Re: Is there a range filter?

From: Wim Lewis (
Date: 2002-03-23 00:00:50 UTC

On Friday, March 22, 2002, at 01:14 PM, Brad Colbert wrote:
> Has anyone had experience using a prismII card with Jouni's
> HostAP driver at a distance greater that 300meters?
> Is there a time(distance) filter? If so, can it be disabled?

802.11 uses link-layer acknowledgements --- that is, the receiver is expected to send an ACK packet whenever it receives a data packet. The timings of the CSMA algorithm take this into account: after a data packet goes by, there's a window in which only the receiver is allowed to transmit (so that it can send the ACK without worrying about contention with other nodes). So if the receiver is too far from the transmitter, the ACK may not get back within its allotted time slot.

I don't know offhand what happens in this case --- I'm sure some people have tried this already. My guess is that your throughput will go way down (since the transmitter will be unnecessarily re-sending lots of packets) and the card will register lots of TX errors (since it thinks its packets aren't being received). But I'd guess that the data will in fact make it across the link, just not efficiently.

You can probably tweak the MAC's timings to get around this, or maybe trick it into thinking all your traffic is multicast/broadcast traffic --- the ACKs are only used for unicast packets. (300m might not be far enough for this to matter anyway. I don't know offhand what 802.11's timings are. 300m is about 1 microsecond at lightspeed, or 2 uS roundtrip.)


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