GSM

BASIC GSMPRINCIPLES

COMPARISON

	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

TCH = 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

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)

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)

COMMON CONTROL CHANNELS

PCH = PAGING CHANNEL

=> Used for paging the Mobile. (Downlink only)

     Reason could be an incoming call or an incoming Short Message.

    

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)

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)

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)

FACCH = FAST ASSOCIATED CONTROL CHANNEL

 => Used for handover. (Uplink & Downlink)

CBCH = CELL BROADCAST CHANNEL

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

      Typical example is 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 !!!!!!!!

(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 centre 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 send through

      the SACCH. 

MOBILE STATIONS ISDN NUMBER (MSISDN)

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

MSISDN = Country Code + National Destination Code + Subscriber number

e.x.   63 + 0918 + 8889999

Maximum length is 15 digits.

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)

=> 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 digit ]                            [ 2 digit ]                          [ 11 digit ]

e.x.  502 + 19 + 2345451

TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)

=> 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.

LOCATION AREA IDENTITY (LAI)

=> 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 digit ]                          [ 2 digit ]                         [ 1 to 65 536 ]

e.x. =  502 + 20 + 60001

CELL GLOBAL IDENTITY (CGI)

=> Is used for cell identification within the GSM network.

LAI   = MCC + MNC + LAC + CI

             Mobile Country Code + Mobile Network Code + Location Area Code + Cell Identity

               [ 3 digit ]                          [ 2 digit ]                         [ 1 to 65 536 ]           [ 1 to 65 536 ]

e.x. =  502 + 20 + 60001 + 50001

BASE STATION IDENTITY CODE (BSIC)

=> Is used to distinguish co channel Frequency used in the neighboring cell.

BSIC = NCC + BCC

             Network Color Code + Base Station Color Code

                [ 1 to 7 ]                          [ 1 to 7 ]

SUBSCRIBER IDENTITY MODULE (SIM)

SIM is 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 management

The PIN number consist of 4 to 8 digit 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 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 are specified :

ID - 1 SIM - Looks like a Credit card

Plug in SIM - Look like a small chip is installed semi permanent in the mobile equipment.

What is the accurate way of setting the RxLevAccessMin parameter

 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

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

TACS	TACS	TACS
10 Watt (40dBm)	20 Watt (43dBm)	1 Watt (30dBm)
4 Watt (36dBm)	8 Watt (39dBm)	0.25 Watt (24dBm)
1 Watt (30dBm)	5 Watt (37dBm)	4 Watt (36dBm)
0.6 Watt (28dBm)	2 Watt (33dBm)	*
*	0.8 Watt (29dBm)	*

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