Overview of GSM Cellular
Network and Operations
Ganesh Srinivasan
NTLGSPTN
Network and switching subsystem
• NSS is the main component of the public mobile network GSM
– switching, mobility management, interconnection to other
networks, system control
• Components
– Mobile Services Switching Center (MSC)
controls all connections via a separated network to/from a mobile
terminal within the domain of the MSC - several BSC can belong
to a MSC
– Databases (important: scalability, high capacity, low delay)
• Home Location Register (HLR)
central master database containing user data, permanent and
semi-permanent data of all subscribers assigned to the HLR
(one provider can have several HLRs)
• Visitor Location Register (VLR)
local database for a subset of user data, including data about all
user currently in the domain of the VLR
Operation subsystem
• The OSS (Operation Subsystem) enables centralized operation,
management, and maintenance of all GSM subsystems
• Components
– Authentication Center (AUC)
• generates user specific authentication parameters on request of
a VLR
• authentication parameters used for authentication of mobile
terminals and encryption of user data on the air interface
within the GSM system
– Equipment Identity Register (EIR)
• registers GSM mobile stations and user rights
• stolen or malfunctioning mobile stations can be locked and
sometimes even localized
– Operation and Maintenance Center (OMC)
• different control capabilities for the radio subsystem and the
network subsystem
Mobile Handset
TEMPORARY DATA
PERMANENT DATA
- Temporary Subscriber Identity
Permanent Subscriber Identity
- Current Location
Key/Algorithm for Authentication.
- Ciphering Data
Provides access to the GSM n/w
Consists of
Mobile equipment (ME)
Subscriber Identity Module (SIM)
The GSM Radio Interface
AIR INTERFACE
BASE TRANSCEIVER STATION
DOW
935
NK
NLI
- 960
z
MH
MOBILE
Hz
5M
89
NK
LI
UP
0
1
-9
The GSM Network Architecture
• Time division multiple access-TDMA
• 124 radio carriers, inter carrier spacing
200khz.
• 890 to 915mhz mobile to base - UPLINK
• 935 to 960mhz base to mobile DOWNLINK
• 8 channels/carrier
GSM uses paired radio channels
890MHz
0
915MHz
124
935MHz
0
960MHz
124
Access Mechanism
– FDMA, TDMA, CDMA
Frequency multiplex
• Separation of the whole spectrum into smaller frequency bands
• A channel gets a certain band of the
spectrum for the whole time
k1
k2
k3
k4
• Advantages:
c
– no dynamic coordination
necessary
– works also for analog signals
• Disadvantages:
– waste of bandwidth
if the traffic is
distributed unevenly
t
– inflexible
– guard spaces
k5
k6
f
Time multiplex
• A channel gets the whole spectrum for a certain amount of
time
• Advantages:
– only one carrier in the
medium at any time
– throughput high even
for many users
• Disadvantages:
– precise
synchronization
necessary
t
k1
k2
k3
k4
k5
k6
c
f
Time and Frequency Multiplex
• Combination of both methods
• A channel gets a certain frequency band for a certain
amount of time
k
k
k
k
1
2
3
4
k5
k6
c
f
t
Time and Frequency Multiplex
• Example: GSM
• Advantages:
– Better protection against
tapping
– Protection against frequency
selective interference
– Higher data rates compared to
code multiplex
• But: precise coordination
required
t
k1
k2
k3
k4
k5
k6
c
f
• GSM combines FDM and TDM: bandwidth
is subdivided into channels of 200khz,
shared by up to eight stations, assigning
slots for transmission on demand.
GSM uses paired radio channels
890MHz
0
915MHz
124
935MHz
0
960MHz
124
Code Multiplex
k1
•
•
•
•
•
Each channel has a unique code
All channels use the same spectrum at the same
time
Advantages:
– Bandwidth efficient
– No coordination and synchronization
necessary
– Good protection against interference and
tapping
Disadvantages:
– Lower user data rates
– More complex signal regeneration
Implemented using spread spectrum technology
k2
k3
k4
k5
k6
c
f
t
Various Access Method
Cells
Capacity & Spectrum Utilization
Solution
The need:
• Optimum spectrum
usage
• More capacity
• High quality of
service
• Low cost
increase capacity
without adding NEW BTS!
I wish I could
What can I do?
Network capacity at required QoS
with conventional frequency plan
Out of
Capacity!!!
Subscriber
growth
Time
Representation of Cells
Ideal cells
Fictitious cells
Cell size and capacity
• Cell size determines number of cells
available to cover geographic area and (with
frequency reuse) the total capacity available
to all users
• Capacity within cell limited by available
bandwidth and operational requirements
• Each network operator has to size cells to
handle expected traffic demand
Cell structure
• Implements space division multiplex: base station covers a certain
transmission 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
Capacity of a Cellular System
• Frequency Re-Use Distance
• The K factor or the cluster size
• Cellular coverage or Signal to interference
ratio
• Sectoring
The K factor and Frequency Re-Use Distance
7
6
K=
i2 +
ij +
1
K = 22 + 2*1 + 12
5
j
K=4+2+1
K=7
2
j2
7
6
3
R
2
i
1
D
5
3
4
D = 3K * R
Frequency re-use distance is based on the cluster size K
D = 4.58R
The cluster size is specified in terms of the offset of the center of a cluster from the
center of the adjacent cluster
The Frequency Re-Use for K = 4
K = i2 + ij + j2
K = 22 + 2*0 + 02
K=4+0+0
D
K=4
D = 3K * R
D = 3.46R
R
i
The Cell Structure for K = 7
7
6
2
1
5
7
6
3
4
1
2
2
1
5
3
7
6
4
2
1
7
6
5
2
1
5
7
6
3
4
4
2
1
5
3
4
3
Cell Structure for K = 4
1
1
1
2
4
4
3
4
2
3
1
4
1
4
3
2
3
1
4
1
2
3
2
4
3
2
3
2
Cell Structure for K = 12
9
8
9
10
2
7
11
3
1
6
9
8
7
3
6
4
5
10
7
1
6
11
3
12
4
5
12
4
5
2
12
3
6
8
11
1
9
11
1
7
4
10
10
2
12
5
2
8
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
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: ½ cell radius leads 4 fold capacity
increase
Tri-Sector antenna for a cell
Cell Distribution in a Network
Rural
Highway
Suburb
Town
Optimum use of frequency
spectrum
• Operator bandwidth of 7.2MHz (36 freq of 200
kHz)
• TDMA 8 traffic channels per carrier
• K factor = 12
• What are the number of traffic channels available
within its area for these three cases
– Without cell splitting
– With 72 cells
– With 246 cells
Re-use of the frequency
One Cell = 288 traffic channels
8 X 36 = 288
72 Cell = 1728 traffic channels
8 X (72/12 X 36) = 1728
246 Cell = 5904 traffic channels
Concept of TDMA Frames and
Channels
c
f
t
• GSM combines FDM and TDM: bandwidth is subdivided
into channels of 200khz, shared by up to eight stations,
assigning slots for transmission on demand.
GSM uses paired radio channels
890MHz
0
915MHz
124
935MHz
0
960MHz
124
GSM delays uplink TDMA frames
The start of the uplink
TDMA is delayed of
three time slots
TDMA frame (4.615 ms)
Downlink TDMA
F1MHz
R1 R2 R3 R4 R5 R6 R7 R8
T1 T2 T3 T4 T5 T6 T7 T8
R
T
R
T
Fixed transmit
Delay of three time-slots
Uplink TDMA
Frame
F1 + 45MHz
GSM - TDMA/FDMA
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
higher GSM frame structures
time
GSM TDMA frame
1
2
3
4
5
6
7
8
4.615 ms
GSM time-slot (normal burst)
guard
space
tail
3 bits
user data
S Training S
user data
57 bits
1 26 bits 1
57 bits
guard
tail space
3
546.5 µs
577 µs
LOGICAL CHANNELS
TRAFFIC
FULL RATE
Bm 22.8 Kb/S
SIGNALLING
HALF RATE
Lm 11.4 Kb/S
BROADCAST
FCCH
SCH
COMMON CONTROL
DEDICATED CONTROL
BCCH
PCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- 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
RACH
AGCH
SDCCH
SACCH
FACCH
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS
Broadcast Channel - BCH
• Broadcast control channel (BCCH) is a base to
mobile channel which provides general information
about the network, the cell in which the mobile is
currently located and the adjacent cells
• Frequency correction channel (FCCH) is a base to
mobile channel which provides information for
carrier synchronization
• Synchronization channel (SCH) is a base to mobile
channel which carries information for frame
synchronization and identification of the base
station transceiver
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)
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
Dedicated Control Channel DCCH
• Slow associated control channel (SACCH) is a bi-directional
channel 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 capacity from the associated TCH
DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms
NORMAL BURST
- NB
FREQUENCY
CORRECTION
BURST - FB
SYNCHRONISATION
BURST - SB
57
3
1
26
142
3
39
3
ACCESS
BURST - AB
TAIL BIT
ENCRYPTION BIT
6
57
1
39
GUARD PERIOD
TRAINING BITS
3
36
8.25
3 8.25
64
41
3
FIXED BITS
FLAG BITS
3 8.25
68.25
SYNCHRONISATION BITS
MIXED BITS
HIERARCHY OF FRAMES
1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )
0
1
2
3
4
5
6
2043
2044 2045 2046 2047
1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S )
LEFT (OR) RIGHT
1 SUPER FRAME = 51 MULTI FRAMES
TRAFFIC CHANNELS
0
1
2
3
4
48
49 50
SIGNALLING CHANNELS
1 SUPER FRAME = 26 MULTI FRAMES
0
1
2
24
25
1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )
0 1 2
3
24 25
1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )
0 1 2
0 1
2
3
4 5
(4.615ms)
0
6
7
0
1
2
3
4
5 6
3
4
0
1
48 49 50
7 0
TDMA FRAME NO.
1
1 TIME SLOT = 156.25 BITS
( 0.577 ms)
1 2 3 4
155 156
1 bit =36.9 micro sec
0 1
2
3
4 5
0
6
7
2
3
4
(4.615 ms)
1
5
6
7 0
GSM Frame
SACCH is
transmitted
in frame 12
0 to 11 and 13 to 24
Are used for traffic data
0
1
0
3
Full rate
channel is
idle in 25
57
2
1
12
2
1
3
26
4
1
24
5
Frame
duration =
120ms
25
6
57
Frame
duration =
60/13ms
7
3
8.25
Frame
duration =
15/26ms
• 114 bits are available for data transmission.
• The training sequence of 26 bits in the
middle of the burst is used by the receiver to
synchronize and compensate for time
dispersion produced by multipath
propagation.
• 1 stealing bit for each information block
(used for FACCH)
LOGICAL CHANNELS
TRAFFIC
FULL RATE
Bm 22.8 Kb/S
SIGNALLING
HALF RATE
Lm 11.4 Kb/S
BROADCAST
FCCH
SCH
COMMON CONTROL
DEDICATED CONTROL
BCCH
PCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- 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
RACH
AGCH
SDCCH
SACCH
FACCH
DOWN LINK ONLY
UPLINK ONLY
BOTH UP &
DOWNLINKS
Location update from the mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH request for location updating
SDCCH authenticate
SDCCH authenticate response
SDCCH switch to cipher mode
SDCCH cipher mode acknowledge
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode
Call establishment from a mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH send call establishment request
SDCCH do the authentication and TMSI allocation
SDCCH send the setup message and desired number
SDCCH require traffic channel assignment
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH receive connect message
FACCH acknowledge connect message and use TCH
TCH conversation continues
Call establishment to a mobile
Mobile looks for BCCH after switching on
Mobile receives paging message on PCH
Generate Channel Request on RACH
Receive signaling channel SDCCH on AGCH
Answer paging message on SDCCH
Receive authentication request on SDCCH
Authenticate on SDCCH
Receive setup message on SDCCH
Receive traffic channel assignment on SDCCH
FACCH switch to traffic channel and send ack (steal bits)
Receive alert signal and generate ringing on FACCH
Receive connect message on FACCH
FACCH acknowledge connect message and switch to TCH
GSM speech coding
AIR INTERFACE
BASE TRANSCEIVER STATION
DOW
935
NK
NLI
- 960
z
MH
MOBILE
Hz
5M
89
NK
LI
UP
0
1
-9
Transmit Path
BS Side
8 bit A-Law
to
13 bit Uniform
8 K sps
RPE/LTP speech Encoder
To Channel Coder 13Kbps
RPE/LTP speech Encoder
To Channel Coder 13Kbps
MS Side
8 K sps,
LPF
A/D
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
GSM Speech Coding
• GSM is a digital system, so speech which is
inherently analog, has to be digitized.
• The method employed by current telephone
systems for multiplexing voice lines over
high speed trunks and is pulse coded
modulation (PCM). The output stream from
PCM is 64 kbps, too high a rate to be
feasible over a radio link.
GSM Frame
SACCH is
transmitted
in frame 12
0 to 11 and 13 to 24
Are used for traffic data
0
1
0
3
Full rate
channel is
idle in 25
57
2
1
12
2
1
3
26
4
1
24
5
Frame
duration =
120ms
25
6
57
Frame
duration =
60/13ms
7
3
8.25
Frame
duration =
15/26ms
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:
– 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 two output
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.
• To further protect against the burst errors common to the
radio interface, each sample is interleaved. The 456 bits
output by the convolution encoder are divided into 8
blocks of 57 bits, and these blocks are transmitted in eight
consecutive time-slot bursts. Since each time-slot burst can
carry two 57 bit blocks, each burst carries traffic from two
different speech samples.
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
3
57 bits
1 26
1
57 bits
3
GSM Protocol Suite
SS
HLR
MM + CM
MSC
VLR
RR
BSC
BTS
Radio interface
Link Layer
• LAPDm is used between MS and BTS
• LAPD is used between BTS-BSC
• MTP2 is used between BSCMSC/VLR/HLR
Network Layer
• To distinguish between CC, SS, MM and RR
protocol discriminator (PD) is used as network
address.
– CC call control management MS-MSC.
– SS supplementary services management MSMSC/HLR.
– MM mobility management(location management,
security management) MS-MSC/VLR.
– RR radio resource management MS-BSC.
• Messages pertaining to different transaction are
distinguished by a transaction identifier (TI).
Application Layer protocols
• BSSMAP between BSC and MSC
• DTAP messages between MS and MSC.
• All messages on the A interface bear a
discrimination flag, 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
is related 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
Mobile Application Part
A Interface
MAP
Q931 BSSAP
TCAP
CCS7 SCCP
CCS7 MTP
SCCP
CCS7 MTP
Um
Base Station System
Q.921
Radio Interface
Q.931
Q.921
A-Bis Interface
GSM protocol layers for
signaling
Um
Abis
MS
A
BTS
BSC
MSC
CM
CM
MM
MM
RR
RR’
BTSM
RR’
BTSM
LAPDm
LAPDm
LAPD
LAPD
radio
radio
PCM
PCM
16/64 kbit/s
BSSAP
BSSAP
SS7
SS7
PCM
PCM
64 kbit/s /
2.048 Mbit/s
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),
LAPDm on radio interface
• In LAPDm the use of flags is avoided.
• LAPDm maximum length is 21 octets of
information. 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 the
transmission coding scheme offered by the
physical layer
LAPDm Message structure
ADDRESS
CONTROL
INFORMATION 0-21 OCTETS
SAPI
N(S)
N(R)
LAPDm on radio interface
• The acknowledgement for the next expected frame in the
indicator 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
called the 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 for layer 2 management on the Abis interface.
• There is no need of a TEI, because there is no need to
distinguish the different mobile stations, which is done by
distinguishing the different radio channels.
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
• 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 user
rate, which may be 300, 1200, … is adjusted to 16
kbit/s
LAPD message structure
FLAG
SAPI
ADRESS
CONTROL
INFORMATION 0 – 260 OCT
FCS
TEI
N(S)
N(R)
FLAG
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.
Presentation of the A-ter interface
TRAU
BSC
LAPD TS1
OAM
Speech TS
Transcoding
CCS7 TS
X.25 TS2
PCM
LINK
Speech TS
CCS7 TS
X.25 TS2
PCM
LINK
MSC
OMC
Presentation on the A-ter
interface
• Signaling messages are carried on specific timeslots (TS)
– LAPD signaling TS between the BSC and the TCU
– SS7 TS between the BSC and the MSC, dedicated for BSSAP
messages transportation.
– X25 TS2 is reserved for OAM.
• Speech and data channels (16kbit/s)
• Ater interface links carry up to:
– 120 communications(E1), 4*30
– 92 communications(T1).
• The 64 kbit/s speech rate adjustment and the 64 kbit/s data rate
adaptation are performed at the TCU.
Presentation of the A interface
Signaling Protocol Model
Presentation on the A-Interface
BSSMAP - deals with procedures that take place logically between the BSS and
MSC, examples:
Trunk Maintenance, Ciphering, Handover, Voice/Data Trunk
Assignment
DTAP - deals with procedures that take place logically between the MS and
MSC. The BSS does not interpret the DTAP information, it simply repackages it
and sends it to the MS over the Um Interface. examples:
Location Update, MS originated and terminated Calls, Short Message
Service, User Supplementary Service registration, activation, deactivation
and erasure
Inter MSC presentation
MS
NSS
CM
CM
MM
BTS
O
A
M
R
R
MM
BSC
BSSAP
O
A
M
L
A
P
D
R
R
L
A
P
D
M
A
P
BSSAP
DTAP/
BSSMAP
T
C
A
P
SCCP
SCCP
SCCP
MTP3
MTP3
MTP3
MTP2
MTP2
MTP2
D
T
A
P
B
S
S
M
A
P
MTP1
Um
Interface
A bis
Interface
A
Interface
MS
BSC
MSC
PD=RR
PD=MM
TI=a
TI=b
PD=CC
DLCI: SAPI=0
Link: SAPI=0
Link: SAPI=3
Channel=C1
TI=A
DLCI: SAPI=3
Channel ID = N1
DTAP
SCCP Ref=R1
Channel=C2
Channel ID = N1
SCCP Ref=R2
TRX:TEI=T1
Radio Interface
Abis Interface
A Interface
PD: protocol discriminator
TI: Transaction Identifier for
RIL3-CC protocol
DLCI: Data Link connection
Identifier
SAPI: Service Access Point
Identifier on the radio
Interface
TEI: Terminal Equipment
Identifier on the Abis I/F
Bearer Services
• Telecommunication services to transfer data
between access points
• Specification of services up to the terminal
interface (OSI layers 1-3)
• Different data rates for voice and data (original
standard)
– Data service
• Synchronous: 2.4, 4.8 or 9.6 kbit/s
• Asynchronous: 300 - 1200 bit/s
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 the
traditional 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
• 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
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
Thank You