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Be Wary Of Potential FHSS Interference

In the old days, frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) were the only two radio-based 802.11 wireless LANs available. At one point, I remember FHSS being among the top choices when deploying a wireless LAN because FHSS access points were less expensive and more available than DSSS access points. Then, the high rate version of DSSS (802.11b) and orthogonal frequency division multiplexing (OFDM) in 5GHz band (802.11a) came along. Of course the fairly recent ratification of OFDM in the 2.4GHz band (802.11g) has also been added to the pool of wireless LANs alternatives.

Not many of the wireless LAN vendors sell FHSS wireless LANs today because the hardware is rather costly, and almost everyone is deploying OFDM-based 802.11g. FHSS networks are still out there, however, in hospitals, warehouses, and manufacturing plants, supporting applications that have been around for a decade or so.

FHSS Basics

FHSS delivers 1Mbps and 2Mbps data rates in the 2.4GHz band. FHSS systems transmit data by hopping from one channel to a different channel (with a total of 79 channels in the U.S.) at least every 0.4 seconds according to a particular hopping sequence that uniformly distributes the signal across the entire 2.4GHz frequency band.

In comparison, 802.11b/g systems are set to a specific frequency, such as channel 6, and only transmit a signal that occupies roughly one third of the 2.4GHz frequency band. The 802.11 standard defines sets of specific hopping sequences that are designed to minimize interference among FHSS systems; however, the 802.11 standard offers no provisions for minimizing interference with 802.11b/g networks.

Impacts of FHSS Interference

Because FHSS wireless LANs transmitting a frame spread there signal power over the entire 2.4-GHz band, FHSS hops all over the narrower 802.11b/g signals. No matter what channel you set in an 802.11b/g access point, the FHSS signal is always present. An 802.11b/g signal only interferes with roughly one third of the FHSS signal, however, which doesn't cause much damage. As a result, FHSS interferes much more with 802.11b/g rather than the opposite. Keep in mind that Bluetooth and some 2.4GHz cordless phones also utilize FHSS, which can cause similar interference to 802.11b/g wireless LANs.

In practice, I've seen cases of FHSS systems interfering with 802.11b/g. I've tested it in lab scenarios and assisted several of my clients with mitigation efforts. For example, I recently performed an RF site survey and wireless LAN design for a hospital, and we found that the existence of a FHSS wireless LAN throughout the emergency ward of the hospital was causing and approximately 40% decrease in throughput on the 802.11b/g network we were testing in the same location.

In most cases, FHSS interference causes lower throughput and not loss of data. FHSS and 802.11b/g networks can't understand each other, so they may transmit 802.11 frames at the same time, which causes bits errors and frame re-transmissions. Data will not be lost unless the severity of interference causes the 802.11b/g system to exceed the frame retransmission limits, which is often set to three tries before the 802.11 radio device gives up. If the retry limit is exceeded, then a higher-level protocol, such as transmission control protocol (TCP) generally provides error recovery.

Tips to Consider

The following are tips to consider when a FHSS system is operating within the same vicinity as your 802.11b.g wireless LAN:

  • When deploying a wireless LAN, always conduct a RF site survey, and analyze the presence of interfering RF sources. Understand that if FHSS networks are found operating in the local area, then you'll probably need to live with lower performance on an 802.11b/g network.

  • Since FHSS spreads signal power over the entire 2.4GHz band, changing the 802.11b/g access point RF channel will not help. So, don't bother trying to find an optimum channel.

  • If the resulting degradation of performance is acceptable, then there's probably no need to take any action. The presence of FHSS interference, however, could prompt you to install 5GHz (802.11a) wireless LANs, depending on the situation. The 5GHz band is relatively free from sources of RF interference, including FHSS systems.

    As usual, you need minimize risks by carefully planning a wireless LAN deployment and make certain that issues, such as the presence of FHSS systems, are not going to be a problem.

    Jim Geier is the principal of Wireless-Nets, Ltd., a consulting firm focusing on the implementation of wireless mobile solutions and training. He is the author of the books, Wireless LANs (Sams) and Wireless Networks - First Step (Cisco Press).