fig1 shows a configuration according to the invention . media gateways mg a ( a - side media gateway ) and mg b ( b - side media gateway ) are provided . according to the present exemplary embodiment , both media gateways are intended to take the form of set - top boxes with subscriber - side connections to the subscriber terminal devices a , b . the two media gateways or set - top boxes mg a and mg b are controlled via a protocol p by call control servers or media gateway controllers mgc a , mgc b respectively . the control protocol p used for this purpose can be a protocol known per se such as the mgcp called h . 323 or the sip protocol . both controllers exchange information via an inter - office signaling protocol known per se . the connection - related signaling information between the two subscriber terminal devices a , b taking part in the connection is routed via the two set - top boxes and the two media gateway controllers ( signaling link ). the present exemplary embodiment shows two media gateway controllers . this number is not however a mandatory requirement for the invention . a plurality of media gateway controllers can be used , as can a single media gateway controller . the invention now provides for the useful data connection ( bearer link ) to be routed via an ip gateway ipgw ( loop connection ). in the case of a purely audio connection the useful data routed via the bearer link is audio and in the case of a video connection it is audio plus video . in the latter case for example there are two useful data streams in a single connection , as fig1 shows . the useful data stream is thus routed indirectly between the packet - based end points ( set - top boxes ) mg a and mg b . the useful data is transmitted with the aid of the real time protocol rtp and the real time control protocol rtcp . the ip gateway ipgw is likewise controlled with the aid of a protocol known per se such as h . 248 by the media gateway controller mgc a responsible for billing . the a - side media gateway controller mgc a is preferably intended to assume the function of the controller responsible for billing . the billing tickets ( call data records , cdr ) for interference - free connections are created here in the a - side controller . they include the required accuracy level ( e . g . billing to an accuracy of 1 s ) and have a sequence of information elements , documenting in the case of the link concerned the beginning , end and type ( audio , video and where appropriate , bandwidth or codec ). further information is also recorded during a connection , such as a change in the type characteristic ( e . g . audio & lt ;-& gt ; video ). complementary information on communication quality , the duration of the communication , audio and video carrier bandwidth / volume , can be provided as cdr in the ip gateway ipgw or reach the media gateway controller responsible for billing via the signaling . an outgoing connection request from the subscriber terminal device a is notified to the subscriber terminal device b via the two media gateway controllers mgc a and mgc b . if the subscriber terminal device b accepts the call , the bearer link is set up in bidirectional mode ( that is , in the forward and return directions ) between the media gateway mg a and the ip gateway ipgw , and between the ip gateway ipgw and the media gateway mg b . billing now starts in the ip gateway . according to the invention , the two media gateways mg a and mg b together with the ip gateway ( ipgw ) ensure that rtp / rtcp packets are transmitted at a minimum separation typically defined as 100 ms , regardless of connection type , encoding method and packetizing length . it is intended that it shall be possible to configure this minimum separation . the mg a , mg b and the ip gateway thus preserve the integrity of the useful data stream so long as ip packets in the useful data stream arrive with this minimum separation . the generation of rtp packets at a definable minimum separation in time is satisfied for voice connections and no further requirement . a packet is generated even if this would not be necessary due to the “ silence suppression ” feature in the audio stream , a still - picture transfer in the video stream , a missing input signal , or the encoding method or packetizing length used . in the interests of quality control of the through - connected rtp carrier , both the ip gateway ipgw and the media gateways mg a and mg b monitor the arrival of said ip packets at the guaranteed minimum separation . if the ip gateway ipgw now detects the non - appearance of an ip packet within the minimum separation time , it waits a certain guard period t 1 ( such as 200 ms ) before defining the connection as inactive after which it stops the transfer of packets to the other end and in the return direction . furthermore a corresponding billing ticket is immediately written and transmitted to the media gateway controller mgc a . the non - appearance of ip packets can also be detected in like manner by the two media gateways mg a and mg b . in this case the transfer of packets is stopped in the return direction and signaling is sent to the associated media gateway controller mgc a or mgc b , which then initiates release of the connection concerned . in order to reduce the signaling load on the controller and network in the context of normal call termination by the a or b side , break signaling can be delayed by a defined guard period t 2 in the media gateways ( e . g . 2 s ). a regular connection release by subscriber terminal device a or b then causes the break in the rtp stream to be detected in the ip gateway ipgw and in the media gateways mg a and mg b . however , the provision of the guard period t 2 enables the connection release to proceed as normal ( call flow ), causing no increase in signaling load . the billing information can be stated online via signaling between ipgw and mgc a or offline in the context of postprocessing in the billing center by including the information directly from the ip gateway . the billing information is stated with the aid of connection identifying information stored in the ip gateway . this identifying information can likewise be transferred for evaluation to equipment other than the controller mgc a responsible for billing . the configuration to which the invention relates will tolerate the failure of different devices without excessively affecting billing . a failure in one of the media gateways mg a or mg b interrupts rtp / rtcp packet transfer in the ip gateway ipgw . this interruption is subsequently signaled by the respective other end , that is , gw a or gw b . the exact billing information is then present in the ip gateway . since the billing information in the ip gateway can be very accurately determined , any billing information recorded in parallel in the controller can be stated online by signaling or offline via separate cdrs from the ip gateway . the simultaneous failure of both media gateways mg a and mg b likewise interrupts the rtp / rtcp packet transfer in the ip gateway . in this case too , the exact billing information is determined in the ip gateway . the interruption is disclosed by signaling from the media gateways and immediate notification from the ip gw to the call control server , after the a or b side becomes available again . since the billing information in the ip gateway can be very accurately determined , any billing information recorded in parallel in the controller can be stated online by signaling or offline via separate cdrs from the ip gateway . the temporary isolation of the media gateway controller responsible for billing ( either mgc a or mgc b ) does not cause a break in the useful data streams . even if the connection is released after this interference , the effect is as if there had been no interference beforehand . if the connection is released during said interference , the information is produced by the media gateway mg a or mg b or via the ip gateway no later than after return of availability by repeating the release signal or another protocol - specific message . the exact billing information is then present in the ip gateway . the billing information can if necessary be stated online by signaling or offline via separate cdrs from the ip gateway . a communication loss between the media gateway controller responsible for billing and the ip gateway causes the connections concerned to be released . this is necessary because the case could arise in which the billing information is no longer available in the ip gateway and the “ on hook ” messages signaled by the media gateway controller mg a or mg b have been lost . high - frequency monitoring by exchange of signaling is still needed for the ip gateway , but not for each of the media gateways . however , since the ip gateway is de facto rather large , this does not lead to any further reduction in the signaling load and the network load . certain network operators require that billing should indeed be stopped in the event of interference , but the connection to the remote end should not be released . this requirement is fulfilled by a configuration and a method according to fig1 . if on the other hand there is a requirement for the connection to be released without fail in the event of interference , this requirement can be addressed by releasing in the ip gateway in isolation from the call controller . an alternative form of control can be achieved by means of administration or configuration parameters . fig2 shows in detail the configuration and method to which the invention relates . in the example shown here it is assumed that following subscriber input during a call controlled by the terminal device and routed via tdm switching centers , a new call is established via the ip network . this second call is a continuation of the first call and enables further performance features , such as additional sections of videotelephony in the connection . this second call can establish not only a voice connection but also a voice plus video connection . the subscriber can toggle at will between audio and audio plus video transmission . exact billing must be available without fail for these ip connections , just as in the case of the tdm call . in particular the operator wishes to be able to bill the different services ( audio , video etc .) separately . the protocol used for signaling to the subscriber is standard sip protocol . the subscriber terminal devices used are sip terminal devices ( sip user a , sip user b ). a media call control protocol is used for controlling the ip gateway . the h . 248 protocol is intended to be used for preference in this case . the call control server mgc a or mgc b ( media gateway controller ) is in the form of a switching processor . a sip proxy server is connected upstream of said switching processor . the signaling messages between subscriber and switching processor are routed via the sip proxy server . the switching processors mgc a or mgc b have a peripheral unit pcu responsible for the ip interface and for handling the ip protocols . they further have a call processing core system which can control both ip call processing and tdm call processing , and also has a built - in billing interface . the interface between the pcu and the core system uses an internal protocol . billing is intended preferably to be performed a side . a call is billed according to the following sequence . a sip : invite message with sdp information ( audio codecs ) is sent from the subscriber ( sip user a ) to the unit pcu . once the a - side unit pcu has received an invite message it creates in the ip gateway , via megaco , the resources for the new call ( add two new ephemeral terminations ). the sdp data from the a side is immediately transferred to the ip gateway . the ip gateway furthermore receives a call reference and uses this in the billing tickets for this connection . the sdp data ( ip address and port ) is handled b side using the data returned from the ip gateway . when the b side has been detected in the core system and the b side has accepted the call , the b side acknowledges the invite message by means of a sip : 200ok message . this 200ok message is used in the a side as a criterion for starting the audio billing . the unit pcu sends the core system a corresponding internal message . the ip gateway ipgw is on the one hand supplied with the sdp data from the b side and on the other is set to send - receive . monitoring of the rtp stream begins . if a further sip : invite message having sdp information ( audio codecs plus video codecs ) reaches the unit pcu from a subscriber and is acknowledged positively by the other party , it is again accepted by means of a sip : 200ok message . the ip gateway is supplied with the new codec information . simultaneously with the sip : 200ok message , the unit pcu sends a message a side to the core system , indicating use of the video service . a further sip : invite message with sdp information ( audio ) is handled in a like manner by the unit pcu . if the other party positively acknowledges and accepts by means of sip : 200ok that the video transmission should be turned off , the unit pcu sends a message to the core system , indicating use of the audio - only service . the ip gateway is then supplied with the new codec information . billing is stopped a side by means of the sip : bye message from one side of the call . simultaneously the call is also released in the ip gateway ( subtract the two terminations ). if there is no network interference , a billing accuracy of 500 ms can be achieved , as network operators usually demand . given such a configuration and such a method , billing errors can be avoided in the event of failures . for instance the failure of the a side in the rtp stream is detected by the b side and reported to the unit pcu , where the call is released . the ip gateway also detects non - appearance of rtp packets from the a side and reports the exact duration of the connection to the unit pcu . the failure of the b side is handled in like manner to the failure of the a side . network interference which interrupts the communication of the pcu to the a and / or b side does not cause the call to be released so long as there is a connection from the unit pcu to the ip gateway and the latter does not report any rtp interference . any “ on hook ” messages are repeated by the terminal devices until the network fault is remedied . the exact time - related data concerning the duration of the rtp connection is determined on the ip gateway , which stores it and signals it to the unit pcu . if the ifp gateway detects disruption to the rtp stream , it signals this to the unit pcu . the call is released and billed with sufficient accuracy . if the pcu / ip gateway connection fails , all calls via this ip gateway are released , otherwise neither exact billing nor the availability of the bearer link can be guaranteed .