GSM system working

1. Radio Transmission Techniques
FDMA
TDMA
CDMA
Channels
Physical channels
Logical channels

2. Frequency
Channel
Time

3. Frequency Time Slot
Channel
Time
0 1 2 3 4 5 6 7

4. Frequency
Code
Time
Code 1
Code 2
Code 3

5. Channels
Physical Channels
Associated with frequency bands, time slots, codes
Physical channels transfer bits from one network
element to another
Logical Channels
Distinguished by the nature of carried information and
the way to assemble bits into data units
Three types
 one-to-one: traffic channels between a BTS and a MS
 one-to-many: synchronization signals from BTS to MSs in a cell
 many-to-one: from MSs to the same BTS

6. Air Interface Layers
Layer 3
Messages Messages
Messages
Logical Layer 2 Logical
Channels Packets Channels
Radio Layer 1 Radio
Transmission Bits Transmission
Terminal Base
Station

7. GSM Frame Structure
Superframe ... 51 Multiframe
6.12 sec
120 msec ... 26 Frames
4.615 msec 0 7 8 Multiframe
Coded data Midamble
0.57692 msec
3 57 1 26 1 57 3 8.25
156.25 bits
Tail bit Stealing Flag Guard period

8. Frame Types
• There are two types of multiframe
– 26 TDMA-frame multiframe is used to carry
TCH, SACCH and FACCH
– 51 TDMA-frame multiframe is used to carry
BCCH, CCH, SDCCH and SACCH

9. Burst and Frames
• The information contained in one time slot
on the TDMA frame is call a burst.
• Five types of burst
– Normal Burst (NB)
– Frequency Correction Burst (FB)
– Synchronization Burst (SB)
– Access Burst (AB)
– Dummy Burst

10. Traffic TCH/F: Full-rate Traffic Channel Two-way
channels TCH/H: Half-rate Traffic Channel
(TCH)
FCCH: Frequency correction
BCH SCH: Synchronization
BCCH: Broadcast control
Base-to-
Signaling PCH: Paging mobile
CCCH
channel AGCH: Access grant
RACH: Random access
SDCCH: Stand-alone dedicated control
DCCH
SACCH: Slow associated control
Two-way
FACCH: Fast associated control

11. Logical Channels
• Control Channels
– Broadcast Channels (BCH)
– Common Control Channels (CCCH)
– Dedicated Control Channels (DCCH)
• Traffic Channels (TCH)
– Full Rate (TCH/F)
– Half Rate (TCH/H)

12. UP / Down-Link
Down-link: the transmission path from Base Station
to Mobile Station
Up-link: the transmission path from Mobile Station
to Base Station

13. Control Channels
• Broadcast Channels (BCH)
– Frequency Correction Channel (FCCH)
– Synchronization Channel (SCH)
– Broadcast Control Channel (BCCH)
• Common Control Channels (CCCH)
– Paging Channel (PCH)
– Random Access Channel (RACH)
– Access Grant Channel (AGCH)
• Dedicated Control Channels (DCCH)
– Stand alone Dedicated Control Channel (SDCCH)
– Cell Broadcast Channel (CBCH)
– Slow Associated Control Channel (SACCH)
– Fast Associated Control Channel (FACCH)

14. Broadcast Channels (BCH)
To help the MH (Mobile Handset)
to turn to a BTS
to listen for the cell information
 to start roaming, waiting for calls to arrive, making calls
Because BTSs are not synchronized with each other,
every time a MH decides to camp to another cell, its
FCCH, SCH, and BCCH must be read.

15. Frequency Correction Channel
(FCCH)
• Provide MH with the frequency reference of
the system
– To enable the Mobile Handset (MH) to
synchronize with the frequency
• Transmission properties
– Transmit on the down-link
– Point to multi-point.

16. Synchronization Channel (SCH)
• MH synchronize with the structure within the
locative cell
– MH can receive information from the proper time slots
on the TDMA structure
• To ensure a GSM BTS is chose
– The Base Station Identity Code (BSIC) can only be
decoded by a GSM BTS
• Transmission properties
– Transmit on down-link
– Point to multi-point.

17. Broadcast Control Channel
(BCCH)
• BTS broadcast cell information to MH
– LAI ( Location Area Identity), to start roaming,
waiting for calls to arrive, making calls
– maximum output power allowed in the cell
– information about BCCH carriers for the
neighboring cells
• MH will perform measurement to BTS
• Transmission properties
– Transmit on down-link
– Point to multi-point

18. Common Control Channels (CCCH)
CCCH support the establishment of a dedicated
communication path (dedicated channel) between
the MH and the BTS
Three types of CCCH
Paging Channel (PCH)
Random Access Channel (RACH)
Access Grant Channel (AGCH)

19. Paging Channel (PCH)
• Used by BTS to page particular MH in the cell
– MH actively listen to PCH to check contact info
within certain time
– Contact could be incoming call or short message
• Contact info on PCH include
– IMSI (MH’s identity number), or
– TMSI (temporary number)
• Transmission properties
– Transmit on down-link
– point to point

20. Random Access Channel
(RACH)
• Used by MH to request a dedicated channel
for call setup
– Shared by any MH attempts to access the
network
– Channel request message contains the reason
for the access attempt
• Transmission properties
– Transmit on up-link
– Point to pint.

21. Access Grant Channel (AGCH)
• The network assigns a signaling channel via
AGCH
– A Stand alone Dedicated Control Channel
(SDCCH) is assigned
• Transmission properties
– Transmit on down-link
– Point to point

22. Dedicated Control Channels
(DCCH)
DCCH are used for transferring nonuser
information between the network and the MH
Messages on DCCH Including
 channel maintenance
 mobility management
 radio resource management
Four kinds of DCCH
Stand alone Dedicated Control Channel (SDCCH)
Cell Broadcast Channel (CBCH)
Slow Associated Control Channel (SACCH)
Fast Associated Control Channel (FACCH)

23. Stand alone Dedicated Control
Channel (SDCCH)
• Transfer signaling information between the
BTS and the MH
• Typically used for location updating prior
to use of a traffic channel
• Transmission properties
– Bidirectional channel, transmit on both up and
down-link
– Point to point.

24. Cell Broadcast Channel (CBCH)
• To carry Short Message Service Cell
Broadcast (SMSCB)
• Use the same physical channel as SDCCH
• Transmission properties
– Transmit on down-link
– Point to multi-point

25. Slow Associated Control
Channel (SACCH)
• Carries control and measurement parameters along
with routine data necessary to maintain a radio
link between the MH and the BTS
– On the uplink, MS sends averaged measurements
(signal strength and quality) of current and neighboring
BCCH
– On downlink, MS receives information about
transmitting power to use and an instruction with time
advance/retard
• Transmission properties
– Bidirection channel, transmit on both up and down link
– Point to point

26. Fast Associated Control Channel
(FACCH)
• An FACCH is used over a TCH where it
steals time slots from a TCH
– a 20 ms segment of speech is stolen to carry
handover signaling information
• Appears on demand

27. Traffic Channels (TCH)
• TCH transport user information (speech/data)
• TCH are bidirectional dedicated channels
between the network and the MH

28. Burstinformation contained in one time slot is a
The
burst
Five types of burst
Normal Burst (NB)
 To carry information on traffic and control channels
Frequency Correction Burst (FB)
 To synchronize the frequency of the mobile
Synchronization Burst (SB)
 To synchronize the frames of the mobile
Access Burst (AB)
 For random and handover access
Dummy Burst
 For padding the frame