Abstract:
Data communication apparatus including a port and a control entity. The control entity is operative to establish a connection with a remote entity over a first path and negotiate with the remote entity using in-band signaling over the first path establishment of a second path allowing the exchange of data between the data communication apparatus and the remote entity. The invention presents advantages from the standpoint of ease of implementation and bandwidth and resource savings. The use of an in-band messaging protocol to negotiate a establishment of the second path can be implemented generally in a straight forward manner. At the same time, the ability to transfer at least part of the connection to the second path avoids the drawbacks that would arise if that part of the connection were constrained to the first path. This feature allows the operator to take advantage of benefits provided by the second path but not available to the first path.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/393,386 to Rabipour et al., filed on Jul. 5, 2002 and U.S. Provisional Patent Application Serial No. 60/395,271 to Rabipour et al., filed on Jul. 12, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to communications networks and, more particularly, to methods and apparatus for increasing the service quality and efficiency with which data is communicated between entities in such networks.  
         BACKGROUND OF THE INVENTION  
         [0003]    According to most existing telecommunications standards, the transmission of speech information over a wireless interface takes the form of compressed speech parameters. Upon receipt of compressed speech parameters at a base station in communication with a mobile unit, the speech parameters are processed by a codec (coder/decoder), which converts (expands) the speech parameters into speech samples, typically at a rate of 64 kilobits per second (kb/s) in order to provide compatibility with the public switched telephone network (PSTN). The speech samples at 64 kb/s are then transmitted over the PSTN towards the called party. The speech samples associated with a given call may share the same link as speech samples associated with other calls by virtue of time division multiplexing (TDM), which provides for fixed-duration time slots to be allotted to individual calls.  
           [0004]    [Note: Does it matter that the descriptions assume a mobile originated call? Do we have state that the descriptions are valid also to mobile terminated call?] 
           [0005]    If the called party is connected directly to the PSTN, such as via a wireline connection, the speech samples having travelled through the network will simply be converted into audio form by a digital telephone unit at the called party site. Of course, the called party may also be a second mobile unit, in which case the speech samples will terminate at a second base station, where a second codec re-converts the speech samples back into compressed speech parameters for transmission to the second mobile unit via a wireless interface. The usage of a source decoder to expand speech parameters into a stream of speech samples, in combination with the use of a destination encoder for re-compression of these samples into a second set of compressed speech parameters, is referred to as operation of codecs in tandem, or “tandem operation”.  
           [0006]    Those skilled in the art will appreciate that when both the called and calling parties are mobile units, the tandem operation described above introduces a degradation in service quality, as errors may be introduced by the decompression and re-compression operations performed by the source and destination codecs, respectively. Such error should in principle be avoidable, as neither codec operation is required by virtue of the second base station requiring the compressed speech parameters rather than the expanded speech samples. Thus, it is of interest to find a solution to the problem of service quality in call connections involving tandem codecs.  
           [0007]    Two classes of solutions to the problem relating to the service quality in call connections involving tandem codecs have already been described and standardized, or are well in their way towards standardization. The earlier of the two methods, called Tandem-Free Operation (TFO), uses an in-band handshaking protocol to detect the presence of tandem codecs, and then proceeds to insert the compressed speech parameters within the 64 kb/s sample stream. This arrangement bypasses the requirement for decompression at the source codec and (re-)compression at the destination codec, which obviates the occurrence of errors at these two stages. As a result, a high quality of service can be achieved for a given end-to-end call between two mobile units. However, the standardized TFO approach provides no bandwidth advantage, as the full bandwidth ordinarily needed for the 64 kb/s sample stream is consumed for transmission of the compressed speech parameters.  
           [0008]    A more recent approach, called Transcoder-Free Operation (TrFO), uses out-of-band signaling to detect call scenarios involving tandem codecs at call set-up time. Thereupon action is taken to put in place a direct end-to-end link to provide for a direct exchange of the compressed speech parameters without the involvement of network transcoders. However, while it provides for a savings and resource reduction compared to the standardized TFO approach, the TrFO implementation suffers from the disadvantage of added cost and complexity due to, for example, the requirement for out-of-band signaling.  
           [0009]    From the above, it will be apparent that there is a need in the industry to provide a solution that is as robust and easy to implement as TFO, while providing the bandwidth and resource savings of TrFO.  
           [0010]    Moreover, the use of TFO has heretofore been limited to enhancing the quality of calls established between two TFO-enabled base station units in a mobile-to-mobile call. When one party is not a TFO-enabled base station unit, e.g., a telephone connected to a common packet-switched network via a network gateway, the use of TFO is not possible. It would therefore be an advantage to exploit the ability of one. party&#39;s TFO capabilities, even when the other party is not a TFO-enabled base station unit.  
           [0011]    In addition, the use of TFO is often limited by the use of backhaul gateways in a network, even when both parties to a call are TFO-enabled base station units. Such gateways compress speech samples into a different format prior to transmittal of the formatted speech samples over a network. Unfortunately, when TFO information is carried within the bit structure of the speech samples, the compression effected by a backhaul gateway results in loss of the TFO information and hence prevents advantageous usage of this facility. Hence, it would be beneficial to be able to allow tandem-free operation in circumstances where a backhaul gateway is used.  
           [0012]    For more information on the TFO and TrFO techniques, the reader is invited to refer to the following documents that are hereby incorporated by reference:  
           [0013]    3 rd  generation partnership project, Technical specification group core network, Out of band transcoder control—Stage 2 (3GPP TS 23.153 V4.4.0 (2001-12));  
           [0014]    3 rd  generation partnership project, Technical specification group core network, Bearer-independent circuit-switched core network, Stage 2 (3GPP TS 23.205 V4.4.0 (2002-03));  
           [0015]    3 rd  generation partnership project, Technical specification group (TSG) RAN3, Transcoder free operation (3GPP TR 25.953 V4.0.0 (2001-03));  
           [0016]    3 rd  generation partnership project, Technical specification group services and system aspects, In-band tandem free operation (TFO) of speech codecs, service description—Stage 3 (3GPP TS 28.062 V5.0.0 (2002-03));  
         SUMMARY OF THE INVENTION  
         [0017]    According to a broad aspect, the invention provides a data communication apparatus, including a port for enabling data communication with a remote entity via a network and a control entity in communication with the port. The control entity is operative to establish a connection with the remote entity over a first communication path through the network and negotiate with the remote entity using in-band signaling over the first communication path establishment of a second communication path between the data communication apparatus and the remote entity allowing the transmission of data from one of the data communication apparatus and the remote entity to the other of the data communication apparatus and the remote entity.  
           [0018]    The invention presents advantages from the standpoint of ease of implementation and bandwidth and resource savings. The use of an in-band messaging protocol to negotiate a establishment of the second communication path can be implemented generally in a straight forward manner. At the same time, the ability to transfer at least part of the connection to the second communication path avoids the drawbacks that would arise if that part of the connection were constrained to the first communication path. This feature allows the operator to take advantage of benefits provided by the second communication path but not available to the first communication path. Those benefits may include increased bandwidth, among others.  
           [0019]    In a specific and non-limiting example of implementation, the first communication path is a circuit-switched path. This path is used by the data communication apparatus to establish a tander-free data connection with the remote entity. Subsequently, the control entity negotiates with the remote entity to transfer the tandem-free data connection over a second communication path in a packet-switched network. The second communication path is defined by the address of the data communication apparatus and by the address of the remote entity.  
           [0020]    During the negotiation, the respective addresses are exchanged via in-band signaling over the first communication path. After the address exchange is effected and any other steps necessary to complete the establishment of the second communication path, the data communication apparatus starts sending data to the address of the remote entity and the remote entity starts sending data to the address of the data communication apparatus. At this point, the transfer of the tandem-free data connection is completed. The negotiation and establishment primarily use in-band signaling, although the use of out-of-band signaling is not excluded.  
           [0021]    In a specific and non-limiting example of implementation, the connection conveys audio information, such as a voice call.  
           [0022]    According to a second broad aspect, the invention provides a gateway, including an interface for allowing establishment of an end-to-end connection between a first remote entity and a second remote entity. The gateway also includes a control entity operative to monitor the end-to-end connection and detect the presence of in-band messages received from the first remote entity, the in-band messages being indicative of an attempt by the first remote entity to enter a tandem-free mode of operation. In the absence of an in-band response message from the second remote entity, the control entity is operative to generate and send an in-band response message to the first remote entity and negotiate therewith establishment of a second connection with the first remote entity, while maintaining the portion of the end-to-end connection between the gateway and the second remote entity.  
           [0023]    According to a third broad aspect, the present invention provides a gateway, including an interface for allowing establishment of a data connection between a first remote entity and a second remote entity. The gateway also includes a processing entity operative to convert data received from the first remote entity and destined for the second remote entity from a first format to a second format different from the first format. Furthermore, the gateway includes a control entity operative to monitor the data connection established between the first remote entity and the second remote entity, detect the presence of in-band messaging information among the data received from the first remote entity in the first format and destined for the second remote entity and cause the in-band messaging information to be sent to the second remote entity separately from the data in the second format.  
           [0024]    According to a third broad aspect, the present invention provides a gateway, including an interface for allowing establishment of a first connection to a first remote entity and a second connection to a second remote entity, the first connection being a TFO connection. The gateway also includes a control entity operative to monitor the second connection; detect the presence of TFO messages received from the second remote entity; and in the presence of in-band TFO messages received from the second remote entity, establish an end-to-end TFO connection between the first and second remote entities.  
           [0025]    These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    In the accompanying drawings:  
         [0027]    [0027]FIG. 1 illustrates an arrangement of network elements in accordance with an example of implementation of a first embodiment of the present inventive concept; and  
         [0028]    FIGS.  2  to  4  illustrate various arrangements of network elements in accordance with respective examples of implementation of a second embodiment of the present inventive concept; and  
         [0029]    [0029]FIG. 5 illustrates an arrangement of network elements in accordance with an example of implementation of a third embodiment of the present inventive concept; and  
         [0030]    FIGS.  6  to  8  illustrate an example of a call scenario in accordance with an example of implementation of a fourth embodiment of the present inventive concept. 
     
    
       [0031]    In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    [0032]FIG. 1 illustrates an arrangement of network elements in accordance with an example of implementation of a first embodiment of the present inventive concept. In this first embodiment, a data communication apparatus is equipped with the functionality to use an in-band messaging protocol in determining whether to transfer any part of an existing connection, which includes in-band messaging, to an alternate communication path.  
         [0033]    With particular reference to FIG. 1, there is shown a data communication apparatus  10 , which can be a base station controller (BSC) in a cellular network such as TDM/CDMA and GSM. In a UMTS network, the data communication apparatus  10  may be referred to as a “core network”. The data communication apparatus  10  includes a transcoder and rate adaptation unit (TRAU)  12 , which is a term used in this specification to generically refer to a resource capable of data or speech compression and/or decompression and preferably of rate adaptation. To this end, the TRAU  12  may include a vocoder, an echo canceller and other functional components (not shown). The data communication apparatus  10  also includes a control entity  22  in communication with the TRAU  12  and equipped with suitable circuitry, software and/or control logic for providing call setup and call processing functionality, such as notification of impending handover, three-way calls, and so on.  
         [0034]    The TRAU  12  includes an interface (not shown) for exchanging compressed speech parameters with a mobile unit  14  over a wireless link  16 . The TRAU  12  is also connected through the interface to a network  18  via a communication link  20 . In a specific example of implementation, the network  18  is a circuit-switched (time-division multiplexed) network across which speech samples are exchanged with a data communication apparatus  30 , e.g., in a format such as G.711, G.722 or G.726. In the specific case of G.711, speech samples are exchanged at a rate of 64 kb/s. The conversion from compressed speech parameters to speech samples and vice versa is effected by a vocoder (not shown) in the TRAU  12 . It should be understood that the network  18  may be a mixed circuit-switched and packet-switched network.  
         [0035]    In addition, the TRAU  12  is equipped with the capability of participating in an in-band messaging protocol. This may be an enhanced version of the standardized tandem-free operation (TFO) protocol, in which case the TRAU  12  can be said to be “enhanced-TFO-capable” or “eTFO-capable”. In the interest of clarity and simplicity, it will be assumed that the in-band messaging protocol is indeed an enhanced version of the standardized TFO protocol (hereinafter eTFO), since this would require only minor modifications to an existing standard. However, the reader skilled in the art will appreciate that there are myriad ways of implementing in-band messaging protocol without necessarily basing oneself on TFO, while remaining within the spirit of the present invention.  
         [0036]    A control entity (not shown) in the TRAU  12 , which is adapted to establish calls through the network  18 , additionally uses the in-band messaging protocol to identify the existence of another eTFO-capable TRAU at the other end of the call and to negotiate a transfer of a portion of the call to a second communication path as will be described herein below.  
         [0037]    Also shown in FIG. 1 is a second data communication apparatus  30 , which can also be a base station controller (BSC) in a cellular network. The data communication apparatus  30  includes a transcoder and rate adaptation unit (TRAU)  26 . In addition to the TRAU  26 , the data communication apparatus  30  includes a control entity  34  equipped with suitable circuitry, software and/or control logic for providing call setup and call processing functionality, such as notification of impending handover, three-way calls, and so on.  
         [0038]    For the purposes of this example, it is assumed that the data communication apparatus  10  is the calling party and that the data communication apparatus  30  is the called party, although the reverse may be the case without departing from the spirit of the present invention. It is also assumed that both TRAUs  12 ,  26  are eTFO-capable in order that a tandem-free connection is possible. Again, the relation between the in-band messaging protocol to TFO is made simply for convenience and need not be strictly adhered to.  
         [0039]    The TRAU  26  in the data communication apparatus  30  is connected to the network  18  via a communication link  28 , while it exchanges compressed speech parameters with a mobile unit  31  over a wireless link  32 . The TRAU  26  further includes a control entity (not shown) which is responsible for communicating with the control entity (not shown) of the TRAU  12  by means of the in-band messaging protocol.  
         [0040]    Moreover, in the embodiment illustrated in FIG. 1, the TRAUs are further connected to a common packet-switched network  42 . Specifically, TRAU  12  has a communication link  40  to the packet-switched network  42  while TRAU  26  has a communication link  44  to the packet-switched network  42 . Thus, it may be possible to establish an alternate communication path between TRAU  12  and TRAU  26  through the packet-switched network  42 . It should be understood that an alternate communication path may also be established through the circuit-switched network  18  or through another network different from the packet-switched network  42  and to which the TRAUs  12 ,  26  are connected.  
         [0041]    In operation, when a connection (e.g., a call) is set up between the data communication apparatus  10  and the data communication apparatus  30 , a circuit-switched communication path  38  is established within the network  18  between communication link  20  of TRAU  12  and communication link  28  of TRAU  26  for the purposes of transmitting speech samples. In accordance with one example of a suitable in-band messaging signaling protocol, the TRAUs  12 ,  26  are eTFO-capable and TFO setup information and TFO speech information can be exchanged using different subsets of bits from among the bits ordinarily used for transmission of speech samples between the TRAU  12  and the TRAU  26  via circuit-switched communication path  38 , a process commonly referred to as bit stealing.  
         [0042]    By virtue of the in-band messaging signaling protocol, each TRAU  12 ,  26  will receive TFO setup information from the other TRAU, which will indicate to the recipient TRAU that a remote TRAU is attempting to enter a tandem-free mode of operation. During the negotiation process, various parameters may be exchanged between the TRAUs  12 ,  26  prior to effecting switch-over of a portion of the circuit-switched communication path  38  to a second communication path  46  (e.g., using asynchronous transfer mode adaptation layer  2 —AAL 2 ) through the packet-switched network  42 . Example of messaging format may be ETSI Standard AMR or EFR.  
         [0043]    For example, each TRAU  12 ,  26  will use the in-band messaging protocol to indicate to the other TRAU whether it has access to the packet-switched network  42 . If both TRAUs  12 ,  26  have a link to the packet-switched network  42 , as is the case in FIG. 1, addresses may be exchanged to allow the transmission of either the compressed or uncompressed speech signal in packet format over the second communication path  46  established through the packet-switched network  42 , as defined by the addresses of the two data communication apparatus  10 ,  30 . Another example of TFO setup information includes a list of codecs supported by the TRAU providing the information. Also during the negotiation process, information could be sent to each of the control entities  22 ,  34  in order to arrange for required changes in the routing of the packets.  
         [0044]    Once the second communication path  46  has been established, part of the connection established via the communication path  38  is transferred to the second communication path  46 . Such transfer may be done in several ways.  
         [0045]    In a first variant, transmission of speech over the second communication path  46  takes place in compressed format, i.e., both TRAUS  12 ,  26  exchange TFO speech information with one another over the packet-switched network. If this is done while suspending the transmission of speech samples via the circuit-switched communication path  38  through the network  18 , this will allow the codecs in both TRAUs  12 ,  26  to be disabled, resulting in resource savings. On the other hand, it may be desirable to continue exchanging speech samples along the circuit-switched communication path  38 , even if only a reduced number of fixed-duration time slots are used. This may be done in the interest of maintaining synchronization between the two TRAUs  12 ,  26  in the event that the second communication path  46  fails and communication must revert back to use of the circuit-switched communication path  38  through the circuit-switched network  18 . Still other variants will retain the circuit-switched connection path  38  in its entirety in order to perform voice quality enhancement functions.  
         [0046]    In a second variant, it is within the scope of the invention to transfer speech samples in their decompressed format (e.g., G.711) across the second communication path  46 . Thus, it will be appreciated that even though the second communication path  46  is established on the basis of the in-band messaging protocol revealing that both TRAUs  22 ,  26  are eTFO-capable and share access to the packet-switched network  42 , it is not a requirement that TFO speech information be sent along the second communication path  46 .  
         [0047]    Those skilled in the art will further appreciate that when necessary, the data format can be altered in a dynamic fashion to meet any particular requirements, such as transmission of dual-tone multi-frequency (DTMF) signals, etc.  
         [0048]    FIGS.  2  to  4  illustrate various arrangements of network elements in accordance with respective examples of implementation of a second embodiment of the present inventive concept. In this second embodiment, a gateway connected to a non-eTFO-capable entity is equipped with the intelligence to emulate a eTFO-capable entity. With particular reference to FIG. 2, TRAU  12  proceeds to send TFO setup information in an attempt to communicate with a remote entity  260  via a gateway  220 . This is effected over a circuit-switched communication path  230  established through a network  240 . The gateway  220  monitors the messages but, in anticipation of a response from remote entity  260 , it does not respond.  
         [0049]    After a timeout period, recognizing that the entity connected at the other end is not eTFO-capable, the gateway  220  can proceed to initiate its own response, with the ensuing handshaking resulting in the transmission of TFO speech information through a packet-switched communication path  250  established through the network  240 . The gateway  220  includes a codec and an internal control entity similar to the internal control entity in the TRAU  12  described earlier with reference to FIG. 1. Note that the signal processing functionality previously associated with the TRAU  12  has been shifted to the gateway  220 . In addition to coding and decoding, such functionality may include echo cancellation, automatic gain control and so on.  
         [0050]    With particular reference to FIG. 3, there is shown a connection between 3G and 2G wireless networks. In this case, a call  360  is established between a mobile unit  310  (e.g., a UMTS mobile unit) and another mobile unit  320  (e.g., a GSM mobile unit) through a network  350 . For this example, it is assumed that the GSM mobile unit  320  has a connection to the network  350  via a GSM TRAU  330 . Ultimately, the execution of the in-band messaging protocol, as described earlier with reference to FIG. 2, will lead to transfer of the rate adaptation operation from TRAU  12  to gateway  220  and also to the transfer of traffic to a packet-switched communication path  340 , resulting in minimization of the transmission bandwidth between the two nodes. In addition, execution of the in-band messaging protocol will result in establishment of tandem-free operation between gateway  220  and the GSM TRAU  330  and thus a virtual end-to-end tandem-free operation for the communication.  
         [0051]    According to one variant, the gateway  220  detects the TFO setup information exchanged between the GSM TRAU  330  and TRAU  12 , but will not react until those negotiations are concluded. However, the GSM TRAU  330  in this example is not linked to a packet-switched network, and thus the protocol will advance only to the extent of tandem-free operation. Gateway  220  can monitor the process to recognize that the full optimization has not been achieved. It can then carry out a dialog with the TRAU  12  to transfer the rate adaptation operation to gateway . 220  and transfer the tandem-free connection to a packet-switched communication path  340  through the network  350 , thus reducing the transmission bandwidth between the two nodes.  
         [0052]    According to another variant, the gateway  220  detects the TFO setup information messages exchanged between the GSM TRAU  330  and TRAU  12  and recognizes that the remote GSM TRAU  330  is incapable of enhanced TFO (eTFO) Gateway  220  will then engage in a two-way handshaking with TRAU  12  and the GSM TRAU  330  to transfer the rate adaptation operation from TRAU  12  to gateway  220 , and to exchange TFO speech information with the GSM TRAU  330 .  
         [0053]    With particular reference to FIG. 4, there is shown a more complex scenario for the signal path, where an original connection originates from a TRAU  420 , traverses a packet-switched network  430  and gateways  440 ,  450 , before connecting to a second TRAU  480  back through the packet-switched network  430 . Once the in-band messaging protocol is exercised through to the exchange of the addresses of the two TRAUs  420 ,  480 , a second path  460  through the packet-switched network  430  is chosen to continue the transmission of the traffic signal. The handshaking sequence is as follows: TRAU  420  and TRAU  480  initiate the in-band messaging protocol, identifying themselves as “endpoint” units. The in-path gateways  440 ,  450  recognize the exchange between two end-point TRAUs  420 ,  480  and allow the transfer to take place.  
         [0054]    [0054]FIG. 5 illustrates an arrangement of network elements in accordance with an example of implementation of a third embodiment of the present inventive concept. According to this third embodiment, a “backhaul” gateway that employs a codec format that is incompatible with standardized tandem-free operation is given the intelligence to allow tandem-froo operation to take place and reduce bandwidth.  
         [0055]    With particular reference to FIG. 5, there is shown a network configuration, in which a TRAU  510  is connected to a remote entity, in this case a mobile switching center (MSC)  520  through a pair of “backhaul” gateways  530 ,  540  at either end of a network  550 . Such gateways  530 ,  540  are likely to operate codecs such as G.729, G.726, or G723.1, which are not compatible with tandem-free operation. In particular, tandem-free operation is facilitated when certain specified bits of a G.711 sample stream are used to transmit the TFO setup information or the TFO speech information. However, the use of a codec that manipulates the G.711 sample stream is likely to distort the information contained therein.  
         [0056]    This will result in the tandeming of two codecs in land-mobile connections, and at least three codecs in mobile-mobile calls. One way to avoid this problem is to provide the backhaul gateways  530 ,  540  with the intelligence to recognize and support the in-band messaging protocol. If this case, transfer of the TFO speech information would be exchanged without bit-stealing the data in the incompatible format exchanged between the backhaul gateways  530 ,  540 . The TFO speech information could then be carried from, say, backhaul gateway  530  to backhaul gateway  540 , whereupon it will be injected back into the G.711 sample stream in place of the incompatible transcoding in backhaul gateways  530  and  540 .  
         [0057]    The mechanism just described with reference to FIG. 5 permits the various scenarios described herein above with reference to FIGS.  1 - 4  to reach their optimal mode of operation despite the presence of backhaul gateways  530 ,  540  with incompatible codecs. For example, in a call scenario that involves a gateway connected to a circuit-switched network, the gateway may need to be provided not only with the functionality to bypass an incompatible codec as described in connection with FIG. 5, but also with the functionality described in connection with FIG. 2, wherein the gateway acquires the functionality of a TRAU, hence allowing enhanced TFO (eTFO) to take place. In such a case, signal processing functionality can be shifted from the TRAU to the edge of a network.  
         [0058]    FIGS.  6  to  8  illustrate an example of a call scenario in accordance with an example of implementation of a fourth embodiment of the present inventive concept. According to this fourth embodiment, an in-band eTFO connection is used as a backup connection while speech samples are transmitted over a packet-switched network. With particular reference to FIG. 6, a call is to take place between parties via two gateways  610 ,  620  both located in City A. Both gateways  610 ,  620  have access to a circuit-switched network  630  that is configured in such a way as to require the call to be routed through City B. The data format exchanged over the network  630  is assumed to be G.711 for the purposes of the present example, although other formats are possible. In addition, both gateways  610 ,  620  are linked via a packet-switched network, say an ATM network  640 .  
         [0059]    [0059]FIG. 6 illustrates the situation during call initiation. The call starts in the normal way with an inter-city path  650  being established over a network  630 . Each gateway  610 ,  620  thus exchanges G.711 data via City B over the path  650 , without involving the packet-switched network  640 . Once the call is established, either one or both gateways  610 ,  620  start probing the path  650  by way of the in-band messaging protocol in order to identify peers, i.e., to determine whether another gateway along the path  650  is also eTFO-compatible. In this case, it is assumed that the gateways  610 ,  620  identify one another as peers and that the gateways  610 ,  620  proceed to establish an in-band eTFO connection over the path  650 .  
         [0060]    [0060]FIG. 7 illustrates the situation once an in-band eTFO connection has been established between the gateways  610 ,  620  via City B. Specifically, the path  650  carries the G.711 data as well as in-band messaging information. However, the in-band messaging information may consist of a reduced amount of in-band messaging information as compared with that required to transmit TFO speech information. In other words, the in-band eTFO connection -may involve the transmission of “dummy” frames, where by “dummy frame” is meant a frame sent by one of the gateways  610 ,  620  that the other gateway will recognize such that the in-band eTFO connection will be maintained, i.e., not dropped. The objective of the eTFO connection in this particular embodiment is to keep the connection over path  650  alive so as to maintain a path that can be used as a fallback position in the event of a disturbance, as will be described in greater detail herein below.  
         [0061]    At this point, the gateways  610 ,  620  proceed to transfer the portion of the connection containing speech samples over to the packet-switched network  640 . The purpose of this negotiation process, which may require out-of-band resources, is for the gateways  610 ,  620  to establish a “short-cut” path therebetween by passing through the packet-switched network, which does not pass through City B.  
         [0062]    [0062]FIG. 8 illustrates the situation when the portion of the path  650  containing G.711 speech samples has been transferred to the short-cut path  660 . The G.711 data now flows through the packet-switched network  640 . Meanwhile, the in-band eTFO connection over the path  650  is still kept alive by sending only basic signaling information. It will be appreciated that the bandwidth used by this residual eTFO connection is small.  
         [0063]    In a scenario wherein the entity at City B via which the eTFO connection is maintained “disturbs” the call such as by attempting a call conferencing or call transfer operation then operation returns to the scenario at FIG. 6, where the G.711 data flow is routed via City B and the connection through the packet-switched network  640  is severed.  
         [0064]    Although various ways of negotiating the establishment of a second communication path using in-band signaling have been described, it is to be understood that variations of the present invention in which recourse is had to out-of-band signaling are within the scope of the present invention. Moreover, it is to be appreciated that once negotiation is complete, the actual establishment of the second communication path may also involve out-of-band resources.  
         [0065]    It will also be appreciated that the functional elements of the TRAUs and gateways described above may be implemented as an arithmetic and logic unit (ALU) having access to a code memory which stored program instructions for the operation of the ALU. The program instructions could be stored on a medium which is fixed, tangible and readable directly by the TRAU or gateway, (e.g., removable diskette, CD-ROM, ROM, or fixed disk), or the program instructions could be stored remotely but transmittable to the TRAU or gateway via a modem or other interface device (e.g., a communications adapter) connected to a network over a transmission medium. The transmission medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented using wireless techniques (e.g., microwave, infrared or other transmission schemes).  
         [0066]    Those skilled in the art should also appreciate that the program instructions stored in the code memory can be compiled from a high level program written in a number of programming languages for use with many computer architectures or operating systems. For example, the high level program may be written in assembly language, while other versions may be written in a procedural programming language (e.g., “C”) or an object oriented programming language (e.g., “C++” or “JAVA”).  
         [0067]    Those skilled in the art will further appreciate that in some embodiments of the invention, the functionality of the TRAUs and gateways may be implemented as pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components.  
         [0068]    While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.