Comms chips

A Tale of Two Throughputs

3:00 PM -- I recently completed a major testing project of most of the MIMO WLAN products on the market. One interesting element in this was reading the boxes the products came in. I noticed liberal use of specs like “270 Mbit/s” and “300 Mbit/s.” Of course, I saw throughput of nowhere close to those numbers. I have started to refer to specs like these as “the throughput the marketing department guarantees the product will never exceed.”

So, what are these numbers, anyway? Are they lying to us?

In a word, no. But they are potentially misleading. Numbers like these most certainly represent throughput that you’ll never see at Layer 7. Under ideal circumstances, through, you may see them at Layer 1. If you’re not an engineer, Layer 7 is the Application layer of the ISO OSI model -- that part of a network’s protocol stack that talks to an application. This is the only throughput that ultimately matters.

Layer 1 is the Physical Layer -- the radio signaling itself. But note that the PHY carries everything -- user data, network protocols, control messages, everything. In short, there’s a lot of overhead in any network architecture; 802.11/Wi-Fi is no exception. And the PHY signaling rate can and will change over time, upshifting and downshifting in response to changing radio conditions resulting from a variety of technical and environmental artifacts.

So, what you get at Layer 7 is a lot less than the peak possible at Layer 1. And, by the way, the same is true on wire. Those are, after all, radio waves moving over copper or fiber. And, yes, given a particular standard, some vendors do a much better job of building products that narrow the gap between Layer 1 and Layer 7 and maximize throughput regardless.

— Craig Mathias is Principal Analyst at the Farpoint Group , an advisory firm specializing in wireless communications and mobile computing. Special to Unstrung

wstargardt 12/5/2012 | 3:44:21 AM
re: A Tale of Two Throughputs I believe that misuse of the Layer 1 raw signalling rate is broader than Craig indicates. In addition to the overhead of higher layers, these channels are also often shared among multiple users under contention access mechanisms, further reducing observed throughput. This is especially evident when using broadband cable and cellular data services.
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