20 Chapters
Medium 9780596008680

Circuit-Switched Telephony

Theodore Wallingford O'Reilly Media PDF

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.

See All Chapters
Medium 9780596008680

Linux as a PBX

Theodore Wallingford O'Reilly Media PDF

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.

See All Chapters
Medium 9780596008680

What Can Go Wrong?

Theodore Wallingford O'Reilly Media PDF

Chapter

15 15

CHAPTER

What Can Go Wrong?

As system designers, integrators, and geeks, we do our homework to become masters in a subject fully before we go headfirst, implementing it in a production environment, right? Well, we aspire to, anyway. But experience says you can never know a hundred percent of what there is to know about a subject… particularly with Voice over IP, which is still changing and evolving. So, once you’ve equipped yourself with the book knowledge, you hit the field. It’s only then that you’ll get a chance to discover whether your VoIP project plan has any remaining flaws. But, before you take that leap of faith, there are a few things you should know about: What can go wrong when implementing VoIP in the enterprise?

Common Problem Situations

The people you call complain about echo

Generally, echo is at its worst when end-to-end latency is high. If end-to-end latency is below 150 ms, echo should be nearly imperceptible. Remove echo by removing latency. Remember that using bigger packet sizes, which are often used with lowbandwidth codecs, can increase latency. If capacity is stopping you from removing latency, increase the capacity on the links that are causing the latency. Steer away from frame-relay and VPN if the link is critical—these technologies provide the slowest links.

See All Chapters
Medium 9780596008680

VoIP Readiness

Theodore Wallingford O'Reilly Media PDF

Chapter 8

CHAPTER 8

VoIP Readiness

Let’s face it: VoIP isn’t exactly new, but IP telephony’s readiness for enterprise consumption is a fairly recent development. When it first appeared on the Internet scene, VoIP offered the ability for people to make free long-distance calls over the

Internet. In fact, products like Internet Phone came with substantial buzz about how they let in-laws with Microsoft Windows have half-duplex speakerphone conversations through their PCs over the Net.

Lack of interoperability, poor quality of service, and a drop in traditional long-distance calling rates ultimately killed the first generation of consumer VoIP software.

The short-lived voice-over-Internet craze of the late 1990s died. VoIP is still what historians might call a disruptive technology—it is changing the status quo—but as it becomes more standardized, quality-driven, and accepted, it also becomes a more sustaining technology, just as the PSTN has been for decades. In this regard, VoIP has proven much more valuable in the enterprise than in the home.

See All Chapters
Medium 9780596008680

Asterisk Manager Socket API Syntax

Theodore Wallingford O'Reilly Media PDF

Appendix C

APPENDIX C

Asterisk Manager Socket API Syntax

AbsoluteTimeout Channel Timeout

Sets an absolute timeout in seconds for the specified channel. The call will be ended after the time has elapsed. The following example limits the call on the current channel to 10 minutes:

Action: AbsoluteTimeout

Channel: SIP/201

Timeout: 600

ChangeMonitor Channel File

Equivalent of ChangeMonitor( ).

Action: ChangeMonitor

Channel: Zap/1-1

File: Zap1-1-incsound

Command command

Execute the specified dial-plan command. The command must include all arguments necessary for it to work.

GetVar Channel Variable

Gets a variable from the specified channel.

Hangup Channel

Hangs up specified channel. Equivalent to SoftHangup( ).

IAXpeers

Lists IAX peers. Equivalent of IAX2 show peers CLI command.

ListCommands

Lists available Manager API commands.

Logoff

Closes the connection to the Manager.

MailboxCount Mailbox

Gets the message count for the specified mailbox.

MailboxStatus Mailbox

Gets the message-waiting indication for the specified mailbox.

See All Chapters

See All Chapters