Abstract:
The invention relates to the field of voice over Internet protocol (VoIP) and more specifically to a system and method of terminating a VoIP call. In certain exemplary embodiments, the present solution provides in a voice over Internet protocol (VoIP) network, a method of terminating a call received from a public switched telephone network (PSTN) phone by a session initiation protocol (SIP) phone via a PSTN gateway, in the absence of a terminating signal from the PSTN gateway, the method comprising: (a) receiving at said SIP phone a sequence of codec frames from said PSTN gateway; (b) determining if a specified portion of each codec frame in said received sequence of codec frames is a specified value; (c) if said specified portion is said specified value, incrementing a counter by an incrementing amount; and (d) when said counter reaches a specified threshold, terminating said call In the absence of a BYE message, session termination of a VoIP call can still be facilitated using the method and apparatus of the present invention.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to the field of voice over Internet protocol (VoIP) and more specifically to a system and method of terminating a VoIP call. 
     2. Description of the Related Prior Art 
     The growth and popularity of the Internet has facilitated voice over Internet Protocol (VoIP), the technology used to transmit voice traffic over a data network using the Internet Protocol. Such a data network may be the Internet or a corporate Intranet. Using the hardware/software integral to VoIP, users are able to use the data network as the transmission medium for telephone calls. Voice traffic is sent in IP packets rather than by traditional public switched telephone network (PSTN) circuits. 
     To connect an originating VoIP device with a destination VoIP device, an auxiliary IP-based protocol developed by the IETF known as Session Initiation Protocol (SIP) may be used. SIP is an application-layer control protocol that can establish, modify, and terminate multimedia sessions or calls with one or more participants. These multimedia sessions include, for example Internet telephony, and similar applications.  FIG. 1  is a signal flow diagram depicting a typical call using SIP. At step  1 ) User A sends an INVITE to User B requesting session setup (i.e. for User B to participate in a media session). At step  2 ) User B responds with an informational message indicating that the SIP phone is ringing. At step  3 ) User B responds with an OK indicating that the User A request was successful. At step  4 ) User A sends an ACK request to confirm final response to the INVITE request. Following step  4 ) the media session occurs (e.g. the telephone conversation between User A and User B). At step  5 ) User B requests session termination by sending a BYE request. At step  6 ) User A responds with OK indicating the session has been successfully terminated. 
     In some cases, User B may be unable to request session termination because User B is unable to generate a BYE message. If this situation occurs, an alternate means of terminating the session is required. 
     SUMMARY OF THE INVENTION 
     The present solution serves to overcome the deficiencies of the prior art by providing an apparatus for and method of terminating a VoIP call in the absence of a terminating signal. 
     In certain exemplary embodiments, the present solution provides in a voice over Internet protocol (VoIP) network, a method of terminating a call received from a public switched telephone network (PSTN) phone by a session initiation protocol (SIP) phone via a PSTN gateway, in the absence of a terminating signal from the PSTN gateway, the method comprising: (a) receiving at the SIP phone a sequence of codec frames from the PSTN gateway; (b) determining if a specified portion of each codec frame in the received sequence of codec frames is a specified value; (c) if the specified portion is the specified value, incrementing a counter by an incrementing amount; and (d) when the counter reaches a specified threshold, terminating the call. Preferably, if the call is a voicemail message, the method further comprises after said step (d): trimming said voicemail message 
     Certain other exemplary embodiments comprise a computer readable medium having stored thereon, computer-executable instructions which, when executed by a processor in a SIP phone cause the SIP phone to implement the method described above. 
     Still certain other exemplary embodiments comprise a carrier wave embodying a computer data signal representing sequences of statements and instructions which, when executed by a processor in a session initiation protocol (SIP) phone, cause the SIP phone to implement the method described above. 
     Yet another exemplary embodiment comprises a session initiation protocol (SIP) phone for terminating a call received from a public switched telephone network (PSTN) phone by said SIP phone via a PSTN gateway, in the absence of a terminating signal from the PSTN gateway, the SIP phone comprising: (a) a processor; (b) a codec communicating with the processor for compressing and decompressing a sequence of codec frames received from said PSTN gateway according to a specified standard; (c) a frame analyzer and counter communicating with the codec; and (d) a memory communicating with the processor and having stored thereon, computer-executable instructions which, when executed by the processor cause said SIP phone to: (i) receive the sequence of codec frames into the frame analyzer; (ii) determine if a specified portion of each codec frame in said received sequence of codec frames is a specified value; (iii) if the specified portion is the specified value, incrementing the counter by an incrementing amount; and (iv) when the counter reaches a specified threshold, terminating the call. 
     Still another exemplary embodiment comprises, in a voice over Internet protocol (VoIP) network, a method of terminating a voicemail message received from a public switched telephone network (PSTN) phone by a session initiation protocol (SIP) phone via a PSTN gateway, the method comprising: (a) receiving a sequence of codec frames from the PSTN gateway; (b) determining if a specified portion of each codec frame in the received sequence of codec frames is a specified value; (c) if the specified portion is the specified value, incrementing a counter by an incrementing amount; and (d) when a BYE message is received from the PSTN gateway: (i) terminating the call; and (ii) deleting a time associated with the incrementing from the voicemail message. 
     The advantage of the present solution is now readily apparent. In the absence of a BYE message, session termination of a VoIP call can still be facilitated using the method and apparatus of the present solution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the solution will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  a signal flow diagram depicting a typical call using SIP; 
         FIG. 2  depicts a sample Voice over IP over Ethernet network; 
         FIG. 3  is a block diagram of a typical VoIP phone; 
         FIG. 4  is a table which details the elements of a G.729A codec frame; 
         FIG. 5  depicts a sample voice over IP over Ethernet packet containing the G.729A codec frame of  FIG. 4 ; 
         FIG. 6  is a flow chart broadly depicting the steps of the present solution; and 
         FIG. 7  is a flow chart depicting in greater detail the steps of the present solution. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  depicts a particular network topology which facilitates VoIP and which incorporates an Enterprise (i.e. corporate) local area network (LAN)  200  connecting to a traditional public switched telephone network (PSTN)  210  via gateway  220 . PSTN  210  accesses PSTN phone  240  via private branch exchange (PBX)  250 . LAN  200  may be an Ethernet or packet based transmission protocol LAN. Ethernet is the common name for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 industry specification and it is often characterized by its data transmission rate and type of transmission medium (e.g., 100 baseT or 100 Mbps over twisted pair). Distributed throughout LAN  200  are Internet telephones  230 , which contain embedded software that allows them to initiate and receive calls through the Internet using standard protocols such as H.323 or SIP (discussed above). Internet telephones  230  in LAN  200  may utilize SIP-based peer-to-peer (P2P) technology whereby all Internet telephones  230  communicate directly with each other and join together dynamically to participate in call routing, call handling and other network-related processes that would otherwise be handled by a central server. Using P2P technology means that there is no traditional SIP infrastructure i.e. a SIP proxy or registration server normally associated with a SIP capable network are not required. In the network of  FIG. 2 , PSTN gateway  220  provides SIP to Signaling System Seven (SS7) conversion for accessing PSTN phone  240 . 
     In a SIP-based telephony network, users place calls via entities known as user agent clients (UACs), an application which initiates a SIP request. Users receive calls via user agent servers (UASs), an application which contacts the end-user when a SIP request is received. Also, a UAS returns a response on behalf of the user since the user may select to accept, reject, or redirect the request. Accordingly, a SIP “user agent” is identified by a particular combination of a UAC and its associated UAS. The basic protocol functionality and its operation are summarized as follows. Calling parties and called parties are identified by SIP addresses which point to “objects” on a network. The objects are “users at hosts,” having appropriate SIP Universal Resource Locators (URLs) as addresses. The SIP URL typically takes a form similar to a mailto or telnet URL, i.e., user@host. The user part is generally provided as a user name or a telephone number. The host part typically comprises a domain name having a Domain Name System (DNS) Server Name, CNAME, or a numeric network address, among others. 
     In the network of  FIG. 2 , PSTN gateway  220  acts as a user agent for calls between a PSTN phone  240  and a selected Internet phone  230  contained in LAN  200 . Depending on the sophistication of PSTN gateway  220 , it may not be able to provide the full functionality of Internet phone  230 . More specifically, it may be unable to generate a BYE message to request termination of the media session. Where a PSTN caller has encountered an auto attendant or the like associated with Internet phone  230  and terminates the call, valuable network resources may be wasted before the auto attendant determines that the call has ended. This problem is heightened if a PSTN caller is attempting to leave a voice message on Internet phone  230 . If the session is not properly terminated, Internet phone  230  will continue to record a message with no content up to the default storage space allotted to a received voice message, thereby unnecessarily utilizing memory of Internet phone  230 , in addition to wasting network resources. 
       FIG. 3  is a block diagram of a typical SIP VoIP phone  300 , a specific example of the Internet phone  230  shown in  FIG. 2 .  FIG. 3  does not illustrate all functional elements of SIP phone  300  for simplicity. SIP phone  300  consists of a microphone  310  and speaker  315  through which a user speaks and listens to a caller respectively. Communicating with microphone  310  and speaker  315  is microprocessor or digital signal processor (DSP)  320  which includes various software modules which will be discussed below. Connected to DSP  320  is storage media  325  for storing, among other things, voice messages received from a caller and/or processor executable instructions for performing the method of the present invention. Also connected to DSP  320  is LAN port  330  for providing access to LAN  200  described in relation to  FIG. 2 . Finally, SIP phone  300  also includes display  335  which serves as a user interface, and battery/power adapter  340  for powering the aforementioned parts of SIP phone  300 . 
     DSP  320  includes a number of modules which define the protocols used to communicate in the VoIP network of  FIG. 2 . These include but are not limited to: (a) Internet Protocol (IP) module  345 ; (b) User Datagram Protocol (UDP) module  350 ; (c) Real-time Transport Protocol (RTP) module  355 , and SIP module  360 . DSP  320  further includes voice activity detection (VAD) module  365  which, when enabled on a voice port or a dial peer, causes SIP phone  300  to not transmit silence over the network, only audible speech. Additionally, DSP  320  comprises a codec  370 , frame analyzer  375  and counter  380  the operation of which will be described in relation to  FIGS. 4 and 7 . As will be appreciated by those in the art, such functions as jitter control, echo cancellation (in accordance with e.g. ITU standard G.165/168) and logical link control (in accordance with e.g. LAN standard IEEE 802.2) are also performed by DSP  320  and are meant to be included in the functionality of SIP phone  300 . 
     As will be appreciated by those in the art, in the telecommunications environment, a codec, such as codec  370 , is a piece of hardware/software that converts an analog voice signal to a digitally encoded version. More specifically, in VoIP networks, the codec samples the audio waveform at regular intervals and generates a value for each sample. These samples are typically taken at 8000 Hz. These individual values are accumulated for a fixed period to create a frame of data. Each service, program, phone, gateway, etc. typically supports several different codecs, and when talking to each other, negotiate which codec they will use. SIP phone  300  of the present solution supports the International Telecommunications Standard (ITU) G.729A standard, although the present solution is not meant to be limited in this regard. G.729A is a reduced complexity 8 kbit/s conjugate-structure algebraic-code-excited linear prediction (CS-ACELP) codec. With the G.729A standard the sampling occurs over 10 milliseconds (ms) and produces a very small frame of data (i.e. 10 bytes).  FIG. 4  is a table which details the 80 bit G.729A codec frame and its component parts. Integral to the present invention are sub-frames GA 1  and GA 2  which will be discussed in relation to  FIG. 7 . 
     As those in the art will also appreciate, the payload (or data frame) generated by the codec is wrapped in an IP header to form an IP packet which is the vehicle used to deliver the payload to its destination. These layers are: IP; UDP; and RTP. In the case of G.729A, to increase the efficiency of the VoIP network, two frames of data are placed in each IP packet (or 20 ms worth of sampling). In order to travel through the IP network, the IP packet is wrapped in another layer by the physical transmission medium e.g. Ethernet.  FIG. 5  depicts a sample voice over IP over Ethernet package  500  comprising an Ethernet preamble  510 , Ethernet header  520 , IP header  530 , payload  540  containing the two G.729A codec frames, Ethernet cyclical redundancy check (CRC)  550  and Ethernet Inter-Frame gap  560 . 
       FIG. 6  is a flow chart broadly depicting the steps for terminating a call in accordance with the present invention. At step  600 , payload  540  contained in Ethernet package  500  is received by SIP phone  300 . At step  610 , the payload is analyzed to determine the presence or absence of a specified value. Based on the analysis, a counter is adjusted at step  620 . The counter value is then reviewed at step  630  to determine if it has reached a specified threshold. If it has reached the specified threshold then the call is terminated at step  640 . It should be understood that the payloads are analyzed in sequence as they are received from PSTN gateway  220  and that the each received payload continues to be analyzed until the specified threshold is met. 
       FIG. 7  is a flow chart depicting in greater detail the steps of the present invention. The VAD described in relation to  FIG. 3  is turned off so that every received IP packet is a straight G.729A codec frame i.e. the present invention looks for silent codec frames to determine if call termination is appropriate. At step  700 , a G.729A codec frame is received at SIP phone  300 . More specifically, each of the two G.729A codec frames are extracted from each sequentially received Ethernet package  500  and individually analyzed. At step  710  the GA 1  and GA 2  sub-frames of a received G.729A codec frame (see  FIG. 4 ) are analyzed by frame analyzer  375  to determine if they contain a value of 1 or 5. These values are indicative of silence (i.e. no audio received from PSTN gateway  220 ). If GA 1  is 1 or 5 and GA 2  is 1 or 5 then, at step  720 , counter  380  is incremented by an amount (L) where L=10. If GA 1  is not 1 or 5 or GA 2  is not 1 or 5 then, at step  730 , counter  380  is decremented by an amount 3 L or 30. It should be understood that counter  380  will not be decremented below zero. Counter  380  will simply remain at zero until a “silent” frame is received. Next at step  740 , the value of counter  380  is reviewed. If the value is less than 5000 then the next G.729A codec frame is received at step  700  and the method outlined above is repeated. If the value is greater than or equal to 5000 then, at step  750 , the call is terminated. In the event that the call is a voice message then optionally, at step  760 , the portion of the voice message represented by the value of counter  380  is deleted. More specifically, the counter value can be interpreted as a count of silent frames that can be discarded. Since each G.729A codec frame is 10 milliseconds in length, a counter value of 5000 would be equivalent to:
 
5000÷10=500 frames
 
500 frames×10 milliseconds/frame=5000 milliseconds or 5 seconds
 
     The present invention has been described in relation to a SIP call where a termination signal is not generated by PSTN gateway  220 . It should be understood that the invention may also be used between SIP phones  300  or PSTN phone  240  and SIP phone  300  where a BYE signal is present but delayed. For example, if a caller has left a voice message, there may be several seconds between the time that the caller ends the call (e.g. places the handset “on hook”) and the BYE message is generated. The present invention can alternately be used to trim the delay from the voice message for the purpose of preserving memory in SIP phone  300 . In this case, whatever time is associated with the counter value is deleted from the voice message e.g. a counter value of 3000 would be equivalent to 3 seconds. 
     Although particular embodiments of the present invention have been described in detail, it should be appreciated that numerous variations, modifications and adaptations may be made without departing from the scope of the invention as defined in the appended claim set. For example, although the preferred embodiment has been described in relation to codec G.729A, it will be understood that invention could be practiced using other codecs such as G.711. In addition, although the preferred embodiment has been described in relation to a plurality of SIP phones using P2P technology, a network in which the SIP phones communicate via a proxy server could also be used to implement the present invention.