Bandwidth Requirements

It may be obvious - however judging from some of the mail I get on the subject it may not be - but the key rule for successful wireless streaming is:

The available bandwidth of the connection must be sufficient to support the bandwidth required by the content.

Figures 6 and 7 illustrate the rule, showing IxChariot plots of a single wireless connection between the "Home" WRT54G and WPC54G client card. Figure 6 starts out with an IxChariot throughput.scr script running TCP with a file size of 100,000 Bytes. You can think of this as simulating a long file download. After about 45 seconds, I started streaming the test video over the same connection to the same client.

Figure 6: Shared data and video stream - 11g WLAN (click image to enlarge)

While it's not a one-to-one match, if you take the bandwidth profile of the test file in Figure 1 and flip it upside down, it corresponds reasonably well to the "bite" taken out of what would otherwise be a straight 21 Mbps throughput line. Another check is that the throughput drops by about 9 Mbps - the average throughput of our test video file.

What was surprising here is that the video stream takes all the bandwidth it needs, dropping the speed of the IxChariot throughput script, i.e. our file download. This happened with the Wireless Quality of Service (QoS) / WMM mechanism in both routers disabled and with no sign or QoS or WMM in the client card properties or settings. So it appears that the available bandwidth for streaming in this particular WLAN is the total bandwidth. This polite behavior isn't guaranteed to occur, however, especially in WLANs that use a mix of clients and APs.

Because there is enough bandwidth to support the video stream, the video plays with no problems - as if there were nothing else happening on the WLAN. But if you were little Susie, running your nightly BitTorrent session, you would notice the drop in Torrent download speed.

Of course, if there isn't enough bandwidth to go around, miracles don't occur. Figure 7 shows what happens when the Linksys router is forced to run in 802.11b mode.

Figure 7: Shared data and video stream - 11b WLAN (click image to enlarge)

This time, the IxChariot script gets only about 5 Mbps of bandwidth, typical of an 11b connection. So when I start streaming the video - which needs an average of 9 Mbps or bandwidth - at about the 15 second mark, the result is not surprising. The stream tries take as much bandwidth as it needs, but comes up short and an unwatchable video results (Figure 8).

Figure 8: What happens when you run out of bandwidth

Tags: Tutorial, Video streaming, WiFi,

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