GSM Mobility Management

Mobility Management is one of the features of the GSM network that allows the mobile phones to make or receive calls while moving one place to another.

Mobile device uses the location update procedure to update the current location information to the cellular network. Mainly there are three different reasons for a mobile station to do a location update.

• Normal location update
• IMSI attach
• Periodic registration

Whenever a mobile station moves from one location to another or comes to an active state (IMSI attach), the mobile station will check the Location Aread Identity (LAI) stored in its SIM against the LAI of the cell. When both LAIs are different, mobile station will initiate a location update process. The network also has a provision for periodic registration to avoid the unnecessary paging of the mobile if MSC never receives the IMSI detach message.

Normal location update

Mobile station will initiate a normal loaction update procedure as when it finds a change of location area. As soon as a mobile station changes the location ares, it will compare the LAI stored in the SIM with the current LAI. If the two differs, then mobile station has to update the new location information to ensure that the mobile station will be paged to corret location.

The location update procedure will start with the mobile station sending the new location information to the network. After the authentication, the MSC will update its VLR. If the mobile station is new to this MSC, then MSC will interact with the HLR to get all the necessary information about the subscriber and then updates its VLR.

IMSI attach

When a mobile station switched on, ie, moved from an idle state to active state, it will try to attach to the network. This process of attachment is called IMSI attachment. During the IMSI atatch procedure, if the MSC finds that the mobile station is new, then MSC will query the corresponding HLR and then it will update the VLR with status “IMSI Attached”. During the process, the HLR will update its own data base and sends a message to the old MSC where the mobile station was located before to update its VLR.

Similarly, when a mobile station is switched off or sim card is removed, the mobile station will sends a message to the VLR and the VLR will mark the mobile station status as detached. Once the status is updated in the VLR, the incoming call to the mobile station will not be paged.

Periodic Registration

It is not necessary that the MSC will always gets the IMSI detach request from the mobile station whenever the later is switched off. So there will be a possibility of unnecessay paging of the mobile whenever there is an incoming call. The periodic registration is provisioned to avoid such unnecessay paging.

After each registration, the mobile station and MSC will reset their timers and when the timer in mobile station expires, the mobile station will perform a location update. If the mobile station does not perform a location update within the time, then the MSC will update the VLR and HLR with status a “detached”.

Standardization of CdPA

Different formats
Format	Numbers
National format	0 9443 124365
International format	91 9443 124365
Unknown format	0091 9443 124365 (Longest unknown format, a complete phone number up to 20 digits)

BCSM Detection Points 50 and 51

Till Camel Phase 4, the O_Not_Reachable and T_Not_Reachable conditons were only able to armed using O_Busy and T_Busy events. In this case, in order to differentiate the Not Rechable condition from the Busy conditions we have to relay on the Release Causes in the ISUP Messages. When ISUP Release Cause 17 identifies the Busy condition, 20 will be used to identify the Not Rechable / Subscriber Not Present condition.

In Camel Phase 4, the Not Rechable conditions can be armed using detection points 50 for originationg and 51 for terminating calls.

O-BCSM Detection Points:

1	Origination_Attempt_Authorized
2	Collected_Info
3	Analyzed_Info
4	Route_Select_Failure
5	O_Called_Party_Busy
6	O_No_Answer
7	O_Answer
8	O_Mid_Call
9	O_Disconnect
10	O_Abandon
50	O_Not_Rechable

T-BCSM Detection Points:

12	Termination_Attempt_Authorized
13	T_Called_Party_Busy
14	T_No_Answer
15	T_Answer
16	T_Mid_Call
17	T_Disconnect
18	T_Abandon
51	T_Not_Rechable

ISUP Release Causes:

Rel Cause Value	ITU-T Description
1	Unallocated (unassigned) number
3	No route to destination
5	Misdialled trunk prefix (national use)
16	Normal call clearing
17	User busy
18	No user responding
19	No answer from user (user alerted)
20	Subscriber absent
21	Call rejected
22	Number changed
27	Destination out of order
28	Invalid number format (address incomplete)
31	Normal, unspecified
34	No circuit/channel available
38	Network out of order
41	Temporary failure
42	Switching equipment congestion
44	Requested circuit/channel not available
47	Resource unavailable, unspecified
50	Requested facility not subscribed
57	Bearer capability not authorized
63	Service or option not available, unspecified
65	Bearer capability not implemented
69	Requested facility not implemented
88	Incompatible destination
95	Invalid message, unspecified
102	Recovery on timer expiry
111	Protocol error, unspecified
127	Interworking, unspecified

Cloud Phone

A new mobile phone solution which allows the user to access the data using cloud technology without changing the handset or device. Since all the datas are stored in one place over the internet, the chances of losing the same is very less.

An UK based company Movirtu has developed an email like concept for Cloud Phone. In this concept, the user has to buy a card from the local service providers and log in to their accounts with the user id and password from any mobile phone where they can make and receive calls, sms, mms, and any other service provides by the service providers. This concept involved two parties, the mobile phone user and the lender. The lender will get the credits for the usage.

Since the user can not log in all the times, the user can not receive the calls all the time. They can inform the log in time in advance to the other parties for receiving the calls. And the user can share their user id and password to their friends or relatives so that they can check the users accounts when ever they have access to some mobile phones.

This concept may not be interesting for the developed or developing countries where normally peoples has more than one hand set. But Movirtu is claiming that their concept will help the users to track all of their calls and messages in one place.

Global USSD

USSD (Unstructured Supplementary Service Data) service is one of the easiest value added service in the GSM world which provides a fast and real time transmission of data when compared to SMS.

Global USSD is a new technology based on traditional USSD messaging for delivering the messages to different operators across the world. This platform also provides a mechanism for creating the mobile services and deploying without integrating to any operators infrastructure.

Global USSD platform serve as an interface between the network operators and the service providers. The Service Delivery Platform will allows to create and deploy new USSD service.

Global Title Translation

As I mentioned in the earlier post, Addressing and Information Routing in SS7 Global Title Translation is used when the message has to deliver to a different network and the point code details of the destination node is not available. The Global Title Translation method reduces the burden of keeping the point code and sub system number of the actual destination. In a larger network, there will be many intermediate GTT to route the information to the next possible node and the final GTT will provide the point code and subs system number of the final destination. 

Addressing and Information Routing in SS7

In SS7, the SCCP layer routes the messages between different network elements using Destination Point Code (DPC) plus the Sub System Number (SSN) or Global Title (GT) plus SSN (Optional) combination. Normally, the application layer expects the SCCP to route the message and also specifies to the lower layer whether to route the message on the DPC or the GT.

SCCP uses the first combination when the application layer is aware of the destination point code and the SSN located at that point code. The second combination is used when the destination entity can be identified by the combination of elements within the GT. Normally the second combination is used when the message has to deliver to a different network and the point code of the end node is not available. In such scenarios, the network determines the destination point code and the SSN by performing a GT Translation.

As per ITU-T SCCP recommendation Q.713, SCCP should support the following elements when GT is used for addressing:

·         Address Indicator

·         GT Indicator

·         Translation Type

·         Numbering Plan

·         Encoding Scheme

·         GT Address Information