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Will phasing in new Cisco devices interfere with network performance?

Wireless expert Lisa Phifer explains how phasing in new Cisco devices might nterfere with network performance while your system continues to run.

I need to keep a Symbol Spectrum24 system running while I install a Cisco AP1242G system. I need to allow old Symbol devices run while slowly phasing in new Cisco 802.11g devices. What can I expect in terms of performance and interference since they both run in the 2.4 range? Will they be fighting each other? Do you have any documentation or white papers on this subject?

Products that followed the original 802.11 standard could transmit data at 1-2 Mbps using one of three physical layers (PHY) specifications: Infrared (IR), frequency hopping spread spectrum (FHSS), and direct sequence spread spectrum (DSSS). Products that implemented the 802.11b amendment could transmit data up to 11 Mbps, but only with DSSS. Products that support the 802.11g amendment still use DSSS for backwards compatibility with 802.11b, but can transmit at rates up to 54 Mbps using orthogonal frequency division multiplexing (OFDM).

Your old Spectrum24 APs use FHSS. Your new AP1242G APs can use both DSSS and OFDM. Because your old and new APs use different modulation techniques, they cannot communicate with each other over the air. However, because they share the 2.4 GHz frequency band, they will interfere with each other.

This is like having two tour guides in the same room, one speaking loudly in English, the other speaking loudly in Japanese. If both guides speak simultaneously, that makes it hard for everyone to hear, requiring the guides to repeat themselves at least some of the time. Likewise, when your FHSS AP transmits at a frequency used by your DSSS/OFDM AP, this "noise" will cause frame loss and retransmission.

The protection mechanism used by 802.11g to coexist with 802.11b cannot be understood by your FHSS APs. Specifically, when an 802.11g device has data to transmit, it first sends a Request To Send (RTS) or Clear To Send (CTS)-to-Self control packet over DSSS. This is like having the Japanese guide speak just enough English so that the two can take turns speaking without interrupting each other. But since your APs do not share a common language, they cannot take turns this way.

FHSS hops around the 2.4 GHz spectrum, following one of 66 defined sequences, each composed of up to 79 frequencies. Many FHSS APs can be installed in close proximity because they will follow different hop sequences. If two FHSS APs happen to hop to the same frequency, the dwell time (and therefore interference between them) is very short. When an FHSS AP hops to a channel used by a DSSS/OFDM AP, it will also encounter narrowband interference, but only briefly. As a result, your old APs should continue to operate with only minor degradation in the presence of your new APs. However, their performance will decline as your new APs become busier and transmit more often.

Unfortunately, DSSS and OFDM are less resistant than FHSS to narrowband interference. Your new APs will be tuned to one of 11 channels in the 2.4 GHz band, where each channel is much wider (+/- 22 MHz) than those FHSS 1 MHz hops. Not only will they encounter interference fairly often, but that interference won't stay put, letting the 802.11g AP re-tune to a clear channel. As a result, your new APs will need to rely on error correction to reduce interference impact. OFDM is far better at error correction than DSSS, so you may want to configure your new APs to use only the higher OFDM rates and resist backing down to lower DSSS rates.

I assume that your new APs must cover the same area as your old APs. But if not, separation of coverage areas -- either by moving APs or turning down their transmit power -- could also help. Another possibility would be to move your new WLAN to the 5 GHz band by using AP1242AGs instead of AP1242Gs, avoiding interference completely.

Because you are dealing with older FHSS equipment, you cannot take advantage of some of the newer adaptive techniques used by more recent FHSS equipment -- notably Bluetooth devices that also the 2.4 GHz band. For further information, see IEEE 802.15.2-2003, coexistence of wireless personal area networks with other wireless devices operating in unlicensed frequency bands.

This was last published in August 2007

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