2.4 GHz Mutual Interference

Is there anything that can be done to filter the
interference between wireless network and the wireless
household phones? Is the only or best solution to scrap
my relatively new 2.4 multi-station phone system for a
newer 5.8GHz model? JAJ

Dear John,

The majority of cordless phones in use today are 2.4GHz
models. These phones reap devastating effects on
802.11b/g WLANs. Some 802.11b WLANs have even been
totally shut down by somebody simply answering a 2.4GHz
cordless phone. Why? The answer lies in the technology.

Most 2.4GHz phones use frequency hopping spread spectrum
(FHSS) technology that operates within the 2.4GHz
spectrum. FHSS hops from frequency-to-frequency across
the entire 2.4GHz spectrum. 802.11b/g WLANs, on the other
hand, use direct sequence spread spectrum (DSSS), which
transmits within approximately one third of the 2.4 GHz
spectrum. Because FHSS jumps across the entire spectrum
while DSSS stays in only one portion, a FHSS 2.4GHz will
clobber an 802.11b/g network, causing interference or
even failure.

The jury is still out on whether 5GHz cordless phones
have any detrimental effects on 802.11a WLANs. If
interference issues do arise, however, they will most
likely be easier to correct than with 802.11b/g. 802.11a
uses more non-conflicting channels within the 5GHz
spectrum as compared to 802.11b/g in the 2.4GHz spectrum.
802.11a enjoys the luxury of 12 independent, non-
overlapping channels. 802.11b/g has 11 channels in the
U.S., but only three don't overlap (channels 1, 6, and
11).

What can you do to eliminate the effects a cordless phone
has on your WLAN? There are a number of solutions, some
of which are guaranteed to work and others that may
significantly lower the interference level. The fail-safe
solutions all consist of replacing equipment.

If your interference is coming from a 2.4GHz phone, the
sure fire solution is to switch the WLAN to 802.11a. This
solution is the most effective, but also the most
expensive.

Before spending lots of money on new hardware, consider
the following less expensive (but possibly less
effective) approaches:

With 802.11b/g systems, try changing the channel on the
access point (at least try 1, 6, and 11). Anecdotally,
people have found that they have the least interference
on channel 11.

With 802.11a systems, change to any one of the channels
that are not in use by the cordless phone.

Change the location of the access point or the cordless
phone base. Maximize the distance between the cordless
phones and WLAN devices, especially WLAN users operating
at the fringes of access point range.

If you have the option, use an external, remote antenna
on your computer. This will let you place the antenna as
far away as possible from a cordless phone.

Operate the cordless phone with the antenna in the
lowered position if possible. This will dampen the RF
signal coming from the cordless phone and reduce the
amount of interference.

Finally, transmitted signal are POLARIZED in the
direction of the antenna. Since most cordless phones
have the antenna VERTICAL to the user, place your
antennae (adapters and base station) HORIZONTAL.

Good Luck!


>-----Original Message-----
>Is there anything that can be done to filter the
>interference between wireless network and the wireless
>household phones? Is the only or best solution to scrap
>my relatively new 2.4 multi-station phone system for a
>newer 5.8GHz model? JAJ
>.
>