Understanding of GSM Technology for Engineering Students

1. Presentation
by
G. Govindarajan
Dy. General Manager (Rtd)
Mahanagar Telephone Nigam Limited (MTNL)
Mumbai
GGRajan

2. GSM Spectrum
Cell Principles
Sectorisation
Frequency Planning
GGRajan

3. RADIO SPECTRUM
GGRajan

4. GSM Frequency Spectrum
1] GSM 900
1] GSM 900
Uplink
Uplink : 890 MHz to 915 MHz
: 890 MHz to 915 MHz
GSM Spectrum
Downlink : 935 MHz to 960 MHz
Downlink : 935 MHz to 960 MHz
ARFCN
ARFCN : 124
: 124
2] EGSM 900
2] EGSM 900
Uplink
Uplink : 880 MHz to 915 MHz
: 880 MHz to 915 MHz 3] GSM 1800 (DCS 1800)
3] GSM 1800 (DCS 1800)
Downlink : 925 MHz to 960 MHz
Downlink : 925 MHz to 960 MHz Uplink
Uplink : 1710 MHz to 1795 MHz
: 1710 MHz to 1795 MHz
ARFCNs : 174
ARFCNs : 174 Downlink : 1805 MHz to 1880 MHz
Downlink : 1805 MHz to 1880 MHz
ARFCNs : 374
ARFCNs : 374
Absolute Radio Frequency Channels : 4] PCS 1900
4] PCS 1900
BW - 200 KHz, TDMA Slots - 8 Uplink
Uplink : 1850 MHz to 1910 MHz
: 1850 MHz to 1910 MHz
Downlink : 1930 MHz to 1990 MHz
Downlink : 1930 MHz to 1990 MHz
ARFCNs : 299
ARFCNs : 299 GGRajan

5. GSM Frequency Spectrum
GSM Spectrum [900 MHz]
Time
Code
• UL band : 890 - 915 MHz
• DL band : 935 - 960 MHz TS-8
• Duplex spacing: 45 MHz
A
FDMA structure: 124 RFs (ARFCN) TS-2
M
•
TD
TS-1
• RF Band width: 200 KHz
Freq
• TDMA structure: 8 Timeslots/ RF RF-1 RF-2 RF-3
FDMA
960 Mhz
124
•
•
RF • 1 2 3 4 5 6 7 8
•
2 TDMA structure of each ARFCN
1
935 Mhz
GGRajan

6. GSM Frequency Spectrum
B
T
S
890 MHz 915 MHz 935 MHz 960 MHz
1 2 3 124 1 2 3 124
UPLINK FREQUENCY DOWNLINK FREQUENCY
GGRajan

7. Cell Principles
CELL : a Base Station
(Transmitter) having number
of RF channels. Usually a
Hexagonal shape.
2
7 3
CLUSTER: Group of cells. 1
Size > 3, 4, 7, 9 or 12 6 4
5
GGRajan

8. Cell Principles
COVERAGE AREA
Each Cell covers a
limited number of
- 70 dBm Mobile Subs within the
Cell boundaries.
- 80 dBm
- 90 dBm
- 100 dBm
GGRajan

9. Cell Principles
CELL RADIUS : Radial distance of a Cell
Approximately : 25 KMs ( Start up )
1 KM ( Mature )
CELL SIZE :
Large Cells are employed in:
Remote areas
Coastal regions
Areas with few subscribers
Small cells are used in:
Urban areas
Areas with high subscriber density
GGRajan

10. Cell Principles
Frequency Reuse
GSM has 124 frequencies for use in network
Allocated to more than one network operator
If 4 operators, then each will get only 31 frequencies
Maximum cell radius is approximately 35 KMs
Network provider must re-use the same frequencies
2
7 3
Q = D/R = 3N (if all cells
1
with same Radius) R
6 4
N - Cluster size, R - Cell Radius
D - Distance between two co-channels 5 D
High Q > Less interference – Less Traffic capacity 1
Low Q > Higher interference – Higher capacity
GGRajan

11. Cell Sectorisation
Omni Antenna 3 Sectored Cell Site 6 Sectored Cell Site
F2
1 1
6 2
3 F1
5 3
2
4
F3
Sectorisation splits a single site into a number of cells
Each sector cell has TX and RX antennas
Each sector behaves as an independent cell
GSM = 4/12 : 4 Cell Sites, each with 3 Sectors
DAMPS = 7/21 : 7 Cell Sites, each with 3 Sectors
CDMA = 1/3 : 1 Cell Site, each with 3 Sectors
GGRajan

12. Cell Sectorisation
3 Sectored Cell Site
GGRajan

13. Frequency Planning
4 / 12 Layout
A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
A1
A2
D1 A3 B1
D2 B2
A1 C1
D3 B3
A2 C2
D1 A3 B1 C3
D2 B2
D3 C1 B3
C2
C3
GGRajan

14. Frequency Planning
A1 B1 C1 A2 B2 C2 A3 B3 C3
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 3 / 9 Layout
19 20 21 22 23 24 25 26 27
28 29 30 31 32 33 34 35 36
A1 A1
A2 A2
A3 B1 A3 B1
B2 B2
C1 B3 C1 B3
C2 C2
C3 A1 C3
A2
A3 B1
B2
C1 B3
C2
C3
GGRajan

15. Frequency Planning
Typical Cellular Radio Plan
GGRajan

16. GSM Architecture
SIM Anatomy
BSS Architecture
NSS Architecture
GGRajan

17. GSM Architecture
GSM SYSTEM
Base Station Switching OMC /NMC
Subsystem Subsystem Subsystem
Base Station Mobile Switching
Controller (BSC) Centre (MSC)
Base Transceiver Registers AUC
Station (BTS) HLR
VLR
EIR
GGRajan

18. GSM Architecture
NSS
MSC/VLR
BSS Architecture
BSC BSC
Abis
BTS
BTS
BTS
BTS
MS
MS MS
MS
GGRajan

19. GSM Architecture
BTS: Base Transceiver Station
NSS Architecture BSC: Base Station Controller
MSC: Mobile Switching Center
PSTN
PSTN VLR: Visiting Location Register
HLR: Home Location Register
GMSC NSS IWF: InterWorking Function
HLR /
AuC
SS7
SS7
VLR STP
MSC EIR
CSE
IWF
SM-SC MSC / VLR
VMS Internet
Internet
BSS BSC
BTS BTS
MS GGRajan

20. GSM Architecture
BTS Functions
◄Encodes, Encrypts, Multiplexes, Modulates and
feeds RF to Antenna
◄Received signals is Decoded, Decrypted and
Demodulate
◄Trans-coding and Rate Adoption Functionality
◄11 power classes from .01 watts (Micro cell) to 320
watts
◄Frequency hopping
NOKIA BTS
◄Up-link radio channel measurements
◄Multiple Antennas for Diversity
LUCENT BTS
GGRajan

21. GSM Architecture
BSC Functions
Allocation of radio
channels
Receives MS
measurements
Inter-cell handovers
from BTS to BTS
Concentrator for low
capacity BTSs
Reduce the number
of connections to MSC
Data stored in BSC
►Carrier frequencies
►Frequency hopping lists
►Power reduction levels
►Levels for cell border calculation
GGRajan

22. GSM Architecture
MSC Functions
Paging, specifically call handling
Dynamic allocation of resources in coordination with BSS
Location registration
Handover management
Billing for all subscribers based in its area
Reallocation of frequencies to BTSs to meet heavy demands
Signaling interface to databases like HLR, VLR.
Gateway to SMS between SMS centers and subscribers
Handle inter working function while working as GMSC
GGRajan

23. GSM Architecture
Visitor Location Register
IMSI & TMSI
IMSI & TMSI MSISDN
MSISDN
VLR Data MSRN
MSRN Location Area
Location Area
MS category
MS category Authentication Key
Authentication Key
Main task of VLR is to reduce the number of queries to HLR
Controls those mobiles Roaming in its area.
One VLR may be in-charge of one or more LA.
VLR gets updated from HLR, whenever a new MS enters its area.
Authentication along with HLR and AUC.
Relays ciphering key from HLR to BSS.
Assign TMSI and periodically changes to secure MS’s identity.
IMSI detach and attach operation
GGRajan

24. GSM Architecture
Home Location Register
►Stores
►Stores MS
MS Parameters,
Parameters, Numbers,
Numbers,
Authentication & Encryption values.
Authentication & Encryption values.
►Current MS status and associated VLR.
►Current MS status and associated VLR.
►One PLMN may contain one or several HLR.
►One PLMN may contain one or several HLR.
Authentication Centre
►Protect against intruders in air interface
►Protect against intruders in air interface
►Authentication (Ki) and Ciphering (Kc) keys stored
►Authentication (Ki) and Ciphering (Kc) keys stored
securely
securely
►Cipher key changes randomly with each phone call
►Cipher key changes randomly with each phone call
►A separate entity and physically included in HLR
►A separate entity and physically included in HLR
GGRajan

25. GSM Architecture
Equipment Identity Register
Stores IMEI for all registered MS and is unique to every MS.
Only one EIR per PLMN.
White list : IMEI, assigned to valid MS.
Black list : IMEI reported stolen
Gray list : IMEI having problems like faulty software,
wrong make of equipment etc.
GGRajan

26. SIM Anatomy
SIM
(Subscriber Identification Module)
A removable card.
Contains an IC chip with a microprocessor.
Contains RAM and ROM.
It is inserted in ME at the time of making or receiving a call.
GGRajan

27. SIM Anatomy
Programs in SIM
1 . IMSI (International Mobile Station Identity)
2 . Ki (Individual Subscriber authentication Key)
3 . A3 (Authentication Algorithm )
4 . A8 [ Cipher Key (Kc) Algorithm ]
IMSI
Ki
5 . A5 (Encryption Algorithm ) A3
A8
• IMSI & Ki are specific to each MS A5
• A3 & A8 can be different for different operators
• A5 - Unique, used across GSM operators
GGRajan

28. SIM Anatomy
SIM Authentication Process
VLR
IMSI
RAND
RAND
Ki Ki Ki Ki
A3 A8 A3 A8
Kc Kc
SRES SRES
SIM in MS NET WORK
GGRajan

29. SIM Anatomy
MSISDN
Numbering for Cellular Mobile Network
ACCESS MSC CODE MOBILE
CODE NUMBER
2-digit 3-digit 5-digit
98/94 etc. ABC XXXXX
GGRajan

30. GSM Architecture - Review
NMC
NMC
VLR VLR HLR
AUC
OMC
OMC
EIR
MSC
MSC Terrestrial Interfaces MSC
MSC
XC IWF EC EC IWF XC
BTS
BTS BSC
BSC BTS
BTS
BSC
BSC
BTS
BTS
PSTN
PSTN
BTS BTS
BTS BTS
BTS BTS
BTS BTS
BTS
BTS
BTS
BTS BTS
BTS GGRajan

31. Physical & Logical Channels
Traffic & Control Channels
TDMA Frames
Super & Hyper Frames
GGRajan

32. GSM Channels
Physical Channels
GSM RF carrier can support up to 8 MS simultaneously
Each MS occupy 1 Time Slot
Information carried in TS is “BURST”
Each TS carrying Burst is a “Physical Channel”
8 Time Slots forms 1 TDMA Frame
GGRajan

33. GSM Channels
Logical Channels
One or more logical channels are transmitted on a Physical channel.
Determined by the information transmitted over it.
Types of Logical Channels
Logical Channels
Logical Channels
Traffic Channels Control Channels
Broadcast
Common Control
Dedicated Control
GGRajan

34. GSM Channels
Traffic Channels on Air Interface
TCH (Traffic Channel)
TCH (Traffic Channel)
NB NB
SPEECH
SPEECH DATA
DATA
TCH/FS
TCH/FS TCH/HS
TCH/HS TCH/9.6
TCH/9.6 TCH/2.4
TCH/2.4
TCH/EFR
TCH/EFR TCH/4.8
TCH/4.8
SACCH
SACCH
NB:
NB: Normal Burst
Normal Burst
TCH
TCH SACCH: Slow Associated Control Chl.
SACCH: Slow Associated Control Chl.
FACCH: Fast Associated Control Chl.
FACCH: Fast Associated Control Chl.
FACCH
FACCH
GGRajan

35. GSM Channels
Traffic channel Types
Traffic channels are used to send Speech or Data services.
May also be used for sending information.
Example: call forwarding and short messages
Traffic channel Types based on transmission rates
TCH/FS (Full rate Speech) - gross bit rate of 22.8 kbps
TCH/HS (Half rate Speech) - gross bit rate of 11.4 kbps
GGRajan

36. GSM Channels
Control Channels
CCH (Control Channel)
CCH (Control Channel)
NB NB/AB
BCCH-
BCCH-
DCCH
DCCH Downlink Only
Downlink Only
NB/DB
SDCCH
SDCCH ACCH
ACCH BCCH
BCCH SYNC. CHLS.
SYNC. CHLS.
SB FB
CCCH
CCCH
FACCH
FACCH SACCH
SACCH SCH
SCH FCCH
FCCH
AB NB
RACH
RACH NB CBCH
CBCH
FB: Freq. Burst
FB: Freq. Burst
Uplink
Uplink Downlink
Downlink SB: Sync. Burst
SB: Sync. Burst
AB: Access Burst
AB: Access Burst
PCH/AGCH
PCH/AGCH DB: Dummy Burst
DB: Dummy Burst
Downlink
Downlink
GGRajan

37. GSM Channels
BCCH
Broadcast Control Channel
Parameters, LAI, CI, Neighbor Cells, Frequencies used in cell etc
are transmitted to MS.
FCCH
(Frequency Correction Channel)
BCCH-
BCCH-
FCCH transmits a constant frequency Downlink Only
Downlink Only
shift of the RF carrier that can be used NB/DB
by the MS for frequency correction. BCCH SYNC. CHLS.
BCCH SYNC. CHLS.
SB FB
SCH SCH
SCH FCCH
FCCH
Synchronization Channel
SCH is used to time synchronize the MSs,
carries TDMA frame number and BSIC
(Base Station Identity Code).
GGRajan

38. GSM Channels
CCCH Types
RACH
Used in uplink by MSs for requesting
a channel for a connection.
CCCH
CCCH
CBCH
AB NB
Used for the transmission of Short
RACH
RACH CBCH
CBCH
Messages as broadcast messages to Uplink
Uplink Downlink
Downlink
all MSs.
PCH PCH/AGCH
PCH/AGCH
Downlink
Downlink
Used in Down link by BTS to page
IMSI, TMSI or IMEI to the MS.
AGCH
Used in downlink to allot DCCH, if MS
requested via RACH.
GGRajan

39. GSM Channels
DCCH
DCCH
DCCH
Dedicated Control CHannel SDCCH ACCH
SDCCH ACCH
Fully duplex, Point to Point channels. FACCH SACCH
FACCH SACCH
Used for signaling between BTS and a certain MS.
Assigned to a single MS for call setup & subs. validation.
DCCH, a single TS on an RF carrier used to convey 8 SDCCHs.
SDCCH
Stand-alone Dedicated Control Channel
A duplex channel which is always allocated to a TCH or SDCCH.
Used for transmission of signaling data, radio link supervision
measurements, TX power control and timing advance data.
GGRajan

40. GSM Channels
ACCH ACCH
ACCH
Associated Control Channel
Can be associated with either SDCCH or TCH. FACCH SACCH
FACCH SACCH
Used for carrying information associated with the process being
carried out on either SDCCH or the TCH.
SACCH
Slow Associated Control CHannel
Conveys power control & timing information in downlink.
SACCH is used for non urgent procedures.
FACCH
Fast Associated Control Channel
Used for handovers, assignments, for call set-up and release.
GGRajan

41. GSM Frames
TDMA Frame
TDMA FRAME - 1 TDMA FRAME - 2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Time Slot
BURST
TIME SLOT – 0.577 ms
GGRajan

42. GSM Frames
TDMA Multi Frame
26-TDMA multi-frame
Succession of 26 TDMA frames, and corresponds to the 26 x 8 BP
or 120 ms.
51-TDMA multi-frame
Succession of 51 TDMA frames, and corresponds to the 51 x 8 BP
cycle.
Superframe
Sccession of 51 x 26 TDMA frames (6.12 sec).
Hyperframe
Numbering period, 2048 x 51 x 26 x 8 Broadcast Period long, or 3
hours, 28 minutes, 53 seconds and 760 milliseconds.
GGRajan

43. GSM Frames
Low Capacity Cell
Combined
BCCH / CCCH / DCCH
0 1 2 3 4 5 6 7
1 to 7 TRAFFIC
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44. GSM Frames
Higher Capacity Cell
Combined
BCCH / CCCH
0 1 2 3 4 5 6 7
1 to 7 TRAFFIC
DCCH
0 1 2 3 4 5 6 7
1 to 7 TRAFFIC
GGRajan

45. GSM Frames
26 Frame Traffic Channel Multiframe
0 . 577 ms
TIMESLOT TS-4
TDMA FRAME
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
4 . 615 ms
MF0 ~ MF11 & MF13 ~ MF24 : Traffic : MF12 : SACCH : MF25 : Idle
Idle
0 1 2 3 --- 12 -- 25
120 ms SACCH
MULTIFRAME
MS needs to send information about signal strength from surrounding cells to the
network, Capacity required to send measurement info is 1/24 the capacity required
to send voice.
GGRajan

46. GSM Frames
51 Frame Control Channel Multiframe
TS-4 0 . 577 ms
TIMESLOT
TDMA FRAME
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
4 . 615 ms
IDLE
0 1 2 3 -- -- -- 12 --- -- 25 -- -- 49 50
MULTIFRAME 235.365 ms
51 Frame structure occurs in several forms, depending on the
type of control channel and the network provider’s requirements
GGRajan

47. GSM Frames
Superframe & Hyperframe
0 1 2 3 4 5 6 7
4 . 615 ms
TDMA Frame
26 Frames Multiframe 51 Frames Multiframe
0 1 2 -- --- -- -- 49 50
0 1 2 3 --- 12 -- 25
120 ms 235.65 ms
Traffic Channel Control Channel
Repeat 51 times Repeat 26 times
0 1 2 3 --- 12 -- 25
Superframe - 6 . 12 Sec - 1326 TDMA frames
0 25 50
2048 Superframes = 2715648 TDMA Frames
0 1 2 ---- Hyperframe
---- ----- 2046 2047
3 H 28 M 53 S 760 ms
GGRajan

48. Mobile Originated Call
Mobile Terminated Call
GSM Call Handover
GGRajan

49. Mobile Call Phases
Phases of Mobile Originated Call
/ Setup phase.
/ Ringing phase.
/ Conversation phase.
/ Release phase.
Phases of Mobile Terminated Call
Routing analysis
Paging.
Call setup.
Call release.
GGRajan

50. GSM Call flow
Base Station Subsystem BSC-1 Voice Call
Switching Centre
HLR
VLR
VLR
MSC
AUC
AUC
EIR
EIR
FIBER LINK
BTS-1
NK
LI
W
M
BTS-2
BTS-1 BSC-2 PSTN
BTS contains TX and RX
BSC Manages Radio resources, Radio channels
and Local connection management GGRajan

51. GSM Call flow
Circuit-Switched Data Call
Public
Switched
Telephone
Network
HLR
MS
MSC /
BSC VLR
BTS Wireless
Network
Internet
IWF
MS
GGRajan

52. Handover Management
Mobile Hand-off
Signal Strength
Handoff Zone
B
B T
T S
S
Distance
GGRajan

53. Handover Management
Four types of handovers in GSM
..Channels (TS) in the same Cell
Channels (TS) in the same Cell (Intra-BTS)
(Intra-BTS)
..Cells under the control of same BSC
Cells under the control of same BSC (Inter-BTS)
(Inter-BTS)
..Cells under the control of different BSCs
Cells under the control of different BSCs (Intra-MSC)
(Intra-MSC)
..Cells under the control of different MSCs
Cells under the control of different MSCs (Inter-MSC)
(Inter-MSC)
GGRajan

54. Handover Management
MSC MSC
BSC BSC BSC
LAI-B
LAI-A
Inter MSC hand Off Intra BSC handoff
Inter BSC hand Off
GGRajan

55. Handover Management
BSC MSCVLR
BTS-A
HLR
BTS-B BSC MSC VLR
•BSC-A informs MSC-A that MS needs handover from BTS-A to BTS-B
•MSC-A informs MSC-B that a handover from BTS-A to BTS-B is underway
•MSC-A commands BSC/BTS-A to proceed with handover to BTS-B. Hanover
from BTS-A to BTS-B is underway
•BTS-A commands MS to change to a specified channel on BTS-B
•BTS-B informs BSC/MSC that hand over is complete
• MSC-B informs the old MSC-A that hand over to BTS-B is complete
GGRajan

56. General Packet Radio Service (GPRS)
Enhanced Data rate for GSM Evolution (EDGE)
GGRajan

57. What is GPRS ?
/ GPRS is a new bearer service for GSM
/ Packet Data Network (PDN) to the Internet.
GPRS benefit from shorter access times and higher data rates.
“online” over a long period of time, but billed for data volume.
GPRS improves the utilization of the radio resources,
Offers volume-based billing,
Higher data transfer rates,
Shorter access times,
Simplifies the access to packet data networks.
GPRS includes GSNs
SGSN : Serving GPRS Support Node
GGSN : Gateway GPRS Support Node
GPRS Register
GGRajan

58. GSM vs GPRS
GPRS Architecture
GSM Core
BSC Network PSTN/ISP
Air Interface Solution GPRS Core
Network IP
SGSN IP GGSN
PDN
Network Solution
GGRajan

59. GPRS System Architecture
GPRS in GSM Networks
Packets Same Radio
BSS for Packet
and Circuit
Traffic
Circuit
BSC
IP
GPRS GGSN
PLMN
SGSN
MSC/VLR Intra
PLMN IP
HLR SS7 Backbone
SGSN
GGSN
GSM PLMN X.25
MSC
GGRajan

60. EDGE Features
EDGE acts as a bolt-on enhancement to 2G and 2.5G, ie GPRS
EDGE is also known as Enhanced GPRS, a superset to GPRS
Can carry data speeds up to 384 Kbps in packet mode
Meet the ITU's requirement for a 3G network
EDGE requires no H/W changes in GSM core networks
BSSs must be modified
EDGE can be introduced in two ways :
1) Packet-switched enhancement (called EGPRS)
2) Circuit-switched data enhancement (called ECSD).
GGRajan

61. GPRS - EDGE compared
Technical Differences - GPRS and EDGE
Technology Data Rates Spectrum Services
Voice and circuit data
GSM/HSCSD 9.6 to 57.6 kbps 200 kHz
services
GPRS 171.6 kpbs 200 kHz Packet data services
EDGE 473.6 kbps 200 kHz Packet data services
GGRajan

62. GGRajan