04- air interface

GSM Air Interface

Confidentiality level on slide master

Version number on slide master

GSM Complete ModelSS

BTSO S S

ISDN

PLMN

PSTN M I N

S C

B G W

H L R

AUC

SMSC-GMSCSMS-IWMSC

E I R

GWMSC

MSC/VLR

BSC

BSS


Version number on slide master17 April 2023

Presentation title in footer3

Idle mode behaviour

What is idle mode?

The MS is said to be in Idle mode when it is powered on but not allocated any dedicated channel.

Idle mode activities :

• PLMN selection• CELL selection & reselectionTune to the control channel of the cell, to listen to the system information messages transmitted in that cell (Camping on the cell)4- It must also be able to register its current location to the network (Location Updating)




Idle Mode: PLMN Selection

PLMN selection (Automatic/Manual mode)

• Automatic mode1- Last registered PLMN

2- Home PLMN

3- According to PLMN list stored on SIM

4- PLMNs with SS > -85 dBm in random order

5- All other PLMNs in order of decreasing SS

• Manual mode

1- Last registered PLMN

2- Home PLMN

3- The MS will indicate the user all available PLMNs




Idle mode: Cell Selection

Why MS must camp on a cell?

• The MS can receive system information

• The MS can initiate a call

• The system can reach the MS

Types of cell selection:

Normal cell selection

• The MS will search all RF channels in its supported frequency band (124 channels in GSM 900)

Stored list cell selection

• The MS will perform the measurements for only the last BA list BCCH carriers stored on SIM before it was powered off.




Idle mode: Cell SelectionScan 124 channels and measure signal strength

Tune to the RF channel with the highest received average signal strength

Determine if it is a BCCH frequency carrier by searching for frequency correction bursts

Tune to the RF-channel with the highest signal strength not already

tried

The MS shall attempt to synchronize to this carrier and read BCCH info

Is it a BCCH carrier

Is it the wanted PLMN ?

Is the cell barred for access ?

Is C1>0 ?

Camp on this cell

No

Yes

No

Yes

No

Yes

Yes

No




Idle mode: Cell reselection

Cell reselection:

The MS detects that it is more suitable to camp on another cell.

Why?

The MS at all times try to camp on the cell which has the highest probability of successful communication on both up and downlinks when accessing the system

When?

MS performs cell reselection if:

• The serving cell becomes barred

• Downlink signaling failure

• C1<0 for serving cell more than 5sec

• C2(neighbor)>C2(serving) (+CRH) more than 5sec




Idle mode: Cell reselection

Cell reselection procedures:

• MS monitors Neighbor’s BCCH carriers as well as the serving one.

• MS measure server BCCH at least once every 30s

• MS measures neighbors BCCH carrier at least once every 5min.

• MS attempts to decode the BSIC parameter at least once every 30s

(for each of

• the six strongest surrounding cells)




The smallest unit of a cellular system

Each cell is assigned unique number call CGI

1- The Cell




BTS

BTS

BTSBTS

BTS

BTSBTS

BTS

BTS BTSBTS

BTS

BTS

Group of cells

Has identity called location area identity (LAI)

May be control by one or more BSC but one MSC

2- The location Area (LA)




MSCVLR

3- The MSC Service Area

Group of LAs controlled by one MSC

The subscriber’s MSC service area is stored in its HLR




MSCVLR

MSCVLR

MSCVLR

3- The PLMN Service

Group of MSC areas controlled by one network operator.




GSM Service Area

PLMN Service Area

MSC Service Area

Location Area

One per operator

5- The GSM Service Area

More operator sign contracts agreeing to work together for international roaming




Cell Identity (CGI)

MCC: Mobile Country Code

LAC: Location Area Code

MNC: Mobile Network Code

MCC (3 digits)

MNC (2 digits)

LAC(4 digits)

CI(4 digits)

CI: Cell Identity

LAI




Idle mode: location updating

Location Updating:

The location updating is initiated by the MS to keep the network updated by it’s location.

Types of Location updating :

1- Normal:

• MS listens to the broadcasted system information.

• It compares the broadcasted LAI with the stored one on the SIM.

• If it is different , a location updating type normal will be initiated.

2- Periodic registration:

• Reduces unnecessary paging of MSs which lost coverage or run out of battery.

• T3212 parameter (1 – 255) determines how often MS should perform periodic location update.




Idle mode: IMSI attach/detach

• When a MS is powered on, an IMSI attach message is sent

to the MSC/VLR

• When a MS is powered off, an IMSI detach message is sent

to the MSC/VLR

• Implicit detach?




Dedicated Mode: Handover

Handover :

• Handover is essential to keep continuity of the call when MS roams along the network and moving from one cell to another between different nodes in the network.

• During a call, the MS is continuously measuring the signal levels of neighboring cells in addition to server’s and reports this results to the BSC through the BTS.

• The BSC, being responsible on supervising the cells, is responsible of handover initiation.

• Good neighbor relations between cells is an important factor in keeping the network performance in the accepted level.

Types of handover :

• Intra Cell

• Intra BSC Handover

• Inter BSC / Intra MSC Handover

• Inter MSC Handover

BSC




Basic definitions

Basic definitions:• Time Slot : TS is the basic radio resource during which the carrier serves one logic channel

MS receives TS by TS and also sends TS by TS. • Burst : the contents of each time slot is called a burst.• Physical channel : the recurrence of the same physical TS in successive frames.• Logical channel : the recurrence of the same TS contents in successive frames.




Basic definitionsPhysical Channels

CH 1 54321 6 70 0 1 Time

CH 3 54321 6 70 0 1

CH 2 54321 6 70 0 1

CH 124 54321 6 70 0 1

GSM band is divided into 124 RF channels, and each channel is divided into 8 time slots using TDMA. These time slots are called “physical channels”.




• TDMA frame




• TDMA frame structure

-Bit rate of the radio carrier is 270.833 Kbps

-Bit duration =1/270.833=3.69 µsec

-One time slot =148 bits+8.25 guard bits=156.25 bits

-Time slot duration =156.25x3.69 µsec= 0.577 msec

-Frame duration=0.577x8= 4.615 msec




• Guard band is used for :

- time dispersion on the propagation path

- Time required for smooth switch on-off in transmitter

1 2 3

• The numbering scheme of the time slots is staggered by 3 time slots to remove necessity for MS to send and receive in the same time

4 5 6

1 2 3 4 5 60

0




Logical Channels

Synchronization Channel

Broadcast Control Channel

Frequency Correction Control ChannelPaging Channel

Random Access Channel

Access Grant Channel

Standalone Dedicated Control Ch

Slow Associated Control Ch

Fast Associated Control Ch

Cell Broadcast Control Ch

BroadcastCommon

Dedicated

Logical Channels

Control ChannelsTraffic Channels

Half Rate Full Rate




• Types of Traffic channels

1. Full rate

- 13 Kbps data rate

2. Enhanced full rate

- 12.2 Kbps data rate

3. Half rate

- increase network capacity

13 Kbps 9.8 Kbps

12.2 Kbps 10.6 Kbps

5.6 Kbps 5.8 Kbps

11.4 Kbps

22.8 Kbps

Data

Redundancy




Traffic Channels

• TCH is allocated to the MS when it is in dedicated mode.

• TCH is intended to carry two types of bi-directional user information : Encoded speech and data .

• Two types of traffic channels are defined• TCH/F: Full rate uses one physical channel• TCH/H: Half rate shares one physical channel with another

TCH/H

• Channel rate:

Full Rate (22,8 kbps gross bit rate on air interface)Half Rate (11,4 kbps gross bit rate)

•Speech versions

FR SPV 1 = “normal Full Rate” (FR) 13 Kbps (speech coding rate FR SPV 2 = “enhanced Full Rate” (EFR) 12.2 Kbps (speech coding rate) FR SPV 3 = “AMR Full Rate” (AMR-FR) ACS (Active Codec Set)

HR SPV 1 = “normal Half Rate” (HR) 5.6 Kbps (speech coding r

HR SPV 3 = “AMR Half Rate” (AMR-HR) ACS (Active Codec Set)




Broadcast Channels

Frequency Correction Channel (FCCH):

• The 1st Burst that MS looks for just after switching on

• It is the repetition of frequency correction bursts

• Bursts containing only zeros are transmitted

• Make sure that this is the BCCH carrier

• Allow the MS to synchronize to the frequency

• Only transmitted DL , TS0




Broadcast Channels

Synchronization Channel (SCH):

• Used to provide synchronization information between MS & BTS in

the time domain.

• Repetition of synchronization bursts

• Transmitted DL only.

• Base Station Identity Code (BSIC)

• TDMA Frame Number (RFN)




Broadcast Channels

Broadcast Control Channel (BCCH):

• When a mobile is turned on, it scans the GSM spectrum to find an

available BTS.

• It is a permanent channel to be scanned, providing network

information (called system information)


• Contains; cell description, cell identity, location area identity,

network identity, BCCH of neighboring cells….




Common Control Channels

Paging Channel (PCH) :

• When the network wants to initiate a communication with the MS,

it looks for the mobile by sending a paging request.

• Incoming call or incoming short message.

• Mobile Identity parameter (IMSI) or (TMSI).


Access Grant Channel (AGCH) :

• Inform about the SDCCH or TCH allocated.


• Channel description, Timing advance….




Common Control Channels

Random Access Channel (RACH) :

• When the MS requests allocation of signaling channel by sending a

Channel Request

• Answering to paging, network contact

• Only transmitted UL




Dedicated Control Channels

Slow Associated Control Channel (SACCH) :

• When an SDCCH or a TCH is allocated, an associated channel is used

by MS & Network in Order to exchange general information

• Transmitted both UL and DL

• Up-Link: Measurement report

• Down-Link: System information, MS power level, timing advance

Fast Associated Control Channel (FACCH) :

• During a hand over, both MS and Network need to send a fast

message to the other

• Works in stealing mode





Dedicated Control Channels

Standalone Dedicated Control Channel (SDCCH) :

• MS and network need to exchange information before initializing

communication.

• Used in authentication, ciphering, setup and assignment procedures


• Transmission of SMS in idle mode.

Cell Broadcast Control Channel (CBCCH)

• It displays general information (weather, addresses, traffic…..) .

• It uses one of the SDCCH channels

• MS must be configured to receive this channel

.




Normal Burst Structure

.It carries information of all logical channels except RACH, SCH and FCCH

Tail Bits Encrypted Bits Training Sequence Encrypted Bits Tail

BitsGuard Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame

156.25 bits in 577 Sec

The tail bits help the equalizer to determine the start and stop points of the transmitted bits. They are three bits at the beginning and at the end of the burst and they are always zeros

The subscriber speech or data is encrypted into 57 bit blocks. Each burst will contain two 57 bits blocks from two different speech segments.

One bit “stealing flag” will be added to each block to indicate whether the burst is stolen for the FACCH signaling or used as a normal traffic channel

The training sequence is inserted in the middle of the burst to help the Viterbi equalizer to create the channel model

The guard period of 8.25 bits length, which is equivalent to about 30 s, is left at the end of each burst, to prevent overlapping between consecutive bursts and to facilitate burst synchronization




8.253571261573

76543210

156.25 bits in 577 m Sec

Normal burst

26e.g. TCH21 51e.g. BCCH21

26e.g. BCCH1

51e.g. TCH21

2048e.g. BCCH21

1 Hyper frame = 2048 Super frames = 2,715,648 TDMA Frames = 3hrs 28 min and 53.76 s

Hyper Frame

12

1

2

2

1 Super frame = 51 TCH Multi frames

1 Super frame = 26 BCCH Multi frames

1 BCCH Multi frame = 51 TDMA Frames1 TCH Multi frame = 26 TDMA Frames




Mapping of logical channels (DL)

CBBBBSF

F S F S F S F S F S IB C C C C C C C C C

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7




Mapping of logical channels (UL)

RRRRRRR

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7




Frame F D T R A TS D T R A TB D T R A TB D T R A TB D T R A TB D T R A TC D T R A TC D T R A TC D T R A TC D T R A TF D T R A TS D T R A T

Frame C D A I A I A I R I A I A I A IC D T R I TC D T R I TC D T R D TC D T R D TC D T R D TC D T R D TC D T R D TF D T R D TS D T R D TC D T R D TC D T R D TC D T R D T

Frame C D I A I A I A R D I A I A I AC D T R D TC D T R D TC D T R D TC D T R D TF D T R D TS D T R D TC A T R D TC A T R D TC A T R D TC A T R D TC A T R D TC A T R D T

Frame C A A I A I A I R D A I A I A IC A T R D TF A T R D TS A T R D TC A T R D TC A T R D TC A T R D TC A T R D TC A T R D TC A T R A TC I T R A TC I T R A T

Frame I I T R A T

Downlink Uplink

51 T

DM

A F

ram

es =

1 B

CC

H M

ult

i-fr

ame




Normal Location update within same MSC/VLR service area

MSC / VLR

BSC

1

1. The Mobile sends an SDCCH allocation request message on the RACCH to the BTS

2

2. The BTS responds with the SDCCH allocation message on the AGCH

3

3

3

3. The mobile sends a location update request message with its IMSI on the allocated SDCCH channel to the MSC/VLR

4

4

4

4. The MSC/VLR updates the location information and sends a Location Update confirmation message on the SDCCH channel to the mobile then the SDCCH channel is released




Normal Location update between 2 MSC/VLR service areas

7

7

7

7. The new MSC/VLR sends a location updating confirmation message to the mobile

HLR

Old MSC

Old BSC

New MSC

New BSC

1

1

1

1. The mobile requests a location update just like the first case

IMSI MGT2

2. The new MSC/VLR receives the IMSI and conclude the MGT

3

3. The MSC/VLR sends a subscriber information request with the IMSI to the HLR

4. The HLR stores the address of the new MSC/VLR

HLR

5

HLR

5. The HLR sends the data to the new MSC/VLR and it is kept there

New MSC

6

6. The HLR sends a location cancellation message to the old MSC/VLR to remove the data

Old MSC




Post installation cycle

•Site integration

•Site testing

•Site optimization




Drive-test Sample

Questions

Thanks

04- air interface

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