Method and system for transcoding avoidance in Border Gateways

This invention relates to a method and system for transcoding avoidance in Border Gateways according to the preamble of claim 1 respectively claim 6.

The architecture framework for Next Generation Network NGN, as defined by ITU-T and ETSI, describes beside others the functional entities of the gateway connecting different networks. The different networks could be either core networks of different network providers or access networks connected to a core network. The Service Stratum contains besides others the functional entity Call/Session Control, whereas the Transport Stratum contains in the Resource and Admission Control Area the Policy Decision Functional Entity PD-FE and in the Transport Area the Policy Enforcing Functional Entity PE-FE. Network operators do assign a specific codec to be used in the interconnecting network (between networks of different network operators) based on bilateral agreement or specify a specific codec to be used in their core network independent of the codec used in the access network, based on criteria such as bandwidth; see EP 1 758 333 Al and WO 2007/022818 Al [1] or to connect IP end devices using different codecs. The network element connecting the different networks is hereafter called border gateway BGW.

A number of different (bit rate) codecs are defined for the transport of voice over IP. However it is not economical feasible to provide all these codecs in IP based devices. That is why network operators provide in their networks network elements with the "transcoding" capability. This makes it possible to "connect" IP end devices with different codecs .

The above mentioned method suffers from the drawback that unnecessary transcoding may be present. Transcoding intro-

duces additional delay and reduces the audio quality caused by converting one coding scheme into another coding scheme.

On the other hand an end to end based codec negotiation has been defined (ITU-T, IETF) to agree, if possible, to a codec available in both end devices, considering that transcoding reduces the voice quality by adding delay and decreasing the audio quality.

Nevertheless this negotiation does not avoid transcoding per se, because the Border Gateways, where the transcoding takes place are not involved in the codec negotiation, see Fig. 1 BGW.

It is therefore a task of the present invention, to provide a method and a system for avoiding unwanted transcoding causing a reduced voice service quality.

WO 2004/006597 Al / EP 1 586 209 Bl [2] discloses the «Avoidance of transcoding or interruption on live load coding exchange in extant connections)). This disclosure does not solve the above mentioned problem, since it is restricted to a media gateway MG between a TDM- and a packet-based network and is related to a media change during an extant connection, e.g. voice to fax and vice versa. An example would be where person A assures by voice communication with a person B, that the person B will be able to receive a confidential fax and only then sends the fax. This can be done by using a normal telephone apparatus combined with a Fax-Device Gr .3.

The object to provide a method and a system for avoiding unwanted transcoding causing a reduced voice service quality is solved by the method steps specified in claim 1 and by a system given with the features in claim 6.

By the introduction of i) an information element containing at least a field

«call/connection identifier» Call-Id and a field «common codec» of type «true/false» (Y/N) in the information exchange between a Physical Entity Call/Session Con ^¬ troller PE-CSCF and a Physical Entity containing the Policy Decision Functional Entity PD-FE and by a table in the Policy Decision Functional Entity PD-FE, where the table elements store the information «common codec» on a per call basis and a procedure for the Policy Decision Functional Entity PD-FE whereby ii) in the involved border gateway BGW in case of «common codec» = true the transcoding is avoided or in case of «common codec» = false an predefined transcoding is performed according to the rule based on origin and destination , and contained in the the Policy Decision Functional Entity; an unwanted transcoding between the two peers A and B can be avoided. This information element («common codec») does not affect other protocols and can easily be implemented in existing systems.

Further advantageous embodiments of the present invention are given in dependent claims.

The invention will be now described in preferred embodiments with reference to the accompanying figures wherein:

Figure 1 Transcoding in the Border Gateways of network A and network B according to the state of the art; Figure 2 Transcoding avoidance in the Border Gateways of network A and network B according to a preferred embodiment of the invention;

Figure 3 Protocol elements and table for «common codec»; Figure 4 Media (voice) /RTP - Codec Analysis in the Policy Decision Functional Entity;

Figure 5 BGW containing Policy Enforcing Functional Entity and Policy Decision Functional Entity;

Figure 6 BGW containing Policy Enforcing Functional Entity and an interface to Policy Decision Functional Entity.

Fig. 2 shows the end-to-end negotiation between two subscribers A and B, where an A-subscriber initiates a call to a B-subscriber and the call is identified by a call/connection identifier «Call-Id».

Each IP end device of the subscribers A, B is connected to an access router R of an access network, where the media stream is routed to a border gateway BGW. The signalling SIP, SDP is routed via an access router R to the Physical Entity «Call/Session Control Function» PE-CSCF, the latter is called for the sake of simplicity just «Call/Session Controller» PE- CSCF. The protocol used between the Call/Session Controller PE-CSCF and the Physical Entity containing a Policy Decision Functional Entity PD-FE, which may or may not be an integra- ted part of the Border Gateway BGW is enhanced by a new information element 12. Further details see Fig. 3. The information element 12 contains a field «common codec» of type«true/false» or «Y/N» or «yes/no», this field is accompanied by another field «Call-Id» identifying the call by a call/connection identifier, see e.g. SIP.

With reference to Fig. 3 the logic of the Policy Decision Functional Entity PD-FE is enhanced by a table containing the information «common codec» for each active call identified by the «Call-Id». The «Call-Id» is used to generate the appropriate address to access (read/write) the said table. Furthermore a procedure is provided to analyze for each RTP stream, if transcoding is to be applied or not by determining the media type and in case of voice accessing the correspon- ding element of the said table to get the «common codec» information and set the appropriate flag for the further processing in the Policy Decision Functional Entity PD-FE.

Protocol 10 (based on Diameter as specified in Q.3301.1 or H.248/SDP which is also a possible candidate) is used to transmit these information elements «Call-Id» and «common codec» from the Call/Session Control Function PE-CSCF to the Border Gateway BGW. The enhancements to the protocol 10 are shown exemplary by using H.248/SDP in table 1.

Messages_ ADD, MODIFY

Message format: MEGACO/x.y. Transaction = . Context = ... Call-Id = ... new info element ADD (MODIFY) H.24!

Table 1: Messages and Message format with additional fields.

If the Policy Decision Functional Entity PD-FE is not integrated in the Border Gateway BGW, then the protocol 11 used between the Policy Decision Functional Entity PD-FE contained in a separate Physical Entity and the Policy Enforcing Functional Entity PE-FE in the Border Gateway BGW is preferably according to an open standard, see Fig. 5 and

6. An example for such a protocol is H.248 (accompanied by SDP) or COPS.

The Policy Decision Functional Entity PD-FE maintains a table to store the new information element «common codec» on a per call basis. If the information «common codec» is true («Y», «yes») , then the Policy Decision Function Entity PD-FE ignores the existing rule stored in the Policy Decision Functional Entity PD-FE for applying the assigned codec (for crossing network borders) for this voice call and instructs the Policy Enforcing Functional Entity PE-FE accordingly. That means for the media «voice» no codec information will be sent to the Policy Enforcing Functional Entity when requesting the creation of the appropriate context.

Fig. 4 shows the new procedure introduced in the Policy Decision Functional Entity PD-FE:

For each RTP stream the media type is determined and in case of voice, the corresponding element in said table is accessed to get the «common codec» information and the flags for the further processing are set, i.e. preparing the instruction to be sent to the Policy Enforcing Functional Entity PE-FE.

The Border Gateways BGW includes either only the Policy Enforcing Functional Entity PE-FE and thus provides the voice related signal processing resources or in addition to the Policy Enforcing Functional Entity PE-FE also the Policy Decision Functional Entity PD-FE.

Session Border Controllers of pre-NGN networks handling media and signalling can be considered as a physical implementation of a NGN Border Gateway containing the Policy Decision Functional Entity PD-FE and the Policy Enforcing Functional Entity PE-FE combined with a Signalling Gateway.

The Policy Enforcing Functional Entity PE-FE of the Border Gateway BGW providing the signal processing functions for

voice, e.g. one or more codecs, is informed either by the integrated or external Policy Decision Functional Entity PD- FE based on the information received from the Call/Session Controller PE-CSCF, where the call set up handling takes place, if a common codec has been negotiated or not.

In another embodiment of this invention the codec could be negotiated between the Policy Decision Functional Entities PD-FE involved in the call.

List of reference numerals

10 Resource control protocol, Rs interface; ITU-T Rec. Q.3301.1 or alternatively H.248/SDP (another possible candidate for the resource control protocol at the Rs interface)

11 protocols H.248/SDP, COPS

12 Common codec Information Element

List of used symbols

BGW Border Gateway

Call-Id Call (Connection) Identifier

Co-Codec Common Codec

COPS Common Open Policy Service, Protocol; IETF RFC 274! CSCF Call/Session Control Function

ETSI European Technical Standardization Institute

IETF Internet Engineering Task Force

ITU-T International Telecommunication Union

PD-FE Policy Decision Functional Entity PE-FE Policy Enforcing Functional Entity

PE-CSCF Physical Entity «Call/Session Control Function»

R Router

RFC Request For Comment

RTP Real Time Transport Protocol, IETF RFC 1889

SBC Session Border Controller

SDP Session Description Protocol, IETF RFC 2327 SIP Session Initiation Protocol, IETF RFC 2543

H.248 MEGACO (Media Gateway Control Protocol/Resource Control Protocol), ITU-T/IETF

References [1] WO 2007/022818 Al

«Method for the manipulation of speech packets in an IP-Gatway»

Siemens Schweiz AG; CH - 8047 Zurich.

[2] WO 2004/006597 Al / EP 1 586 209 Bl «Avoidance of transcoding or interruption on live load coding exchange in extant connections)) Siemens Aktiengesellschaft, DE - 80333 Munich