The introduction of new communication systems including third generation mobile networks (3 G) and broadband IP networks will result in a wide range of services being available to users. Not least amongst these services will be the possibility for multimedia (MM) calls between users, allowing video telephony and the exchange of data.
There are circumstances in which authorised agencies such as the police and intelligence services must be able to monitor calls including multimedia calls. Such lawful interception is required in order to be able to collect information on those suspected of involvement in criminal or terrorist activities. The lawful interception of traditional voice call has been handled in two ways:
1) The voice streams coming from the subscribers involved in a call to be intercepted are mixed together by monitoring equipment located in one of the “switches” involved in the call. The mixed stream is sent, by establishing an ancillary call, to the monitoring centre. Thus the mixed stream, i.e. the complete conversation between the parties, can be played for example using an ordinary loudspeaker in the monitoring centre.
2) The voice streams coming from the subscribers involved in the intercepted call are not mixed, but rather two connections are established from the monitoring equipment to the monitoring centre, each carrying one leg of the call. This allows the monitoring centre to record the voices of the two call parties separately and/or mix the voice streams in the monitoring centre.
The lawful interception of multimedia calls is more problematic than for voice calls. The protocols used to set up a multimedia call between terminals require handshaking between the participating terminals. The handshaking is used to agree upon parameters describing the payload of the call and how the payload is to be transported. The parameters to describe the payload include a used codec and codec options (e.g. video codecs such as H263 and MPEG4 include a number of optional features, the main purpose of which are to either improve the picture quality or decrease the used bandwidth, or both). Transport parameters include for example payload format, e.g. the format of the RTP-packet to be used to carry a data stream in IP based transport network, or H223 logical channel parameters used in narrowband multimedia H.324. H223 logical channel parameters include parameters specifying whether payload frames are allowed to be segmented into several H223 multiplex frames, whether the payload frames are numbered, etc.
FIG. 1 illustrates for example a handshake between two terminals according to the ITU-T H.245 protocol (where “OLC” designates Open Logical Channel signaling messages). In the lawful interception scenario, it is not possible to involve the monitoring centre in the handshaking process as two terminals are already involved in the process and in any case it is undesirable to alert a terminal associated with a call to the interception action. For multimedia calls therefore, according to current interception processes, normal multimedia equipment (e.g. mobile handsets) cannot be used in the monitoring centre to decode and display the media. Interception can only be achieved using specialist equipment installed at the monitoring centre.