METHOD AND ARRANGEMENT FOR LIMITING PAGING LOAD IN A MOBILE COMMUNICATION SYSTEM

Abstract: The invention relates to a method and an arrangement for limiting the paging load in a mobile communication system. The method is charaterized in that the number of calls initiated during a given time interval is counted and the number of calls to be initiated is limited to a predetermined maximum value. The mobile communication system adapted to the implementation of the method comprises counting means for counting the number of calls to be initiated during a given time interval, timing means for measuring the desired time interval, and comparison means for limiting the number of calls to be initiated to a predefined maximum value.

Description: METHOD AND ARRANGEMENT FOR LIMITING PAGING LOAD IN A MOBILE COMMUNICATION SYSTEM FIELD OF THE INVENTION The invention relates to a method and an arrangement for limiting the paging load in a mobile communication system when mobile stations are being paged. BACKGROUND OF THE INVENTION Location information on a mobile station is needed in mobile com¬ munication systems for routing of calls and for other services. Figure 1 in the attached drawings shows a simplified example of the structure of a GSM mo¬ bile communication system as a block diagram. The units shown in the Figure are also found in other mobile networks, but may have different names. A mo¬ bile station MS in standby or active mode is in continuous connection with a Base Transceiver Station BTS. A Base station subsystem BSS comprises a Base Station Controller BSC and its subordinate base stations BTS. Usually a plurality of base station controllers BSC are subordinate to a Mobile Services Switching Centre MSC. The Mobile Services Switching Centre MSC switches messages to other mobile services switching centres. Location information on a mobile station MS and other subscriber data are in permanent storage in a Home Location Register HLR of the system and temporarily in the Visitor Lo¬ cation Register VLR in whose area the mobile station MS is located at each particular moment. The visitor location register VLR comprises subscriber data required for all mobile stations MS in the area of the VLR. The geographical area supervised by a visitor location register is di- vided into one or more Location Areas LA1 to LA3, within each of which a MS can move freely without notifying the VLR. One or several base stations BTS may operate in each location area. Base stations BTS continuously transmit information on themselves and their environment, such as a Base Station Identity Code BSIC and a Location Area Identifier LAI. On the basis of the lat- ter, a mobile station MS locked to a base station BTS knows in which particu¬ lar location area LA it is located. When the base station BTS is being changed and the mobile station MS detects that the location area identifier LAI of the base station has changed, the mobile station sends a location update request to the network. The location area of the mobile station MS is updated in the visitor location register VLR within whose area the mobile station is located at each particular moment. Information on the VLR within whose area the MS is located is transmitted to the home location register HLR. A visitor location register VLR may lose the subscriber data due to VLR failure, software update, or restart following equipment maintenance. To recover from this, the visitor location register VLR has to request update of necessary subscriber data from the home location register HLR. Accurate lo¬ cation information can, however, be received from mobile stations only. In this case a mobile station MS has to be paged in all location areas LS of the visitor location register VLR involved. If a visitor location register serves a large num- ber of subscribers, paging a mobile stations causes significant overload mainly in three sectors: 1. between the mobile services switching centre MSC and the base station subsystem BSS, 2. in the base station subsystem BSS signalling process, and 3. on radio channels. Location information is not updated until the data are needed, i.e. when service has to be offered to a mobile sub- scriber, e.g. in case of an incoming call, or when the mobile station establishes connection in order to make a call or to update its location. In practice the lo¬ cation update often occurs simultaneously for each MS. Prior art systems can¬ not prevent overload in a mobile system in this situation. Normal network op¬ eration can also cause overload in a mobile system if an exceptionally large number of subscribers is located in a particular location area LA, and they load the base station subsystem BSS with calls and location updates. BRIEF DESCRIPTION OF THE INVENTION It is an object of this invention to prevent mobile system overload while storing and/or updating location information relating to mobile stations in a visitor location register. It is a further object of the invention to prevent mobile system over¬ load when a large number of subscribers is located in a particular location area. This new kind of call load limiting is achieved with the method of the invention, which is characterized in that the number of calls initiated during a given time interval is counted and the number of calls to be initiated is limited to a predetermined value. The invention also relates to an arrangement of limiting call load in a mobile system. The arrangement is characterized in that the mobile system comprises counting means for counting the number of calls to be initiated during a given time interval, means for measuring the desired time interval, and comparison means for limiting the number of calls to be initiated to a pre¬ determined maximum value. The invention is based on the idea that initiated during a given time interval the number of calls is limited to a number proportional to the perform- ance of the mobile system. In the method of the invention the number of calls initiated in order to call mobile stations is counted. Time measurement is started when a call is initiated. After a predetermined time from initiation of the call it can be re¬ moved from the number of initiated calls. New calls are initiated on request until the number of calls counted is equivalent to a maximum allowed number of calls to be initiated during the interval. The situation corresponds to a maxi¬ mum call load that must not be exceeded. New calls cannot be initiated until a predetermined time has passed from the start of a call that was initiated ear¬ lier, and that call has been removed from the number of calls counted. The advantage of such a paging load limitation is that the radio units in a mobile system do not become overloaded and thus other system operations are not jeopardised. It is a further advantage of the arrangement of the invention that changes in an existing system can be implemented with relatively small changes, e.g. in the software of the mobile services switching centre MSC. The changes are compatible with existing systems in all respects. Thus cen¬ tres in which the change of the invention has been implemented, and centres where this change has not been implemented, can flexibly be used simultane¬ ously in the network. DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail with reference to the accompanying drawings, in which Figure 1 shows the essential parts of a mobile system applicable to the invention, Figure 2 is a block diagram of a mobile services switching centre implementing a preferred embodiment of call load limiting conforming with the invention, and Figure 3 shows a preferred embodiment of the method of the inven¬ tion as a flow diagram. DETAILED DESCRIPTION OF THE INVENTION The present invention can be applied in connection with any mobile system. The invention is described below in greater detail by means of an ex¬ ample in connection with the Pan-European digital mobile communication system GSM. Figure 1 shows the previously described simplified structure of a GSM network. As to a more precise description of the GSM system, reference in made to the GSM recommendations and the publication "The GSM System for Mobile Communications", M. Mouly & M. Pautet, Palaiseau, France, 1992, ISBN:2-9507190-0-7. The invention is described below with reference to call limiting in a single mobile services switching centre. It is advantageous to limit the calls of a mobile services switching centre MSC especially when updating the mobile station MS location information stored in the visitor location register VLR by inquiries directed to the mobile stations. The method of the invention can also be applied to limiting calls that pass via a given base station subsystem BSS. In the method of the invention the number of calls to be initiated is counted. The number of calls N max to be initiated during a predetermined time interval T int under study, measured by means of a call-specific timer, is limited (N max /Ti rvt ). The number N max of allowed calls is preferably set to a smaller value than the system call load processing capacity, also allowing for other signalling or a decrease in system performance. The time interval T, nt under study and the number of calls N^,, to be initiated can be set by an operator and can be set e.g. separately for each mobile services switching centre MSC. The proc¬ essing capacity is determined by the weakest unit of the network as regards call load, e.g. the base station subsystem BSS, the calling channel of a single base station BTS, or exceptionally, the mobile services switching centre MSC. The method of the invention is implemented e.g. by means of counting, timing, and comparison means arranged in connection with a mobile services switching centre MSC. The counting means can be e.g. counters. E.g. timers can be used as timing means, and their number should be ade¬ quate for timing of each call to be initiated. The current values of the counting means, e.g. counters, are compared by means of the comparison means with a preset counter threshold value. The primary embodiment of the invention is described in the follow- ing with reference to Figure 2. Figure 2 shows a block diagram of a mobile services switching centre MSC conforming with the invention, showing only the components of a mobile services switching centre relevant to the invention. The mobile services switching centre of Figure 2 comprises a digital switch 81 , via which the mobile services switching centre MSC is linked to base station subsystems BSS, other mobile services switching centres MSC, a public tele- phone network PSTN, a visitor location register VLR etc. Switch 81 connects calls to a mobile station MS. The operation of the mobile services switching centre MSC is controlled by a call control computer 82, which also controls the operation of a switch SW. A register 84 contains preset parameters: the time interval T int to be studied and the number of calls N max to be initiated. A counter 85 counts the number of calls initiated in the mobile system MSC concerned. In addition there is a number of call-specific call timers 86^86^, for measuring the time lapsed from the initiation of each call. The call control computer 82 uses these timers, the register, and the counter in accordance with the inven¬ tion for call load limiting, as will be explained below. Figure 3 shows a primary embodiment of the method of the inven¬ tion as a flow diagram with regard to one mobile services switching centre MSC. When the method is implemented or when the variables are redefined, the call counter counting proportionally to the number of calls to be initiated is initialised to the value N^ (step 21 in Figure 3). The initial value of the call counter together with the threshold value of the call counter limits the number of calls initiated during the time interval under study. When the mobile services switching centre MSC has to send a call to a certain mobile station MS (step 22), a check is made to see if the value of the call counter has reached the preset threshold value, in this case 0 (step 23). When the value of the call counter exceeds 0, a predetermined call step, in this case 1 , is deducted from the value of the call counter (step 24). The time interval chosen to be studied is measured with a call- specific call timer which has not been in use, i.e. a new call counter is chosen for each call to be initiated. In step 25 the call counter is initialised using the value T jnl of the time interval under study and in step 26 the call and the corre¬ sponding call timer are initiated. Time measurement of call timers in operation is monitored (step 28). When the time under study to be measured is termi¬ nated in a call counter, a call step, in this case 1 , is added to the value of the call counter (step 29). Once the call counter reaches a preset threshold value, 0 in Figure 3, the party requesting the call is notified that the call cannot be initiated (step 27). Using the GSM system as an example, this notification can be effected by a "No Paging Response" message. New calls cannot be initiated until the time interval under study is terminated in one of the call timers and the value of the call counter has been increased so as to exceed the threshold value. The operation of the embodiment of Figure 3 can be illustrated by the following example. The numerical values used in the example have been chosen merely to clarify the idea of the invention. If the initial value N max of the call counter is initialised e.g. to the value 50 and the time interval T ιnt under study is set to 1 s, 50 calls per second are allowed to be initiated in the mobile services switching centre. Each initiated call decreases the call counter value by one. During one second after the first call request a total of 50 calls can be initiated. Should a 51 st call request be made during this second, initiation of a new call is prevented and termination of the time interval under study of the first call is awaited. In other embodiments of the method the call counter can also count in the opposite direction as compared with the above downward counting counter. The initial set value of the call counter and the threshold value have to be set properly, e.g. by exchanging the numerical values of the initial set value and the threshold value of the embodiment of Figure 3. When an upward counting counter is used, the counter value is increased by a call step before the call is initiated and the call counter is decreased by a call step when the call timer reaches a predetermined timer threshold value. The call step value must be chosen so that the call counter threshold value set will be reached by this call step with sufficient call load. Time measurement of call timers can be implemented by upward or downward counting. In each case, monitoring the termination of the call timer counting must be arranged appropriately. In the embodiment of Figure 3, downward counting call timers are used. As each initiated call initiates is own call timer, a sufficient number of call timers must be available for measuring the time interval under study of the allowed calls. Another embodiment of the method of the invention is especially suited to limiting calls in a location area of a mobile system in normal mode. In this embodiment the number of calls to be initiated is counted separately for each location area. The time interval under study T ιnt and the number of calls N.^,, to be initiated can be set separately for e.g. each base station controller BSC in accordance with their performance. The arrangement of limiting the call load in a location area can be located in a mobile services switching centre MSC or some other part of the network, e.g. in connection with the base station subsystem BSS. In the ar¬ rangement of this embodiment each location area has its own counter. A com- parator can be arranged for each counter. Each location area has its own reg¬ ister comprising parameters preset by an operator. The method of the invention is in continuous operating mode in a mobile system. When mobile station MS location information stored in a visitor location register VLR is being updated, the paging load rapidly decreases after restart of the visitor location register VLR and thus the method does not es¬ sentially limit normal network operation. The drawings and the description are intended to illustrate the in¬ ventive idea. As to the details, the method and arrangement of call load limit¬ ing of the invention may vary within the scope of the appended claims.

Claims: CLAIMS 1. A method for limiting the paging load in a mobile communication system, characterized in that in the method the number of calls initiated during a given time interval is counted, and the number of calls to be initiated is limited to a predetermined maximum value. 2. A method as claimed in claim 1, characterized in that said limitation of the number of calls to be initiated comprises prevention of initiation of new calls when said number of counted calls equals said prede¬ termined maximum value. 3. A method as claimed in claim 1, characterized in that said number of counted calls to be initiated is increased/decreased when a new call is initiated, the time lapsed from the initiation ofthe call is measured, and said number of counted calls to be initiated is decreased/increased, respectively, when said given time interval has lapsed from the initiation of the call. 4. A method as claimed in claim 3, characterized in that a call counter counting the number of initiated calls is set to a predetermined value in connection with implementation, and when initiating a call, the follow¬ ing operational steps are taken: the value of the call counter is compared with a predetermined counter threshold value, when the value of the call counter is equal to the counter threshold value, call initiation is prevented and a message concerning the prevention of the operation is created to the subscriber, when the call counter value differs from the counter threshold value, the call counter value is decreased/increased by a predetermined call step, a call-specific call counter measuring said given time interval is set to a prede¬ termined value and the call timer and mobile station (MS) paging is initiated, and the call counter value is increased/decreased, respectively, by a call step when the call-specific call timer reaches a predetermined timer threshold value. 5. A method as claimed in claim 1, 2 or 4, characterized in that said predetermined maximum value limiting the number of calls to be initi¬ ated is smaller than the maximum number of calls which the mobile communi¬ cation system is able to handle simultaneously. 6. A method as claimed in any one of the preceding claims, characterized in that the method is used for limiting the call load in a location area (LA). 7. A method as claimed in any one of claims 1 to 5, charac¬ terized in that the method is used for limiting the call load of a mobile services switching centre (MSC) when mobile station (MS) location data stored in a visitor location register (VLR) are updated by paging the mobile station (MS) and by inquiring the location area (LA) ofthe mobile station (MS). 8. An arrangement for limiting the call load in a mobile communica¬ tion system, characterized in that the mobile system comprises counting means for counting the number of calls to be initiated dur- ing a given time interval, timing means for measuring the desired time interval, and comparison means for limiting the number of calls to be initiated to a predefined maximum value. 9. An arrangement as claimed in claim 8, characterized in that the counting means, the timing means, and the comparison means are located in connection with the mobile services switching centre MSC.

Inventors: LAHTINEN LAURI, LAHTINEN, LAURI
Applicants: LAHTINEN LAURI, NOKIA TELECOMMUNICATIONS OY, LAHTINEN, LAURI
Application year: 1997
Main classification: H04Q7
IPC codes: H04M3, H04W68