Reading the Specification Sheet

Early testing of 802.11 devices focused on range and throughput because there was little else that could be tested. In some environments, range may be an important factor, and one that can be determined in part by reading the specification sheet for a card.

Range is largely a function of receiver sensitivity, which is a measurement of the weakest signal that a receiver can correctly translate into data. Better sensitivity measurements result in improved range. (Improving sensitivity can also help other performance factors, too, but range is the easiest one to discuss.) Most vendors have focused on improving performance, and the resulting improvements (Atheros' XR, and Broadcom's BroadRange) have been announced with great fanfare.

Not all vendors publish complete specification sheets. Cisco discloses a great deal of information, with receiver sensitivities disclosed at each data rate in all frequency bands supported. (The card's 5 GHz performance has a slight dependence on the frequency.) Many vendors simply report the supported data rates without any indication of sensitivity.

Sensitivity Comparison

As an example, compare the sensitivity of some common 802.11b cards. Sensitivity is defined by each 802.11 PHY. For the direct sequence rates, it is defined as the received power at which the input frame error rate is 8%, for 1024 byte frames. The standards require that the sensitivity be -76 dBm or better for 11 Mbps, and -80 dBm for 2 Mbps.[*] Lower sensitivity is better because it means the card can receive weaker signals than required.

[*] This corresponds roughly to a bit error rate of .001% (10-5).

The sensitivities reported in Table 16-1 were taken from data sheets and user manuals for four well-known 802.11b cards and a new a/b/g card. The Cisco Aironet 350 had a reputation for pulling in weak signals well, which is entirely justified by the data. At 11 Mbps, it was sensitive to a signal half as strong as the Orinoco Gold card, and nearly a quarter of the signal required by the Microsoft card. However, the march of technology has improved sensitivities at higher bit rates. All the older-generation cards are less sensitive than the Atheros-based Cisco tri-mode card currently on the market.

Table 16-1. Sensitivity (in dBm) for various cards

Card	11 Mbps	5.5 Mbps	2 Mbps	1 Mbps
Cisco 350	-85	-89	-91	-94
Orinoco Gold (Hermes)	-82	-87	-91	-94
Linksys WPC11 (Prism)	-82	-85	-89	-91
Microsoft MN-520	-80	-83	-83	-83
Cisco CB-21 (a/b/g); 802.11b performance only	-90	-92	-93	-94

 

Delay Spread

When radio waves bounce off objects, several echos of the wave will converge on the receiver. The difference between the first wave's arrival and the last arrival is the delay spread. Receivers can pick through the noise to find the signal, but only if the delay spread is not excessive. Some vendors also quote the maximum delay spread on their data sheets. Table 16-2 reports the delay spread for three of the cards listed above.

Table 16-2. Delay spread (in ns) for various cards

Card	11 Mbps	5.5 Mbps	2 Mbps	1 Mbps
Cisco 350	140	300	400	500
Orinoco Gold (Hermes)	65	225	400	500
Cisco CB-21 (a/b/g); 802.11b performance only	130	200	300	350

Cards rated for higher delay spreads are capable of dealing with worse multipath interference. Again, the Cisco Aironet 350 was an extremely capable card for its day, capable of dealing with over twice the time-smearing as the Hermes-based card.