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May 2007 SIP Magazine
Volume 2 / Number 3
SIP Magazine May 2007 Issue

SIP and Open Source

By Richard "Zippy" Grigonis, Feature Articles


In recent years, open source telephony software has made as great an impact on the telecom scene as has Internet telephony itself. SMBs (Small and Medium-sized Businesses) are tantalized by the prospect of setting up a $900 to $2,500 IP PBX. Established PBX makers, however, are horrified that open source software has “moved the decimal point” over two places, and that an ever-expanding group of nameless, faceless people scattered around the globe are working diligently — and at little or no salary — to usurp their longtime dominance of the industry.

The earliest and premier open source PBX software package was Asterisk, which runs under another famous open source software project, Linux. Mark Spencer, the president of Digium (which makes I/O and other hardware that’s compatible with Asterisk) got the Asterisk ball rolling about seven years ago. Asterisk now offers just about anything a business would want in terms of a PBX, such as voicemail, IVR, auto-attendant, overhead paging, call parking, VoIP, PRI compatibility with many central office switches and codecs.

Ironically, Asterisk’s native protocol is not SIP, but IAX (Inter Asterisk Exchange) — IAX2, to be exact, a protocol based on UDP (User Datagram Protocol) protocol. That’s not a problem, since Asterisk can work with SIP too. Asterisk can act as a SIP client, server and media gateway. In Asterisk, every call is placed or received on a separate logical “channel” that connects the Asterisk server and some other VoIP server, such as one in a company’s branch office. The two primary Asterisk VoIP channels are for IAX and SIP. Ironically, IAX is better at penetrating any given company’s NAT (Network Address Translation) firewall barrier than SIP (or Ye Olde H.323 protocol for that matter), since it needs only a single port, UDP 4569.

SIPfoundry (http://www.sipfoundry.org), an international open-source community dedicated to speeding the adoption of SIP applications as well as the underlying technology, has focused more on SIP. To be specific, an open source, native SIP and Linux-based PBX called sipX. SIPfoundry (news - alert) calls it an ECS (Enterprise Communications Server). The system is based on SIP URI addresses that the company over time believes will replace standard PSTN phone numbers. sipX has a modular architecture and supports the exchange of just about any kind of real-time information — voice, video, IM, collaboration, etc. The modular sipX system runs on standard Intel servers and allows 12 different server processes to coexist on one server, or they can be distributed to different hardware systems. It also offers call traffic load balancing and high availability redundancy between call control components.

Instead of being based on a B2BUA (Back-To-Back User Agent, which is a SIP logical entity that can receive and process INVITE messages as a SIP User Agent Server), sipX implements a true SIP proxy architecture. This enables one to revel in some of the more interesting aspects of proxy servers, such as forking (A forking proxy can forward a SIP request to multiple SIP addresses and return the responses to the sender.)

The sipX package has the “look and feel: of an IT application, and it supports “plug and play” IP phone management. It also supports Pingtel’s ACD Call Center server.

Speaking of Pingtel (http://www.pingtel.com), they’ve taken SIPfoundry’s raw open source code and have fashioned it into enterprise-level PBX and SIP router solutions. Pingtel (news - alert) retains the SIPfoundry term ECS or Enterprise Communications Server, but Pingtel’s SIPxchange ECS is much more. Pingtel has added enterprise-grade quality, reliability, support, documentation to the original SIPfoundry system, in much the same way that Red Hat has “spruced up” the open source Linux code produced by the Fedora open source community. (Indeed, SIPfoundry was founded by Pingtel, the ReSIProcate community and some members of the Vovida community.)

Despite its many improvements, the SIPxchange ECS is still capable of providing low-cost IP PBX, key system, branch office, and call center solutions, any of which are capable of integrating with legacy TDM and IP networks.

It looks like the Pingtel’s business model of improving upon and packaging the open source telephony architecture — adding bells, whistles, documentation, support and professionalism — could the be the future method of choice for monetizing open source telephony.

Richard Grigonis is Executive Editor of TMC’s IP Communications Group.



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