Wardriving for wireless LANs 2

[Derek Fawcus]

On Sat, Jun 01, 2002 at 02:27:31PM +0100, Owen Lewis wrote:
> 
> WLAN operates for now) in the 2.4 GHz ISM band. This band is used by a wide
> range of radio systems in addition to IEEE 802.11
> systems. No licence is required to operate in this band and most equipment
> is unregulated also.

Nope use of this band is regulated and requires a licence.

Radio amatures have access to the band (secondary status if I recall).
As I recall,  they are allowed upto 300 W of transmitted power,  now
add some antenna gain, and cook a few pidgeons.

WLAN deies have access to the band under a _class licence).  Amongst
other things,  this specifies the amount of power (100mW EIRP in UK)
that can be used.  These licences cover the device as sold,  and
manufactures have to get their devices approved.

 [ Actually I'm not sure if it's the class licence that specifies the
   power levels,  or if it's covered in the ETSI document which the
   class licence references. ]

Any modification of the device (or use of excessive antenna gain)
potentially makes the operation unlicenced,  and hence illegal.

> Users of the band must accept interference as they find it.

True.  I wonder what'll happen if more licenced use of the band for
commercial communication services is allowed - i.e. the services
it's been suggested BT intend to provide.

> WLAN deconflicts its comms from other users along the following lines.
> 
> 	-	The system is frequency hopping, using (by govt requirement, I
> understand) 80 channels only and with a fixed algorithm determining the
> channel hop sequence.

That would be the FH MAC (which only gives 1 and 2 Mbps) - Hmm,  not sure
if 2 Mbps is possible with the FH MAC.  There is also the DS MAC,  this
is the one used by all devices offering 5.5 and 11 Mbps.

The DS MAC does not (usually) frequence hop - I belive there is an option
to do this,  but I don't know of any manufacturer who has produced a
frequency hopping DS MAC.

When using the DS MAC the spreading code is supposed to allow 'channel'
seperation.  Given the band range allowed in the UK,  there are only
three completely non overlapping channels available within the allocated
range.  The other 11 (of approx 14) bands overlap to some extent.

> 	-	EDC orders the retransmission of any packet not correctly received.
> 
> As WLAN splatters its momentary communication across the whole ISM band,

FH does that,  DS splatters across about 1/3rd of the band.

> it is impossible to use the band and not be constantly receiving parts
> of other people's communications. However, one may be unaware of this
> unless the density of WLANs is particularly high.

Well you notice it fairly soon,  as the perfomance degrades noticibly.

> Now, to return to the topic....
> 
> The 2.4 - 2.5 GHz band is assigned for general, unlicensed, use by the
> public. That (IANAL caveat) should be sufficient to prevent any prosecution
> being brought for the reception, deliberate or otherwise of whole or
> fragmentary communications transmitted in that band.

As I stated above - use of this band _is_ licenced.

DF