This report summarizes the results of comparison testing between six Voice-over-IP products:
Pingtel Instant Xpress (a software phone on a Dell PC)
Cisco 7960 (a full-featured VoIP phone)
Cisco 7905 (a streamlined-functionalityphone)
Cisco 7935 "ConfPhone" (a conferencingphone)
Cisco ATA186 "POTS Adapter" (a device that allows a plain ordinary telephone to be connected to the Internet for VoIP use).
All these phones are real products being marketed today. All were tested under a variety of network conditions which are nearly identical to what they might encounter when operated in the open Internet. Although it is possible to upgrade network infrastructure to provide QoS guarantees, this will not be helpful in most cases. In the general case, "anyone-talking-to-anyone" communications, product performance could be affected by network disturbances. These disturbances include jitter, packet drop, reorder, duplication, and others. The report includes sound clips so you can hear and judge for yourself the voice quality results of representative conditions. We used our Maxwell to impose these real-world network conditions. The test methodology is also described.
Maxwell makes it very easy to perform side-by-side product, product-version, regression and -interoperability comparisons under controlled and realistic network conditions.
Some VoIP phone manufacturer's documentation recommends that the user's network be designed within certain quality-of-service conditions, such as jitter not to exceed 30ms, so we used those as a starting point in these measurements. We picked 25ms and 30ms. The Pingtels performed well at jitter levels far in excess of these, as you can hear from the sound clips.
The sound clips also demonstrate how combinations of network disturbances or impairments affect the phones. Individual impairments may not affect voice quality, however, in combination with other impairments, voice quality is degraded. For example, at an average jitter of 25ms, we found no audible distortion in the Cisco 7960 phones unless we also added reordering.
The sound clips are in WAV format, which most desktop computers can play. All are digitized at 8KHz, 16-bit resolution, monophonic. For reference, CD-quality sound is 44.1KHz, 16-bit resolution, stereo. You can listen to the recordings and judge for yourself. Reference recordings are also included for `best case' network conditions. You will need a PC with a sound card. It is best to listen with good headphones, rather than typical desktop PC speakers. Actual sound quality effects are more accurate iif the sound comes from a source near your ears, just as it does with a regular phone. Good headphones also block ambient noise, allowing you to hear just the recording.
To read the full report and hear the comparisons, follow this link: networkreporter.com/results/voip
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