Abstract:
A signal state management (SSM) system avoids both the overhead of maintaining call state and complex signaling in a packet network gateway, while simultaneously providing a more scalable system by not placing too great a state or processing burden on the signaling or call processing server. The SSM system sends an event/action table from a call agent to the gateway that directs the gateway to detect and accumulate specified signaling events generated by a telephone. The gateway accumulates the signaling events according to the event/action table and notifies the call agent only when the signaling events specified in the event/action table are detected. The call agent then sends a new event/action table to the gateway that commands the gateway to detect and accumulate a new set of signaling events. After notifying the call agent, the gateway operates in a quarantine where all signaling events are queued. The gateway remains in the quarantine state until the new event/action table is received from the call agent.

Description:
[0001]    This application is a continuation of Ser. No. 09/107,071 filed on Jun. 29, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to managing signaling states in packet network telephony gateways and more particularly to handling telephony signaling in a way that minimizes state overhead in the gateway while allowing high scalability.  
           [0003]    Packet telephony gateways are sometimes optimized to work in concert with common channel signaling protocols. For example, packet gateways operate with Common Channel Signaling System #7 (CCS#7) or with Integrated Services Digital Network (GSDN) PRI. These protocols allow signal processing entirely outside the gateway itself, such as in a dedicated call control system. This has the advantage of avoiding the need for maintaining any call state in the gateways.  
           [0004]    For some types of telephony systems, such as those attached directly to analog telephones or to older trunk systems such as Ear and Mouth (E&amp;M) or Channel Associated Signaling (CAS) systems, the gateway is involved in at least low level telemetry aspects of call signaling. In CAS-like scenarios, the gateway also needs to disentangle the channel associated signaling from truly in-band signaling, such as Dual Tone Multi Frequency (DTMF) signaling.  
           [0005]    The types of telemetry signals received by the gateway from a telephone include on-hook and off-hook transitions, hook flash, progress tone detection (and possibly analysis) and DTMF detection and possible generation. For trunks connected via in-band signaling to PBXs, analog switches, etc., a gateway needs to hear various progress signals like busy, network congestion, etc. The gateway then needs to process (i.e. “progress tone analysis”) and convert the signals into events.  
           [0006]    The simplest approach is to back-haul telemetry-like signaling to a signaling or call processing server and treat the signaling in the call processing server identically as in the common channel signaling case. Unfortunately, this fails to scale for a couple of reasons. The number of independent signaling channels can be extremely large, possibly on the same order of magnitude as the number of stations on a local carrier&#39;s network. The number of messages generated can also be very large; many more than 2-3 per call as in the common channel signaling case. For example, every digit pressed while dialing a phone number can constitute a message that must be sent to the call process server. The back-haul signaling approach substantially increases network traffic between the gateway and the call processing server and requires additional computing resources to transmit, receive and process the back-hauled signaling.  
           [0007]    Thus, a need remains for reducing the overhead required to maintain call states in the gateway while simultaneously providing gateway intelligence that allows the gateway to process telephone events more efficiently.  
         SUMMARY OF THE INVENTION  
         [0008]    A signal state management (SSM) system according to the invention avoids both the overhead of maintaining call state and complex signaling in a gateway, while simultaneously providing a more scalable system by not placing too great a state or processing burden on the signaling or call processing server. Besides providing a more scalable system, the SSM system has the ability to create (possibly a dynamic) adaptation based on several varied classifications. Classifications can include user identity, type of service subscribed, time of day, type of gateway, location of telephone instruments within the network, etc. The system manages signaling without adding to the programming complexity, and processing burden of the packet gateway device.  
           [0009]    The SSM system sends an event/action table from a call agent to the gateway. The event/action table directs the gateway to detect and accumulate specified signaling events generated by a telephone. The gateway detects and accumulates the signaling events according to the event/action table and notifies the call agent only when certain signaling events specified in the event/action table are detected. The call agent then acknowledges the notification by sending a new event/action table to the gateway that directs the gateway to detect and accumulate a new set of signaling events. The gateway then accumulates signaling events according to the new event/action table and only notifies the call agent when the signaling events specified in the new event/action table are detected.  
           [0010]    After notifying the call agent, the gateway operates in a quarantine state where all signaling events generated by the telephone are queued by the gateway. The gateway remains in the quarantine state until the new event/action table is received from the call agent.  
           [0011]    The event/action table can include a digit map that directs the gateway to accumulate signaling events into a dial string. The dial string is continuously compared with the digit map. Signaling events are added to the dial string if the current dial string is underqualified and only partially matches the digit map. The dial string is sent in a single message to the call agent current dial string matches a complete entry in the digit map. The event/action table specifies the events that the gateway is to detect such as tones, hook transitions and telephone digits. The event/action table also specifies actions that the gateway takes when the specified events are detected, such as ignore, send immediately and accumulate. 
       
    
    
       [0012]    The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention, which proceeds with reference to the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a signaling state management system according to the invention.  
         [0014]    [0014]FIG. 2 is a table showing the types of actions specified in an event/action table used with the signaling state management system shown in FIG. 1.  
         [0015]    [0015]FIG. 3 is a table showing events that can be specified in the event/action table according to the invention.  
         [0016]    [0016]FIG. 4 is a table showing actions that can be specified in the event/action table according to the invention.  
         [0017]    [0017]FIG. 5 is a BNF syntax for specifying an entry in the digit map.  
         [0018]    [0018]FIG. 6 is a block diagram showing how the digit map in FIG. 5 is used in the signaling state management system.  
         [0019]    [0019]FIG. 7 is a block diagram showing how the event/action table and digit map are used in the signaling state management system.  
         [0020]    [0020]FIG. 8 shows an example of how the signaling state management system handles an aborted call.  
         [0021]    [0021]FIG. 9 shows an example of how the signaling state management system handles a credit card call. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring to FIG. 1, a packet-based telephony system  12  includes circuit oriented telephones  28  that generate conventional telephony signals. A residential gateway  26  connects the telephones  28  to a packet network  14 . The residential gateway  26  conducts the conversions between data packets containing audio data (audio packets) in the packet network  14  and audio signals coming from the telephones  28 .  
         [0023]    A Realtime Transport Protocol (RTP) is used over a link  30  to transfer the audio packets between the residential gateway  26  and a trunk gateway  22 . A call agent  24  controls signaling events from telephone  28  through the residential gateway  26  via the signaling state management system according to the invention. A signaling and control protocol, such as SGCP can be used over link  32  and/or link  33 . Call agent  24  communicates to another call agent  20  over link  34  using a conventional signaling protocol such as CCS#7. The call agent  20  connects to a telephone carrier Signal Transfer Point (STP) switch  16 .  
         [0024]    The residential gateway  26  is typically part of a router, personal computer, etc. and connects to standard telephones  28 . The call agent  24  is typically located in a host server computer. The call agent  20  is a computer that controls the trunk gateway  22  through a protocol such as SGCP. The call agents  20  and  24  do not pass the actual audio data but control how the gateways  22  and  26 , respectively encode, packetize, and route the audio data in packet network  14 . The general hardware and software used in gateways  22  and  26  and call agents  20  and  24  for processing audio packets is known and is, therefore, not described in further detail. The call agent  24  that controls the residential gateway  26  is alternatively referred to as a signaling or call processing server  24 . The residential gateway  26  is alternatively referred to as simply gateway  26 .  
         [0025]    The signaling state management (SSM) system according to the invention maintains synchronization with the control path used by the call agent  24  to control the gateway  26 . There is little or no permanent configuration state in the gateway  26 . The invention allows individual low level signals generated by telephones  28  to either be sent to the call agent  24 , processed locally by the gateway  26 , or entirely suppressed, under control of the call agent  24 . For example, the gateway  26  can maintain a purely slave status in keeping with the simple control model.  
         [0026]    An event/action table  36  is sent from the call agent  24  to the gateway  26 . The event/action table  36  treats each signal from telephones  28  as a signaling event  56 . Each event  56  is looked up in the table  36  by the gateway  26 . The table  36  maps predefined events to predefined actions. The mapping and the types of actions are defined to be as stateless as possible to simplify processing in gateway  26 . One key to the invention is that the event/action table  36  may be downloaded by the call agent  32  piggybacked on any control command or response from the call agent  24  to the gateway  26 .  
         [0027]    Digit strings may be accumulated by the gateway  26  without sending each digit to the call agent  24  for processing. A digit map  50  performs regular expression matching on a digit string from telephones  28  to determine if the accumulated digit string must be sent to the call agent  24 . The digit map  50  is downloaded, when necessary, by the call agent  24  with the event/action table  36 .  
         [0028]    The gateway  26  has only a small, fixed set of actions defined by the event/action table  36 , thus keeping the processing very simple. Typical actions include ignore, accumulate, send to call agent, invoke digit map, and swap audio to endpoint. When the specified event in the event/action table  36  or digit map  50  occurs, the gateway  26  notifies the call agent  24  with an event list  66 .  
         [0029]    Referring to FIG. 2, the event/action table  36  maps telemetry-style signaling events from any one of the telephones  28  to actions taken by the gateway  26 . The actions are of three types: suppress, send, or local processing. The event/action table  36  is sent from the call agent  24  to the gateway  26  whenever the call processing server  24  needs to change the mapping of events to actions. The event/action table  36  may be piggybacked on any control message and may be encoded in any number of ways depending on the protocol it is embedded in. As an example, the event/action table  36  can be encoded in any existing voice over IP (VoIP) signaling or control protocol, such as Simple Gateway Control Protocol (SGCP) or H.323 RAS. The invention is not limited to use only with the SGCP syntax. However, for explanation purposes, some signaling state management operations are described in the context of the SGCP environment.  
         [0030]    Table  37  in FIG. 2 identifies the actions in the gateway  26  that are controlled by the call agent  24  via the event/action table  36 . When the event/action table  36  sends an Ignore action, the gateway  26  ignores the specified event. An Accumulate action causes the gateway  26  to add the event to the event list  66  to be sent to the call agent  24 . The gateway  26  sends the event list  66  to the call agent  24  when an event specified by a Send action in the event/action table  36  is detected. The Send action causes the gateway  26  to send the specified event and all previously accumulated events to the call agent  24 . An Apply Digit Map action causes the gateway  26  to apply events to the digit map  50  that is described in further detail below. A Swap Audio action causes the gateway  26  to reassign a current audio output to a next audio source in a list. This action is used for hold, transfer, conference and 3-way calling functions.  
         [0031]    [0031]FIGS. 3 and 4 show detailed tables of the events and actions implemented in the event/action table  36 . An event table  38  in FIG. 3 provides a list of events  40  that can be specified by the call agent  24 . Each event  40  is identified by a code  42  and include fax tones, modem tones, continuity tones, hook transitions and digit collection.  
         [0032]    Each event  40  can be qualified by a requested action, or by a list of actions  46  defined in action table  44 . The actions  46 , when specified, may be encoded in a variety of ways such as a list of keywords, an ASN. 1  data structure, etc. The actions  46  have associated codes  48 . The actions  46  include notifying the call agent  24  immediately (N), accumulating events (A), treating the event according to a digit map (D), swapping calls (S) and ignoring the event (I). The swap action is used for handling hook-flash locally at the gateway  26 . The telephone  28  may be connected to two calls at the same time. The gateway  26  does not want to notify the call agent  24  each time a user hits hool-flash in order to switch between the two calls. Thus, the call agent  24  uses the Swap action to allow the gateway  26  to handle hook-flash locally.  
         [0033]    When no action  46  is specified, the default action is to notify the event to the call agent  24  immediately (N). This means that, for example, fit and ft(N) are equivalent. Events that are not listed are ignored.  
       Digit Map  
       [0034]    Of significant importance to the invention is a digit map  50  that allows events associated with dialing of telephone numbers, PINs, credit card numbers, etc., to be “batched” and sent as one message by the gateway  26  to the call agent  24 . Conventional signaling protocols require the gateway  26  to perform one of the following:  
         [0035]    1. Send every single digit as it is entered, resulting in considerable overhead in the call agent. This is how legacy telephony switches and most ISDN systems operate.  
         [0036]    2. Have an explicit “Send” user interface action, like a cellular phone.  
         [0037]    3. Rely on a timeout to guess when the user is finished dialing digits.  
         [0038]    4. Have a complete dial plan locally in the gateway  26  so the number can be parsed locally. This is infeasible for most numbering plans, especially international plans.  
         [0039]    Referring to FIGS. 5 and 6, the digit map  50  is represented as regular expressions. Any regular expression syntax can be used, such as that used by a Unix egrep utility. The digit map  50  is encoded in the control protocol used between the call agent  24  to the gateway  26 , such as in the SGCP protocol.  
         [0040]    The digit map  50  is stored in the gateway  26  and used to detect predefined events  56  such as digits, letters or timers. The gateway  26  adds the event parameter code  42  (FIG.3) as a token to the end of an internal state variable called the “current dial string”. The current dial string  58  is derived by accumulating signaling events  56  from telephone  28 . The current dial string  58  is applied to the digit map  50  by a processor  60 . The processor  60  attempts to match the current dial string  58  to each regular expression in the digit map  50  in lexical order. If the result is under-qualified (current dial string partially matches at least one entry in the digit map), nothing further is done by the gateway  26 . If the result matches, or is over-qualified (i.e. no further digits could possibly produce a match), the digit string  62  is sent to the call agent  24  by processor  60 .  
         [0041]    For example, the digit map  50  may tell the gateway  26  to accumulate a specified group of digits and then send the accumulated digits to the call agent  24 . The call agent  24  may determine the received accumulated digits are credit card numbers. The call agent  24  then sends a new digit map  50  that directs the gateway  26  to look for a PIN, a telephone number, or anything else the call agent deems appropriate at this stage of the signaling state for this call.  
         [0042]    Referring to FIG. 7, an example of an event/action table  36  is as follows: 
           R:hu ( N ),  hf ( S,N ) 
         [0043]    The call agent  24  in the event/action table  36  commands immediate notification (N) of a “on hook transition” (hu) for the telephone  28 . The call agent  24  also requests a call swap (S) and immediate notification (N) when a “flash hook” event (hf) occurs on the telephone  28 . The gateway  26  stores the event/action table  36  and monitors for the events hu and hf from the telephone  28 . Events detected by the gateway  26  are provided to the call agent  24  in the event list  66 . The event codes in the event list  66  are the same as those used in the event/action notification request from the call agent  24 . In a first event list  66 , the gateway  26  notifies the call agent  24  of two flash-hooks (hf) and an on-hook transition (hu). 
         
       O:hf, hf, hu 
     
         [0044]    In a second event/action table  36 , the call agent  24  requests immediate notification (N) of an “on hook” transition (hu). The call agent  24  also requests the gateway  26  to treat digits 0-9 and timers according to a digit map (D). The digit map  50  is sent along with the event/action table  36 . The entries in the event/action table  36  are as follows: 
           R:hu ( N ), [0-9# T] (D) 
         [0045]    Events that have been accumulated according to the digit map  50  are grouped in a single string by processor  60 . The events that correspond with an event/action table  36  or match the digit map  50  are sent in the event list  66  to call agent  24 . In a second event list  66 , the gateway  26  notifies the call agent  24  of an off-hook transition (hf) and then notifies the call agent  24  of a string of digits (8295555) generated by the telephone  28  within a time (T) specified by a timer. 
         
       O:hf 
     
           O: 8295555 T   
       Synchronization with Control Path  
       [0046]    In order to prevent race conditions between the detection of signaling events  56 , reporting it to the call agent  24 , receiving a new event/action table  36 , and the detection of other signaling events  56 , the SSM system ensures two key properties or invarients are always met. After detecting an event, all further events are “quarantined” or otherwise “queued up” until either disposed of locally by the gateway  26 , or the gateway  26  has sent the event to the call agent  24  and received a response, possibly with a new event/action table  36 .  
         [0047]    Quarantining events means the gateway  26  stops processing signaling events according to the event/action table  36 . Subsequent events are stored while the gateway  26  waits to hear back from the call agent. The stored events are processed after an acknowledgment is sent back from the call agent  24 . The reason the gateway  26  waits to respond to the new events, is because the call agent  24  may change the event/action table  36 . This ensures each one of the events  56  is processed according to an event/action table  36  synchronized with the call agent  24 . The call agent  24  can provide a new event/action table  36  or digit map  50 , or both, in any response sent to the gateway  26 . These two invariants ensure that every event  56  is processed in the context of the current state of the whole system, as seen by the call agent  24 . Unlike a simple backhaul protocol, the SSM system is tightly synchronized with the rest of the gateway control and may piggyback event/action tables on any control message.  
         [0048]    For example, a race condition can occur during three-way telephone calling. When a first call is established, hitting hook-flash generates a dial tone for establishing a second call. If the two calls are now established, hitting hook-flash switches between the two already established calls. The user may have already established the first call and is about to establish the second telephone call. If a hook-flash happens after the second call is established, the gateway  26  should switch to the second call. If a hook-flash happens before the second call is established, the gateway should establish a dial-tone.  
         [0049]    The gateway  26  is not allowed to interpret the hook-flash until the call agent  24  has acknowledged the last notified event. Thus, the SSM system avoids the hook-flash race condition described above. Because the call agent  24  is always aware of the call state context in which to interpret any signaling event, race conditions inherent in other management schemes are avoided.  
         [0050]    [0050]FIG. 8 shows an example of how the signaling state management system operates for an aborted call. The events described in FIG. 8 occur when a user picks up the telephone  28 , dials a phone number, and then decides to abandon the call before completing the call. This shows two critical aspects of the invention. First, the digit map  50  processing by the gateway  26  avoids sending messages to the call agent  24  on every digit press. Second, the quarantining of signaling events  56  by the gateway  26  only processes an on-hook event, caused by the user hanging up, only after the call agent  24  acknowledges the prior notification with the entered digits.  
         [0051]    In step  80 , the call agent  24  sends the event /action table  36  and a digit map  50  (if appropriate) to the gateway  26 . This primes the gateway  26  with the information needed for processing the signaling events  56  from telephone  28 . Signaling events  56  are discarded until the gateway  26  receives the event/action table  26 . The gateway  28  then acknowledges receipt of the table to the call agent  24  in step  82 . All messages are acknowledged by call agent  24 . In step  84 , a user picks up the telephone  28  creating an off-hook event. The event/action table  36  includes a command to notify the call agent  24  immediately when an off-hook event occurs. The gateway  26  accordingly notifies the call agent  24  of the off-hook event and then quarantines all further events received from the telephone  28 .  
         [0052]    The gateway  26  at this point will not take any other action, other than accumulating further signaling events  56 , until there is an acknowledgement from the call agent  24 . This is because the action that the gateway  26  may depend on the next event/action table  36  that the call agent  24  may send in response to the off-hook notification. The call agent  24  in step  86  acknowledges the notification from the gateway  26  by changing the event/action table  36  and digit map  50 . The digit map  50  tells the gateway to look for a phone number.  
         [0053]    The gateway in step  88  receives a digit from the telephone  28 . The gateway does not notify the call agent  24  yet because the current accumulated dial string does not match the entire digit map  50 . The gateway  26  in steps  90  and  92  continues to add digits to the current dial string  58  and apply the digits to the digit map  50 . A fully qualified match of the current dial string  58  with the digit map  50  occurs in step  94 . Accordingly, the gateway  26  sends the current dial string to the call agent  24 .  
         [0054]    In step  96 , a user decides for some reason not to make the telephone call and hangs up the telephone  28 . The gateway  26  accordingly receives an on-hook event. The gateway  26  cannot notify the on-hook event to the call agent  24  at this point, because on-hook may warrant a different action depending on the next event/action table  36  sent by the call agent  24 . The on-hook event is, therefore, quarantined by the gateway  26  until an acknowledgment (ACK) of the dial string notification is sent by the call agent  24 .  
         [0055]    The call agent  24  sends the acknowledgement in step  98  that includes a new event/action table  36 . The new event/action table  36  directs the gateway  26  to ignore further digits because the user has completed dialing the phone number. The new event/action table  36  also directs the gateway to immediately notify the call agent  24  if an on-hook event “hu(N)” is detected.  
         [0056]    If the call had been to an emergency service, such as 911, the call agent  24  may have loaded an event/action table  36  that commands the gateway  26  to ignore events such as on-hook to ensure the user remains connected. In this case the following interaction would not occur until the emergency service released the call.  
         [0057]    In step  100 , the gateway  26  has received the acknowledgement from the call agent  24  in the form of a new event/table  36 . The on-hook event is therefore, no longer quarantined and, in turn, notified by the gateway  26  to the call agent  24 . The call agent  24  then stops processing the call identified in the dial string previously sent by the gateway  26 . The call agent  24  resets to a clean state in step  100 . In step  102  the call agent  24  acknowledges the on-hook event by sending a new event/action table  36  to gateway  26 . The new event/action table  36  has the effect of resetting the gateway to the initial state in step  80 .  
         [0058]    Referring to FIG. 9, a second call example starts out in steps  104 - 110  identically as steps  80 - 86 , respectively, in the call described in FIG. 8. However, in steps  112 - 116  the user has dialed the access number for a credit card call. The gateway in step  118  notifies the call agent  24  of the current dial string when it fully matches the current digit map  50 . The call agent  24  determines from analyzing the dial string that the dialed number is a credit card call and that a credit card number has to be obtained before connecting the call. In step  120 , the acknowledgement from call agent  24  to the gateway  26  includes a new digit map  50  that directs the gateway  26  to accumulate a string of any 9 digits before sending another notification to the call agent  24 .  
         [0059]    The gateway  26  in steps  122 - 126  accumulates a string of 9 digits from telephone  28 . In step  128  the gateway  26  notifies the call agent  24  of the dial string that terminates the digit map  50 . The call agent  24  after analyzing the credit card number, allows the call to proceed. The call agent  24  in step  130  acknowledges the notification from gateway  26  with a new event/action table  36  that commands the gateway to ignore further digits from the telephone  28 . The telephone call then proceeds normally. This example shows the ability of the SSM system to change the digit map  50  at any time, depending on the state of the call.  
         [0060]    The SSM system operates with any type of packet based network, such as Internet Protocol (IP), Frame Relay, Asynclhronous Transfer Mode (ATM), etc. By controlling all signaling states with the call agent  24 , the gateway  26  can have relatively simple intelligence. However, the bit mapping performed by the gateway  26  substantially reduces the processing and communication bandwidth required to process gateway events. Thus the SSM system allows intelligence in the packet gateway  26  to be drastically simplified while allowing the entire packet telephony system  12  to scale much better than heavy-weight signaling techniques such as H.323.  
         [0061]    Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.