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1 In a key system, each line has a corresponding button on each telephone, and lines are accessed by pressing |
the desired line key. |
108 |
| |
Chapter 7: Outside Connectivity |
The [globals] section contains two variables, named LOCAL and TOLL.2 The purpose |
of these variables is to simplify management of your dialplan should you ever need to |
change carriers. They allow you to make one change to the dialplan that will affect all |
places where the specified channel is referenced: |
[globals] |
; These channels are the same to asterisk as |
; any PJSIP enpoint, so they'll be configured |
; similar to telephone sets. |
; Each carrier will have their own configuration |
; requirements (although they'll all be similar) |
LOCAL=PJSIP/my-itsp |
TOLL=PJSIP/my-other-itsp |
The [external] section contains the actual dialplan code that will recognize the |
numbers dialed and pass them to the Dial() application:3 |
[external] |
exten => _NXXNXXXXXX,1,Dial(${LOCAL}/${EXTEN}) ; 10-digit pattern match for NANP |
exten => _NXXXXXX,1,Dial(${LOCAL}/${EXTEN}) ; 7-digit pattern match for NANP |
exten => _1NXXNXXXXXX,1,Dial(${TOLL}/${EXTEN}) ; Long-distance pattern match |
; for NANP |
exten => _011.,1,Dial(${TOLL}/${EXTEN}) ; International pattern match for |
; calls made from NANP |
; This section is functionally the same as the above section. |
; It is for people who like to dial '9' before their calls |
exten => _9NXXNXXXXXX,1,Dial(${LOCAL}/${EXTEN:1}) |
exten => _9NXXXXXX,1,Dial(${LOCAL}/${EXTEN:1}) |
exten => _91NXXNXXXXXX,1,Dial(${TOLL}/${EXTEN:1}) |
exten => _9011.,1,Dial(${TOLL}/${EXTEN:1}) |
In any context that would be used by sets or user devices, you would use an include |
=> directive to allow access to the external context: |
[sets] |
include => external |
It is critically important that you do not include access to the exter‐ |
nal lines in any context that might process an incoming call. The |
risk here is that a phishing bot could eventually gain access to your |
outgoing trunks (you’d be surprised at how common these phish‐ |
ing bots are). |
We cannot stress enough how important it is that you ensure that |
no external resource can access your toll lines. |
2 You can name these anything you wish. |
3 For more information on pattern matches, see Chapter 6. |
Fundamental Dialplan for Outside Connectivity |
| |
109 |
The PSTN |
The public switched telephone network (PSTN) has existed for over a century. It is |
the precursor to many of the technologies that shape our world today, from the inter‐ |
net to MP3 players. |
The use of old-school PSTN circuits in Asterisk systems is no longer common. The |
technical complexities, costs, and limitations of obsolete technology are only justified |
in situations where a reliable internet connection is not available (and even then, tra‐ |
ditional circuits will often be a problematic choice). Even the carriers themselves have |
largely switched to VoIP for their internal backbones. |
The PSTN Has Retired |
More than any technical factor, perhaps the most significant nail in the PSTNs coffin |
is the fact that most of the technical experts in the field of traditional telephony are |
near or past retirement age, and the new kids have no interest in this sort of thing. |
Point being: you will increasingly find that carriers no longer have the skilled staff |
required to deploy traditional PSTN services. All the cool kids are learning VoIP |
(which is ultimately just a networking technology), and all the carriers put their best |
and brightest on the VoIP/SIP side of the business. |
So, while it used to be true that you couldn’t beat a PRI circuit for reliability, that is no |
longer the case. In fact, many companies deliver PRI circuits across a SIP connection, |
which is a kludge Asterisk has no use for. |
Where the PSTN might still hold sway for a few years more is in telephone numbers. |
If VoIP had been invented without the PSTN preceding it, it’s unlikely that something |
like a phone number would have ever been invented. Still, we’ve got them, and we use |
them, and the reason we do so is perhaps not so much due to any usefulness they pro‐ |
vide, but rather due to the fact that they are managed by a complex, multinational |
consortium of standards bodies and curators who ensure the integrity of the global |
call routing plan. |
To put the value of this in perspective, it might be worth considering that if the inter‐ |
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