1 1
DLCI Data Link Connection Identifier
Discriminator
0 0 0 0 0 0 0 1
OCTET
DLCI
2
3 2 Length
Indicator 1
n n Layer 3
Messages
Octet
Length
Indicator 1
Layer 3
Um Interface
Octet
Bit no.: 8 7 6 5 4 3 2 1
C2 C1 0 0 0 S3 S2 S1
C2 C1 identifies signaling-radio channel
(00: SDCCH/FACCH, 01: SACCH)
S3-S1 is the SAPI on the radio interface
BSSMAP DTAP
Label SIF F CK F I B
FSN F B I B
BSN LI > 2 SIO
x3h
BTS
A-Interface BSSMAP
104
BTS
BSSMAP Format
Disc. 0 = BSSMAP
Length indicator
BSSMAP message type
BSSMAP message Information Element Identifier
Length of Information Elements
Content of Information Elements
Information Element Identifier
Content of Information Elements
Fixed Length Info. Element Format
Variable Length Info. Element Format
OCTET 1
OCTET 2
OCTET n
BSSMAP Messages, part 1
Assignment messages: (Setup of traffic channels)
- Assignment request
- Assignment complete
- Assignment failure
Release messages:
- Clear command (Release of traffic channels)
- Clear complete
- Clear request
- SAPI "n" clear command (Control of layer 2 links with SAPI not equal
- SAPI "n" clear complete to 0 on the radio interface)
- SAPI "n" reject
0000- - - -
0001
0010
0011
0010- - - -
0000
0001
0010
0011
0100
0101
Message type Code
BSSMAP Messages, part 2
Handover messages:
- Handover request (To BSC: Request for handover to that BSC)
- Handover required (To MSC: Inter-BSC/MSC handover required)
- Handover request ack (To MSC: Acknowledge of Handover request)
- Handover command (To BSC: Contains the new radio channel/BTS
to which the MS should switch)
- Handover complete (To MSC: Commanded handover complete)
- Handover failure (To MSC: Commanded handover unsuccessful)
- Handover performed (To MSC: BSC has performed intern. handover)
- Handover candidate (To BSC: MSC requests list of MS that could
enquire be handed over to another cell)
- Handover candidate (To MSC: Answer to Handover candidate
response enquire)
- Handover required reject (To BSC: Required handover unsuccessful)
- Handover detect (To MSC: Commanded handover successful)
0001- - - -
0000
0001
0010
0011
0100
0110
0111
1000
1001
1010
1011
Message type Code
BSSMAP Messages, part 3
General messages:
- Reset (Initialisation of BSS or MSC due to failure)
- Reset Acknowledge
- Overload (Processor or CCCH overload)
- Trace invocation (Start production of trace record)
- Reset Circuit (Initialisation of single circuit due to failure)
- Reset Circuit
acknowledge
Terrestrial resource messages:
- Block (Management of circuits/time slots
- Blocking acknowledge between MSC and BTS)
- Unblock
- Unblocking acknowledge
0011- - - -
0000
0001
0010
0011
0100
0101
0100- - - -
0000
0001
0010
0011
Message type Code
BSSMAP Messages, part 4
Radio resource messages:
- Resource request (Available radio channels in the BSS cells)
- Resource indication
- Paging (Paging of MS)
- Cipher mode command (Commands start of cyphering)
- Classmark update (Change of MS power class)
- Cipher mode complete (Ciphering is successfully initiated)
- Queuing indication (Indicates delay in assignment of traffic
channel)
- Complete layer 3 (Contains first message received from MS;
information sets up SCCP-connection at A-interface)
0101- - - -
0000
0001
0010
0011
0100
0101
0110
0111
Message type Code
- Circuit identity code (Traffic channel on A-interface)
- Radio channel identity (Description of channels allocated to MS)
- Resource available (Available radio channels in a cell)
- Cause (Reason for event/failure)
- Cell identifier (Identity of radio cell)
- Priority (Indicates the priority of a request)
- Layer 3 header (Protocol discriminator and transaction identifier
information to be used on the radio interface)
- IMSI
- TMSI
- Encryption information (Encryption algorithm and key)
- Channel type (Speech, data incl. speed or signalling; full or half rate)
- Periodicity (Defines periodicity of a particularly procedure)
- Number of MSs (No. of handover candidates to be sent to MSC)
- Current radio environment (Measurement data on radio cells for handover)
- Environment of BS “n” (Data in order of priority on the n preferred new cells
for handover)
BSSMAP Information Elements, part 1
BSSMAP Information Elements, part 2
- Classmark information type 2 (MS power class + SMS capability)
- Interference band to be used (Indicates acceptable interference level)
- RR Cause (Reason for RR release)
- Trace number (Reference number for a trace record)
- Layer 3 information (Contains transparent RR, MM or CM message)
- DLCI (Indicates the layer 2 link to be used on the radio
interface)
- Downlink DTX flag (Disabling of discontinuous transmission)
- Cell identifier list (Identifies the cells within a BSS)
- Response request (Requests a response on a Handover required
messages)
- Resource indication method (How the BSS shall transfer resource info)
- Classmark information type 1 (MS power class)
BTS
A-Interface DTAP
112
• The Direct Transfer Application sub-Part (DTAP) is
used to transfer call control and mobility
management messages to and from the MS;
» The layer-3 information in these messages is not
interpreted by the BSS.
BTS
Direct Transfer Application sub-Part (DTAP)
113
BTS
Disc. 1 = DTAP
Length indicator
BSSMAP message type
BSSMAP message Information Element Identifier
Length of Information Elements
Content of Information Elements
Information Element Identifier
Content of Information Elements
Fixed Length Info. Element Format
Variable Length Info. Element Format
OCTET 1
OCTET 2
OCTET n
DTAP Format
114
BTS
• Layer 3 of the DTAP messages has the same
format as BSSMAP messages.
• The DTAP messages and information
elements are identical to the the transparent
MM and CM listed in the A-bis section.
DTAP messages and elements
115
BTS
MSC BSC
SCCP CR (BSSMAP Comp layer 3 info (CM Service Request
SCCP CC (BSSMAP Cipher mode command)
SCCP DT1 (BSSMAP Cipher mode complete)
SCCP DT1 (DTAP (Setup [CM]))
SCCP DT1 (DTAP (Call Proceeding [CM]))
SCCP DT1 (BSSMAP Assignment request)
SCCP DT1 (BSSMAP Assignment complete)
SCCP DT1 (DTAP (Alerting [CM]))
SCCP DT1 (DTAP (Connect [CM]))
SCCP RLC
SCCP DT1 (DTAP (Disconnect [CM]))
SCCP DT1 (DTAP (Release [CM]))
SCCP DT1 (DTAP (Release Complete [CM]))
SCCP DT1 (BSSMAP Clear command)
SCCP DT1 (BSSMAP Clear complete)
SCCP RLSD
.
.
.
.
. .
.
.
. Active call
A-Interface signalling example
[MM]))
BTS
Inter-MSC Signalling MAP
Base
Station
Controller
BSC
Public Switched
Telephone Network
Base Mobile Station
(MS)
Mobile
Switching
Centre
MSC
(PSTN)
Station Subsystem
(BSS)
A-Inter
A-bis
Um
2 Mbit/s PCM
Air Interface
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Base
Transceiver
Station
BTS
Equip. Id
Register
EIR
Authen.
Centre
AUC
Visitor
Location
Register
VLR
Home
Location
Register
HLR Mobile
Switching
Centre
MSC
MAP
ISUP /
TUP
MAP
MAP MAP
ISUP /
TUP ISUP /
TUP
MAP
The System.
118
• DTAP - Direct Transfer Application Part
• BSSMAP - BSS Management Application Part
• CM - Call Management
• MM - Mobile Management
• RR - Radio Resource Management
• BTSM - BTS Management
• SCCP - Signalling Connection Control Part
• MAP - Mobile Application Part
• TCAP - Transaction Capability Application Part
• ISUP - ISDN User Part
• MTP - Message Transfer Part
DTAP CM
MM
RR
Sig. layer 2
Layer 1 (air)
Sig. layer 2
Sig. layer 1
MTP
SCCP
BSSMAP RR
(CM+MM)
MS BSC
MTP MTP
SCCP SCCP
CM
MM
BSSMAP TCAP
MAP
I
S
U
P
MSC
Sig. layer 2
Layer 1 (air)
BTS
(CM)
(MM)
(RR)
(CM)
(MM)
(RR)
(CM)
(MM)
(LAPDm) (LAPDm) (LAPD)
Sig. layer 2
Sig. layer 1
(LAPD)
BTSM BTSM RR'
Um Interface Abis Interface A Interface Inter-MSC
Protocol Architecture
119
Signalling System Number 7
MTP Level 3
Physical
Data Link
Network
Transport
Session
Presentation
Application
OSI Layer SS7 Levels
7
6
5
4
3
2
1 MTP Level 1
MTP Level 2
Physical MTP Level 1
Data Link 2 MTP Level 2
MTP Level 3 Network 3
I
S
U
P
SCCP
TCAP
T
U
P
IN, MAP
I
S
U
P
Transport
Session
Presentation
Application
SCCP
• Level 4/User Parts
• SCCP
Signalling Connection Control Part (SCCP)
• The SCCP itself has users called Subsystems (SS).
• The SCCP provides additional functions to the MTP for an OSI
network service.
» In particular, the non circuit related data transfer between signalling
end points is supported by the SCCP.
• Special protocol functions are provided by SCCP.
» Segmentation.
• Allows messages of any great length to be transmitted.
» Addressing and Routing.
• See next slide.
SCCP Addressing and Routing.
• The SCCP provides its own routing function.
» As address parameter the SCCP can use.
• DPC and SSN
› Routing based on MTP DPC and SSN in Global title (GT).
• Global title (GT)
› Routing based on global title
• E.g. Routing based on MTP DPC and SSN in Global title.
Link Set Link Set A
Link Set B
DPC 1
DPC 2
DPC 3
First Route Second Route
Link set A
Link set B
Link set B
Link set B Link set A
Link set A
MTP DPC = 2
SCCP Addressing and Routing.
» When global title (GT) is used different information cant be available.
• Translation type, numbering plan, encoding scheme, nature of address,
Address and Point Code.
• If for example , a destination number of the ISDN or IMSI numbering plan is
used, then the SCCP defines the DPC on the basis of a “GT Translation
Table” that is available within the signalling point.
Link Set Link Set A
Link Set B
DPC 1
DPC 2
DPC 3
First Route Second Route
Link set A
Link set B
Link set B
Link set B Link set A
Link set A
GT = 3
DPC 2
DPC 1
DPC 3
GT 3
GT 2
GT 1
123
SCCP Subsystem number (SSN)
• SSN 01h = SCCP Management message (SCMG).
• The MAP layer consist several of Application Service Elements, so
more than one SSN are alocated.
» The SSN for MAP are:
• 05h = MAP
• 06h = HLR
• 07h = VLR
• 08h = MSC
• 09h = EIR
• 0Ah = AUC
• Within a INAP node, it is the choice of the network operator to which
SSN(s) he will assigned to INAP.
» Any addressing scheme supported and not reserved by the SCCP may be
used.
• 91h = GMLC
• 93h = gsmSCF
• 94h = SIWF
• 95h = SGSN
• 96h = GGSN
124
SCCP Management message (SCMG)
• The SCCP management function (SCGM).
» SCMG are taking care of handling of errors and other problems on
subsystems level.
• Subsystem-Prohibited.
• Subsystem-Status-Test.
• Subsystem-Out-of-Service.
• Subsystem Congested.
» The SCMG messages (SSA, SSP, SST, SOR, SOG) contain
mandatory fixed parameters. These parameters are defined in the
data field of the UDT and XUDT message.
125
SCCP four classes of service
• Basic connectionless Class (Class 0).
» Data are transparent independently of each other and may therefore be
delivered out of sequence. This corresponds to a pure connectionless
network service.
• Sequenced connectionless Class (Class 1).
» In protocol class 1 the features of class 0 are complemented by a sequence
control.
• Basic Connection-oriented Class (Class 2).
» Bi-directional transfer of NSDUs is done by setting up a temporary or
permanent signalling connection. This corresponds to a simple connection-
oriented network service.
• Flow control connection-oriented Class (Class 3).
» In protocol class 3 the features of class 2 are complemented by the
inclusion of flow control.
126
Short descriptions SCCP Message Types, Class 0 and 1.
•Unitdata (UDT).
» Used by a SCCP wanting to send
data in a connectionless mode.
•Unitdata Service (UDTS).
» A Unitdata Service message is
used to indicate to the originating
SCCP that a UDT it sent cannot
be delivered to its destination. A
UDTS message is sent only when
the option field in that UDT is set
to "return on error".
•TCAP uses the connectionless protocol classes of SCCP only.
» In connection with INAP the protocol class 1 is used exclusively.
» In connection with MAP the protocol class 0 and 1 is used.
127
Connectionless Data Transfer
UDT
UDT
UDT
UDT
128
Signalling System Number 7
Level 4/User Parts
TCAP
MTP Level 3
Physical
Data Link
Network
Transport
Session
Presentation
Application
OSI Layer SS7 Levels
7
6
5
4
3
2
1 MTP Level 1
MTP Level 2
Physical MTP Level 1
Data Link 2 MTP Level 2
MTP Level 3 Network 3
I
S
U
P
SCCP
TCAP
T
U
P
INAP, MAP
I
S
U
P
Transport
Session
Presentation
Application
SCCP
TCAP
Transaction Capabilities Application Part (TCAP)
• Definition
» The overall objective of TCAP is to provide means for the transfer of
information between nodes, and to provide generic services to
applications, while being independent of any of these.
» Transaction Capabilities in the SS7 protocol are functions that control
non-circuit-related information transfer between two or more signalling
nodes via a signalling network.
• For use between
» Exchanges
» An exchange and a network service centre
» Network service centres
130
TCAP / SCCP classes
• ITU-T has only specified the use of SCCP class 0 and 1 (connectionless transfer) This means that the intermediate service part (ISP) is empty/not needed because no layer 4, 5 and 6 functions are required for control of SCCP.
131
The ASN.1 notation
• TCAP is build on ANS.1 abstract notation.
• The ASN.1, abstract syntax notation one, is a formal language for
defining high level protocol information by means of user defined
data types. It was standardized by CCITT in 1984 but is now also
adopted by ISO in conjunction with BER as part of OSI
applications. ASN.1`s prime use has been in the definition of
application protocols, but is also used in definition of presentation
protocols. The grammar of the syntax is the Backus-Naur Form
(BNF) as used in other programming languages.
TCAP Applications
• Mobile services (MAP)
» e.g.
• Location updating/roaming
• Non-call related supplementary services
• Charging information
• Supplementary services (INAP/CAMEL)
» e.g.
• Number conversion (800, VPN).
• Credit check.
• Prepaid/calling card
• Prepaid roaming.
• Operation and Maintenance (OMAP)
Messages Structure when TCAP is used.
• TCAP is an user of SCCP.
MAXIMUM 272 OCTETS
SIF F CK F I B
FSN F B I B
BSN LI > 2 SIO
Optional part Mandatory
variable part
Mandatory
fixed part MTC SLS OPC DPC
Messages
Component
Messages
Component Length
Transaction
information element
Messages
Length
Messages
Type Tag
Higher layer (INAP, CAMEL or MAP)
The messages type (MTC)
is always UDT or UDTS, if the
higher layer is MAP CAMEL or INAP
MTP
SCCP
TCAP
TCAP sub-layers
• TCAP is divided into two sub-layers
• Transaction sub-layer » Is a simple transport service for the component sub-layer
(comparable to an “envelope” containing a group of components to be processed at the remote end)
• Component sub-layer » Deals with individual actions or data, called components
(e.g. one mobile phone)
TCAP Message types
• Transaction Sub-layer
» Unidirectional (used when no need to establish a transaction)
» Begin (initiate transaction)
» End (terminate transaction)
» Continue (continue transaction)
» Abort (terminate transaction in abnormal situation)
• Component Sub-layer
» Invoke (request operation to be performed at remote end)
» Return Result(last) (successful completion of operation, contains last/only result)
» Return Error (reports unsuccessful completion of operation)
» Reject (incorrect component received at remote end)
» Return Result (contains part of result of operation)
136
TCAP Structure
Message type tag
Total message length
Transaction portion information element
Component portion tag
Component type tag
Component length
Component portion information
element
Component
Abort Cause
Dialogue Portion
Component Portion
Destination Transaction Identifier
Originating Transaction Identifier
Tag
Length
Contents
Tag
Length
Contents
Invoke ID
Linked ID
Operation Code
Sequence
Error Code
Problem Code
- Unidirectional
- Begin
- End
- Continue
- Abort
- Invoke
- Return Result
(Last)
- Return Result
(Not Last)
- Return Error
- Reject
• The table shows where ORIG and
DEST TID is used.
Transaction portion information element
• The application on higher layer (MAP and INAP) are using the ORIG and
DEST TID within the “Transaction portion info element” to differ between
the transactions send to and from the application.
» ORIG TID (Originating Transaction Identifier).
• The originating transaction ID is assigned by the node sending a message, and is
used to identify the transaction at that end.
» DEST TID (Destination Transaction Identifier).
• The destination transaction ID identifies the transaction at the receiving end.
Unidirectional
Begin
Continue
End
Transaction
Abort
ORIG ID DEST ID
YES NO
YES YES
YES NO
NO NO
YES NO
Component portion information element
• The application on higher layer (MAP, CAMEL and INAP) are
using the Invoke ID and Linked ID within the “Component portion
info element” to correlate the question and answer within
transactions send to and from the application.
» Invoke ID
• An Invoke ID is used as a reference number to identify uniquely an
operation. It is present in the Invoke component and in any reply to the
Invoke (Return Result, Return Error or Reject), enabling the reply to be
correlated with the invoke.
» Linked ID
• A Linked ID is included in an invoke component by a node when it
responds to an operation invocation with a linked operation invocation.
The node receiving the Linked ID uses it for correlation purposes, in the
same way that it uses the invoke ID in Return Result, Return Error and
Reject components.
139
TCAP Signaling Example.
BEGIN ORIG TID: 75 INVOKE INVOKE ID: 0
CONTINUE ORIG TID: 18 DEST TID: 75 INVOKE INVOKE ID: 0
CONTINUE ORIG TID: 75 DEST TID: 18 RET RES L INVOKE ID: 0
CONTINUE ORIG TID: 18 DEST TID: 75 INVOKE INVOKE ID: 1
CONTINUE ORIG TID: 18 DEST TID: 75 INVOKE INVOKE ID: 2
CONTINUE ORIG TID: 75 DEST TID: 18 RET RES L INVOKE ID: 2
CONTINUE ORIG TID: 75 DEST TID: 18 RET RES L INVOKE ID: 1
END DEST TID: 75 INVOKE INVOKE ID: 0
Application Begin
Data send
Data received
Data send
Data send
Data received
Data received
Application End
140
BTS
Signalling System Number 7
Level 4/User Parts
MAP
MTP Level 3
Physical
Data Link
Network
Transport
Session
Presentation
Application
OSI Layer SS7 Levels
7
6
5
4
3
2
1 MTP Level 1
MTP Level 2
Physical MTP Level 1
Data Link 2 MTP Level 2
MTP Level 3 Network 3
I
S
U
P
SCCP
TCAP
T
U
P
INAP, MAP
I
S
U
P
Transport
Session
Presentation
Application
SCCP
TCAP
MAP
BTS
• MAP = Mobile Application Part
• Users: MSC, HLR, VLR, EIR
• Applications:
» Location updating/roaming
» Incoming call routing information (MSRN)
» Subscriber service information
» Non-call related supplementary services
» Short message service delivery
» MS equipment identity (IMEI)
» Charging information
MAP
Called party
address
Message
type
Calling party
address
Protocol
class 0, 1
Pointers
MAP
Info. TCAP part Sub-system Number
(part of the address field)
SCCP User Data
Format of SCCP message for MAP information
Label SIF F CK F I B
FSN F B I B
BSN LI > 2 SIO
x3h
143
MAP information in TCAP
Message type tag
Total message length
Transaction portion information element
Component portion tag
Component type tag
Component length
Component portion information
element
Component
Tag
Length
Contents
Tag
Length
Code
Parameter
• The inter-MSC interface are:
» The MSC-VLR interface.
» The MSC-HLR interface.
» The HLR-VLR interface.
» The MSC-MSC interface.
» The MSC-EIR interface.
» The VLR-VLR interface.
» The MSC-ISDN/telephone network interface.
BTS
Inter-MSC Signalling
145
BTS
Operation Value
1 Update location area
2 Update location
3 Cancel location
4 Provide roaming number
5 Detach IMSI
6 Attach IMSI
7 Insert subscriber data
8 Delete subscriber data
9 Send parameters
10 Register SS
11 Erase SS
12 Activate SS
13 Deactivate SS
14 Interrogate SS
15 Invoke SS
16 Forward SS notification
17 Register password
18 Get password
19 Process unstructured data
MAP Operations Part 1
146
BTS
20 Send info for incoming call
21 Send info for outgoing call
22 Send routing information
23 Complete call
24 Connect to following address
25 Process call waiting
26 Page
27 Search for mobile subscriber
28 Perform handover
29 Send end signal
30 Perform subsequent handover
31 Allocate handover number
32 Send handover report
33 Process access signalling
34 Forward access signalling
35 Note internal handover
36 Register charging information
37 Reset
38 Forward check SS indication
MAP Operations Part 2
Operation Value
147
BTS
39 Authenticate
40 Provide IMSI
41 Forward new TMSI
42 Set ciphering mode
43 Check IMEI
45 Send routing info for SM
46 Forward short message
47 Set message-waiting data
48 Note MS present
49 Alert service centre
50 Activate trace mode
51 Deactivate trace mode
52 Trace subscriber activity
53 Process access request
54 Begin subscriber activity
MAP Operations Part 3
Operation Value
148
BTS
Home side
Visited
side
INVOKE (Update Location)
BEGIN
INVOKE (Insert Subscriber Data)
CONTINUE
RETURN RESULT
CONTINUE
(Update Location)
END
(Insert Subscriber Data)
RETURN RESULT
MAP - signalling Location Updating example
BTS
GSM Signalling procedures
BTS
• Call Management
» Ordinary Call Control (as usual)
• Mobility Management
» Location Updating (Roaming)
» Authentication
• Radio Resource Management
» Paging
» Network Access
» Encryption
» Radio Signal Control
» Radio Signal Measurements
» Handover
Special signalling procedures for GSM
Area 1
Area 2
BTS BTS
BTS BTS
•An MM procedure
•Reasons for roaming:
» MS has detected that it
has entered into a new
location area (by listening
to Broadcast system info)
•Types of roaming:
» Inside same VLR area
• The HLR does not
need to know
» Another VLR area
• In this case the
HLR is informed
MSC
VLR
MSC
VLR
SONOFON
M N
Location Updating (Roaming)
MS
BT
S
BS
C
MS
C VLR 1 VLR 2 HLR
Location Update Request (old LAI, TMSI) [MM] Update Location Area (old LAI, new LAI, TMSI)
Send Parameters (old LAI, TMSI)
Send Parameters Result (IMSI)
Update Location (IMSI, MSRN, VLR no)
Cancel Loc (IMSI, VLR' no)
Cancel Location Ack
Update Location Ack
Insert Subscriber Data (IMSI, service inf)
Subscriber Data Acknowledge
Update Location Area Ack
Forward New TMSI (TMSI) Location Update Accept (new LAI, TMSI) [MM]
TMSI Reallocation Complete [MM]
TMSI Ack Channel Release [RR] Clear Command [RR]
Um/A-bis/A signalling
MAP signalling
Location Updating (Roaming)
• A RR procedure
• The Handover process is the
situation where a Mobile
Station changes from being
served by one Antenna to
another
• Handovers take place during
a call
• Handover are done
automatically
• Crossing the boundary of two
adjacent cells is the typical
example of a Handover
Handovers.
BTS BTS
• Major types of handovers
» Intra BSC
» Inter BSC
» Inter MSC
• Purpose of handover » Poor quality
connection
• Avoid loosing contact to the mobile station
» Fault in the MS or BTS/BSC
» Network management
Intra BSC
Inter BSC
Inter MSC
BSC
MSC - A
MSC - B
BSC
BSC
BTS
BTS
BTS BTS
SONOFON
M N
SONOFON
M N
SONOFON
M N
TRAU Signaling
Send handover report
Um/A-bis/A signalling
MAP signalling
ISUP/TUP signalling
MS
BT
S
BSC-
A
MSC-
A
BSC-
B
MSC-
B
VLR-
B
Handover Required (new BTS) [RR]
Perform handover (new BTS, Ch type)
Handover Request (new BTS)
Handover Req Ack (Radio Ch)
Allocate handover number
(MSRN)
(MSRN)
Radio Ch Ack (Radio Ch, MSRN)
IAM (MSRN)
ACM Handover Command (Radio CH) [RR]
Handover Complete [RR]
Clear Command[RR]
Clear Complete[RR]
Send End Signal
Answer
Measurement Result [RR]
Handover, example
BTS
• A CM procedure
• Distinguish two types
» Mobile Terminating Call
• i.e. a call from the fixed network to a Mobile
Station
» Mobile Originating Call
• i.e. a call from a Mobile Station to the fixed
network
Call Setup
BTS
• Problems and answers
» Where in the world is the Mobile Station
• Look it up in the HLR
• (The HLR may have to ask the VLR)
» How to Make the Mobile Station Aware that a
call is waiting
• Page it in the cell where it is located
» What does the MS do when being paged ?
• Asks for a Radio channel
• Tells the system that it is ready
• Now the usual setup flow follows
Mobile Terminating Call
IAM (MSISDN, service)
BTS BSC
MS
C VLR HLR
GMS
C
ISD
N
Send Routing Info (MSISDN)
Roaming Number (MSRN) Routing Info (MSRN)
Provide Roaming Number (IMSI)
IAM (MSRN, service)
Send Info Incoming Call (MSRN, service)
Page (IMSI, TMSI, LAI) Paging Request (TMSI)
[RR]
Paging Response (TMSI) [RR] Page Result
Complete Call (service)
Channel Request
[RR] Immediate Assign [RR]
SETUP (service) [CM]
Call Confirm [CM]
Assign Command [RR] Assign Req [RR]
Assign Complete [RR]
Alerting [CM] ACM Connect [CM]
ANM Connect Ack [CM]
Complete Call Result
MS
Um/A-bis/A signalling
MAP signalling
ISUP/TUP signalling
Incoming Call
BTS
• Problems and Answers
» How the mobile gets in contact with the network
• Switch the MS on
• Request a channel
• Tell the network what kind of service is wanted
» How does the network respond
• Verifies the Mobile identity (authentication)
• Assigns a traffic channel
• And then everything proceed as usual
Mobile Originating Call
BTS BSC MSC VLR ISDN
Immediate Assign [RR]
CM Service Req (IMSI, transact) [MM] Process Access Request
Authenticate (RAND) Authentication Request (RAND) [MM]
Authentication Response (SRES) [MM] Authentication Response (SRES)
Set ciphering mode (key)
Ciphering Mode Command (key) [RR]
Ciphering Mode Complete [RR]
Access Request Ack
SETUP (service, called number) [CM] Send info for o/g (service, called number)
Complete call Call Proceeding [CM]
Assign Request [RR] Assign Command [RR]
Assign Complete [RR] IAM
ACM
ANM
Alerting [CM]
Connect [CM]
Connect Ack [CM]
CM copy [MM]
MS Channel Request/Required [RR]
Um/A-bis/A signalling
MAP signalling
ISUP/TUP signalling
Outgoing Call from MS
161
Signalling System Number 7
Level 4/User Parts
INAP
MTP Level 3
Physical
Data Link
Network
Transport
Session
Presentation
Application
OSI Layer SS7 Levels
7
6
5
4
3
2
1 MTP Level 1
MTP Level 2
Physical MTP Level 1
Data Link 2 MTP Level 2
MTP Level 3 Network 3
I
S
U
P
SCCP
TCAP
T
U
P
INAP, MAP
I
S
U
P
Transport
Session
Presentation
Application
SCCP
TCAP
INAP
CS1 / IN / INAP
• Abbreviations.
» CS1 = Intelligent Network Capability Set 1.
» IN = Intelligent Network.
» INAP = Core Intelligent Network Application Part
• The intelligent Network (IN) is a control architecture for
telecommunication network service.
» The goal of the IN service control architecture is to provide a
framework, which allows the Network Operator to create , to
control and to manage services more efficiently, economically
and rapidly as the present network architecture allows.
CS1 / IN / INAP
• INAP supports interactions between the following three Functions
» - Service Switching Function (SSF).
» - Service Control Function (SCF).
» - Specialized Resource Function (SRF).
• INAP provides a set of predefined Messages and parameters that can be used in the intelligent Network predefined functions, however Extension Fields is allowed.
» Those parameter are operator specific, and therefore not known by the MPA.
• INAP is not like the MAP where all transactions shall be ended by TC_END. In INAP a TC_END shall no longer be maintained if both part involved in the transaction knows that no more data is required.
» Those cases are called pre-arranged end. The MPA doesn't know those cases, and therefore not able to to make a correct sequence recording.
164
Short descriptions INAP Operations Part 1
•Initial DP. Direction: SSF -> SCF
» This operation is used after a TDP to indicate request for service.
•Assist Request Instructions. Direction: SSF -> SCF or SRF -> SCF
» This operation is used when there is an assist or a hand-off procedure and may be
sent by the SSF or SRF to the SCF. This operation is sent by the SSF or SRF to the
SCF, when the initiating SSF has set up a connection to the SRF or to the assisting
SSF as a result of receiving an Establish Temporary Connection or Connect (in
case of hand-off) operation from the SCF.
•Establish Temporary Connection. Direction: SCF -> SSF
» This operation is used to create a connection to a resource for a limited period of
time (e.g. to play an announcement, to collect user information); it implies the use of
the assist procedure.
•Disconnect Forward Connection. Direction: SCF -> SSF
» This operation is used to disconnect a forward temporary connection or a
connection to a resource.
165
Short descriptions INAP Operations Part 2
•Connect To Resource. Direction: SCF -> SSF
» This operation is used to connect a call from the SSP to the PE containing the SRF.
•Connect. Direction: SCF -> SSF
» This operation is used to request the SSF to perform the call processing actions to
route or forward a call to a specified destination. To do so, the SSF may or may not
use destination information from the calling party (e.g., dialed digits) and existing
call setup information (e.g., route index to a list of trunk groups), depending on the
information provided by the SCF.
•Release Call. Direction: SCF -> SSF
» This operation is used to tear down an existing call at any phase of the call for all
parties involved in the call.
•Request Report BCSM Event. Direction: SCF -> SSF
» This operation is used to request the SSF to monitor for a call-related event (e.g.,
BCSM events such as busy or no answer), then send a notification back to the SCF
when the event is detected.
166
Short descriptions INAP Operations Part 3
•Event Report BCSM. Direction: SSF -> SCF
» This operation is used to notify the SCF of a call-related event (e.g., BCSM events
such as busy or no answer) previously requested by the SCF in a Request Report
BCSM Event operation.
•Request Notification Charging Event. Direction: SCF -> SSF
» This operation is used by the SCF to instruct the SSF on how to manage the
charging events which are received from other FEs and not under control of the
service logic instance. The operation supports the capabilities to cope with the
interactions concerning charging.
•Event Notification Charging. Direction: SSF -> SCF
» This operation is used by the SSF to report to the SCF the occurrence of a specific
charging event type as previously requested by the SCF in a Request Notification
Charging Event operation. The operation supports the capabilities to cope with the
interactions concerning charging.
167
Short descriptions INAP Operations Part 4
•Collect Information. Direction: SCF -> SSF
» This operation is used to request the SSF to perform the originating basic call
processing actions to prompt a calling party for destination information, then collect
destination information according to a specified numbering plan (e.g., for virtual
private networks).
•Continue. Direction: SCF -> SSF
» This operation is used to request the SSF to proceed with call processing at the DP
at which it previously suspended call processing to await SCF instructions (i.e.,
proceed to the next point in call in the BCSM). The SSF continues call processing
without substituting new data from SCF.
•Initiate Call Attempt. Direction: SCF -> SSF
» This operation is used to request the SSF to create a new call to one call party using
address information provided by the SCF.
•Reset Timer. Direction: SCF -> SSF
» This operation is used to request the SSF to refresh an application timer in the SSF.
168
Short descriptions INAP Operations Part 6
•Furnish Charging Information. Direction: SCF -> SSF
» This operation is used to request the SSF to generate, register a call record or to
include some information in the default call record. The registered call record is
intended for off-line charging of the call.
•Apply Charging. Direction: SCF -> SSF
» This operation is used for interacting from the SCF with the SSF charging
mechanisms. The Apply Charging Report operation provides the feedback from the
SSF to the SCF.
•Apply Charging Report. Direction: SSF -> SCF
» This operation is used by the SSF to report to the SCF the occurrence of a specific
charging event as requested by the SCF using the Apply Charging operation.
•Call Gap. Direction: SCF -> SSF
» This operation is used to request the SSF to reduce the rate at which specific
service requests.
169
Short descriptions INAP Operations Part 7
•Activate Service Filtering. Direction: SCF -> SSF
» When receiving this operation, the SSF handles calls to destination in a specified
manner without sending queries for every detected call. It is used for example for
providing Tele-voting or mass calling services. Simple registration functionality
(counters) and announcement control may be located at the SSF. The operation
initializes the specified counters in the SSF.
•Service Filtering Response. Direction: SSF -> SCF
» This operation is used to send back to the SCF the values of counters specified in a
previous Activate Service Filtering operation.
•Call Information Report. Direction: SSF -> SCF
» This operation is used to send specific call information for a single call to the SCF as
requested by the SCF in a previous call Information Request.
•Call Information Request. Direction: SCF -> SSF
» This operation is used to request the SSF to record specific information about a
single call and report it to the SCF (with a call Information Report operation).
170
Short descriptions INAP Operations Part 8
•Send Charging Information. Direction: SCF -> SSF
» This operation is used to instruct the SSF on the charging information to be sent by
the SSF. The charging information can either be sent back by means of signalling or
internal if the SSF is located in the local exchange. In the local exchange this
information may be used to update the charge meter or to create a standard call
record. The charging scenario supported by this operation is scenario 3.2 (refer to
Annex B where these are defined).
•Play Announcement. Direction: SCF -> SRF
» This operation is to be used after Establish Temporary Connection (assist procedure
with a second SSP) or a Connect to Resource (no assist) operation. It may be used
for in-band interaction with an analogue user, or for interaction with an ISDN user. In
the former case, the SRF is usually collocated with the SSF for standard tones
(congestion tone etc.) or standard announcements. In the latter case, the SRF is
always collocated with the SSF in the switch. Any error is returned to the SCF. The
timer associated with this operation must be of a sufficient duration to allow its
linked operation to be correctly correlated.
171
Short descriptions INAP Operations Part 9
•Prompt And Collect User Information. Direction: SCF -> SRF
» This operation is used to interact with a user to collect information.
•Specialized Resource Report. Direction: SRF -> SCF
» This operation is used as the response to a Play Announcement operation when the
announcement completed report indication is set.
•Cancel. Direction: SCF -> SRF or SCF -> SSF
» This generic operation cancels the correlated previous operation or all previous
requests. The following operations can be cancelled ”Play Announcement and
Prompt And Collect User Information”.
•Activity Test. Direction: SCF -> SSF
» This operation is used to check for the continued existence of a relationship
between the SCF and SSF. If the relationship is still in existence, then the SSF will
respond. If no reply is received, then the SCF will assume that the SSF has failed in
some way and will take the appropriate action.
IN Signalling procedures
The System.
• SSF = Service Switching Function
• SCF = Service Control Function
• SRF = Specialized Resource Function
• SSP = Service Switching Point
• SMP = Service management Point.
• SCP = Signalling Control Point
Local exchange
Local exchange
LAN / WAN IP
IP
SSF
SSF
SCF
SRF
IP
Mobile switching
Center
The System
• Service Management Point (SMP).
» Management of
• data.
• statistic.
» Introduction of new services.
» Administration of the SCP.
• Service Switching Point (SSP)
» Access point for the service user. *
» Execution of service functions.
* service user: A user of an IN Service, the so-called customer.
• Service Control Point (SCP).
» Call control and routing.
• Selection code dependent,
origin dependent, state
dependent.
• Intelligent Periphery (IP).
» Announcement.
» Speech recognition.
• E.g. voice dialling.
» Speech synthesis.
175
Simple IN call service
• The dialled number will be translated into anther number.
» Process:
• The service user are dialling e.g. 800 or free number.
• At the the IN the number is converted into an E.164 number.
• The call is then forwarded to the destination.
SSP (SSF)
SCP
(SCF) Local Exchange
IAM (CLD CLG) TC_BEGIN InitialDP (CLD CLG)
TC_END Connect(CLD CLG)
IAM (CLD CLG)
ISUP signalling
INAP signalling
176
Simple IN call service with Busy/No answer Monitoring
• The dialled number will be translated into anther number.
» Process:
• Same as last slide, but in case of busy or No answer. The call will be forwarded to an alternative number (e.g another phone or voice mail)
SSP (SSF)
SCP
(SCF) Local Exchange
IAM (CLD CLG) TC_BEGIN InitialDP (CLD CLG)
TC_CONTINUE RequestReportBCSMEvent, connect
IAM (CLD CLG)
REL(Busy)
TC_CONTINUE EventReportBCSMEvent(Busy)
TC_CONTINUE Connect (New CLD)
IAM (New CLD CLG)
CON ISUP signalling
INAP signalling
177
Simple IN call service with announcement.
» Process:
• Same as last slide, but in this case the call are forwarded to an
(e.g. Waiting announcement or voice mail)
SSP (SSF) Local Exchange
IAM (CLD CLG) TC_BEGIN InitialDP (CLD CLG)
TC_CONTINUE ConnectToResource PlayAnnouncement
Internal IP CON
TC_CONTINUE SpecializedRescourceReport
Setup & PA
AnnCompleted
TC_CONTINUE DisconnectForwardConnection, ReleaseCall
Release Rel
ISUP signalling
INAP signalling
178
Virtual Private Network Call (VPN)
MSC/SSP
(SSF)
SCP
(SCF) MS
ISUP signalling
INAP signalling
IAM (CLD:6198 CLG: 26126134) TC_BEGIN InitialDP
(CLD:26126198 CLG: 26126134 ORIG CLD 6198
TermAttemptAuthorized 26126198)
TC_END Connect (CLD:26126198 ORIG: CLD 6198
CLG: 6138 ORIG CLG: 26126134)
IAM (CLD:26126198, CLG: 6138,
ORIG: CLD 6198, ORIG CLG: 26126134)
ACM
ANM
Now the usual Call flow follows
179
Outgoing Prepaid Call from MS
MSC VLR ISDN
CM Service Req (IMSI, transact)
[MM] Process Access
Request Authenticate
(RAND)
Authentication Request (RAND) [MM]
Authentication Response (SRES) [MM] Authentication Response
(SRES)
Set ciphering mode (key) Ciphering Mode Command (key)
[RR] Ciphering Mode Complete
[RR]
Access Request
Ack
SETUP (service, called number)
[CM] Send info for o/g (service, called number)
Complete
call Call Proceeding
[CM] Assign Command
[RR] Assign Complete
[RR] IAM
ACM
ANM
Alerting [CM] Connect
[CM] Connect Ack
[CM]
CM copy [MM]
MS
Um/A-bis/A signalling
MAP signalling
ISUP/TUP signalling
SC
P
INAP signalling
InitialDP (CollectInfo)
ApplyCharging (Continue)
BTS
TRAU Transcoder Rate Adaptation Unit
BTS
• TRAU - Transcoder / Rate Adaptation Unit
Functions:
» Conversion of speech from 64 kbit/s on PCM (A-law)
to 13/6.5 kbit/s on the GSM radio interface
» Intermediate rate adoption of data from V.110 frames
to the special TRAU frames on the A-bis interface
TRAU
182
13Kbit/s speech channel
T
R
A
U
BTS BSC MSC
BTS BSC MSC
BTS BSC MSC
T
R
A
U
T
R
A
U
Um A-bis A-Interface
Possible Locations of the TRAU.
BTS
• Bandwidth: 13 kbit/s
• Encoding algorithm: Regular Pulse Excitation
with Long Term Prediction (RPE LTP):
» Speech is sampled 8000 times per second
» Each sample is converted into a 13 bit digital value
» Every 20 ms a 260 bit segment is generated (13 kbit/s)
» The segment is divided by importance into 182 class 1
bits and 78 class 2 bits
» For protection, the 182 class 1 bits are mapped into
378 bits
» The resulting 456 bits (378 + 78) are divided into 8x57
bits
» The data are transmitted in 4 consecutive TDMA
blocks
• Resulting overall delay is 57.5 msec.
GSM Speech Encoding
BTS
• TRAU is controlled by BTS
• In-band signalling used, if TRAU not at BTS
• Control functions:
» Shift between speech and data
» Shift between full rate and half rate channels
» Timing of speech frames (BSS - MS)
» Comfort noise (Discontinuous Transmission)
Control of TRAU
Synchronization
0000000000000000 D/C/T - Bits
user data/control bits User data bits
D - Bits
Control bits
1 C1 - - - - - - - - - - C15
TS 0 TS 31
C1 - C4
C5
C6 - C11
C12 - C15
Frame type: Speech/Idle speech/Data/O&M
Channel type: Full rate/half rate
Speech: Time alignment (250/500 us)
Data: Intermediate RA (8 or 16 kbit/s)
Speech: Frame indicators (BFI, SID, TAF)
C16
C17 - C21
T1 - T4
Speech: Spare
Speech: Time alignment bits
Speech: Frame indicator (SP)
0 16 32 304 320 Bit
TRAU
frame
16 kbit/s
PCM frame 2 Mbit/s TS 1 TS 2
TRAU Signalling
BTS
GSM 3G
187
• As early as 1994, a Special Mobile Group started to think about a
High Speed Data upgrade for GSM.
• The first step was HSCSD (High Speed Circuit Switched Data).
» HSCSD is a circuit-switched extension to GSM.
• The next step was GPRS.
» GPRS is a packet-switched extension to GSM.
History
188
• HSCSD (High Speed Circuit Switched Data).
» HSCSD invented the principle of timeslot bundling to achieve higher
throughput rates.
» HSCSD is the simplest high speed data upgrade for GSM.
» HSCSD provides GSM users with a bandwidth of up to 57.6 Kbps.
» HSCSD does not require a hardware upgrade within BSS or core
network (NSS), but different mobile stations.
• Even though HSCSD is easy to implement into the GSM
network hardly any operator have decided to implement it.
» The commercial implementations of HSCSD barely exceed a speed of
38.4 Kbps.
» The most common implementation is 14,4 Kbps which only requires
one full rate TCH.
What is HSCSD?
189
• GPRS (General Packet Radio Service) is a packet oriented data
service for IP and X.25 over GSM networks.
• GPRS provides data speeds up to 170 Kbps.
» Normally GSM only provides 9.6 Kbps, however, HSCSD provides GSM
users with a bandwidth up to 57.6 Kbps.
• GPRS provides an “always on” functionality, without continuous
consumption of resources.
• GPRS is a step stone to 3rd generation networks.
» EDGE. Almost similar to GRPS, but three times faster.
» UMTS.
What is GPRS?
190
• EDGE is mainly concerned with the modulation scheme on the
Air-Interface.
» Originally, EDGE was the abbreviation for Enhanced Data rates for GSM
Evolution. Nowadays, EDGE is the acronym for Enhanced Data rates for
Global Evolution.
• EDGE is using frequency modulation scheme 8-PSK in order to
increase the Data speed.
» Applying 8-PSK-modulation to such a network implies shrinking of the cell
size.
» GSM and GPRS are using the same modulation scheme GPSK.
• Not only is a new core network required, but also additional BTSs
and a new cell structure.
» EDGE requires a major hardware upgrade and this is extremely costly to the
operator.
Why not choose EDGE if it is almost similar to GRPS?
191
• Introduction of a new modulation technique – 8PSK, 8 Phase Shift Keying. 8PSK enables air interface bitrates roughly 3 times higher than traditional GMSK (Gaussian Minimum Shift Keying)
• However, the major disadvantage of 8-PSK modulation is that it includes amplitude modulation.
Q
I
Start
+90
(same bit)
-90
(diff bit)
Q
I (1,1,1)
(0,1,1)
(1,0,0)
(1,0,1)
(0,0,1)
(0,0,0)
(0,1,0)
(1,1,0)
8PSK: 1 Symbol = 3 bits GMSK: 1 Symbol = 1 bit
EGPRS (Enhanced GPRS)
BTS
The GPRS network and it’s new elements.
193
From GSM to GPRS Network
Um MAP
ISUP
MAP MAP
ISUP
GSM
A A-bis
BSS
PSTN/ISDN
R
Gi
Gp
Gb Gs
Gf
Gr
PDN
Private
Backbone
Gn
Gn
GPRS
Gc
PDN
194
• Handles:
» PDP contexts for Mobile Stations.
» Determines Quality of Service assigned to the user.
» Routes packets to Mobile Stations.
» “Pages” Mobile Stations when data is to be sent.
• Stores:
» Subscriber data for all Mobile Stations in the location area.
• Security:
» Authentication by means of identity or equipment check.
» P-TMSI is allocated by SGSN.
» Ciphering. (Not only in ”Um as in GSM” but all the way down to SGSN).
SGSN (Service GPRS Support Node)
195
• Handles:
» Gateway to the Internet.
» Routes IP packets to the appropriate SGSN.
• If the Mobile Station changes the SGSN during ready mode, the GGSN is
used as data packet buffer.
• Stores:
» Subscriber data for active Mobile Stations.
• Security:
» Firewall.
» Screening.
GGSN (Gateway GPRS Support Node)
196
• New fields have been added to HLR in order to serve the GPRS Network.
» IMSI is still the reference key.
• SGSN Number.
› The SS7 address of SGSN currently serving the MS.
• SGSN Address.
› The IP address of SGSN currently serving the MS.
• MS purged for GPRS.
› Indicates that MM and PDP context of the MS are deleted from SGNS.
• GGSN List.
› The GGSN number and optional IP address are related to the GGSN which will be contacted when activity from the MS is detected.
• Each IMSI “subscriber” record contains zero or more of the following PDP context.
› PDP Type. (e.g.. X25 or IP).
› PDP Address. (Note: This field will be empty if dynamic IP add is used.).
› QoS Profile. (Qos profile for this PDP context).
› VPLMN Address allowed.
› Access Point name. (A label according DNC naming list).
HLR (Home Location Register)
197
• In GPRS, LA is divided into RA. Each RA contains one or more cells.
LA = Location Area.
LAI = MCC+MNC+LAC
RA = Routing Area (Subset of LA)
RAI = LAI+RA
PCU = Packet Control Unit.
CCU = Channel Codec Unit.
LA 1 LA 2 RA 1
RA 3
RA 5
RA 2
RA 4
BTS + CCU
• In a RA, the RAI is broadcasted
as System Information.
• When an MS is crossing an RA
border the MS will initiate an RA
update procedure.
• New elements (CCU , PCU) are
added to the BSS in order to
support new coding schemes
introduced by GPRS.
BSS
BTS
The GPRS GPRS Air and A-bis Interface
199
BTS
CS-4
CS-3
CS-2
CS-1
Coding Schemes in GPRS
• To achieve higher throughput rates per timeslot than plain GSM,
GPRS introduces three new coding schemes.
» CS-1. Throughput =< 8kbit/s. Also provided by GSM.
» CS-2. Throughput =< 12kbit/s.
» CS-3. Throughput =< 14.4kbit/s.
» CS-4. Throughput =< 20kbit/s.
• Due to unpredictable environment of the radio transmission the
distance between MS and the cell impacts the QoS.
» The different CS are therefore not always available.
200
• Channel Codec Unit (CCU).
» The existing CCU used in GSM is upgraded to handle GPRS.
• CH Coding (CS-2 …. CS-4).
• Radio Channel Management (Signal, Strength, Quality and TA).
• Packet Control Unit (PCU).
» The PCU is a very important function for the interfaces in GPRS.
• Communication with CCU using in-band signalling. (One can say that the
PCU is the TRAU of the GPRS network).
• PDCH scheduling.
• Segmentation (LLC to RLC blocks).
• Error Handling (Retransmission of data packets).
TA = Timing Advance.
PDCH = Packet Data Channel.
TRAU = Transcoder Rate Adaptation Unit (Part of a GSM Network).
LLC = Logical Link Control which is part of the GPRS protocol stack..
RLC = Radio Link Control which is part of the GPRS protocol stack..
CCU & PCU
201
PCU (Packet Control Unit)
• Interface the new GPRS core network to the existing GSM BSS. » Converting packet data coming from the SGSN in so called PCU-frames that
have the same format as TRAU-frames. These PCU-frames are transparently routed through the BSC and towards the BTS. The BTS needs to determine the respective coding scheme and other options before processing a PCU-frame.
• Takes over all GPRS radio related control functions from the BSC.
202
• Three different classes of mobile stations have been defined.
• Class A.
» The Mobile Station class A supports simultaneous monitoring and operation of
packet-switched and circuit-switched services.
• Class B.
» The Mobile Station class B supports simultaneous monitoring but not
simultaneous operation of circuit-switched and packet-switched services.
• Class C.
» The Mobile Station class C supports either circuit-switched or packet-switched
monitoring and operation at a given time.
The Mobile Station
203
GPRS Protocols
204
• Like MM in GSM, GMM are used to keep track of the current location of an MS and to initiate security procedures.
• GMM is a function that is mainly handled between the mobile station and the SGSN. However, the HLR is also involved.
• There are various scenarios defined in GPRS to update a subscriber's location within the network. The most important ones are:
» Routing Area Update (Intra-SGSN and Inter-SGSN) » GPRS Attach and Detach » Cell Update (only while in GMM-Ready State)
• The GMM cell update procedure replaces in GPRS what is known as
handover procedure in circuit-switched GSM.
• Due to the fact that a GPRS MS is not constantly “connected” to the network, the GMM has introduced a new state, called “Ready State”.
GMM (GPRS Mobility Management)
205
•Like MM in GSM, GMM handles the roaming and authentication procedure. However, due to the fact that a GPRS MS is not constantly using the resources in Um, the GMM has introduced a new state, called “Ready State”.
• Idle Mode. (MS off or not attached yet.). » If the MS is on, and is a Class B or Class C MS, the MS
will listen to the network, but not make any updating of
where the MS is. It is not possible to page an MS.
• Ready Mode. (MS is able to send and receive data).
» Cell updating is necessary.
» If no activity within the timer (T3314 / Default = 44s) the
MS will fall back to a stand-by state.
» NOTE: an MS can be forced back to standby mode due
to lack of recourses.
• Standby Mode. (MS is listening to the Network).
» Only RA update and periodic update is necessary.
» It is possible to page the MS.
Idle
Ready
Standby
GPRS
Attach GPRS
Detach
Ready
Timer
expired
Data
transfer or
reception
GPRS Mobility Management
206
• GPRS Attach/Detach
» Made towards the SGSN
» The MS must provide its identity (P-TMSI/IMSI) and an indication of which type
of attach that is to be executed (GPRS / combined GPRS/IMSI)
» After GPRS attach the MS is in ”Ready” state and MM contexts are established
in the MS and the SGSN.
• Routing Area Update
» When a GPRS-attached MS detects a new RA (Routing Area)
» When the periodic RA update counter has expired
• Cell Update
» When the MS enters a new cell inside the current RA and the MS is in ”Ready”
state
• Combined RA/LA update
» Only if option Gs-interface i simplemented
GMM Procedures
207
• Allow transfer of user data packets between the MS and some
external packet data network.
• Before data transmission start, SM involves a handshaking
procedure between the MS, SGSN and teh GGSN.
» Establish a PDP context between the MS and the GGSN (includes the
negotiated QoS profile)
Session Management (SM)
208
• Identifies the transaction parameters of an active session of a
GPRS mobile station.
» Note that a GPRS mobile station may support multiple simultaneous
sessions and activated PDP-contexts
• Can be initiated by the network or the MS (in ”Standby” or
”Ready” state)
• Cannot be activated before a GMM context exists. (A GPRS
mobile station first needs to register itself towards the SGSN
before a PDP context activation procedure can be initiated).
• Can be deactivated on request of the MS or the SGSN or the
GGSN by means of the PDP context deactivation procedure
PDP Context Activation
209
• Takes care of the allocation and maintenance of radio
communication paths
• Paging
» The paging procedure moves the MM state to ”Ready” to allow the SGSN to
forward downlink data to the BSS
• TBF Establishment/Release
» A Temporary Block Flow (TBF) is a physical connection used by two RR entities
to support unidirectional transfer of user data or signalling.
» The TBF is an allocated radio resource on one or more Packet Data Channels
(PDCH)
» A TBF is temporary and is maintained only for the duration of the data transfer.
Radio Resource (RR) Management Procedures
210
•In GPRS, the RLC protocol, and the MAC Protocol is in charge of all
radio related control functions on the air interface.
•In GPRS, the LLC Protocol is in charge of transmission between SGSN
and the Mobile Station.
» Delivery of data units to the higher layer in the correct sequence.
•In GPRS, the SNDCP Protocol is in charge of Segmentation and
compression of Data.
» SNDCP represents the highest layer of the GPRS protocol stack. Therefore,
SNDCP provides an interface function between the GPRS protocol stack and the
different packet data protocols.
Physical Layer
MAC
LLC
GMM
RLC
SM IP / X25
SNDCP
GPRS Um protocol stack
= Radio Link Control.
= Medium Access
Control.
= Logical Link Control.
= SubNetwork Dependent Convergence .
GPRS Air Interface
211
• In GSM the A-bis is a well known structured protocol.
• In GPRS the A-bis has more or less become the wild west for the
vendors.
» Each vendor is making his own version of the A-bis protocol.
• Due to the principle of PTCH bundling as well as the new GMM
the known BTMS (BTS Management Protocol for GSM) has been
changed to BSSGP.
= BSS GPRS.
= Frame Relay
Layer1
Network
Service
Relay
A-bis protocol stack
BSSGP
Layer1
RLC
MAC
LLC
GMM/SM
Gb protocol stack
GPRS A-bis Interface
212
Physical Layer (GSM RF)
• The channel coding fucntions (see fig. Below)
• Cell selcetion/reselection
• Setting of Timing Advance
• Perform measurements on the neighbouring cells
• Four channel coding schemes are defined (CS1-4):
213
• The RLC/MAC protocol is defined between the mobile station and
the PCU (Packet Control Unit).
• In charge of all radio related control functions on the air interface.
• Performs segmentation of LLC frames into radio blocks
RLC/MAC (Radio Link Control/Medium Access Control)
214
LLC (Logical Link Control)
• LLC provides different types of services to different upper layer applications, namely SNDCP GMM/SM and SMS.
• Provides the transport frames for the data transfer between MS and SGSN
• Encapsulation of higher layer protocol data units into LLC data
units. (This applies in particular to data units from SNDCP which
are tailored to fit into one LLC data unit. )
• Delivery of data units to the higher layer in the correct sequence.
• Ciphering and Deciphering (if enabled)
215
• Interface function between the GPRS protocol stack and the
different packet data protocols like IP.
• The SNDCP is applicable between the MS and the SGSN.
» Within the SGSN, there is a relay function from the SNDCP towards the
GPRS Tunneling Protocol (GTP).
• Segmentation of user data packets (max. 1520 octets)
» Compression of Packet Data (optional)
• Relies completely on the error recovery and transmission
capabilities of LLC and therefore provides no means for these
functions
• nn
SNDCP (Sub-Network Dependent Convergence Protocol)
216
GPRS Air Interface
R
BTS
SNDPC
LLC
MAC/RLC MAC/RLC Info H Info H Info H
Segment H FSC Towards the SGSN.
E.g. WWW or E-mail IP
Segmentation and compressing
H = Header.
B = Normal Burst, see the GSM
recommendation.
FSC = Frame Check Sequence.
Towards the PCU
Physical Layer 1.
B B B B
LLC frame Max size 1600 octets.
Encryption, error detecting and
retransmission.
RLC Block 20 to 50 octets of data
Into normal radio bursts (58bit*8) = 456bit.
217
L1bis (WAN) L1 (LAN) L1bis (WAN)
MAC
RLC
NS
BSSGP
NS
BSSGP
L2 (PPP)
IP
UDP/TCP
GTP
BSS SGSN Gb
RELAY
RELAY
LLC
LAPG
PLL
RFL
GPRS Gb Interface
SNDCP
218
• BSSGP (Base Station Subsystem GPRS Protocol) » Transparent transfer of signaling and data PDU's between the SGSN and the
PCU
» Administration of the packet-switched link resources between SGSN and PCU.
» Initiation of packet-switched paging for a particular mobile station if requested by the SGSN.
• NS (Network Service) - consists of two sublayers: » The Network Service Control Protocol.
• Provides for virtual connections (NS-VC) between the SGSN and the PCU. These virtual connections need to be administrated by the Network Service protocol
» The Frame Relay Protocol.
• The Network Service is a packet-switched protocol: A single virtual connection may use resources from 0 kbit/s up to the entire bandwidth of the transmission link
» Transports BSSGP PDUs between BSS and SGSN.
Gb protocol layers
219
GPRS Network
Um MAP
ISUP
MAP MAP
ISUP
GSM
A A-bis
BSS
PSTN/ISDN
R
Gi
Gp
Gb Gs
Gf
Gr
PDN
Private
Backbone
Gn
Gn
GPRS
Gc
PDN
220
• Gn - GSN backbone network
» Private IP network intended for GPRS data/signalling only
» Connects the GPRS Support Nodes (GSNs) together within a GPRS PLMN
• Gp – inter-PLMN backbone network » PLMN to PLMN connection (i.e. roaming) via Border Gateways (BG)
» Packet data Network (public Internet or leased line)
• Gi » Interface to external packet data network (IP)
• Gs – SGSN to MSC/VLR » Used to perform IMSI attach and GPRS attach simultaneously
» Combined paging procedures, where all paging is done form SGSN
• Gr – SGSN to HLR » SGSN must contact the HLR whenever a new subscriber enters one of its
Routing Areas
• Gd – SGSN to SMS » Used if SMS is forwarded over GPRS channels
• Gf – SGSN to EIR » Used to check the IMEI number
Other GPRS interfaces
221
L1 (LAN) L1bis (WAN) L1 (LAN)
Frame Relay
BSSGP
L2 (PPP)
IP
UDP/TCP
GTP
L2 (PPP)
IP
UDP/TCP
GTP
IP/X.25
SGSN GGSN Gn
RELAY
LLC
LAPG
Gn Interface
SNDCP
222
• Allows multi-protocol packets to be tunnelled through the GPRS
backbone between GPRS Support Nodes (GSNs).
• Responsible for the transmission of both, signaling information and
application data.
• Based on an IP-protocol stack and uses UDP as transport layer
(OSI layer 4)
• Also takes care of the transfer of charging information. In that
function, GTP is called GTP'
GTP (GPRS Tunnel Protocol)
223
• TCP (Transmission Control Protocol)
» Manages the segmentation of a message or file into smaller packets that are
transmitted over the Internet and received by a TCP layer that reassembles the
packets into the original message.
» A connection-oriented protocol, which means that a virtual connection is
established between the two peers of a TCP-transaction.
» Usually, TCP is used together with IP.
• UDP (User Datagram Protocol )
» Offers a limited amount of service compared to TCP. Most importantly:: UDP
does not provide segmentation or sequencing functions. The application needs
to take of these functions.
» UDP is an alternative to TCP if network applications need to save processing
time.
UDP/TCP
224
I’m still
confused, but
on a higher
level!
Now I know why!
Excellent, I will be
the champ!
To
difficult!
Not good!
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Evaluation