802.11a: First Glimpse – part 1

It’s finally in silicon. The long-awaited 54Mbps technology that’s expected to light up the 5GHz radio spectrum is available and being tested by at least one chip supplier and its customers. Now, the lively – and hitherto theoretical – debate on the relative capabilities of 802.11a and 802.11b can be waged on the basis of real-world observation.

Atheros Communications paid a visit to 802.11 Planet late last week and gave us a demonstration of its single-card, “reference design” 802.11a components, based on early production runs of the Atheros AR5000 chipset, which will be shipping in quantity to OEM customers soon. Atheros has been using the hardware, based on its two-chip solution, in its own internal network for some time.

The demo consisted of the famous Madonna BMW video beamed from a notebook server to an 802.11a access point and then to another notebook workstation. We carried the workstation around our offices with some freedom within a range of 75 feet or so with no deterioration in quality until very substantial impediments (heavy concrete walls) interrupted the signal.

Following the film credits, product manager James Chen discussed with us the company’s early, in-house testing findings, at some length. Today, we’ll look at three key issues: 802.11a’s effective range, power consumption, and the cost of the hardware. Tomorrow, we’ll look at some additional topics, including a “turbo” mode that allows data links at up to 108 Mbps!

Circles within circles
Pre-release popular wisdom holds that 802.11a should offer a much smaller radius of coverage than does 802.11b technology, which is generally held to be effective up to about 300 feet or so. Theoretical calculations put 802.11a coverage at roughly one-fourth of that range. But in systematic empirical tests carried out at Atheros’s offices, technicians found that 11a operates with acceptable reliability to well over 200 feet. Moreover, throughout most of its range, including the maximum, it offers a throughput advantage over 802.11b at the same distances.

According to Chen, typical 802.11b deployments use a cell radius of about 65 feet (based on access points with power in the 15dBm range), which ensures reliable coverage at the maximum data link rate of 11Mbps. Beyond 100 feet or so, an 802.11b link steps down to 5.5 Mpbs, and at roughly 175 feet, to 2 Mbps.

With 802.11a, on the other hand, Atheros testing established stepdown points at about 20 feet (from 54Mbps to 48Mbps), 40 feet (to 36 Mbps), 75 feet (to 24 Mbps), 85 feet (to 18 Mbps), 135 feet (to 12 Mbps) and 175 feet (to 6 Mbps).

The points Chen chose to stress are that at the 65-foot “sweet spot” distance, 802.11a appears to hold about a 3.5x data-rate advantage, and even out at the 225-foot distance, it betters 802.11b by about 3 to 1. (At certain other points in the curve, the advantage is less.)

Cost questions
Based on these range-performance figures, Atheros now calls into question another commonly held notion: that 802.11a is less power-efficient than its 2.4GHz cousin. The company’s argument is based not on power consumption per time unit, but power consumption per megabyte of data transfer.

Assuming radio activity of anywhere between 10 and 30 percent of operating time, Atheros calculates that the energy cost of sending/receiving a megabyte of data (at maximum data rate) is between four and nine times greater for 802.11b equipment than for 802.11a.

Then there’s the question of the cost of the hardware itself. Product manager Chen was quick to point out that as a chip supplier, Atheros does not control pricing at the end-user level. (Although the company’s decision to use relatively inexpensive CMOS chip-making technology was consciously aimed at minimizing hardware costs.) Nonetheless, Chen said, the companies that will be selling products based on the Atheros chipset – which include Intel, Proxim, and TDK – have stated intentions to price their hardware “similarly” to 802.11b equipment. We won’t have to wait long to find out what this means.

Check this column tomorrow for more about 802.11a

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