Abstract:
Malfunctions in a communications network may introduce an unacceptably low level of reliability for many users, thereby slowing further adoption of Internet Protocol (IP) telephony, for example. In an example embodiment of the present invention, a method and corresponding apparatus for supporting a call in a presence of a fault in a network is provided. The method includes supporting a primary protocol to service a call between near-end and far-end access nodes associated with two or more callers. Signaling information in the primary protocol supporting the call may be identified and used to establish a backup protocol between the near-end and far-end access nodes. The primary protocol may be monitored for a fault and, in an event a fault occurs, supporting the call using the backup protocol. As a result, IP telephony may be transported in a more reliable manner, thereby reducing the number of dropped and uncompleted calls.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The popularity and convenience of Internet communications, as well as an increasing availability of broadband Internet connections, has resulted in a transformation of existing applications and services. Migration of traditional Public Switched Telephone Networks (PSTN) to Internet Protocol (IP) telephony is one such example. Also known as Voice Over IP (VoIP), IP telephony can provide real-time delivery of voice, video, and other multimedia content (herein collectively “data”), across a network using IP. Voice information is converted into packets and transmitted between or among users over an IP network, effectively allowing telephone calls to be made over the Internet. 
         [0002]    IP telephony offers a number of advantages over PSTN networks, such as reduced costs and new features due to convergence of voice and data. However, in order for IP telephony to continue to displace PSTN service, it must have the same high reliability that users of traditional PSTN systems have come to expect. 
         [0003]    IP telephony is session-based rather than connection based as used in PSTN systems. A signaling protocol, such as Session Initiation Protocol (SIP), may be used to create, modify, and terminate sessions with one or more participants. Sessions may include IP telephone calls, multimedia conferences, and multimedia distribution. Participants can be people or automation components, such as a voicemail server. Because SIP is an application layer protocol, it is transparent to the underlying data link layer. 
         [0004]    IP telephony traffic is often carried over high-speed networks, such as a passive optical network (PON). PONs have emerged as a popular network architecture owing to their compelling economic advantages over other network architectures. In addition, a PON&#39;s point-to-multipoint architecture has a significant density advantage over point-to-point copper connections used with traditional PSTN systems. 
         [0005]    A PON system can malfunction in a way such that the system may not be able to complete a new voice call, or inadvertently terminate in session calls. Existing error detection techniques, such as those described in various PON protocol standards, may not detect this type of malfunction or, if detected (e.g., generic system failure message), may not be identified. These types of malfunctions may introduce an unacceptably low level of reliability for many users thereby slowing further adoption of IP telephony. 
       SUMMARY OF THE INVENTION 
       [0006]    A method and corresponding apparatus for supporting a call in a presence of a fault in a network according to an example embodiment of the invention may support a communications protocol among multiple communications protocols to service a call between near-end and far-end communications devices. The example method may identify parameters of the call in a primary protocol and instantiate a backup protocol between a near-end access node and a far-end access node associated with the communications devices based on at least one of the parameters. The example method may further monitor the primary protocol for a fault and, in an event of a fault, support the call using the backup protocol. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0008]      FIG. 1  is a network diagram of a Passive Optical Network (PON) employing an example embodiment of the invention; 
           [0009]      FIG. 2  is a block diagram of an example portion of a network employing an example embodiment of the invention; 
           [0010]      FIG. 3  is a more detailed block diagram of a portion of a network employing an example embodiment of the invention; 
           [0011]      FIG. 4  is a flow diagram performed in accordance with an example embodiment of the invention; 
           [0012]      FIGS. 5-7  are flow diagrams illustrating procedures performed in accordance with an example embodiment of the invention; and 
           [0013]      FIG. 8  is a block diagram of the internal structure of a computer in which example embodiments of the invention may be implemented. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    A description of example embodiments of the invention follows. 
         [0015]    Network service providers are increasingly deploying fiber optic transmission media deeper and deeper into the network infrastructure. As a result, fiber-optic media is beginning to replace copper twisted pair media in many applications. For example, communications signals formerly transmitted across copper wires are now transmitted across a fiber-optic media. Consequently, new applications such as Voice over Internet Protocol (VoIP) are becoming increasingly commonplace. Although VoIP is attractive from a feature and price point of view, continued acceptance requires, in part, the same high reliability which users have become accustomed to with respect to traditional switching networks. 
         [0016]      FIG. 1  is a network diagram  100  depicting aspects of an example embodiment of the invention illustrating a signaling protocol for use with VoIP communications for creating, modifying, and terminating sessions between one or more participants. Example protocols include session initiation protocol (SIP), described in IETF RFC 3261, “SIP: Session Initiation Protocol,” June 2002, http://www.ietf.org/rfc/rfc3261.txt, International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Recommendation H.323, “Packet-based multimedia communication systems,” November 2000, and ITU-T Recommendation H.225, “Call signaling protocols and media stream packetization for packet-based multimedia communication systems,” November 2000, all of which are incorporated herein by reference in their entirety. 
         [0017]    A typical Passive Optical Network (PON)  100  includes an optical line terminal (OLT)  115 , a passive optical splitter/combiner (OSC)  125 , and at least one optical network node, such as an optical network terminal (ONT)  135   a - n  or an optical network unit (ONU)  160   a - n . The PON may also include an element management system (EMS) in communication with, for example, the OLT  115 . Note that as used herein, an ONU and an ONT may be used interchangeably unless indicated otherwise. Further note that “Data” as used herein refers to voice, video, analog, or digital data. The ONT  135   a - n  may be in optical communication with multiple end users  140   a - n . Data communications  110  may be transmitted to the OLT  115  from a wide area network (WAN)  105 . Content server(s)  107  or other user networks  106  provide communications signals to and from the WAN  105 . 
         [0018]    A SIP telephone network may be implemented using, in part, a PON  100 . In the PON  100 , communication of downstream data  120  and upstream data  150  transmitted between the OLT  115  and the ONTs  135   a - n  may be performed using standard communications protocols known in the art. For example, multicast may be used to transmit the downstream data  120  from the OLT  115  to the ONTs  135   a - n , and time division multiple access (TDMA) may be used to transmit the upstream data  150  from an individual ONT  135   a - n  back to the OLT  115 . Note that the downstream data  120  is power divided by the OSC  125  into downstream data  130  matching the downstream data  120  “above” the OSC  125  but with power reduced proportionally to the number of paths onto which the OSC  125  divides the downstream data  120 . It should be understood that the term downstream data (e.g., downstream data  120 ,  130 ) refers to optical traffic signals that travel from the OLT  115  to the ONT(s)  135   a  and end user(s)  140   a - n , and “upstream data” (e.g., upstream data  145   a ,  150 ) refers to optical traffic signals that typically travel from the end users  140   a - n  and ONTs  135   a - n  to the OLT  115  via optical communications paths, such as optical fiber links  131 ,  127 , and in some networks, link  135 . 
         [0019]    The PON  100  may be deployed for fiber-to-the-premise (FTTP), fiber-to-the-curb (FTTC), fiber-to-the-node (FTTN), and other fiber-to-the-X (FTTX) applications. The optical fiber  127  in the PON may operate at bandwidths such as 155 megabits per second (Mbps), 622 Mbps, 1.25 gigabits per second (Gbps), and 2.5 Gbps or other bandwidth implementations. The PON may incorporate asynchronous transfer mode (ATM) communications, broadband services such as Ethernet access and video distribution, Ethernet point-to-multipoint topologies, and native communications of data and time division multiplexing (TDM) formats or other communications suitable for a PON. End users  140   a - n  may receive and provide communications from and to, respectively, the PON and may be connected to Internet Protocol telephones, video devices, Ethernet units, digital subscriber lines, computer terminals, wireless access, as well as any other conventional customer premise equipment. 
         [0020]    The OLT  115  generates, or passes through, downstream communications  120  to an OSC  125 . After flowing through the OSC  125 , the downstream communications  120  are transmitted as power reduced downstream communications  130  to the ONTs  135   a - n , where each ONT  135   a - n  may filter and replicate data  130  intended for a particular end user  140   a - c . The downstream communications  120  may also be transmitted to, for example, another OSC  155  where the downstream communications  120  are again split and transmitted to additional ONT(s)  160   a - n  and end user(s)  140   n.    
         [0021]    Data communications  137  may further be transmitted to and from, for example, end user(s)  140   a - n  in the form of voice, video, data, and/or telemetry over copper, fiber, or other suitable connection  138  as known to those skilled in the art. The ONTs  135   a - n  may transmit upstream communication signals  145   a - n  back to the OSC  125  via fiber connections  133  using transmission protocols known in the art, such as Internet Protocol (IP) enabling applications such as VoIP. The OSC  125 , in turn, combines the ONT&#39;s  135   a - n  upstream signals  145   a - n  and transmits a combined signal  150  back to the OLT  115  which may, for example, employ a time division multiplex (TDM) protocol to determine from which ONTs  135   a - n  portions of the combined signal  150  are received. The OLT  115  may further transmit the communication signals  112  to a WAN  105 , back downstream to other ONTs connected to the OLT, or both. 
         [0022]    Communications between the OLT  115  and the ONTs  135   a - n  occur using a downstream wavelength, for example 1490 nanometer (nm), and an upstream wavelength, for example of 1310 nm. The downstream communications  120  from the OLT  115  to the ONTs  135   a - n  may be provided at 2.488 Gbps, which is shared across all ONTs. The upstream communications  145   a - n  from the ONTs  135   a - n  to the OLT  115  may be provided at 1.244 Gbps, which is shared amongst all ONTs  135   a - n  connected to the OSC  125 . Other communication data rates known in the art may also be employed. 
         [0023]    Communications between end users  140   a - n  may travel across multiple PONs and multiple networks  105 ,  106 . For example, another PON  100 , such as that represented by OLT  117 , OSC  119 , and ONT  121 , may be connected to the WAN  105  or to other user networks  106  and may operate in a manner similar to that described above with respect to OLT  115 , OSC  125 , ONTs  135   a - n , and end users  140   a - n . Thus, communications may travel between end users within the same PON or may traverse multiple PONs and/or networks. 
         [0024]    A SIP network may be enhanced by providing error detection and backup connection techniques in an event errors are detected. A phone call between two or more users, for example, End Users  140   a  and  140   n , may be supported using downstream communications  130 ,  132  and upstream communications  145   n ,  147  via a primary protocol. The ONT(s)  121 ,  160   n  associated with the End Users  140   a ,  140   n  may be configured to identify call parameters in the primary protocol and, based on the parameter, instantiate a backup protocol between the End Users  140   a ,  140   n . The primary protocol may be monitored for faults, and in the event a fault occurs, the called can be switched to the backup protocol, thereby preventing the call from being dropped. 
         [0025]    Instantiating a backup protocol is defined herein as creating, modifying, and terminating sessions with one or more participants such as is described in Request For Comments (RFC) 3261, “SIP: Session Initiation Protocol,” June 2002, incorporated herein by reference in its entirety. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. 
         [0026]    An example method and corresponding apparatus for supporting a communications protocol among multiple communications protocols to service a call between near-end and far-end communications devices may include identifying parameters (e.g., source or destination identification (ID)) of the call in a primary protocol. The example method may include instantiating a backup protocol between a near-end access node and a far-end access node associated with the communications devices based on at least one of the parameters. The example method may further include monitoring the primary protocol for a fault and, in an event of a fault, support the call using the backup protocol. Example communications protocols may include ATM adaptation layer 1 (AAL1), AAL2, TDM, ATM, Internet protocol (IP), wireless, or similar such protocols known in the art. Example network nodes may include an ONT, ONU, or OLT. 
         [0027]    An alternative embodiment may further include identifying the far-end source ID of an incoming call or destination ID of an outgoing call and providing the ID to disable a ‘call busy state’ to enable instantiation of the backup protocol to support caller ID blocking. A unique identifier may be used to identify the source ID or destination ID. Identifying parameters of the call may also include parsing Session Initiation Protocol (SIP) packets to identify the parameters of a call, including a source ID or destination ID using at least one of the following identifiers: a media access control (MAC) address, IP address, or telephone number 
         [0028]    In another example embodiment, the technique may further include configuring the primary and backup protocols prior to establishing the call via the primary protocol. The backup protocol may be instantiated after establishing the call via the primary protocol. In this way, the backup protocol may be activated prior to a call being inadvertently dropped. The example embodiment may further include reestablishing the call via the backup protocol after a drop of the call. The active protocol may use a SIP where a “ping” rate on the SIP is increased to determine its availability in order to reestablish the call via the primary protocol. The primary protocol may be monitored while the call is being serviced by the backup protocol, and if the primary protocol becomes available again during the call, the call may be returned to the primary protocol. 
         [0029]    In yet another example embodiment, identifying parameters may include identifying a call priority identifier among the identified parameters, such that a 911 call, enhanced 911 call, or emergency service call may be recognized. The technique may further include determining service usage fees based on instantiating the backup protocol or supporting the call using a backup protocol. 
         [0030]    In still another example embodiment, a computer program product may be used to support a call in a network where the computer program product includes a computer readable medium having computer readable instructions stored thereon, which, when loaded and executed by a processor, cause a processor to support any combination of the foregoing procedures. 
         [0031]    It should be noted that a call may refer to a traditional telephone voice call, but should not be viewed as limited to such, and may also include transmission of any variety of multimedia content such as video, audio, multimedia conferencing, gaming, distant education, and the like. 
         [0032]      FIG. 2  is a block diagram of a SIP telephone network  200  configured to support communications such as a telephone call  260   a - b  between at least one near-end communications device  220   a  and at least one far-end communications device  220   b  according to an example embodiment of the invention. The network  200  may include one or more PONs connected to a WAN  210 . The WAN  210  may support multiple communications protocols such as AAL1, AAL2, TDM, ATM, IP, various wireless protocols, or similar such protocols known in the art. The WAN  210  may also include other networks (not shown) such that the call  260   a - b  travels across multiple networks. A call  260   a - b  between two communications devices  220   a - b  takes place in the form of downstream communications and upstream communications as was described with reference to  FIG. 1 . 
         [0033]    The network nodes  205   a - b  include communications modules  230   a - b , redundancy set-up modules  240   a - b , and fault recovery modules  250   a - b . The communications modules  230   a - b  are configured to support a communications protocol from among multiple communications protocols to service the call between the near-end and far-end communications devices  220   a - b . A primary protocol  265  may be initially used to service the call  260   a - b . The redundancy set-up module  240   a - b  identifies parameters  275  of the call  260   a - b  in the primary protocol  265 , such as a source ID or destination ID. Based on the identified parameters  275 , the redundancy set-up module  240   a - b  also instantiates a backup protocol  270  between the near-end device  220   a  and the far-end device  220   b , thus, effectively providing a backup connection for the call  260   a - b . The fault recovery module  250   a - b  may be used to monitor the primary protocol  265  for a fault. In the event a fault occurs with the primary protocol  265 , the backup protocol  270  may be used to continue supporting the call  260   a - b , thereby transparently preventing call termination and providing an additional level of reliability. It should be understood that the redundancy set-up modules  240   a - b  and fault recovery modules  250   a - b  can be configured to operate independently or in a cooperative manner, such as through an exchange of state information or signaling procedure. 
         [0034]    In an alternative embodiment, the backup protocol  270  may be initially used to service the call  260   a - b . For example, if the primary protocol is  265  designated to be the default communications protocol and that protocol is unavailable, the backup protocol may be initially used to make any subsequent calls  260   a - b . The primary protocol  265  may be monitored, and should it become available, the call  260   a - b  may be switched back to the primary protocol  265 . 
         [0035]    The one or more PON(s)  200  may include an OLT  245   a - b , OSC  235   a - b , and network node  205   a - b , such as an ONT. In a case where the near-end communications device  220   a  and the far-end communications device  220   b  are within the same PON (e.g., ultimately connected to the same OLT), the call may take place within the same PON, and traverse the same physical link within that PON. However, if the communications devices  220   a - b  are not within the same PON (i.e., not connected to the same OLT), at least two PONs may be used conduct the call  260   a - b , as is the case depicted in  FIG. 2 . In this case, the call may or may not traverse the same physical link. 
         [0036]      FIG. 3  is a more detailed block diagram of the SIP telephone network  300  depicted in  FIG. 2 . The network includes near-end and far-end communication devices  320   a - b , network nodes  305   a - b , and a network such as a WAN  310 . Note that a SIP telephone network  300  may be configured, in part, as one or more PONs as was shown in  FIG. 2  and as discussed above; however, in  FIG. 3 , the OSC and OLT have been omitted for the sake of brevity. 
         [0037]    In an example embodiment, the network node  305   a - b  may be, for example, an ONT and may include a redundancy set-up module  340   a - b , communications module  330   a - b , fault recovery module  350   a - b , call registration module  332   a - b , fee determination unit  334   a - b , and wireless interface  336   a - b . The redundancy set-up module  340   a - b  may further include a configuration module  346   a - b , parsing module  344   a - b , and priority identification unit  342   a - b . The fault recovery module  350   a - b  may further include an activation module  352   a - b.    
         [0038]    In this embodiment, a call  360   a - b  may, for example, be initiated by a user at the near-end communications device  320   a  to a user at the far-end communications device  320   b . The call registration module  332   a  associated with the near-end communications device  320   a  (i.e., caller) may identify the destination ID of the far-end communications device  320   b  (i.e., callee). Upon receiving the call  360   b  at the callee&#39;s network node  305   b , the call registration module  332   b  associated with the far-end communications device  320   b  may identify a call parameter, such as a source ID of the calling device. 
         [0039]    Traditional telephone calls typically do not allow more than one simultaneous connection per call. In the event a second call is received while another call is in progress, a call busy state is indicated. To enable both primary and backup connections simultaneously, in one example embodiment, the call registration module  332   a - b  provides the source and destination ID to the redundancy set-up module  340   a - b  in order to disable a “call busy state” in order to enable instantiation of the backup protocol  370  to support caller ID blocking. 
         [0040]    The parsing module  344   a - b  may be used to parse packets to identify various call parameters  375 . For example, the parsing modules  344   a - b  may be used to parse SIP packets to identify a call parameter  375 , such as a MAC address, IP address, telephone number, or other such identifier, and may associate a unique identifier with a corresponding source ID or destination ID. 
         [0041]    Continuing to refer to  FIG. 3 , the configuration module  346   a - b  may be used to configure the primary protocol  365  and backup protocol  370  for the call  360   a - b . The primary protocol  365  and backup protocol  370  may be configured prior to establishing the call via, for example, the primary protocol. The primary protocol  365  and backup protocol  370  may use various communications protocols, such as, for example, AAL1, AAL2, SIP, TDM, ATM, TDM, ATM, IP, or similar such protocols. In addition, or alternatively, the wireless interfaces  336   a - b  may be used to enable use of various wireless protocols, such as TDMA, code division multiple access (CDMA), global system for mobile communications (GSM), or similar wireless protocols known in the art. 
         [0042]    After the call  360   a - b  has been established using the primary protocol  365 , the configuration module  346   a - b  may instantiate the backup protocol  370 , thus, providing a backup connection for the call  360   a - b . After the backup protocol  370  has been instantiated, the activation module  352   a - b  may activate the backup protocol  370  at any time in order to provide a seamless transition between protocols. That is, because the call  360   a - b  effectively maintains two simultaneous connections (i.e., primary and backup protocol), in the event the primary connection is dropped, the activation module  352   a - b  may reestablish the call via the backup protocol  370 . 
         [0043]    While the call  360   a - b  is being maintained by the backup protocol  370 , the activation module  352   a - b  may also be configured to monitor the primary protocol  365  and, in the event the primary protocol  365  becomes available during the call  360   a - b , the call  360   a - b  may be switched back or returned to the primary protocol  365 . For example, if the primary protocol  365  uses SIP, the activation module  352   a - b  may increase a ping rate on the SIP to determine its availability in order to reestablish the call via the primary protocol  365 . 
         [0044]    The ability to monitor the primary protocol  365  while the backup protocol  370  is being used to support the call  360   a - b  may be advantageous in the situation where the primary protocol  365  and backup protocol  370  are associated with different usage fees. For example, calls  360   a - b  serviced using a SIP protocol may be less expensive than calls using, for example, an ATM protocol. Thus, a more expensive backup protocol may be used only when necessary. In addition, the network node  305   a - b  may also include a fee determination unit  334   a - b  that may be used to determine service usage fees based on instantiation or support of the call  360   a - b  using the backup protocol  370 . 
         [0045]    The priority identification unit  342   a - b  may also identify a call priority identifier among the call parameters  375 . For example, the call priority identifier may represent or be associated with a 9-1-1 call, enhanced 9-1-1 calls, or similar such emergency call. Enhanced 9-1-1 refers to the an emergency calling system that automatically associates a physical address with a calling party&#39;s telephone number. The priority identification unit  342   a - b  may also be configured to undertake further action based on the identified call priority. 
         [0046]      FIG. 4  illustrates, in the form of a flow diagram, an example embodiment of the present invention. It should, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the example embodiment. For example, some of the flow diagrams presented herein, including  FIG. 4 , may be performed in an order other than that which is described. It should be appreciated that not all of the flow diagrams are required to be performed, that additional flow diagram(s) may be added, and that some may be substituted with other flow diagram(s). 
         [0047]    The procedure  400  depicted in  FIG. 4  begins ( 405 ) and supports a communications protocol from among multiple protocols to service a call between at least two communications devices ( 410 ). The procedure  400  may identify call parameters in a primary protocol ( 415 ). The procedure  400  may then instantiate a backup protocol between at least two access nodes associated with the at least two communications devices based on the identified call parameters ( 420 ). After the primary protocol and backup protocol have been established, the procedure may monitor the primary protocol for a fault ( 425 ). In the event of a fault ( 430 ), the procedure may support the call using the backup protocol ( 435 ). The procedure may end ( 440 ) when, for example, the calling parties terminate the call. 
         [0048]    Some or all of the steps in the procedure  400  may be implemented in hardware, firmware, or software. If implemented in software, the software may be (i) stored locally with the OLT, ONT, End User communications device, or some other remote location such as a server or the EMS, or (ii) stored remotely and downloaded to the OLT, ONU, End User communications device, or the EMS during, for example, start  405 . The software may also be updated locally or remotely. To begin operations in a software implementation, the OLT, ONU, End User communications device, or EMS loads and executes the software in any manner known in the art. 
         [0049]      FIG. 5  is a flow diagram of a procedure  500  illustrating an alternative example embodiment of the procedure described with reference to  FIG. 4 . In this embodiment, support of a communications protocol to service a call between communications devices ( 505 ) may include determining if the primary protocol uses a SIP protocol ( 510 ). If not, the procedure returns ( 520 ) to  FIG. 4 . If a SIP protocol is in use, the procedure may parse packets to identify particular call parameters ( 515 ), such as a MAC address, IP address, telephone number, or the like. These parameters may be used to associate a unique identifier with a particular communications device or access node. The procedure then returns ( 520 ) to  FIG. 4 . In an alternative embodiment, the same procedure may be used except that a backup protocol is examined for use of a SIP protocol. 
         [0050]      FIG. 6  is a flow diagram of a procedure  600  according to another alternative example embodiment. In this embodiment, identifying call parameters in a primary protocol ( 605 ) may further include determining if, for example, a call&#39;s caller ID is blocked ( 610 ). If not, the procedure returns ( 625 ). If caller ID is blocked ( 610 ), the procedure may identify a source ID of an incoming call or destination ID of an outgoing call ( 615 ) and disable a call busy state ( 620 ). The procedure then returns ( 625 ) to  FIG. 4 . 
         [0051]      FIG. 7  is a flow diagram of a procedure  700  illustrating yet another alternative embodiment. The procedure  700  may instantiate the backup protocol after establishing the call via the primary protocol ( 710 ). After the backup protocol and been instantiated, it may be activated prior to a call being dropped ( 715 ), thereby providing a redundant connection. The procedure  700  may determine if an established call has been dropped ( 720 ) and, if not, the procedure  700  returns ( 745 ) to  FIG. 4 . If the call is dropped from the primary protocol, the call may be reestablished via the backup protocol ( 725 ). 
         [0052]    The procedure  700  may determine if the active protocol uses SIP ( 730 ) and, if so, the procedure  700  may increase the SIP&#39;s ping rate to determine its availability for the purpose of, for example, reestablishing the call via the primary protocol ( 735 ). For example, in the situation where SIP is the primary protocol and TDM/ATM is the backup protocol, and due to an error of some sort, the call is switched to the backup protocol. The procedure may increase the SIP&#39;s ping rate (e.g., increasing the rate to 1 second) such that when the primary protocol becomes available, the call may be switched back to use the SIP. 
         [0053]    The procedure  700  may continue to monitor the primary protocol during support of the call by the backup protocol and, in the event the primary protocol becomes available to call, support of the call may be returned to the primary protocol ( 740 ). The process  700  may then return ( 745 ). 
         [0054]    It should be readily appreciated by those of ordinary skill in the art that the aforementioned procedures are merely exemplary and that the example embodiments are in no way limited to the number of actions or the ordering of steps described above. 
         [0055]    In another alternative example embodiment where the primary protocol uses a SIP, the SIP network may be configured to implement “keep alive” messages at a non-standard rate. For example, rather than a standard rate of say 10 minutes, the keep alive message rate may be increased to 1 second such that the messages cover the entire span of an active call from one device associated with an ONT to another device associated with another ONT. This much finer level of granularity allows the ONT to determine error conditions quickly. The ONT may further report an alarm and/or initiate corrective action. 
         [0056]    In this embodiment, if both ONT&#39;s have access to the same backup protocol (e.g., TDM/ATM), a backup protocol may be instantiated and activated in less time that it takes for a call to be dropped. Consequently, the backup protocol is provided only when the primary protocol becomes problematic. That is, the primary and backup protocols do not need to be simultaneously available. For example, consider a SIP network where each ONT uses SIP as the primary protocol and each ONT has access to the same backup protocol (e.g., TDM). In the event a problem occurs with the primary protocol (i.e., SIP), the primary protocol can be abandoned, and the originating ONT can automatically call the other ONT(s) using the backup protocol via a TDM/ATM network using an AAL1 or AAL2 connection. 
         [0057]    In the event there are no problems with the primary protocol, the backup protocol need not be enabled. Furthermore, if a call is not in progress and the SIP network is not available, the ONT may initiate new calls using the backup TDM network until the SIP network is restored. 
         [0058]    Although the apparatus and method described herein discuss transporting downstream and upstream communications signals using a fiber optic media in a PON, the example embodiments are not meant to be limited to such a media or network architecture. It should be understood that the example embodiments can be implemented, in part, or in its entirely, using alternative physical media such a coaxial (or similar) wire such as a cable media commonly used to deliver television, VoIP, and/or Internet services to an end user&#39;s premise. 
         [0059]    Further, the Sessions Initiation Protocol as well as the instructions to transmit and receive SIP messages may reside on a computer-readable medium. The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device. Volatile media includes dynamic memory, such as main memory. Transmission media includes fiber optics, copper wires and coaxial cables, including the wires that comprise bus. Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. 
         [0060]    Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, or any other optical medium, RAM, PROM, EPROM, FLASH, or any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
         [0061]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.