Private Line Automatic Ringdown

Although not directly related to dial-peer configuration, Private Line Automatic Ringdown (PLAR) configurations rely heavily on existing dial peers to complete a call. Ports configured with PLAR capabilities automatically dial a number as soon as the port detects an off-hook signal. The most obvious use for PLAR configurations is emergency phones in locations such as company elevators or parking garages. The configuration in Example 8.13 designates x1101 (shown in Figure 8.8) as a PLAR extension that will immediately dial x1102 as soon as a user lifts the receiver.

CME_A

x1101

x1101

CME_A

x1102

Figure 8.8 PLAR Configuration Diagram

Example 8.13 FXS PLAR Configuration

Key Topic

CME_A(config)# voice-port 0/0/0

CME_A(config-voiceport)# connection ? plar Private Line Auto Ringdown tie-line A tie line trunk A Straight Tie Line

CME_A(config-voiceport)# connection plar ? WORD A string of digits including wild cards tied dedicated tie to this number

CME_A(config-voiceport)# connection plar 1102

The FXS voice port 0/0/0 is now hard-coded to dial the number 1102 as soon as a user lifts the handset.

PLAR can be useful in a variety of other circumstances as well. One common scenario is using FXO connections to the PSTN, as shown in Figure 8.9.

Receptionist

Receptionist

CME_A

Figure 8.9 FXO PSTN Connections

CME_A

Figure 8.9 FXO PSTN Connections

Although the destination-pattern command from dial-peer configuration mode is very useful for dictating what is able to go out the PSTN FXO ports, it is not too useful for handling what comes in the FXO ports. When the CME_A router shown in Figure 8.9 receives an incoming call from the PSTN, the call information sent from the PSTN carrier does not include dialed number information (this is known as Dialed Number Identification Service [DNIS]). It does include caller ID information (known as Automatic Number Identification [ANI]), but this does not help the router to know where to send the call when it is received. As a result, calls into the CME_A router will hear a second dial tone played after they have dialed into the CME_A router from the PSTN. This is essentially the router saying, "Yes, I've received your call, please tell me what to do now." If the caller on the phone were to dial 1500, the CME_A router would forward them to the receptionist. However, the likelihood of a PSTN caller doing this is very slim. This is where PLAR comes to the rescue. Example 8.14 configures two analog FXO ports as PLAR connections for incoming calls.

Example 8.14 FXO PLAR Configuration

CME_A(config)# voice-port 2/0/0

CME_A(config-voiceport)# connection

plar 1500

CME_A(config-voiceport)# exit

CME_A(config)# voice-port 2/0/1

CME_A(config-voiceport)# connection

plar 1500

CME_A(config-voiceport)# exit

By entering the connection plar 1500 command under both FXO ports, the router receives incoming calls from the PSTN and immediately forwards them to the receptionist phone rather than playing a second dial tone.

Note Keep in mind that configuring PLAR connections for incoming calls is something you would only need to do for analog FXO trunks. Digital PSTN connections (such as T1 or E1) receive DNIS information for incoming calls, which the router can use for Direct Inward Dial (DID) services.

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