The Wireless Question: 802.11a vs. 11g

By Jim Geier | Posted April 26, 2004
In my previous tutorial, I discussed how to setup a home wireless LAN. Let's take a closer look at the range and performance that you can expect from a single access point in a home office environment.

For this testing, I setup a typical home variety dual-band wireless LAN router in the center of the first floor of a 2,700 square feet two story buidling. The router implements 802.11a and 802.11g, and channels were set to 6 and 52, respectively. Transmit power was set to maximum for both. Standard omni-directional antennas and default router configurations were in use. In order to measure signal strength, noise level, signal-to-noise ratio (SNR), packet retries, data rate, and throughput, I made use of Airmagnet Trio.

In order to achieve best results, I made sure that RF interference was kept to a minimum. There were no microwave ovens or cordless phones in use, and Airmagnet indicated no other access points in the area.

Best-Case Results
I first took measurements within a couple feet of the router. This of course is where the signals levels are highest, where you'll get optimum performance. The following are readings at this point:

 

802.11a

802.11g

Signal Strength

-36dBm

-36dBm

Noise Level

-95dBm

-86dBm

SNR

59dB

50dB

Packet Retries

0%

3%

Data Rate

48Mbps

48Mbps

Throughput

2478pps

2164pps

Even though both 802.11a and 802.11g were operating at 48Mbps at this point, the throughput of 802.11a was higher at 2478 packets-per-second (pps). The reason for this is that the noise level (-86dBm) in the 2.4GHz band (802.11g) was considerably higher, which resulted in lower signal to noise ratio (SNR) and corresponding higher packet retries. 3% of the 802.11g packets sent had to be retransmitted.

Worst-Case Results
I took many other measurements from different places around the home, such as in the far reaches of the basement and within bedrooms upstairs. The following shows the poorest readings that were found, which was in the corner of the master bedroom on the second floor:

 

802.11a

802.11g

Signal Strength

-68dBm

-64dBm

Noise Level

-95dBm

-90dBm

SNR

27dB

24dB

Packet Retries

0%

8%

Data Rate

36Mbps

48Mbps

Throughput

1877pps

1628pps

The signal strengths for both 802.11a and 802.11g are plenty high enough to maintain continuous associations. The data rate of 802.11a, however, shifted down to 36Mbps due to relatively low signal strength. The lower noise level in the 5GHz band, though, kept 802.11a packet retries very low (0%) as compared to the higher retry value (8%) found with 802.11g. This led to 802.11a having higher throughput at 1877pps, despite the fact that the 802.11a data rate had downshifted to a lower rate than 802.11g.

What does all this mean?
The most significant conclusion that I draw from this testing is that a single 802.11a or 802.11g router (or access point) is enough to provide good performance throughout an entire home. This assumes, however, that you can install the router in a central location.

The router may need to be next to a wall on one side of the house, nevertheless, if that's where you connect to the broadband service. This could make signal levels on adjacent corners of the house too weak for some applications. The presence of RF interference from operating microwave ovens and cordless phones will also causes packet retries to increase, which lowers throughput.

Something else that these results indicate is that 802.11a offers higher throughput in all parts of the home. At best, throughput of 802.11a was 16% better, which slightly increases file transfer speed. Keep in mind, through, that 802.11a will not experience the dramatic drop in throughput that 802.11g does when RF interference is present.

Which one is best for Home Offices?
802.11g makes most sense for use in homes. This standard supports laptops with 802.11b radio cards, and 802.11g wireless clients will have good performance and be able to interoperate with the prevalent 802.11b/g wireless networks found in enterprises and public hotspots. If you're a heavy user of 2.4GHz cordless phones, though, strongly consider using 802.11a.

Article courtesy of Wi-Fi Planet.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers training focusing on wireless LANs.

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