Abstract:
A method and apparatus for transferring data associated with synchronous transfer mode (STM) calls through an asynchronous transfer mode (ATM) network are described. The apparatus includes a voice interface control unit, a signaling interface and a TDM peripheral device associated with each edge node in the ATM network. The voice interface control unit controls the ingress and egress of STM calls to the network. The signaling interface sends and receives common channel signaling messages associated with calls originating in the STM network and passes message content to the voice interface control unit. The TDM peripheral device converts STM network and passes message content to the voice interface control unit. The TDM peripheral device converts STM calls to ATM cells, and vice versa. The method disclosed involves enabling the network interface controls at edge nodes in the ATM network with a capacity to receive, interpret and send common channel signaling messages associated with STM cells, while using native ATM signaling, such as P-NNI within the ATM network. The advantage is the ability to transfer STM data through an ATM network without modifying the ATM network itself. The ATM network can therefore be utilized to its fullest extent as a multi-service backbone network. SVC and cross-connect caching can also be used, which further enhances network performance and resource utilization.

Description:
TECHNICAL FIELD 
     This invention relates generally to synchronous transfer mode (STM) call completions and, in particular, to the completion of calls which originate and terminate in an STM network but at least a portion of the call connection is completed using an asynchronous transfer mode (ATM) network. 
     BACKGROUND OF THE INVENTION 
     The public switched telephone network (PSTN) is a synchronous transfer mode (STM) network in which time division multiplex (TDM) switches are interconnected by transport links commonly referred to as “trunks”. Each trunk supports a plurality of “channels” which are time slots used by individual calls. Although the STM network is reliable and robust, the demand for voice and other voice-band data services is taxing the capacity of current network facilities. 
     Consequently an interest has developed in using alternate facilities to complete voice grade connections. Many local exchange carriers (LECs) are experiencing tandem network congestion and are seeking economical ways to grow their tandem networks. Inter-exchange carriers (IECs) would like to consolidate their voice and data traffic on a single multi-service network. The currently most attractive alternate facility is a network which operates in an asynchronous transfer mode (ATM) protocol. ATM has a now matured to an extent that it provides a viable alternative to STM facilities. 
     ATM uses a fixed data packet size of 53 octets called a “cell”. A cell includes a header of 5 octets and a payload of 48 octets for transferring user information. There are known methods for converting STM voice and voice-band call data to ATM cells and vice versa. 
     Call setup and control in the PSTN is commonly effected using an out-of-band signaling network known as a common channel signaling network. Most of the North American PSTN is equipped to operate with a common channel signaling protocol called Signaling System  7  (SS 7 ). ATM networks, however, use a different signaling protocol in which signaling messages are transported through the network in cells like those used for carrying payload data. The signaling systems of the PSTN and ATM networks are therefore incompatible and STM calls cannot be transferred directly to or from an ATM network. 
     Methods and apparatus for transferring voice and voice-data services over ATM networks are known, however. One ATM network architecture designed for that purpose is described in U.S. Pat. No. 5,568,475 entitled ATM NETWORK ARCHITECTURE EMPLOYING A COMMON CHANNEL SIGNALING NETWORK, which issued Oct. 22, 1996 to Doshi et al. In the ATM network architecture described by Doshi et al, each switch in the ATM network is equipped with a signal processor that is capable of sending and receiving common channel signaling messages. The signal processor translates STM trunk identification information into ATM Permanent Virtual Circuit (PVC) information to permit STM calls to be transported through the ATM network using virtual trunks. While this ATM architecture provides an option for a migration of PSTN voice services to an ATM protocol, it appears to suffer from certain drawbacks. First, the architecture requires that all ATM switches be enabled with SS 7  signaling capability and that the SS 7  network overlay the entire ATM network. Second, the use of PVCs in the ATM network has the potential to tie up ATM resources unnecessarily, preventing those resources from being used for other purposes, even during off-peak calling hours. 
     There therefore exists a need for a method and apparatus for transferring STM calls in a multi-service ATM network which requires only a small investment in infrastructure and ensures efficient use of ATM network resources. 
     Simply transferring STM calls through an ATM network to relieve congestion in the PSTN is not in itself enough, however. In North America, the current call setup rate is 4,000,000 calls per busy hour and PSTN usage is expected to double over the next five years. Despite the current call volume, call setup within an STM node requires only 20 msec, and service is delivered with 99.999% availability. 
     There therefore exists a need for a method and apparatus for transferring STM calls in a multi-service ATM network which ensures that calls transferred through the ATM network are rapidly set up so that current PSTN service levels are sustained. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method of transferring STM calls through an ATM network by provisioning ATM edge nodes with apparatus which is adapted to send, receive and interpret common channel signaling messages related to STM calls and to dynamically map STM trunks to switched virtual circuits in a multi-service ATM network. 
     It is a further object of the invention to provide a method for transferring STM calls through a multi-service ATM network in which native ATM signaling is used in the ATM network. 
     It is yet a further object of the invention to provide a method of transferring STM calls through an ATM network in which cached switched virtual circuits (SVCs) are used for STM calls. 
     It is a further object of the invention to provide a method of transferring STM calls through an ATM network in which cached cross-connections are also used for STM calls. 
     It is another object of the invention to provide an apparatus for transferring STM calls through an ATM network, the apparatus including a voice interface control unit associated with each edge node in the ATM network. 
     It is a further object of the invention to provide an apparatus for transferring STM calls through an ATM network which includes a signaling interface to receive and send common channel signaling messages associated with STM calls. 
     It is yet a further object of the invention to provide an apparatus for transferring STM calls through an ATM network in which the voice interface control unit dynamically maps a trunk channel carrying the cell to an SVC in the ATM network. 
     It is yet a further object of the invention to provide an apparatus for transferring STM calls through an ATM network in which the voice interface control unit exchanges messages with a peer associated with a destination edge node in the ATM network to enable the peer voice control unit to set up egress of the call from the ATM network to a trunk in the STM network. 
     In accordance with a first aspect of the invention, there is provided apparatus for enabling the transport of synchronous transfer mode (STM) calls through an asynchronous transfer mode (ATM) network, the apparatus being associated with each edge node in the ATM network designated to serve as a gateway for the STM calls, comprising in combination: 
     a signaling interface adapted to receive and send common channel signaling messages associated with STM calls transferred to the ATM network; 
     a voice interface control unit adapted to route calls through the ATM network when an STM call seeks admission at the associated edge node, and to set up egress of a call from the ATM network when the associated node is an ATM destination node for the call; and 
     a TDM peripheral device for terminating STM trunks and converting STM calls to ATM cells and vice versa. 
     In accordance with a further aspect of the invention, there is provided a method of transferring synchronous transfer mode (STM) calls through a multi-service asynchronous transfer mode (ATM) network comprising: 
     provisioning ATM edge nodes for the ingress and egress of the STM calls, each edge node having an associated TDM peripheral adapted to convert STM calls to ATM cells and vice versa, a signaling interface adapted to send and receive common channel signaling messages related to STM calls transferred to an associated edge node for admission to the ATM network, and a voice interface control unit which is adapted to route STM calls through the ATM network and setup the egress of STM calls routed through the network by another voice interface control unit; and 
     using native ATM signaling to set up switched virtual circuits between edge nodes for the STM calls. 
     In accordance with yet a further aspect of the invention, there is provided a method of transferring STM calls through an ATM network, comprising the steps of: 
     receiving a common channel signaling message at an edge node in the ATM network, the common channel signaling message providing information regarding a calling party and a called party as well as a circuit identification code identifying a trunk channel for delivery of the call to the ATM network, to permit the call to be transferred through the ATM network; 
     translating at least a portion of a called number in the common channel signaling message into a destination edge node in the ATM network; 
     allocating a switched virtual circuit (SVC) to transfer the STM call through the ATM network to the destination edge node associated with the called number; 
     dynamically mapping the SVC to a port associated with the trunk channel; 
     downloading a first cross-connecting the SVC to the port; 
     passing the common channel signaling information to the destination edge node; 
     translating at the destination edge node at least a portion of the called number into an egress STM port for the call; 
     downloading a second cross connect from the SVC to the egress STM port from the call; and 
     transferring the STM call through the ATM network using the cross-connects and the SVC until a common channel signaling message is received indicating that the call has been terminated. 
     The invention provides a method and apparatus for transferring STM calls through an ATM network which permits resources in the ATM network to be efficiently used while ensuring that the rapid call setup time now provided by the PSTN is maintained. 
     In accordance with the method, voice interface control units are provided at edge nodes in the ATM network. The voice interface control units are enabled to communicate with PSTN switches using common channel signaling messages. A signaling interface terminates Message Transfer Part (MTP) layers  2  and  3  for the voice interface control unit. The signaling interface is adapted to receive common channel signaling messages in Integrated Services Digital Network User Part (ISUP) protocol. In response to instructions from the voice interface control unit, it is also enabled to formulate and send ISUP messages. If Local Number Portability (LNP) or other PSTN call features are to be supported, the signaling interface must also be enabled to communicate and receive common channel signaling messages in Transaction Capability Application Part (TCAP) protocol. Likewise, the signaling interface must be enabled to formulate and send TCAP messages in response to instructions from the voice interface control unit in order to query databases for call routing information, or number translation. 
     In order to efficiently use resources in the ATM multi-service network it is preferable that the edge nodes in the network be equipped to use cached SVCs and cached cross-connects to facilitate call setup while ensuring efficient network resource use. 
     In all implementations of the invention, native ATM signaling such as BISUP or P-NNI are used in the ATM network. This permits the ATM network to be used for the purposes of offering different high-bandwidth data services while permitting efficient and reliable transfer of STM calls to relieve congestion in inter-exchange carrier or local exchange carrier networks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further explained by way of example only and with reference to the following drawings, wherein: 
     FIG. 1 a  is a schematic diagram of a first preferred embodiment of the apparatus in accordance with the invention showing the apparatus in relation to a portion of a public switched telephone network; 
     FIG. 1 b  is a schematic diagram of a second preferred embodiment of the apparatus in accordance with the invention showing the apparatus in relation to a portion of a public switched telephone network; 
     FIG. 2 is a schematic diagram of a preferred interface between a voice interface control unit in accordance with the invention and a switch control element of an ATM edge node in an ATM network; 
     FIG. 3 is a schematic diagram showing application program interface messages exchanged between the voice interface control unit and the switch control element in accordance with a preferred embodiment of the invention; 
     FIG. 4 is a schematic diagram of a call control message flow between call originating and terminating components of the PSTN and the apparatus in accordance with the invention at edge nodes in an ATM network during STM call setup; 
     FIG. 5 is a schematic diagram showing a call control message flow during call setup in which cached SVCs are used for call completion through the ATM network; 
     FIG. 6 is a schematic diagram showing call control message flow in a call release sequence in which the SVC used for call completion through the ATM network is released; and 
     FIG. 7 is a schematic diagram of a call control message flow in a call release sequence in which the SVC used for the STM call is cached on call release for use by subsequent calls between the same edge nodes in the ATM network. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention relates to a method and apparatus for transferring synchronous transfer mode (STM) network calls through an asynchronous transfer mode (ATM) network. In accordance with the method, edge nodes in the ATM network are equipped with associated voice interface control units and time division multiplex (TDM) peripherals which convert STM calls to ATM cells and vice versa. The voice interface control unit uses a signaling interface to provide a link between a common channel signaling control network of the STM network. The signaling interface handles the Message Transfer Part (MTP) layers of the common channel signal messaging. In the ATM network, and the edge nodes which serve as gateways for voice and voice-data traffic are substantially unmodified. This permits the ATM network to function as a multi-service network which supports application interfaces for other types of data services. The voice and voice-data traffic is therefore completely transparent to the ATM network. It is preferable that the edge nodes be configured to support switched virtual circuit (SVC) caching to enable rapid call setup in order to comply with current PSTN call setup performance and to ensure that setup delays are not experienced during peak call hours. Performance can be further enhanced using cached cross-connects. SVC caching and cross-connect caching are described in applicants&#39; copending application entitled METHOD AND APPARATUS FOR CACHING SWITCHED VIRTUAL CIRCUITS IN AN ATM NETWORK which was filed on Apr. 2, 1998, the disclosure of which is incorporated herein by reference in its entirety. 
     FIG. 1 a  is a schematic diagram of the principal components of an apparatus in accordance with the invention shown in one of several potential arrangements with edge nodes of an ATM network  10 . The ATM network  10  serves as a transport backbone for many varied data services. The PSTN  12  includes a plurality of time division multiplex switches  14 , commonly referred to as service switching points (SSPs). Only two SSPs  14   a, b  are shown in FIG.  1 . The SSPs  14   a, b  shown in FIG. 1 are tandem service switching points used to route interswitch calls in the PSTN. Such SSPs are found, for example, in local exchange carrier (LEC) tandem networks as well as in inter-change carrier (IEC) networks. 
     The apparatus in accordance with the invention is primarily intended for relieving congestion in the PSTN  12  and for permitting both LECs and LECs to grow their networks economically. 
     As is well understood, in the art, the PSTN  12  includes and is largely controlled by an out-of-band signaling system known as a common channel signaling system. The common channel signaling system includes a plurality of signal transfer points (STPs)  16   a-c  which transfer common channel signaling messages between network nodes over common channel signaling links  18   a-j  The network nodes include, for example, SSPs  14   a,b  and a service control point (SCP)  20 . The SSPs  14   a,b  exchange call control information over the common channel signaling links  18   b,c,g  and h for call setup and call release. The STPs  16   a,b,c  minimize the number of signaling links  18   a-j  required by serving as tandem switches in the signaling network to route messages to their destination. The SSPs  14   a,b  are also enabled to send query messages to the SCP  20  in order to obtain routing information for special service calls. Query messages may also be sent to the SCP  20  for number translations to support network functions such as Local Number Portability (LNP). 
     The ATM network  10  operates with an asynchronous transfer mode protocol in which connections are established using virtual circuits. Virtual circuits share facilities in a prioritized schedule determined by each circuit&#39;s quality of service (QOS). The ATM network  10  includes a plurality of switching nodes  22   a,b.  The switching nodes  22   a,b  generally include an ATM switch fabric  24   a,b  controlled by a switch control element  26   a,b . Auxiliary line cards (AX LC)  28   a,b  provide ingress/egress ports for transport links  30   a,b  in the ATM network  10 . The construction of ATM switches  22  and ATM networks  10  is well known in the art. 
     The apparatus in accordance with the invention provides an interface between the ATM switches  22   a,b  and the PSTN  12  to provide gateways for transferring voice-grade traffic to and from the ATM network  10 . The apparatus in accordance with the invention includes a voice interface control unit  32   a,b  and a signaling interface  34   a,b.  A prior art device commonly referred to as a TDM peripheral  36   a,b  is required to terminate STM trunks  38  and to convert STM voice and voice data calls into ATM cells, as is well understood in the art. The TDM peripheral  36   a,b  generally includes Digital Signal Processing (DSP) service circuits which are responsible for tone generation/detection; digit collection/generation on the capture of Dual Tone Modulated Frequency (DTMF), or Multiplex Frequence (MF) signaling, etc. The DSP is also responsible for the collection of call progress digits. Other DSP functions include the playback of recorded announcements, voice bridging for multi-cast and conference calling; fax/modem detection; silence suppression and comfortable noise generation; echo cancellation, etc. The TDM peripheral  36  also includes an adaptor section which provides a connection point for STM DS 0  and T 1 , T 3  and larger sizes up to  0 C 3  trunks, as well as functionality to convert STM calls to ATM calls. 
     There are several options for connecting the TDM peripheral  36   a,b  to the ATM switch fabric  24   a,b.  The preferred option is a direct fibre interface which connects to the ATM switch fabric. This option delivers an integrated ATM multi-service switch while reducing cost. To implement this option, the TDM peripheral  36   a,b  must include a fibre interface which emulates the AX LCs  28   a,b . Since all traffic handled by the TDM peripheral is admitted as committed bit rate (CBR) traffic, traffic management is simple and memory requirements are therefore minimized. This permits a TDM peripheral  36   a,b  in accordance with the invention to be inexpensively constructed. 
     The voice interface control unit  32   a,b  controls voice traffic ingress to and egress from the ATM network  10 . In order to accomplish the ingress/egress of voice traffic, the voice interface control unit  32   a,b  manages the switched virtual circuits (SVCs) required for voice calls. In the method in accordance with the invention, voice calls are routed over SVCs at a CBR to ensure efficient management of network resources in a multi-service ATM network. By using SVCs rather than permanent virtual circuits (PVCs), unused network capacity is available for other services supported by the network. As is well known in the art, SVCs tie up network resources only so long as they are in use. Once released, the network resource is freed for use by any other service. The voice interface control unit  32   a,b  also manages address translation between North American Dialing Plan (NADP) addresses, also referred to as E.164 addresses, and an ATM destination address required to route the call through the ATM network  10 . There are several options for address translation and routing, as will be explained below in more detail. 
     The voice interface control unit  32   a,b  interfaces with the switch control element  26  to request and confirm SVC setup. It also provides the necessary information for SVC setup such as the ATM called address when voice calls are routed through the ATM network  10 . In addition, the voice interface control unit  32   a,b  manages the cross-connections between an STM trunk channel identified by a circuit identification code (CIC), and an SVC set up to carry a call through the ATM network  10 . The CIC is received in an Integrated Services Digital Network User Part (ISUP) message. ISUP is a subset of the common channel signaling protocol known as SS 7 . The voice interface control unit  32   a,b  preferably also manages SVC caching and selection as described in applicants&#39; copending patent application referenced above. 
     As shown in FIG. 1 a,  the voice interface control unit  32   a,b  may be closely coupled with the switch control element. In a second preferred embodiment shown in FIG. 1 b,  the voice interface control unit  32   b  is a stand-alone computing machine with its own ATM transfer link  33   a,b  to an AX LC  28   a,b.  In the embodiment shown in FIG. 1 b,  all communications between the switch control element  26   a,b  and the voice interface control unit  32   a,b  are through the ATM switch fabric  24   a,b.  In all other respects, the embodiment shown in FIGS. 1 a  and  1   b  are identical. 
     In accordance with the method of the invention, the voice interface control unit  32   a,b  is owner of an address assigned to the TDM peripheral  36   a,b  and registers that address with the switch control element  26   a,b  using processes which will be described below in more detail with reference to FIG.  3 . The address registered is a concatenation of the ATM switch prefix address plus the common channel signaling point code assigned to the voice interface control unit  32   a,b.    
     An interface is required between the voice interface control unit  32   a,b  and the switch control element  26   a,b.  Several options may be adopted for a signaling protocol at the interface. For example, ATM User Network Interface (UNI) signaling can be used. ATM UNI is a standard ATM signaling message protocol but the functionality for this application is limited because a flexible cross-connect download command is absent and the protocol has performance drawbacks. As an alternative, B-ISUP could be used but the number of signaling layers to be treated detract from its appeal. Likewise, the ATM signaling system Protocol for Network to Network Interface (P-NNI) is not preferred because constructing a designated transit list required for P-NNI is considered to be unduly complex for this application. 
     Proprietary switch messaging could be used but requires the implementation of certain features to be described below such as the download of half-leg virtual circuit releases, which are generally not supported by ATM switching equipment. 
     Several standards-based options could also be used for this application. For example, one standards-based option is the use of ATM native service Application Programming Interface (API) primitives to which are added a custom cross-connect download message. The ATM API primitives are a set of standard primitives that can call the native ATM SVC service. The API published by the ATM forum is intended for use by the applications in a host on the user side of UNI to request the SVC services. The API is suitable for supporting non-ATM services on an ATM multi-service switch because the voice control unit  32   a,b  using API primitives emulates the functionality of an application host requesting ATM SVC services from the ATM multi-service switches  22   a,b.  FIG. 2 shows a schematic diagram of the API interface  40  which permits message exchange between the switch control element  26   a,b  and the voice interface control unit  32   a,b.    
     FIG. 3 is a schematic diagram of the principal messages exchanged between the voice interface control unit  32   a,b  and the switch control element  26   a,b  using the API interface  40 . API interface  40  permits the voice interface control unit  32   a,b  to register addresses in an initialization or update process. During the process, the voice interface control unit  32   a,b  registers as application agent for the TDM peripheral  36   a,b  and informs the switch control element  26   a,b  of the address which it has been assigned. When using the “address register” primitive, the voice interface control unit  32   a,b  passes the concentrated address (ATM switch prefix and point code) to the switch control element  26   a,b.    
     The switch control element  26   a,b  on receipt of the “address register” message records the address in its message control tables. Thereafter, whenever a signaling message or a call is received having an address matching the registered address, it is passed to the voice interface control unit  32   a,b  for further processing. 
     In addition to address registration, the API interface  40  permits the voice interface control unit  32  to request SVC services. This includes SVC setup which is initiated by an SVC Request command in which the voice interface control unit  32   a,b  requests an SVC using the “add-cd” (address) prefix of the destination ATM switch plus the point code of the destination voice interface control unit  32   a,b  and the “add-cg” (address prefix of the origination ATM switch plus the point code of the origination voice interface control unit  32   a,b ). The contents of the address variables will be explained below in more detail. When the voice interface control unit  32   a,b  sends an SVC request, the switch control element  26   a,b  normal responds with an SVC confirmation in which the address of the calling and called parties are returned along with a virtual circuit identification code (VCCI). An SVC request is used by the voice interface control unit  32   a,  for example, when a call is received from the PSTN by the TDM peripheral  36   a.    
     If an incoming SVC setup is received by the ATM switch fabric  24   a,  the switch control element  26   a  examines the called party address and determines that it is owned by the voice interface control unit  32   a.  The switch control element  26   a  after processing the SVC for the incoming ATM ports sends an SVC Indication message to the voice interface control unit  32   a  which includes the called party address (add-cd), the calling party address (add-cg) and the VCCI. The calling party and called party addresses are the concentrated address described above, ATM switch prefix plus voice interface control unit point code. On receipt of the SVC Indication message, the voice interface control unit  32   a  responds with an SVC Response message in which the address parameters are repeated. The use of the SVC indication and response messages will be described below in more detail. 
     In the method in accordance with the invention, it is frequently necessary for the voice interface control unit  32   a,b  to request a half-leg cross-connection download from the switch control element  26 . A half-leg cross-connect download is used to complete the cross-connect which has been setup with only one port in the cross-connect table of a resource manager for the ATM switch fabric  24   a,b.  In order to accomplish this, an “TXconnect set” command is used (FIG.  3 ). The TXconnect set message carries a TDM port identification number and a VCCI which includes an ATM port number/virtual path identifier/virtual circuit identifier. On receipt of an Xconnect set, the switch control element  26   a,b  performs the requested half-leg cross-connect by connecting the TDM port with the reserved ATM port/vpi/vci and returns an acknowledge (ACK) message indicating that the operation has been successfully completed. 
     It may also be necessary for the voice interface control unit  32   a,b  to disconnect a half-leg cross-connect for reasons which will be explained in more detail below with reference to FIGS. 6 and 7. In order to accomplish this, the voice interface control unit  32  uses the “TXconnect rel” message (FIG.  3 ). The TXconnect rel message carries the same parameters of port identification number and VCCI. Only the TDM port is released. The ATM port/vpi/vci that corresponds to the VCCI is still reserved as a half-leg cross-connect. The TXconnect rel message does not have to be acknowledged and the voice interface control unit  32   a,b  doe snot expect an acknowledgment. 
     The signaling interface  32   a,b  is logically closely coupled with the voice interface control unit  32   a,b  although they are physically separated. The DSO signaling link(s)  18   d,j  of the common channel signaling system preferably respectively terminate on DSO ports of the TDM peripheral  36   a,b.  Common channel signaling messages received on link  18   d,j  are passed to the signaling interface  34   a,b  by the TDM peripheral  36   a,b.  The signaling interface  34   a,b  provides the narrow band signaling interface and protocols which permit the common channel signaling link  18   d,j  to be terminated. The signaling interface  34   a,b  is responsible for terminating SS 7  MTP 2  and MTP 3  layers. It receives ISUP messages and removes the data required by the voice interface control unit  32   a,b  for call processing. The signaling interface  34   a,b  packs the data into ATM cells using any appropriate internal messaging protocol which permits the voice interface control unit  32   a,b  to determine the type and content of the message. The ATM calls are then forwarded through the ATM switch fabric  24   a,b  to the voice interface control unit  32   a,b,  which uses the data to set up or release SVCs for a call. The voice interface control unit  32   a,b  may also send instructions and data packed in ATM cells to the signaling interface  34   a,b  to request that the signaling interface  34   a,b  formulate and send an ISUP message required for call control in the PSTN  12 . 
     The signaling interface  34   a,b  may also be enabled to originate and terminate Transaction Capabilities Application Part (TCAP) protocol messages and, if required, formulate TCAP queries in response to requests by the voice interface control unit  32   a,b.  This permits the voice interface control unit  32   a,b  to query the SCP  20  if the voice interface control unit  32   a,b  determines that a call requires special treatment or address translation. 
     Call Control Messaging 
     Call Setup with SVC Setup 
     In order to illustrate the use of the apparatus in accordance with the invention, several detailed call control message flows illustrating call setup and call release are described below. 
     FIG. 4 is a schematic diagram of a high level overview of the message flow involved in a call setup using the method and apparatus in accordance with the invention. In the message flow shown in FIG. 4, in step  1   s  an ISUP IAM message originating from an SPC (not illustrated) in a LEC network, for example, arrives at STP  16   c  on signaling link  18   i.  The STP  16   c  examines the destination point code of the message and forwards the message over signaling link  18   j  in the second part of step  1   s  to the TDM peripheral  36   b.  On receipt of the ISUP message, the TDM peripheral  36   b  passes the ISUP message to the signaling interface  34   b.  The signaling interface  34   b  removes call control information from the ISUP message and packs that information into ATM cells which are through ATM switch fabric  24   b  to the voice interface control unit  32   b  which extracts the call control information and interprets the ISUP message. This entire process is schematically represented by step  2   s  in FIG.  4 . After interpreting the ISUP message data, the voice interface control unit  32   b  determines that the ISUP message is an IAM requesting admission to the ATM network  10  by the voice call from the LEC network. In response to the message, the voice interface control unit  32   b  examines the called number to determine whether call feature treatment is required. In this example, the call is a simple non-featured call and call feature treatment is not required. If call feature treatment were required, the voice interface control unit  32   b  would send ATM cells to the signaling interface  34   b  requesting that it formulate a TCAP message addressed to the SCP  20  to obtain the required translation or routing information. The voice interface control unit  32   b  is also enabled to send an ISUP Release (REL) message to release the call to the originating PSTN SSP if the called number relates to a service which the voice interface control unit  32   b  is not enabled to provide. The originating PSTN SSP may then forward the call into the PSTN network  12  for a normal STM call completion. 
     A virtual circuit is required to route the call to the destination edge node  22   a.  In order to establish the required virtual circuit, the voice interface control unit  32   b  must determine an appropriate destination edge node in the ATM network for egress of the call. The identification of the destination edge node is determined using an E.164 address to ATM address mapping table, shown in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 E.164 Address to ATM Address Mapping Table. 
               
             
          
           
               
                   
                 Destination Voice Interface 
                 Destination ATM 
               
               
                 Called Party Number 
                 Control Unit Point Code 
                 Node Address 
               
               
                   
               
               
                 613-xxx-xxxx 
                 P2 
                 AESA2 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                   
               
             
          
         
       
     
     The E-164 address is the called number from the ISUP IAM message. The called number is used to determine the point code of the destination voice interface control unit  32   a  and the address of the destination ATM node. After the destination address is located, the voice interface control unit  32   b  consults Table 2 to locate an available VCCI to be assigned to the call. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 VCCI Resource Table 
               
             
          
           
               
                   
                   
                 Peer Voice Interface 
                 Peer ATM Node 
               
               
                 VCCI 
                 In Use (1) 
                 Control Unit Point Code 
                 Address 
               
               
                   
               
               
                 x1 
                 0 
                   
                   
               
               
                 x2 
                 1 
                 P3 
                 AESAx 
               
               
                 . . . 
                 . . . 
                 . . .  
                 . . .  
               
               
                   
               
             
          
         
       
     
     Table 2 is examined to find an unoccupied VCCI for use in transferring the call through the ATM network  10 . The voice interface control unit  32   b  then marks the VCCI as “in use” and formulates an SVC Request which is forwarded to the switch control element in step  3   s.  In the SVC Request, the voice interface control unit  32   b  indicates the incoming STM port for the trunk channel carrying the STM call which it locates using a circuit identification code (CIC) that it obtains from the ISUP IAM message. The trunk to STM port ID mapping is performed using a table lookup, the table having a structure similar to Table 3. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Trunk to STM Port ID Mapping Table 
               
             
          
           
               
                   
                 CIC 
                 STM Port ID 
               
               
                   
                   
               
               
                   
                 001 
                 A1 
               
               
                   
                 002 
                 A2 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                   
               
             
          
         
       
     
     On receipt of the SVC Request message in step  3   s,  the switch control element  26   b  processes the incoming request by validating the required parameters and consulting routing tables to located preferred routes to the called ATM edge note  22   a.  The preferred routes are used to select the outgoing port through which the SVC is to be set up. The switch control element  26   b  then compiles resource information for both the incoming call from the TDM peripheral  36   b  and the outgoing switched virtual circuit. Switch control element  26   b  passes the call resource information of incoming/outgoing port ID in step  4   s  to a resource manager (not illustrated) which controls the ATM switch fabric  24   b  and the AX LCs  28   b.  The resource manager performs call admission control and allocates the resources required for the call. The resource manager also downloads the cross-connect from the voice trunk channel in trunk  38   b  on TDM peripheral  36   b  to the ATM switch fabric and downloads a cross-connect from the ATM switch fabric  22   b  to the outgoing port on the AX LC  28   b  using resource tables well known in the art. If any of the resource allocations fail, a reject signal is returned to the switch control element  26   b.  Otherwise, the resource manager sends a “confirmation” message back to the switch control element  26   b.  The confirmation message includes an index to indicate that the requested allocation was successful. Due to space constraints, the confirmation message is not shown. 
     The switch control element  26   b  then formulates an ATM signaling message for an SVC setup and send it in step  5   s  via the signaling in an ATM network  10  to a next switch in the TM network  10  to initiate the SVC setup. The SVC setup proceeds across the ATM network  10  in accordance with procedures well understood in the art. Call allocation resource and SVC setup must be accomplished at each node in the call path. After the ATM network  10  is traversed by the SVC Request message, it is received by the switch control element  26   a.  The switch control element  26   a  analyzes the SVC Request message and requests the resource manager of ATM switch fabric  24   a  in step  6   s  to admit and allocate the resource on the incoming ATM AX LC  28   a.  The switch control element  26   a  then sends an incoming SVC Indication message to the voice interface control unit  32   a  in step  7   s,  because the called address in the SVC setup request belongs to the voice interface control unit  32   a.    
     On receipt of the SVC Indication message, the voice interface control unit  32   a  determines that it has an incoming SVC and it updates its VCCI resource table with an “in use” indication (Table 2) and it creates a record of the SVC in a memory table, the structure of which is shown in Table 4. The originating ATM node address and the originating PC stored in Table 4 are extracted from the add-cg field of the SVC setup message. When these values are stored in Table 4, the “occupied” field is set to zero because the voice interface control unit does not know at that time if the SVC will be used for a call. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 SVC Record Table 
               
             
          
           
               
                 Peer Voice 
                   
                   
                   
                   
               
               
                 Interface 
               
               
                 Control Unit 
                 Peer ATM Node 
                   
                   
                 Trunk 
               
               
                 Point Code 
                 Address 
                 VCCI 
                 Occupied 
                 Characteristics 
               
               
                   
               
               
                 P1 
                 AESA1 
                 x1 
                 1 
                 DSO/CBR 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
               
             
          
         
       
     
     Then the voice interface control unit  32   a  sends a response to switch control element  26   a  to accept the SVC. Otherwise, the voice interface control unit  32   a  may reject the incoming SVC based on any predetermined control policy, if a control policy is instituted. For example, certain originating addresses may be disallowed. 
     In step  8   s,  the switch control element  26   a  returns an ATM signaling message back to the switch control element  26   b  indicating a successful SVC setup. When the switch control element  26   b  receives the successful SVC setup message, it informs the voice interface control unit  32   b  in step  9   s,  using an SVC confirmation message (FIG.  3 ), that the SVC to the destination was set up as requested. When the voice interface control unit  32   b  receives confirmation that it now has a virtual connection to the destination edge node  22   a,  it updates its VCCI resource table (Table 2) to record the destination voice interface control unit&#39;s PC and destination ATM address. The voice interface control unit  32   b  then sends an ISUP IAM message to the destination voice interface control unit in step  10   s  to the voice interface control unit  32   a.  The ISUP IAM may be sent via the signaling interface  34   b,  the signaling link  18   j,  STP  16   c,  signaling link  18   g,  STP  16   b,  signaling  18   c,  STP  16   a  and signaling link  18   d,  etc. Alternatively and preferably, the ISUP IAM information is packed in the ATM cells and sent through the ATM network to the voice interface control unit  32   a.  In the ISUP IAM information, regardless of the path over which it is transferred, the CIC is set to the VCCI of the SVC set up for the call, i.e. CIC=x1. If the ISUP message is routed through the ATM network  10 , it is preferably routed over a pre-established PVC designated for carrying ISUP call control messages. 
     When the voice interface control unit  32   a  receives the IAM information, it uses the CIC (VCCI=x1) to check its ATM incoming SVC table (Table 4) to verify that the VCCI=x1 originated from the same point code as the SVC being held for the call. The voice interface control unit  32   a  then performs a routing table lookup to determine which trunk(s) serves the called number in the ISUP message. In this example, the table (not illustrated) indicates that the call should be routed to trunk  38   a  and an SSP in a terminating PSTN network (not illustrated). Using a table similar to Table 3, the voice interface control unit  32   a  maps the trunk ID to an STM port ID (port A) on the TDM peripheral  36   a.  In step  11   s,  the voice interface control unit  32   a  sends an TXconnect set request to the switch control element  26   a  requesting a half-leg cross-connect download to cross-connect the SVC identified by the VCCI=X1 with outgoing port A on the TDM peripheral  36   a.  On receipt of the TXconnect set message in step  11   s,  the switch control element  26   a  uses an internal mapping table, shown as Table 5 to obtain the incoming port/vpi/vci associated with the SVC identified by the VCCI sent in the TXconnect set message. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 VCCI Mapping Table in Switch Control Element 
               
             
          
           
               
                   
                 Originating ATM Switch 
                   
                   
               
               
                 Originating PC 
                 Address 
                 VCCI 
                 Incoming Port/vpi/vci 
               
               
                   
               
               
                 P1 
                 AESA1 
                 x1 
                 C1/vp1/vc150 
               
               
                 . . . 
               
               
                   
               
             
          
         
       
     
     The switch control element  26   a  then requests the resource manager of ATM switch fabric  24   a  to download the cross-connect to interconnect AX LC  28   a  and the TDM peripheral  36   a.  If the cross-connect download is successful, the switch control element  26   a  informs the voice interface control unit  32   a  of the successful cross-connect. The voice interface control unit  32   a  then formulates ATM cells which are forwarded in step  12   s  to the signaling interface  34   a  instructing the signaling interface  34   a  to prepare an ISUP IAM message to be sent over signaling link  18   d  via STP  16   a  and signaling link  18   a  to the terminating PSTN network SSP that an incoming call is arriving on trunk  38   a.  The IAM message is sent by the signaling interface  34   a  in step  13   s.    
     On receipt of the IAM message, the destination SSP (not illustrated) examines the called number to determine the destination of the call. The destination SSP forwards the call towards its destination if it does not serve the called number. In any event, the destination SSP will return an ISUP Address Complete (ACM) message if the called party&#39;s telephone is determined to be available and an ISUP Answer (ANM) message when the called party goes off-hook. The path of the ACM and ANM messages is indicated by steps  14   s  in which the ACM and ANM messages are sent via signaling link  18   b  to STP  16   a  and via signaling link  18   d  to the signaling interface  34   a.  The signaling interface  34   a  extracts the call control information from the respective messages and packs the information extracted from each message in ATM cells which are forwarded through the ATM switch fabric  24   a  to the voice interface control unit  32   a.  The voice interface control unit  32   a  forwards the respective messages through the ATM network  10  to its peer voice interface control unit  32   b  which repeats the steps of forwarding the messages back to the originating SSP. Thereafter conversation ensues between the calling and called parties. 
     Call Setup with Cached SVC&#39;s 
     FIG. 5 presents a high-level overview of a call setup sequence in which cached SVCs are used in accordance with the method described in applicants&#39; copending patent application incorporated herein by reference. The call setup shown in FIG. 5 commences when an IAM is received in step  1   c  at STP  16   c  via signalling link  18   i.  The STP  16   c  forwards the IAM over signaling link  18   j  in the second part of step  1   c  to the TDM peripheral  36   b,  which passes the message to signaling interface  34   b.  The signaling interface terminates the MTP layers of the message and extracts call control information required by the voice interface control unit  32   b  to process the admission request. The signaling interface  34   b  then forwards the information in step  2   c  through ATM switch fabric  22   b  to the voice interface control unit  32   b.  On receipt of the ISUP IAM message, the voice control unit examines the called number to determine if special call treatment is required. In this example, the call is an unfeatured voice call, so the voice interface control unit  32   b  sends a request to its SVC selector (not illustrated) to determine whether a cached virtual circuit exists from the AX LC  28   b  to a AX LC  28   a.  As explained in applicants&#39; copending application, the SVC selector maintains a table of cached SVCs (Table 4) by destination point code and ATM switch prefix. In this example, the SVC selector returns an unoccupied SVC to the required destination which may be used for the connection. The voice interface control unit  32   b  in step  3   c  therefore requests the switch control element  26   b  to download a half-leg cross-connect from the port which terminates trunk  38   b  to the SVC identified by VCCI=x, as described above with reference to FIG.  4 . In response, the switch control element  26   b  in step  4   c  requests the resource manager of ATM switch fabric  24   b  to download the cross-connect. On confirmation that the cross-connect was successful, switch control element  26   b  responds in step  5   c  to the voice interface control unit  32   b  with an acknowledgement that the cross-connect was successful. Consequently, the voice interface control unit  32   b  formulates ATM packets containing ISUP IAM information which it forwards through the ATM network  10  in step  6   c  to the voice interface control unit  32   a  or through the SS7 network as an IAM message. The CIC carried in the message is the VCCI of the selected SVC. On receipt of the ISUP IAM information, voice interface control unit  32   a  checks its Table 4 to verify that it has an SVC identified by the given VCCI to the point code of voice interface control unit  32   b,  and on positive verification performs a table lookup to determine an outgoing port on TDM peripheral  36   a  to which the call should be routed. In step  7   c  the voice interface control unit  32   a  requests that switch control element  26   a  download a half-leg cross-connect to link the SVC to the port on TDM peripheral  36   a  which terminates the voice trunk  38   a.  The switch control element  26   a  in step  8   c  requests that the resource manager download the cross-connect. When the resource manager verifies that the cross-connect was successfully downloaded, switch control element  26   a  in step  9   c  returns confirmation to the voice interface control unit  32   a.  On receipt of the confirmation, the voice interface control unit in step  10   c  formulates ATM cells instructing the signaling interface  34   a  to send an ISUP IAM message to the destination SSP information the SSP that it has an incoming call on trunk  38   a.  The signaling interface  34   a  formulates the ISUP IAM message and forwards it in steps  11   c  to STP  16   a  over signaling link  18   d.  The STP  16   a  forwards the message over signaling link  18   a  to the SSP. On receipt of the ISUP IAM message, the SSP examines the called number to determine where the call should be terminated. If the called number is not served by the SSP, the SSP formulates another IAM message and forwards it to a terminating switch. In any event, if the called party line is available an ACM message is returned from the SSP to the originating SSP in steps  12   c  in the manner described above with relation to FIG.  4 . An ISUP ANM message is also returned via the same path after the called part answers the call, as is well understood in the art. 
     The difference in the call setup message flow using cached SVCs is the establishment of a switched virtual circuit through the ATM network  10  and the confirmation that the SVC has been set up (steps  5   s  and  8   s  in FIG.  4 ). Those two steps can be quite time-consuming if a plurality of nodes are transversed in the route between edge nodes  22   b  and  22   a.  The use of cached SVCs therefore significantly improves call setup time while ensuring that a minimum of unused ATM network resources are reserved, as explained in detail in applicants&#39; copending patent application. 
     Message Flow in Call Release Sequences 
     Call Release with SVC Release 
     FIG. 6 shows a schematic high-level diagram of a call release sequence in which an SVC is released on call termination. In step  1   r,  an ISUP REL message is received via signalling link  18   i  at STP  16   c,  indicating that the calling party has gone on-hook. The STP  16   c  forwards the message in the second part of step  1   r  to TDM peripheral  36   b.  The TDM peripheral  36   b  passes the message to the signaling interface  34   b  which terminates the MTP layers of the message and extracts the call control information which it forwards in ATM packets in step  2   r  to the voice interface control unit  32   b.  On receipt of the ATM packets containing the REL message information, the voice interface control unit  32   b  in step  3   r  returns ATM packets instructing the signaling interface  34   b  to formulate a ISUP RLC message which is sent back to the origination SSP in steps  4   r.  Subsequently, the voice interface control unit  32   b  releases the call and passes the SVC to the SVC selector which together with the SVC caching manager determines that the SVC used for the call should be released, because either SVCs are not cached or the number of cached SVCs exceeds a predetermined threshold. 
     The voice interface control unit  34   b  therefore sends a request in step  5   r  to the switch control element  26   b  to release the SVC with VCCI=x. On receipt of the request, the switch control element in step  6   r  requests the resource manager of ATM switch fabric  24   b  to release the cross-connect from the TDM peripheral  36   b  to the AX LC  28   b.  The switch control element  26   b  in step  7   r  therefore formulates an ATM signaling message which it forwards through the ATM network  10  requesting release of the SVC. The corresponding entries in Tables 2 and 4 are also deleted. As each ATM switch in the ATM network  10  is traversed, the resources allocated to the SVC are released and the SVC is deleted from local tables. When switch control element  26   a  receives the release request, it downloads a cross-connect release in step  8   r  and formulates a release complete ATM signaling message which it returns towards switch control element  26   b  in step  9   r.  The switch control element  26   b  then returns a release complete indication to the voice interface control unit  32   b  in step  10   r.  The switch control element  26   a  informs the voice interface control unit  32   a  in step  11   r  that the SVC has been released. The voice interface control unit  32   a  then deletes the corresponding entries in its Tables 2 and 4. 
     The voice interface control unit  32   b  formulates ISUP Release (REL) message information which it places in ATM packets and forwards in step  12   r  through ATM network  10  to the voice interface control unit  32   a.  On receipt of the ISUP REL message information, the voice interface control unit  32   a  verifies that VCCI=x has been released. It then returns an ISUP Release Complete (RLC) message in step  13   r  to its peer voice interface control unit  32   b.  Thereafter, the voice interface control unit  32   a  prepares ATM packets which are forwarded in step  14   r  through the ATM switch fabric  24   a  to the signaling interface  34   a  instructing the signaling interface  34   a  to prepare an ISUP Release message addressed to the terminating SSP. The signaling interface  34   a  prepares the ISUP REL message and forwards it in steps  14   r  over signaling links  18   d  and  18   a  to the SSP. On receipt of the REL message, the terminating SSP formulates an ISUP RLC message which it returns by the same path in steps  16   r.  On receipt of the RLC message, the signaling interface  34   a  terminates the MTP layers and extracts the call control information which it returns to the voice control unit  32   a  in step  17   r.    
     The release of the SVC is thereby complete and confirmed so that the resources reserved by the ATM SVC are thereafter available to other users of the ATM network  10 . 
     Call Release with Cached SVC 
     FIG. 7 shows an overview of a call release sequence in which the SVC used to complete the call through the ATM network  10  is cached for future use by another call to the same destination. In the call sequence shown in FIG. 7, an ISUP REL message is received over signaling link  18   i  by STP  16   c  in step  1   h  from an originating PSTN SSP (not illustrated). The STP  16   c  examines the destination point code and determines that the REL message should be forwarded in the second part of step  1   h  to the TDM peripheral  36   b.  On receipt of the message, the TDM peripheral  36   b  passes it to the signaing interface  34   b.  The signaling interface  34   b  terminates the MTP layers of the message and extracts the call control information which it packs in ATM cells and forwards through the ATM switch fabric  24   b  to the voice interface control unit  32   b  in step  2   h.    
     On receipt of the REL message, the voice interface control unit  32   b  advises its SVC caching manager of the call release. The caching manager, as explained in applicants&#39; copending patent application, examines its cache tables and determines that the SVC should be cached. The caching manager therefore instructs the switch control element in step  3   h  that the half-leg cross-connect from the port on TDM peripheral  36   b  to the AX LC  28   b  should be released. The SVC segment from AX LC  28   b  to the destination is cached for future use, and is not released. Thereafter, the voice interface control unit  32   b  in step  4   h  instructs the signaling interface  34   b  to formulate an ISUP RLC message and return it to the LEC SSP. The signaling interface  34   b  formulates the RLC message and returns it in steps  5   h  to the originating SSP. The voice interface control unit  32   b  then formulates ATM packets containing ISUP REL message information which it forwards in step  7   h  to the peer voice interface control unit  32   a.  On receipt of the ISUP REL message, the voice interface control unit  32   a  verifies that the VCCI is still in Table 4 and in step  8   h  sends a TXconnect rel message (FIG. 3) to the switch control element  26   a  requesting that it download a half-leg disconnect of the cross-connect from the AX LC  28   a  to the DS0 port on TDM peripheral  36   a.  Subsequently, the voice interface control unit  32   a  formulates ATM packets containing ISUP RLC message information which it returns in step  9   h  to its peer voice interface control unit  32   b.  The voice interface control unit  32   a  then formulates ATM cells instructing, in step  11   h.  the signaling interface  34   a  to prepare an ISUP REL message towards the destination SSP. The signaling interface  34   a  formulates the ISUP REL message and forwards it in steps  12   h  through STP  16   a  to the terminating SSP via signaling links  18   d  and  18   a.  On receipt of the ISUP REL message, the terminating SSP formulates and returns an ISUP REL message in steps  13   h  to the TDM peripheral  36   a.  TDM peripheral  36   a  passes the ISUP Release Complete message to the signaling interface  34   a.  The signaling interface  34   a  terminates the MTP layers of the message and extracts the call control information which it returns in step  14   h  to the voice interface control unit  32   a.    
     At the conclusion of the steps shown in FIG. 7, a cached virtual circuit exists between AX LC  28   b  and AX LC  28   a.  When a next call arrives requiring an SVC from the same ATM source to the same ATM destination, or vice versa, the call can be rapidly set up in accordance with the sequence of events described above in relation to FIG. 5 using the cached SVC. 
     As will be understood by those skilled in the art, the method and apparatus described above enable the support of PSTN features for calls routed for transfer through an ATM network. The method and apparatus also have the advantage that separate call control and ATM SVC control operate to minimize the impact of transferring voice services to ATM networks. This permits the ATM network  10  to be available for other data services. Since native signaling is used in the ATM network, equipping ATM edge nodes to admit STM calls is easily and inexpensively effected. In addition, if the method of SVC caching and cross-connect caching are practised in accordance with the teaching of applicants&#39; copending patent application, voice calls may be rapidly established through the ATM network which permits service providers to maintain expected call connect times even during peak calling hours. 
     Changes and modifications to the above-described embodiments will no doubt be apparent to those skilled in the art. The scope of the invention is therefore intended to the limited solely by the scope of the appended claims.