The Hitchhiker's Guide to Asterisk | ||
---|---|---|
<<< Previous | Introduction to Asterisk | Next >>> |
To summarize, a channel generally consists of either an analog signal running on POTS (or Plain Old Telephone System) or some combination of codec and signaling protocol, ie. GSM and SIP, or, ULAW and IAX.
A channel is a voice path equivalent to a phone line between two points. There are many different ways the information can be sent, but often is split into two groups -- analog and digital. Analog data is the type of signal that has been used on the phone system since it was invented. It can be prone to noise and echo, and can not be sent as-is over a digital network in its raw form. Digital data consist of ones and zeros. Analog data as picked up from a microphone must be converted into a series of discrete levels, or quantized, to be able to form a digital signal. Once the data is in a digital state it will require a fair amount of bandwidth (relatively speaking) to send as-is; 64kbits/sec for uncompressed voice data sampled at 8KHz with 8bits resolution.
Obviously it is desirable to fit as many calls as possible into a data network. This is done by encoding it into a form that takes up much less space using a codec (short for COder/DECoder). Some examples of these are ulaw, alaw, gsm, ilbc and g.729. Codecs determine the sustained data bit rate which is required for each channel. Different codecs have different advantages but are independent of the type of protocol that is used to establish the channel. The codec converts the analog voice signal to a digitally encoded one. The quality, databit rate required and the computational requirements vary from one codec to the next.
Sending data to another phone would be easy if the data found its own way there and knew what to do at the other end. Unfortunately it doesn't which is why we use a signaling protocol to encapsulate the voice data. The common signaling protocol used today is SIP (an acronym for Session Initiation Protocol). Other protocols that Asterisk support include IAX, H.323 and CAPI. CAPI is a special case in that it is used within a computer system to deal with ISDN interfaces.
<<< Previous | Home | Next >>> |
Introduction to Asterisk | Up | Key Components |