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Circuit-Switched Telephony

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Chapter 4

CHAPTER 4

Circuit-Switched Telephony

Conventional telephone networks, whether public (PSTN) or private, bear several things in common. First, the phones used to make calls across them almost always use one- or two-pair physical connections. Second, the call-management device nearest the end user, be it a key system or a PBX, usually provides a dedicated, singlepurpose circuit for each phone. The voice applications delivered by legacy systems are rigidly tied to the lower layers of the network. For instance, you can’t get plain old telephone service from a cable company or a satellite provider because they can’t provision copper telephone lines to your premises. Finally, the capacity of the data links used to carry traditional telephone calls rarely increases over time. It remains fixed, forever tied to the quantity of cable pathways between one point and the next.

These traits are common among legacy voice setups, whether they consist of heavyduty TDM-bus PBX systems or just a few analog phones connected to the PSTN.

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AGI Commands

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Appendix B

APPENDIX B

AGI Commands

ANSWER

Answers the current channel if not already answered.

Returns -1 upon failure, or 0 upon success.

CHANNEL STATUS [channel]

Returns the status of the specified channel. If none is given, returns the status of the current channel. Here is what the status codes mean:

0: Channel is on hook and available.

1: Channel is on hook, but reserved.

2: Channel is off the hook but no digits have been dialed.

3: Digits have been dialed.

4: The line connected to this channel is ringing.

5: A called line connected to this channel is ringing.

6: A called line connected to this channel has a call in progress.

7: A called line connected to this channel is busy.

DATABASE DEL family key

Deletes a value in the Asterisk database for the specified family and key.

Returns 1 if successful, 0 if not.

DATABASE DELTREE family [keytree]

Deletes a family or and/or keytree within a family in the Asterisk database.

Returns 1 if successful, 0 if not.

DATABASE GET family key

Retrieves a value in the Asterisk database for the specified family and key.

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PSTN Trunks

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Chapter 12

CHAPTER 12

PSTN Trunks

While private trunks connect voice switches on your private network, PSTN trunks serve another purpose: connecting your PBX or your VoIP network to the outside world. They can be analog phone lines, digital phone lines like T1s, ATM connections, or VoIP based, depending on what kinds of service are available from your

PSTN carrier.

Legacy telephony purists will balk at the use of the word trunk to describe a T1 or an

ATM connection, arguing that a trunk is nothing more than a phone line connecting two switches. In fact, the definition has grown to mean any connection between two voice networks. A 5-mile-long T1 between two old-school PBXs is a trunk, and so is a UDP pathway between two VoIP servers. Even in non-voice scenarios, the word trunk is used to describe a pathway between two switches—take VLAN trunks as an example.

The way you think about trunk connections is different when they’re PSTN trunks.

While privately owned trunks are relatively cheap or free, PSTN trunks incur service fees. Careful design, utilization, and monitoring of PSTN trunks is important to your bottom line. PSTN trunks can also offer calling features that let you do things that may be less easy to do with private trunks: features like distinctive ring and threeway calling can be integrated into your voice network to simplify your PBX design or to enable functions that you otherwise couldn’t provide.

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Linux as a PBX

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Chapter

3 3

CHAPTER

Linux as a PBX

Evaluating VoIP for enterprise or for your home phone setup means a lot of experimentation, and you’ll need to build a test server with which to hone your VoIP skills.

That test server should be something you can get a lot out of without spending a bundle or committing to a specific vendor’s commercial VoIP platform before you’ve done your homework. Free telephony software lets you do that homework.

Free Telephony Software

If you were learning engine repair instead of VoIP, you probably wouldn’t use a Ferrari for your experiments. You would want something more forgiving and easier to work on, like a nice Dodge Omni. Luckily, there’s Asterisk PBX software—the very open, roomy-under-the-hood telephony server. Like a Dodge Omni, Asterisk is easy to work on, support is a snap to find, and experimenting is cheap. In fact, Asterisk is free

(although its development is supported by Digium, Inc., http.//www.digium.com). So is its source code.

But like a Ferrari, Asterisk is very powerful. Asterisk supports several Voice over IP communication protocols: H.323, SIP, IAX, and others (see Chapter 7 for more on these). Using these protocols, it can support just about any IP telephone, as well as traditional analog and digital telephones. Asterisk has some industrial-strength features like call-queuing, conference calling, voice mail, and caller ID.

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Asterisk Reference

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Chapter

17 17

CHAPTER

Asterisk Reference

Earlier in the book, we covered compiling and installing Asterisk, installing a legacy interface card and drivers, and setting up some simple PBX applications. Later on, we used Asterisk configurations to illustrate some common enterprise telephony concepts. This chapter is geared toward the person who’s comfortable with the earlier material in the book and wants a deeper understanding of Asterisk.

Asterisk is a deep subject that touches disciplines of networking, code-writing, protocols, and standards. This chapter won’t make you an Asterisk expert, but it should help you go a step or two beyond the essentials we’ve already covered. We’ll cover channel configuration—with an emphasis on Zaptel and SIP channels, dial-plan syntax elements like variables and string processing, and the commands you can use in building an Asterisk dial-plan.

How Asterisk Is Supported

Asterisk’s principal sponsor is a firm called Digium, based in the United States. The company provides development leadership and commercial support for the open source system. It also manufactures and distributes the Wildcard interface devices and iAXY ATA used in some of the projects in this book. A number of independent consultants provide commercial support for Asterisk. Subscribing to the Asterisk

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