Called mobile subscriber present status

Details Called mobile subscriber present status Called mobile subscriber present status

1998-10-14

2001-05-22

Banks-Harold, Marsha (Department: 2745)

Telecommunications

Radiotelephone system

Special service

C455S432300, C455S421000

Reexamination Certificate

active

06236853

ABSTRACT:

This application claims priority under 35 U.S.C. §§119 and/or 365 to 9703759-2 filed in Sweden on Oct. 15, 1997; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates generally to the problem of providing information about a called subscriber's status in a mobile communications system, and more particularly to the problem that occurs when a caller is unable to determine whether the subscriber being called is either out of the service area or has merely switched off his mobile phone.
RELATED ART
In
FIG. 1
is shown a mobile communications system such as GSM (“Global System for Mobile communications”), AMPS or PDC. The background discussion here will focus on a system which corresponds to GSM. However, it can be appreciated by a person skilled in the art that there are many similarities between mobile systems and many of the problems in one system also exist in others.
A typical system, as in GSM, has a particular geographic area covered by each system operator. This geographic area is divided into cells, drawn as hexagons for ease of design and cell planning. These hexagons symbolize each “cell”. Each cell will also have an antenna for transmitting and receiving radio signals from the various mobile stations (“MS”) within their coverage area. The mobile station is the physical equipment used by a subscriber to gain access to the Public Land Mobile Network (“PLMN”), which is a mobile network for a given operator. The PLMN is established and operated by an administration or its licensed operators for the specific purpose of providing land mobile communication services to the public.
Every telephone network needs a certain structure in order to route incoming calls to the correct exchange and finally to the called subscriber. In a mobile network this structure is of even greater importance because of the mobility of its subscribers. The links between a GSM/PLMN network and other PSTN (“Public Switched Telephone Network”) ISDN (“Integrated Services Digital Network”) or PLMN networks will be on the level of international or national transit exchanges. All incoming calls for a PLMN network will be routed to one or more Gateway Mobile Switching Centers (“Gateway MSC” or “GMSC”). A Gateway MSC works as an incoming transit exchange for the PLMN. It is the place which has the interrogation call routing function for mobile terminated calls. It enables the system to route calls to their final destination: the called mobile stations. In a GSM/PLMN network, all mobile terminated calls will be routed to a Gateway MSC.
A Mobile Switching Center (“MSC”) area represents the part of the network that is covered by one MSC. In order to route a call to a mobile subscriber, the path through the network links to the MSC in the MSC area where the subscriber is currently located.
A Service Area (“SA”) is the part of the network that is defined as an area in which a mobile station is obtainable, due to the fact that the MS is registered in a Visitor Location Register (“VLR”). The VLR is a database commonly located in the MSC (and referred to as MSC/VLR) temporarily containing information about all the MS's currently located in the MSC area. As soon as an MS roams into a new MSC area, the VLR connected to that MSC will request data about the MS from the Home Location Register (“HLR”). The VLR contains both subscriber information (received from the HLR) and information relating to the Location Area in which the MS is currently situated.
The HLR is also a database. When someone buys a subscription from an operator, he will be registered in the HLR of that operator. The HLR contains subscriber information, such as supplementary services and authentication parameters. Furthermore, there will be information about the location of the MS, i.e. in which MSC area the MS resides presently. This information changes as the MS moves around. The MS will send location information (via the MSC/VLR) to its HLR, thus providing means to receive a call.
In many systems the PLMN will be divided into one or several MSC/VLR Service Areas (it is often the case that the MSC Area and the Service Area cover exactly the same part of the network, while the MSC and VLR are implemented in the same node). Each MSC/VLR Service Area is divided into several Location Areas (“LA”). An LA is a part of the MSC/VLR Service Area in which a mobile station may move freely without updating location information to the MSC/VLR exchange that controls the LA. It is also the area where a paging message is broadcast in order to find the called mobile subscribe. An LA can have several cells and depend on one or more Base Station Controllers (“BSC”), but it belongs to only one MSC/VLR. It can also be identified by the system by using a Location Area Identity (“LAI”).
The Location Area is divided into a number of Cells. The cell is an area of radio coverage that the network identifies with the Cell Global Identity (“CGI”). The MS distinguishes between cells using the same carrier frequencies by use of the Base Station Identity Code (“BSIC”).
It is often the case that a Subscriber A, calling from e.g. the PSTN, wishes to call a Subscriber B in a particular PLMN (e.g. GSM). The connection is set up from Subscriber A's local exchange to the Gateway MSC of the GSM/PLMN network. The GMSC analyzes the number for Subscriber B. Using an interrogation function, the GMSC sends the Mobile Station ISDN (“MSISDN”) number, with a request for a Mobile Station Roaming Number (“MSRN”), to the Home Location Register. The MSISDN is a number which uniquely identifies a mobile telephone subscription in the PSTN numbering plan. The HLR knows what Service Area Subscriber B is located in and translates the dialed mobile subscriber number (e.g. MSISDN) into a GSM/PLMN subscriber identity (e.g. IMSI): i.e. MSISDN→IMSI.
In order to provide a temporary number for routing, the HLR sends the IMSI of Subscriber B to the current MSC/VLR and requests it to temporarily allocate a MSRN to Subscriber B and return it. At reception of the MSRN, the HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered. The GSMC, now in possession of the correct MSRN, can now set up the incoming call to the Service Area where subscribe B is currently located.
The VLR points out the Location Area Identity (“LAI”) for the called subscriber: i.e. IMSI→LAI. The MSC/VLR then sends a paging message to all transmitters within the Location Area. The Mobile Station may be idle and listening to the paging channel of one of the cells belong to the Location Area. It will receive the paging message, recognize the IMSI and send a response. After the call set-up, procedures are performed and the assignment of a traffic channel is done, the call is connected over the air path to the Mobile Station.
It is, however, possible that the MS is turned off. It is considered “detached”. It cannot be reached by the network, because it doesn't answer a paging message. It also doesn't inform the system about possible changes of Location Area. Similarly, when the MS is turned on, it is called “attached”. Both attach and detach are related to the International Mobile Subscriber Identity (“IMSI”). For a correct identification over the radio path and through the GSM/PLMN network, a specific identity is allocated to each subscriber. This identity is the IMSI and is used for all signalling within the PLMN. It is stored in the Subscriber Identity Module (“SIM”, as smart card located in the MS), as well as in the HLR, system registration, and in the VLR, temporary registration. The IMSI consists of 3 parts, a Mobile Country Code, a Mobile Network Code, a Mobile Station Identification Number, and has a maximum length of 15 digits.
As mentioned above, a mobile station in an active state is marked “attached” by a flag on the IMSI. When the MS powers off, the MS sends a last message to the network, containing a request for the detach procedure. On reception of a “detached” mes

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