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SUDACADGSM
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History Early 1980s, country isolated analog cellular telephone
systems (interoperability problem) 1982, CEPT (Conference of European Post and
Telecommunications ) established a WG to develop a newpublic land mobile system to span Europe
GSM: Groupe Speciale Mobile (French)
Proposed criteria Good speech quality Low cost for terminals and service International roaming Handheld terminals
Support for introduction of new services Spectral efficiency Compatibility with ISDN
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History (cont)
1989, GSM development responsibility transferred to the
European Telecommunications Standards Institute (ETSI)
1990, GSM phase 1 published
1991, first commercial service launched WG language changed from French to English, and GSM
became Global System for Mobile Communications
1994, phase 2 data/fax services launched
1995, phase 2 standard completed
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Technology
GSM uses a TDMA/FDMA combination
More channels of communication are available
All channels are digital
GSM uses higher frequency bands Provides additional capacity
And higher subscribers densities
GSM is capable for international roamingthrough agreements between GSM operatorsworldwide
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GSM Frequency Bands
450 MHz
Upgrade of older analog cellular systems in Scandinavia
900 MHz
Original band used everywhere except NA and most of SA 1800 MHz
New band used everywhere except NA and most of SA
1900 MHz
PCS band used in NA and most of SA
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Cells and frequency reuse
Service area divided into cells
Available frequencies divided into groups
Frequency used per cell
Same frequency reused in other far away cell
200 kHz, time shared by 8 users
Uplink and downlink separated
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Technical data Multiple Access Method
TDMA / FDMA
BS to MS frequencies
935960 MHz
MS to BS frequencies
890915
Duplexing
FDD
Channel spacing
200 kHz
Modulation
GMSK
Portable TX power, maximum /average (mW)
1000 / 125
Power control
handset and BSS
Speech coding and rate (kbps)
RPE-LTP / 13
Speech Channels per RF channel
8
Channel rate (kbps)
270.833
Channel coding Rate
1/2 convolutional
Frame duration (ms)
4.615
Duplex spacing 45 MHz
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Network Elements
Mobile Station (MS)
Base Transceiver Station (BTS)
Base Station Controller (BSC)
Base Station Subsystem (BSS) Mobile Switching Center (MSC)
Equipment Identity Register (EIR)
Authentication Center (AuC)
Home Location Register (HLR) Visitor Location Register (VLR)
Network and Switching Subsystem (NSS)
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SIM
Subscriber Identity Module or Smart Card
Contains a computer chip and some non-volatile memory
Inserted into a slot in the base of the handset
The memory held info include Subscriber identity number
Telephone number
Original network to which the subscriber belongs
Can be moved from one handset to another
A handset reads the info off the smart card and transmitsit to the network
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MS
Mobile Station
Starting point of a mobile wireless network
Can contain
Mobile Terminal (MT)
GSM cellular handset
Terminal Equipment (TE)
PC or Personal Digital Assistant (PDA)
Can be
Two devices (MT & TE) interconnected with a P-t-P interface
A single device with both functions integrated
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BTS
Base Transceiver Station
A subscriber call request is sent by the MS tothe BTS
Includes all the necessary radio equipment forradio transmission within a cell Antennas, signal processing devices, amplifiers
Responsible for
Establishing the link to the MS Modulating/Demodulating radio signals between
the MS and the BTS
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BSC
Base Station Controller
The controlling component of the radio network
Manages the BTSs
Reserves radio frequencies for communications
Handles the handoff between BTSs when an MS roams
from one cell to another
Responsible for paging the MS for incoming calls
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BSS
Base Station Subsystem
A GSM network is comprised of many BSSs
Each BSS is controlled by a BSC
The BSS performs the necessary functions for
Monitoring radio connections to the MS
Voice coding/decoding
Rate adaptation to/from the wireless network
A BSS can contain several BTSs
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MSC
Mobile Switching Center
A digital switch that sets up connections to
the other MSCs and to the BSCs
The MSCs form the wired (fixed) backbone of
a GSM network and can switch calls to the
PSTN
An MSC can connect to several BSCs
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EIR
Equipment Identity Register
A database that stores the internationalmobile equipment identities (IMEIs) of all the
MSs in the network The IMEI is an equipment identifier assigned
by the manufacturer of the MS
The EIR provide security features such asblocking calls from handsets that have beenstolen
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HLR
Home Location Register
The central database for all users to register tothe GSM network
Stores subscribers static information such as International mobile subscriber identity (IMSI)
Subscribed services
Subscriber authentication key It also stores dynamic subscriber info such as
the current location of the mobile subscriber
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AuC
Authentication Center
A database associated with the HLR
Contains
The algorithms for subscribers authentication
The necessary encryption keys to safeguard
the user input
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VLR
Visitor Location Register
A distributed database that temporarily storesinformation about the MSs that are active in thegeographic area for which the VLR is responsible
A VLR is associated with each MSC in the network When a new subscriber roams into a location area, the
VLR copies subscriber info from the HLR to its localdatabase
This HLR-VLR relationship avoids Frequent HLR database update
Long distance signaling of the user info
Hence allowing faster access to subscriber info
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GSM database
The HLR, VLR, and AuC comprise themanagement database that support roaming(including international roaming) in the GSM
network They authenticate calls while the GSM
subscribers roam between the privatenetwork and the PLMN
They store subscriber identities, currentlocation area, and subscription levels
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NSS
Network and Switching Subsystem
The heart of the GSM system
Connects the wireless network to the standard
wired network
Responsible for calls handoff between BSSs
Perform services such as
Charging
Accounting
Roaming
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GSM network structure
MSC/VLR
HLR
GMSC/VLR
BSC
AuC
NSS
EIR
BTS
BSC
MS
cell
cell
MSBTS
BSS
Other Network
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GSM Interfaces (1)
Various interfaces used for communication betweennetwork elements
A separate interface exists between each pair ofelements
Each interface requires its own set of protocols
Communication over the interfaces occurs in a sequentialmanner
MS to BTS, BTS to BSC, BSC to MSC
And also to the different databases
Communication may traverse multiple MSCs
GMSC is the gateway towards other networks
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Interfaces (2) Um
Air interface (MS to BTS)
Traffic
Voice: 13kbps, Data: 9.6kbps
Signaling
Link Access Procedure-D mobile (LAPDm)
Abis
BTS to BSC
Traffic
16kbps
Signaling LAP-D signaling protocol
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Interfaces (3)
TRAU: Transcoder Rate Adapter Unit
BSC to MSC
A interface
Traffic
Translates between the 16 kbps on the BTS sideand the 64 kbps on the GMSC side
Signaling SS7 protocol, which defines call set-up and call
services across the interface
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Interfaces (4) B
MSC-VLR
No traffic
Signaling
MAP: Mobile Application Part of the SS7 stack
C
MSC-HLR
No traffic
Signaling MAP
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Interfaces (5) D
HLR-VLR
No traffic
Signaling: MAP
E
MSC-MSC
Traffic: 64 kbps
Signaling: MAP, ISUP
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Interfaces (6) F MSC-EIR
No traffic Signaling
G VLR-VLR
No traffic Signaling: MAP
H HLR-AuC
No traffic
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Interface to external world
GMSC-PSTN
GMSC-ISDN
GMSC-PDN
Traffic
64 kbps
Signaling ISUP
TUP
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GSM interfaces
MS
BSC GMSC
MSC
PSTN, ISDN, PDNUm
B
C
D
A
BTS
VLR
E
VLR HLR
Abis
AuC
F
H
EIR
B
D
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Representation of Cells
Ideal cells Fictitious cells
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Cell size and capacity
Cell size determines number of cells availableto cover geographic area and (with frequencyreuse) the total capacity available to all users
Capacity within cell limited by availablebandwidth and operational requirements
Each network operator has to size cells to
handle expected traffic demand
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Cell structure
Implements space division multiplex: base station covers a certaintransmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures:
higher capacity, higher number of users
less transmission power needed more robust, decentralized
base station deals with interference, transmission area etc. locally
Problems:
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side(GSM) - even less for higher frequencies
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Capacity of a Cellular System
Frequency Re-Use Distance
The K factor or the cluster size
Cellular coverage or Signal to interference
ratio
Sectoring
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i
j
1
2
3
4
5
6
7
Frequency re-use distance is based on the cluster size K
The cluster size is specified in terms of the offset of the center of a cluster from the
center of the adjacent cluster
K = i2 + ij + j2
K= 22+ 2*1 + 12
K = 4 + 2 + 1
K = 7
D = 3K * R
D = 4.58R
1
2
35
6
7
D
R
The K factor and Frequency Re-Use Distance
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K = i2 + ij + j2
K= 22+ 2*0 + 02
K = 4 + 0 + 0
K = 4
D = 3K * R
D = 3.46R i
D
R
The Frequency Re-Use for K = 4
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1
2
3
4
1
1
1
1
1
12
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
3
2
Cell Structure for K = 4
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1
11
1
2 2
22
3
3
3
3
4
4 4
45
5 5
5
6
6 6
6
7
7
7
7
8 8
889
99
9
10
1010
10
1111
1111
1212
12 12
Cell Structure for K = 12
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Increasing cellular system capacity
Cell sectoring
Directional antennas subdivide cell into 3 or 6
sectors
Might also increase cell capacity by factor of 3 or 6
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Increasing cellular system capacity
Cell splitting
Decrease transmission power in base and mobile
Results in more and smaller cells
Reuse frequencies in non-contiguous cell groups
Example: cell radius leads 4 fold capacity
increase
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Tri-Sector antenna for a cell
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Highway
TownSuburb
Rural
Cell Distribution in a Network
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f h f
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One Cell = 288 traffic channels
72 Cell = 1728 traffic channels
246 Cell = 5904 traffic channels
Re-use of the frequency
8 X 36 = 288
8 X (72/12 X 36) = 1728
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Concept of TDMA Frames and Channels
f
t
c
GSM combines FDM and TDM: bandwidth is subdividedinto channels of 200khz, shared by up to eight stations,
assigning slots for transmission on demand.
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GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
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/
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1 2 3 4 5 6 7 8
higher GSM frame structures
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz124 channels (200 kHz)
uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 s577 s
guard
space
guard
spacetail user data TrainingS S user data tail
3 bits 57 bits 26 bits 57 bits1 1 3
GSM - TDMA/FDMA
LOGICAL CHANNELS
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TRAFFIC SIGNALLING
FULL RATE
Bm 22.8 Kb/S
HALF RATE
Lm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCHRACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL
FACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS
B d Ch l BCH
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Broadcast Channel - BCH
Broadcast control channel (BCCH) is a base tomobile channel which provides general informationabout the network, the cell in which the mobile iscurrently located and the adjacent cells
Frequency correction channel (FCCH) is a base tomobile channel which provides information forcarrier synchronization
Synchronization channel (SCH) is a base to mobilechannel which carries information for framesynchronization and identification of the basestation transceiver
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Common Control Channel - CCH
Paging channel (PCH) is a base to mobile channel used
to alert a mobile to a call originating from the network
Random access channel (RACH) is a mobile to base
channel used to request for dedicated resources Access grant channel (AGCH) is a base to mobile
which is used to assign dedicated resources (SDCCH or
TCH)
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Dedicated Control Channel - DCCH
Stand-alone dedicated control channel (SDCCH)
is a bi-directional channel allocated to a specific
mobile for exchange of location update
information and call set up information
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Dedicated Control Channel - DCCH
Slow associated control channel (SACCH) is a bi-directionalchannel used for exchanging control information between base
and a mobile during the progress of a call set up procedure. The
SACCH is associated with a particular traffic channel or stand
alone dedicated control channel Fast associated control channel (FACCH) is a bi-directional
channel which is used for exchange of time critical information
between mobile and base station during the progress of a call.
The FACCH transmits control information by stealing capacityfrom the associated TCH
LOGICAL CHANNELS
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TRAFFIC SIGNALLING
FULL RATE
Bm 22.8 Kb/S
HALF RATE
Lm 11.4 Kb/S
BROADCAST COMMON CONTROL DEDICATED CONTROL
FCCH SCH BCCH
PCHRACH
AGCH
SDCCH SACCH FACCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL
FACCH -- FAST ASSOCIATED CONTROL CHANNEL
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS
Location update from the mobile
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Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH authenticate
SDCCH switch to cipher mode
SDCCH request for location updating
SDCCH authenticate response
SDCCH cipher mode acknowledge
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode
Call establishment from a mobile
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Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH do the authentication and TMSI allocation
SDCCH require traffic channel assignment
SDCCH send call establishment request
SDCCH send the setup message and desired number
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH acknowledge connect message and use TCH
TCH conversation continues
FACCH receive connect message
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GSM speech coding
AIR INTERFACE
UPLINK
890
-915
MHz
DOWNLI
NK935
-960M
Hz
MOBILE
BASE TRANSCEIVER STATION
h
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Transmit Path
BS Side
8 bit A-Law
to
13 bit UniformRPE/LTP speech Encoder
To Channel Coder 13Kbps
8 K sps
MS Side
LPF A/DRPE/LTP speech Encoder
To Channel Coder 13Kbps
8 K sps,
Sampling Rate - 8K
Encoding - 13 bit Encoding (104 Kbps)
RPE/LTP - Regular Pulse Excitation/Long Term Prediction
RPE/LTP converts the 104 Kbps stream to 13 Kbps
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GSM Speech Coding
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GSM Speech Coding
Speech is divided into 20 millisecond samples,
each of which is encoded as 260 bits, giving a
total bit rate of 13 kbps.
Regular pulse excited -- linear predictive coder(RPE--LPC) with a long term predictor loop is
the speech coding algorithm.
The 260 bits are divided into three classes:
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e 60 b ts a e d ded to t ee c asses
Class Ia 50 bits - most sensitive to bit errors.
Class Ib 132 bits - moderately sensitive to bit errors.
Class II 78 bits - least sensitive to bit errors.
Class Ia bits have a 3 bit cyclic redundancy code added for error
detection = 50+3 bits.
132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136.
Class Ia + class Ib = 53+136=189, input into a 1/2 rate convolution
encoder of constraint length 4. Each input bit is encoded as twooutput bits, based on a combination of the previous 4 input bits. The
convolution encoder thus outputs 378 bits, to which are added the 78
remaining class II bits.
Thus every 20 ms speech sample is encoded as 456 bits, giving a bit
rate of 22.8 kbps.
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GSM Protocol Suite
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BTS
Radio interface
HLR
MSC
VLR
BSC
RR
MM + CM
SS
Link Layer
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Link Layer
LAPDm is used between MS and BTS
LAPD is used between BTS-BSC
MTP2 is used between BSC-MSC/VLR/HLR
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Network Layer
To distinguish between CC, SS, MM and RR protocoldiscriminator (PD) is used as network address.
CC call control management MS-MSC.
SS supplementary services management MS-MSC/HLR.
MM mobility management(location management, securitymanagement) MS-MSC/VLR.
RR radio resource management MS-BSC.
Messages pertaining to different transaction are
distinguished by a transaction identifier (TI).
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Application Layer protocols
BSSMAP between BSC and MSC DTAP messages between MS and MSC.
All messages on the A interface bear a discriminationflag, indicating whether the message is a BSSMAP or
a DTAP. DTAP messages carry DLCI(information on type of
link on the radio interface) to distinguish what isrelated to CC or SMS.
MAP protocol is the one between neighbor MSCs.MAP is also used between MSC and HLR.
GSM Functional Architecture and Principal Interfaces
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Q.921
Radio Interface
Q.931
Q.921
MAP
TCAP
CCS7 MTP
CCS7 SCCP
Mobile Application Part
Q931 BSSAP
SCCP
CCS7 MTP
A Interface
A-Bis Interface
Um
Base Station System
GSM Functional Architecture and Principal Interfaces
GSM protocol layers for signaling
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GSM protocol layers for signaling
CM
MM
RR
MM
LAPDm
radio
LAPDm
radio
LAPD
PCM
RR BTSM
CM
LAPD
PCM
RRBTSM
16/64 kbit/s
Um Abis A
SS7
PCM
SS7
PCM
64 kbit/s /2.048 Mbit/s
MS BTS BSC MSC
BSSAPBSSAP
Protocols involved in the radio interface
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Protocols involved in the radio interface
Level 1-Physical
TDMA frame
Logical channels multiplexing
Level 2-LAPDm(modified from LAPD)
No flag
No error retransmission mechanism due to real time constraints
Level 3-Radio Interface Layer (RIL3) involves three sub layers
RR: paging, power control, ciphering execution, handover
MM: security, location IMSI attach/detach
CM: Call Control(CC), Supplementary Services(SS), Short Message
Services(SMS),
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LAPDm on radio interface
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LAPDm on radio interface
In LAPDm the use of flags is avoided.
LAPDm maximum length is 21 octets ofinformation. It makes use of more bit to
distinguish last frame of a message. No frame check sequence for LAPDm, it uses
the error detecting performance of thetransmission coding scheme offered by thephysical layer
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LAPDm on radio interface
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LAPDm on radio interface
The acknowledgement for the next expected frame in theindicator N(R ).
On radio interface two independent flows(one for signaling,and one for SMS) can exist simultaneously.
These two flows are distinguished by a link identifier calledthe SAPI(service access point identifier).
LAPDm SAPI=0 for signaling and SAPI=3 for SMS.
SAP1=0 for radio signaling, SAPI=62 for OAM and SAPI=63 forlayer 2 management on the Abis interface.
There is no need of a TEI, because there is no need todistinguish the different mobile stations, which is done bydistinguishing the different radio channels.
Protocols involved in the A-bis interface
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Protocols involved in the A bis interface
Level 1-PCM transmission (E1 or T1) Speech encoded at 16kbit/s and sub multiplexed in
64kbit/s time slots.
Data which rate is adapted and synchronized.
Level 2-LAPD protocol, standard HDLC Radio Signaling Link (RSL)
Operation and Maintenance Link (OML).
Level 3-Application Protocol
Radio Subsystem Management (RSM)
Operation and Maintenance procedure (OAM)
Presentation of A-bis Interface
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Presentation of A bis Interface
Messages exchanges between the BTS and BSC. Traffic exchanges
Signaling exchanges
Physical access between BTS and BSC is PCM
digital links of E1(32) or T1(24) TS at 64kbit/s. Speech:
Conveyed in timeslots at 4X16 kbit/s
Data:
Conveyed in timeslots of 4X16 kbit/s. The initial userrate, which may be 300, 1200, is adjusted to 16kbit/s
LAPD message structure
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FLAG ADRESS CONTROL INFORMATION 0260 OCT FCS FLAG
SAPI TEI
N(S) N(R)
LAPD message structure
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LAPD
The length is limited to 260 octets of information.
LAPD has the address of the destination terminal, to
identify the TRX, since this is a point to multipoint
interface. Each TRX in a BTS corresponds to one or several
signaling links. These links are distinguished by TEI
(Terminal Equipment Identities).
SAPI=0, SAPI=3, SAPI=62 for OAM.
Signaling Protocol Model
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Presentation on the A-Interface
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Presentation on the A Interface
BSSMAP- deals with procedures that take place logically between the BSSand
MSC, examples:
Trunk Maintenance,Ciphering, Handover, Voice/Data Trunk
Assignment
DTAP- deals with procedures that take place logically between the MSand
MSC. The BSSdoes not interpret the DTAPinformation, it simply repackages it
and sends it to the MSover the Um Interface. examples:
Location Update,MS originated and terminated Calls, Short Message
Service, User Supplementary Service registration, activation, deactivation
and erasure
I t MSC t ti
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Inter MSC presentation
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O
AM
L
A
P
D
BTS
MTP2
SCCP
MTP3
L
A
P
D
O
AM
RR
DT
A
P
BS
S
M
A
P
BSSAP
BSC
MTP1
MTP3
MTP2
SCCP
MTP2
MTP3
SCCP
BSSAP
DTAP/BSSMAP
T
CA
P
MM
CM M
A
P
NSS
R
R
MM
CM
MS
Um
Interface
A bis
Interface
A
Interface
B S i
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Bearer Services
Telecommunication services to transfer databetween access points
Specification of services up to the terminal interface(OSI layers 1-3)
Different data rates for voice and data (originalstandard)
Data service
Synchronous: 2.4, 4.8 or 9.6 kbit/s
Asynchronous: 300 - 1200 bit/s
Tele Services
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Tele Services Telecommunication services that enable voice communication via
mobile phones.
All these basic services have to obey cellular functions, security
measurements etc.
Offered services.
Mobile telephony
primary goal of GSM was to enable mobile telephony offering thetraditional bandwidth of 3.1 kHz.
Emergency number
common number throughout Europe (112); Mandatory for all
service providers; Free of charge; Connection with the highest
priority (preemption of other connections possible). Multinumbering
several ISDN phone numbers per user possible.
Performance characteristics of GSM
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Communication
mobile, wireless communication; support for voice and data
services
Total mobility
international access, chip-card enables use of access points of
different providers
Worldwide connectivity one number, the network handles localization
High capacity
better frequency efficiency, smaller cells, more customers per cell
High transmission quality
high audio quality and reliability for wireless, uninterrupted
phone calls at higher speeds (e.g., from cars, trains)
Security functions
access control, authentication via chip-card and PIN
Di d t f GSM
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Disadvantages of GSM
No full ISDN bandwidth of 64 kbit/s to the user
Reduced concentration while driving
Electromagnetic radiation
Abuse of private data possible High complexity of the system
Several incompatibilities within the GSM standards
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Thank You