1.basic gsm principles

8/12/2019 1.Basic Gsm Principles

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B SICGSMPRINCIPLES


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COMP RISON

TACS GSM 900 DCS 1800

Uplink 890 - 905 MHz 890 - 915 MHz 1710 - 1785 MHz

Downlink 935 - 950 MHz 935 - 960 MHz 1805 - 1880 MHz

Duplex Distance 45 MHz 45 MHz 95 MHz

Carrier Separation 25 kHz 200 kHz 200 kHz

Number of Channels 15 Mhz / 25 kHz = 600 25 MHz / 200 kHz = 124 75 MHz / 200 kHz = 374

Channels defined in Switch 1, 2, .599, 600 1, 2, .123, 124 512 , 513 .884, 885

Access Method FDMA TDMA TDMA


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GSM 900DCS 1800TACS

1 Control

Channel

1 Voice Channel

10 Control

Channels

1 Voice Channel


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Logical Channels:1 VOICE CHANNEL

10 CONTROL CHANNELS


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TCH = Traffic Channel

Full rate- Used for speech at 13 kbits/s orsending data at 9.6 kbits/s

Half rate- Used for speech at 6.5 kbits/s orsending data at 4.8 kbits/s

Enhanced Full rate- Used for speech at 13kbits/s or sending data at 9.6 kbits/s but withalmost Land line quality

BACK


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BCH

CCCH

DCCH

CBCH

NEXT


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BCH = Broadcast Channels

FCCH = FREQUENCY CORRECTION CHANNEL- To tell the Mobile that this is the BCCH carrier

- To able the Mobile to synchronize to the frequency(Downlink only)

SCH = SYNCHRONISATION CHANNEL- Used for sending BSIC (Base station Identity Code)- Give TDMA frame number to the Mobile.

(Downlink only)


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BCH = Broadcast Channels

BCCH = BROADCAST CONTROLCHANNEL

- Used for sending information to the mobile likeCGI (Cell Global identity), LAI (Location Area

Identity), BCCH carriers of the neighboring cells,

maximum output power allowed in the cell and

other broadcast messages like barred cell.

(Downlink only)BACK


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CCCH = Common Control Channels

PCH = PAGING CHANNEL- Used for paging the mobile; Reason could be an incoming call or

an incoming short message(Downlink only)

RACH = RANDOM ACCESS CHANNEL- Used for responding to the paging (terminating), location

updating or to make call access (originating) by asking for asignaling channel.(Uplink only)

AGCH = ACCESS GRANT CHANNEL- Used to allocate SDCCH to the mobile.

(Downlink only)BACK


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DCCH = Dedicated Control Channels

SDCCH = STANDALONE DEDICATED CONTROLCHANNEL

- Used for allocating voice channel (TCH) to the mobile(call setup) and Location updating.

- Send Short Text message to Idle Mobile

(Uplink & Downlink)

FACCH = FAST ASSOCIATED CONTROL CHANNEL- Used for handover

(Uplink & Downlink)


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SACCH = SLOW ASSOCIATED CONTROL CHANNEL- Used for sending information to the mobile like CGI (Cell

Global Identity), LAI (Location Area Identity), BCCH of all the

neighbors and TA (Timing Advance)

- Send Short Text message to Busy Mobile

(Downlink)

- Used for sending signal strength & bit error rate

measurement of the serving cell and signal strength of theBCCHs of the neighboring cells.

(Uplink)

BACK

DCCH = Dedicated Control Channels


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CBCH = Cell Broadcast Channel

Used for sending short messages to al l the

mobi les with in a geog raph ic area.

Typical example: traffic congestion in a major road ora major accident in an area. Up to 93 characters can

be sent.

If the mobile is in the Idle mode then the short

message will be send through the CBCH. If themob i le is Bu sy , i t wi l l not be sent.

NOT TO BE CONFUSED WITH SMS


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What is an SMS?

Short Message Service

SMS messages are short TEXT messages up to 160characters in length that you can send or receive. Themessages are not sent straight to the other mobile but

is sent to message center operated by the Network

Provider


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If the mobile was switched off or is at outside of thecoverage area, the message is stored in the Message

Service Center. The message will be offered to thesubscriber when the mobile is switched on again or

has reentered the coverage area again.

If the mobile is in the idle mode the short message

will be send through the SDCCH. If the mobile is busythe short message will be sent through the SACCH.

BACK

What is an SMS?


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TS 0 TS 1 TS 2 TS 3 TS 4

200 kHz

4.615 ms

0.577 ms

FCCH,SCH,

BCCH

PCH,

RACH,

AGCH

SDCCH,SACCH,

CBCH

TCH,SACCH,

FACCH

TCH,SACCH,

FACCH

TCH,SACCH,

FACCH

TCH,SACCH,

FACCH

TCH,SACCH,

FACCH

TCH,SACCH,

FACCH

1 TDMA FRAME

TS 5 TS 6 TS 7


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C RD DT TT TT TT TT TT T



C RD DA AT IA AT IA AT I

S RD DT TT TT TT TT TT T

F RD DT TT TT TT TT TT T





B RD DT TT TT TT TT TT T

B RD DT TT TT TT TT TT T

B RD DT TT TT TT TT TT TB RD DT TT TT TT TT TT T

S RD DT TT TT TT TT TT T

F RD DT TT TT TT TT TT T


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C D T T T T T T

C D T T T T T T

C D T T T T T T

C D A T A T A T

S D T T T T T T

F D T T T T T T

C D T T T T T T

C D T T T T T T

C D T T T T T T

C D T T T T T T

B D T T T T T T

B D T T T T T T

B D T T T T T TB D T T T T T T

S D T T T T T T

F D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D A I A I A I

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T T

R D T T T T T TR D T T T T T T

R D T T T T T T

R D T T T T T T

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

4.615 ms 4.615 ms

1 Frame

(Downlink - BTS transmit)

1 carrier = 200 kHz

1 Frame

(Uplink - Mobile transmit)

1 carrier = 200 kHz

R = RACHA = SACCH

T = TCH

F = FCCHS = SCH

B = BCCH

C = PCH or

AGCH

D = SDCCH

A = SACCH

T = TCH


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Downlink.

Uplink

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

5432107654321076543210765 76.

3 TIMESLOTS

3 * 0.577ms = 1.73ms

3 TIMESLOTS

3 * 0.577ms = 1.73ms

3 TIMESLOTS

3 * 0.577ms = 1.73ms

TCH UP-DOWNLINK OFFSET

This means that the mobile does not transmit and receive at the same

time.

Also n ote that :

in TS 0 : All the Logical Channels will repeat itself after 51 frames

in TS 1 : All the Logical Channels will repeat itself after 102 frames

in TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames


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MSISDN

MOBILE STATIONS ISDN NUMBERIs the mobile number used in a GSM PLMN

(Public Land Mobile Network)

MSISDN = Country Code + National DestinationCode + Subscriber number

Maximum length is 15 digitse.g.: 63 + 0918 + 8889999


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IMSI

INTERNATIONAL MOBILE SUBSCRIBER IDENTITYIs the subscriber number used over radio path for all signaling in the

GSM PLMN.This number is stored in SIM (Subscriber Identity Module), HLR (Home

Location Register, and VLR (Visitor Location Register)

IMSI = MCC + MNC + MSIN= Mobile Country Code + Mobile Network Code + Mobile

Identification Number

[ 3 digits ] [ 2 digits ] [ 11 digits ]

e.g.: 502 + 19 + 2345451


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TMSI

TEMPORARY MOBILE SUBSCRIBER IDENTITY

Is used for the subscriber's confidentiality. Since the TMSI hasonly local significance (within MSC/VLR), the structure of theTMSI can be chosen by the Vendor. But the size must be 1/2 of

the size of IMSI. Each time a mobile request for location updating

or call setup, MSC/VLR allocates to the IMSI a new TMSI, so the

TMSI is used on the signaling path, protecting the IMSI identity.

Plus since the TMSI is half the size of IMSI, we can page twicethe amount compared to IMSI.


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LAI

LOCATION AREA IDENTITYIs used to uniquely identify each location area in the GSM PLMN.

When the system receives an incoming call it knows in whichlocation area it should page the mobile and does not page the

entire network.

LAI = MCC + MNC + LAC= Mobile Country Code + Mobile Network Code + Location

Area Code[ 3 digits ] [ 2 digits ] [ 1 to 65536 ]

e.x. = 502 + 20 + 60001


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CGI

CELL GLOBAL IDENTITY

Is used for cell identification within the GSM network

CGI = MCC + MNC + LAC + CI

e.x. = 502 + 20 + 60001 + 50001

Mobile

Country Code

[ 3 digits ]

+ Mobile

Network Code

[ 2 digits ]

Location

Area Code

[ 1 to 65536 ]

Cell

Identity

[ 1 to 65536 ]

+ +=


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BSIC

BASE STATION IDENTITY CODE

Is used to distinguish co channel frequencyused in the neighboring cellBSIC = NCC + BCC

= Network Color Code + Base Station

ColorCode

[ 0 to 7 ] [ 0 to 7 ]


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SIM

SUBSCRIBER IDENTITY MODULEUsed to provide storage on subscriber related information as following :

- IMSI (International Mobile Subscriber Identity)- Temporary network data like TMSI, LAI, Location update status

- Subscriber Authentication Key (Ki)and Ciphering Key (Kc)which are

used for security purposes

- BCCH information : List of carrier frequencies to be used for cell

selection

- Forbidden PLMN- Language preference

- PIN number (Personal Identification Number) and PIN error counter

- PUK number (Personal Unlock Key) and PUK error counter


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Pin Management

The PIN number consist of 4 to 8 digitsand it is loaded by the serviceactivator an subscription time. Afterwards the PIN number can be

changed as many times an user wishes including the length of the PIN

number. The user can disable the PIN function but again can beinhibited at subscription time by a authorized person. If an incorrect PIN

is entered, an indication is given to the user. After 3consecutive

incorrect entries, the SIM is blocked, even if if the SIM is removed or the

mobile is switch off and on. If the SIM card is blocked the user cannot

access the network. The unblocking of the SIM card can only be done

by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is givento the user at subscription time. If an incorrect PUK is entered more than

10 times then the PUK will not work anymoreand the SIM card will

continue to be blocked until taken to the mobile vendor service center.


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Two Physical Types of SIM:

ID-1 SIM -looks like acredit card

Plug in SIM -looks likea small chip is installed

semi permanently inthe mobile equipment


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When the mobile is in idle mode it must always

be camped to a BCCH carrier!

For the PLMN to know in which location area the mobile is so that itcan page the mobile when an incoming call or short text message is

received.

The mobile can initiate a call by accessing the network on RandomAccess Channel (RACH) of the cell which it camped on.To receive system information from the PLMN like traffic congestionand major accidents.

WHY?!

THERE ARE THREE REASONS:


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An Idle Mobile Does Four Tasks:

PLMN SELECTION

CELL SELECTION

CELL RESELECTION

LOCATION UPDATING


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PLMN SELECTION

If there is no registered PLMN or theregistered PLMN is not available (nocoverage) then the mobile will try to select

another PLMN either

BACK

When the mobile is switched on it will selectthe registered PLMN in the mobile if there

exists one.

manuallyautomatically ordepending on its mode.


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Automatic Mode

The automatic mode uses a list of PLMNS in an orderof priority. The priority will be:

The last network on which you were registeredHome PLMNEach PLMN stored in the SIM card in priority orderOther PLMN with signal level above -85 dBm in

random orderAll other PLMN in decreasing signal strength

BACK


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CELL SELECTION

Once the mobile is switched on and the registeredhome PLMN was selected (e.x. SMART), it will next

search for a BCCH frequency list, stored in its memoryor in its SIM card. The list can have up to 32 BCCHfrequenciesfor the mobile to scan. This reduces the timeof cell selection, compared to scanning the wholefrequency band. If this feature is turned off at the switch

then the mobile has to scan the entire frequency bandfor the strongest BCCH carrier.


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CELL SELECTION

The BCCH frequency list is called BA (BCCH Allocation) listand there are 2 types, ACTIVE AND IDLE. Idle is a list of BCCHused for scanning when the mobile is in an idle mode and Active isa list of BCCH used during mobile busy mode. Why 2 List ???

When the mobile is in idle mode it may want to scan a longer list ofBCCH and tune to the strongest whereas when in Active mode thelist of BCCH should be shorter (correspond to defined neighbors) sothat the mobile will scan the short list and get a more accuratesignal strength measurements to achieve better handoverperformance. It is also to reduce the time spend by the mobile to

decode the BSIC.

Recommendation:ACTIVE MODE LIST SHOULD NOT BEMORE THAN 15 BCC FREQUENCIES.


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CELL SELECTION

If there is no BA list stored in the Mobile or SIM card then themobile will scan all the 124 GSM channels and 374 DCS channeland arrange the frequencies in a DESCENDING order of signalstrength. It will take the mobile 3 to 5 seconds to scan the wholeband. After which it will tune to the strongest frequency. The mobilewill check if this is a BCCH carrier by looking out for the frequencycorrection burst send by the FCCH (Frequency CorrectionChannel). If it is the BCCH carrier than mobile tunes to this carrier toread the SCH (Synchronization Channel) for the BSIC parameter.

Next it will read the BCCH for system information like CGI (CellGlobal identity), LAI (Location Area Identity), BCCH carriers of theneighboring cells (BA List), maximum output power allowed in thecell and other broadcast messages like barred cell.


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CELL SELECTION

Next the mobile will compare if the selected cell belongsto a forbidden PLMN stored in its SIM card. It will look at the2 digit Network Mobile Code transmitted by the BCCH on the

LAI (Location Area Identity). If those 2 digits ware registeredas forbidden in the SIM card then the mobile will not selectthis cell. The mobile then will tune to the second strongestBCCH carrier and subsequently does the same process overand over again until it finds the right cell. Once it finds the

right cell it will start using the BA (BCCH allocation) listtransmitted by the BCCH carrier for cell reselection, will bediscussed later.


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CELL SELECTION

Lets say the chosen PLMN is correct, able to read the FCH, SCHand BCCH and the chosen Cell is accessible (no cell barring),

NO!!

THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTIONCRITERIONWHICH MUST BE CALCULATED BY THE MOBILE AND IF

THE C1 VALUE IS GREATER THAN 0THEN THE MOBILE CAN CAMP

ON THIS CELL OR ELSE THE NEXT CELL WITH C1 > 0 WILL BE

SELECTED.

DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ON

THIS SITE?


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C1-CELL SELECTION CRITERIONC1 = A - Max (B,0)

and C1 > 0 for the mobile to camp on this BTS

RxLev = Signal strength received by the mobileRxLevAccMin = Minimum Signal level to be received by the mobile from BTS (BCCH)

before it could access the BTSMsTxPwrMaxCCH = Maximum Transmit Power allowed to access the BTS (using

RACH)P = Mobile Class power

C1 = (RxLev - RxLevAccMin) - Max(MsTxPwrMaxCCH - P,0)

Ex :C1 = ( -80 - (-100) ) - Max (33 - 33, 0)= -80 + 100 - Max (0, 0)

= 20

C1 > 0 so mobile will camp on this site

Shortcut: If RxLevel > RxLevAccMin then Mobile can camp on this site

where:

A = RxLev - RxLevAccMin

B = MsTxPwrMaxCCH - P


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Minimum Signal level that must be received by the mobile

from BTS (BCCH) before it could access the BTS

RxLevAccessMin (Nokia)ACCMIN (Ericsson) SSACC (TACS) (Uplink)

= -102 (GSM900) = -100 (DCS1800)

General rule:The signal received by the mobile

should be 2dB higher than the mobile sensitivity

WHY?!


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RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss +Interference Margin

Mobi le Sensi t iv i ty =-104 for GSM900 and -102 for DCS1800

Body loss =3 dB recommended by ETSI and 5 dB recommended by Ericsson for

GSM 900=3 dB recommended by ETSI and 3 dB recommended by

Ericsson for DCS 1800

Mult ipath loss=Signal loss from base station due to reflection by buildings, etc

before reaching mobile. Normally the Multipath loss is around 3 dB but can be

overcome by Antenna Diversity which has gain around 3 dB too. (Space diversity = 3

dB, 90 degrees polarized diversity = 3 dB, 45 degrees slant polarized diversity = 4.5dB)

Interference Margin =Margin allocated to overcome C/I and C/N, the

recommended value is 2 dB

What is the accurate way of setting the

RxLevAccessMin parameter ?


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RxLevAccessMin = Mobile Sensitivity + Body loss +

Multipath loss + Interference Margin

RxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 withETSI standard)= - 99 dBm

RxLevAccessMin = -104 + 5 + 0 + 2 (for GSM 900 withEricsson standard)

= - 97 dBm

RxLevAccessMin = -104 + 3 + 0 + 2 (for DCS 1800)= - 99 dBm

What is the accurate way of setting the

RxLevAccessMin parameter ?


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Class 1 10 Watt (40dBm) 20 Watt (43dBm) 1 Watt (30dBm)

Class 2 4 Watt (36dBm) 8 Watt (39dBm) 0.25 Watt (24dBm)

Class 3 1 Watt (30dBm) 5 Watt (37dBm) 4 Watt (36dBm)

Class 4 0.6 Watt (28dBm) 2 Watt (33dBm) *

Class 5 * 0.8 Watt (29dBm) *


Mobile Sensitivity -113 dBm -104 dBm -102 dBm

BTS Sensitivity -116 dBm -107 dBm -106 dBm


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MsTxPwrMaxCCH (Nokia)CCHPWR (Ericsson) PLC (TACS)

= 33dBm

(GSM900)

= 30dBm

(DCS1800)

Maximum Transmit Power allowed to access the

BTS (using RACH) - mobile is idle

= 0 (28dBm)

(DCS1800)


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MsTxPwr (Nokia)MsTxPwr (Ericsson) PLVM (TACS)

= 33dBm

(GSM900)

= 30dBm

(DCS1800)

Maximum Transmit Power allowed to use in a BTS

during busy status (using TCH) - mobile is busy

= 0 (28dBm)

(DCS1800)


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Minimum Transmit Power allowed to use in a BTS

during busy status (using TCH)mobile is busy

MsTxPwrMin (Nokia)

= 13dBm (GSM900) = 13dBm (DCS1800)


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Switch on the Mobile

For 2 to 3 seconds the mobile will scan all the 124

channels in GSM900 and 374 channels in DCS1800

The mobile will synchronize to this carrier

and read the BCCH info like LAI, CGI

Tune to the second

strongest channel

Mobile will compare the signal strengthof the 124 channels and tune to the strongest

Mobile will check if it is a BCCH carrier ?

Does the BCCH belong to the wanted PLMN,

E.x: Smart, Globe, Islacom ?

Is the cell barred from accessing ?

Is C1 >0 ?

Camp on this site !!!

Yes

Yes

Yes

No

Yes

No

No

No


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An Idle Mobile Does Four Tasks:

PLMN SELECTION

CELL SELECTION

CELL RESELECTION

LOCATION UPDATING


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After the cell has been successfully selected, the mobile nowwill start reselection tasks. It will continuously make measurementson its neighboring cells (as indicated by the BA list) to initiate cell

reselection if necessary. At least 5 measurement samples per

neighboring cellare needed. A running average of the received

signal level will be maintained for each carrier in the BA list.

CELL RESELECTION

1) Perform Cell Reselection Measu remen t First


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All system information messages sent on the current BCCH on theserving cell must be read by the mobileevery 30 secondsto monitor

changes in cell parameters (ex: MsTxPwrMax). The mobile also has to read

the 6 strongest BCCHevery 5 minutesto receive its cell parameters (ex:MsTxPwrMax). The 6 strongest can be seen from the BA list which has the

updated measurement of the 32 BCCH carrier. The neighboring list for the

best 6 neighbors is updatedevery 60 seconds,which means the mobile

has to measure each neighbor by 10 seconds. The mobile also has to read

the BSIC of the 6 strongest BCCHevery 30 secondsto confirm that it is still

monitoring the same cells. If a new BSIC is detected, then the BCCH of thisBSIC will be read to receive the cell parameters.

CELL RESELECTION


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CELL RESELECTION

1) Cell Reselection Criter ia

The mobile will reselect and camp on another cell if any of the following criteria is satisfied:

The serving cell is barred.

C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss ishigh and the mobile needs to change cell.The Mobile has unsuccessfully tried to access the network as defined by the MAXRET(Ericsson) parameter or MaxNumberRetransmissions (Nokia).

MAXRET is the maximum number of retransmission a mobile can do when it is accessing

the system. It is defined per cell.

Assuming that one of the criteria above was satisfied then the mobile will select a cell with a

better C1.

However if the cell belongs to a different location areathen the C1 for that cell has to

exceed a reselection hysterisis parameter calledCRH (Ericsson) or CellReselectHyseteris

(Nokia) for the reselection to happen!!


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CELL RESELECTIONIf the mobile is moving in a border area between location areas, it might repeatedly changebetween location areas. Each change requires location updating and cause heavy signaling load and

risk paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used. Thecell in a different location area will only be selected if the C1 of that cell is higher than the C1 of the

current serving cell by the value of the Reselect Hysteresis.

Since the Value of CRH maybe different for each cell, the CRH used for comparison will be the onebroadcast by the serving cell. If the value is set very low then the mobile will Ping-Pong between

location areas which will increase signaling load. If the value is set very high the mobile may camp in

the wrong cell too long.

Recommendations:Set the value to 6

C1 = 1

CRH = 4

C1 = 6

Serving cell Target cellLA 1 LA 2


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B SIC COMP RISONMOBILE IDLEWhen the mobile is idle, it listens to the best cell to camp. Mobiledecides to choose the cell by

itselfwithout the help of BSC. This is done by comparing signal strength of each BCCH

frequency and if found the strongest then it will camp once the C1 > 0 (Cell Selection Criterion).

If after camping to this cell, it finds that a neighboring cell is much better then it will change to

that neighboring cell. If the new cell is in same location area the mobile does not have to inform

the BSC about its new cell but if the new cell is from a different different location area then the

mobile will perform a location updating to inform the BSC.

MOBILE BUSYA mobile is considered busy when there is a call going on (speech, data or fax) or it is in the

middle of a call setup. At this stage the mobile cannot decide by itself whether it is necessary

for the mobile to handover to a better cell. Only BSC can determine if a mobile has to changeto another cell other than the serving cell. BSC makes the decisionbased on measurement

reports sent by both Mobile and BTS. This decision making is called locating. In a busy state,

mobiles can receive Short Text Message (SMS) but cannot receive Cell Broadcast Messages.

1.basic gsm principles

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