Abstract:
Providing a systematic and comprehensive mechanism for applying echo cancellation within a telecommunication switching system by a local switching system such as a PBX. Echo cancellation circuits are deployed throughout the telecommunication switching system using different types of echo cancellation circuits with each type having different capabilities with respect to a time offset of an echo return signal relative to when an voice signal was generated by a talker. The echo cancellation circuits deployed within the local telecommunication switch are an integral part of trunk circuits and provide the largest time offset. Further, the echo cancellation circuits deployed within in the local telecommunication switch are capable of controlling echoes in either direction with respect to the integral trunk circuit and may be used as service circuits if not needed by the integral trunk circuit. A systematic and comprehensive mechanism further comprises terminating echoes at the edges of networks having long transmission delays such as a wide area network (WAN) comprising a combination of ATM and/or IP switching networks. The mechanism enables uniformity in policing echoes at the “near end” at the appropriate points in the PBX network or in a network of PBX&#39;s.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to telecommunications switching systems and, in particular, to providing echo control across local exchange carrier systems, interexchange carrier systems, and business communication switching systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Within the prior art, echoes within telephone switching systems are well known. Such echoes are normally caused by hybrid unbalanced conditions at a four-wire to two-wire conversion points in a local exchange carrier network or a telephone set or both. In addition, within a telephone set, acoustical feedback can cause echoes as well. There are two basic factors that determine whether echoes are perceived by humans or not. These two factors are highly interrelated. The first factor is the signal level of the echo return signal (also referred to as acoustic to acoustic echo path loss) which is defined as the level of the returned echo signal relative to the transmitted voice signal. The second factor is the time offset of the echo return signal relative to when the voice signal was generated by the talker. FIG. 1 illustrates in graphic form the manner in which loss (manifested in the relative strength of an originating signal and the strength of the returned echo signal) can be utilized to migrate the affects of echo. The lines such as lines  101  and  102  illustrate the echo path delay in milliseconds plotted against the acoustic-to-acoustic echo path loss in dB along the horizontal scale. Plotted on the vertical scale is the rating given by an average group of listeners with the percentage indicating the members of the group who believe that the resulting speech was good or better. The definition and use of the average group is defined in the book entitled Transmission Systems for Communications, Bell Telephone Laboratories, 5 th  Edition, 1982. Examining line  102 , it can be seen that an average group of listeners finds an echo of 5 milliseconds very acceptable if the difference in the echo path loss is in excess of 30 dB. Conversely, if the echo path delay is 5 milliseconds and there is no loss, line  102  shows that only 30 percent of an average group would find this an acceptable telephone conversation. Even for a large echo delay of 1200 milliseconds as illustrated in line  103 , if the echo path loss is 60 dB, 90 percent of an average group find that this amount of delay is acceptable. Contrast this against 1.5 milliseconds of echo path delay as illustrated by line  101  with no echo path loss. In this situation, only 70 percent of an average group would find acceptable a delay of 1.5 milliseconds with no echo path loss.  
           [0003]    The human perception of echoes verses echo path loss has been well understood within the telephone industry for many years. The designers of prior art telephone switching systems have utilized the manipulation of path loss (referred to as the loss plan technique) to mitigate negative human perception of echoes. The loss plan technique was particularly effective when the national telephone system was controlled by the Bell System. The Bell System was able to implement the loss plan technique effectively. This technique was also aided by the fact that the majority of the prior art telephone switching equipment was circuit-switched equipment or time division multiplex, both of these types of switching systems have low delay times (on the order of a few milliseconds), because of this, the loss plan technique was capable of controlling the perception of echoes.  
           [0004]    However, even in prior art switching systems, it has been necessary from time to time to utilize external echo cancellation circuits for severe cases. Indeed, the perceptual effects of echoes due to time offset as well as a high echo return signal are known. When echo returns are high, but delay is low, the perceptual effect is a side tone effect similar to the high side tones experienced in some European countries. On the other hand, the barrel perceptual effect which is encountered when two telephone sets are offhook at the same time occurs from relatively low time offsets in the range of 30-40 msec. When delays in the echo path are long, the perceptual effect is similar to the effect of bouncing ones voice off a mountain.  
           [0005]    Echo cancellers (also referred to as echo cancellation circuits) for switching networks are normally finite impulse response digital filters that are implemented using DSP or ASIC circuits. These filters have the advantage that the device resources needed are roughly linearly proportioned to the echo cancellation tail length. An echo cancellation tail length is the time period relative to the reference between the end of the speech burst at the transmitting end and receipt of the end of the echo return at the transmitting end. The cost of an echo canceller is determined to a large extent by the length of the echo cancellation tail for which the echo canceller can compensate. Because the cost of echo cancellers increases as the echo tail length capability increases, it is highly desirable not to utilize echo cancellers that have an echo cancellation tail length greater than what is needed. Another type of echo canceller is an infinite impulse response filter which requires fewer resources than the finite impulse response digital filter but has stability problems.  
           [0006]    The prior art telephone switching systems have approached the echo problem in two basic ways. The first is that adopted by the interexchange carriers which is to put an echo canceller on every link going to the local exchange carriers. The second method that has been adopted by most PBX (also referred to as business communications systems or enterprise switching systems) manufacturers has been to add echo cancellers to links to a local exchange carrier only when the need has arisen in the field. The technique utilized by the interexchange carriers is economic for these carriers since their connection to the local exchange carriers is only via high capacity digital trunks. Interexchange carriers deploy echo cancellers at the point of termination between their networks and local exchange carrier networks to avoid having problems with echoes generated in the local exchange networks being perceived by users as an interexchange carrier problem. For a variety of reasons that are described in the following paragraphs, PBX manufacturers are not free because of economic constraints to adopt the method used by the interexchange carrier nor will their prior art technique of adding echo cancellers on a need based scheme work either. A PBX is in many cases placed in the network between a local exchange carrier and an interexchange carrier. A PBX experiences the same echo environment as that seen by an interexchange carrier, and could be indicted by users as causing echo problems which actually occur in local exchange carrier networks. If not dealt with by the PBX, then, these problems are perceived by customers as being problems within the PBX.  
           [0007]    PBX and other types of intermediate switch manufacturers face a number of problems with respect to echoes due to the changing environment in which PBXs are being used. The prior art PBX normally connected to telephones that were part of the PBX system (referred to as intercom telephones), local exchange carriers and occasionally to interexchange carriers. However, the prior art PBXs rarely were utilized to communicate a number of calls from a telephone connected to the local exchange carrier to an interexchange carrier. In this case, the PBX resides between the local exchange carrier and the interexchange carrier, and the echo problems of the local exchange carrier are assumed by the customers to be caused by the PBX. Where in reality, the problem is in the local exchange carrier with the delay through the interexchange carrier simply making these echoes perceptually more pronounced. One such situation is where the PBX is used as a call center system and has a number of remote call center agents connected through a local exchange carrier to the PBX. The PBX is receiving “800” type calls from the interexchange carrier and then is re-routing these calls via the local exchange carrier to the remote call center agents. The problem becomes particularly severe where the PBX is interconnected to the local exchange carrier via analog trunks.  
           [0008]    [0008]FIG. 2 illustrates a prior art situation where PBX  201  and PBX  203  utilize a connection via local exchange carrier  202  to form a PBX network. The problem occurs in an example where telephone  218  of local exchange carrier  206  is engaged in a telephone call with telephone  212  local exchange carrier  204  via PBX  203 , local exchange carrier  202  and PBX  201 . If local exchange carrier  204  has an excessive amount of echo path delay this echo path delay is accentuated for a user of telephone  218  and may be attributed by the user of telephone  218  as a defect in PBX  203 . In addition, local exchange carrier  206  may also have excessive echo path delay, and the problem is compounded for both telephones  212  and  218  with the user of each telephone assuming that their respective PBX is malfunctioning.  
           [0009]    Another situation where PBXs are exposed to the echoes originating in local exchange carriers causing problems is where the PBX utilizes an ATM network or an IP connection to complete a call from the PBX to a distant station. An IP connection in particular introduces a large delay into the transmission path due to switching and encoding times.  
         SUMMARY OF THE INVENTION  
         [0010]    The aforementioned problems are solved and a technical advance is achieved in the art by an apparatus and method that provide a systematic and comprehensive mechanism for applying echo cancellation within a telecommunication switching system by a local switching system such as a PBX. Echo cancellation circuits are deployed throughout the telecommunication switching system using different types of echo cancellation circuits with each type having capabilities with respect to a time offset of an echo return signal (echo tail length) relative to when a voice signal was generated by a talker. Advantageously, the echo cancellation circuits deployed within the local telecommunication switch having the largest echo tail lengths are an integral part of digital trunk circuits. Further, the echo cancellation circuits deployed within in the local telecommunication switch are capable of controlling echoes in either direction with respect to the integral trunk circuit and may be used as service circuits if not needed by the integral trunk circuit.  
           [0011]    Advantageously, the systematic and comprehensive mechanism further comprises terminating echoes at the edges of networks having long transmission delays such as a wide area network (WAN) comprising a combination of ATM and/or IP switching networks. For example, a remote network controller or soft phone eliminates echoes before transmitting voice information into the WAN. Similarly, a PBX eliminates echoes in voice information received from local exchange carriers before transmitting the voice information into the WAN via the PBX. Advantageously, the termination of echoes at the edges of networks having long transmission delays allows for the use of echo cancellation circuits having shorter echo tail lengths than if echo cancellation circuits were used to terminate the echoes after transmission through the networks having long transmission delays. The method of the present invention enables uniformity in policing echoes at the “near end” at the appropriate points in the PBX network or in a network of PBX&#39;s.  
           [0012]    Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    [0013]FIG. 1 illustrates a prior art graph that defines the relationship between human perception of echoes with respect to echo path loss and echo path delay;  
         [0014]    [0014]FIG. 2 illustrates a prior art switching system configuration;  
         [0015]    [0015]FIG. 3 illustrates, in block diagram form, a system for implementing an embodiment the invention;  
         [0016]    [0016]FIG. 4 illustrates, in block diagram form, a system for implementing an embodiment the invention;  
         [0017]    [0017]FIG. 5 illustrates, in block diagram form, an echo cancellation circuit;  
         [0018]    FIGS.  6 - 9  illustrate, in flow chart form, operations of a PBX; and  
         [0019]    [0019]FIG. 10 illustrates, in block diagram form, an IP Trunk for implementing an embodiment of the invention 
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 3 illustrates a system for implementing the invention. Local exchange carriers  311  and  329  comprise local offices  319  and  321 - 324  plus telephones connected to these local offices. Interexchange carrier  312  may be a plurality of interexchange carriers such as AT&amp;T or Sprint. Wide area network (WAN)  313  can be a variety of communication media including but not limited to the Internet or an Intranet of a corporation in combination with ATM switching networks. PBX  300  comprises control computer  301 , switching network  302 , line circuits  303 , telephone  327 - 328 , Internet Protocol (IP) trunk  308 , ATM trunk  307 , and trunks  304 ,  306 , and  309 . PBX  300  also provides control and switching for remote switching network  316  and its attached soft phones  317  and  318 . Remote switching network  316  has analog telephones, digital telephones and softphones connected to it. Soft phone  314  is also capable of being connected through WAN  313  to PBX  300 .  
         [0021]    In FIG. 3, WAN  313  comprises ATM switches and IP switching and transmission elements. WAN  313  has the longest transmission delays for transmitting voice information of any of the units illustrated in FIG. 3. In WAN  313 , ATM switches have transmission delays measured in tens of milliseconds; whereas, IP switches and elements in the aggregate may have transmission delays measured in hundreds of milliseconds. As was set forth in the background of the invention section, the effects of an echo in voice information being transmitted through WAN  313  results in a variety of undesirable acoustic effects. In accordance with one aspect of the invention, the interfaces and trunk elements which interface with WAN  313  eliminate the echoes before transmission of the voice information to WAN  313 . Softphone  314  provides an echo cancellation circuit having an echo tail length of advantageously four milliseconds so as to eliminate the effects of an acoustic echo in the handset utilized with softphone  314 . Thus, softphone  314  eliminates the near-end echo caused by acoustic echoes before these echoes are transmitted to WAN  313 . Remote switching network  316  provides near-end echo cancellation for the analog and digital telephone connected to it before transmitting voice information via WAN  313 . Since the echo cancellation functions provided by remote switching network  316  are designed to eliminate echoes resulting from the attached telephones, these echo cancellation functions can have echo tail length capabilities of approximately eight milliseconds since the echo path lengths to these sets are short.  
         [0022]    Within PBX  300 , ATM trunk  307  and IP trunk  308  provide, advantageously, echo cancellation circuits that have adjustable tail lengths so that the tail length for each individual call can be adjusted. FIG. 10 illustrates IP trunk  308  in greater detail. ATM trunk  307  would be of a similar design as that for IP trunk  308  with the similar external interface circuit to external interface circuit  1003  being modified for the ATM protocol requirements that are well known to one skilled in the art. The adjustable echo tail lengths allow a echo cancellation circuit to provide echo cancellation functions for a larger number of calls. Control computer  301  determines the adjustable echo tail length in an echo cancellation circuit in trunks  307  or  308  based on where the call is coming from. For example, telephone  326  attached to local exchange carrier  329  may require a longer tail length than the tail length required for telephone  327  attached directly to PBX  300  via line circuit  303 . Note that a later example sets forth the proposition that neither telephone  327  nor  328  causes any echo; however, it is well-known to those skilled in the art that a telephone connected directly to a PBX may be at some distance from the PBX and may indeed have an echo. For example, if telephone  327  is an analog set, a four wire to two wire hybrid function is provided in line circuit  303 . Note, that the four wire refers to the environment within switching network  302  or on digital trunks such as digital trunk  304 . Here, both directions of transmission are separate; where as a two wire refers to the connection to telephone  327  where both direction of the transmission simultaneously occur over the same pair of wires. It is at the hybrid, where four to two wire conversion occurs, that, if the hybrid is not properly balanced, echo returns can occur.  
         [0023]    Because of the large number of calls that can be handled by ATM trunk  307  and IP trunk  308 , the echo cancellation circuits of these trunks may at times lack the processing capability of providing long enough echo tails for all of the calls being handled via these trunks. In that case, as is discussed in the next paragraph, the capabilities of digital trunks  304  and  309  can be utilized to provide echo cancellation functions for some of the calls being processed by ATM trunk  307  and IP trunk  308 .  
         [0024]    To provide near-end cancellation of the echoes being received from local exchange carrier  329  via trunks  338  and  339 , digital trunks  309  and  304  are utilized. These digital trunks have echo cancellation circuits that have advantageously maximum echo tail lengths of 96 milliseconds that should handle the echoes resulting from local exchange carrier  329  before the voice information is transmitted via PBX  300  from local exchange carrier  329  to WAN  313 . Note, one skilled in the art could readily envision that the echo tail lengths cancellation capabilities provided by digital trunks  304  and  309  could be increased. The echo cancellation circuits of the digital trunks are also controlled by control computer  301  to provide echo tails of varying lengths. This capability allows the echo cancellation circuits to provide the echo cancellation functions for a larger number of calls than if the echo tail length was fixed. The echo return delays encountered at the interface to trunks  338  and  339  connected to the local exchange carriers may in the majority of calls be of moderate or short lengths. However, because policies and policies of echoes within a given local exchange carrier are not uniform and are for the most part not subject to regulation, pathological echo paths may be encountered on given calls where delay paths can be long resulting in the need for long tail length cancellation to cover all cases. As will be described later, calls being received via analog trunk  306  utilize the echo cancellation circuits of digital trunks  304  or  309  as needed to provide near-end echo cancellation. In addition, if a call received via analog trunk  306  from local exchange carrier  329  is being transported through switching network  302  to WAN  313  via either ATM trunk  307  or IP trunk  308 , then the echo cancellation functions of trunks  307  and  308  can be utilized to provide echo cancellation functions.  
         [0025]    As is well-known in the art, interexchange carrier  312  normally provides echo cancellation with respect to local exchange carrier  311 . However, if interexchange carrier  312  does not provide echo cancellation, then, PBX  300  may use digital trunks  304  and  309 . In addition, ATM trunk  307  and IP trunk  308  can be utilized if the echo being received from interexchange carrier  312  is within the echo tail length capacity of the echo cancellation circuits of these trunks. Note, it is possible to have the echo cancellation circuit of digital trunk  309  or digital trunk  304  cascaded with the cancellation circuits of either ATM trunk  307  or IP trunk  308 . In this situation, both echo cancellations perform their normal operation and do not interfere with each other. The echo canceller first traversed in a connection does the initial cancellation, if it has sufficient echo tail length cancellation capabilities relative to the echo encountered. Consider now in greater detail the trunk circuits of PBX  300 . Analog trunk  306  is a prior art analog trunk that has no echo cancellation within the trunk itself. Within the prior art, if objectionable echoes should be encountered by the communication of calls from analog trunk  306  to local exchange carrier  329 , an external echo canceller would be inserted in link  339 . Digital trunks  304  and  309  provide integrated echo cancellation circuits that have long echo cancellation tail lengths which advantageously may be  96  msec. Interexchange carrier  312  may provide sufficient echo cancellation for its connections to local exchange carrier  311 . However, local exchange carriers  311  and  329  may not provide adequate echo cancellation in their internal or external operations. Nor, in general, does WAN  313  provide any echo cancellation functions.  
         [0026]    Echo cancellation circuits  332  and  334  of digital trunks  309  and  304 , respectively, are versatile. For example, echo cancellation circuit  332  of digital trunk  309  can cancel echoes being caused by communication through local exchange carrier  329  to telephone  326  via local offices  322 - 324 . This type of echo cancellation is referred to as forward echo cancellation because it is eliminating echoes being received on the digital trunk&#39;s outgoing link  338 . Echo cancellation circuit  334  is identical in design to echo cancellation circuit  332 . In addition, echo cancellation circuits  332  and  334  can be utilized to eliminate echoes in the reverse direction. This type of echo cancellation is referred to as reverse echo cancellation because it is eliminating echoes being received from switching network  302 . The following is an example of echo circuit  334  being utilized in the reverse direction. If analog trunk  306  is communicating a call from telephone  326  in local exchange carrier  329  to interexchange carrier  312 , it is necessary to eliminate the echo being caused by local exchange carrier  329  in PBX  300 . This is done by echo circuit  334  eliminating the echo that is received via switching network  302 , analog trunk  306  and local exchange carrier  329 .  
         [0027]    In addition, not every call being communicated through digital trunk  304  requires echo cancellation nor is it provided as is determined by control computer  301 . For example, if telephone  327  is communicating a call to telephone  336  of local exchange carrier  311 , interexchange carrier  312  eliminates any echo resulting from local exchange carrier  311 . Hence, echo cancellation circuit  334  does not provide any echo cancellation; thus, saving valuable resources that can be utilized to cancel other echo sources.  
         [0028]    Not only can echo cancellation circuits  332  and  334  be utilized both in the forward and reverse direction but they can be utilized as service circuits for eliminating echoes in calls not being communicated by their respective digital trunks. An example of this type of echo control is when telephone  327  is on a call with telephone  326  of local exchange carrier  329  via local offices  323 - 324 , analog trunk  306 , switching network  302 , and line circuits  303 . To eliminate the echo caused by local exchange carrier  329 , the output of analog trunk  306  into switching network  302  is routed to echo cancellation circuit  334  via switching network  302 . Echo cancellation circuits  334  eliminate the echo before the path is returned to switching network  302  and switched to telephone  327 . Advantageously, digital trunk  304  can be eliminating echoes for a variety of calls being performed by analog trunk  306  as well as supplying any necessary echo cancellation with respect to interexchange carrier  312 . Note, this is also true of digital trunk  309 . Similarly, digital trunks  304  and  309  can provide additional echo cancellation for ATM trunk  307  and IP trunk  309 .  
         [0029]    Advantageously, the amount of echo cancellation provided by PBX  300  with respect to local exchange carriers  311  and  329  can be tailored to the amount of echo being received back by using different types of trunk circuits or by providing no echo cancellation. For example, connections via local office  323  to telephones such as telephone  325  may not have any echo; whereas, calls routed via local office  324  to a telephone connected directly to it such as telephone  326  may require echo cancellation. Control computer  301  can utilize the information of where the call is being routed within local exchange carrier  329  to provide or not provide echo cancellation.  
         [0030]    [0030]FIG. 4 illustrates PBX  300  placed in the prior art situation that is depicted in FIG. 2. PBX  300  is communicating through a private line to PBX  401  via local exchange carrier  402 . PBX  401  has only analog trunks to interconnect to local office  319  and local office  321  of local exchange carrier  311 . As previously described with respect to FIG. 3, PBX  300  interconnects to local exchange carrier  329  via analog trunk  306  and digital trunk  309 . For sake of example, it is assumed that PBX  401  does not have external echo cancellation circuits in its connections to local exchange carrier  311 . As a first example, consider the situation where telephone  325  is communicating via local office  323  and analog trunk  306  on a telephone call established by PBX  300  to telephone  336  via digital trunk  304 , local exchange carrier  402 , PBX  401  and local office  319  of local exchange carrier  311 . Assume that no echo is present on the path from switching network  302  to telephone  325 , but an echo does occur in local exchange carrier  311  with respect to telephone  336 . PBX  300  eliminates this echo by utilizing echo cancellation circuit  334  in the forward direction to eliminate the echo that is being received back from local exchange carrier  402  that is actually caused by local exchange carrier  311 . By PBX  300  utilizing digital trunk  304  in the forward direction, a user of telephone  325  has no perception of an echo having occurred.  
         [0031]    Consider a second example that also illustrates the utilization of the echo cancellers in PBX  300 . Consider where a telephone call is established from telephone  326  via local office  324 , local office  323 , analog trunk  306  by PBX  300  to telephone  336  via digital trunk  304 , local exchange carrier  402 , PBX  401 , and local office  319  of local exchange carrier  311 . In this example, local exchange carrier  329  causes an echo on its portion of the telephone path, and local exchange carrier  311  introduces an echo in its portion of the telephone path. PBX  300  eliminates the echo resulting from local exchange carrier  311  by utilizing digital trunk  304  in the forward direction. To eliminate the echo being caused by local exchange carrier  329 , PBX  300  utilizes a portion of echo cancellation circuit  332  as a service circuit to eliminate the echo being received by analog trunk  306  being received from local office  323 . This is done by the voice communication received from local office  323  being routed through switching network  302  to echo cancellation circuit  332  which eliminates the echo and transmits the results back through switching network  302  to digital trunk  304 .  
         [0032]    [0032]FIG. 5 illustrates, in block diagram form, echo cancellation circuit  332  of FIGS. 3 and 4. Echo cancellation circuits  334  of FIG. 3 and echo cancellation circuit  1002  of FIG. 10 are of a similar design. DSP  501  is illustrated as being a single DSP, but could be a group of DSPs. The echo canceller operations are performed by implementing a finite impulse response digital filter within DSP  501 . The implementation of a finite impulse response digital filter for performing echo cancellation is well known by those skilled in the art. DSP  501  is responsive to information from control computer  301  designating the length of the echo tail to allocate resources to perform an echo canceller function that has a tail length as determined by control computer  301 . Principally, the determination of the echo tail length is one of allotting memory within DSP  501  that is to be used to perform a particular echo cancellation function on a particular call. DSP  501  controls the operations of the different circuit blocks illustrated in FIG. 5 utilizing control information received from control computer  301  via interface  504  and cable  508 . DSP  501  also transmits control information to control computer  301  via the same path. DSP  501  is also performing all of the control functions required of digital trunk  309 . When echo cancellation circuit  332  is operating in the forward direction that is eliminating echoes being received via trunk  338 , the information being received via cable  519  of link  342  is directed to DSP  501  via multiplexer  507 , cable  511 . The output of DSP  501  is transmitted to switching network  302  via cable  512 , de-multiplexer  502 , cable  514 , multiplexer  503 , cable  509 , and interface  504 . When echo cancellation circuit  332  is operating in the reverse direction, that is eliminating echoes being received from switching network  302  that are caused by external carriers or telephones, DSP  501  receives its input from interface  504 , cable  516 , multiplexer  507 , and cable  511 . DSP  501  transmits its output to interface circuit  316  via link  342  by transmission via cable  512 , de-multiplexer  502 , cable  513 , multiplexer  506  and cable  518 . When echo cancellation circuit  332  is being used as a service circuit by PBX  300 , DSP  501  receives its input via interface  504 , cable  516 , multiplexer  507 , cable  511 . DSP  501  transmits the results back to switching network  302  via cable  512 , de-multiplexer  502 , cable  514 , multiplexer  503 , cable  509 , and interface  504 .  
         [0033]    FIGS.  6 - 9  illustrate, in flowchart form, the operations performed by control computer  301  to eliminate echoes. After being started in block  601 , decision block  602  determines if a call is being set up. If the answer is no, control is transferred to block  603  which performs normal processing before returning control back to decision block  602 . If the answer in decision block  602  is yes, decision block  604  determines whether the call is an internal call between telephones connected directly to PBX  300 . If the answer is yes, control is transferred to block  603 . In the following description, the terms inward and outward trunks are defined in the following manner. If a call is set up from telephone  325  of FIG. 3 into PBX  300  via analog trunk  306 , analog trunk  306  is the inward trunk circuit. If this call is being set up to internal telephone  328 , then there is no outward trunk. However, if the call from telephone  325  is being set up to telephone  336  via digital trunk  304 , then, digital trunk  304  is the outward trunk circuit. If the answer in decision block  604  is no, decision block  605  determines whether the call is a call that is going to be communicated via WAN  313 . If the answer is yes, control is transferred to decision block  901  of FIG. 9. If the answer is no in decision block  605 , control is transferred to decision block  606 . Decision block  606  examines internal tables that have been administered to determine if the outward path of the call requires echo cancellation. If the answer is no, control is transferred to decision block  701  of FIG. 7. If the answer in decision block  606  is yes, decision block  607  determines if the inward call path, is designated as requiring echo cancellation. If the answer is no, control is transferred to decision block  801  of FIG. 8. If the answer in decision block  607  is yes, decision block  608  determines if the inward trunk circuit has echo cancellation. At this point, it has been determined that both the outward and inward paths require echo cancellation operations. If the answer is no in decision block  607 , block  611  obtains utilization of a portion of another digital trunk to serve as a service circuit to provide the echo cancellation operations before transferring control to decision block  612 . If the answer in decision block  608  is yes, then block  609  properly utilizes the internal echo canceller of the inward trunk circuit before transferring control to decision block  612 . Decision block  612  determines if the outward trunk circuit has echo cancellation. If the answer is no, block  613  uses another digital trunk as a service circuit before transferring control to block  616 . If the answer in decision block  612  is yes, block  614  utilizes the internal echo canceller of the outward trunk circuit before transferring control to block  616 . Block  616  performs normal call processing.  
         [0034]    If the answer in decision block  606  is no that the outward call path is not designated as requiring echo cancellation, then it must be determined if the inward call path requires echo cancellation. If the answer in decision block  606  is no, control is transferred to decision block  701  of FIG. 7. Decision block  701  determines if the inward call path is designated as requiring echo cancellation. If the answer is no, control is transferred to block  706  which performs normal processing before returning control back to decision block  602  of FIG. 6. If the answer in decision block  701  is yes, decision block  702  determines if the inward trunk circuit has echo cancellation. If the answer is yes, block  703  utilizes this internal echo canceller before transferring control to block  706 . If the answer in decision block  702  is no, decision block  704  determines if the outward trunk circuit for the call has an echo canceller. If the answer is yes, control is transferred to block  707  which utilizes the echo canceller of the outward trunk circuit before transferring control to block  706 . If the answer in decision block  704  is no, control is transferred to block  708  which uses the echo canceller of another digital trunk as a service circuit before transferring control to block  706 .  
         [0035]    Returning to FIG. 6, if the answer in decision block  607  is no that the inward call path is not designated as requiring echo cancellation, then it must be determined if the outward call path requires echo cancellation. If the answer in decision block  607  is no, control is transferred to decision block  801  of FIG. 8. Decision block  801  determines if the outward call path is designated as requiring echo cancellation. If the answer is no, control is transferred to block  806  which performs normal processing before returning control back to decision block  602  of FIG. 6. If the answer in decision block  801  is yes, decision block  802  determines if the outward trunk circuit has echo cancellation. If the answer is yes, block  803  utilizes this internal echo canceller before transferring control to block  806 . If the answer in decision block  802  is no, decision block  804  determines if the inward trunk circuit for the call has an echo canceller. If the answer is yes, control is transferred to block  807  which utilizes the echo canceller of the inward trunk circuit before transferring control to block  806 . If the answer in decision block  804  is no, control is transferred to block  808  which uses the echo canceller of another digital trunk as a service circuit before transferring control to block  806 .  
         [0036]    Returning to FIG. 6, if the answer in decision block  605  is yes, that the call is being communicated via WAN  313 , then, control is transferred to decision block  901  of FIG. 9. Decision block  901  determines whether the call path either inward or outward requires echo cancellation. In most situations, echo cancellation is only provide for outward calls. If the answer is no, control is transferred to decision block  903  which performs normal processing before returning control back to decision block  602  of FIG. 6. If the answer is yes, decision block  904  determines the required echo cancellation capacity that is required of the DPS unit controlling the echo circuit. This determination is based on the amount of memory that is required for the echo tail length and the processing capacity. Decision block  906  then determines if the internal echo canceller has the required capabilities. For example, if the call is being routed through IP trunk  308 , the determination would be made if echo circuit  1002  of FIG. 10 has sufficient capacity. If the answer is yes, decision block  909  performs the setup operation to initialize the DSP of the internal echo canceller for the proper echo tail length before transferring control to decision block  911 . Decision block  911  performs normal processing before returning control back to decision block  602  of FIG. 6. Returning to decision block  906 , if the answer is no, block  907  selects and uses a digital trunk such as digital trunk  309  as a service circuit. Then, decision block  908  sets up the echo tail length of the echo canceller of the digital trunk before transferring control to decision block  911 .  
         [0037]    [0037]FIG. 10 illustrate, in block diagram form, IP trunk  308  in greater detail. Internal interface circuit  1001  provides the interface to switching network  302 , external interface circuit  1003  provides the interface to WAN  313 , and echo cancellation circuit  1002  provides the echo cancellation functions. Internal interface circuit  1001  provides for interfacing to the time slot interchange subsystem of switching network  302  and in addition, provides for call conferencing and gain adjustments for telephone calls. The operations of internal interface circuit  1001  are well known to those skilled in the art.  
         [0038]    External interface circuit  1003  provides for interfacing to WAN  313 . These functions comprise all functions required for voice-over-IP such as compression of digital samples received from echo cancellation circuit  1002  and packetization of the received digital samples for transmission to WAN  313 . External interface circuit  1003  is responsive to packets received from WAN  313  to perform depacketization, decompression, and error recovery, etc. The operations of external interface circuit  1003  are well known to those skilled in the art.  
         [0039]    Echo cancellation circuit  1002  is identical in design to echo cancellation circuit  332 .  
         [0040]    Of course, various changes and modifications to the illustrative embodiment described above will be apparent to those skilled in the art. These changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the following claims except insofar as limited by the prior art.