Abstract:
A pool of echo cancellers provides echo cancellation on PCM digital transmissions on an as needed basis. A dynamic port device operating under the direction of call processing identifies the transmissions requiring echo cancellation and routes the identified transmissions through echo cancellers. The echo cancellation can be performed on an as needed basis without having to dedicate an echo canceller to each DS0 channel. The dynamic port device can provide multiplexing up to SONET carrier levels immediately following selective echo cancellation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/031,082, filed Jun. 28, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to the field of telephone networks and communications, and more particularly to echo cancellation in telecommunications systems. 
     The presence of reflected voice signals or “echo” in telephone lines is a well-known phenomenon. Modern telephone systems employ echo cancellers at various points in a transmission system to eliminate such undesirable reflected voice signals. An early example of an echo canceller is described in U.S. Pat. No. 3,500,000, issued Mar. 10, 1970. 
     Hybrid circuits are a common source of impedance mismatch that gives rise to the signal reflection problem that may be heard as an echo of the speaker&#39;s own voice. In addition to hybrid circuits, telephone systems have other inherent sources of reflection and signal feedback that can give rise to undesirable echo transmissions. For example, speaker phones and “hands-free” mobile phones can acoustically couple or “feedback” a portion of the sound from the phone&#39;s loudspeaker into its microphone. Conventional echo cancellers can eliminate undesirable echoes from any such additional sources, when the echo signals are correlated, as well as from the ordinary hybrid circuit. 
     To facilitate an understanding of the echo phenomenon, reference is made to FIG. 1 showing a simplified transmission system of the prior art, which is designated generally by reference numeral  10 . The system  10  is shown connecting telephone A to telephone B through a network N. Phone A is connected by line  11  to a hybrid circuit H A  which in turn is connected to an echo canceller E A  by line  12 . The echo canceller E A  is connected to the network N by line  13 . Similarly, phone B is connected through hybrid circuit H B  and echo canceller E B  to the network N via lines  14 ,  15  and  16 . The lines  11  and  14  typically consist of conventional two-wire subscriber loops (or “local loops”) through which analog voice signals are conducted in both directions. The hybrid circuits H A  and H B  separate the two-way voice signals on lines  11  and  14  to provide separate transmit and receive signals on the respective pairs of the four-wire lines  12  and  15 . A hybrid circuit can be part of the subscriber&#39;s equipment or part of the phone company&#39;s equipment. 
     Whether an echo is perceptible, and therefore objectionable, depends upon the delay from original transmission to receipt of the reflected signal. In the example of FIG. 1, if a party using phone A is speaking, the signal must travel the distance from phone A to hybrid circuit H B  on the opposite side of the network N and be reflected back to phone A. To prevent the return of such echo signal to phone A, echo canceller E B  superimposes an inverted copy of the echo signal on the line  16  to cancel the actual echo signal reflected by hybrid circuit H B . The echo canceller E B  senses the duration for transmission from it to hybrid H B  and reflection back to precisely time the cancellation function. Thus, the party speaking into phone A will not hear any annoying echoes. Similarly, echo canceller E A  may be employed to remove the echo of speech transmitted by phone B and caused by signal reflection at hybrid circuit H A . 
     More recently, digital transmission has become commonplace in telecommunications networks. As a result, sophisticated digital echo cancellers have been developed to subtract out echoes caused by reflections at various points in the transmission system. Such digital echo cancellers are well known in the art, an illustrative example being described in U.S. Pat. No. 5,418,849. 
     In addition to transmitting digitized voice signals, telephone systems are being used increasingly for digital data transmission, as when computers communicate with each other. A telephone technology known as Integrated Services Digital Network (ISDN) provides uniform standards and protocols for computers to send and receive digital data through the twisted-pair copper wires of the conventional local loop at relatively high transmission rates compared to “modem” technology. An important application for ISDN technology is to provide a relatively high-speed connection to the Internet via the two-wire local loop of a conventional telephone. Unlike digital voice transmissions, ISDN data transmissions do not require echo cancellation. Conventional digital echo cancellers must be disabled so that they can pass ISDN data and other digital data transmissions without applying echo cancellation. 
     One end of a digital transmission system is depicted in the simplified block diagram of FIG.  2  and designated generally by reference numeral  20 . A phone A used by a “near-end talker” is connected to a HYBRID circuit by a conventional subscriber loop  21  for sending and receiving analog voice signals. The HYBRID circuit provides separate communication paths  22  and  23  for “send” and “receive” signals, respectively. A conventional device known as a CODEC (coder-decoder) converts analog signals on send line  22  to digital signals on send line  24 , and converts digital signals on receive line  25  to analog signals on receive line  23 . A digital echo canceller  26  communicates with the telephone network (not shown) via send line  27  and receive line  28 . Pulse-code-modulated (PCM) signals are communicated on lines  24 ,  25 ,  27  and  28  in accordance with network standards. The network interconnects the near-end talker using phone A with a far-end talker (not shown). 
     Echo canceller  26  is employed to eliminate the echo of the far-end talker&#39;s voice reflected on send path  22  by the HYBRID circuit. The far-end talker&#39;s voice signal is received on line  28  by the echo canceller  26 , sensed internally and passed through as an output on line  25 . From the signal on line  28  the echo canceller  26  estimates an echo signal expected to be returned on line  24 . The echo canceller  26  then subtracts the estimated echo signal from the actual echo signal. The resulting signal, which may include some “residual” echo, is further processed internally by the echo canceller  26  to produce an essentially echo-free output on line  27 . 
     In the United States, a digital multiplexing system is employed in which a first level of multiplexed transmission, known as T 1 , combines 24 digitized voice channels over a four-wire cable (one pair of wires for “send” signals and one pair for “receive” signals). The conventional echo canceller  26  of FIG. 2 is shown operating on a single PCM voice transmission line prior to multiplexing (or “muxing”) for network transmission. The digital coding produced by the CODEC on line  24  provides 8,000 samples per second of the analog signal on line  22 , each sample being represented by an 8-bit binary number. Thus, the transmission rate on line  24  is 64,000 bits per second (64 kbps). 
     The conventional bit format on the T 1  carrier is known as DS 1  (i.e., first level multiplexed digital service or digital signal format), which consists of consecutive frames, each frame having 24 PCM voice channels (or DS 0  channels) of 8 bits each. Each frame has an additional framing bit for control purposes, for a total of 193 bits per frame. The T 1  transmission rate is 8,000 frames per second or 1.544 megabits per second (Mbps). The frames are assembled for T 1  transmission using a technique known as time division multiplexing (TDM), in which each DS 0  channel is assigned one of 24 sequential time slots within a frame, each time slot containing an 8-bit word. 
     Transmission through the network of local, regional and long distance service providers involves sophisticated call processing through various switches and a hierarchy of multiplexed carriers. At the pinnacle of conventional high-speed transmission is the synchronous optical network (SONET), which uses fiber-optic media and is capable of transmission rates in the gigabit range (in excess of one billion bits per second). After passing through the network, the higher level multiplexed carriers are demultiplexed (“demuxed”) back down to individual DS 0  lines, decoded and connected to individual subscriber phones. 
     Echo cancelling is commonly applied at the DS 0  level. It has been conventional practice to provide 24 echo cancellers per T 1  line so that each DS 0  channel has a dedicated echo canceller. However, as digital data transmission over telephone lines has increased (e.g., for ISDN data traffic), the percentage of DS 0  channels needing echo cancellation has decreased. Unlike digitized voice, such digital data communication in a DS 0  channel does not require echo cancellation. When digital data is detected, typically the call processing system has had to route the call to special trunk groups not equipped with echo cancellers, or when echo cancellation is equipped on a dedicated basis, has had to disable the echo canceller on that particular DS 0  channel. 
     Echo cancellation may be applied at various points within a transmission system. It is common to apply echo cancellation on the network side (rather than subscriber side or “access side”) of a conventional voice circuit switch operating on T 1  lines. By way of illustration, FIG. 3 shows such a switch in a block diagram. The switch is designated generally by reference numeral  30  and includes an access-side port device  31 , a switch core  32  and a network-side port device  33 . Such switches are common in the public telephone network and facilitate the basic routing and interconnection of ordinary telephone calls and data communications over telephone lines. Multiplexers  34  and  35  are provided on the network side of the switch  30  to mux up the signals to higher rates for transmission through conventional high-speed media. For example, DS 3  transmission is typically carried by a coaxial cable and combines 28 DS 1  signals at 44.736 Mbps. An OC 3  optical fiber carrier, which is at a low level in the optical hierarchy, combines three DS 3  signals at 155.52 Mbps, providing a capacity for 2016 individual voice channels in a single fiber-optic cable. SONET transmissions carried by optical fiber are capable of even higher transmission rates. 
     The switch  30  is simplified in FIG. 3 to show it operating on a single DS 1  line  36 , but it will understood that switching among many such lines actually occurs so that calls on thousands of individual subscriber lines can be routed through the switch on their way to their ultimate destinations. Port device  31  demultiplexes the signals on DS 1  line  36  to provide 24 corresponding DS 0  appearances to ports of the switch core  32 . The switch core  32  includes a complex matrix of electronic switches and control circuits that route the individual DS 0  lines on the access side to other DS 0  lines on the network side. The signals emerging on the network side of the switch core  32  are muxed back up to the DS 1  level and transmitted further on line  37 . DS 1  carrier line  37  and other such lines (not shown) are muxed up to the DS 3  level by multiplexer  34  for transmission on line  38 . Similarly, DS 3  carrier line  38  and other such lines (not shown) are muxed up to an optical transmission level, such as OC 3 , by multiplexer  35  for transmission by SONET carrier  39 . 
     FIG. 4 schematically depicts an arrangement of echo cancellers as commonly employed in prior art systems. A voice circuit switch  40  is shown in block diagram form but will be understood to include a switch core and port devices like those of the switch  30  of FIG.  3 . The arrangement of FIG. 4 also shows two of a plurality of multiplexers  41  and  42 , each muxing up 28 DS 1  transmissions to the DS 3  level. A group of 28 echo canceller cards, designated collectively by reference numeral  43 , services the lines entering multiplexer  41 , and a group of 28 echo canceller cards, designated collectively by reference numeral  44 , services the lines entering multiplexer  42 . The switch  40  has a plurality of T 1  lines  45  entering from the access side. A corresponding number of T 1  lines  46  emerge from the switch  40  arranged in groups of 28 to correspond to respective multiplexers. 
     Each of the echo canceller cards of the groups  43  and  44  contains 24 echo cancellers since each card services one T 1  transmission line carrying 24 voice channels. Typically, the circuitry of each echo canceller is implemented in a single integrated circuit chip. Thus, it will be appreciated that each T 1  line  46  has a dedicated echo canceller card, and each DS 0  channel has a dedicated echo canceller chip. As an alternative to the arrangement of FIG. 4, some prior art systems have voice circuit switches with internal echo cancellers dedicated on a DS 0  basis. However, whether the echo cancellers are of the internal or external type, the prior art systems typically provide a dedicated echo canceller for each DS 0  channel. In some cases, groups of T 1  lines are not equipped with echo cancellers and are used exclusively for digital data transmissions. 
     In many instances individual echo cancellers in prior art systems are maintained in a disabled state and merely pass through the DS 0  signal transmissions without applying echo cancelling. This occurs either because the transmission delay is sufficiently short that echo cancellation is not needed or because digital data is being carried by the DS 0  line. Also, echo cancellation may have been applied at a different point in the transmission network and thus is not needed at this particular point in the system. Because each echo canceller is dedicated to a particular DS 0  line, a significant percentage of echo cancelling equipment remains quiescent at all times. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an echo canceller system for use in a digital telephone transmission system is provided in an efficient equipment architecture. Echo cancellers are pooled and selectively interconnected by call processing control through a pool switch matrix to individual transmission lines only in the event that a determination is made that the line requires echo cancellation. 
     The echo canceller pooling arrangement of the present invention permits efficient use of echo cancellers on an as needed basis. A relatively small number of echo cancelers can effectively service a relatively large number of individual transmission lines. 
     The pool switch matrix optionally can be configured to dynamically route either access-side transmissions or network-side transmissions to echo canceller inputs to cancel echoes coming from either direction. 
     The invention optionally can provide additional system efficiencies, such as combining multiplexer stages in a port device on one side of a voice circuit switch to enable direct connection of a fiber-optic cable to the multiplexed output of the port device. 
    
    
     The presently preferred way of carrying out the invention is described in detail below with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a telephone system of the prior art employing echo cancellation; 
     FIG. 2 is a simplified block diagram of one end of a digital transmission system of the prior art employing echo cancellation; 
     FIG. 3 is a simplified block diagram of the components of a conventional voice circuit switch in a portion of a multiplexed transmission system; 
     FIG. 4 is a simplified block diagram of a prior art arrangement of dedicated echo cancellers in a multiplexed transmission system; 
     FIG. 5 is a schematic block diagram of an echo canceller system of the present invention; 
     FIG. 6 is a schematic block diagram of a dynamic port device employed in the system of the present invention; 
     FIG. 7 is a schematic block diagram of one example of an implementation of a pool switch matrix depicting a simplified routing scheme for selectively making interconnections with an echo canceller pool in accordance with the present invention; and 
     FIG. 8 is a schematic block diagram of a second example of an implementation of a pool switch matrix in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 5, an echo canceller system according to one embodiment the present invention is depicted and designated generally by reference numeral  100 . The system  100  includes a switch core  102  of a switch for routing incoming transmissions on lines  104  to outgoing transmissions on lines  106  and vice versa. The switch core  102  preferably comprises the conventional DS 0  switch fabric and call processing functionality of the prior art switch core  32  depicted in FIG.  3 . However, it will be appreciated from the following description that the invention is not limited to application with a DS 0  level voice circuit switch. 
     In the embodiment of the invention depicted in FIG. 5, the transmissions on lines  104  and  106  are at the DS 0  level. A port device  108  is provided on the access side of the switch core  102  for providing the individual DS 0  appearances on lines  104  to the switch core  102 . A dynamic port device  110  for facilitating the echo cancelling system  100  of the present invention is provided on the network side of the switch core  102  in communication with DS 0  lines  106 . Port device  108  communicates with a plurality of TDM digital transmission lines on the access side, only one of which is shown and designated by numeral  112 . Typically, line  112  carries signals at a relatively low level of the digital network hierarchy, which in this example is the DS 1  level. 
     The dynamic port device  110  performs a number of sophisticated functions including optionally multiplexing up several carrier levels. For example, the device  110  may include multiplexing capability up to a level compatible with SONET transmission through a fiber-optic cable  114 . If the cable  114  is an OC 3  carrier (155.52 Mbps transmission rate), an electrical synchronous transport signal (STS) is carried by line  116  at the compatible STS- 3  transmission rate. Fiber-optic transmitters for converting electronic signals into light signals are well known in the art. 
     In accordance with an important feature of the present invention, the dynamic port device  110  performs an echo canceller pooling function, which will now be described with reference to FIG.  6 . The dynamic port device  110  includes a pool switch matrix  120  in communication with DS 0  lines  106  emerging from the switch core  102  (FIG.  5 ). A like number of DS 0  lines  122  emerge from the pool switch matrix  120  and enter a multi-stage multiplexer system  124 , shown for simplicity as a single box labelled “MUX” in FIG.  6 . An echo canceller pool  126  is provided in communication with the pool switch matrix  120  through lines  128 . Operation of the pool switch matrix  120  is managed by control circuitry  130  issuing switch commands through control lines  132 , which are shown grouped together for convenience of illustration. 
     The control circuitry  130  is in communication with the service provider&#39;s call processing system  134 , which determines which of the DS 0  lines  106  require echo cancellation. For example, ISDN digital data calls contain information that identifies the transmission as digital data rather than digitized voice. Upon detecting a digital data transmission, the call processing system  134  directs the control circuitry  130  to allow the particular DS 0  input line  106  to connect through the pool switch matrix  120  to a corresponding DS 0  output line  122  without echo cancellation. When the call processing system  134  determines that a particular DS 0  line  106  requires echo cancellation, it directs the control circuitry  130  to disconnect and interconnect selected lines in the pool switch matrix  120  to route the incoming signals on the particular DS 0  line  106  through an available echo canceller in the pool  126  and then back out to a corresponding network-side DS 0  line  122 . 
     It will be appreciated that the embodiment of FIGS. 5 and 6 is merely illustrative of the invention, which can be implemented in various configurations. For example, the echo canceller pooling function can be performed on the access side of the switch core  102  by a dynamic port device in place of the conventional port device  108 . In another implementation, SONET transmission can be provided on both sides of the system  100  with each port device  108  and  110  performing the required multiplexing. 
     FIG. 7 schematically depicts the operation of the pool switch matrix  120 . It will be understood that the actual switch fabric is implemented by semiconductor switches and logic circuitry using conventional switch technology. For simplicity, only six DS 0  input lines  106   A  through  106   F  and only six corresponding DS 0  output lines  122   A  through  122   F  are expressly shown arriving at and emerging from the switch matrix  120 , though many more such lines are provided. It will also be understood that the individual DS 0  lines represent conventional four-wire send/receive lines. Standard interface connectors  136  and  138  interconnect DS 0  lines  106  and  122 , respectively, with the pool switch matrix  120 . The lines  106  are referred to as “input” lines because their send paths may include reflected signals requiring cancellation by the echo canceller pool  126 . However, it will be understood that lines  106  also have outgoing transmissions on their receive paths. Likewise, the “output” lines  122  actually have both incoming and outgoing transmission on separate wire pairs. 
     The echo canceller pool  126  contains an array of echo cancellers, which may be provided on printed circuit boards assembled in racks (not shown). A first echo canceller  140  and a second echo canceller  142  are expressly shown, others being depicted in dashed outline. Each of the echo cancellers in the pool  126  is connected to the pool switch matrix  120  by corresponding lines, through a standard interface  144 . Echo canceller  140  is connected to the switch matrix  120  via input line  146  and output line  148 . It will be understood that input line  146  actually includes pairs of send/receive lines comparable to lines  24  and  25  of FIG.  2  and that output line  148  actually includes pairs of send/receive lines comparable to lines  27  and  28  of FIG. 2, the terms “input” and “output” being used in the context of FIG. 7 to correspond to the echo transmission and the echo-free transmission, respectively. Similarly, echo canceller  142  is connected to the pool switch matrix  120  via input line  150  and output line  152 . 
     In the simplified example of FIG. 7, call processing has determined that lines  106   B  and  106   E  are carrying digitized voice signals that require echo cancellation, and that lines  106   A ,  106   C ,  106   D  and  106   F  are carrying transmissions (either voice or data) that do not need echo cancellation. Accordingly, control signals are sent into the switch matrix  120  to electronically disconnect input line  106   B  from output line  122   B  at point  154  and disconnect input line  106   E  from output line  122   E  at point  156 . Transmissions on input lines  106   A ,  106   C ,  106   D  and  106   F  pass through the matrix  120  to corresponding output lines  122   A ,  122   C ,  122   D  and  122   F  without rerouting through echo cancellers. Input line  106   B  is routed through the switch matrix  120  with intermediate electronic switching at points  158  and  160  to arrive at the interface  144  on line  162  where connection is made to input line  150  of echo canceller  142 . Thus, the signals on DS 0  line  106   B  pass through echo canceller  142  and return to the switch matrix  120  on line  152 . Echo canceller output line  152  is connected through interface  144  to routing line  164 , which is interconnected with DS 0  output line  122   B  through matrix switch points  154 ,  158  and  160 . Similarly, routing line  166  interconnects DS 0  input line  106   E  with input line  146  to echo canceller  140 , and routing line  168  interconnects DS 0  output line  122   E  with output line  148  from echo canceller  140 . 
     Thus, it will be appreciated that echo cancellation is selectively applied only to the transmissions on lines  106   B  and  106   E  in the particular simplified example. In the generalized operation of the echo cancelling system, the control circuitry  130  (FIG. 6) dynamically orchestrates routing through the pool switch matrix  120  selecting idle echo cancellers from the pool  126  to apply echo cancellation on an as needed basis. Only those DS 0  lines that the call processing system  134  identifies as requiring echo cancellation are connected to an echo canceller in the pool  126 . The DS 0  input lines  106  that do not need echo cancellation remain connected through the matrix  120  to corresponding output lines  122  via routing lines and normally closed intermediate switch points. For example, multiple intermediate switch points  172 ,  174  and  176  are schematically depicted on routing line  178  interconnecting DS 0  input line  106   A  to DS 0  output line  122   A . Of course, many additional switch points and routing lines are provided in the matrix  120  but for simplicity are not shown. 
     As an example of a contemplated application, dynamic port device  110  (FIGS. 5 and 6) may be configured to directly serve fiber-optic cable  114  operating at the OC 3  level. Thus, 2016 individual DS 0  lines  106  pass through the pool switch matrix  120  and are muxed up to the OC 3  level. If it is determined that the statistical probability of echo cancellation being needed within the 2016 individual circuits or channels is sufficiently low that at most one in three channels will simultaneously require echo cancellation, then the echo canceller pool  126  can be equipped with 672 echo cancellers to effectively serve the entire fiber-optic cable  114 . 
     In the FIG. 7 embodiment the pool switch matrix  120  is depicted with each output line  122  emerging from the interface  138  at a connection point or port which corresponds in position to the connection point in interface  136  of the corresponding input line  106 . For example, input line  106   A  enters the uppermost port  180  in the interface  136 , and output line  122   A  emerges from the uppermost port  182  in the interface  138 . Although such a scheme may have advantages, present switch technology readily permits dynamic reconfiguration of internal routing paths so that matrix connection to input and output line pairs need not be juxtaposed in corresponding physical positions in the input and output interface devices  136  and  138 . An alternative arrangement will now be described. 
     With reference to FIG. 8 another embodiment of a pool switch matrix will be described, with similar numerals designating similar elements previously described. The pool switch matrix  220  has an interface  236  for receiving a random set of access-side transmissions on lines A, B, C, D, E and F, and an interface  238  for connection to corresponding network-side transmission lines, which in this illustrative example are ordered C, E, F, B, A and D at a particular point in time. 
     An echo canceller pool  226  is provided with a plurality of echo cancellers, two of which are expressly shown and labelled  240  and  242 . The echo cancellers  240  and  242  communicate with the switch matrix  220  through an interface  244  via respective input lines  246  and  250  and output lines  248  and  252 . 
     Routing lines in the switch matrix  220  interconnect the access-side lines A-F at interface  236  to corresponding network-side lines at interface  238  or to an echo canceller on an as needed basis. For example, access-side line A enters the switch matrix  220  at port  280  and corresponding network-side line A emerges from the switch matrix  220  at port  282 . The interconnection between access-side port  280  and network-side port  282  is provided by routing lines  283 ,  284  and  285  and intermediate switch points  286  and  287 . This path through the matrix is dynamically set up by switch matrix logic circuitry under the ultimate direction of the call processing system and associated control circuitry. In this case the send transmission on access-side line A has been determined not to require echo cancellation. 
     As an example, the send transmission on access-side line F has been determined to require echo cancellation. Thus, the switch matrix  220  sets up routing lines to run the transmissions on channel F through echo canceller  242 . Thus, echo signals on the send wire of access-side line F are removed from the send wire of network-side line F prior to muxing up for network transmission. 
     The ability to dynamically reconfigure the pool switch matrix  220  provides additional options not available with prior art dedicated echo canceller systems. For example, echo cancelling can be applied in either direction. FIG. 8 illustrates this optional feature as to channel B. Transmissions from a long-haul carrier on the network side arriving at port  290  are routed through the switch matrix  220  by routing lines  291  and  292  to interface  244  where connection is made to the input line  246  of echo canceller  240 . The output of echo canceller  240  on line  248  is routed through the switch matrix  220  by routing lines  293  and  294  to port  295  where connection is made to the access-side line for channel B. Thus, echo cancellation can be performed on echo signals arriving at the switch matrix from either direction on an as needed basis. 
     Although the echo canceller pools  126  and  226  of FIGS. 7 and 8 are illustrated as provided separately from their respective switch matrix devices  120  and  220 , it is contemplated that the echo cancellers optionally can be integrated with the switch fabric, thus eliminating the interface connection devices  144  and  244 . In such an optional architecture, the echo canceller input and output lines will consist of terminating routing lines in the switch matrix. 
     Those skilled in the art will appreciate that the addition of a switch matrix to the system is more than offset by the significant reduction in the number of echo cancellers employed. Further, by performing echo cancellation at the port device, several stages of multiplexing can be collapsed and assembled in a common port device to facilitate direct connection to a fiber-optic cable. 
     Although a preferred embodiment of the present invention has been described in detail, it will be understood that various alternatives and modifications thereof are within the spirit and scope of the invention as set forth in the appended claims.