Dag wrote :
> Hi,
>
> Since Jean is so active these days, I would like to ask him a question
> which might be interesing for others as well.
>
> Can BT live together with WLAN technologies using FHSS, or will BT
> interfere with it? I have heard somewhere that BT is much more aggressive
> than WLAN, and might therefore give bad performance for a WLAN installation
> when people start carrying BT devices in their pockets.
>
> Is there any truth in this, or will BT and WLAN (802.11 FHSS) live happily
> side by side?
>
> -- Dag
First let's start with what I think about BT :
o As a point to point link between two device, it's bloated
and far too complex (the spec is 1000 pages, plus another 500 pages of
profiles). 14 packet type at the MAC layer and more than 40 packet
types at the LLC layer, half a dozen different modes of operation. And
IrDA was complex ?
o As a low speed Wireless USB, you may get it to work on good
days (I mean, after much comliance testing).
o As a networking technology, it's crap. No native IP support,
no broadcasting and many limitations. And there is bugs in the spec
(some procedure are kind of fragile).
o User interface (how the user interact with the technology)
is the biggest issue, and BT doesn't seem to have worked much on
that. For most user, apart some preconfigured configuration (like
bundle phone dongle + headset), it might just be too complex and
impossible to understand.
Now, to your question. BT use the same band as 802.11 (and
other WLAN). And the two won't like each other at close range.
802.11 will suffer because it's packet are long and BT is
jumping around fast, so there is a high probability of getting your
packets hit. My simulation show a 50% throughput degradation for a FH
system in the worst conditions (not published yet).
BT will suffer as well. BT has a low power and poor channel
rejection, so when tramsitting on the same channel BT will always
loose the packet but even if 802.11 transmit in an adjacent channel,
the BT receiver will get swamped by the high power transmissions of
802.11. Furthermore, the BT spec is fragile and there is many points
where the designers have assumed that no packet get lost (when
switching mode in particular), so this will create delays in
establising connections and making discoveries.
Of course, this apply only if the BT network is close to a
802.11 node. If the 802.11 node is further away than a few meter,
there won't be any interference.
But don't worry, BlueTooth is not there yet, and we can
continue to play safely with our WLAN (now at 11 Mb/s). BT hardware is
predicted to be available next summer, but they surely won't debug the
software and provide interoperability for Chrismas 2000. After that,
802.11 will be so cheap that BT might be irrelevant...
Note : you can buy some 802.11 FH for $75/node retail
(WebGear). Projected BOM of first generation BT is $20/$25 (so when
you add margins and profits or manufacturer and retailer, it doesn't
cut the mustard).
On the other hand, a question for you Dag : I'm very curious
about USB FIR dongle. What do you think of it ? When available ? If
they can get prices in the $50 region, that might be a hit...
Jean
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