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Emergency Dispatch/911
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353
The second priority in this example hangs up the channel, while the third priority
places a call to the local public safety answering point, whose phone number has
been stored in
${LocalPSAP} with a SetVar command in priority 1.
This is fine when you have only one SIP trunk channel at your disposal for 911 calls,
but if you have more than one, you wouldn’t want to automatically hang up a chan-
nel based on some proprietary setting in the dial-plan. The ChanIsAvail( ) command
will help us determine which of several SIP trunk channels can be used to place the
call—kind of like an outbound hunt group. Consider this dial-plan snippet:
exten => 911,1,ChanIsAvail(SIP/VOICEPULSE1&SIP/VOICEPULSE2)
exten => 911,2,SetVar(LocalPSAP=440-361-9000)
exten => 911,3,Dial(${AVAILCHAN}/${LocalPSAP})
exten => 911,4,Hangup( )
exten => 911,102,SoftHangup(SIP/VOICEPULSE1)
exten => 911,103,Dial(SIP/VOICEPULSE1/${LocalPSAP})
In this example, priority 1 checks to see if either of the two named SIP channels is
available (these correspond to SIP peers in sip.conf). If one is available, its channel
name is stored in
${AVAILCHAN} and processing continues at the next priority, 2.
Priority 2 sets the phone number of the public safety dispatcher as in the previous
example. Priority 3 dials that number to connect the 911 call on the available SIP
channel.
If no channels are available in priority 1, the priority becomes 1+101, or 102. Since
no channels are available, priority 102 hangs up a channel arbitrarily so priority 103
can place the emergency call.
Administrator Tools
In legacy telephony, the tools of the system administrator ranged from a primitive
serial terminal all the way up to a dedicated desktop software application that
allowed administrative control over the phone system. Traditional PBX CPUs, such
as those made by old-schoolers Isoetec, AT&T, and Executone, used a 2,400-baud
serial connection to an RS-232 port for management of channels, trunks, and sub-
scribers.
Later model phone systems offered dedicated Windows-based software for manag-
ing these things. Most recently, the administrator tools have become web-based.
Administering phone systems has become easier and easier, even as PBX systems
have grown increasingly feature rich. Commercial softPBX platforms extend tele-
phony administration further than ever before by using desktop and web-based
applications with CTI to give administrators point-and-click power.
Asterisks administrative interface
Asterisk doesn’t come with any administrative applications other than the (rather
limited) Astman. As a result, if you want to administer Asterisk, you’re going to be

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