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Session Initiation Protocol Phones

Mobile phone users know acceptable service involves three factors: an inexpensive plan, decent coverage and an affordable phone that can handle your needs with panache. Same with voice over IP. VoIP's protocols are coming along, and its coverage area is as big as the Internet, but the high price of VoIP phones has been a deterrent. Now that last obstacle is beginning to disappear. More vendors are adopting SIP (Session Initiation Protocol) as the standard for IP phone connectivity, which is increasing competition and bringing about potentially higher production volumes and lower prices.

With all that in mind, we decided to test low-priced SIP phones. We invited Alcatel, Avaya, Cisco Systems, Grandstream Networks, ipDialog, Mitel Networks, Nortel Networks, Pingtel, Polycom, Siemens, Snom Technology and Zultys Technologies to send their SIP phones to our Syracuse University Real-World Labs. We asked for phones that cost no more than $200. And we made it clear we would test interoperability among all the phones and performance over a simulated network, with degradation. Knowing these guidelines, only three vendors agreed to participate: ipDialog, Snom and Zultys.

The last time we tested SIP phones, we did not restrict participation based on price, nor did we undertake any performance tests. This time, we did conduct performance tests. We used an Agilent Telegra VQT with phone adapters to test voice quality and Shunra Software's Storm network-emulation device to introduce jitter and packet loss into the network. The phones fared well in spite of some poor network conditions. On most Internet connections, all would perform satisfactorily.

Our interoperability tests featured Pingtel's SIPxchange SIP server, which we thought would be a good reference for SIP implementation because Pingtel has been in the business for some time and recently made its SIP server an open-source product (you can download it at In addition, we used Pingtel's Xpressa SIP phone in our test bed. In general, we found that the phones we tested can handle features like hold, transfer and conferencing without any difficulties, even in a mixed, multivendor environment. However, message-waiting notifications posed some problems. We left voicemail messages for all the phones, and we couldn't get the MWI (message waiting indicator) to light on the ipDialog and Zultys phones. We worked with ipDialog, Zultys and Pingtel but were unable to solve the problem.

These less expensive phones have smaller screens and fewer feature buttons. And though they don't support a wide range of codecs, they do support both the G.711 and G.729 codecs for toll-quality audio. Most of the higher-priced phones we tested last year had full-duplex speakerphones, but only ipDialog offers a full-duplex speakerphone at this low price.

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