We judge a VoIP service's performance on how calls sound under baseline conditions, as well as during data uploads and data downloads. The overall weighted average is based on calls made under these three conditions. Baseline conditions are given the highest weight of 66 percent; audio quality during data uploads and data downloads each factor 17 percent of the weightings. The scale for the voice-quality ratings is from 0 to 10.0, with a perfect score of 10.0 equaling our reference analog connection.
(Higher scores are better)
|Overall weighted average||All PCs off||During download||During upload|
In our experience, nearly all the VoIP services we've tested deliver baseline audio quality that is almost indistinguishable from regular analog (landline) telephone connections. (We define baseline quality as the audio quality of the VoIP service when the telephone adapter (TA) is the only device sending and receiving substantial amounts of data over the local network on our tests. During these tests, the only other devices permitted to transmit and receive network traffic are the broadband modem and router.) In the case of AT&T CallVantage, the audio quality is so clear that it equals that of what we're used to from analog connections--even during heavy network traffic conditions. The only other VoIP service we've tested in which the same can be said is Vonage.
The Achilles' heel of the VoIP services we've tested, however, is that all of them exhibit a certain amount of background noise during calls. This noise amounts to a very faint hiss with CallVantage, which is perceptible only by the person on the CallVantage end of the call; the person on the other end did not hear the noise during testing. Users less sensitive to such auditory distractions might not even notice it, and it did not affect our ability to make or receive calls.
Unique among the VoIP services we've reviewed, AT&T CallVantage is the only service to come with a TA that is integrated into a wireless broadband router: The Linksys WRT54GP2A lets you connect your phone to it for use on the CallVantage service, and it's also an 802.11b/g wireless and four-port wired Ethernet router. Combining the TA and router into a single device permits the TA to have much more control over how the voice data packets are given priority in relation to other network traffic. This helps minimize the potential loss of VoIP audio quality when the local network bandwidth starts filling up with other types of data traffic, such as files being uploaded to the Internet from a PC.
Network bandwidth is a significant issue for most small-business and home-broadband users, whose upstream data throughput is not large enough to support both voice and data packets simultaneously. With many of the VoIP services we've tested, the end result is that VoIP audio quality is seriously degraded--often to the point of unintelligibility--during phone calls when a user is simultaneously generating a lot of upstream traffic from a PC (such as uploading photographs to an online photo-finishing service) while conducting a VoIP call. CallVantage did not suffer from this problem because the Linksys TA/router is designed specifically to avoid such issues; audio quality was maintained despite the additional traffic we placed on the network.
The trade-off is that the Internet data throughput speeds of your PC(s) drop precipitously when you make or receive VoIP calls. Upstream throughput saw more than an eightfold drop in performance; our 1.37MB photo upload to Ofoto.com, which usually takes only about 35 seconds, took more than five minutes to complete when a VoIP call was active. Even downstream throughput took more than a 50 percent nosedive. These throughput performance drops are the worse we've seen from any VoIP service we've tested, which is good news for voice traffic. CallVantage obviously puts its highest priority in maintaining the integrity of audio quality, even at the expense of data throughput. We saw a similar effect with Vonage, but its throughput performance hit wasn't as severely while still maintaining the audio quality. How frequently you use your PC for Internet access or upload files from your computer will help dictate if you can live with this limitation.
We also measure how quickly calls are connected by timing how long it takes from the moment the last digit of a phone number is dialed to the moment we hear ringing. CallVantage was the speediest of all the VoIP services we've tested in connecting calls. In fact, its consistent connect time of about two seconds matched that of our reference analog telephone line.
Since the Linksys WRT54GP2A is also an 802.11b/g wireless router, we tested how well it performs as a wireless router, using the CNET Labs test procedures for access points. In 802.11g mode, the WRT54GP2A-AT wireless router has one of the fastest throughput rates (25Mbps) we've seen from an 802.11g wireless router. It also has excellent range capabilities, able to sustain an 11.3Mbps throughput rate at a distance of 200 feet in 802.11g mode. In mixed mode (802.11g and 802.11b), its performance drops somewhat, as is typical for most mixed-mode routers. The throughput rate of 15.1Mbps in mixed mode is respectable, but other routers have speedier mixed-mode performance.
Performance analysis written by CNET Labs manager Daniel A. Begun and lab technician Matthew Wood.
Find out more about how we test VoIP.
Although it lacks features and ease of installation, AT&T CallVantage shines in services and support. With your account, you get 24/7 technical or repair assistance through a toll-free phone number, and 8 a.m. to 9 p.m., toll-free billing or service-management support. In addition, the CallVantage Web site hosts separate forms for both types of issues, and the installation guide and the user guide are downloadable directly from the contact page.