Russell Bryant - Asterisk: The Definitive Guide
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- Book:Asterisk: The Definitive Guide
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This bestselling guide makes it easy, with a detailed roadmap that shows you how to install and configure this open source software, whether youre upgrading
your existing phone system or starting from scratch.
Ideal for Linux administrators, developers, and power users, this updated edition shows you how to write a basic dialplan step-by-step, and brings you up to
speed on the features in Asterisk 11, the latest long-term support release from Digium. Youll quickly gain working knowledge to build a simple yet inclusive system.
* Integrate Asterisk with analog, VoIP, and digital telephony systems
* Build an interactive dialplan, using best practices for more advanced features
* Delve into voicemail options, such as storing messages in a database
* Connect to external services including Google Talk, XMPP, and calendars
* Incorporate Asterisk features and functions into a relational database to facilitate information sharing
* Learn how to use Asterisks security, call routing, and faxing features
* Monitor and control your system with the Asterisk Manager Interface (AMI)
* Plan for expansion by learning tools for building distributed systems
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Utility is when you have one telephone, luxury is when you have two, opulence is when you have threeand paradise is when you have none.
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While tomes could be written about the technologies in use in telecom networks, the material included here was chosen based on our experiences in the community, which helped us to define the specific items that might be most useful. Although this knowledge may not be strictly required in order to configure your Asterisk system, it will be of great benefit when interconnecting to systems (and talking with people) from the world of traditional telecommunications.
The purpose of the public switched telephone network (PSTN) is to establish and maintain audio connections between two endpoints in order to carry speech.
] most of the sounds we make when speaking tend to be in the range of 2503,000 Hz. Since the purpose of the telephone network is to transmit the sounds of people speaking, it was designed with a bandwidth of somewhere in the range of 3003,500 Hz. This limited bandwidth means that some sound quality will be lost (as anyone whos had to listen to music on hold can attest to), especially in the higher frequencies.
An analog phone is composed of five parts: the ringer, the dialpad, the hybrid (or network), and the hook switch and handset (both of which are considered parts of the hybrid). The ringer, the dialpad, and the hybrid can operate completely independently of one another.
When the central office (CO) wants to signal an incoming call, it will connect an alternating current (AC) signal of roughly 90 volts to your circuit. This will cause the bell in your telephone to produce a ringing sound. (In electronic telephones, this ringer may be a small electronic warbler rather than a bell. Ultimately, a ringer can be anything that is capable of reacting to the ringing voltage; for example, strobe lights are often employed in noisy environments such as factories.)
Ringing voltage can be hazardous. Be very careful to take precautions when working with an in-service telephone line.
Many people confuse the AC voltage that triggers the ringer with the direct current (DC) voltage that powers the phone. Remember that a ringer needs an alternating current in order to oscillate (just as a church bell wont ring if you dont supply the movement), and youve got it.
In North America, the number of ringers you can connect to your line is dependent on the Ringer Equivalence Number (REN) of your various devices. (The REN must be listed on each device.) The total REN for all devices connected to your line cannot exceed 5.0. An REN of 1.0 is equivalent to an old-fashioned analog set with an electromechanical ringer. Some electronic phones have RENs of 0.3 or even less. If you connect too many devices that require too much current, you will find that none of them will be able to ring.
).
1209 Hz | 1336 Hz | 1477 Hz | ] | |
697 Hz | A | |||
770 Hz | B | |||
852 Hz | C | |||
941 Hz | * | # | D | |
[] Notice that this column contains letters that are not typically present as keys on a telephone dialpad. They are part of the DTMF standard nonetheless, and any proper telephone contains the electronics required to create them, even if it doesnt have the buttons themselves. (These buttons actually do exist on some telephones, which are mostly used in military and government applications.) |
When you press a button on your dialpad, the two corresponding frequencies are transmitted down the line. The far end can interpret these frequencies and note which digit was pressed.
The hybrid is a type of transformer that handles the need to combine the signals transmitted and received, across a single pair of wires from the PSTN and two pairs of wires in the handset. One of the functions the hybrid performs is regulating sidetone , which is the amount of your transmitted signal that is returned to your earpiece; its purpose is to provide a more natural-sounding conversation. Too much sidetone, and your voice will sound too loud; too little, and youll think the line has gone dead.
]
The hook switch can also be used for signaling purposes. Some electronic analog phones have a button labeled Link that causes an event called a flash . You can perform a flash manually by depressing the hook switch for a duration of between 200 and 1,200 milliseconds (nominally about 600 ms). If you leave it down for longer than that, the carrier may assume youve hung up. The purpose of the Link button is to handle this timing for you. If youve ever used call waiting or three-way calling on an analog line, you have performed a hook-switch flash for the purpose of signaling the network.
The handset is composed of the transmitter and receiver. It performs the conversion between the sound energy humans use and the electrical energy the telephone network uses. All handsets (including those in digital and VoIP phones) are analog.
).
The Tip lead is the positive polarity wire. In North America, this wire is typically green and provides the return path. The Ring wire is the negative polarity wire. In North America, this wire is normally red. For modern Cat 5 and 6 cables, the Tip is usually the white wire, and Ring is the colored wire. When your telephone is on-hook, the ring wire will have a potential of 48V DC with respect to Tip. Off-hook, this voltage drops to roughly 7V DC.
Analog telephony is almost dead.
]
One of the primary challenges when transmitting analog signals is that all sorts of things can interfere with those signals, causing low volume, static, and all manner of other undesired effects. Instead of trying to preserve an analog waveform over distances that may span thousands of kilometers, why not simply measure the characteristics of the original sound and send that information to the far end? The original waveform wouldnt get there, but all the information needed to reconstruct it would.
This is the principle of all digital audio (including telephony): sample the characteristics of the source waveform, store the measured information numerically, and send that data to the far end. Then, at the far end, use the transmitted information to generate a completely new audio signal that has the same characteristics as the original. The reproduction is so good that the human ear cant tell the difference.
The principal advantage of digital audio is that the sampled data can be mathematically checked for errors all along the route to its destination, ensuring that a perfect duplicate of the original arrives at the far end. Distance no longer affects quality, and interference can be detected and eliminated.
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