Background to the invention In today's mobile telecommunications networks, the operator of a mobile network is able to maintain tight control over the calls made and services used by its subscribers when they are at home, i. e. when they are registered with the operator's own network.

For example, if a subscriber exceeds his calling credit limit the operator can notify the subscriber of this situation and/or can prevent the subscriber from making further calls until his outstanding account is settled. Similarly, where a subscriber makes use of a top-up account, e. g. using scratch cards to credit his account, an operator can restrict the subscriber's access when the subscriber's credit drops to zero. Such charging control is possible because of the introduction into mobile networks of Intelligent Network (IN) nodes which use the so-called Intelligent Network Application Part (INAP) protocol to communicate with one another. Problems arise however when a mobile subscriber roams outside of his home network and registers with a foreign network. Charging mechanisms currently in use for roaming subscribers are slow and cumbersome, resulting in the barring of roaming services to certain classes of subscriber (e. g. pre-paid subscribers). Furthermore, current charging mechanisms do not lend themselves to flexible charging schemes such as are desired by network operators.

In order to overcome the problems of existing charging mechanisms, a mechanism referred to as Customised Applications for Mobile network Enhanced Logic (CAMEL) has been standardised by ETSI (further enhancements to CAMEL are in the process of being standardised according to CAMEL phase 2 and phase 4). CAMEL provides for the transfer of charging related information in near real time between a Service Control function (SCF) associated with a Cost Control Function (CCF) (the CCF is usually running at a Cost Control Node (CCN) located in a subscriber's home network) and a Service Switching Function (SSF) typically running at, or associated with, an MSC or GMSC (in the case of a GSM network) with which a subscriber to be charged is registered. The SSF may be located in the same network as the SCF or in a different network. A protocol known as the CAMEL Application Part (CAP) protocol has been defined for the purpose of transporting CAMEL messages between a CCF and a SSF.

A feature of CAMEL is that it allows charging elements to be sent from an SCF to an SSF prior to and during a call involving a mobile terminal registered with an MSC/GMSC. A charging element defines a tariff for a call or part of a call or for some other service (e. g. data transfer), and is used by the SCF to calculate a charge. The charging element may be sent to a subscriber's terminal, e. g. according to the Advice Of Charge (AoC) service, by the SSF.

A charging element is typically sent to an SSF in advance of the point in time when that charging element is to be brought into use. This may be during a call set up procedure, with the element being used immediately upon completion of the connection, or may be during a call in which case the charging element sent from the SCF is usually accompanied by a tariff switch interval (tsw) which specifies the time after receipt of the element by the SSF at which the element is to be brought into use (replacing the previously used charging element). Charging elements and tsw intervals are sent from a SCF to a SSF as a result of SendChargingInformation (SCI) operations. An SCI operation causes a timer, having a value tsw, to be started at the SSF. However, whilst upon expiry of the timer the new charging element is brought into use, it does not cause a request to be sent to the SCF asking for a new charging element and tsw interval to be issued. This presents a problem for the AoC service (but not for prepaid services or the AoC service used in conjunction with a prepaid service).

In order to overcome this problem, use is made of the ApplyCharging (ACH) and ApplyChargingReport (ACR) operations which are normally intended for pre-paid services. At the same time as an SCI operation is initiated by a SCF, an ACH operation is also initiated. The prepaid timer in ACH is"misused"as a separate tsw interval timer. Receipt of the ACH tsw interval causes a second timer to be started at the SSF.

This timer will expire at substantially the same time as the SCI timer interval, and its expiry triggers an ACR operation. This causes a request to be sent to the SCF for a new charging element and tsw interval. In practice, no tsw interval may be sent with the SCI operation. Rather, the SSF knows to look for the tsw interval in the ACH operation.

However, the result remains that two separate timers are used.

Summary of the Present Invention.

Perhaps the main problem with the introduction of enhanced versions of CAMEL is the extra load which they place on network nodes. This is due in part to the requirement for two separate timers in each SSF charging operation.

It is an object of the present invention to overcome or at least mitigate the disadvantages of known charging management mechanisms. In particular, it is an object of the present invention to reduce the processing load resulting from an SSF. These and other objects are achieved by including a new parameter in ACH operation which causes a report to be sent from the SSF to the SCF upon expiry of the SCI initiated timer, and including a new parameter in ACR which indicates that the tsw interval has expired. Alternatively, the objects may be achieved by defining a new Detection Point (DP) at the SSF, causing a report to be sent to the SCF upon expiry of the tsw interval.

According to a first aspect of the present invention there is provided a method of controlling tariff switching for a mobile station in a mobile telecommunications network using CAMEL, the method comprising the steps of : carrying out an SCI operation including sending a charging element from a responsible SCF to a responsible SSF; carrying out an ACH operation including sending a timer expiry report request from said SCF to said SSF, wherein as part of the SCI operation or the ACH operation a tsw interval is sent from said SCF to said SSF; upon receipt of the tsw interval at the SSF, starting a timer at the SSF; and upon expiry of said timer, sending said charging element to the mobile station and/or bringing the charging element into use, and sending a timer expiry report from said SSF to said SCF.

As a result of the sending of the timer expiry report request as part of the ACH operation, the sending of a timer expiry report to the SCF is triggered by the expiry of the SCI initiated timer. There is therefore no requirement for a second timer, and the processing load resulting from the SSF is reduced.

Preferably, the tsw interval is sent to the SSF as part of the ACH operation.

Preferably, said charging element is an Advice of Charge (AoC) element. Upon expiry of the tsw timer, the charging element is sent from the SSF to the mobile station, e. g. in a FACILITY message.

Preferably, said timer expiry report is sent as part of an ApplyChargingReport operation which is triggered by the expiry of said timer. The timer expiry report is introduced as a new parameter into the ApplyChargingReport to indicate to the receiving SCF that the ACR is reported because of the expiry of the SCI timer.

The SCI and ACH operations may be carried out prior to a call established to or from the mobile station, or may be carried out during such a call. It will be appreciated that these operations may be repeated several times during a given call.

According to a second aspect of the present invention there is provided apparatus for controlling tariff switching for a mobile station in a mobile telecommunications network using CAMEL, the apparatus comprising: first processing means implementing a SCF; second processing means implementing a SSF; wherein in use said first processing means is arranged to carry out an SCI operation including sending a charging element from the SCF to said SSF, and to carry out an ACH operation including sending a timer expiry report request to said SSF, and wherein as part of the SCI operation or the ACH operation a tsw interval is sent from said SCF to said SSF; and said SSF being arranged to start a timer upon receipt of said tsw interval and, upon expiry of said timer, to send said charging element to the mobile station and/or bring the charging element into use, and send a timer expiry report to said SCF.

Typically, the first processing means is located at a Charge Control Node, whilst the second processing means is located at an MSC or GMSC with which the mobile station is registered.

According to a third aspect of the present invention there is provided a method of controlling tariff switching for a mobile station in a mobile telecommunications network using CAMEL, the method comprising the steps of : carrying out an SCI operation including sending a charging element and a tsw interval from a responsible SCF to a responsible SSF; upon receipt of the tsw interval at the SSF, starting a timer at the SSF, and arming a detection point which is triggered by expiry of the timer; and upon expiry of said timer, sending said charging element to the mobile station and/or bringing the charging element into use, and sending a timer expiry report from said SSF to said SCF as a result of the triggering of said detection point.

Brief Description of the Drawings Figure 1 illustrates a telecommunication system comprising two interconnected PLMNs ; Figure 2 illustrates Advice of Charge related signalling in the system of Figure 1 according to one example; Figure 3 is a flow diagram illustrating an Advice of Charge procedure in the system of Figure 1; and Figure 4 illustrates Advice of Charge related signalling in the system of Figure 1 according to a second example.

Detailed Description of Certain Embodiments There is illustrated in Figure 1 a telecommunications system comprising two interconnected Public Land Mobile Networks (PLMNs). For the purposes of the following illustration, a first of the PLMNs 1 represents the home network of a subscriber using a mobile telephone 2 or wireless data terminal 3, and is referred to as the Home PLMN (HPLMN). The second of the PLMNs 4 is referred to as the Visited PLMN (VPLMN). Only those nodes of the networks 1,4 relevant to the following discussion are illustrated in Figure 1. Both of the networks 1,4 are Global System for Mobile communications (GSM) based networks and support GPRS. Illustrated in the VPLMN 4 are a Mobile Switching Centre (MSC) 5 (which is co-located with a Visitor Location Register (VLR)) and a GPRS Serving Node (GSN) 6 which are responsible for connecting and routing voice and data calls respectively. Both the MSC/VLR 5 and the GSN 6 are coupled in use to the subscriber equipment 2,3 via Base Station Sub-systems (BSS) 7,8. The networks 1,4 are coupled together via Gateway nodes although these are not illustrated in Figure 1.

Illustrated in the HPLMN 1 is a Home Location Register (HLR) 9. The HLR 9 maintains a record of all of the subscribers of the HPLMN 1, including the International Mobile Subscriber Identity (IMSI) for each subscriber and which is used by a subscriber to register with a network. A Service Control Function (SCF) is implemented at a Cost Control Node (CCN) 10 which, together with a HLR 9, forms part of a Home Subscriber System. For each subscriber, the SCF 10 records details of the tariff to which the subscriber subscribes. For example, a subscriber may subscribe to a "business tariff'which defines the cost of voice calls in terms of caller location, called party destination, duration, and time of day. The cost of data calls may also be defined using these same conditions, and additionally the volume of data sent to and/or received by the subscriber during a data call and the Quality of Service (QoS) involved.

Associated with each MSC 5 is a Visitor Location Register (VLR) which maintains a record of subscribers currently registered with the associated MSC 5. The record includes subscribers for whom the MSC 5 is a home MSC, as well as subscribers for whom the MSC 5 is a foreign network. In the event that the subscriber using the mobile telephone 2 roams outside of the coverage area of his HPLMN 1 and into the coverage area of the VPLMN 4, his telephone 2 will seek to register with an MSC 5 of the VPLMN 4 after the MSC 5 has determined that the subscriber is a subscriber of a foreign network and has not already been registered in the associated VLR. A similar arrangement and procedure apply to the subscriber equipment 3 and the GSN 6.

For the purpose of the following discussion, it is assumed that the subscriber 2 has access to an Advice of Charge (AoC) service whereby the subscriber's terminal is notified prior to and during a call of the call tariff in use at any given time. The subscriber's terminal may use the received tariff information to compute and display real time charge information. In the event that a subscriber initiates a call, e. g. a data call to a subscriber of the same or a different network, a Service Switching Function (SSF) implemented by the SGSN 6 contacts the SCF (of the CCN 10) of the HPLMN 1 during a call setup phase. The correct SCF is identified to the SSF in an INSERT SUBSCRIBER DATA message previously received from the HLR 9 during registration. Signalling between the SSF and the SCF is carried out according to the CAMEL Application Part (CAP) protocol. Figure 2 illustrates operations and signalling during and following the setup phase.

The SCF first identifies an initial AoC element which will determine the call tariff immediately following call setup. The SCF also identifies the subsequent AoC element to be applied to the call and a timer switch interval (tsw) defining the time after call set up when the subsequent AoC element is to be brought into use. The SCF then initiates an ApplyCharging (ACH) operation which causes the tsw interval to be sent from the SCF to the SSF. This operation also causes a requestReportTsw parameter (i. e. flag) to be sent to the SSF. This parameter is proposed as a new CAMEL parameter and its purpose will be described below. The SCF then initiate a SendChargingInformation (SCI) operation which causes the initial and a subsequent AoC element to be sent to the SSF.

In the event that the subscriber initiated call is answered, the SSF causes the initial AoC element to be sent to the subscriber's terminal in an ANSWER message. The tsw timer is also started by the SSF. Upon expiry of the timer, the SSF causes a further FACILITY message containing the subsequent AoC element to be sent to the mobile terminal. As a result of the ACH operation carried out by the SCF, expiry of the tsw timer also causes an ApplyChargingReport (ACR) operation to be initiated by the SSF.

The requestReportTsw parameter previously received from the SCF causes a parameter reportTsw to be sent to the SCF from the SSF in the ACR operation. Inclusion of this parameter notifies the SCF that the ACR operation has resulted from expiry of the tsw timer. The SCF initiates further ACH and SCI operations, with the latter causing a new AoC element to be sent to the SSF and the former causing a new tsw interval and a requestReportTsw parameter to be sent to the SSF.

The process described above is repeated, with a new timer being started and the new AoC element being sent to the mobile terminal upon expiry of the timer. Several new AoC elements and respective tsw intervals may be sent during a given call, until such time as the call is disconnected.

Figure 3 is a flow diagram illustrating the method of managing charging in a mobile network as described above.

An alternative to using a requestReportTsw parameter to trigger the sending of a reportTsw and hence to notify the SCF of the expiry of the tsw timer, is to introduce a new detection point at the SSF. This detection point is referred to here as "DPTswexpired". The DPTswexpired is armed by a RequestReportBCSM (RRB) operation by the SCF and reported to the SCF with an EventReportBCSM (ERB) operation. Expiry of the tsw timer at the SSF will thus be notified to the SCF, which will then know to send the new AoC element and tsw interval to the SSF. The signalling associated with this alternative embodiment of the invention is illustrated in Figure 4.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the invention.