Basic Gsm Principles

BASIC GSM PRINCIPLES

BASIC GSM PRINCIPLES

COMPARISONCOMPARISON

TACS GSM 900 DCS 1800Uplink 890 - 905 MHz 890 - 915 MHz 1710 - 1785 MHzDownlink 935 - 950 MHz 935 - 960 MHz 1805 - 1880 MHzDuplex Distance 45 MHz 45 MHz 95 MHzCarrier Separation 25 kHz 200 kHz 200 kHzNumber of Channels 15 Mhz / 25 kHz = 600 25 MHz / 200 kHz = 124 75 MHz / 200 kHz = 374Channels defined in Switch 1, 2, ….599, 600 1, 2, ….123, 124 512 , 513 ….884, 885Access Method FDMA TDMA TDMA

GSM 900DCS 1800 TACS

1 Control Channel

1 Voice Channel

10 Control

Channels

1 Voice Channel

Logical Channels: Logical Channels:

1 VOICE CHANNEL

10 CONTROL CHANNELS

TCH = Traffic ChannelTCH = Traffic Channel

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

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

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

BACK

BCH

CCCH

DCCH

CBCH

NEXT

BCH = Broadcast ChannelsBCH = 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)

BCH = Broadcast ChannelsBCH = Broadcast Channels

BCCH = BROADCAST CONTROL CHANNEL

- Used for sending information to the mobile like CGI (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

CCCH = Common Control ChannelsCCCH = 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 a signaling channel.

(Uplink only)

AGCH = ACCESS GRANT CHANNEL- Used to allocate SDCCH to the mobile. (Downlink only) BACK

DCCH = Dedicated Control ChannelsDCCH = Dedicated Control Channels

SDCCH = STAND ALONE DEDICATED CONTROL CHANNEL

- 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)

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 the BCCHs of the neighboring cells.

(Uplink)BACK

DCCH = Dedicated Control ChannelsDCCH = Dedicated Control Channels

CBCH = Cell Broadcast ChannelCBCH = Cell Broadcast Channel

Used for sending short messages to all the mobiles within a geographic area.

Typical example: traffic congestion in a major road or a 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 the mobile is Busy, it will not be sent.

NOT TO BE CONFUSED WITH SMS

What is an SMS?What is an SMS?

Short Message Service

SMS messages are short TEXT messages up to 160 characters in length that you can send or receive. The messages are not sent straight to the other mobile but is sent to message center operated by the Network Provider

If the mobile was switched off or is at outside of the coverage area, the message is stored in the Message Service Center. The message will be offered to the subscriber 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 busy the short message will be sent through the SACCH.

BACK

What is an SMS?What is an SMS?

TS 0 TS 1 TS 2 TS 3 TS 4

200 kHz4.615 ms

0.577 ms

FCCH,SCH,BCCHPCH,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

C RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TC RD DA AT IA AT IA AT IS RD DT TT TT TT TT TT TF RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TC RD DT TT TT TT TT TT TB RD DT TT TT TT TT TT TB RD DT TT TT TT TT TT TB RD DT TT TT TT TT TT TB RD DT TT TT TT TT TT TS RD DT TT TT TT TT TT TF RD DT TT TT TT TT TT T

C D T T T T T TC D T T T T T TC D T T T T T TC D A T A T A TS D T T T T T TF D T T T T T TC D T T T T T TC D T T T T T TC D T T T T T TC D T T T T T TB D T T T T T TB D T T T T T TB D T T T T T TB D T T T T T TS D T T T T T TF D T T T T T T

R D T T T T T TR D T T T T T TR D T T T T T TR D A I A I A IR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR 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 = SACCHT = TCH

F = FCCHS = SCHB = BCCHC = PCH or AGCHD = SDCCHA = SACCHT = TCH

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 TIMESLOTS3 * 0.577ms = 1.73ms



TCH UP-DOWNLINK OFFSETTCH UP-DOWNLINK OFFSET

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

Also note that :

in TS 0 : All the Logical Channels will repeat itself after 51 framesin TS 1 : All the Logical Channels will repeat itself after 102 framesin TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames

MSISDNMSISDN

MOBILE STATIONS ISDN NUMBER

Is the mobile number used in a GSM PLMN (Public Land Mobile Network)

MSISDN = Country Code + National Destination Code + Subscriber number

Maximum length is 15 digits

e.g.: 63 + 0918 + 8889999

IMSIIMSI

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY

Is 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

TMSITMSI

TEMPORARY MOBILE SUBSCRIBER IDENTITY

Is used for the subscriber's confidentiality. Since the TMSI has only local significance (within MSC/VLR), the structure of the TMSI 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 twice the amount compared to IMSI.

LAILAI

LOCATION AREA IDENTITY

Is used to uniquely identify each location area in the GSM PLMN. When the system receives an incoming call it knows in which location 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

CGICGI

CELL GLOBAL IDENTITY

Is used for cell identification within the GSM network CGI = MCC + MNC + LAC + CI

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

MobileCountry Code

[ 3 digits ]

+ MobileNetwork Code

[ 2 digits ]

LocationArea Code

[ 1 to 65536 ]

CellIdentity

[ 1 to 65536 ]

+ +=

BSICBSIC

BASE STATION IDENTITY CODE

Is used to distinguish co channel frequency used in the neighboring cell

BSIC = NCC + BCC = Network Color Code + Base Station

Color Code

[ 0 to 7 ] [ 0 to 7 ]

SIMSIM

SUBSCRIBER IDENTITY MODULE

Used 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

Pin ManagementPin Management

The PIN number consist of 4 to 8 digits and it is loaded by the service activator 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 be inhibited at subscription time by a authorized person. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive 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 given to the user at subscription time. If an incorrect PUK is entered more than 10 times then the PUK will not work anymore and the SIM card will continue to be blocked until taken to the mobile vendor service center.

Two Physical Types of SIM:Two Physical Types of SIM:

ID-1 SIM - looks like a credit card

Plug in SIM - looks like a small chip is installed semi permanently in the mobile equipment

When the mobile is in idle mode it must always be camped to a BCCH carrier! 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 it can page the mobile when an incoming call or short text message is received.

The mobile can initiate a call by accessing the network on Random Access Channel (RACH) of the cell which it camped on.

To receive system information from the PLMN like traffic congestion and major accidents.

WHY?!

THERE ARE THREE REASONS:THERE ARE THREE REASONS:

An Idle Mobile Does Four Tasks:An Idle Mobile Does Four Tasks:

PLMN SELECTION

CELL SELECTION

CELL RESELECTION

LOCATION UPDATING

PLMN SELECTIONPLMN SELECTION

If there is no registered PLMN or the registered PLMN is not available (no coverage) then the mobile will try to select another PLMN either

BACK

When the mobile is switched on it will select the registered PLMN in the mobile if there exists one.

manuallyautomatically or

depending on its mode.

Automatic ModeAutomatic Mode

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

The last network on which you were registered

Home PLMN

Each PLMN stored in the SIM card in priority order

Other PLMN with signal level above -85 dBm in random order

All other PLMN in decreasing signal strength

BACK

Manual ModeManual Mode

In the manual mode the mobile will try to connect to the Home PLMN first. If it is unsuccessful then it will provide a list of available PLMN and ask the user to choose one. If the second chosen PLMN is not successful then the mobile will make an indication to the user to select another PLMN. Until the users selects another network a message “No Access” will be displayed. If there is no GSM or DCS coverage at all then a message “No Network” will be displayed.

BACK

CELL SELECTIONCELL SELECTION

Once the mobile is switched on and the registered home PLMN was selected (e.x. SMART), it will next search for a BCCH frequency list, stored in its memory or in its SIM card. The list can have up to 32 BCCH frequencies for the mobile to scan. This reduces the time of cell selection, compared to scanning the whole frequency band. If this feature is turned off at the switch then the mobile has to scan the entire frequency band for the strongest BCCH carrier.


The BCCH frequency list is called BA (BCCH Allocation) list and there are 2 types, ACTIVE AND IDLE. Idle is a list of BCCH used for scanning when the mobile is in an idle mode and Active is a 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 of BCCH and tune to the strongest whereas when in Active mode the list of BCCH should be shorter (correspond to defined neighbors) so that the mobile will scan the short list and get a more accurate signal strength measurements to achieve better handover performance. It is also to reduce the time spend by the mobile to decode the BSIC.

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


If there is no BA list stored in the Mobile or SIM card then the mobile will scan all the 124 GSM channels and 374 DCS channel and arrange the frequencies in a DESCENDING order of signal strength. It will take the mobile 3 to 5 seconds to scan the whole band. After which it will tune to the strongest frequency. The mobile will check if this is a BCCH carrier by looking out for the frequency correction burst send by the FCCH (Frequency Correction Channel). If it is the BCCH carrier than mobile tunes to this carrier to read the SCH (Synchronization Channel) for the BSIC parameter. Next it will read the BCCH for system information like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells (BA List), maximum output power allowed in the cell and other broadcast messages like barred cell.


Next the mobile will compare if the selected cell belongs to a forbidden PLMN stored in its SIM card. It will look at the 2 digit Network Mobile Code transmitted by the BCCH on the LAI (Location Area Identity). If those 2 digits ware registered as forbidden in the SIM card then the mobile will not select this cell. The mobile then will tune to the second strongest BCCH carrier and subsequently does the same process over and over again until it finds the right cell. Once it finds the right cell it will start using the BA (BCCH allocation) list transmitted by the BCCH carrier for cell reselection, will be discussed later.


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

NO!!

THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTION CRITERION WHICH MUST BE CALCULATED BY THE MOBILE AND IF THE C1 VALUE IS GREATER THAN 0 THEN 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?

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

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

RxLev = Signal strength received by the mobile

RxLevAccMin = Minimum Signal level to be received by the mobile from BTS (BCCH) before it could access the BTS

MsTxPwrMaxCCH = 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 - RxLevAccMinB = MsTxPwrMaxCCH - P

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

General rule: The signal received by the mobile should be 2dB higher than the mobile sensitivity

WHY?!

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

Mobile Sensitivity = -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

Multipath 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 = 4.5 dB)

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 ?What is the accurate way of setting the RxLevAccessMin parameter ?

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

RxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 with ETSI standard)

= - 99 dBm

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

= - 97 dBm

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

= - 99 dBm

What is the accurate way of setting the RxLevAccessMin parameter ?What is the accurate way of setting the RxLevAccessMin parameter ?

TACS GSM 900 DCS 1800Class 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) *

TACS GSM 900 DCS 1800Mobile Sensitivity -113 dBm -104 dBm -102 dBmBTS Sensitivity -116 dBm -107 dBm -106 dBm

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

= 33dBm (GSM900)= 33dBm (GSM900)

= 30dBm(DCS1800)= 30dBm(DCS1800)

Maximum Transmit Power allowed to access the BTS (using RACH) - mobile is idle

= 0 (28dBm) (DCS1800)= 0 (28dBm) (DCS1800)

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

= 33dBm (GSM900)= 33dBm (GSM900)

= 30dBm(DCS1800)= 30dBm(DCS1800)

Maximum Transmit Power allowed to use in a BTS during busy status (using TCH) - mobile is busy

= 0 (28dBm) (DCS1800)= 0 (28dBm) (DCS1800)

Minimum Transmit Power allowed to use in a BTS during busy status (using TCH) – mobile is busy

MsTxPwrMin (Nokia)

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

Switch on the Mobile

For 2 to 3 seconds the mobile will scan all the 124channels 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 secondstrongest channel

Mobile will compare the signal strength of 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

An Idle Mobile Does Four Tasks:An Idle Mobile Does Four Tasks:

PLMN SELECTION

CELL SELECTION

CELL RESELECTION

LOCATION UPDATING

After the cell has been successfully selected, the mobile now will start reselection tasks. It will continuously make measurements on its neighboring cells (as indicated by the BA list) to initiate cell reselection if necessary. At least 5 measurement samples per neighboring cell are needed. A running average of the received signal level will be maintained for each carrier in the BA list.

CELL RESELECTIONCELL RESELECTION

1) Perform Cell Reselection Measurement First

All system information messages sent on the current BCCH on the serving cell must be read by the mobile every 30 seconds to monitor changes in cell parameters (ex: MsTxPwrMax). The mobile also has to read the 6 strongest BCCH every 5 minutes to 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 updated every 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 BCCH every 30 seconds to confirm that it is still monitoring the same cells. If a new BSIC is detected, then the BCCH of this BSIC will be read to receive the cell parameters.



1) Cell Reselection CriteriaThe 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 is high 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 area then the C1 for that cell has to exceed a reselection hysterisis parameter called CRH (Ericsson) or CellReselectHyseteris (Nokia) for the reselection to happen!!

CELL RESELECTIONCELL RESELECTION If the mobile is moving in a border area between location areas, it might repeatedly change between 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. The cell 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 one broadcast 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 = 1CRH = 4

C1 = 6

Serving cell Target cell LA 1 LA 2

BASIC COMPARISONBASIC COMPARISON

MOBILE IDLEWhen the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell by itself without 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 change to another cell other than the serving cell. BSC makes the decision based 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.

Basic Gsm Principles

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