Abstract:
The delivery of emergency services to users of a set of communication devices in a packet-switched network is enabled by a method that determines a routing key corresponding to a particular directory number that is associated with a particular communication device, and stores the directory number and the corresponding routing key in a database accessible to a packet switch. The steps of determining and storing are executed during a provisioning phase, in the absence of an emergency call placed by the communication device. The routing key is indicative of routing instructions to be followed by the packet switch upon receipt of a future emergency call placed by the communication device. When the call is received by the packet switch, it will consult the database to obtain the routing key associated with the directory number and send the call as well as the routing key over a path dedicated to emergency calls.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims the benefit under 35 U.S.C. 119(e) of Provisional U.S. Patent Application Ser. No. 60/617,656, to William Crago et al., filed on Oct. 13, 2004, hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of telecommunications in general and, more specifically, to the delivery of emergency services to users of communication devices in a network that provides voice-over-packet services.  
       BACKGROUND  
       [0003]     The current Emergency Services Enhanced 9-1-1 (E911) infrastructure in North America is based on a distribution of Primary Public Safety Answering Points (P-PSAPS, hereinafter referred to solely as PSAPs) accessible from the Public Switched Telephone Network (PSTN) via a special group of telephone lines dedicated solely to emergency use. The same applies to other emergency codes used in other parts of the world, e.g., E112 in continental Europe, E999 in England, etc.  
         [0004]     In the current infrastructure, each individual telephone number is assigned a corresponding PSAP that is nearest to the physical location of the user of that telephone number. The physical location of the user is assessed on the basis of the area code and local exchange of the telephone number. When a user places an emergency call, the calling partys telephone number is transmitted with the call, and on the basis of the originating telephone number, the emergency call will be routed to the designated PSAP along one of the dedicated emergency lines. Upon receipt at the PSAP, the call is answered by a trained responder. By virtue of an automatic location identification (ALI) database which maps each telephone number to an address, the responder is able to obtain the physical location of the calling party and dispatch a police officer, firefighter or ambulance as necessary.  
         [0005]     With the advent of the Internet, society has witnessed the expansion of a global packet-switched network into an ever increasing number of homes and businesses. This has put ever increasing numbers of users into contact with one another, usually at little cost for unlimited use. Meanwhile, advances have been made in delivering voice communication over packet networks, driven primarily by the cost advantage of placing long-distance calls over the Internet as opposed to the leased lines of the worlds telcos. Technology dealing with the delivery of real-time voice calls over a packet-switched network is generally known as voice-over-packet or voice-over-Internet-Protocol (voice-over-IP), and often simply referred to as “VoIP”.  
         [0006]     From a purely technological standpoint, the successful deployment of VoIP has several challenges, typically related to latency and congestion. Still, despite these and other technical drawbacks, many consumers have opted to subscribe to VoIP services, motivated by significant cost savings in the area of long-distance calling. This has led to a trend, whereby some residential and business consumers have actually chosen to abandon their “basic” PSTN connection in favour of a VoIP connection, not only to satisfy their long distance requirements but also to conduct local, day-to-day telephony. One area where this shift to all-VoIP paradigm can be problematic for consumers (and VoIP service providers) is in the delivery of emergency services.  
         [0007]     Specifically, the call delivery technology is fundamentally different for VoIP, and as a result, the dialing of 9-1-1 during a VoIP connection does not work today in the same way as for a basic PSTN connection. For example, VoIP users can select their own telephone numbers, which may comprise an “area code” and a “local exchange” that are unrelated to the physical location from which calls will be placed. If the VoIP user dials 9-1-1, the call may be directed to a PSAP located in a different part of the country, significantly reducing the value of the emergency services being provided.  
         [0008]     Recognizing these defects, some VoIP service providers have enhanced their offerings in the area of emergency services. For example, there are consumer VoIP solutions which allow the transfer of a 9-1-1 call from the VoIP network to the nearest PSAP in accordance with level i1 of the service levels proposed by the National Emergency Number Association (NENA) and the Voice over the Net (VON) coalition.  
         [0009]     However, in accordance with this and other i1-compliant solutions, emergency calls are not delivered via the dedicated emergency lines and trunks described above, but instead arrive at the nearest PSAP via the PSAP&#39;s ordinary, i.e., administrative, lines. Since many PSAPs are unprepared to handle calls over ordinary telephone lines, this creates a variety of problems, ranging from the low priority typically given to administrative calls, to the possibility of having an emergency call answered by improperly trained staff such as a receptionist or, worse still, by an auto-attendant during off-normal hours.  
         [0010]     Against this background, it is clear that further improvements are needed in the delivery of emergency services to persons dialing 9-1-1 from a VoIP-enabled telephone or device.  
       SUMMARY OF THE INVENTION  
       [0011]     A first broad aspect of the present invention seeks to provide a method of enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The method comprises determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the corresponding routing key in a database accessible to a packet switch in the packet-switched network. The steps of determining and storing are executed in the absence of an emergency call placed by the particular communication device.  
         [0012]     A second broad aspect of the present invention seeks to provide a method of enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The method comprises determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the corresponding routing key in a database local to a packet switch in the packet-switched network.  
         [0013]     A third broad aspect of the present invention seeks to provide a method of enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The method comprises determining the identity of an emergency zone corresponding to a particular directory number that is associated with a particular communication device, and providing the particular directory number and the identity of the corresponding emergency zone to a packet switch in the packet-switched network. At the packet switch, and on the basis of the identity of the corresponding emergency zone, a routing key corresponding to the particular directory number is determined.  
         [0014]     A fourth broad aspect of the present invention seeks to provide a method of enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The method comprises determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the routing key corresponding to the particular directory number in a database accessible to a packet switch in the packet-switched network. The routing key corresponding to the particular directory number is indicative of routing instructions to be followed by the packet switch upon receipt of a future emergency call placed by the particular communication device.  
         [0015]     Other broad aspects of the present invention seek to provide computer readable storage media containing a program element for execution by a computing device to implement one or more of the above methods.  
         [0016]     According to yet another broad aspect, the present invention seeks to provide a registration entity for enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The registration entity comprises a control entity and an I/O for communicating with a packet switch in the packet-switched network. The control entity is operative to execute the steps of determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the routing key corresponding to the particular directory number in a database accessible to the packet switch. The steps of determining and storing are executed for the particular communication device in the absence of an emergency call placed by the particular communication device.  
         [0017]     According to still another broad aspect, the present invention seeks to provide a network entity for enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The network entity comprises a control entity and an I/O in communication with the control entity. The control entity is operative to execute the steps of determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the corresponding routing key in a database local to a packet switch in the packet-switched network.  
         [0018]     According to another broad aspect, the present invention seeks to provide a packet switch for enabling the delivery of emergency services to users of a set of communication devices in a packet-switched network, each of the communication devices being associated with a respective directory number. The packet switch comprises a control entity and an I/O in communication with the control entity. The control entity is operative to execute the steps of determining a routing key corresponding to a particular directory number that is associated with a particular communication device, and storing the particular directory number and the routing key corresponding to the particular directory number in a database. The routing key corresponding to the particular directory number is indicative of routing instructions to be followed by the packet switch upon receipt of a future emergency call placed by the particular communication device.  
         [0019]     According to yet another broad aspect, the present invention seeks to provide a computer-readable storage medium for storing data for access by an application program being executed at a packet switch in a packet-switched network. The memory comprises a plurality of records, each record identifying a directory number associated with a respective communication device in the packet-switched network, and a routing key corresponding to the directory number. The routing key corresponding to a particular directory number is indicative of routing instructions to be followed by the packet switch upon receipt of a future emergency call placed by the communication device associated with the particular directory number.  
         [0020]     These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     In the accompanying drawings:  
         [0022]      FIGS. 1A  to  1 C show in schematic form, various embodiments of an architecture of network elements suitable for the delivery of emergency services;  
         [0023]      FIGS. 2A  to  2 J show interaction of the various network elements in the architecture of  FIG. 1A  during a provisioning phase;  
         [0024]      FIGS. 3A and 3B  conceptually illustrate the contents of a table maintained by a network element forming part of the architecture of  FIG. 1 , in accordance with two specific embodiments of the present invention;  
         [0025]      FIGS. 4A  to  4 F show interaction of the various network elements in the architecture of  FIG. 1A  during a call handling phase. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0026]     With reference to  FIG. 1A , there is shown a network element architecture suitable for the delivery of emergency services in accordance with an embodiment of the present invention. A packet-switched network  100 , which may or may not be the public Internet, comprises a backbone to which users have access via customer premises equipment  102  such as a modem  104  in combination with a residential gateway  106 . In some embodiments, the modem  104  and the residential gateway  106  may be combined into a single unit at the customer premises.  
         [0027]     A VoIP customer desirous of obtaining telephony services via the packet-switched network  100  may be provided with a special-purpose VoIP telephone or device that connects directly to the residential gateway  106 . In another embodiment, and as illustrated in  FIG. 1A , the VoIP customer utilizes a conventional analog telephone  110  which connects to the residential gateway  106  using an analog terminal adapter (ATA)  112 . The ATA  112  permits the VoIP customer to re-use conventional telephony hardware in a VoIP environment, thus obviating the need to purchase and maintain a second telephone strictly for IP telephony purposes. Generally speaking, however, it is immaterial to the present invention whether the VoIP customer uses a special-purpose VoIP telephone or a conventional analog telephone  110  coupled to an ATA  112 .  
         [0028]     In a typical residential application, the packet-switched network  100  is accessed by the modem  104  in the customer premises equipment  102  upon establishing a connection to a modem  108  belonging to a network service provider, commonly an Internet service provider (ISP). The connection is made via an access infrastructure, examples of which include but are not limited to copper telephone lines (for an ADSL modem)  104  and coax cable (for a cable modem  104 ). It should be further understood that the present invention applies to the delivery of emergency services not only in a residential context but in other contexts such as business and corporate applications, where access to the packet-switched network  100  may be provided by a server that in some cases is directly connected to the packet-switched network  100 .  
         [0029]     A VoIP service provider maintains a registration entity  190 , which VoIP customers may access via the packet network  100 . In a non-limiting example of implementation, the registration entity  190  may be embodied as a server having a control entity and an I/O. Initially, potential VoIP customers contact the VoIP service provider via the registration entity  190 . A given VoIP customer registers with the registration entity  190  and obtains a VoIP telephone number N (hereinafter referred to as a “directory number”). The VoIP customer, identified by the directory number N, can then begin to place calls into (and receive calls from) the packet-switched network  100 . The registration entity  190  performs various other functions which will be described in further detail later on.  
         [0030]     At the edge of the packet-switched network  100  there is provided a network element  114 , which may be referred to as a packet switch or softswitch, and which comprises suitable circuitry, software and/or control logic for providing various communication services to VoIP customers. Examples of such communication services include but are not limited to call waiting, call forwarding, and so on. In addition, the network element  114  comprises suitable circuitry, software and/or control logic for exchanging calls with entities outside the packet-switched network  100 . Where a call is placed by a VoIP customer, there are at least two circumstances that require the call to pass through the network element  114 , namely, (i) a call placed to a telephone number that is reachable only via the Public Switched Telephone Network (PSTN)  116  and (ii) an emergency call.  
         [0031]     In the former case, the network element  114  detects when a VoIP customer in the packet-switched network  100  is attempting to reach a destination that can only be reached via the PSTN  116 , in which case the call is routed via a network  124  to one of a plurality of gateways  118 A,  118 B that connect to the PSTN  116 .  
         [0032]     In the latter case, the network element  114  detects when a VoIP customer in the packet-switched network  100  has dialed (either explicitly or via a speed dial function or in some other way) an emergency number such as “9-1-1”. In such a case, the call, hereinafter referred to as an emergency call, is routed to one of the gateways  118 A,  118 B, which connect not only to the PSTN  116  as described above, but also to a network of dedicated emergency lines and Public Safety Answering Points (PSAPs), hereinafter collectively referred to as an E911 network  122 .  
         [0033]     In addition to the above, when handling an emergency call, the network element  114  comprises circuitry, software and/or control logic suitable for outpulsing a “routing key” associated with the directory number of the VoIP customer having placed the emergency call. The routing key accompanies the emergency call as it is routed by the network element  114  to the appropriate one of the gateways  118 A,  118 B. Further detail regarding routing keys and the operation of the network element  114  will be given later on in this description.  
         [0034]     In a specific example of implementation, the network element  114  is the Multimedia Communication Server  5200  from Nortel Networks Limited, Brampton, Ontario, Canada, although it should be understood that the present invention applies equally to other makes, models and types of packet switches or softswitches that have (or can be configured to have) the ability to assign a routing key to a VoIP customers directory number.  
         [0035]     As previously mentioned, the network element  114  is connected to the gateways  118 A,  118 B via the network  124 . In some embodiments, the network  124  may be part of the packet-switched network  100  while in other embodiments it may not. In still other embodiments, rather than being connected via the network  124 , the network element  114  may be connected to each gateway  118 A,  118 B by a respective communication link that can be optical fiber, coaxial cable, wireless, free-space optical, etc. It is noted that the network  124  (or the communication link(s), as the case may be) carries multiple telephone calls simultaneously. In an embodiment of the present invention, emergency calls are treated differently from non-emergency calls and therefore it is envisaged that distinct virtual trunk groups will be established for either type of call (non-emergency and emergency), as well as for each of the gateways  118 A,  118 B.  
         [0036]     In addition to communicating with the network element  114  via the network  124 , the gateways  118 A,  118 B in  FIG. 1A  also communicate with components of the PSTN  116  and the E911 network  122 . Specifically, gateway  118 A is connected to the PSTN  116  via a plurality of high-capacity switches  128 A,  128 B and is also connected to the E911 network  122  via a plurality of high-capacity switches  132 A,  132 B. Similarly, gateway  118 B is connected to the PSTN  116  via a plurality of high-capacity switches  148 A,  148 B and is also connected to the E911 network  122  via a plurality of high-capacity switches  152 A,  152 B. It should be understood that the specific architecture shown in  FIG. 1A  is merely for purposes of illustration; in other architectures that are within the scope of the present invention, there may be more or fewer gateways, and not all gateways need be connected to both the PSTN  116  and the E911 network  122 .  
         [0037]     In a specific example of implementation, each or either of the gateways  118 A,  118 B may be embodied as the Communication Server  2000  from Nortel Networks Limited, Brampton, Ontario, Canada, although it should be understood that the present invention applies equally to other makes, models and types of gateways.  
         [0038]     An example of a basic function of the gateways  118 A,  118 B is to allow non-emergency calls originated in the packet-switched network  100  to be completed via the PSTN  116  (which is circuit-switched) and vice versa. Another example of a basic function of the gateways  118 A,  118 B is to take emergency calls originated in the packet-switched network  100  and to route them into the E911 network  122 , which is circuit-switched (much like the PSTN  116 ).  
         [0039]     Continuing with the description of the architecture in  FIG. 1A , switch  132 A is connected via a first portion of the E911 network  122  to a first plurality of PSAPs, including PSAP  138 A and PSAP  138 B, while switch  132 B is connected via a second portion of the E911 network  122  to a second plurality of PSAPs, including PSAP  138 C and PSAP  138 D. Similarly, switch  152 A is connected via a third portion of the E911 network  122  to a third plurality of PSAPs, including PSAP  138 E and PSAP  138 F, while switch  152 B is connected via a fourth portion of the E911 network  122  to a fourth plurality of PSAPs, including PSAP  138 G and PSAP  138 H. Of course, this distribution of PSAPs is not to be considered as limiting.  
         [0040]     Each switch routes a received call in accordance with a connection map. For an emergency call received at a given one of the switches  132 A,  132 B,  152 A and  152 B, the call will specify a desired PSAP to be reached. The identity of the desired PSAP may be expressed in the form of a “E911 telephone number”. Thus, for example, switch  132 B will recognize an emergency call that has an associated “E911 telephone number” which specifies either PSAP  138 C or PSAP  138 D, and will route the emergency call accordingly. The E911 telephone number of may correspond to the telephone number of a specific PSAP along a dedicated in the E911 network  122 , and is usually held confidential by the local exchange carrier.  
         [0041]     In addition, switches  132 A,  132 B,  152 A and  152 B may have a further ability to forward an emergency call towards a specialized entity other than the PSAPs shown in the drawings. The desirability of doing so arises when a trained responder at a PSAP determines that a special agency (e.g., police, fire or ambulance) may need to be contacted. Forwarding of the emergency call may be done in accordance with a forwarding table that maps plural emergency telephone numbers to each directory  11  number. Each emergency telephone number mapped to a given directory number is associated with a respective forwarding code that signifies either “police”, “fire” or “ambulance”. During an actual call received at a PSAP, a particular forwarding code would be applied by a trained responder at the PSAP in question and sent to switch  132 B. Upon receipt of the particular forwarding code, switch  132 B is operative to look up the directory number of the call in question and to forward the emergency call towards the appropriate agency using the emergency telephone number for the forwarding code in question. In practice, the trained responder may enter into a three-way conference before the call forward is complete.  
         [0042]     It has already been mentioned that PSTN calls and emergency calls are received from the network  124  over different virtual trunk groups. This makes it a simple task for a particular one of the gateways  118 A,  118 B to determine towards which network (i.e., the PSTN  116  or the E911 network  122 ) to direct a given call. However, in the case of an emergency call received by, say, gateway  118 A, there is still a question of whether to route the call towards switch  132 A or towards switch  132 B. To this end, gateway  118 A maintains a connection map  134 A which associates each potential received routing key with one of the switches, either switch  132 A or switch  132 B. In addition, gateway  118 A may convert the routing key into a format more understandable to the switches  132 A,  132 B. One example of a more understandable format is the “E911 telephone number” format mentioned above. The E911 telephone number accompanies the emergency call as it is routed by the gateway  118 A to the appropriate one of the switches  132 A,  132 B.  
         [0043]     In an analogous fashion, gateway  118 B maintains a connection map  134 B and also may convert received routing keys into E911 telephone numbers. Further detail regarding E911 telephone numbers and the operation of the gateways  118 A,  118 B will be given later on in this description.  
         [0044]     Switches  132 A,  132 B,  152 A and  152 B currently operate entirely within the circuit-switched domain. However, this does not rule out the possibility of the switches  132 A,  132 B,  152 A and  152 B being retrofitted with the functionality of an IP gateway that would allow an IP connection from the network element  114  directly to the switches  132 A,  132 B,  152 A and  152 B via dedicated virtual trunk groups, thus bypassing the need for gateways  118 A and  118 B in this intermediate position. This possibility is envisaged in  FIG. 1C . It is noted that a set of gateways  198 A,  198 B is still used to connect the network element  114  to the legacy switches  128 A,  128 B,  148 A,  148 B leading to the PSTN  116 .  
         [0045]     Returning to  FIG. 1A , the PSAPs  138 A to  138 H are connected to an ALI database  140 . The ALI database  140  is a known database that stores street addresses and associated telephone numbers, thus enabling a PSAP operator to obtain the street address corresponding to a given directory number from which an emergency call has originated. The ALI database  140  is connected to a 9-1-1 database management system (9-1-1 DBMS)  150 , which maintains a mapping of street addresses to “emergency zones”, such as a municipality, county or district, for example. The 9-1-1 DBMS  150  is accessible to the registration entity  190 , either by a direct link or via the packet-switched network  100 .  
         [0046]     The architecture in  FIG. 1A  also comprises a street address guide (SAG)  160 , which is accessed by the registration entity  190 , either by a direct link or via the packet-switched network  100 . The street address guide  160  provides validation of a street address in order to determine whether a particular entry corresponds to a realistic address.  
         [0047]     In accordance with an embodiment of the present invention, certain steps are performed for each VoIP customer during a provisioning phase, which occurs before the placement of an emergency call by that VoIP customer, and is now described with reference to the signal flow diagrams in  FIGS. 2A through 2H , which correspond to steps  2 -A through  2 -H. In fact, it may be advantageous to perform the following steps during the same general time frame as when the VoIP customer obtains his or her directory number N.  
         [0048]     At step  2 -A, the VoIP customer provides a service address to the registration entity  190 . The service address, which may differ from the billing address, is typically the geographic location of the VoIP customer, which may be the civic (street) address where the VoIP customer is located, although it is envisaged that in some embodiments it may be the latitude/longitude of the VoIP customer or some other form of localization data. The manner in which the VoIP customer provides the service address to the registration entity  190  is not material to the present invention and may include the usage of the web, email, snail mail, etc. It is noted that step  2 -A may be performed at the same time as when the VoIP customer is first assigned a directory number N and in fact it is envisaged that the execution of step  2 -A may even be made a condition for the delivery of VoIP services.  
         [0049]     At step  2 -B, the registration entity  190  validates the service address supplied by the user. This can be achieved by running the service address through a street address guide (SAG)  160  that is available to the VoIP service provider. Validation provides an assurance that the service address given by the user is a valid address, i.e., really exists, and therefore will be capable of being meaningfully associated with an emergency zone and its designated PSAP. If validation at step  2 -B is unsuccessful, then the VoIP customer may be asked to re-enter the service address with a greater degree of precision or may be prompted to resolve an ambiguity by choosing the service address from a list of two or more address choices. Step  2 -B may also be performed interactively with the VoIP customer and may involve the intervention of a customer service representative.  
         [0050]     Provided validation at step  2 -B is successful, the registration entity  190  proceeds to step  2 -C, which consists of supplying the validated street address to the 9-1-1 DBMS  150 . The 9-1-1 DBMS  150  has the functionality of identifying an emergency zone associated with the service address. In one embodiment, the 9-1-1 DBMS  150  maintains a mapping that associates postal codes (zip codes) to emergency zones. Thus, a given service address having a given postal code will map to a corresponding emergency zone.  
         [0051]     At step  2 -D, the 9-1-1 DBMS  150  returns a file processing confirmation  209  to the registration entity  190 . The file processing confirmation  209  may identify the emergency zone (hereinafter denoted  210 ) associated with the service address in question.  
         [0052]     At step  2 -E, which may actually be executed before step  2 -D, the 9-1-1 DBMS  150  provides the directory number N and the validated street address to the ALI database  140  for storage therein.  
         [0053]     At step  2 -F, the 9-1-1 DBMS  150  updates the forwarding tables at the switches  132 A,  132 B,  152 A,  152 B, with routing information  202  for the purposes of eventual call transfer to dispatch agencies (police, fire, ambulance) as per established routines. In an example, the individual emergency telephone numbers corresponding to police, fire and ambulance agencies which are associated with emergency zone  210  are entered into the forwarding table in association with directory number N.  
         [0054]     At step  2 -G, which may actually be executed before step  2 -F, the registration entity  190  consults a call routing list (CRL)  188 , which associates emergency zones  210  to individual “routing keys”  214 . The result of step  2 -G is the obtaining of a routing key  214  that corresponds to the emergency zone  210 . By virtue of the association between each directory number N and its emergency zone  210 , and by virtue of the association between each emergency zone  210  and its routing key  214 , it will be apparent that each directory number N will be associated with a routing key  214 . Also, since more than one emergency zone may be serviced by the same PSAP, a plurality of directory numbers N will share the same routing key  214 .  
         [0055]     At step  2 -H, the registration entity  190  provides the directory number N and the associated routing key  214  (obtained at step  2 -G) to the network entity  114 . The network entity  114  enters this information into a table  178  local to the network entity  114 . The table  178  may be stored in the network entity  114  or otherwise directly accessible thereto.  
         [0056]      FIG. 3A  shows a specific, non-limiting example of the table  178  that is local to the network entity  114 . Basically, the table  178  comprises a plurality of records  204 , each containing a directory number N and a related routing key  214 . A particular routing key  214  comprises information that defines a route to be taken by an emergency call in order to reach a particular PSAP. In one embodiment, not to be considered as limiting, the routing key  214  comprises a gateway identifier  214 A and a routing code  214 B. Further detail regarding the purpose and effect of fields  214 A,  214 B will be given later on in this specification.  
         [0057]     In an alternative embodiment of steps  2 -G and  2 -H, shown in  FIGS. 2-1  and  2 -J, the registration entity  190  provides the directory number N and the associated emergency zone  210  to the network entity  114 , and it is the network entity  114  that consults a call routing list (CRL)  188  in order to obtain the appropriate routing key  214  for the emergency zone  210  in question. In this case, and with reference to  FIG. 3B , the table  178 ′ local to the network element  114  would comprise a plurality of records, each containing a directory number N, a related emergency zone  210  and a related routing key  214 .  
         [0058]     With additional reference now to the diagrams of FIGS.  4 -A to  4 -F, placement of an emergency call and operation of the various elements in the architecture of  FIG. 1  in a “call handling” phase is now described.  
         [0059]     At step  4 -A, the network element  114  detects an emergency call  400  received from a VoIP customer associated with a particular directory number N.  
         [0060]     At step  4 -B, the network element  114  consults the table  178  (or  178 ′) which is local to the network element  114  and retrieves the routing key  214  for the directory number N. As previously mentioned, the routing key  214  contains a gateway identifier  214 A, which identifies the destination gateway towards which the emergency call  400  should be routed. Let this destination gateway be gateway  118 A. In addition, the routing key  214  contains a routing code  214 B which, when interpreted by gateway  118 A, will identify (i) a destination switch towards which gateway  118 A should route the emergency call  400  and (ii) the destination PSAP for the emergency call  400 . For the purposes of this example, let the destination switch be switch  132 B and let the destination PSAP be PSAP  138 C.  
         [0061]     At step  4 -C, the network element  114  routes the emergency call  400  onto the virtual trunk group assigned to the destination gateway, in this case gateway  118 A. In addition, as part of step  4 -C, the network element  114  forwards the routing code  213 B along with the emergency call  400 . In an alternative embodiment, the network element  114  forwards the routing key  214  in its entirety. The forwarded information accompanies the emergency call  400  as it is routed to gateway  118 A.  
         [0062]     At step  4 -D, the gateway  118 A receives the emergency call  400  from the network element  114 . The emergency call  400  is accompanied by at least the routing code  214 B. Gateway  118 A reads the routing code  214 B in order to learn (i) the identity of the destination switch (in this case switch  132 B) towards which the emergency call  400  should be routed by gateway  118 A and (ii) the identity of the destination PSAP (in this case PSAP  132 C) towards which the emergency call  400  should be routed by the destination switch  132 B. Additionally, gateway  118 A obtains the E911 telephone number corresponding to the destination PSAP  138 C, hereinafter denoted  250 . The destination gateway  118 A then proceeds to route the emergency call  400  to the destination switch  132 B and forwards the E911 telephone number  250  along with the emergency call  400 .  
         [0063]     At step  4 -E, the destination switch (in this case switch  132 B) routes the received emergency call  400 . Routing is performed on the basis of the E911 telephone number  250  received from gateway  118 A, resulting in the emergency call  400  being transferred onto a dedicated line leading towards the destination PSAP (in this case PSAP  138 C) over the E911 network  122 .  
         [0064]     At step  4 -F, once the incoming emergency call  400  is received at the destination PSAP  138 C, it is handled by a trained responder. With knowledge of the directory number N (which follows the emergency call  400  from its inception), the responder obtains the validated service address associated with the directory number N. In one embodiment, the responder queries the ALI database  140  upon receipt of the emergency call  400  in order to obtain the validated service address. In an alternative embodiment, the validated service address is pushed by the ALI database  140  during a previous step. Specifically, after step  4 -E described above, receipt of the emergency call  400  by switch  132 B could be followed by switch  132 B supplying the directory number N to the ALI database  140 , which then pushes the validated service address to the destination PSAP  138 C. In either case, the responder learns the exact geographic location of the caller and can dispatch emergency personnel if necessary.  
         [0065]     In an example scenario, the responder may determine that a particular type of emergency agency (police, ambulance, fire) needs to be dispatched. A forwarding code can be dialed back to switch  132 B from which the emergency call  400  originated. The forwarding code triggers switch  132 B to use its internal forwarding table in order to forward the emergency call  400  to a particular emergency telephone number where the appropriate agency can be reached. Since the updating of the forwarding table was done in the provisioning phase at step  2 -F (as described earlier), the emergency call  400  will be automatically forwarded to the agency of the appropriate type that is geographically in the best position to handle the emergency call  400 .  
         [0066]     Of course other embodiments of explicit routing using a routing key  214  are within the scope of the present invention. For instance, it is envisaged that the routing code  214 B mentioned above may comprise only the E911 telephone number  250  corresponding to the destination PSAP. In such a scenario, a gateway that receives the emergency call and the associated E911 telephone number  250  would access a local table to obtain the identity of the switch that is connected to the destination PSAP. In fact, the functionality of consulting a local table could be relegated to network element  114 , such that it is the network element  114  that determines the ports that need to be used by the gateway when routing the emergency call in question, in order that the call reach the destination PSAP.  FIG. 1B  shows such an embodiment, where the connection maps ( 134 A,  134 B in  FIG. 1A , formerly executed by the gateways  118 A,  118 B, respectively) have been consolidated into a single connection map executed at the network element  114 .  
         [0067]     From the above description, it will be noted that the assignment of a routing key  214  to each directory number N permits independence of the directory number N and the destination PSAP. In other words, there need not be any relationship between the “area code” or “local exchange” of the directory number N and the destination PSAP, which is unlike the case with the traditional telephony infrastructure. As a result, VoIP service providers can assign arbitrary directory numbers to their customers, while ensuring that emergency services will be dispatched effectively by the appropriate PSAP for each customer. Moreover, as has been shown using the example of FIGS.  4 -A to  4 -F, emergency calls  400  can be directed to the appropriate PSAP over a dedicated emergency circuit in the E911 network  122 , rather than over an administrative line, thereby maximally assuring a prompt response by trained personnel.  
         [0068]     Those skilled in the art will appreciate that in some embodiments, the functionality of parts of the network element  114  and/or the registration entity  190  may be implemented as pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components. In other embodiments, parts of the network element  114  and/or the registration entity  190  may be implemented as an arithmetic and logic unit (ALU) having access to a code memory (not shown) which stores program instructions for the operation of the ALU. The program instructions could be stored on a medium which is fixed, tangible and readable directly by the network element  114  and/or the registration entity  190 , (e.g., removable diskette, CD-ROM, ROM, or fixed disk), or the program instructions could be stored remotely but transmittable to the network element  114  and/or the registration entity  190  via a modem or other interface device (e.g., a communications adapter) connected to a network over a transmission medium. The transmission medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented using wireless techniques (e.g., microwave, infrared or other transmission schemes).  
         [0069]     While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.