Page 214 - Asterisk™: The Future of Telephony

• G.711
• G.726
• G.729A
• GSM
• iLBC
• Speex
• MP3
We will then conclude the chapter with a discussion of how voice traffic can be routed
reliably, what causes echo and how to deal with it, and how Asterisk controls the au-
thentication of inbound and outbound calls.

The Need for VoIP Protocols

*
The basic premise of VoIP is the packetization of audio streams for transport over
Internet Protocol-based networks. The challenges to accomplishing this relate to the
manner in which humans communicate. Not only must the signal arrive in essentially
the same form that it was transmitted in, but it needs to do so in less than 150 milli-
seconds. If packets are lost or delayed, there will be degradation to the quality of the
communications experience, meaning that two people will have difficulty in carrying
on a conversation.

The transport protocols that collectively are called “the Internet” were not originally
designed with real-time streaming of media in mind. Endpoints were expected to re-
solve missing packets by waiting longer for them to arrive, requesting retransmission,
or, in some cases, considering the information to be gone for good and simply carrying
on without it. In a typical voice conversation, these mechanisms will not serve. Our
conversations do not adapt well to the loss of letters or words, nor to any appreciable
delay between transmittal and receipt.
The traditional PSTN was designed specifically for the purpose of voice transmission,
and it is perfectly suited to the task from a technical standpoint. From a flexibility
standpoint, however, its flaws are obvious to even people with a very limited under-
standing of the technology. VoIP holds the promise of incorporating voice communi-
cations into all of the other protocols we carry on our networks, but due to the special
demands of a voice conversation, special skills are needed to design, build, and maintain
these networks.
The problem with packet-based voice transmission stems from the fact that the way in
which we speak is totally incompatible with the way in which IP transports data.

* This word hasn’t quite made it into the dictionary, but it is a term that is becoming more and more common.
It refers to the process of chopping a steady stream of information into discrete chunks (or packets), suitable
for delivery independently of one another.

186 | Chapter 8: Protocols for VoIP