We all know that there are trade-offs when using wireless communications. We would never tolerate the quality of our cell phones on land lines and we know that our wired Ethernet connections have better raw throughput than wireless.

In both cases, though, flexibility and mobility trump reliability and quality. But when it comes to wireless LANs (WLAN) it might be time to reconsider some long-held beliefs. For certain applications, what we accept as “normal” behavior on WLANs might be deemed close to a communications breakdown.It’s all about packet loss.

Considered an abnormal condition on wired connections, it is “the nature of the beast” on WLANs. As waves propagate they lose strength, travel through obstructions and/or encounter interference. Thus, it is not to be expected that every packet that is transmitted will necessarily be picked up by its intended recipient. This fact of wireless life has not been a problem. But is it? Our conversations with a recent WLAN vendor client has caused us to reconsider the situation.

For connection-oriented protocols, packet loss is but a nuisance. While overall performance can degrade if excessive retransmissions are required, the data stream stays intact because higher-level sequence numbers are matched to assure that, eventually, every packet is received. But what about stateless, UDP streams where there is no connection-level intelligence?

When a receiver processes UDP packets, it has no way of knowing whether there are missing packets. It just processes what it has. As this situation applies to say general-purpose VoIP and video over IP, it is nothing more than a nuisance. The loss of a single packet might not even be noticed by the listener as our senses have the ability to “fill in” audible and visual sources to some extent. A string of dropped packets usually results in the inevitable “you are breaking up.” As with TCP traffic the “higher level sequencer” exists — only this time it consists of the conversation participants rather than the protocol.

With video over IP, we have a similar situation — our eyes can ignore or fill in for a momentary “glitch” in the video. With video, however, even the loss of a single packet can cause a more significant degradation than with VoIP. Because of the vast amount of data required to represent full motion video at roughly 30 (video) frames per second, virtually every video transport will use data compression to reduce the stream to a more manageable level. A common approach is to use a “key frame” that contains complete video information followed by a number of frames that transmit only the modifications to that key frame (and are thus less data intensive). All good, but if your “key frame” is the one that is lost, a small glitch could turn into a big glitch — known in the trade as an “artifact.”