Patent Publication Number: US-7212522-B1

Title: Communicating voice over a packet-switching network

Description:
RELATED APPLICATIONS AND CLAIM OF PRIORITY 
   This application is a continuation of and claims priority to U.S. patent application Ser. No. 10/409,994, filed on Apr. 8, 2003, now U.S. Pat. No. 6,768,733 entitled “COMMUNICATING VOICE OVER A PACKET-SWITCHING NETWORK”, which is a continuation of and claims priority to U.S. patent application Ser. No. 09/163,312, now issued as U.S. Pat. No. 6,570,869, filed on Sep. 30, 1998, entitled “COMMUNICATING VOICE OVER A PACKET-SWITCHING NETWORK”, the contents both of which are hereby incorporated by reference in their entirety for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates to telecommunications and more particularly to packet switched networking systems capable of carrying voice traffic. 
   BACKGROUND OF THE INVENTION 
   Recent legislative changes in the United States have promoted competition in the telecommunication industry and spurred demand for new services at lower prices. These trends are pressuring major telecommunications carriers to increase capacity while reducing the cost of providing service. Consequently, major carriers around the world are looking to packet technologies, such as Internet Protocol (IP), frame relay, and Asynchronous Transfer Mode (ATM), to replace circuit-switched technologies in the Public Switched Telephone Network (PSTN) for providing voice capability. In addition, IP, frame relay, ATM, and other packet-based technologies offer narrow-band and broad-band services to selected customers on the same network, providing the same platform for integrated voice, data, and video services from low bandwidth to very high bandwidths. 
   Over the decades, however, major voice carriers have invested heavily in developing a Signaling System 7 (SS7) signaling and switching infrastructure to offer reliable telephone service. This infrastructure includes countless systems for billing, provisioning, maintenance, and databases that are compatible only with SS7. These systems are commonly referred to “legacy systems,” a term that also includes other proprietary protocols such as ISDN_PRI, DPNSS, ISUP, TUP, NUP, H.323, and SIP. Due to the substantial investment in the legacy systems, it is desirable to keep the legacy systems in operation, yet still take advantage of the newer packet technologies. 
   These legacy systems, however, do not handle the protocols for the newer packet-switching networks, and, due to the age of many of the legacy systems, it is difficult and expensive to upgrade or replace the legacy systems to support the newer packet-switching protocols. 
   Accordingly, there exists a need for establishing and carrying voice calls that are originated or terminated by legacy systems over a packet-switching network. There is also a need for a way to seamlessly integrate legacy SS7-type systems and newer packet-switching networks. 
   Moreover, certain demographic trends are motivating telephone call carriers to integrate their systems with packet-switched networks. Certain countries are known to generate an above-average amount of long-distance telephone traffic. For example, residents of Israel are known to consume long-distance telephone services at a rate far greater than the average of residents in other industrialized nations. Long-distance telephone services carried over the PSTN are expensive. Voice calls carried over the globally accessible packet-switched network known as the Internet, however, are generally free. Accordingly, local telephone companies and other call access providers in certain countries are acutely interested in finding ways to integrate the PSTN with the Internet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is a diagram of a packet-switching network carrying voice signals; 
       FIG. 2  is a block diagram of a signaling unit; 
       FIG. 3  is a block diagram of a software architecture of a signaling unit; 
       FIG. 4  is a call flow diagram illustrating an establishment of a voice call and voice call release over a packet-switching network; 
       FIG. 5(   a ) is a diagram of another packet-switching network carrying voice signals; 
       FIG. 5(   b ) is a diagram of still another packet-switching network carrying voice signals; and 
       FIG. 5(   c ) is a diagram of yet another packet-switching network carrying voice signals. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A telecommunications method, network, and devices for carrying voice over a packet-switching network are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
   Network Overview 
     FIG. 1  depicts a telecommunications network that carries voice calls from an originating node  100  to a terminating node  160  over a packet-switching network  130 , in which the voice signaling processing is separated from the processing of the voice data. More specifically, the voice signaling aspects of establishing and handling voice calls over packet-switching network  130  are provided by one or more signaling units, for example, the originating signaling unit  120  and the terminating signaling unit  140 . The aspects relating to the voice traffic of a voice call are handled by one or more coding units, for example, the originating coding unit  110  and the terminating coding unit  150 . 
   For purposes of illustration,  FIG. 1  depicts a network configuration in which the originating coding  110  and the terminating coding unit  150  are coupled to respective signaling units, namely originating signaling unit  120  and terminating signaling unit  140 . As described in more detail hereinafter, however, the signaling processing functionality for the originating signaling unit  120  and the terminating signaling unit  140  can be incorporated into a single signaling unit. Even though the signaling units and the coding units are generally described herein in terms of being separate devices, which can be geographically remote from one another, a signaling unit and a coding unit may also be incorporated as respective subsystems of a single computer system. Thus, the present invention is not limited to the configuration depicted in  FIG. 1 . 
   Originating node  100  can be implemented as a Private Branch eXchange (PBX), a telephone switch, a “smart phone” capable of generating voice calls, a wireless PBX, or a legacy telecommunications system. Similarly, terminating node  160  can also be a PBX, telephone switch, telephone, a wireless PBX, or legacy telecommunications system. 
   Packet-switching network  130  is a network designed to carry information in the form of digital data packets. In such a network, data to be transmitted is subdivided into one or more individual packets of data, each having a unique identifier and a destination address. Each packet is individually routed or switched to the destination address, and individual packets for a single body of data may traverse the packet-switching network by different routes. In fact, the individual packets may even arrive at the destination in a different order from which they were shipped, to be reassembled at the destination in the proper sequence based on the packet identifiers. Packet-switching network  130  can be implemented as an IP network, an ATM network, a frame relay network, or by any other packet-switching technology. In some implementations, the packet-switching network  130  may even be overlaid on the PSTN. One example of packet-switching network  130  is the global packet-switching network known as the Internet. 
   The telecommunication network of  FIG. 1  includes an originating coding unit  110  and a terminating coding unit  150  functioning as gateways between the respective originating node  100  and the terminating node  160  and the packet-switching network  130 . The originating coding unit  110 , coupled to the originating node  100  by a trunk such as a T1 line or an E1 line, converts multiplexed voice data produced by originating node  100  into packets for the packet-switching network  130 . The voice data produced by originating node  100  may be, for example, Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) information. The originating coding unit  110  can also be configured to support voice data encoding and decoding as well as associated functions such as echo cancellation, voice activity detection, and voice compression. Similarly, the terminating coding unit  150  is also configured to convert between multiplexed voice data and voice data packets as well as the encoding and decoding functions. 
   While a major purpose of the origination coding unit  110  is to terminate the bearers from PBX  100 , in some embodiments the originating coding unit  110  is also configured to extract or “groom” the signaling data associated with the incoming voice call from originating node  100 . This signaling data is then transmitted or “backhauled” over a backhaul signaling link  112  to a signaling apparatus such as originating signaling unit  120 . The backhaul signaling link  112  can be implemented in various ways, including by an IP connection over Ethernet or other Local Area Network (LAN) technology such as token ring. The signaling data in the voice call can be Channel Associated Signaling (CAS), in which the signaling bits are isolated, time stamped, packaged in IP or ATM packets, and shipped to the originating signaling unit  120 . 
   Similarly, the terminating coding unit  150  is coupled by a backhaul signaling link  152  to a signaling apparatus such as terminating signaling unit  140 . The terminating coding unit  150  is configured for receiving signaling messages from the terminating signaling unit  140  and appropriately transmitting them to the terminating node  160 . Preferably, the coding units are implemented to be symmetrical, supporting the functionality of both the originating coding unit  110  and the terminating coding unit  150  as described herein. In fact, a single coding unit can performing the both the originating and terminating functionality for the same call. 
   Alternatively, the signaling data can be Common Channel Signaling (CCS), such as an ISDN PRI, in which case the signaling data is directly transported to the originating signaling unit  120 . In an embodiment wherein originating node  100  implements a CCS signaling such as U.S. SS7 signaling, the signaling data can be directly transmitted over link  113  to the originating signaling unit  120  bypassing the originating coding unit  110  entirely. Similarly, when terminating node  160  implements such signaling, the signaling data can be directly transmitted over link  153  from the terminating signaling unit  140  to the terminating node  160 , bypassing the terminating coding unit  150 . By these techniques, the originating signaling apparatus  120  is advantageously capable of receiving the signaling data associated with the voice call in a flexible manner, suitable for interfacing with diverse legacy systems. 
   The originating signaling unit  120  and the terminating signaling unit  140  implement a “virtual switch” and are responsible for processing and routing the signaling messages that are exchanged to set up and tear down a voice connection. Thus, the signaling units perform such functions as call resolution, call routing, bearer selection, and generation of call detail records (CDRs) for billing management. In one embodiment, the signaling units also convert the legacy protocols of the originating node  100  and the terminating node  160 , such as DPNSS, ISDN_PRI, SS7/C7 (including ISUPs, TUPs, and NUPs), H.323, SIP, or CAS, into messages for communicating with one another and for controlling a coding unit over control links  114  and  154 . Control links  114  and  154  can be implemented over IP or ATM and, in fact, on the same channel as the respective backhaul signaling link  112  and  152 , respectively. Through the control link, a coding unit is controlled by a signaling unit, for example, to establish a bearer channel for the voice data over the packet-switching network  130 . 
   In the configuration depicted in  FIG. 1 , a voice call from originating node  100  is received by the originating coding unit  110 , which, if necessary, extracts the signaling data associated with the voice call and transmits the signaling data over the backhaul signaling link  112  to originating signaling unit  120 . In response, the originating signaling unit  120  obtains the network address of the originating coding unit  110  within the packet-switching network  130  by accessing configuration data stored on the originating signaling unit  120 , by querying the originating coding unit  110  over the control link  114 , or by inquiring another computer system (not shown) such as domain name server (DNS). 
   Next, the originating signaling unit  120  determines which terminating signaling unit  140  should receive the call by accessing internal routing tables or querying external systems. After the originating signaling unit  120  has performed this call routing capability, the originating signaling unit  120  transmits a signaling message, including information for establishing the voice and the network address of the originating coding unit  110 , through network  132  to terminating signaling unit  140 . 
   In response, the terminating signaling unit  140  determines which bearer should receive the call. After performing the bearer selection, the terminating signaling unit  140  controls the terminating coding unit  150  to establish a bearer channel for the voice through packet-switching network  130  and repackages the signaling messages for terminating node  160  over backhaul signaling link  152 . In this manner, a voice call that is originated from a legacy system  100  or terminated by a legacy system  160  is seamlessly established over the packet-switching network  130  without having to upgrade or replace the legacy systems. 
   Hardware Overview 
   In a preferred embodiment, the signaling units are implemented by protocol converters that are configured to act as a virtual switch, but in alternative embodiments, especially where protocol conversion is not required, the signaling units are implemented directly as a virtual switch. A protocol converter is a telecommunications device capable of processing and converting at least those messages for establishing a connection between different protocols. For example, a protocol converter can convert messages between the DPNSS protocol and the ISDN protocol. In one configuration, the protocol converters are coupled to signaling network  132 , which can be the same as the packet-switching network  130 , and communicate with each other according to a common protocol regardless of the protocol of the respective legacy originating and terminating nodes. Consequently, legacy systems employing different protocols can communicate with one another voice over a packet-switching network. 
   In a preferred embodiment, protocol converters that implement originating signaling unit  120  and the terminating signaling unit  140  each comprise three abstract machine components that are instantiated for each call handled by the protocol converter. These abstract machine components are referred to as an originating call control (OCC)  122 , a universal call model (UCM)  124 , and a terminating call control (TCC)  126 . The originating call control (OCC)  122 , instantiated at the start of the call, converts signaling messages between the protocol of the originating side, for example, DPNSS, and a non-protocol specific universal protocol. 
   The universal call model (UCM)  124 , typically instantiated at the start of the call, handles calls in the converted universal protocol, arranges for messages to be routed ultimately to the other protocol converter, and controls the originating coding unit  110  over a control link  114 . The control link  114  can be implemented in various ways, including by an IP connection over a LAN. In an alternative embodiment, only two abstract machine components for the OCC  122  and the TCC  126  are implemented, with the functionality for the UCM  124  being distributed over the OCC  122  and the TCC  126 . 
   The terminating call control (TCC)  126 , typically instantiated after routing analysis has determined the route, converts signaling messages between the universal protocol and the protocol that provides connectivity to the terminating signaling unit  140 , which may in fact be different from the protocol of the terminating node  160 . For the example, the protocol of the terminating signaling unit can be an extension of Integrated Services Digital Network User Part (ISUP), described in more detail hereinafter and referred to as “XISUP”, while the protocol of the terminating node  160  is a legacy protocol such as DPNSS. Likewise, the protocol converter that implements the terminating signaling unit  140  includes an OCC  142 , a UCM  144 , and a TCC  146 . 
   One implementation of a protocol converter is described in more detail in the commonly assigned, co-pending U.S. patent application Ser. No. 08/904,295 entitled “Universal Protocol Conversion,” filed on Jul. 31, 1997 by Lev Volftsun, Clay H. Neighbors, David S. Turvene, Fred R. Rednor, Anatoly V. Boshkin, and Mikhail Rabinovitch, the entire contents of which are hereby incorporated by reference as if fully set forth herein. The above-referenced patent document discloses structural and functional details of an embodiment of a protocol converter that can be used to implement the originating signaling unit  120  and terminating signaling unit  140 . For purposes of context in this document, however, an overview of such structures and functions in an alternative embodiment is now provided. 
   Referring to  FIG. 2 , the hardware components, computer system  200 , of a protocol converter include a bus  202  or other communication mechanism for communicating information between internal components of the computer system  200 . A central processing unit (“CPU”)  204 , comprising one or more processors, is coupled with bus  202  for processing information. Computer system  200  also includes a main memory  206  coupled to bus  202  for storing information and instructions to be executed by CPU  204 . Main memory  206  typically includes a random access memory (“RAM”) or other dynamic storage device, for storing temporary variables or other intermediate information during execution of instructions to be executed by CPU  204 . Main memory  206  may also include a read only memory (“ROM”) or other static storage device for storing static information and instructions for CPU  204 . A storage device  208 , such as a magnetic disk, magnetic tape, or optical disk, is provided and coupled to bus  202  for storing information and instructions. 
   In some implementations, computer system  200  includes a video card  210  coupled to bus  202  for controlling display unit  212 , such as a cathode ray tube (CRT), a liquid crystal display (LCD), a video monitor or other display device, to display information to a computer user. An input device  214  is coupled to bus  202  for communicating information and command selections from a user to CPU  204 . Typically an input device includes a keyboard with alphanumeric, symbolic, and cursor direction keys for receiving input from a user in the form of commands and data entry and communicating the input to CPU  204 . The input device typically includes a cursor control input device, such as a mouse or a trackball, integral with or separate from the keyboard, for controlling cursor movement on display unit  212 , and communicating direction information and command selections to CPU  204 . A cursor control input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. In other implementations, these devices are connected to the computer system via a local area network such as Ethernet. 
   Computer system  200  also includes a communication interface  218  coupled to bus  202  and comprising, for example, a plurality of I/O cards  218   a  through  218   j . Ten I/O cards  218   a  through  218   j  are shown in  FIG. 2 , but any number of I/O cards, modems, transceivers, or other I/O devices may be used. Communication interface  218  provides a two-way data communication coupling to one or more coding units and zero or more other signaling units. Some of the I/O cards  218   a – 218   j  can be coupled directly to SS7 or DPNSS links via multiplexer/digital cross connect (not shown). 
   At least one of the I/O cards, for example I/O card  218   a , is coupled to a coding unit through control link  220 . Communication interface  218  may include an integrated services digital network (ISDN) card, terminal adapter, or modem for providing a data communication connection to a corresponding type of telephone line. As another example, communication interface  218  may include a local area network (LAN) card to provide a data communication connection to a compatible LAN, for example an Ethernet network. Wireless links, such as infrared, for communication interface  218  may also be implemented. In any such implementation, communication interface  218  sends and receives electrical, electromagnetic or optical signals that carry digital or analog data streams representing various types of information, in the form of carrier waves transporting the information. 
   This configuration enables the use of a computer system  200  for establishing voice connections in a packet-switching network. For example, such functionality is provided by computer system  200  in response to CPU  204  executing one or more sequences of one or more instructions arranged in main memory  206 . Such instructions may be written into main memory  206  from another computer-readable medium, such as storage device  208 . Execution of the sequences of instructions contained in main memory  206  causes CPU  204  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  206 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
   The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to CPU  204  for execution. Such a medium may take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device  208 . Volatile media include dynamic memory, such as main memory  206 . Transmission media include coaxial cables, copper wire and fiber optics, including the wires that constitute bus  202 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
   Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to CPU  204  for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer and downloaded to computer system  202 . The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A communications interface  218  local to computer system  200  can receive the data on a telephone line or other network or telecommunication link and place the data on bus  202 . Bus  202  carries the data to main memory  206 , from which CPU  204  retrieves and executes the instructions. The instructions received by main memory  206  may optionally be stored on storage device  208  either before or after execution by CPU  204 . The received instructions may be executed by CPU  204  as it is received, and/or stored in storage device  208 , or other non-volatile storage for later execution. In this manner, computer system  200  may obtain application code in the form of a carrier wave. 
   Software Architecture 
     FIG. 3  schematically illustrates a software architecture relating to protocol conversion implemented on a computer system  200  of a protocol converter that implements originating signaling unit  120  and terminating signaling unit  140 . The software architecture includes an I/O subsystem  300  for handing OSI Layer 2 (data link layer) messages and a protocol conversion engine  310  for handling messages at OSI Layer 3 (network layer). I/O subsystem  300 , containing I/O channel controllers  302 ,  304 , and  306 , is configured for handling incoming connection requests and other incoming messages. For example, I/O subsystem  300  can convert OSI Layer 2 frames and packets that transport the message into an OSI Layer 3 networking protocol data unit, which is a populated data structure that represents the contents of the messages. Specifically, I/O subsystem  300  may be configured to convert LAP-D (Link Access Protocol-D) frames or Ethernet frames into protocol data units. Moreover, the I/O subsystem  300  is also responsible for converting protocol data units generated by the protocol conversion engine  310  into frames and packets as appropriate for transmission in the telecommunications network. Each I/O channel controller  302 ,  304 , and  306  is responsible for messages from a network channel handled by a corresponding I/O card of communications interface  218 . 
   The protocol conversion engine  310  includes a plurality of protocol adapters, implemented to support respective protocols or protocol families, and a number of call instances corresponding to each active call. A protocol adapter is a software module responsible for interfacing the protocol conversion engine  310  with the I/O subsystem  300 . Specifically, a protocol adapter, when loaded and executed, is configured to connect with I/O subsystem  300  in order to route protocol-specific messages between an I/O channel and the appropriate call instance. Multiple instances of the same protocol adapter may be loaded and executed, each of which is associated with a respective I/O channel. Although the protocol adapters are fundamentally bi-directional, it is convenient to refer to an originating protocol adapter  312 , an external protocol adapter  314 , and a terminating protocol adapter  316 , based on their particular function during a call. Thus, a protocol adapter can be employed as an originating protocol adapter  312  for one call and as a terminating protocol adapter  316  for another call. 
   An originating protocol adapter  312  is capable of decoding an incoming message to determine the call with which the message is associated. Each message transmitted on a signaling channel contains a protocol-dependent value that serves to disambiguate messages for different calls from the same logical signaling channel. Every telecommunications protocol provides some means for matching up a message with an associated call, for example, a specific call identifier embedded in the message (e.g., the Call Reference field used in ISDN_PRI) or the bearer channel identifier (e.g. with DPNSS and SS7/ISUP), but the present invention, which is capable of supporting many different protocols, is not limited to any particular means of matching up messages with the associated calls. Also, each call is identified internally with a unique integer identifier for the signaling unit, referred to as a “Global Call Reference,” which is generated when the call is first instantiated. The Global Call Reference distinguishes simultaneously handled calls from one another. When concatenated with a network or other identifier of the signaling unit, the Global Call Reference serves to create a Universal Call Reference for the call that is unique for the network. 
   Accordingly, the originating protocol adapter  312  is configured to associate the Global Call Reference with a corresponding call instance  320 . The corresponding call instance  320  is responsible for processing the call, including converting, if necessary, the protocol from the originating side to be compatible with the protocol at the terminating side. If the originating protocol adapter  312  can locate the corresponding call instance  320  for the message, then the protocol adapter  312  routes the message to the call instance  320  for further processing as described hereinafter. On the other hand, the originating protocol adapter  312  may not be able to find the corresponding call instance  320 , for example, because the message is the first message pertaining to a call. In that case, the originating protocol adapter  312  is designed to instantiate a new call instance  320  corresponding to the particular phone call and to route the message into the new call instance for further processing. 
   When a new call instance  320  is instantiated, for example by originating protocol adapter  312 , an appropriate channel for the call is determined based on an analysis of the content of the incoming messages and the path by which the message arrived. The logic for selecting a channel may be static or dynamic. For example, static logic may be implemented by a hard-coded table in a configuration file resident in storage device  208 , and dynamic logic may be set up at run-time based on such factors as channel availability. A combination of static and dynamic techniques may be used as well. The channel is associated with a particular terminating protocol adapter  316  and, hence, indicates the proper I/O channel controller  306  and protocol on the terminating side. The terminating protocol adapter  316  can route messages from associated call instances to the corresponding I/O channel controller  306 , to a network node, and ultimately to the destination telephone. In accordance with the bi-directional nature of protocol adapters, a terminating protocol adapter  316  is configured also to receive protocol-specific messages from the terminating side of the network and pass them to the appropriate call instance. Likewise, an originating protocol adapter  312  can receive messages from a call instance  320  and pass them onto the corresponding I/O channel controller  302  for transmission to the appropriate destination. 
   An external protocol adapter  314  is a special kind of protocol adapter for interconnecting the protocol converter with an external system involved with the call. For example, the external protocol adapter  314  enables external systems to be involved in real-time Intelligent Networking (IN) call control such as Transaction Control Application Part (TCAP) communications with a C7 network Service Control Point (SCP). As another example, external protocol adapter  314  can employ a proprietary protocol for communicating with an external Fraud Control System involved in non-real-time control over the call. For implementing voice over packet-switching networks, the external protocol adapter  314  is used for real-time communication with a coding unit such as originating coding unit  110  and terminating coding unit  150 . Accordingly, the external protocol adapter  314  is responsible for connecting with the appropriate I/O channel controller  304  in the I/O subsystem  300  for sending and receiving messages with the coding unit and routing the messages to and from the proper call instance  320 . In addition, external protocol adapter  314  is capable of instantiating one or more new call instances based upon a message received from the external channel. In such an event, the other protocol adapters  312  and  316  are directed to initiate a call from a logical “originating” node to a terminating node. 
   A call instance  320  is instantiated by an originating protocol adapter  312  (or an external protocol adapter  314 ) for processing a call and performing protocol conversion, if necessary. A call instance  320  may be implemented in a variety of ways, including by a separate process, thread, or an interruptible flow of execution that can be resumed. When the call instance  320  is instantiated, memory for originating call control (“OCC”)  322 , universal call model (“UCM”)  324 , and terminating call control (“TCC”)  326  is allocated and initialized. The call instance  320  also contains a call context  328 , which is a region of memory for storing information about the current call. Some call-related information that persists beyond the duration of the call may be stored in a database in main memory  206  or storage device  208  to implement billing records. 
   Preferably, OCC  322 , UCM  324 , and TCC  326  are implemented as state machines by objects in an object-oriented programming language such as C++ or by other data structures in other programming languages. A state machine is an automaton that transitions from one of a finite number of states to another of those states in response to particular inputs. The output of a state machine occurs upon a state transition and is based on the destination state and typically also on the input and/or source state. The OCC  322 , UCM  324 , and TCC  326  state machines model a call from the perspective of the originating protocol, a universal protocol, and the terminating protocol, respectively. 
   OCC  322  models a call from the perspective of the originating protocol. More specifically, OCC  322  receives messages in the originating protocol from originating protocol adapter  312  and, in response, transitions from one state to another state, resulting in outputting a non-protocol specific (i.e., universal protocol specific) message to UCM  324 . Conversely, OCC  322  receives non-protocol specific messages from UCM  324  and, by responsively transitioning from one state to another, outputs originating protocol specific messages to originating protocol adapter  312 . 
   Likewise, TCC  326  models the call from the perspective of the terminating protocol. More specifically, TCC  326  receives non-protocol specific messages from UCM  324  and, by responsively transitioning from one state to another, outputs terminating protocol specific messages to terminating protocol adapter  312 . Conversely, TCC  326  receives messages in the terminating protocol from terminating protocol adapter  316  and, in response, transitions from one state to another state, resulting in outputting a non-protocol specific (i.e., universal protocol specific) message to UCM  324 . 
   UCM  324  manages the call according to a universal call model that uses the universal protocol produced by OCC  322  and TCC  326 . For the most part, UCM  324  merely passes the universal protocol messages between the OCC  322  and TCC  326 , thereby implementing a protocol conversion of the originating protocol into the terminating protocol via a universal protocol. UCM  324  may conditionally send messages to OCC  322  and TCC  326 , however, based on the capabilities and requirements of the originating and terminating protocols, respectively. For example, some protocols require acknowledgement messages to be sent in response to a call setup message and others protocols do not. In this case, UCM  324  is configured to determine whether the originating protocol needs the acknowledgement message and cause one to be generated, if needed. 
   Since UCM  324  is positioned to intercept messages passed between the OCC  322  and the TCC  326 , UCM  324  can perform different kinds of message processing other than mere protocol conversion. In accordance with one embodiment of the present invention, UCM  324  is configured to implement feature transparency. Specifically, if the primary communication network  130  is unable to handle a particular feature even after protocol conversion, UCM  324  arranges for the feature to be communicated over the auxiliary communication network  132  using external I/O channel controller  304 , as described in more detail hereinafter. 
   A Common Signaling Protocol 
   In one embodiment, the signaling units  120  and  140  communicate with each other over a network connection  132  using a common signaling protocol, XISUP. The network connection  132  can be implemented over a packet-switching network. This common signaling protocol XISUP is an extension of Integrated Services Digital Network User Part (ISUP), which contains a universal call reference (UCR) in the message header for uniquely identifying the current voice call and a new Information Entity (IE) to carry coding unit related data between the signaling units. More specifically, the common protocol message header includes the fields listed in TABLE 1. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               FIELDS OF COMMON PROTOCOL MESSAGE HEADER 
             
          
         
         
             
             
             
          
             
               Field 
               Length 
               Description 
             
             
                 
             
             
               Protocol ID 
               1 
               Designates the protocol type for this packet, 
             
             
                 
                 
               useful for distinguishing from other Ethernet 
             
             
                 
                 
               packets. 
             
             
               Version 
               1 
               Version of the common protocol, for facilitating 
             
             
                 
                 
               a phase development. 
             
             
               Length 
               2 
               Number of bytes in the packet, not including the 
             
             
                 
                 
               length of the Protocol ID or the Length field. 
             
             
               Dest. SU 
               4 
               Address of the signaling unit that this message is 
             
             
                 
                 
               directed toward, for example, an IP address. 
             
             
               Org. SU 
               4 
               Address of the signaling unit that is sending this 
             
             
                 
                 
               message, for example, an IP address. 
             
             
               UCR 
               8 
               The Universal Call Reference specifies a system- 
             
             
                 
                 
               wide identifier for a call. The UCR is generated 
             
             
                 
                 
               by the signaling unit that received the initial call 
             
             
                 
                 
               set up for the call and is passed in all messages 
             
             
                 
                 
               to identify the call in a signaling unit. 
             
             
               Msg Type 
               1 
               Message type, encoded according to existing 
             
             
                 
                 
               ISUP message types (e.g., IAM, ACM, ANM, 
             
             
                 
                 
               REL, and RLC). 
             
             
               Msg Data 
               variable 
               Message data, encoded according to the message 
             
             
                 
                 
               type as per the existing ISUP specifications. 
             
             
                 
             
          
         
       
     
   
   Since the XISUP protocol is established for the facilitating integration and intercommunication between different signaling units on a packet-switching network for establishing voice call over the packet-switching network, the XISUP protocol need not support each and every ISUP message to achieve the desired functionality. However, the XISUP protocol preferably supports at least the following ISUP messages, with variation as explained herein, as set forth in TABLE 2. 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               ISUP MESSAGES SUPPORTED BY XISUP PROTOCOL 
             
          
         
         
             
             
             
          
             
                 
               Message 
               Description 
             
             
                 
                 
             
             
                 
               Initial Address 
               Same as the ISUP IAM, except with a 
             
             
                 
               Message (IAM) 
               new IE for a Connection Descriptor, that 
             
             
                 
                 
               includes the network address of the 
             
             
                 
                 
               originating coding unit 110 in. 
             
             
                 
               Address Complete 
               Same as to the ISUP ACM, except with 
             
             
                 
               Message (ACM) 
               a new IE for the Connection Descriptor. 
             
             
                 
               Subsequent Address 
               Same as to the ISUP SAM. 
             
             
                 
               Message (SAM) 
             
             
                 
               Answer Message 
               Same as to the ISUP ANM. 
             
             
                 
               (ANM) 
             
             
                 
               Release Message 
               Same as to the ISUP REL. 
             
             
                 
               (REL) 
             
             
                 
               Release Complete 
               Same as to the ISUP RLC. 
             
             
                 
               Message (RLC) 
             
             
                 
                 
             
          
         
       
     
   
   Establishing A Voice Call Over a Packet-Switching Network 
     FIG. 4  is a call flow diagram illustrating messages transmitted and received by originating coding unit  110 , originating signaling unit  120 , terminating signaling unit  140 , terminating coding unit  150 , in establishing a voice call between originating node  100  and terminating node  160  through a packet-switching network  130 . The left side of  FIG. 4  depicts the call flow of messages performed by originating node  100 , originating coding unit  110 , and originating signaling unit  120  on the originating side of the call. The right side of  FIG. 4  depicts the call flow of messages performed by terminating signaling unit  140 , terminating coding unit  150 , and terminating node  160  on the terminating side of the call. 
   When a user initiates a voice call, a “setup” connection request message  402  is generated and ultimately transmitted by originating node  100  to originating coding unit  110 , which grooms off and backhauls the signaling message to originating signaling unit  120  via backhaul signaling link  112 . Alternatively, the set up message  420 , in the case of SS7, is delivered directly to originating signaling unit  120  from the trunk via link  113 . This setup connection request message is protocol-specific and varies from protocol to protocol. For example, in the DPNSS protocol, the connection request message is an ISRM_C message, but, as another example, the connection request message would be an Initial Address Message (IAM) in the Signaling System 7 (SS7) family of protocols. In yet another example, in the ISDN family of protocols such as ISDN_PRI, the connection request message is a Setup message. In most protocols, the connection request message contains a call identifier, an originating number, and a terminating number. The originating number may be the number of the originating telephone, and the terminating number may be altered in the course of determining the terminating telephone. 
   The setup connection request message  402 , when received by originating signaling unit  120  through control channel  222  from originating coding unit  110  or directly from originating node  100 , comes into I/O card  218   c  in communications interface  218 . A corresponding I/O channel controller  302 , executing as part of I/O subsystem  300  on computer system  200 , unpacks the message into a protocol data unit and submits it to the corresponding, originating protocol adapter  312 . Since this message is a set up connection request  402 , the originating protocol adapter  312  cannot match the call identifier with an existing call instance. Consequently, the originating protocol adapter  312  instantiates a call instance containing OCC  122 , UCM  124 , and TCC  126  as specific embodiments of the OCC  322 , UCM  324 , and TCC  326  described above in connection with  FIG. 3 . Once the call instance is created, the protocol-specific connection request message is routed by the originating protocol adapter  312  to OCC  122 . 
   OCC  122  of the originating signaling unit  120  is initially in an “idle” state. At point  402  in the call flow diagram, OCC  122  receives the protocol-specific connection request message from originating protocol adapter  312 . In response, OCC  122  transitions from the idle state to a protocol-specific state generally indicative of waiting for a ring. For example, an OCC  122  implemented for the DPNSS protocol would enter a “wait for NAM” state. During the transition, OCC  122  performs various operations including the unpacking of the message into its component information, storing the information in the call context  328 , and outputting of an internal, universal protocol “[Call]” message to UCM  124 . In this example, the call context  328  includes, among other information, the originating telephone number, the destination telephone number, and a flag that indicates the presence of a feature. In this example, the feature flag is “false.” Depending on the protocol, OCC  122  may perform other tasks such as sending back a “proceeding” message  404  to the originating node  100  to acknowledge that the setup message  402 . 
   When the UCM  124  of originating signaling unit  120  received the universal protocol “[Call]” message, the UCM  124  selects the bearer channel to be seized by the coding unit based upon a bearer channel specifically requested or merely “preferred” in the setup message, or upon an available channel on the associated trunk (for example, T1 or E1 line). Accordingly, the UCM  124  generates and transmits a create connection message  406 , specifying the bearer channel, through external protocol adapter  314 , I/O channel controller  304 , control link  114  to originating coding unit  110 . The create connection message  406  can be implemented according to an appropriate protocol such as a CRCX message in the Simple Gateway Control Protocol (SGCP), a proposed International Engineering Task Force (IETF) standard submitted by Bellcore and Soliant Internet Systems. 
   In response to the create connection message  406 , the originating coding unit  110  seizes an endpoint for a bearer channel specified in the create connection message  406  and associates a network address for the selected bearer channel, which can be an IP address plus a port number. The originating coding unit  110  thereupon responds back to the originating signaling unit  120  over the control link  114  with a message  408  that includes the network address of the originating coding unit  110 . The create connection message  408  can also contain parameters that indicate the capabilities of the originating coding unit  110 , for example, the encoding and compression types the originating coding unit  110  supports. 
   When this message  408  with the network address is received by the originating signaling unit  120 , the originating signaling unit  120  resolves the call routing through network  132  to the terminating signaling unit  140 , based on dynamic techniques, static techniques such as provisioned lookup tables, or a combination of dynamic and static techniques. Once the appropriate terminating signaling unit  140  is identified, TCC  126  of the originating signaling unit  120  generates and transmits an XISUP IAM message  410  to the terminating signaling unit  140  with the standard ISUP IAM information plus the Universal Call Reference and the network address of the bearer channel on the originating coding unit  110 . 
   Upon receipt of the XISUP IAM message  410  from the originating signaling unit  120 , the terminating signaling unit  140  resolves the call routing down to the terminating coding unit  150  and terminating node  160  level. In addition, the terminating signaling unit  140  issues a CRCX message  412  via SGCP and control link  154  to terminating coding unit  150  for setting up a connection from the endpoint for terminating node  160  to the bearer channel port of the originating coding unit  110 . In addition, the CRCX message  412  may contain the parameters that indicate the capabilities of the originating coding unit  110 . 
   The terminating coding unit  150  optionally initiates an H.245 negotiation session  414  with the originating coding unit  110  using the network address of the bearer channel on the originating coding unit  110  passed in the CRCX message  412 . During the negotiation session  414 , the originating coding unit  110  and the terminating coding unit  150  negotiate appropriate compression and decoding levels. Alternatively, the terminating coding unit  150  may use the capability parameters of the originating coding unit  110  in the CRCX message  412  and its own capabilities parameters to determine the common capabilities. Accordingly, the terminating coding unit  150  establishes a bearer channel circuit  416  on the bearer packet-switching network  130 . The bearer channel circuit  416  may be one-way (terminating coding unit  150  to originating coding unit  110 ) or two-way. If successful, the terminating coding unit  150  responds back with a connection message  418  to terminating signaling unit  140  over control link  154 . The connection message  418  contains the network address of the terminating coding unit  150  and the negotiated parameters or the common capabilities as determined above. 
   Upon successful setup of the bearer channel or virtual circuit between the originating coding unit  110  and the terminating coding unit  150  as indicated by the connection message  418 , the terminating signaling unit  140  sends a call setup message  420  via backhaul signaling link  152  to the terminating node  160 . In response, the terminating node  160  sends an alerting message  422 , backhauled to the terminating signaling unit  140 , which is converted to an XISUP ACM message  424  to the originating signaling unit  120 . The XISUP ACM message  424  contains the standard ISUP ACM information plus the results of the H.245 negotiation, and for unidirectional protocols, such as RTP, the network address of the terminating coding unit  150 . 
   Thereupon, the originating signaling unit  120  generates and sends a modify connection request MDCX message  426  to the originating coding unit  110  to cross connect the user side bearer with the network side bearer and thereby set up an end to end bearer path. The MDCX message  426  also contains the parameters negotiated between the originating coding unit  110  and the terminating coding unit  150 ; If the earlier establishment of the bearer channel  416  was one-way, then the connection is modified to include the reverse direction (from originating coding unit  110  to terminating coding unit  150 ), thereby becoming a bi-directional connection. In addition, in response to the ACM message  424 , the originating signaling unit  120  sends an Alerting message  428  to originating node  100 . 
   When the person being called picks up the ringing telephone, this action results in the terminating node  160  sending a connect message  430  to the terminating coding unit  150 . The connect message  430  is backhauled to the terminating signaling unit  140 , which sends in response an XISUP ANM message  432  to the originating signaling unit  120 . The originating signaling unit  120 , in response, generates and transmits via the backhaul signaling link  112  a connect message  434  to the originating node  100  and ultimately to the originating telephone. At this point, the voice call is active. 
   Extensions and Alternatives 
   While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, an originating signaling unit can control multiple originating coding units, a terminating signaling unit can control multiple terminating coding units, and a single signaling unit can control an originating coding unit and a terminating coding unit, even on the same voice call. 
   As one example,  FIG. 5(   a ) illustrates a configuration wherein a single signaling unit  500  handles the voice call signaling processing for both the originating coding unit  110  and the terminating coding unit  150 . In this configuration, the OCC  502  is responsible for communicating the signaling data with the originating coding unit  110  over link  112 , and the TCC  506  is responsible for communicating the signaling data with the terminating coding unit  150  over link  152 . The UCM  504  is responsible for controlling and querying the originating coding unit  110  over link  114  and the terminating coding unit  150  over link  154 . 
   Referring to  FIG. 5(   b ), a single coding unit  510  can be controlled by a single signaling unit  500 , wherein the coding unit  510  uses an external connection through packet-switching network  130  to connect the originating and terminating channels. This connection can even be internal as shown in  FIG. 5(   c ).