ATech Machine LIBRA-02 M Technical Information PDF herunterladen (Seite 653)

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APPENDIX B

Protocols for VoIP

The Internet is a telephone system that’s gotten uppity.

—Clifford Stoll

The telecommunications industry spans over 100 years, and Asterisk integrates most—

if not all—of the major technologies that it has made use of over the last century. To

make the most out of Asterisk, you need not be a professional in all areas, but under-

standing the differences between the various codecs and protocols will give you a

greater appreciation and understanding of the system as a whole.

This appendix explains Voice over IP and what makes VoIP networks different from

the traditional circuit-switched voice networks that were the topic of Appendix A. We

will explore the need for VoIP protocols, outlining the history and potential future of

each. We’ll also look at security considerations and these protocols’ abilities to work

within topologies such as Network Address Translation (NAT). The following VoIP

protocols will be discussed (some more briefly than others):

• IAX

• SIP

• H.323

• MGCP

• Skinny/SCCP

• UNISTIM

Codecs are the means by which analog voice can be converted to a digital signal and

carried across the Internet. Bandwidth at any location is finite, and the number of

simultaneous conversations any connection can carry is directly related to the type of

codec implemented. We’ll also explore the differences between the following codecs

in regard to bandwidth requirements (compression level) and quality:

• G.711

• G.726

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