Abstract:
The invention comprises a system and method for transmitting and receiving VoDSL voice traffic that avoids, where possible, the transcoding of a speech signal multiple times. The invention advantageously provides enhanced Quality of Service (QoS) by opportunistically avoiding signal degradation where VoDSL to VoDSL gateway communications may be provided.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Technical Field of the Invention 
     This invention generally relates to the field of communication systems and, more particularly, to a Voice over Digital Subscriber Line (VoDSL) gateway to gateway communication system for use in the Public Switch Telephone Network (PSTN). 
     2. Description of the Background Art 
     Digital Subscriber Line (DSL) is a technology that enables simultaneous voice and data communication via a single transmission medium, such as a conditioned copper pair. DSL also provides a relatively high data rate for such data communications. Traditionally, if a subscriber wanted to receive and transmit voice and data simultaneously, or to have more than one voice line, the subscriber required more than one telephone line such additional telephone lines depended upon the availability of extra copper pairs being available from the local service provider&#39;s Central Office (CO) to the subscriber&#39;s business or home. With the advent of DSL, an additional telephone line is no longer required since DSL technology allows the simultaneous operation of a plurality of telephones and/or at least one data device over a single copper pair. 
     In providing VoDSL service, the convention is to digitize and compress, at the subscriber&#39;s premise, one or more voice inputs from respective telephones using a conventional voice compression algorithm. The compressed voice traffic is then multiplexed with any data traffic. The multiplexed voice and data traffic is then packetized into an Asynchronous Transfer Mode (ATM), Internet Protocol (IP) or frame relay format to be transmitted over a copper loop (or other medium) using DSL. At the CO, a Digital Subscriber Line Access Multiplexer (DSLAM), operates as a packet concentrator to multiplex a number of packetized voice/data signals and send the multiplexed signals to a VoDSL gateway, where the voice traffic is decompressed and converted to standard 64kb/s signals using Pulse Code Modulation (PCM). The VoDSL gateway interfaces with the Public Switch Telephone Network (PSTN) using typically the Bell Communications Research&#39;s Generic Requirements GR-303 or Technical Requirements TR008 protocol, and the PCM signal will be sent through the PSTN using Time Division Multiplexing (TDM) via Digital Signal level Zero (DS0) channels on a trunk, also known as a T 1 . 
     After being routed through the PSTN, the voice traffic is provided to a second VoDSL gateway that operates to compress, multiplex and packetize the received 64kb/s DS0 signal. The resulting signal is then transmitted to the subscriber&#39;s premises where the voice traffic will be decompressed, demultiplexed, depacketized and undigitized again. 
     SUMMARY OF THE INVENTION 
     The invention comprises a system and method for transmitting and receiving VoDSL voice traffic that avoids, where possible, the transcoding of a speech signal multiple times. The invention advantageously provides enhanced Quality of Service (QoS) by opportunistically avoiding signal degradation. 
     A method for routing voice traffic using a Public Switch Telephone Network (PSTN) according to the invention comprises the steps of: receiving, at a VoDSL gateway servicing a first subscriber, compressed voice traffic from the first subscriber for routing to a second subscriber; determining whether the second subscriber is served by a respective second VoDSL gateway; processing the received voice traffic in a manner dependent on whether the second subscriber is served by the respective second VoDSL gateway; and communicating with the second subscriber. 
     A gateway to gateway communication system according to the invention comprises: a Voice over Digital Subscriber Line (VoDSL) gateway, for communicating voice traffic between a first subscriber and a second subscriber via a Public Switch Telephone Network (PSTN); the VoDSL gateway, in response to a second VoDSL gateway communicating a signaling message via the PSTN, transmits the voice traffic in a transport packet to the second VoDSL gateway; and the VoDSL gateway, in response to a failure to detect the second VoDSL gateway, encoding and transmitting the voice traffic to the second subscriber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a high level block diagram of a communications system including the present invention; 
     FIG. 2 depicts a diagram of an Asynchronous Transfer Mode Adaptation Layer 2 (AAL2) packet data structure useful in understanding the operation of the communications system in FIG. 1; 
     FIG. 3 depicts a diagram of a transport packet data structure useful in understanding the operation of the communications system in FIG. 1; 
     FIG. 4 depicts a graphical representation of the transmission of a V. 120 packet over the Public Switch Telephone Network (PSTN); 
     FIG. 5 depicts a call flow diagram useful in understanding an embodiment of the present invention; and 
     FIG. 6 depicts a high level block diagram of an embodiment of a controller suitable for use within a Voice over Digital Subscriber Line (VoDSL) gateway. 
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described within the context of a pair of subscribers (A and B) communicating via respective Digital Subscriber Line (DSL) services. It should be noted that the present invention does not require a specific DSL service type, such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive DSL (RADSL), Single-line DSL (SDSL), Integrated Services Digital Network (IDSL) and the like. Therefore, those skilled in the art and informed by the teachings of the present invention will be able to readily adopt any appropriate DSL service type for use with the present invention. Moreover, the teachings of the invention are applicable to systems including a mix of DSL subscribers and non-DSL subscribers. 
     FIG. 1 depicts a high level block diagram of a communications system including the present invention. Specifically, the system of FIG. 1 comprises a first Integrated Access Device (IAD)  110  having a plurality of telephone ports  116   1 ,  116   2 , up to  116   N  (collectively telephone ports  116 ) and at least one data port  118 . Each of the telephone ports  116  comprises, for example, an RJ11 jack and associated conductors that is operatively coupled to a respective one of a first plurality of telephones  112   1, 112   2 , up to  112   n  (collectively telephones  112 ) for carrying voice traffic. The IAD  110  also comprises at least one data port  118 , for example a RJ45 jack and associated conductors for carrying data traffic (e.g., Ethernet data), operatively coupled to at least one computing device  114  at subscriber site A. 
     A first DSL service (DSL A ) utilizing a copper pair  120  physical layer is connected to the first IAD  110 . The first DSL service (DSL A ), and any other DSL services (not shown), are connected to a first Digital Subscriber Line Access Multiplexer (DSLAM)  130 . That is, the first DSLAM  130  may be operatively coupled to other respective pluralities of IADs or Residential Gateways (RGs). However, to simplify the discussion of the present invention, the first DSLAM  130  is shown as being coupled to only a single respective IAD  110  through a DSL service (DSL A ) utilizing a first copper pair  120 . 
     A first VoDSL gateway  150  is connected to the first DSLAM  130  via Asynchronous Transfer Mode (ATM A ) service utilizing a first carrier  140 , such as an Optical Carrier level 3 (OC3) or Digital Signal level 3(DS3) carrier. The first VoDSL gateway  150  gains access to a first switch  159  local to subscriber A within the Public Switch Telephone Network (PSTN) 160 via a first trunk  152 . The first trunk  152  comprises, illustratively, a T 1  having twenty-four Digital Signal level Zero (DS0) channels. The PSTN supports a communication between the first switch  159  coupled to a second switch  161 , where the second switch  161  is local to a second subscriber B. A second trunk  162 , supports communication between the first switch B  161  and a second VoDSL gateway  170 . A second trunk  162  supports communication between second local switch  161  and a second VoDSL gateway  170 . A second carrier  180  having ATM (ATM B ) service connects the second VoDSL gateway  170  to a second DSLAM  190 . Connecting a second IAD  210  to the second DSLAM  190  is a DSL (DSL B ) service utilizing, for example, a second copper pair  200 . 
     At subscriber site B, the second IAD  210  has a plurality of telephone ports  216   1 ,  216   2 , up to  216   N  (collectively telephone ports  216 ) and at least one data port  218 . The telephone ports  216  are operatively coupled to a first plurality of telephones  212   1 ,  212   2 , up to  212   n  (collectively telephones  212 ), and the at least one data port  218  is operatively coupled to at least one computing device  214 . 
     It should be noted that the operation of the first IAD  110 , DSLAM  130 , and VoDSL gateway  150  are similar to the operation of, respectively, the second IAD  210 , second DSLAM  190  and second VoDSL gateway  170 . Differences in operation will be described in more detail. 
     In the case of a voice communication from subscriber A to subscriber B, the first IAD  110  digitizes the voice signals from the telephones  112  and compresses those voice signals in a conventional manner using a voice compression algorithm. The compressed voice signal is in turn packetized and then multiplexed with any data signal from the at least one computing device  114  by the first IAD  110 . The resulting multiplexed signal is formatted according to, for example, the ATM packet structure using the ATM Adaptation Layer 2 (AAL2) protocol. The AAL2 packet data structure as adapted to the present invention will be described in more detail below with respect to FIG.  2 . 
     The AAL2 packets are transmitted to the first DSLAM  130  via the first DSL service (DSL A ). Specifically, in the embodiment of FIG. 1, the first DSL service (DSL A ) provides an ATM over DSL service. At the first DSLAM  130 , received voice packets and data packets are separated. The data packets are routed, for example, to the Internet or other computing network (not shown) while the voice packets are routed to the first VoDSL gateway  150  in the AAL2 protocol format over first carrier  140  having ATM (ATM A ) service. 
     The first VoDSL gateway  150  is depicted in FIG. 1 as including an optional controller  150 C, which will be described in more detail below with respect to FIG.  6 . Briefly, the optional controller  150 C comprises general purpose computing elements operative to implement the methods of the present invention. While not explicitly shown in FIG. 1, such an optional controller may also be included within the second VoDSL gateway  170 . 
     FIG. 2 depicts a diagram of an AAL2 packet data structure useful in understanding the operation of the communications system in FIG.  1 . Specifically, FIG. 2 shows the packet data structure of the Common Part Sub-layer (CPS) of an AAL2 packet  200  that may be used in the communications system of FIG. 1 between the first IAD  110  and first VoDSL gateway  150  and between the second IAD  210  and second VoDSL gateway  170 . Any differences between the standard AAL2 packet structure and the packet structure of FIG. 2 comprise data structure modification according to the present invention. 
     The CPS of an AAL2 packet  200  comprises an ATM Header  202  for routing the packets through an ATM network, an AAL2 header  203  and AAL2 payload fields  212   1 ,  212   2  up to  212   n  (collectively AAL2 payload) including packets associated with a compressed voice communication. An AAL2 header  203  comprises a User-to-User Indicator (UUI) fields  204   1 ,  204   2 , up to  204   m  for providing a link between the CPS sub-layer and a Service Specific Convergence sub-layer (SSCS) of the AAL2 packet, Header Error Check (HEC) fields  206   1 ,  206   2 , up to  206   n  for identifying errored cells, Length Indicator (LI) fields  208   1 ,  208   2  up to  208   n  for identifying the length of the packet payload  212  associated with each individual user and Channel Identifier (CID) fields  210   1 ,  210   2  up to  210   n  for identifying the individual channels within the AAL2 packet. 
     The above described packet structure may be transported as payload within a transport data packet structure as depicted in FIG.  3 . Specifically, FIG. 3 depicts the data structure of a V.120 packet  300  that may be used in the communications system of FIG.  1  and is useful in understanding how an AAL2 payload  212  is encapsulated in a V.120 packet  300 . The V.120 packet data structure is more thoroughly described in International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) V.120, which is hereby incorporated by reference in its entirety. 
     A V.120 packet  300  comprises four fields including a flag field  302 , an address field  304 , a control field  306  and a payload field  308  containing an AAL2 payload  212 . Within the context of the present invention, the V.120 packet data structure may be used to transport the AAL2 data structure depicted in FIG. 2 within a telecommunication network requiring V.120 compliance. The use of FIG. 3 is described more fully in a discussion of FIG.  4 . 
     FIG. 4 depicts a graphical representation of the transmission of a V.120 packet  300  over the PSTN 160. More specifically, FIG. 4 shows the process in which a plurality of compressed voice conversations are each encapsulated in a V.120 packet  300  and transmitted over individual DS0 channels on a trunk over the PSTN 160. 
     Referring to FIG. 4, at the first VoDSL gateway  150 , an arriving AAL2 packet payload  212  is extracted. Each payload  212  is placed in a V.120 payload  308 . This V.120 packet is communicated over the PSTN on at least one Logical Link (LL) dedicated to carrying voice traffic between the first VoDSL gateway  150  and the second VoDSL gateway  170 . For illustrative purposes, a Logical Link Identifier (LLI) of  20  will be assigned to carry voice traffic between the first VoDSL gateway  150  and the second VoDSL gateway  170 . 
     At the second VoDSL gateway  170 , the V.120 payload  308  is extracted and placed back into an AAL2 payload  212 . In this manner, the AAL2 packets are routed over the PSTN 160 as packets rather than being converted from packets to PCM signals. Hence no degradation in converting from packet to PCM and again from PCM to packet at the gateways. 
     The AAL2 packet is routed over a second carrier  180  having ATM (ATM B ) service to a second DSLAM  190 . The second DSLAM transmits the AAL2 packet on a DSL (DSL B ) service utilizing a second copper pair  200  where it reaches the second IAD  210  and is depacketized, uncompressed and converted to an analog voice signal. It should be noted, that although FIG. 4 depicts the transmission of the V.120 packet occurring in one direction, it occurs in both directions. 
     FIG. 5 depicts a call flow diagram useful in understanding an embodiment of the present invention. Party A initiates a telephone voice call by picking up the telephone  112  and dialing party B&#39;s phone number. At step  502 , the first LAD  110  communicates a Setup message to the first VoDSL gateway  150  indicating party B&#39;s telephone number, the voice compression rate, and whether silence suppression is being used. 
     At step  504 , after receiving the Setup message, the first VoDSL gateway  150  determines the called party&#39;s telephone number (i.e., party B&#39;s telephone number), the voice compression rate and whether silence suppression is being used. The first VoDSL gateway  150  interfaces with the first switch  159  using, for example, the known GR303 or TR 008  communication interfaces to communicate party B&#39;s telephone number to first switch  159 . 
     At step  503 , if the first VoDSL gateway  150  can communicate with first switch  159 , the first VoDSL gateway  150  communicates a Call_Proc message to the first IAD  110  indicating that the first VoDSL gateway  150  is able to process the call with first switch  159 . At step  506 , first switch  159  examines the called party&#39;s telephone number to determine the local switch that serves party B. A determination is made that second switch  161  serves party B, and a call is established with second switch  161 . 
     At step  508 , second switch  161  determines that the Setup message needs to be routed through the second VoDSL gateway  170 . The second VoDSL gateway  170  communicates the Setup message to the second IAD  210  in step  510 . The setup message is interpreted by the second IAD  210 , which responsively causes party B&#39;s telephone to ring. 
     In response to a ringing signal being communicated to party B by the second IAD  210 , the second IAD  210  communicates an Alert message to the second VoDSL gateway  170  at step  512 . The second VoDSL gateway  170 , in turn, communicates the Alert message to second switch  161  at step  514 . After receiving an Alert message from the second VoDSL gateway  170 , second switch  161  communicates the Alert message to first switch  159  at step  516 . At step  518 , the first switch  159  communicates the Alert message to the first VoDSL gateway  150  which, in turn, communicates the Alert message to the first IAD  110  at step  520 . 
     When party B goes “off hook”, the second IAD  210  communicates a Connect message to the second VoDSL gateway  170  in step  522  indicating that party B is off hook and that a connection is established. The second VoDSL gateway  170  communicates the Connect message to second switch  161  in step  524 . Second switch  161  communicates the Connect message to first switch  159  at step  526 . At step  528 , first switch  159  communicates the Connect message to the first VoDSL gateway  150 . 
     At step  530 , after receiving a Connect message, the first VoDSL gateway  150  encapsulates a VoDSL_GWY_Connect message with a V.120 frame and communicates it to the second VoDSL gateway  170  on a logical link dedicated to carrying signaling messages between the gateways. For illustrative purposes, it is assumed a logical link identifier of  10  has been assigned to carry signaling messages between the gateways. The VoDSL_GWY_Connect message contains parameters such as a call reference, party A&#39;s telephone number, party B&#39;s telephone number, a voice compression rate indicator, and a silence suppression indicator. 
     The first VoDSL gateway  150  expects to receive a response to the VoDSL_GWY_Connect message within a specific time interval. If there is no response to the VoDSL_GWY_Connect message within a specified time interval, the first VoDSL gateway  150  will assume there is no VoDSL gateway at the other end and will place a call using the conventional method of Pulse Code Modulation (PCM). 
     When the second VoDSL gateway  170  receives the VoDSL_GWY_Connect message, the second VoDSL gateway  170  determines, based on parameters contained in the message, whether a call can be established without making changes to the calling parameters. If the second VoDSL gateway  170  can not make the modification (for example the second VoDSL gateway  170  uses a different voice compression algorithm and can not do a conversion), then second VoDSL gateway  170  will communicate a VoDSL_Gateway_NACK message to the first VoDSL gateway  150 . The VoDSL_Gateway_NACK message only contains a call reference Information Element (IE). Upon receiving the VoDSL_Gateway_NACK message, the first VoDSL gateway  150  will then implement the call using PCM. 
     At step  532 , it is established that there is compatibility between the first VoDSL gateway  150  and the second VoDSL gateway  170 . The second VoDSL gateway  170  communicates a VoDSL_GWY_Connect_NACK message containing a call reference IE to the first VoDSL gateway  150 . The second VoDSL gateway has acknowledged the receipt of the first VoDSL gateway&#39;s  150  VoDSL_GWY message and can establish a call with party B. 
     At step  534 , the first VoDSL gateway  150  communicates a Connect message to the first IAD  110 . This indicates that continuity has been established with party B over a DS0 channel. At step  536 , an information flow is established between party A and Party B using the process described in FIG.  4 . 
     When a party hangs up, a Disconnect message is communicated and the connection is broken for the communication link. At step  538 , the first IAD  110  communicates a Disconnect message to the first VoDSL gateway  150  and disconnects the call between the first IAD  110  and the first VoDSL gateway  150 . At step  540 , the first VoDSL gateway  150  communicates a Disconnect message to the first switch  159  disconnecting the call between the first VoDSL gateway  150  and the first switch  159 . 
     The first switch  159  communicates the Disconnect message to the second switch  161  disconnecting the call between first switch  159  and second switch  161  at step  542 . At step  544 , the second switch  161  communicates a Disconnect message to the second VoDSL gateway  170  breaking the call. Finally, the second VoDSL gateway  170  communicates the Disconnect message to the second IAD  210  disconnecting the call between party A and party B. 
     The above-described invention advantageously provides a means of communicating voice traffic between VoDSL gateways in packetized form. Moreover, the invention advantageously does not require a conversion of the voice traffic from a packet format to a PCM format between gateways. In this manner, the invention provides a substantial improvement over prior art VoDSL gateway-to-gateway communication; thereby providing an improved voice quality signal. The invention advantageously provides enhanced Quality of Service (QoS) by opportunistically avoiding signal degradation where VoDSL to VoDSL gateway communications may be provided. 
     FIG. 6 depicts a high level block diagram of an embodiment of the optional controller  150 C suitable for use within a Voice over Digital Subscriber Line (VoDSL) gateway. Specifically, FIG. 6 depicts a high level block diagram of a VoDSL gateway controller  150 C suitable for use in VoDSL gateway  150  of the communication system  100  of FIG.  1 . The VoDSL gateway controller  150 C comprises a microprocessor  620  as well as memory  630  for storing a VoDSL processing method  632 . The microprocessor  620  cooperates with conventional support circuitry  640  such as power supplies, clock circuits, cache memory and the like as well as circuits that assist in executing the software methods of the present invention. As such, it is contemplated that some of the process steps discussed herein as software processes may be implemented with hardware, for example, as circuitry that cooperates with the microprocessor  620  to form various steps. 
     The VoDSL gateway controller  150  also comprises input/output circuitry  610  that forms an interface between the microprocessor  620 , the DSLAM  130 , the PSTN 160 and other VoDSL circuitry (not shown). 
     Although the VoDSL controller  150 C is depicted as a general purpose computer that is programmed to perform VoDSL control and processing functions in accordance with the present invention, the invention can be implemented in hardware, in software, or a combination of hardware and software. As such, the processing steps described above with respect to the various figures are intended to be broadly interpreted as being equivalently performed by software, hardware, or a combination thereof. It will be appreciated by those skilled in the art that the VoDSL controller  150 C provides sufficient computing functionality to implement the invention as described above. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.