GSM Traffic Cases

GSM Traffic Cases

  u   MS can either be in detached mode or it can be in attached mode.

  u  When the MS is in detached mode, there is no traffic case involved with that.

  u  While in attached mode, the MS can either in idle state or in active state.

  u  Changing from idle to active state will involve different types of traffic cases:

1  .     Location updating.

2  .     Call setup.

3  .     Handover.

     1. Location updating

Need for Location updating:

l  It should be possible for the MS to receive a call from the network at any time at any location.

l  While the MS moves freely within a network, the network should know about its whereabouts, in terms of its location and the cell with which it is attached.

l  The MS should update the network whenever it changes the location and is called Location Updating.

Three types of location updating:

l  Location updating normal.

l  IMSI attach.

l  Periodic Registration.

(A)   NORMAL

n  A location area is the area handled by one or more BTSs where the MS can move around without updating the network.

n  A location area is controlled by one or more BSCs but strictly by one MSC.

n  MS gets the location area identity of the serving cell, through listening the BCCH.

n  MS compares the Location Area Identity (LAI) to the one stored in the MS through the SIM card & if LAI differs from the one stored in the SIM card, the MS decides to do a location update, type normal.

Location Updating, type normal. MS already registered in VLR

1. The MS listens to the system information on BCCH Channel, compares the LAI with the one stored in SIM card. When it finds the difference, it decides to do an location update.

2a. The MS sends a channel request message through RACH channel.
2b. The message received by the BTS is forwarded to the BSC. The BSC allocates a SDCCH, if there is one idle, and tells the BTS to activate it.
2c. BTS give acknowledgement on AGCH channel.

3.  The MS sends a location updating request message which contains the identity of the MS, the identity of the old location area and the type of updating.

4a. The authentication parameter is sent to the MS. If the MS is not already registered in this   MSC/VLR the appropriate HLR or the previously used MSC/VLR must be contacted to retrieve MS subscriber data and authentication parameters.
4b. MS sends an answer calculated using the received authentication parameter.

5.   If authentication is successful, the VLR is updated.

6.   The MS receives an acceptance of the location updating.

7a. The BTS is told to release the SDCCH.
7b. The MS is told to release the SDCCH and switches to idle mode.


During the location change, if the MS is busy in call, it receives the information about the new LAI on the SDCCH. The location updating (above said procedure) takes place after the call is released.

(B) IMSI attach:

n  IMSI attach is used by the MS to notify the system that it was powered on, provided it is still in the same location area as it was when it entered the detached state.
n  This procedure is to be used only when the IMSI detach flag is set in the VLR.
n If the flag is set in the HLR, switching to active mode requires a normal location updating of the  MS.

1. The MS requests a SDCCH ( The point number 1 , 2a, 2b , 2c all are same as in Normal Updating).
2. The system receives the IMSI attach message from the MS.
3. The MSC sends the IMSI attach message to the VLR. The VLR removes the IMSI detached flag and resumes normal call handling for the MS.
4. The VLR returns the IMSI attach acknowledge message to the MSC.
5. The MS also receives an acknowledge message.


NOTE: - This procedure is to be used only when the IMSI detach flag is set in the VLR. If the flag is set in the HLR, switching to active mode requires a normal location updating of the MS.

GSM IDENTITES

GSM IDENTITIES

GSM has total 8 identities. 

1. MOBILE STATION ISDN NUMBER (MSISDN) 
2. INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)
3. TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)
4. INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI)
5. MOBILE STATION ROAMING NUMBER (MSRN)
6. LOCATION AREA IDENTITY (LAI)
7. CELL GLOBAL IDENTITY (CGI)
8. BASE STATION IDENTITY CODE (BSIC)

1. MOBILE STATION ISDN NUMBER (MSISDN):-

• These are the digits dialed when calling a mobile subscriber.
• In GSM 900/GSM 1800, the MSISDN consists of the following:

MSISDN = CC + NDC + SN

   CC = Country Code

   NDC = National Destination Code

  SN  = Subscriber Number

• A NDC is allocated to each PLMN. In some countries, more than one NDC may be required for each PLMN.
• The international MSISDN number may be of variable length.
• The maximum length is 15 digits, prefixes not included.

2. INTERNATIONAL MOBILE SUBSCRIBER  IDENTITY (IMSI):-

• The IMSI is a unique identity allocated to each subscriber to allow correct identification over the radio path and through the network.
• All network related subscriber information is connected to the IMSI.
• The IMSI is stored in the SIM, as well as in the HLR and in the serving VLR.
• The IMSI consists of three different parts:

IMSI = MCC + MNC + MSIN

MCC = Mobile Country Code.

MNC = Mobile Network Code

MSIN = Mobile Station Identification Number.

• According to the GSM specifications, IMSI has a maximum length of 15 digits.

3. TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI):-

• Is a temporary number used instead of IMSI to identify a MS.
• Is used for the subscriber’s confidentiality on the air interface.
• Has only local significance (that is, within the MSC/VLR area).
• Is changed at certain events or time intervals.
• The TMSI structure can be chosen by each operator but should not consist of more than four octets (8 digits).

4. INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI):-

• IMEI is used for equipment identification and uniquely identifies a MS as a piece or assembly of equipment.
• The IMEI consists of the following:-

 IMEI = TAC + FAC + SNR + spare

•  TAC = Type Approval Code, determined by a central  GSM body.
•   FAC = Final Assembly Code, identifies the manufacturer.
•   SNR = Serial Number, an individual serial number of six digits uniquely identifies all equipment with in each TAC and FAC.
• spare = A spare bit for future use. When transmitted by the MS this digit should always be zero.

5. MOBILE STATION ROAMING NUMBER (MSRN):-

• Is used during the call setup phase for mobile terminating calls.
• Each mobile terminating call enters the GMSC in the PLMN. The call is then re-routed by the GMSC, to them MSC where the called mobile subscribers are located. For this purpose, a unique number (MSRN) is allocated by the MSC and provided to the GMSC.
• The MSRN is seized for the call setup phase only and released immediately afterwards
• 
  
• (MSRN) consists of three parts:-

MSRN = CC + NDC + SN

      CC = Country Code

      NDC = National Destination Code

      SN = Subscriber Number

6. LOCATION AREA IDENTITY (LAI):-

• Used for paging, to indicate to the MSC in which  Location Area (LA) the MS is currently situated.
• Also used for location updating of mobile subscribers.
• The LAI contains the following:-

LAI = MCC + MNC + LAC

MCC = Mobile Country Code , same as IMSI MCC

MNC = Mobile Network Code, same as IMSI MNC

LAC = Location Area Code, the maximum length of LAC is 16 bits, enabling 65,536 different location areas to be defined in one PLMN.

7. CELL GLOBAL IDENTITY (CGI):-

• Is used for cell identification within a location area.
• Done by adding a Cell Identity (CI) to the components of a LAI.
• CI has a maximum length of 16 bits.
• CGI consists of -

CGI = MCC + MNC + LAC + CI

8. BASE STATION IDENTITY CODE (BSIC):-

• Allows a mobile station to distinguish between different neighboring base stations.
• BSIC consists of:-

BSIC = NCC + BCC

NCC = Network Color Code (3 bits), identifies the PLMN. NCC does not uniquely identify the operator. 

NCC is primarily used to distinguish between operators on each side of a border.

BCC = Base Station Color Code (3 bits), identifies the Base Station to help distinguish between BTS using the same BCCH frequencies.

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GSM INTERFACE

GSM  INTERFACE

Interface means in which medium the information flow from one node to other node : like how the information flow from  mobile to BTS and which type of interface is used?

From	To	Interface	Technology used
MS	BTS	Air / Um / Radio interface	FDMA + TDMA Hybrid
BTS	BSC	Abis   Interface	LAPD Technology
BSC	MSC	A   Interface	SS7 Technology
MSC	VLR	B   Interface	MAP/ TCAP ( SS7 Layers)
MSC	HLR	C   Interface	MAP/ TCAP ( SS7 Layers)
VLR	HLR	D   Interface	MAP/ TCAP ( SS7 Layers)
MSC 1	MSC 2	E   Interface	MAP/ TCAP + TUP + ISUP
MSC	EIR	F   Interface	MAP/ TCAP ( SS7 Layers)
VLR 1	VLR 2	G   Interface	MAP/ TCAP ( SS7 Layers)
HLR	AuC	H   Interface	INTERNAL TECHNOLOGY

GSM CHANNELS

Channel Concepts In GSM

Channel concepts:

• In GSM 900 band, there are 124 carriers.
• In GSM 1800 band, there are 374 carriers.
• In GSM 1900 band, there are 299 carriers.
• The carrier separation in the above bands are 200 kHz..
• Each  carrier is divided into 8 timeslots (TS).
• Each timeslot (TS) is referred as physical channel.
• Physical channels are further divided into different logical channels.
• Logical channels are separated into two categories:-

      1. Traffic Channels.

      2. Signaling/Control channels. 

Traffic Channels:-

Traffic channels (TCH) carry speech and data traffic.

There are two types of Traffic channels:

¤. Full Rate Channel (TCH/F) at the gross rate of 22.8 kbits/sec.

¤. Half Rate Channel (TCH/H) at the rate of 11.4 kbits/sec.

Traffic Channel / Full:-

•  Gross rate of 22.8 kbits/sec.

•  Use when load on the network is low.

•  1 Timeslot is given to 1 user.

• TCH/F carries user speech at 13 kbps and data at 9.6  kbps,4.8 kbps and 2.4 kbps.

• User data rates for the TCH/H are 4.8 kbps and 2.4 kbps.

Traffic Channel / Half:-

•  Gross rate of 11.4 kbits/sec.

•  Use when load on the network is high.

•  1 Timeslot is given to 2 user.

Signaling Channels are divided into three

categories:-

1. Broadcast Channels.

2. Common Control Channels.  

3. Dedicated Control Channel.

Broadcast Channels (BCH):-

These channels are transmitted down link only.

Helps the MS to tune always to the current network.

Acts like the light from a light house for the MS.

Three types of Broadcast channels available in GSM:-

i.  Broadcast Control Channels (BCCH)

ii. Frequency Correction Channel  (FCCH).

iii. Synchronization Channel (SCH).

i. Broadcast Control Channel ( BCCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

•  Give the information of LAI ( Location Area Identity) & CGI ( Cell Global Identity).

•   Tell about BSIC ( Base Station Identity Center).

•   Provide neighboring  frequency data.

•   Also deals with power i.e. how much power is given to MS.

ii. Frequency Correction Channel ( FCCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

•  Used for frequency synchronization.

iii. Synchronization Channel ( SCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

• Used for time and frame synchronization.

Common Control Channels (CCCH):-

Point to multipoint bi-directional channels. 

Used to establish connections between MS and BS.

Before a DCCH is assigned to a MS.

Three types of Common Control channels available:-

i.  Paging Channel  (PCH).

ii. Random Access Channel (RACH).

iii. Access Grant Channel (AGCH).

i. Paging Channel ( PCH):-

•  It is a Downlink Channel.

•  Also Unidirectional.
•  With the help of PCH Network, MTC sends the information to MS via MSC, BSC & BTS
to immediately  report back to network as there may be any incoming call or SMS.

ii. Random Access Channel ( RACH):-

•  It is a uplink Channel.

•  It is used by MS to request for SDCCH.

iii. Access Granted Channel ( AGCH):-

•  It is a Downlink Channel.

•  Also Unidirectional.
•  Its is an Acknowledgement for RACH.

Dedicated control Channel ( DCCH):-

Used for call set up procedure.

Used for signaling and control after call.

Establishment.

Also used for SMS transmission.

Four types of Dedicated control channel available:-

i.  Stand alone Dedicated Control Channel (SDCCH).

ii.  Slow Associated Control Channel (SACCH).

iii. Fast Associated Control Channel (FACCH).

iv. Cell Broadcast Channel (CBCH).

i. Stand alone Dedicated Control Channel (SDCCH):-

•  It is the most important channel.

•  Its a bi- directional channel.

•   Used for call setup.

•   It is also used in authentication.

•   IMSI number checking is done by SDCCH.

•   Its is also used for SMS transfer in Ideal Mode.

•   Its also used in location updation.

•   Also handles user Encryption Process.

ii. Slow Associated Control Channel (SACCH):-

• Its a bi- directional channel.

• It handle the SMS transfer in Dedicated / Active Mode and also Measurement Report in

    dedicated mode.

• Also deals with power control.

• Also handle call waiting.

• used to convey transmit power level control instructions and timing advance information

     to the GSM mobile.

• In the uplink, SACCH carries received signal strength, TCH quality information and

    measurement report of neighbouring cells etc. 

• SACCH reports neighbour cell information such as signal level to the serving cell 

    which helps make handover decision.

•  SACCH message may be sent once every 480 ms (i.e. 2 Sec) duration.

iii. Fast Associated Control Channel (FACCH):-

•  Its a bi- directional channel.

•  It handle Handover Process.

•  FACCH information message can easily be transmitted in 20 ms duration. 

 (As compare to SACCH it is too much  Fast).

• It is used to send fast message like hand over message.

iv. Cell Broadcast Channel (CBCH):-

•  Its a unidirectional channel.

•  It is only downlink.

•  Used by the Operators to Broadcast SMS (like flash SMS, Activity Promotion through SMS)