In North America, E9-1-1 is an integral part of the phone service. All LEC telephony service providers have to provide the functionality, as mandated by federal, state, provincial, or local laws. To do this, the vast majority of North American telephony service providers rely on an overlay network to handle the voice and data traffic outside of the switched dial tone network.
Note As of December 1999, only two major RBOCs rolled out an E9-1-1 solution that uses SS7 for part of the routing of 9-1-1 calls.
This network is composed of analog MF signaling trunks dedicated to handling 9-1-1 traffic (for the aggregation and routing of the calls), and either analog MF signaling trunks or ISDN lines (PRI or BRI) to deliver the calls to the Public Safety Answering Point (PSAP).
There is a misconception that 9-1-1 call handling is a function of the PSTN. In most cases, the only part of the PSTN used for handling 9-1-1 calls is the line side of the CLASS 5 switch serving the local customer. The telephony service provider’s E9-1-1 responsibilities are to:
• Automatically route 9-1-1 calls to the appropriate PSAP.
• Forward the calling party phone number to the appropriate PSAP.
• Provide an address database link to PSAP for retrieving Automatic Location Identification (ALI) (this may not be provided by the telephony Service Provider).
Unlike PSTNs, 9-1-1 calls are routed on the calling number, not the called number. Figure 4-33 highlights the 9-1-1 call flow through a likely E9-1-1 architecture.
Note The information is provided solely to support our examples and does not represent the actual E9-1-1 topology for the San Francisco bay area.
It depicts the voice and information flows associated with a PSTN subscriber dialing 9-1-1 from a phone line directly connected to a CLASS 5 switch. Also, the following examples do not highlight the specifics of an Enterprise Telephony subscriber making a 9-1-1 call.
The top part of the drawing shows the signaling and bearer connections between our two CLASS 5 switches for PSTN traffic. We include this detail to reinforce the fact that the PSTN SS7 fabric and associated switched bearer traffic trunks are not involved in routing 9-1-1 calls.
Figure 4-33 Typical E9-1-1 Network Topology
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Example 1:
In this example a Santa Clara subscriber dials 9-1-1. We will follow the call until it is connected to the call taker.
Assumptions:
• The line’s number is 408.222.1235.
• The serving switch is physically located in San Jose, even though it is also serving customers in Santa Clara.
• For each phone number in the geographical area served by the Silicon Valley area E9-1-1 tandem switch, a database provides an association between the caller’s ANI, Emergency Service Number (ESN), and ALI. This association is created through collaboration between the Local Exchange Carrier (using the service records showing where that phone line is installed) and civil authorities (using the Master Street Address Guide (MSAG)). In other words, based on the phone number, the tandem will learn which PSAP receives the call. In our case, the ESN associated with the Santa Clara PSAP is 1.
Process:
1. The Santa Clara subscriber goes off-hook and obtains dial tone from San Jose CLASS 5 switch.
2. The subscriber dials 9-1-1.
3. The San Jose switch routes the call to outgoing trunks connected to the E9-1-1 tandem Silicon Valley area. Please note that the PSTN call path is not taken.
4. The San Jose switch sends the ANI (408.222.1235) through MF signaling on the Centralized Automatic Message Accounting (CAMA) trunk.
5. The tandem queries the Selective Routing (SR) database for the ESN associated with the ANI.
6. The tandem receives ESN=1. This prompts the routing of the call toward the Santa Clara PSAP.
7. The tandem sends the ANI information to the Santa Clara ANI/ALI controller.
8. At this point, the voice path is cut through in both directions. The ANI/ALI controller provides ringing progress tone.
9. The ANI/ALI controller queries the database for the ALI corresponding to the ANI.
10. As the Santa Clara call taker answers the call, the voice, ANI, and ALI of the caller are presented.
Example 2:
In this example, a San Jose subscriber places the 9-1-1 call.
Assumptions:
• The line’s number is 408.222.1234.
• The serving switch is physically the same as in Example 1.
• The ESN associated with the San Jose PSAP is ESN2.
Process:
1. The San Jose subscriber goes off-hook and obtains a dial tone from the San Jose CLASS 5 switch.
2. The subscriber dials 9-1-1.
3. The San Jose switch routes the call to outgoing trunks connected to the E9-1-1 tandem Silicon Valley area. Please note that the PSTN call path is not taken.
4. The San Jose switch sends the ANI (408.222.1234) through the MF signaling on the CAMA trunk.
5. The tandem queries the Selective Routing (SR) database for the ESN associated with the ANI.
6. The tandem receives ESN=2. This prompts the routing of the call toward the San Jose PSAP.
7. The tandem sends ANI information to the San Jose ANI/ALI controller.
8. At this point, the voice path is cut through in both directions. The ANI/ALI controller provides ringing progress tone.
9. The ANI/ALI controller queries the database for the ALI corresponding to the ANI.
10. As the San Jose call taker answers the call, the voice, ANI, and ALI of the caller are presented.
We can see that even though the callers in examples 1 and 2 are served out of the same switch using the same trunks from the serving switch to the E9-1-1 tandem, the routing of this call is different from example 1.
Note The E9-1-1 tandem trunks are dedicated to the handling of 9-1-1 calls. No switched traffic is sent on these trunks.
Rather than cover the San Francisco customer, just note that the customer is served by a different E9-1-1 tandem, a different database (although it could be the same), and that it could not be routed to the San Jose or Santa Clara PSAP. This last point is important: E9-1-1 tandems are typically not interconnected!
From a practical point of view, this means that if a 9-1-1 call is delivered to the wrong PSAP, it could not be transferred back to the appropriate PSAP if they are served by different E9-1-1 tandems. For WAN deployment of a VoIP solution, this may require that the gateways to the LEC be evaluated for their E9-1-1 tandem accessibility.