User guide

• Private telephony in a rural community (Section 6)
• Connecting the local telephone network to the PSTN (Section 7)
• Interconnecting two remote communities (Section 8)
Finally, we conclude with a list of useful references and pointers to resources to learn more (Section 9).
3. The magic potion
Three magic elements will enable you to deploy a flexible and self-managed telephony infrastructure:
VoIP, open standards and free and open source software.
3.1 VoIP
A general definition of Voice over IP (also known as IP telephony) is the
possibility of carrying telephone conversations as IP packets. When we
talk about VoIP, we refer to Internet telephony in the broad sense.
We are not referring to any of the concrete mechanisms available to carry voice signals over the
Internet. Dozens of technical alternatives allow such conversations to take place.VoIP alternatives can
easily be divided into two main groups: closed-proprietary and open systems. In the first group we find
the very popular Skype and the legendary Cisco's Skinny (SCCP).
2
Among the second group we find
open standards based on SIP,
3
H.323
4
and IAX2.
5
The H.323, a protocol developed by the ITU, was initially the most popular protocol because it was
widely used in the core backbones of the largest carriers. SIP has increased its popularity as VoIP has
moved to the "local loop."
6
Lately we have seen another newcomer, IAX, which is heavily based on
community development, has learned from the past, and seems to be able to address many of the
limitations of its predecessors. Although IAX is not an official standard protocol (RFC),
7
it is enormously
respected by the community and has all the pre-requisites to become a de-facto replacement of SIP.
One of the key issues with all traditional VoIP protocols is the wasted bandwidth used for packet
headers. This problem becomes more important in developing regions where access to international
2. The Skinny Client Control Protocol is a proprietary terminal control protocol originally developed by Selsius Corporation and
now owned and defined by Cisco Systems, Inc. One of the most famous Skinny clients is the Cisco 7900 series of IP
phones.
3. The Signalling Initiation Protocol (SIP) is a protocol from the IETF that describes how to handle sessions with one or more
participants.
4. H.323 is an umbrella recommendation from the ITU-T that defines the protocols to provide audio-visual communication
sessions on any packet network. It is the protocol used by famous applications as Netmeeting.
5. IAX2 is a the Inter- Asterisk protocol used by Asterisk , an open source PBX server. It enables VoIP connections between
Asterisk servers and IAX2 clients .
6. The local loop is the physical link that connects a customer to the edge of the telecommunication service provider.
7. Request for Comments (RFCs) is a series of numbered Internet informational documents and standards
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