Abstract:
A voice playout buffer for a dual PHY-based integrated access device platform has a plurality of voice signal buffer sections. A respective buffer section has a capacity in excess of the number of digitized voice signal bytes contained in a respective cell-based communication signal. A respective buffer section sequentially stores digitized voice signals of a single TDM channel into successive storage locations of a first portion thereof at a first data rate, and reads out digitized voice signals at a second, higher data rate for transport over an ATM cell bus. The storage capacity of a buffer section accommodates a communications control processor writing new outgoing digitized voice signal bytes into the first portion of the voice signal buffer section for transport over a TDM communication link, prior to digitized voice signals newly received from the TDM communication link being written into the first portion of the voice signal buffer section.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is a division of copending U.S. patent application Ser. No. 10/262,402, filed Oct. 1, 2002, by Paul McElroy et al, entitled: “Dual-PHY Based Integrated Access Device” (hereinafter referred to as the &#39;402 application), and claims the benefit of co-pending Provisional U.S. Patent Application Serial No. 60/390,337, filed Jun. 21, 2002, entitled: “Highly Integrated Dual-PHY Voice CoProcessor,” by P. McElroy, each application assigned to the assignee of the present application and the disclosures of which are incorporated herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates in general to digital telecommunication systems, subsystems and components therefor, and is particularly directed to a new and improved voice playout buffer architecture for an integrated access device (IAD) platform, that employs a highly integrated time division multiplexed (TDM), asynchronous transfer mode (ATM) cell based architecture, to provide enhanced interfacing flexibility for multiple and diverse signaling protocols, and effectively reduces the cost and constraints as to choice of host processor used in conventional digital signal processor (DSP)-based IADs.  
         BACKGROUND OF THE INVENTION  
         [0003]    In an effort to accommodate the diverse (e.g., voice and data signaling) requirements of a variety of telecommunication service providers and their customers, manufacturers of digital communication equipment currently offer what are known as integrated access devices (IADs), that allow a user to interface multiple types of digital voice and data signaling circuits with a (wide area) network. Unfortunately, current IAD designs are constrained by the lack or limited availability of reasonably priced and versatile communication control processors.  
           [0004]    A fundamental shortcoming of these conventional controller chips is the fact that they are digital signal processor (DSP)-based, consume large amounts of power, and are procurable from essentially one semiconductor fabrication source. Being DSP-based means that the functionality of an IAD using such control chips is heavily dependent on embedded software. In addition, these chips have only a small number voice and data interface ports, which are typically permanently dedicated to specified signaling modes, thereby limiting their flexibility and efficiency in the face of dynamic signaling requirements.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention detailed in the &#39;402 application, these and other shortcomings of conventional IADs are effectively obviated by a new and improved ‘DSP-less’ IAD architecture, that is configured as a dual PHY-based signal transport ASIC, and offers enhanced interfacing flexibility for multiple and diverse types of digital communication circuits. To this end, the dual PHY based IAD architecture of the invention comprises a multi-protocol communication interface (MCI) and an associated communication or host network processor (HNP). The MCI is configured to execute diverse types of digital communication signaling interface functions with a plurality of communication ports, under the control of supervisory control signals supplied via a generic, host processor interface. Advantageously, the host processor may be implemented using any one of a variety of reasonably priced, commercially available network processor chips.  
           [0006]    A first, wide area communication network (WAN) port of the MCI terminates a WAN with a bidirectional digital cross-connect switch (XCS) and provides both ATM and high level data link control (HDLC) connectivity with the WAN. A second, voice TDM port terminates the digital cross-connect switch with a voice TDM circuit and provides digital transport connection to various TDM communication transceivers, such as analog codecs and T1 (including fractional T1) transceivers. This TDM port may be configured as a conventional TDM mode port and supports standard TDM signaling control parameters, including Frame Sync, transmit and receive clock and data signals. The TDM port is additionally coupled to an adaptive clocking unit which is operative (during ATM mode operational mode) to adjust clock (Clk) and frame sync (Fs) to incoming cell delivery timing over an internal TDM bus from a bidirectional voice gateway.  
           [0007]    The adaptive clocking unit may be configured as a digital phase locked loop (DPLL)-based adaptive clock recovery mechanism, of the type disclosed in co-pending U.S. patent application Ser. No. 09/999,463, filed Oct. 31, 2001, by A. Ghobrial et al, entitled: “Method and Apparatus for Providing Reliable Voice and Voice-Band Data Transmission Over Asynchronous Transfer Mode (ATM) Network” (hereinafter referred to as the &#39;463 application), assigned to the assignee of the present application and the disclosure of which is incorporated herein.  
           [0008]    Coupled with the internal TDM bus are an echo canceler and ADPCM voice compression operator, preferably cascaded within TDM bus in the manner disclosed in co-pending U.S. patent application Ser. No. 10/095,375, filed Mar. 12, 2002, by B Mitchell et al, al, entitled: “Echo Canceler and Compression Operators Cascaded in Time Division Multiplex Voice Communication Path of Integrated Access Device for Decreasing Latency and Processor Overhead” (hereinafter referred to as the &#39; 375  application), assigned to the assignee of the present application and the disclosure of which is incorporated herein. The internal TDM bus is also coupled to a dual tone multifrequency detector (DTMF) unit which contains a plurality of DTMF detectors, that may be selectively dedicated to tone sensing functions for signaling operations on the TDM bus. The DTMF unit also provides the MCI with the ability to detect dial tone.  
           [0009]    A third, UTOPIA port terminates a dual UTOPIA L2 PHY interface with a byte-wide, ATM cell-based UTOPIA bus, that serves as the principal ‘data’ transport path with the host network processor. The dual UTOPIA L2 PHY interface and its associated UTOPIA bus operate at a very high frequency (on the order of 25 MHz, which equates to a data transport rate on the order of 200 Mbps) relative to network and terminal rates, that typically have data rates on the order of only 1.5-2.0 Mbps (e.g., a WAN rate of 2304 kbps). As such, signaling transport communications between the MCI and the host network processor may be considered to effectively quasi-instantaneous, so that participation by the host processor in the transport of both digitized voice and data communication signals over any of the routing paths among the signaling ports of the MCI will not burden (slow down) the operational speed of any of the external communication circuits to which the IAD is ported.  
           [0010]    The dual UTOPIA L2 PHY interface has two separate PHY portions or layers (PHY0 for data, and PHY1 for voice), each PHY layer being byte-wide, containing separate transmit (TX) and receive (RX) buses. The PHY1 portion has the higher priority of the two PHY portions and is exclusively used for voice ATM cell transfers between bidirectional voice playout buffers of a multi-channel voice playout buffer unit and the host network processor, and for ATM voice cell transfers between the host network processor and the WAN. The data PHY portion (PHY0) is used for data ATM cell transfers between the host network processor and TX and RX data first-in, first-out registers (FIFOs) serving the WAN and an auxiliary V.35 circuit path.  
           [0011]    A fourth, auxiliary NxPORT terminates an external port of a bidirectional multiplexer (mux/demux) with an auxiliary (Nx56/64) digital communication path, over which non cell-based (e.g., V.35) digital communications are conducted with an auxiliary digital communication device. A fifth communication port is a TDM legacy port, that terminates a voice gateway with a legacy voice TDM communication link, to provide TDM connectivity with the internal TDM bus containing the TDM transport path-cascaded echo canceler and ADPCM voice compression operator.  
           [0012]    The internal TDM bus is further coupled to the bidirectional digital cross connect switch. This internal TDM voice interconnect path makes the MCI compatible with legacy IAD architectures, where TDM-IN and TDM-OUT interfacing are used. In such a legacy TDM mode, the TDM port by-passes ATM signal processing paths that use the dual UTOPIA L2 PHY interface and UTOPIA bus to the host network processor.  
           [0013]    The TDM voice gateway is also coupled to plurality of bidirectional voice playout buffers of a multi-channel voice playout buffer unit containing thirty-two channels of bidirectional FIFOs, each being sized to store a full ATM voice cell payload, as well as accommodate transport delay to and from the host network processor. This serves to provide for an effectively continuous flow and conversion of TDM communication signals on the TDM bus with ATM cells interfaced with dual UTOPIA L2 PHY interface over a full duplex ATM cell bus therebetween.  
           [0014]    The voice playout buffer unit, to which the present application is particularly directed, contains a plurality of (e.g., 32 voice channel-associated) bidirectional, first-in, first-out registers (FIFOs), each of which is sized (e.g., has a 64 byte capacity) to store a standard 44-byte payload of a full ATM voice cell (53 bytes), and also provide sufficient capacity to accommodate expected worst case transport delay to and from the host network processor. As successively received voice sample data is written into a playout buffer from the internal TDM bus, a voice pointer (VP) is successively incremented, when it points to the forty-third byte location, 44 bytes of TDM voice data are ready to be immediately encapsulated into a 53 byte ATM packet and burst-routed over the PHY1 port of the dual PHY layer to the host processor for delivery to a downstream WAN circuit. For optimizing direct memory access (DMA) transfer efficiency of as many playout buffers (up to 32 channels) that currently have data for the host processor, the playout buffer unit employs a single write interrupt. At this single interrupt ATM cells for up to 32 channels of data are loaded in processor memory under DMA control.  
           [0015]    The fact that each individual voice playout buffer has a sixty-four byte capacity means that for a 44 byte data field of a respective 53 byte ATM cell, there is a twenty-byte window within which the host processor must return a response ATM voice packet for the POTS channel of interest. ATM encapsulation of a respective 44 byte data field includes a four byte AAL2 header, a HEC byte and a four byte ATM header, as customarily employed in the art to realized a standard 53 byte ATM cell. Within the AAL2 header, the channel identification byte (CID) byte may be made programmable, so as to provide selective mapping to timeslots of a TDM frame, and thereby accommodate variations among different vendor equipments.  
           [0016]    In the return direction from the host processor, the ATM and AAL2 headers are stripped off and the 44 bytes of voice payload data are written into successive locations of the playout buffer, as pointed to by a cell pointer (CP), beginning with the location of the first byte of the 44 bytes that had just been burst out over the PHY bus to the processor. As long as the voice pointer (VP), which has been and continues to be incremented at the relatively slower TDM rate, has not reached the end (byte location  63 ) of the playout buffer and begun ‘wrapping around’ to the lowest byte location, and with the contents of the first 44 byte locations of the playout buffer having been read out to the processor and therefore stale, return voice cell data from the processor may be written into those same (stale data) byte locations ( 0 - 43 ) of the playout buffer from which the previous burst was received.  
           [0017]    As result, since it operates at a considerably higher speed than the TDM bus, the host processor is expected to return a response ATM voice cell containing 44 bytes of TDM data to the playout buffer, well prior to voice pointer reaching the end of the twenty cell window of the playout buffer, even though there may be some byte differential (one to twenty bytes, in the present example of a 64 bytes capacity playout buffer) between the current location of the voice pointer (VP) and that of the cell pointer (CP). This flexibility offered by the practical size of the playout buffer greatly reduces the cost and complexity of the digitized voice transport path. Namely, as long as this ‘turnaround’ differential remains within the twenty byte window, continuity of voice packet flow (with no overflow and no underflow) will be effectively maintained throughout the call. If a return cell is not ready to send, the host processor resends the last transmitted cell, to maintain continuous voice cell flow.  
           [0018]    The digital cross-connect switch is used to provide external communication signaling port terminations with the WAN and the voice TDM circuit, and includes a TDM voice port through which the TDM voice circuit is coupled to the internal TDM bus. It further includes an ATM port and an HDLC port which respectively provide connectivity between the WAN port and a WAN ATM transceiver and a WAN HDLC transceiver. The digital cross-connect switch also has a sixth, Nx port that is coupled to the mux/demux. The mux/demux is coupled to an NxPORT HDLC transceiver.  
           [0019]    The digital cross-connect switch has two modes of operation: direct DS0-mapping mode, and ATM/HDLC transceiver interface mode. In DS0-mapping mode, the internal dual ATM PHY conversion and transport functionality of the MCI is effectively bypassed, with DS0 time slots on the voice TDM link directly mapped through the cross-connect switch to the WAN, using a user-controlled mapping scheme. DS0 time slots on the voice TDM link are directly mappable to the voice port, so that they may be coupled to the internal TDM bus. DS0 time slots may also be directly mapped via to Nx mux/demux for Nx56/64 clear channel (V.35) operation.  
           [0020]    In ATM/HDLC transceiver interface mode, the cross-connect switch couples the WAN port to the appropriate one of ATM and HDLC transceiver ports, which are respectively coupled to a WAN ATM transceiver and a WAN HDLC transceiver. For ATM mode communications incoming from the WAN toward the network processor, the WAN ATM transceiver couples to a WAN receive (RX) FIFO incoming ATM cells from the cross-connect switch. The WAN RX FIFO may have a relatively small depth, such as one that accommodates only two ATM cells, due to the considerably higher speed of the UTOPIA L2 PHY bus. ATM cells supplied to the WAN RX FIFO are forwarded via a permanent virtual circuit (PVC) router to the (PHY0) portion of the dual UTOPIA L2 PHY interface for transport over the UTOPIA bus to the network processor.  
           [0021]    The PVC router is preferably implemented using multibit table entries in internal memory to steer the flow of ATM data cells of various virtual circuits within the MCI for voice and data signaling transport. The PVC routing table supports entries for transmit and entries for receive, and specifies to/from which interface the ATM cell of interest is delivered. In a customary manner, the PVC router is configured to analyze the contents of a respective packet presented to it and then selectively route the packet to the appropriate output port based upon the results of that analysis.  
           [0022]    For incoming ATM voice cells from the WAN, routing to the network processor is from the RX FIFO to the PHY1 portion of the dual UTOPIA L2 PHY interface; transmitted WAN voice routing from the processor toward the WAN is from the PHY1 portion of dual PHY layer to a voice WAN TX FIFO. For incoming voice calls from the TDM2 network, routing flows from the voice playout buffer unit to the PHY1 portion of dual PHY interface. Conversely, for outgoing ATM voice calls to the TDM2 network, routing is from the PHY1 portion of the dual PHY interface to the voice playout buffer unit.  
           [0023]    For ATM data cells received from the WAN by way of the ATM transceiver, routing of data to the network processor is from the WAN RX FIFO to the PHY0 port of the dual UTOPIA L2 PHY interface, whereas transmitted WAN data routing from the processor flows from the PHY0 portion of dual PHY layer to a WAN data (D) TX FIFO and to the WAN ATM transceiver.  
           [0024]    For HDLC traffic received from the WAN via an HDLC receiver, routing to the network processor is from the WAN RX FIFO to the PHY0 port of the dual UTOPIA L2 PHY interface  130 , whereas transmitted WAN data from the processor is from the PHY0 portion of the dual PHY layer to the WAN data transmit (DTX) FIFO and HDLC transceiver. For incoming auxiliary V.35 routing, the PVC router directs data entries in an V.35 RX FIFO to the PHY0 portion of the dual UTOPIA L2 PHY interface, and for outgoing auxiliary V.35 routing, the PVC router directs the AAL5 encapsulated data from the PHY0 portion of the dual UTOPIA L2 PHY interface into the V.35 TX FIFO.  
           [0025]    In the transmit direction (outgoing to the WAN from the network processor), the WAN ATM transceiver selectively interfaces to the WAN, either ATM data cells from the DTX FIFO or ATM voice cells from a voice transmit (VTX) FIFO. The VTX FIFO may also have a relatively small depth of 128 bytes due to the considerably higher speed of the UTOPIA L2 PHY bus. On the other hand, the data TX FIFO may have a much larger depth (e.g., on the order of 2K bytes), for buffering a relatively large number of cells or frames of data (such as a full size Ethernet frame with ATM overhead); this serves to accommodate transmission priority given to the voice TX FIFO, and helps to alleviate UTOPIA PHY0 backpressure at the host processor.  
           [0026]    The host processor monitors conventional buffer ‘watermarks’ in the transmit FIFOs, to keep the transmit FIFOs full during transmission. To avoid backing up a packet into the host processor&#39;s UTOPIA PHY interface FIFO structure, or ‘starving’ one of the transmit FIFOs in the MCI, the host processor waits for watermark confirmation before sending a new frame of data to the data transmit FIFO.  
           [0027]    The WAN ATM transceiver employs a priority-based, quality of service (QoS) steering mechanism to controllably interface either (PHY1-sourced) voice ATM cells buffered in the voice cell transmit FIFO, or (PHY0-sourced) data cells buffered in the data cell transmit FIFO. The QoS controller gives priority to (PHY1) voice cells, and continuously examines the voice cell transmit FIFO to determine whether it has voice cells awaiting transmission. If so (and the data transmit FIFO is not currently being read out), the QoS controller immediately couples the voice cell transmit FIFO to the WAN ATM transceiver, so that voice cells may be read out of the VTX FIFO to completion. However, if the data transmit FIFO is currently being read out, then upon completion of this operation, the QoS controller outputs any ATM voice cells buffered in the voice transmit FIFO to the WAN ATM transceiver for transmission over the WAN. However, if the voice cell transmit FIFO does not contain voice cells, the QoS controller allows any data cells buffered in the data transmit FIFO to be coupled to the WAN ATM transceiver for application to the WAN.  
           [0028]    For HDLC mode communications incoming from the WAN toward the network processor, the WAN HDLC transceiver interfaces ATM cells containing HDLC frames to the WAN RX FIFO. To provide ATM-compatibility with the dual UTOPIA L2 PHY interface, an ATM encapsulation mechanism performs HDLC-ATM conversion of the incoming frames, stripping off HDLC information and encapsulating the data using, for example, ATM Adaptation Layer 5 (AAL5) for storage in the RX FIFO. The AAL5 encapsulated frame buffered in the RX FIFO are read out and routed to the data (PHY0) portion of the dual UTOPIA L2 PHY interface for transport to the network processor. In the transmit direction to the WAN, ATM cells containing AAL5-encapsulated HDLC data interface from the host processor are buffered into the DTX FIFO by the PVC router and then converted by the ATM encapsulation mechanism back into HDLC frames. The WAN HDLC transceiver then outputs the HDLC frames through the XCS for application to the WAN.  
           [0029]    The NxPORT HDLC transceiver is configured similar to the WAN HDLC transceiver and provides the ability to interface ATM cell traffic on the PHY0 portion of the dual UTOPIA L2 PHY interface with an auxiliary digital communication path. In the receive direction from the Nx communication path toward the network processor, the NxPORT HDLC transceiver interfaces ATM-encapsulated data cells to a V.35 RX FIFO. These ATM-encapsulated cells contain the contents of the auxiliary protocol (e.g., V.35) data frames (e.g., FRP or PPP) that are coupled to the Nx mux/demux. ATM-encapsulation is used by NxPORT HDLC transceiver to provide ATM-compatibility with the dual UTOPIA L2 PHY interface.  
           [0030]    In the transmit direction to the Nx communication path from the host processor, ATM cells containing AAL5-encapsulated HDLC data, are buffered into a V.35 TX FIFO by the PVC router. The host processor monitors buffer watermarks in the V.35 TX FIFO, to keep the V.35 TX FIFO full during V.35 mode transmission, and waits for watermark confirmation before sending a new frame, to avoid back into the host processor&#39;s UTOPIA PHY interface FIFO structure, or ‘starving’ the V.35 TX FIFO. Outgoing ATM cells buffered in the V.35 TX FIFO from the PVC router are converted by the ATM encapsulation mechanism back into V.35 data. The NxPORT HDLC transceiver then outputs the V.35 data to the Nx mux/demux for application to the auxiliary (Nx56/64) digital communication path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 diagrammatically illustrates the overall architecture of a non-limiting, but preferred, embodiment of the dual PHY-based integrated access device of the &#39;402 application;  
         [0032]    [0032]FIG. 2 diagrammatically illustrates a bidirectional playout buffer of the voice playout buffer unit of the present invention and employed in the multi-protocol communication interface of the IAD architecture of FIG. 1;  
         [0033]    [0033]FIG. 3 highlights a DS0 cross-connect path  3000  between ports of the digital cross-connect switch of the dual PHY-based integrated access device shown in FIG. 1;  
         [0034]    [0034]FIG. 4 highlights a DS0 cross-connect path  4000  between a voice TDM port and an internal TDM bus port of the digital cross-connect switch of the dual PHY-based integrated access device shown in FIG. 1;  
         [0035]    [0035]FIG. 5 highlights a further TDM transport path  5000  between a mux/demux and an NxPort of the digital cross-connect switch of the dual PHY-based integrated access device shown in FIG. 1;  
         [0036]    [0036]FIG. 6 highlights an ATM voice cell transport path  6000  from the WAN port to a TDM port in the dual PHY-based integrated access device shown in FIG. 1;  
         [0037]    [0037]FIG. 7 highlights an ATM voice cell transport path  7000  from the TDM port to the WAN port in the dual PHY-based integrated access device shown in FIG. 1;  
         [0038]    [0038]FIG. 8 highlights an ATM data cell transport path  8000  from the WAN port to the network processor transport direction, in the dual PHY-based integrated access device shown in FIG. 1;  
         [0039]    [0039]FIG. 9 highlights an ATM data cell transport path  9000  from the network processor to the WAN port transport direction, in the dual PHY-based integrated access device shown in FIG. 1;  
         [0040]    [0040]FIG. 10 highlights an HDLC transport path  10000  from the WAN port to the network processor transport direction, in the dual PHY-based integrated access device shown in FIG. 1;  
         [0041]    [0041]FIG. 11 highlights an HDLC transport path  11000  from the network processor to the WAN port transport direction, in the dual PHY-based integrated access device shown in FIG. 1;  
         [0042]    [0042]FIG. 12 highlights a V.35 NxPORT HDLC transport path  12000  from an NxPORT interface port to the network processor, in the dual PHY-based integrated access device shown in FIG. 1; and  
         [0043]    [0043]FIG. 13 highlights a V.35 NxPORT HDLC transport path  13000  from the network processor to the NxPORT interface port, in the dual PHY-based integrated access device shown in FIG. 1;  
     
    
     DETAILED DESCRIPTION  
       [0044]    Before detailing the dual PHY-based integrated access device described briefly above, it should be observed that such device resides primarily in a prescribed arrangement of conventional digital communication circuits and components, and an attendant host communications microprocessor, and application software therefor, that controls the operations of such circuits and components. In a practical implementation, the invention may be readily constructed of field programmable gate array (FPGA)-configured, digital application specific integrated circuit (ASIC) chip sets. Consequently, in the drawings, the configuration of such circuits and components, and the manner in which they may be interfaced with various telecommunication circuits have, for the most part, been illustrated by readily understandable block diagrams, which show only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagrams of the Figures are primarily intended to show the various components of the invention in convenient functional groupings, so that the invention may be more readily understood.  
         [0045]    Attention is now directed to FIG. 1, which diagrammatically illustrates the overall architecture of a non-limiting, but preferred, embodiment of the dual PHY-based integrated access device of the &#39;402 application and employing the voice playout buffer of the present invention. As shown therein this new architecture comprises two essential components: 1—a multi-protocol communication interface (MCI)  100 ; and 2—an associated communication processor  200 , hereinafter referred to as a host network processor (HNP). The MCI  100  has no intelligence of its own, but performs digital communication signaling interface functions in accordance with supervisory control inputs supplied by way of a generic, host processor interface (HPI)  160  from HNP  200 . Advantageously, the host processor may be implemented using any one of a variety of commercially available network processor chips, such as, but not limited to, RISC/CISC based processors with integrated memory controllers, chip select logic, I/O debug interfaces, ATM and Ethernet interfaces of the type available from vendors including Motorola, Infineon, Texas Instruments, IDT, and Virata/Globespan.  
         [0046]    In order to provide signal transport and network processor control interconnectivity, MCI  100  contains a plurality of signaling interface ports P 1 -P 6 , of which ports P 1 -P 5  interface digital communication signals with the HNP  200  and various external communication paths, and port P 6  of which interfaces control signals with the HNP  200 . In particular, a first, wide area communication network (WAN) port P 1  terminates a WAN  10  with a first port  111  of a conventional bidirectional digital cross connect switch (XCS)  110 , and provides both ATM and high level data link control (HDLC) connectivity with the WAN  10 .  
         [0047]    A second, voice TDM or TDM2 port P 2  terminates a second port  112  of the digital cross connect switch with a voice TDM circuit  20 , and provides digital transport connection to various TDM communication transceivers, such as analog codecs and T1 (including fractional T1) transceivers. Port P 2  may be configured as a conventional TDM mode port and supports standard TDM control parameters, including Frame Sync, transmit and receive clock and data signals. In addition, port P 2  is coupled to an adaptive clocking unit  260 , which is operative (during ATM mode operational mode) to adjust clock (Clk) and frame sync (Fs) to incoming cell delivery timing over an internal TDM bus  210  from a bidirectional voice gateway  150 .  
         [0048]    For this purpose, adaptive clocking unit  260  may be configured as a digital phase locked loop (DPLL) based adaptive clock recovery mechanism, of the type disclosed in the above-referenced &#39;463 application. As described therein, this DPLL-based adaptive clock recovery mechanism produces a recovered clock based upon a DPLL&#39;s phase detector&#39;s count of the number of high frequency service clock cycles that occur between transitions in an input signal representative of instances of receipt of ATM cells written into a cell jitter buffer and subject to cell delay variations through the cell transport path, and a reference clock signal whose frequency is a prescribed fraction of that of the output clock.  
         [0049]    Installed within the internal TDM bus  210  is a cascaded arrangement of a TDM transport path-cascaded echo canceler  270  and ADPCM voice compression operator  280 , which are preferably of the types disclosed in the above-referenced &#39;375 application. As described therein, this cascaded compression and echo cancellation arrangement implements G.726 ADPCM voice compression and G.168 echo cancellation by operating directly on the TDM encoded voice stream. Producing a processed digitized voice signal stream in this manner relieves the host processor of having to use data bus cycles to download processed digitized voice samples.  
         [0050]    The TDM bus  210  is also coupled to a dual tone multifrequency detector (DTMF) unit  250  which contains a plurality of DTMF detectors, that may be selectively dedicated to tone sensing functions for signaling operations on the TDM bus. For example, for a 32 TDM voice channel example of the present embodiment, the DTMF unit  250  may include a practical number of DTMF detectors (e.g., sixteen) for any DS0 via the bidirectional digital cross connect switch (XCS)  110 , to provide DTMF detection where required for digital collection and analysis. In addition, the DTMF unit  250  provides the MCI with the ability to detect dial tone.  
         [0051]    A third, UTOPIA port P 3  terminates a dual UTOPIA L2 PHY interface  130  with a byte-wide, ATM cell-based UTOPIA bus  30 . This bus serves as the main ‘data’ or communication signal transport path with the host network processor. The dual UTOPIA L2 PHY interface  130  and its associated UTOPIA bus  30  operate at a very high clocking frequency (on the order of 25 MHz, which equates to a data transport rate on the order of 200 Mbps) relative to network and terminal rates, which have data rates on the order of only 1.5-2.0 Mbps (e.g., a WAN rate of 2304 kbps). As such, signaling transport communications between the MCI  100  and the host network processor  200  may be considered to effectively quasi-instantaneous, so that participation by the host processor in the transport of both digitized voice and data communication signals over any of the routing paths among the signaling ports of the MCI will not burden (slow down) the operational speed of any of the external communication circuits to which the IAD is ported.  
         [0052]    For this purpose, the dual UTOPIA L2 PHY interface  130  contains two separate PHY portions (PHY0 for data, and PHY1 for voice), each PHY layer being byte-wide and containing separate transmit (TX) and receive (RX) buses. The PHY1 portion is dedicated to voice signaling and has the higher priority of the two PHY portions. Conversely, the PHY0 portion (associated with data transport) is the lower priority of the two portions. The voice PHY portion (PHY1) of the dual UTOPIA L2 PHY interface  130  is used for voice ATM cell transfers between bidirectional voice playout buffers of a multi-channel voice playout buffer unit  290  of the present invention and the host network processor  200 , and for ATM voice cell transfers between the host network processor  200  and the WAN via a voice WAN FIFO  330 , as will be described. The data PHY portion (PHY0) of the dual UTOPIA L2 PHY interface  130  is used for data ATM cell transfers between the host network processor and sets of TX and RX data FIFOs, serving the WAN and an auxiliary V.35 circuit path, as will be described.  
         [0053]    A fourth, NxPORT P 4  of the MCI  100  terminates an external port  143  of a bidirectional multiplexer (mux/demux)  140  with an auxiliary (Nx56/64) digital communication path  40 , over which non cell-based (e.g., V.35) digital communications are conducted with an auxiliary digital communication device. The fifth communication port P 5  is a TDM legacy port, that terminates a first port  151  of the gateway  150  with a legacy voice TDM communication link  50 . As pointed out above, gateway  150  provides TDM connectivity with a TDM bus  210  containing the TDM transport path-cascaded echo canceler  270  and ADPCM voice compression operator  280 .  
         [0054]    The TDM bus  210  is further coupled to TDM voice port  113  of bidirectional digital cross connect switch (XCS)  110 . This internal TDM voice interconnect path makes the MCI compatible with legacy IAD architectures, such as those which employ a Motorola  860  processor. The TDM legacy port PS readily supports these architectures where TDM-IN and TDM-OUT interfacing are used. In such a legacy TDM mode, port PS is TDM-coupled to port P 2 , bypassing ATM signal processing paths that use the dual UTOPIA L2 PHY interface  130  and UTOPIA bus  30  to the host network processor.  
         [0055]    A second port  152  of the TDM voice gateway  150  is coupled via a link  212  to port  291  of bidirectional voice playout buffers of a multi-channel voice playout buffer unit  290 . As will be described, for the 32 voice channel example here, the voice playout buffer unit  290  comprises 32 channels of bidirectional first-in, first-out registers (FIFOs). Each FIFO is sized (e.g., has a 64 byte capacity) which is sufficient to store a full ATM voice cell payload (44 bytes), as well as accommodate transport delay to and from the host network processor, to allow for an effectively continuous interfacing/flow and conversion of TDM communication signals on the TDM bus  210  with ATM cells interfaced with dual UTOPIA L2 PHY interface  130  over a full duplex ATM cell bus  214  therebetween. The remaining port P 6  of the MCI terminates a control signal bus  60  with a generic, host processor interface (HPI)  160 , through which control signals are interfaced with the HNP  200  for configuring and managing the functionality of the MCI.  
         [0056]    As pointed out briefly above, the digital cross-connect switch (XCS)  110 , which may of conventional construction, provides first and second external communication signaling port terminations  111 /P 1  and  112 /P 2  with the WAN  10  and voice TDM circuit  20 , respectively. In addition to its two external ports  111  and  112 , XCS  110  includes a third, TDM voice port  113 , through which TDM voice circuit  20  is coupled to the internal TDM bus  210 . XCS  110  has a fourth, ATM port  114 , and a fifth, HDLC port  115 , which respectively provide connectivity between the WAN port  111  and a WAN ATM transceiver  220 , and a WAN HDLC transceiver  230 . The digital cross connect switch  110  further includes a sixth, Nx port  116 , that is coupled to a first internal port  141  of mux/demux  140 . A second internal port  142  of mux/demux  140  is coupled to an NxPORT HDLC transceiver  240 .  
         [0057]    The digital XCS  110  has two modes of operation: 1—direct DS0—mapping mode, and 2—ATM/HDLC transceiver interface mode. In DS0-mapping mode, the internal dual ATM PHY conversion and transport functionality of the MCI is effectively bypassed; instead, DS0 time slots on the voice TDM link  20  at port  112  are directly mappable to port  111  and WAN  10 , based upon a user-controlled mapping scheme. In addition, DS0 time slots on the voice TDM link  20  at port  112  are directly mappable to the voice port  113 , so that they may be coupled to the internal TDM bus  210 . DS0 time slots at port  112  may also be directly mapped via port  116  to port  141  of Nx mux/demux  140  for Nx56/64 clear channel (V.35) operation. As noted above, DTMF detector unit  250  coupled to internal TDM bus  210  may be used to analyze DTMF and dial tone signals.  
         [0058]    In ATM/HDLC transceiver interface mode, XCS  110  couples the WAN port  111  to the appropriate one of ATM and HDLC transceiver ports  114  and  115 , which are respectively coupled to WAN ATM transceiver  220  and WAN HDLC transceiver  230 . Considering first, ATM mode communications, in the receive direction (incoming from the WAN toward the network processor), the WAN ATM transceiver  220  is configured to interface, over an eight bit wide receive bus  222  to a receive (RX) FIFO  310 , incoming ATM cells that have been coupled thereto via port  114  of XCS  110 . As a non-limiting example, RX FIFO  310  may have a relatively small depth (e.g., 128 bytes, which accommodates two ATM cells or 106 bytes) due to the considerably higher speed of the UTOPIA L2 PHY bus. ATM cells supplied to RX FIFO  310  are output via a permanent virtual circuit (PVC) router  120  to the data (PHY0) portion of the dual UTOPIA L2 PHY interface  130 , for transport over UTOPIA bus  30  to the network processor.  
         [0059]    The PVC router  120  is preferably implemented using multibit table entries in internal memory to control or ‘steer’ the flow of ATM data cells of various virtual circuits within the MCI for voice and data signaling transport. For the 32 channel example of the present embodiment, the PVC routing table supports 32 entries for transmit and 32 entries for receive, and specifies to/from which interface the ATM cell of interest is delivered. In a customary manner, PVC router  120  is configured to analyze the contents of a respective packet presented to it and then selectively route the packet to the appropriate output port based upon the results of that analysis.  
         [0060]    For incoming ATM voice cells from the WAN  10 , routing to the network processor is from the RX FIFO  310  to the PHY1 port of the dual UTOPIA L2 PHY interface  130 , whereas transmitted WAN voice routing from the processor is from the PHY1 portion of dual PHY layer to the voice WAN FIFO  330 . For incoming voice calls from the TDM2 network  20 , routing is from the cell bus  214  serving the voice playout buffer unit  290  to the PHY1 portion of interface  130 , whereas outgoing voice calls to the TDM2 network  20 , routing is from the PHY1 portion of interface  130  over the cell bus  214  to the voice playout buffer unit  290 .  
         [0061]    For ATM data cells received via ATM transceiver  220  from the WAN  10 , routing to the network processor is from the RX FIFO  310  to the PHY0 port of the dual UTOPIA L2 PHY interface  130 , whereas transmitted WAN data routing from the processor is from the PHY0 portion of dual PHY layer to the WAN DTX FIFO  320  and to WAN ATM transceiver  220 . For HDLC traffic received via HDLC receiver  230  from the WAN  10 , routing to the network processor is from the RX FIFO  310  to the PHY0 port of the dual UTOPIA L2 PHY interface  130 , whereas transmitted WAN data routing from the processor is from the PHY0 portion of the dual PHY layer to the WAN DTX FIFO  320  and to HDLC transceiver  230 .  
         [0062]    For incoming auxiliary V.35 routing, the PVC router  120  directs data entries in the V.35 RX FIFO  340  to the PHY0 portion of the dual UTOPIA L2 PHY interface  130 ; for outgoing auxiliary V.35 routing, PVC router  120  directs the AAL5 encapsulated data from the PHY0 portion of the dual UTOPIA L2 PHY interface  130  into the V.35 TX FIFO  350 .  
         [0063]    In the transmit direction (outgoing to the WAN from the network processor), WAN ATM transceiver  220  selectively interfaces to the WAN data, either ATM data cells via a data byte bus  321  from a data transmit (DTX) FIFO  320  (which is coupled via PVC router  120  to the data portion (PHY0) of the dual UTOPIA L2 PHY interface  130 ), or ATM voice cells via a voice byte bus  331  from a voice transmit (VTX) FIFO  330  (which is coupled via PVC router  120  to the voice portion (PHYL) of the dual UTOPIA L2 PHY interface  130 ). As a non-limiting example, like RX FIFO  310 , VTX FIFO  330  may have a relatively small depth of 128 bytes due to the considerably higher speed of the UTOPIA L2 PHY bus.  
         [0064]    On the other hand, DTX FIFO  320  may have a much larger depth (e.g., on the order of 2K bytes), for buffering a relatively large number of cells or frames of data (such as a full size Ethernet frame with ATM overhead); this serves to accommodate transmission priority given to the VTX FIFO  330 , and helps to alleviate UTOPIA PHY0 backpressure at the host processor. Via processor interface  160 , the host processor monitors conventional buffer levels or ‘watermarks’ in the transmit FIFOs, in order to keep the transmit FIFOs full during transmission. To avoid undesirably backing up a packet into the host processor&#39;s UTOPIA PHY interface FIFO structure, or ‘starving’ one of the transmit FIFOs in the MCI  100 , the processor waits for watermark confirmation before sending a new frame of data to the DTX FIFO  320 .  
         [0065]    The WAN ATM transceiver  220  employs a priority-based, quality of service (QoS) steering mechanism  225 , which selectively interfaces either (PHY1-sourced) voice ATM cells buffered in voice cell transmit FIFO  330 , or (PHY0-sourced) data cells buffered in data cell transmit FIFO  320 . QoS controller  225  is configured to give priority to (PHY1) voice cells. For this purpose, QoS controller  225  continuously examines the voice cell transmit FIFO  330  to determine whether it has voice cells awaiting transmission. If so (and the data transmit FIFO  320  is not currently being read out), the QoS controller  225  immediately couples the voice cell transmit FIFO  330  to WAN ATM transceiver  220 , so that voice cells may be read out of the VTX FIFO  330  to completion. However, if data transmit FIFO  320  is currently being read out, then upon completion of this operation, QoS controller  225  outputs any ATM voice cells buffered in FIFO  330  to the WAN ATM transceiver  220  for transmission over the WAN  10 . So long as the voice cell transmit FIFO  330  does not contain voice cells, however, QoS controller  225  allows any data cells buffered in the data FIFO  320  to be coupled to WAN ATM transceiver  220  for application to WAN  10 .  
         [0066]    For HDLC mode communications, in the receive direction (incoming from the WAN toward the network processor), WAN HDLC transceiver  230  is configured to interface over an eight bit wide receive bus  221  to RX FIFO  310 , ATM cells containing the contents of incoming HDLC frames that have been coupled thereto via port  115  of XCS  110 . In order to provide ATM-compatibility with the dual UTOPIA L2 PHY interface  130 , WAN HDLC transceiver  230  employs an ATM encapsulation mechanism  235 , which performs HDLC-ATM conversion of the incoming frames (which may employ frame relay (FR) protocol, or point-to-point protocol (PPP), as non-limiting examples). The ATM encapsulation mechanism  235  is operative to strip off HDLC information and then encapsulate the remaining contents of the data using, for example, ATM Adaptation Layer 5 (AAL5) for storage in RX FIFO  310 . The contents of the AAL5 encapsulated frame buffered into the RX FIFO  310  are read out and routed via PVC router  120  to the data (PHY0) portion of the dual UTOPIA L2 PHY interface  130 , for transport over UTOPIA bus  30  to the network processor.  
         [0067]    In the transmit direction (to the WAN from the network processor), ATM cells containing AAL5-encapsulated HDLC data, as transported over the data portion (PHY0) of the dual UTOPIA L2 PHY interface  130  from the host processor, are buffered into the DTX FIFO  320  by the PVC router  120 . They are then coupled over byte-wide bus  322  from the DTX FIFO  320  and converted by the ATM encapsulation mechanism  235  back into HDLC frames. WAN HDLC transceiver  230  then outputs the HDLC frames to port  115  of XCS  110  for application to WAN  10 .  
         [0068]    As pointed out above, MCI  100  contains an additional (NxPORT) HDLC transceiver  240 , which is configured similar to WAN HDLC transceiver  230  and provides the ability to interface ATM cell traffic on the PHY0 portion of the dual UTOPIA L2 PHY interface  130  with an auxiliary (e.g., Nx56/64) digital communication path  40 . For this purpose, in the receive direction (incoming from the Nx communication path  40  toward the network processor), NxPORT HDLC transceiver  240  is configured to interface ATM-encapsulated data cells over an eight bit wide receive bus  341  to a (V.35) RX FIFO  340 . These ATM-encapsulated cells contain the contents of auxiliary protocol (e.g., V.35) data frames (e.g., FRP or PPP) that are coupled thereto via port  142  of Nx mux/demux  140 . Like FIFOs  310  and  330 , described above, V.35 RX FIFO  340  may have a relatively small depth of 128 bytes.  
         [0069]    As in the case of WAN HDLC transceiver  230 , ATM-encapsulation is used by NxPORT HDLC transceiver  240  to provide ATM-compatibility with the dual UTOPIA L2 PHY interface  130 . For this purpose, NxPORT HDLC transceiver  240  contains an ATM encapsulation mechanism  245  which performs HDLC-ATM (AAL5) conversion of the incoming frames (which may employ frame relay (FR) protocol, or point-to-point protocol (PPP), as non-limiting examples). The AAL5-encapsulated V.35 data is buffered in V.35 RX FIFO  340 , and then read out and routed via PVC router  120  to the data (PHY0) portion of the dual UTOPIA L2 PHY interface  130 , for transport over UTOPIA bus  30  to the network processor.  
         [0070]    In the transmit direction (to the Nx communication path  40  from the host processor  200 ), ATM cells containing AAL5-encapsulated HDLC data, as transported over the data portion (PHY0) of the dual UTOPIA L2 PHY interface  130 , are buffered into a V.35 TX FIFO  350  by the PVC router  120 . Like DTX FIFO  320 , V.35 TX FIFO  350  may have a depth on the order of 2K bytes, to accommodate buffering a full size Ethernet frame with ATM overhead), and alleviate UTOPIA PHY0 backpressure at the host processor. As with the DTX FIFO  320 , via interface  160 , the host processor monitors buffer watermarks in the V.35 TX FIFO, to keep the V.35 TX FIFO full during V.35 mode transmission, and waits for watermark confirmation before sending a new frame, to avoid back into the host processor&#39;s UTOPIA PHY interface FIFO structure, or ‘starving’ the V.35 TX FIFO. Outgoing ATM cells buffered into the V.35 TX FIFO  350  from the PVC router  120  are coupled over byte-wide bus  351  from the V.35 TX FIFO  350  and converted by the ATM encapsulation mechanism  245  back into V.35 data. The NxPORT HDLC transceiver  240  then outputs the V.35 data to port  142  of Nx mux/demux  140  for application to auxiliary (Nx56/64) digital communication path  40 .  
         [0071]    As described briefly above, the voice playout buffer unit  290  contains a plurality of (e.g., 32 voice channel-associated) bidirectional, first-in, first-out registers (FIFOs), each of which is sized (e.g., has a 64 byte capacity) to store a standard 44-byte payload of a full ATM voice cell (53 bytes), and also provide sufficient capacity to accommodate expected worst case transport delay to and from the host network processor; this serves to ensure effectively continuous interfacing/flow and conversion of TDM communication voice data on the TDM bus  210  with ATM cells that are interfaced with dual UTOPIA L2 PHY interface  130  over the full duplex ATM cell bus  214  therebetween.  
         [0072]    This may be readily understood by reference to FIG. 2, which diagrammatically illustrates an individual one of 32 (64 byte) bidirectional playout buffers  400 - 1 , . . . ,  400 - 32  that reside within the voice playout buffer unit  290 . For transmitting and receiving ATM cells via the full duplex ATM cell bus  214 , the voice playout buffer  400  is coupled to an ATM cell port  292 . For transmitting and receiving TDM data with respect to the TDM bus  210 , the voice playout buffer is coupled to a TDM port  291 .  
         [0073]    Consider the flow of TDM voice traffic received from the TDM bus  210  (as sourced from the TDM2 port P 2  that terminates port  112  of the digital cross connect switch with voice TDM circuit  20 ). For purposes of simplification, let it be initially assumed that all of the playout buffers are cleared or reset, so that received TDM voice traffic from TDM bus  210  are written into successive byte locations of the playout buffer  400 , beginning with the lowermost or ‘0’th byte location (as pointed to by a voice pointer (VP)), which is incremented through successive storage locations of the playout buffer, at the rate of the received data clock. As successively received voice sample data is written into the playout buffer from the TDM bus  210 , the voice pointer (VP) will eventually point to the forty-third byte location. At this time, 44 bytes of TDM voice data are ready to be immediately encapsulated into a 53 byte ATM packet and burst-routed via PVC  120  and the dual PHY layer  130  to the host processor for delivery to a downstream WAN circuit. For optimizing DMA transfer efficiency of as many playout buffers (up to 32) that currently have data for the host processor, the playout buffer unit employs a single write interrupt. At this single interrupt ATM cells for up to 32 channels of data are loaded in processor memory under DMA control.  
         [0074]    The fact that each individual voice playout buffer  400  has a sixty-four byte capacity means that for a 44 byte data field of a respective 53 byte ATM cell, there is a twenty-byte window within which the host processor must return a response ATM voice packet for the POTS channel of interest. ATM encapsulation of a respective 44 byte data field includes a four byte AAL2 header, a HEC byte and a four byte ATM header, as customarily employed in the art to realized a standard 53 byte ATM cell. Within the AAL2 header, the channel identification byte (CID) byte may be made programmable, so as to provide selective mapping to timeslots of a TDM frame, and thereby accommodate variations among different vendor equipments.  
         [0075]    In the return direction from the host processor, the ATM and AAL2 headers are stripped off and the 44 bytes of voice payload data are written into successive locations of the playout buffer, as pointed to by a cell pointer (CP), beginning with the location of the first byte of the 44 bytes that had just been burst out over the PHY bus to the processor. As long as the voice pointer (VP), which has been and continues to be incremented at the relatively slower TDM rate, has not reached the end (byte location  63 ) of the playout buffer and begun ‘wrapping around’ to the lowest byte location, and with the contents of the first 44 byte locations of the playout buffer having been read out to the processor and therefore stale, return voice cell data from the processor may be written into those same (stale data) byte locations ( 0 - 43 ) of the playout buffer from which the previous burst was received.  
         [0076]    Thus, if the host processor has (and due to its considerably higher speed is expected to have) returned a response ATM voice cell containing 44 bytes of TDM data to the playout buffer, before the end of the twenty byte window of the playout buffer has been reached, there can expected to be some byte differential (one to twenty bytes, in the present example of a 64 bytes capacity playout buffer) between the current location of the voice pointer (VP) and that of the cell pointer (CP). This flexibility offered by the practical size of the playout buffer greatly reduces the cost and complexity of the digitized voice transport path. Namely, as long as this ‘turn-around’ differential remains within the twenty byte window, continuity of voice packet flow (with no overflow and no underflow) will be effectively maintained throughout the call. If a return cell is not ready to send, the host processor will resend the last transmitted cell, to maintain continuous voice cell flow.  
         [0077]    Having described the overall architecture of the dual PHY-based signal integrated access device of the present invention, the following discussion will review the various communication signal (voice and data) flow paths through the IAD for its various modes of operation. Although these communication signal flow paths have been discussed in the context of the components through which they pass, using respective Figures to show each communication path in a bold overlay format on the architecture diagram of FIG. 2 is believed to facilitate an appreciation of the versatility and flexibility of the invention relative to the limited capabilities of conventional DSP-based IAD platforms, described above.  
         [0078]    TDM Voice Time Slot Cross-Connect Mapping Mode (FIGS.  3 - 5 )  
         [0079]    In this mode of operation, the IAD essentially provides DS0 cut-through or ‘patching’ together of voice time slots of external TDM circuits, so that the ATM cell transport functionality of the dual PHY MCI is effectively bypassed. FIG. 3 shows a DS0 cross-connect path  3000  between ports P 1  and P 2  of the digital cross-connect switch  110 . As described above, path  3000  is used in DS0-mapping mode to map DS0s on the voice TDM link  20  directly through the cross-connect switch to the WAN  10 , using a user-controlled mapping scheme. FIG. 4 shows a DS0 cross-connect path  4000  between port P 1  and the internal TDM bus port  113  of the XCS  110  to the internal TDM bus  210  and voice gateway  150 , so that DS0 time slots on the legacy TDM link  50  may be coupled to WAN  10 . FIG. 5 shows a further TDM path  5000  between mux/demux  140  and the NxPort  116  of the cross connect switch  110 , for directly mapping DS0 time slots for Nx56/64 clear channel (V.35) operation.  
         [0080]    2—TDM Voice—ATM WAN Communication Mode (FIGS. 6 and 7)  
         [0081]    In this mode, for the WAN to TDM link transport direction, shown by a path  6000  in FIG. 6, the XCS  110  couples the WAN port  111  to the ATM transceiver port  114  for connection to the WAN ATM transceiver  220 . ATM voice cells from the WAN are buffered from WAN ATM transceiver  220  into the WAN RX FIFO  310 . ATM voice cells in the RX FIFO  310  are then output via the PVC router  120  to the PHY1 portion of the dual UTOPIA L2 PHY interface  130  and UTOPIA bus  30  to the network processor. The voice ATM cells are then returned over the PHY1 portion of the PHY interface to the designated playout buffer associated with the destination channel, which the forty-four voice bytes per cell are loaded for that channel. From the playout buffer the store TDM data is read-out via gateway  150  and transported over internal TDM bus  210  for delivery via XCS  100  to the TDM voice link  20  at port P 2 .  
         [0082]    [0082]FIG. 7 shows a path  7000  for voice signal transport from the TDM link to the WAN as ATM cells. For incoming voice calls from the TDM2 network  20 , routing is from port P 2  of the cross connect switch XCS  110  over the internal TDM bus  210  and the voice gateway to the playout buffer unit. As described above, as groups of forty-four TDM bytes are read out of the voice playout buffer, they are assembled into ATM cells for transport over the ATM cell bus  214  to the PHY1 portion of interface  130 , and transported to the host processor. In the WAN direction, the ATM voice cells are returned from the processor over the PHY1 portion of the dual UTOPIA L2 PHY interface  130  and routed to VTX FIFO  330 .  
         [0083]    As pointed out above, the WAN ATM transceiver  220  employs a priority-based, quality of service (QoS) steering mechanism  225 , which gives priority to PHYL-sourced voice ATM cells buffered in the voice cell transmit FIFO  330  over PHY0-sourced data cells buffered in data cell transmit FIFO  320 . So long as the DTX FIFO  320  is not currently being read out, the QoS controller  225  immediately couples the voice cell transmit FIFO  330  to WAN ATM transceiver  220 , so that voice cells may be read out of the VTX FIFO  330  to completion.  
         [0084]    3—Data ATM Communication Mode (FIGS. 8 and 9)  
         [0085]    In this mode, for the WAN to the network processor transport direction, shown by a path  8000  in FIG. 8, the XCS  110  couples the WAN port  111  to the ATM transceiver port  114  for connection to the WAN ATM transceiver  220 . ATM data cells from the WAN are buffered from WAN ATM transceiver  220  into the WAN RX FIFO  310 . ATM data cells in the RX FIFO  310  are then extracted via the PVC router  120  to the PHY0 portion of the dual UTOPIA L2 PHY interface  130  and UTOPIA bus  30  to the network processor.  
         [0086]    The outgoing ATM WAN data path is shown at  9000  in FIG. 9, wherein outgoing ATM data cells from the processor are steered by the PVC router  120  off the PHY0 portion of the PHY interface and into the WAN DTX FIFO  320 . As pointed out above, read out of the WAN DTX FIFO  320  is controlled by the QoS controller  225 , which gives priority to voice cells awaiting transmission in the VOICE WAN VTX FIFO  330  until completion. However, if the voice cell transmit FIFO  330  is empty, the QoS controller  225  allows any data cells buffered in the data FIFO  320  to be coupled to WAN ATM transceiver  220  for application to the WAN.  
         [0087]    4—HDLC Communication Mode (FIGS. 10 and 11)  
         [0088]    For HDLC traffic received via the WAN HDLC transreceiver  230  from the WAN  10 , via the frame port  115  of XCS  110 , routing to the network processor is over a path  10000  shown in FIG. 10 from the RX FIFO  310  to the PHY0 portion of the dual UTOPIA L2 PHY interface  130 . Transmitted WAN HDLC data routing from the processor traverses a path  11000 , shown in FIG. 11, from the PHY0 portion of the dual PHY layer to the WAN DTX FIFO  320  and HDLC transceiver  230 , for application to frame port  115  of the XCS  110  and delivery to the WAN.  
         [0089]    5—V.35 Communication Mode (FIGS. 12 and 13)  
         [0090]    The NxPORT HDLC transceiver  240  is configured similar to the WAN HDLC transceiver  230  and provides the ability to interface ATM cell traffic on the PHY0 portion of the dual UTOPIA L2 PHY interface  130  with an auxiliary (e.g., Nx56/64) digital communication path  40 . FIG. 12 shows a path  12000  for the receive direction from the Nx communication path  40  toward the network processor. Here, the NxPORT HDLC transceiver  240  interfaces ATM data cells as encapsulated by ATM encapsulation mechanism  245  to V.35 RX FIFO  340 . These ATM-encapsulated cells contain the contents of auxiliary protocol (e.g., V.35) data frames (e.g., FRP or PPP) that are coupled to Nx mux/demux  140 . The AAL5-encapsulated V.35 data buffered in V.35 RX FIFO  340  is read out and routed via PVC router  120  to the data (PHY0) portion of the dual UTOPIA L2 PHY interface  130 , for transport over UTOPIA bus  30  to the network processor.  
         [0091]    [0091]FIG. 13 shows a path  13000  in the transmit direction to the Nx communication path  40  from the host processor  200 . Here, ATM cells containing AAL5-encapsulated HDLC data, as transported over the data portion (PHY0) of the dual UTOPIA L2 PHY interface  130 , are buffered into a V.35 TX FIFO  350  by the PVC router  120 . ATM cells buffered in the V.35 TX FIFO  350  from the PVC router  120  are coupled from the V.35 TX FIFO  350  and converted by the ATM encapsulation mechanism  245  back into V.35 data. The NxPORT HDLC transceiver  240  then outputs the V.35 data to port  142  of Nx mux/demux  140  for application to the auxiliary (Nx56/64) digital communication path  40 .  
         [0092]    As will be appreciated from the foregoing description, shortcomings of conventional DSP-based IADs can be effectively obviated by using a relatively high speed, dual PHY based transport path to interface a multi-protocol communication interface with a reasonably priced host network processor available from a variety of processor chips vendors. As the signaling transport speed of the dual PHY based path is an order of magnitude greater than that of any of the plurality of communication paths with which the IAD is interfaced, the IAD of the invention provides effectively real time support for different communication requirements, including TDM, ATM, HDLC, and the like.  
         [0093]    While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.