[www.connect802.com] Receive Sensitivity

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.