Overview of the GSM System

1
Overview of the GSM System
PLMN
EIR
MSC area
MSC area
MSC area
PLMN
HLR
MSC area
MSC area
HLR
EIR
EIR
PLMN
MSC area
MSC area
EIR
MSC area
MSC area
HLR
MSC area
MSC area
MSC area
MSC area
HLR
HLR
EIR
MSC area
MSC area
HLR
EIR
EIR
MSC area
MSC area
PLMN
MSC area
HLR
MSC area
GSM
MSC area
HLR
EIR
The GSM system is made up of sub-networks called
Public Land Mobile Network's (PLMN). Each member
country has one or more PLMN depending on its
size.
2
Hierarchy of Areas
Cell
Location Area (locating paging area)
MSC Service Area (area controlled by one MSC)
PLMN ( one or more per country)
GSM Service Area ( all member countries)
Location Area Identity (LAI)
3 digits
3 digits
2 Octet (max)
MCC
MNC
LAC
Mobile Country Code
Mobile Network Code
Location Area Code
3
The MSC Area

• The MSC area consists of one MSC and several
  BSS's
• The MSC provides the external interface, either
  directly or through a Gateway MSC
• Each MSC is connected to a Visitor Location
  Registry (VLR)
• The MSC also has access to a Home Location
  Registry (HLR) and Equipment Identification
  Registry (EIR)

VLR
HLR
EIR
MSC
BSS
BSS
BSS
BSS
BSS
4
The GSM Network Model
VLR
EIR
HLR
VLR
AC
G
D
H
C
B
F
MSC
MSC
Ai
E
A
Di
MS Mobile Station BS Base Station BSC
Base Station Controller MSC Mobile Switching
Center HLR Home Location Registry VLR Visitor
Location Registry AC Authentication
Center EIR Equipment Identity Registry PSTN Publ
ic Switched Telephone Network ISDN Integrated
Services Digital Network
BSC
Abis
Um
MS
5
The GSM System Hierarchy
Public Land Mobile Network
MSC
BSC
TRAU
BTS
6
The TRAU Unit

• The Transcoding Rate and Adaptation Unit (TRAU)
  is typically located between the MSC and BSC.
• It could also be placed between the BSC and the
  BTS's
• It converts the 64 kbps PCM-speech into 16 kbps
  compressed speech 13 kbps speech 3 kbps
  overhead
• It uses speech vocoding technique.
• There is an equivalent unit in the Mobile Station
  (MS)

7
The TRAU Unit (cont.)

• The TRAU unit could be physically located with
  the MSC to save transmitting 64 kbps/speech
  connection
• If the connection is "data connection" (rather
  than speech), the unit is turned off
• In the MS, the same vocoding technique is used to
  convert analog signal into digital speech at 13
  kbps (full rate)
• The unit could also operate at 6.5 kbps (half
  rate)

MSC
TRAU
BSC
Full
64 kbps
16 kbps
13 kbps
8 kbps
6.5 kbps
Half
8
GSM Typical Voice Connection
PSTN
64 kbps
64 kbps
MSC
GW
TRAU
16 kbps
BSC
BTS
16 kbps
22.8 kbps
9
Inter-Working Facility (IWF)
Protocol Translator
MSC
IWF
BSC
BS
Point-to-Point Protocol (PPP) Circuit-Switched
Different Data Protocols
10
GSM (voice)/GPRS (data)
Um
A
Voice
BSC
MSC/VLR
GMSC
TRAU
V A S
I N

• Circuit Data could be transmitted over GSM voice
  channels
• The GPRS is a complete IP private network that
  connects many cell sites

HLR/AuC/EIR
Gb
Data
SGSN
GGSN
GPRS
11
GSM Air Interface

• Base Station Sub-System (BSS)
• Base Station Structure
• Mobile Station Transceiver
• Time-Frequency Arrangement

12
Base Transceiver Station

• Typically the Base Transceiver Stations exist in
  clusters of three (120o sectors). The diameter
  of the cell is between 300 m and 35 km).
• Each BTS has a different Cell Identity (CI)
• Some BTS could serve what is called "Umbrella
  Cells" to serve fast mobile units.

BTS
BTS
BTS
120-sectored Cell
Umbrella Cell
13
Base Transceiver Station (cont.)

• Each BTS has several Transmit/Receive (Tx/Rx)
  units.
• The maximum number of Tx/Rx units per BTS is 16
• The BTS also has control circuits for operation,
  management and clock distribution

14
The Transmit/Receive Module

• The Tx/Rx unit consists of five sections
• Data interface unit to provide interface with the
  BSC
• Baseband signal processing unit
• Frequency Hopping and Radio frequency control
  module
• Tx/Rx RF section
• Control unit

To BSC
Signal Processing
Frequency Hopping
Data Interface
Diversity
Control
15
The Mobile Station (MS)

• The user identity is separate from the equipment
  identity.
• The user information is stored in the SIM
  (Subscriber Information Module). Also known as
  the smart card
• Different processing blocks are used to process
  the voice/data

Radio Transceiver
16
Time-Frequency Plan
17
Time-Frequency Plan (cont.)

• Each sector is served by one BTS
• The BTS can support up to 16 Tx/Rx units
• Each Tx/Rx unit operates on a single carrier
  frequency and supports up to 8 voice circuits
• The frequency carriers are arranged in an ABCD
  frequency plan
• The channel bandwidth is 200 kHz
• The minimum frequency separation within one BTS
  is 4x200800 kHz

18
Frequencies within the Same Cell
19
The Antenna Assembly Unit
Ftransmit
Power Amplifier
Duplexer
coupler
Freceive
LNA
LNA Low Noise Amplifier
20
The TDMA Frame
The basic GSM physical channel consists of one
slot to transmit on and a corresponding slot to
receive on. The TDMA frame (on one carrier)
consists of 8 time slots
21
Transmit/Receive Time-Frequency Map
22
The Power-Time Template
23
The Guard Period

• The guard gap is equivalent to 8.25 bits (1
  bit3.69 ms).
• This time is equivalent one-way propagation delay
  over 9.3 km
• Different mobiles in the cell are forced to
  advance or retard their transmission to avoid
  overlap
• The gap gives extra protection and also allow for
  amplifier ramping up and ramping down as shown in
  the next slide.

24
Bursts from Different Users
25
Frame Hierarchy
Hyper Frame 3 hr, 28 min., 53 s, 760 ms
Data
Signaling
26
Air Interface Channels

• Physical and Logical Channels
• Frequency Correction Burst
• Synchronization
• Normal Traffic Channels
• GPRS Physical Channels

27
Logical Channels
Traffic
Signaling
Voice
Data
Broadcast
Common
Dedicated
Broadcast control
Random Access
Stand Alone
Full
Half
Full
Half
TCH/H
TCH/F
TCH/HS
TCH/FS
SDCCH
BCCH
RACH
4.8
9.6
6.5
13
Frequency correction
Access Granting
Slow Association
2.4
4.8
2.4
FCCH
AGCH
SACCH
Fast Association
Synchronization
Paging
SCH
PCH
FACCH
28
Different Transmission Bursts
29
The Frequency Correction Burst

• 3-bits Tail for extra guard time
• 142-bits Fixed Sequence This is an all-zero
  sequence 000000. This sequence causes the GMSK
  modulator to produce a spectral line for easy
  frequency tracking.
• 8.25-bits Gap Guard time identical to the one
  used in the normal transmission burst.

30
The Synchronization Burst

• The synchronization burst has a structure similar
  to the normal burst.
• 3-bits Tail for extra guard time
• 64-bits Synchronization Synchronization
  sequence.
• 39-bits Coded Data The Base Station Information
  Code (BSIC), the Base station Color Code (BCC)
  and the National Color Code (NCC).
• 8.25-bits Gap Guard time identical to the one
  used in the normal transmission burst.

31
The Normal Traffic Burst
T Trail Bits to mark the end of the power ramp
up and the beginning of power ramp down. Data
The two 57-bit coded data belong to two
different speech frames. S Stealing Flag to
separate data from the training sequence. Train
A mid-amble synch and training sequence
(5-16-5). Gap Guard time (approx. 30.4 msec)
32
The Training Sequence (Mid-Amble)
The 16-bit signature sequence is unique to the
base station
33
The Training Sequence (cont.)

• There are 8 unique training sequences in the
  entire system
• Each sector is assigned one of these 8 sequences
• The sequences are repeated geographically in a
  manner similar to the frequency reuse pattern
• The training sequence play dual role
• It is used to estimate the channel
• It is used to tie specific MS's to
• specific BTS
• Each mobile terminal tracks its serving base
  using the training sequence of the station
• Each base station tracks its constituent MS using
  the same sequence

Wrong sequence
Correct sequence
34
The Random Access Burst

• 8-bits Tail 8 bits tail for extra guard time
• 41-bits Synchronization The synchronization
  sequence has the same significance as the
  training sequence.
• 36-bits Coded Data This is a short message
  containing data required for synchronization.
• 68.25-bits Gap Very long guard time to account
  of an initial differential delay over large cell
  (up to 75 km one way or 35 km two-ways).

35
Frequency Hopping

• GSM uses Slow Frequency Hopping to improve the
  radio link quality.
• SFH is mandatory when requested by the base
  station
• The Hopping Rate equals the TDMA Frame Rate
  216.7 hops/s.
• Two algorithms
• Cyclic hopping
• Random Hopping
• FCCH, SCH and BCCH can't hop
• Two implementations
• Baseband hopping
• Synthesizer hopping
• For a set of N frequencies, GSM allows for 64N
  hopping sequences

36
Frequency Hopping Example
37
Power Control

• The MS must set its power level as commanded by
  the Base Station. SACCH channel assignment .
• The power adjustment is performed over 16 steps
  of 2 dB each.
• The base station may (optionally) control its own
  transmitted power to reduce interference to
  mobiles in other cells.
• When the base station power is controlled, it
  also adjusts the power over 16 steps with 2 dB
  per step.