text: fig1 illustrates an asynchronous transfer mode ( atm ) node comprising a plurality of computing units ( cu ) 1 , 2 , 3 that each can serve as an atm signaling message processor , a switch element ( se ) 4 , and a plurality of sites with customer premises equipment ( cpe ) 5 though 8 which are end - points of switched virtual circuits ( svcs ). the cus 1 through 3 can be any standard workstation programmed to run q . 2931 signaling software . they are used to process atm signaling messages which enter and leave the node through the se 4 . the se 4 has a switching fabric ( not shown ) for interconnecting network links in accordance with the atm protocol - suite . the se 4 is connected to all the cus 1 through 3 through a high - speed link 9 that could be an atm link or an ethernet link for example . the cpes 5 through 8 are connected to the se 4 by high - bandwidth atm connections 10 through 13 . the atm user network interface ( uni ) protocol is implemented across the connections 10 through 13 . fig1 also illustrates the hardware elements within the se 4 that help implement atm signaling . these elements include a plurality of atm ports 14 through 17 that each occupy a card of a slot ( it is to be noted that more than one port can occupy a card ) leading into the backplane of the se ( cards , slots and backplane not shown ). one card / slot combination can support multiple ports . it is to be noted that none of the cards require a local embedded computing system for signaling processing purposes . there also is an atm signaling session dispatch logic ( assdl ) 18 , which is implemented in a preferred embodiment as an integrated circuit ( ic ) on a card 19 that also occupies a slot leading into the backplane of the se 4 ( slot and backplane not shown ). it is to be noted that the assdl 18 need not be implemented as an ic occupying a card -- it can be implemented as an asic on a regular network card containing ports , or can be implemented as a stand - alone platform that is separate from the se 4 or the cus 1 through 3 . on the same card 19 as the assdl circuit 18 are ics 20 through 23 that are each connected to a port 14 through 17 respectively . the ics 20 through 23 implement the signaling atm adaptation layer ( saal ) protocols required by the q . 2391 signaling protocol -- the service specific coordination function ( sscf ) and the service specific connection - oriented protocol ( sscop ). the assdl 18 includes an active server table 24 and a call reference table 25 . the active server table 24 contains information about the present signaling processing load being handled by each cu 1 through 3 . this information can include the sizes of , and the priorities of , the processing loads presently being handled by the cus 1 through 3 . the call reference table 25 maps identifiers of svc sessions that are currently being supported by the node on one hand , to the cu 1 , 2 or 3 that is handling each svc session on the other hand . the identifiers of svc sessions can be obtained from the call reference field in each q . 2391 pdu , and are hereinafter referred to as call - reference ids . it will be noted that the card 19 contains the parts of the se 4 that exclusively carry out functions related to atm signaling . when a request to create an svc is generated at one of the cpes , for example the cpe 5 , a message carrying the request is sent through the atm link 10 to the se 4 . at the se 4 , the message is received by the port 14 , stripped of its lower layer protocol headers ( i . e . headers that implement the physical layer protocols , atm layer protocols , common part atm adaptation layer ( cp - aal ) protocols , etc . ), and is sent to the card 19 containing the ics 20 through 23 and the assdl 18 . at the ics 20 through 23 , the message is further stripped of its sscop and sscf headers , so that only a q . 2391 program data unit ( pdu ) remains by the time the message reaches the assdl 18 . the assdl 18 then reads a field within the q . 2391 pdu that contains the call - reference id for the svc session . the assdl 18 uses the call - reference id in scanning the call reference table 25 to determine whether or not the svc being requested is already being supported by one of the node &# 39 ; s cus 1 through 3 . if a mapping between the call - reference id and a given cu is found in the call reference table 25 , the requested svc is already being supported by one of the cus 1 through 3 , and the q . 2391 pdu is dispatched to the supporting cu , as described below . if no entry in the call - reference table 25 maps the call - reference id of the requested svc to one of the cus 1 through 3 , the assdl 18 realizes that the newly arrived pdu is carrying a new svc session setup request . the assdl 18 therefore analyses the active server table 24 to select a cu 1 , 2 or 3 to provide signaling processing for this new svc session . the assdl 18 can use a variety of algorithms to select a cu , depending on what information is held in the active server table 24 . a preferred algorithm would select the cu with the most processing capacity available . in a preferred embodiment , the assdl 18 selects the cu 1 , 2 or 3 , that is presently supporting the smallest number of svc sessions . in another preferred embodiment , the cu that has received the smallest number of messages in a recent period of time is selected . it will be obvious to those skilled in the art that several other cu - selection algorithms can be implemented by varying the information held in the active server table 24 . however the selection process takes place , the selected cu , such as the cu 3 for example , will handle all signaling processing on the node for the duration of the svc session . to ensure the cu 3 receives all messages associated with the given svc , the assdl 18 creates a new entry inside the call reference table 25 that maps the call - reference id of the new svc to the selected cu 3 . when a subsequent q . 2391 pdu belonging to the same svc session is received at the se 4 , the assdl 18 will once again obtain the call - reference id from the pdu , as was done when the first pdu associated with this svc session entered the node . this time however , as a result of the newly made entry into the call reference table 25 , the assdl 18 will use the call - reference id to determine that the cu 3 has already been assigned to support the above - mentioned svc session . the assdl 18 will automatically dispatch the pdu to the cu 3 . the cu 3 will thus continue to process signaling messages passing through the se 4 for the above - mentioned svc session , until the cpe 5 indicates that the svc session is ending . when this happens , the entry mapping the cu 3 to the svc of cpe 5 is removed from the call reference table 25 , and the assdl 18 will no longer automatically dispatch messages associated with the given svc session to the cu 3 . it is to be noted that the flow of q . 2391 response - pdus from the cus 1 through 3 to the ports 14 through 17 , and out over the links 10 through 13 , also take place through the assdl 18 . the ics 20 through 23 , along with other hardware on the se 4 , add on all headers necessary for the signaling message to be sent to other nodes in the network . it also is to be noted that the assdl 18 does not process a q . 2391 pdu , except for identifying the svc session with which the pdu is associated by reading the call reference id from the pdu , selecting a cu to process the pdu , and dispatching the pdu to a cu . this means that there is no need to deploy high - performance embedded computing systems for any of the ports 14 through 17 . this also means that the assdl 18 can operate at a very high speed . it also is to be noted that no queuing of q . 2391 pdus takes place in the assdl 18 or anywhere else on the se 4 . any queuing of these pdus is off - loaded onto the cus 1 through 3 . in a preferred embodiment , the assdl 18 on one hand , and the cus 1 through 3 on the other hand , use a protocol to exchange information that helps the assdl 18 perform more efficiently . it is to be noted , that those skilled in the art will realize it is possible to implement the aforementioned architecture and methods without establishing such a protocol . the assdl 18 for example can read q . 2391 pdus to decide whether or not svcs are going to be created or released , and thus update the call reference table 25 without receiving information from any of the cus 1 through 3 . the assdl 18 can also use several dispatch algorithms , such as round - robin algorithms , that do not require information from the cus 1 through 3 . the use of a protocol between the cu 1 through 3 and the assdl 18 is nonetheless desirable , if the following objectives are to be achieved : ( 1 ) better link integrity between the assdl 18 and the cus 1 through 3 , that decreases the probability that a q . 2391 pdu will not be incorrectly handled ; and , ( 2 ) richer information exchange between the cus 1 through 3 and the assdl 18 , that allows for more sophisticated cu - dispatch algorithms to be used . a protocol between the assdl 18 and the cus 1 through 3 , hereinafter referred to as the assdl protocol , is thus implemented in a preferred embodiment . fig2 illustrates a header 26 that is added to a q . 2391 pdu 27 in order to implement the assdl protocol . the assdl header 26 , which is attached to each q . 2391 pdu 27 flowing between the assdl 18 and the cus 1 through 3 , carries the information required to implement the assdl protocol . in a preferred embodiment , the assdl header 26 comprises a fault protection field 28 , a server load indicator field 29 , a connection release field 30 and a link integrity field 31 . the fault - protection field 28 is used to guarantee better link integrity between the assdl 18 and the cus 1 through 3 . the cus 1 through 3 insert heart - beat messages into the fault protection field 28 of each pdu they send to the assdl 18 . according to the assdl protocol , if the assdl 18 cannot receive the heart - beat message from a cu before a heart - beat time - out expires , the assdl 18 will assume that the cu has malfunctioned and initiates fault - protection fall - back procedures to assign another cu to serve the svcs being supported by the faulty cu . in such an event , the affected cu is also marked within the active server table 24 as being inoperative , so as to prevent new svcs from attempting to rely on the cu for signaling processing services . no new signaling messages are forwarded to the affected cu until it successfully sends heart - beat messages to the assdl 18 . when this occurs , the affected cu is marked within the active server table 24 as being operative . the server load indication field 29 is used to allow a greater variety of more sophisticated q . 2391 dispatch algorithms to be implemented . more specifically , the field 29 is filled by the cus 1 through 3 , and then read by the assdl 18 to update the entries inside the active server table 24 that contain cu - status information . as mentioned previously , this information is used by the assdl 18 to decide which cu 1 , 2 or 3 will be assigned signaling processing responsibilities for newly supported svc sessions . the information placed in this field thus defines which cu - selection algorithms can be used . if the assdl 18 selects the least loaded cu to support a new svc session for example , the server load indication field 29 must indicate the size of the processing loads of each cu . it the assdl 18 selects the cu that has had the least signaling traffic in a recent period of time to support a new svc session , the server load indication field 29 must indicate the number of messages recently handled by each cu . the connection release field 30 is used to guarantee better link integrity between the assdl 18 and the cus 1 through 3 . it is filled by cus 1 through 3 once the q . 2391 signaling software receives a q . 2391 release message and decides to terminate a given svc session . when such a message is placed in the connection release field 30 , the assdl 18 will remove from the call reference table 25 the entry mapping the given svc session to one of the cus 1 through 3 . this feature of the assdl protocol is invaluable when , for example , an incoming message indicates that a request to release the svc has been made , but the q . 2391 software on the cu serving the svc session will eventually turn down the request . if the assdl 18 only associates and disassociates an svc from its cu using the connection release field 30 , and thus only releases svcs when instructed to do so by the q . 2391 signaling software on the cus 1 through 3 , this situation will not lead to the assdl 18 incorrectly removing an entry from the call reference table 25 . if on the other hand , the assdl 18 disassociates an svc from its cu whenever it handles a q . 2391 pdu 27 containing a request to release the svc , entries will be mistakenly removed the call reference table 25 in such situations . the link integrity field 31 also guarantees better link integrity between the assdl 18 and the cus 1 through 3 . in a preferred embodiment , the link integrity field 31 can be filled with a count of the number of q . 2391 pdus that have already been received or sent by either the assdl 18 or the cus 1 through 3 , in supporting a given svc session . by allowing the assdl 18 and the cus 1 through 3 to share this information with each other , the assdl protocol gives them the opportunity to compare the number of pdus each believes has been processed for a given svc session . this comparison , in turn , can lead to the detection of communication errors such as missing pdus , and thus trigger the timely invocation of error re - transmission services . it is to be noted that the signaling processing methods that are performed using the architecture of fig1 and the above described assdl protocol , collectively have several advantages over signaling processing methods available to prior art one - processor - per - card and one - processor - per - node architectures . processing power , and the ability to support a greater number of svc sessions , can be gracefully added to each node by simply adding more cus to a given se . atm signaling messages can be processed more rapidly than even in the one - processor - per - card architecture , since excessive traffic through a single port can be dynamically load - shared between several cus under the architecture of fig1 rather than being rammed through a card &# 39 ; s single processor . furthermore , by not requiring high - performance embedded computing systems on each card of the node , the signaling processing method described above reduces the costs of port cards and of software upgrade and support , relative to the costs generated by the methods available under the one - processor - per - card architecture . upgrade and support costs are reduced because the cus do not need to be embedded in cards and thus can be implemented by widely supported industry - standard workstation technology . finally , the architecture described above , when used in conjunction with the assdl protocol , allows for greater fault - tolerance and more sophisticated signaling dispatch algorithms , by providing dynamic control over computing - system - to - svc mappings . it will be appreciated by those skilled in the art that numerous other modifications and variations of the above - described architecture and methods are possible . the methods and architectures described above can be applied to any node that forms part of an svc , and is not restricted to nodes that provide uni interfaces for cpes . the contents of the active server table 24 , the call reference table 25 and the fields of the assdl header 26 can be varied to contain slightly different information . a further variation is possible as shown in fig3 which illustrates a modified version of the atm node of fig1 . the only major difference between the nodes of fig1 and fig3 is that parts inside the card 19 of fig1 which as mentioned before comprise the parts of the se 4 that carry out functions related to atm signaling , are moved off the se 4 and onto an external platform 32 . according to this embodiment , signaling messages coming from the cpes 5 though 8 , are automatically routed across a switching fabric 33 of the se ( present but not shown on the node of fig1 ) and a high - speed atm link 34 , to the platform 32 . at the platform , the ics 20 through 23 and the assdl 18 deal with the signaling messages in substantially the same manner as described above . it also is possible to install the components inside the card 19 of fig1 onto another cu , such as one of the cus 1 , 2 or 3 .