WO2008014045A2

WO2008014045A2 - An internet protocol multimedia subsystem network element and method of operation therefor

Info

Publication number: WO2008014045A2
Application number: PCT/US2007/069338
Authority: WO
Inventors: Ashley L. Border
Original assignee: Motorola, Inc.
Priority date: 2006-07-27
Filing date: 2007-05-21
Publication date: 2008-01-31
Also published as: EP2050226A4; CN101496352B; GB2440381A; CN101496352A; GB0615026D0; WO2008014045A3; GB2440381B; EP2050226A2

Abstract

An Internet Protocol Multimedia Subsystem, IMS, (109) comprises a network element (115), such as Session Border Controller, which implements a Back to Back User Agent (B2BUA). A remote station (101) implements a User Agent Client (UAC) (111) which can transmit Session Interface Protocol (SIP) session negotiation requests intended for a User Agent Server (UAS) (113). The B2BUA receives the SIP session negotiation requests and evaluates a negotiation frequency criterion. Only the SIP session negotiation requests for which the negotiation frequency criterion is met are forwarded to the UAS (113).

Description

AN INTERNET PROTOCOL MULTIMEDIA SUBSYSTEM NETWORK ELEMENT AND METHOD OF OPERATION THEREFOR

Field of the invention

The invention relates to an Internet Protocol Multimedia Subsystem (IMS) network element and method of operation therefor and in particular, but not exclusively, to an IMS of a cellular mobile communication system.

Background of the Invention

The Internet Protocol (IP) Multimedia Subsystem (IMS) provides a standards-based network architecture enabling multi-media communications for fixed and mobile telecommunications services.

The aim of IMS is not only to provide new services but to provide all the services, current and future, that the Internet provides. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals, IMS uses open standard IP protocols, defined by the Internet

Engineering Task Force (IETF) . So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols. This is why IMS truly merges the Internet with the cellular world; it uses cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services .

Typically in a data communication network the act of communication between end users (or end-user devices) is composed of two parts, namely signalling and data. The IMS system is no different in this respect. Signalling mechanisms allow users to locate and initiate communication with other users or applications. Once end- to-end communication is established, the data part (i.e. media) flows across the network.

Like any other network or computer system, an IMS has a finite set of resources (processing power, persistent storage and network bandwidth) which it can draw on to conduct its work. Every user and application within the system is a drain on those resources. It is the task of the network operator to provision and manage these resources in such a fashion that the end user receives the level of service they have paid for. In this instance it is the actual transmission of data (and the resultant experience) which the user is actually buying. The signalling part is a necessary overhead; whilst essential it is not revenue generating in itself. Therefore any mechanism able to provide an equivalent level of service to the user but with a reduced signalling overhead will be more cost effective for the network operator.

One of the principle signalling mechanisms used with the IMS Network is the Session Initiation Protocol (SIP) which is defined in IETF standard RFC 3261. As its name suggests, SIP is used in the first instance to initiate sessions (e.g. calls) between users. It provides the information necessary to enable a user (or device) to announce their presence on the network and to locate and request sessions with other users (or devices) . Encapsulated within the SIP protocol is the Session Description Protocol (SDP) . Using SDP, devices are able to negotiate with the network (and target device) to gain access to media bandwidth which can provide suitable Quality of Service (QoS) . Dependent on the bandwidth and processing requirements, the end user devices will employ the use of a range of audio and video encoder/decoders (codecs) to process their media stream.

Typically, when users are within a fixed IP Connectivity Access Network (IP-CAN), a session will remain at the same bandwidth and class of codec as originally negotiated during the initial session setup. However, in a mobile IP-CAN where one or more parties in the call are moving, they will be subject to variable radio propagation conditions which may affect the quality of the call. The variable conditions demand repeated decisions to upgrade or downgrade codec class in a continual trade-off between call quality, bandwidth use, processing power and battery life. Each decision to change codec will result in an end-to-end SIP (and SDP) message exchange through the IMS network. This continual exchange will impact many devices in the IMS network and significantly increase the signaling overhead.

Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, reduced signalling, reduced overhead, reduced resource consumption, reduced complexity and/or improved performance would be advantageous. Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a network element for an Internet Protocol

Multimedia Subsystem, IMS, the network element comprising a Back to Back User Agent, B2BUA, the B2BUA being arranged to: receive Session Interface Protocol, SIP, session negotiation requests from a User Agent Client, UAC, of a remote station supported over a radio air interface; evaluate a negotiation frequency criterion for the SIP session negotiation requests; and forward only the SIP session negotiation requests for which the negotiation frequency criterion is met to a remote User Agent Server, UAS.

The invention may allow an improved IMS. In particular, signalling overhead may be reduced for a communication from the remote station to a device implementing the UAS. In particular, a filtering process may be provided which allows only a subset of the SIP session negotiation requests to be forwarded to the AUS. The approach may allow reduced signalling overhead and reduced resource consumption while compensating for varying propagation conditions.

The SIP session negotiation requests may relate to a request to change a setting or parameter of a SIP session supporting the communication from the remote station. Specifically, the SIP session negotiation requests may be requests for a change to a SIP session setting which is more or less reliable for communications over the air interface. For example, the SIP session negotiation requests may relate to a change of a source encoding scheme, such as a change to a different voice or video codec .

According to another aspect of the invention, there is provided an Internet Protocol Multimedia Subsystem, IMS, comprising a network element including a Back to Back User Agent, B2BUA, the B2BUA being arranged to: receive Session Interface Protocol, SIP, session negotiation requests from a User Agent Client, UAC, of a remote station supported over a radio air interface; evaluate a negotiation frequency criterion for the SIP session negotiation requests; and forward only the SIP session negotiation requests for which the negotiation frequency criterion is met to a remote User Agent Server, UAS.

According to another aspect of the invention, there is provided a method of operation for a network element for an Internet Protocol Multimedia Subsystem, IMS, the network element comprising a Back to Back User Agent, B2BUA, the method comprising the B2BUA performing the steps of: receiving Session Interface Protocol, SIP, session negotiation requests from a User Agent Client, UAC, of a remote station supported over a radio air interface; evaluating a negotiation frequency criterion for the SIP session negotiation requests; and forwarding only the SIP session negotiation requests for which the negotiation frequency criterion is met to a remote User Agent Server, UAS.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 is an illustration of a cellular communication system comprising some embodiments of the invention;

FIG. 2 illustrates an example of a SIP signalling flow in accordance with prior art;

FIG. 3 illustrates an example of some functional blocks of a network element in accordance with some embodiments of the invention;

FIG. 4 illustrates an example of a SIP signalling flow in accordance with some embodiments of the invention; and

FIG. 5 illustrates an example of a flowchart of a method of operation in accordance with some embodiments of the invention .

Detailed Description of Some Embodiments of the Invention The following description focuses on embodiments of the invention applicable to an IMS of e.g. a UMTS cellular communication system but it will be appreciated that the invention is not limited to this application but may e.g. be applied to other cellular communication systems.

FIG. 1 illustrates an example of some network elements of a UMTS cellular communication system having an IMS core network. For brevity and clarity, only the elements of the system required for the description of the embodiments are illustrated in FIG. 1.

FIG. 1 illustrates a first remote station 101 which communicates with a second remote station 103. In the specific example, the communication is a voice communication but it will be appreciated that the described principles are equally applicable to other communications, such as video communications. In the specific example, the voice communication is furthermore a Voice over IP (VoIP) communication.

The first remote station 101 is a UMTS remote station which is supported by a first Radio Access Network (RAN) 105. Specifically, the first RAN 105 comprises a UMTS base station which supports the communication over the air interface of the UMTS cellular communication system. Similarly, the second remote station 103 is supported by a second RAN 107 which specifically comprises a different UMTS base station supporting the communication to and from the second remote station over the air interface of the UMTS cellular communication system. It will be appreciated that the remote stations 101, 103 may in other examples be supported by the same RAN and may specifically be supported by different (or the same) base stations of a single RAN. It will also be appreciated that the described principles are equally applicable to for example communication between the first remote station 101 and a fixed terminal, such as a terminal coupled to the Internet via a gateway between the UMTS cellular communication system and the Internet.

In the system of FIG. 1, the RANs 105, 107 are coupled together by a core network which comprises an IMS 109. The IMS network is an Internet Protocol (IP) based network and the communication within the IMS network and between the RANs 105, 107 use the IP protocol. In particular, the Session Initiation Protocol (SIP) is used to initiate and manage multimedia sessions including the VoIP calls.

In the specific example where a VoIP communication is performed, the first remote station 101 implements a SIP User Agent Client (UAC) 111 which has initiated and set up the SIP session supporting the VoIP call. The UAC 111 is responsible for initiating SIP requests in order to change SIP session parameters.

Similarly, the second remote station 103 implements a SIP User Agent Server (UAS) 113 which can receive the SIP requests and which can enter into a negation process with the UAC 111 to change the SIP session parameters as requested by the UAC 111. It will be appreciated that although FIG. 1 for clarity and brevity shows only the first remote station 101 comprising a UAC 111 and the second remote station 103 comprising a UAS 113, the first remote station 101 can also comprise a UAS and the second remote station 103 can also comprise a UAC. In other words, the SIP session parameter changes can also be changed from the second remote station 103.

In the example of FIG. 1, the IMS 109 comprises a Session Border Controller (SBC) 115 at the interface point between the 1^st RAN 105 and the IMS 109. An SBC is a device used in some VoIP networks to exert control over the signaling and media streams involved in setting up, conducting, and tearing down calls. SBCs are put into the signaling and/or media path between calling and called party. Ultimately, SBCs allow their owners to control the kinds of call that can be placed through the networks on which they reside, and also overcome some of the problems that e.g. firewalls cause for VoIP calls. In the example, the SBC 115 is in the path of both the signaling and media communications between the first and second remote station 101, 103.

In the example of FIG. 1 the SBC 115 is coupled to a SIP server 117 which is further coupled to the 2^nd RAN through the IMS 109. It will be appreciated that although FIG. 1 illustrates direct connections between the SBC 115 and the SIP server 117 and between the SIP server 117 and the 2^nd RAN 107 this is merely for illustrative purposes and that practical connections are typically achieved via a number of intervening IMS network elements. Specifically, the 2^nd RAN 107 may be coupled to the IMS 109 via an SBC similar to the SBC 115 of the first RAN 105.

Typically an IMS will contain more than one physical embodiment of a SIP Server and the SIP server 117 can be seen as a representation of all the SIP servers associated with the first and second remote station 101, 103. A SIP server is both a logical and physical function. Each instance of a SIP server within an IMS network might perform either a passive or active role in the exchange of SIP signaling. Typical functions for SIP servers include (but are not limited to) User Registration, Authorization, Call Routing and Billing.

In the system, the signaling supporting the VoIP communication from the first remote station 101 to the second remote station 103 is thus performed through the SBC 115.

In the example, the SBC 115 comprises a Back to Back User Agent (B2BUA) which acts as a SIP signalling proxy for the UAC 111. A proxy resides on a node (in the example the SBC 115) within the IMS network directly within the flow of SIP messaging between two parties. It represents the calling party (the UAC) on one side and the called party (the UAS) on the other. To the caller, the B2BUA looks like the called party. To the called party the B2BUA looks like the caller.

The B2BUA can specifically be considered to correspond to a cascade of a UAC and a UAS. In effect, the B2BUA function is able to observe the signalling between the two parties and make decisions and modifications of the SIP signalling flow. This function allows the network operator to perform many IMS related tasks including value added services, call management and security. In the example, the B2BUA resides on the SBC 115 but it will be appreciated that it may reside on other nodes in other embodiments .

In the most part, SIP proxy devices are obliged to pass on their signalling messages in order to preserve integrity in the signalling path. This includes SIP/SDP negotiations. As previously described, in an IMS network supporting a mobile remote station, the varying radio conditions for the remote station can significantly impact the quality of the air interface communication. In order to address this, SIP allows a dynamic renegotiation of various aspects of the communication. Specifically, SIP allows a renegotiation to be performed between the UAC 111 and the UAS 117 to change the source encoding scheme.

For a VoIP communication, a number of voice codec schemes can be used including different voice codecs and different coding parameters. The sensitivity to adverse propagation conditions vary for these with some coding schemes being resilient to poor radio conditions whereas others require good radio conditions but may provide a better speech quality. SIP allows a UAC to dynamically initiate a negation with the UAS in order to change the currently used codec.

Thus, in a mobile SIP system, a change in the propagation conditions may result in the UAC requesting a new codec to be used. Specifically, if propagation conditions improve, the UAC may request that the codecs are changed to a less resilient but higher quality codecs (typically with a higher data rate) and if the propagation conditions degrade, the UAC may request that the codecs are changed to more resilient but lower quality codecs (typically with a lower data rate) .

However, each request for a change of codec initiates a negotiation process wherein a number of SIP messages are exchanged between the involved parties and intervening network elements as illustrated in FIG. 2. This increases the signalling overhead and thus the loading of the IMS network. Furthermore, for e.g. fast moving mobile remote units, the radio propagations change quickly and accordingly a large number of codec changes may occur resulting in a large volume of SIP signalling. This is illustrated in FIG. 2 which shows how a fast changing radio environment can incur a large number of media codec negotiations throughout the network. In the specific example, eight codec changes occur within the illustrated time interval. In addition, to the high network bandwidth resource usage, this also increases the computational burden on the involved elements and in particular increases the computational burden on the UAS .

In the system of FIG. 1, the B2BUA comprises a filtering function which performs a filtering of the SIP/SDP signalling. Specifically, the B2BUA implements a filter or throttling algorithm which reduces the number of codec change scheme requests which are forward to the UAS 113 from the UAC 111. Thus, in the system, only a subset of the SIP messages originated by the UAC 111 and requesting a change of codec is forwarded to the UAS 113. The filtering function of the B2BUA can accordingly suppress some of the codec change requests from the UAC 111. This can result in a reduction of the (downstream) signalling in the IMS 109 and may reduce computational burden on the UAS 113 as only a subset of codec change negotiations and codec changes are required.

The operation of the filtering of SIP/SDP messages is preferably such that the end users' experience remains unaffected by the variation in radio conditions whilst minimising the impact of downstream signalling overhead.

FIG. 3 illustrates an example of some functional blocks of the B2BUA. The B2BUA comprises a B2BUA UAS 301 which operates as a UAS for the UAC 111. Thus, SIP session negotiation requests are transmitted from the UAC 111 to the B2BUA UAS 301 which from the UAC 111 appears as a UAS of the 2^nd remote station 113.

The B2BUA furthermore comprises a B2BUA UAC 303 which operates as a UAC for the UAS 113 of the 2^nd remote station (or for any intervening UAS) . Thus, in order to change parameters of the SIP session, such as a change of the codec, the B2BUA UAC 303 transmits SIP session negotiation requests to the UAS 113 thereby initiating a negotiation procedure.

The B2BUA also comprises a filter processor functionality 305 which interfaces between the B2BUA UAS 301 and the B2BUA UAC 303. The filter processor 305 filters the SIP session negotiation requests from the UAC 111 such that only a subset of these is forwarded to the UAS 113. Specifically, the filter processor 305 evaluates a negotiation frequency criterion for the SIP session negotiation requests and forwards only the SIP session negotiation requests which meet the criterion. Thus, for each SIP session negotiation request a criterion is evaluated and if the criterion is met, the request is forwarded to the UAS 113. If the criterion is not met, the request is not forwarded to the UAS 113 by the B2BUA.

It will be appreciated that any suitable criterion for evaluating whether to forward a request can be implemented by the filter processor 305.

FIG. 4 illustrates a specific example of the operation of the B2BUA. The example corresponds to the example scenario of FIG. 2 and illustrates the effect of the throttling performed by the B2BUA. The example corresponds to an example where the VoIP voice codec is dynamically changed between a more resilient codec and a less resilient codec depending on the radio conditions. Specifically, whenever the measured receive level increases above a given threshold, the UAC 111 requests a change to a less resilient but higher quality codec. When the receive level falls below a given threshold, the UAC 111 requests a change to a more resilient but lower quality codec. The request to change the codec is instigated by the UAC 111 transmitting a SIP INVITE message requesting a codec change.

The SIP INVITE message is received by the B2BUA UAS 301. It is then forwarded to the filter processor 305 which evaluates the negotiation frequency criterion. If the criterion is met, the SIP INVITE message is forwarded to the UAS 113 via the SIP server 117.

In the example, the criterion is met for the first SIP INVITE message which accordingly is forwarded to the UAS 113 resulting in a codec change negation process proceeding. Specifically, the UAS 113 returns a SIP 200 OK message acknowledging the codec change request.

Thus, in the example where the criterion is met for a SIP session negotiation request, the B2BUA proceeds to forward all messages between the UAC 111 and the UAS 113 for the instigated negotiation process. In other words, if the initial request message meets the criterion this instigates a full negotiation process.

If the criterion is not met, the SIP session negotiation request is throttled and no message is transmitted to the UAS 113. In the example of FIG. 4, the criterion is not met for the second and third message and accordingly no SIP session negotiation request are transmitted towards the UAS 113 thereby substantially reducing the signaling overhead in the IMS 109.

It will be appreciated that the operation of the B2BUA in the case where the criterion is not met may be different in different embodiments.

For example, in some embodiments, the B2BUA may transmit a negation rejection message back to the UAC 111 resulting in the SIP session change being abandoned. In the example of FIG. 4, the B2BUA proceeds to perform the negation and to change the codec. Specifically, the B2BUA transmits a 200 OK message back to the UAC 111 and proceeds to change the codec. Thus, the B2BUA changes the VoIP communication between the UAC 111 and the B2BUA to use the new codec whereas the VoIP communication between the B2BUA and the UAS 111 continue to use the old codec. Thus, in this case the B2BUA also implements a transcoder for converting the voice data from one codec scheme to the other.

Thus, in this embodiment, the B2BUA additionally performs a conversion process and simultaneously supports one SIP session setting between the UAC 111 and the B2BUA UAS 301 while at the same time supporting a different SIP session setting between the B2BUA UAC 303 and the UAS 113. The first SIP session setting corresponds to the setting of the latest SIP session change request received from the UAC 111 whereas the second SIP session setting corresponds to the setting of the last SIP session change request sent to the UAS 113.

This approach can allow the codec to be dynamically changed to suit the air interface radio conditions without resulting in a large amount of signaling overhead. In the specific example, eight end-to-end negotiations are reduced to three negotiations for the given time interval. The reduction in signaling may in particular be significant in fast changing propagation environments wherein frequent codec changes occur.

In the described system, the B2BUA is arranged to initiate calls to the UAS 113 with a nominal, default set of parameters consistent with pre-determined quality of service requirements. Thus, specifically, the B2BUA is set up to initiate a call to the UAS 113 using a default codec and with predetermined bandwidth parameter etc.

It will be appreciated that the specific throttling algorithm and negotiation frequency criterion can be implemented to suit the specific preferences and requirements of the individual embodiment.

Specifically, a high rate of SIP/SDP re-negotiations can be blocked by the B2BUA whereas a low rate of negotiation requests are allowed to pass through the network.

As a specific example, the negotiation frequency criterion can comprise a requirement that an averaged frequency of the forwarded SIP session negotiation requests is below a threshold. For example, the B2BUA can measure the frequency of the SIP session negotiation requests and can low pass filter the determined frequency using a suitable time constant. If the low pass filtered frequency falls below a given value, the SIP session negotiation requests can be allowed through to the UAS 113 and if the low pass filtered frequency is above the threshold, the SIP session negotiation requests are blocked .

As another example, the B2BUA can alternatively or additionally require that the duration since the previous SIP session negotiation request was forwarded to the UAS 113 exceeds a given threshold. This will allow a minimum time between negotiation requests being sent and thus guarantee a maximum rate of negation requests and thus a maximum signalling overhead.

In some embodiments, the negotiation frequency criterion includes a requirement that a maximum number, N, of SIP session negotiation requests are allowed to be forwarded within a given time interval, T. Thus, the B2BUA employs a filter algorithm to only allow N re-negotiations to pass through over a configurable time T.

For example, for a given a time period T, a remote station may move from a good radio propagation environment, to a poor one and back to a good environment. In an attempt to cope with the change in radio propagation conditions the UAC 111 performs a set of SIP negotiations from a non-resilient codec, to a resilient codec, and back to a non-resilient codec. In other words there are two discrete negotiations in the time interval T. If the same occurred over a second interval T immediately following the first, there would be a total of four negotiations initiated by the UAC 111. However, if the B2BUA filter is configured to only pass through one negotiation request for every elapsed time period T, the number of end-to-end negotiations is reduced by a factor of two.

In some embodiments, the negotiation frequency criterion implements different sub-criterions for different SIP session negotiation requests. Specifically, different criterions may be applied to a change from a lower quality of service parameter to a higher quality of service parameter than for a change from a higher quality of service parameter to a lower quality of service parameter. This may allow a biasing of the system towards a desired quality of service.

For example, if the communication between the B2BUA and the second remote station 103 is currently performed using a less resilient but higher quality codec, a SIP request from the UAC 111 may always be allowed through in order to ensure high quality. However, a SIP request from the UAC 111 to change from the high quality codec to the low quality codec may only be allowed through after a given time interval has passed since the previous SIP request was forwarded. This may bias the communication between the B2BUA and the second remote station 103 towards a higher quality and may in particular prevent that the voice communication on this link only uses a low quality codec if the communication from the first remote station 101 uses the low quality codec.

The application of the negotiation frequency criterion may result in some SIP requests being blocked such that the next SIP request meeting the negotiation frequency criterion is identical to the previously SIP request forwarded to the UAS 113. For example, following a forwarded request to change the codec to the high quality codec, a subsequent request to change to the low quality codec may be blocked. The next request initiated by the UAC 111 will be a request to change to the high quality codec. However, although this request may be later than the previous request by the required duration, there is no need to forward the message. Accordingly, the B2BUA can in some embodiments further block SIP requests which are identical to the last request that was forwarded. In some embodiments, the negotiation frequency criterion may be dynamically changed. Thus, the B2BUA can dynamically monitor a parameter and change the negotiation frequency criterion accordingly.

Specifically, the negotiation frequency criterion may be modified in response to the loading or congestion of the IMS. For example, if the loading is less than a given threshold, there is plenty of resource for the full signalling and the B2BUA may allow all requests to be forwarded. However, for increasing load, an increasingly strict requirement may be used in order to reduce the signalling overhead and free up more resource. For example, the number of requests allowed to be forwarded in a given time interval may be reduced for increasing load.

In some embodiments, the negotiation frequency criterion may be modified in response to an amount of received SIP session negotiation requests. For example, the B2BUA may implement a self-tuning negotiation rate allowance. Thus, frequent negotiation requests in a time interval T can result in a decrease in forwarded requests N (e.g. to a configurable minimum) . Conversely, infrequent negotiation requests in time interval T can result in an increase in N (to a configurable maximum) .

It will be appreciated that the described approach provides a number of advantages, including e.g. one or more of the following:

^■ A reduced signalling overhead can be achieved. ^■ A reduction in fixed network bandwidth requirements can be achieved.

^■ A reduction in wireless network bandwidth requirements can be achieved. ■ The effect of varying radio conditions can be minimised within the IMS core.

^■ A reduction in the computational burden of core IMS network elements can be achieved.

^■ A reduction in the computational burden in non IMS core network elements and in particular in the end-user devices can be achieved.

^■ A reduction in capital expenditure for the network operator can be achieved.

^■ A reduction in operational expenditure for the network operator can be achieved.

FIG. 5 illustrates a method of operation of the B2BUA in accordance with some embodiments of the invention.

The method initiates in step 501 wherein the B2BUA UAS 301 receives air interface SIP session negotiation requests from the UAC 111.

Step 501 is followed by step 503 wherein the filtering processor 305 evaluates the negotiation frequency criterion for the SIP session negotiation requests.

Step 503 is followed by step 505 wherein the B2BUA UAC 303 forwards only the SIP session negotiation requests for which the negotiation frequency criterion are met to the UAS 113. It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors .

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A network element for an Internet Protocol Multimedia Subsystem, IMS, the network element comprising a Back to Back User Agent, B2BUA, the B2BUA being arranged to : receive Session Interface Protocol, SIP, session negotiation requests from a User Agent Client, UAC, of a remote station supported over a radio air interface; evaluate a negotiation frequency criterion for the SIP session negotiation requests; and forward only the SIP session negotiation requests for which the negotiation frequency criterion is met to a remote User Agent Server, UAS.

2. The network element of claim 1 wherein the B2BUA is further arranged to support a first SIP session setting for the UAC while supporting a different SIP session setting for the UAS, the first SIP session setting corresponding to a SIP setting of a blocked SIP session negotiation request and the different SIP session setting corresponding to a SIP setting of a forwarded SIP session negotiation request.

3. The network element of claim 1 wherein the negotiation frequency criterion comprises a requirement that an averaged frequency of forwarded SIP session negotiation requests is below a threshold.

4. The network element of claim 1 wherein the negotiation frequency criterion comprises a requirement that a duration since a SIP session negotiation request was forwarded to the UAS exceeds a threshold.

5. The network element of claim 1 wherein the negotiation frequency criterion comprises a requirement that a maximum number, N, of SIP session negotiation requests are forwarded within a time interval, T.

6. The network element of claim 1 wherein the negotiation frequency criterion comprises a first sub- criterion for negotiation of a change from a higher quality of service setting to a lower quality of service setting and a different sub-criterion for negotiation of a change from the lower quality of service setting to the higher quality of service setting.

7. The network element of claim 1 wherein the negotiation frequency criterion comprises a requirement that a SIP session negotiation request is only forwarded to the UAC if it is different than a latest forwarded SIP session negotiation request.

8. The network element of claim 1 further comprising means for adapting the negotiation frequency criterion in response to a loading of the IMS.

9. The network element of claim 1 further comprising means for adapting the negotiation frequency criterion in response to an amount of received SIP session negotiation requests .

10. A method of operation for a network element for an Internet Protocol Multimedia Subsystem, IMS, the network element comprising a Back to Back User Agent, B2BUA, the method comprising the B2BUA performing the steps of: receiving Session Interface Protocol, SIP, session negotiation requests from a User Agent Client, UAC, of a remote station supported over a radio air interface; evaluating a negotiation frequency criterion for the SIP session negotiation requests; and forwarding only the SIP session negotiation requests for which the negotiation frequency criterion is met to a remote User Agent Server, UAS.