How we did it and who was invited

How we tested the wireless LAN switches for their VoIP capabilities.

Network World invited 18 vendors to participate in this test: Airespace, Aruba, Avaya, Bluesocket, Chantry (purchased by Siemens after our tests concluded), Cisco, Colubris, Enterasys Networks, Extreme Networks, Foundry Networks, Legra, Meru Networks, Nortel, Proxim, Reefedge, Symbol, Trapeze and Vernier. Four vendors - Aruba, Chantry, Cisco, and Colubris - accepted our invitation.

We tested VoIP over wireless LANs (WLAN) in terms of voice quality and roaming times. For this project, the system under test (SUT) consisted of two 802.11b-capable access points and (optionally) a switch or router connecting the access points.

Besides the SUT, the test bed consisted of up to 14 WLAN handsets and the SVP Server H.323 gateway from SpectraLink Corp. as well as test and measurement equipment from VeriWave. We also used an impairment generator from Spirent Communications in some roaming tests.

VeriWave developed a VoIP Analysis Suite especially for this project. This application describes audio quality using R-value, an ITU specification derived from measurements of packet loss, jitter, and delay. The VeriWave software computes R-value from direct measurements of these other metrics. The tool also measures roaming times.

As noted in the R-value specification, there is a very strong correlation between subjective voice scoring methods such as Mean opinion scores and R-values.

We measured voice quality under a variety of conditions: With QoS disabled and enabled; with background data traffic present and absent; with various numbers of concurrent calls; and with all calls sent through a single access point, or sent across two access points and through a switch.

The simplest tests were baselines involving a single access point with QoS disabled. We brought up one call and measured audio quality over a 30-second period. Because the SpectraLink handsets use G.711 codecs, the amount of bandwidth consumed by the handsets is a constant regardless of the amount of noise on the line. Because the amount of data used by one call is relatively small - about 67K bit/sec in each direction, or 134K bit/sec total on one 802.11b channel - we did not conduct one-call tests with background data. In this test, we measured R-value, average and maximum delay, and jitter.

We re-ran the test with six and seven concurrent calls, both with and without background data present. In tests with data, we configured the VeriWave test instrument to generate a stream of User Datagram Protocol (UDP) packets at a rate of 1M bit/sec.

We then enabled QoS enforcement and repeated all the various test cases. Once this we complete, we then re-ran all tests - both with and without QoS - using two access points, with half the handsets on each access point.

To test roaming, we began by having all handsets associate to one access point and brought up calls. We then powered up a second access point and verified that no handsets "pre-roamed" prior to launching our test. Once the test measurement was under way, we powered off the first access point, thus forcing all handsets to roam to the second access point.

The VeriWave test equipment calculated roaming time at the application layer. It noted the interval between the last packet seen on a call on the first access point to the first packet seen from the other phone on the second access point. This application-layer focus meant the access points not only had to re-associate handsets but also resynchronize with the SpectraLink call server.

Back to Clear Choice Test: Voice over wireless LANs

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Copyright © 2005 IDG Communications, Inc.