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January 1, 2005

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
To Infinity... and Beyond!
News from the wireless marketplace...
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Essential Wi-Fi

Technology and Engineering

A fundamental specification of an 802.11 card is its receive sensitivity. The receive sensitivity is the minimum power level at which a signal can be reliably received. For example, a NIC manufacturer may indicate that their particular card has a receive sensitivity of –96 dBm at 1Mb/sec. If the actual RF energy present at that card were less than –96 dBm, then the card would no longer be able to differentiate between signal and noise. The NIC would not detect the incoming packet at all, and the packet would be lost.  But how do vendors measure receive sensitivity and what are the implications of their methods for assessing an 802.11 card's performance?

We asked a major vendor of 802.11 hardware how they measured receive sensitivity in their cards. They told us that to measure receive sensitivity, the WLAN card is placed into an RF-shielded room. This guarantees that the test signal will be the only RF transmission in the room, and no background noise in the environment will interfere with the test. The test receiver is placed on a rotating turntable so that measurements can be taken (and then averaged) for all possible horizontal orientations of the receiving antenna. The vendor then transmits packets at weaker and weaker power levels. As the power level decreases, the bit error rate as measured by the card increases. The receive sensitivity of the card will be the minimum power level at which the bit error rate remained below a certain threshold. Therefore, a lower receive sensitivity value (-93 dBm) is better than a higher one (-85 dBm), since it means that the card was able to “reliably receive” data at lower power levels.

Of course, different data rates, having more and less complex encoding and modulation methods, and being more and less resistant to corruption, will result in different receive sensitivities. As data rate increases, receive sensitivity decreases. To put it another way, the higher the data rate, the stronger the signal strength must be for the packet to be reliably received. This is why 802.11 cards drop to lower data rates when interference is present or when they are at the edges of their coverage range. For example, an 802.11b card might have specifications like this:

Receive sensitivity -95 dBm at 1 Mbps
Receive sensitivity -91 dBm at 2 Mbps
Receive sensitivity -89 dBm at 5.5 Mbps
Receive sensitivity -85 dBm at 11 Mbps

While receive sensitivity might seem like a reliable way of comparing two vendors’ cards, we know of no organization that certifies the veracity of the vendor’s results. Therefore, there is the potential for vendors to manipulate the thresholds of their tests to influence their chipset’s receive sensitivity numbers. For example, a vendor that uses a BER threshold of one error in every 1,000,000,000 bits) will end up with lower receive sensitivities than a vendor that uses a BER threshold of one error in every 100,000,000 even though the second vendor’s card may actually be better at receiving bits.  Fortunately, some vendors make their BER threshold available in their card's documentation.

To Infinity... and Beyond!

FCC Ruling Paves Way for Ground-to-Air Data Transmission

On December 15th, the FCC announced that it will auction licenses for 4 MHz of spectrum in the 800 MHz band that is currently used exclusively for Verizon's Airfone service.  These licenses will cover "voice, data, broadband Internet, etc..."

Although the air-to-ground band has existed for years, Verizon was the only vendor to make use of it.  Verizon's Airfone service is slow, limited almost exclusively to voice (although it is possible to dial up through it, the data rates are too low to be useful), and expensive.  By comparison, a new system, designed for data from the ground up, could provide access at rates of approximately 100 Kbps to 400 Kbps.  The FCC announced that Verizon's would be granted a non-renewable five-year license to continue offering its Airfone service, but that it would be limited to 1 MHz of the 4 MHz band.

In-air data services currently exist, but must either piggyback on top of Verizon's system (as Tenzing does) or use expensive satellite uplinks (as Connexion does).  A dedicated ground-to-air system would make higher data rates and cheaper access possible.

The FCC will auction the licenses for the 4 MHz spectrum in three possible allocations: a 1 MHz and a 3 MHz band, two overlapping 3 MHz bands (with a 2 MHz overlap in the middle of the band), or a 3 MHz and a 1 MHz band.  Whichever configuration receives the highest combination of gross bids will win, and no one vendor is allowed to acquire both licenses.  Some vendors have argued that a 1 MHz band is not competitive with a 3 MHz band, and so the 1/3 and 3/1 options in effect create a monopoly.  They argue that the FCC should have offered only the 3/3 configuration.

In the same press release, the FCC announced that it was considering allowing passengers to use standard wireless handsets and other devices via a picocell in the plane.  Currently, FCC regulations allow certain wireless transmitters (such as the 802.11 Access Points that have been installed in some commercial airplanes) but cell phones are explicitly prohibited.

At 802.11, we see this announcement as having great potential for convergence of wireless voice and data services.  An 802.11 AP on the plane could provide access to an 800 MHz ground-to-air uplink.  If VoIP-capable cell phones were ubiquitous, passengers could use the data connection to route their voice calls to the ground and a cellular picocell would not be necessary.  It remains to be seen whether that will actually develop.

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