I did some research on the subject, and it seems that you are right.
The RSSI Value can only be read at the receiving end. This does make a
lot of sense, but I read a paper about the Algorithm I am trying to
implement, and the Author stated it would be possible to read the RSSI-
Value at the senders end. Since the author is a professor at a German
university, I figured he knew what he was talking about. I reworked my
way through the Paper and realized, that there is another thing that
you can't just read at the senders end. He uses the hopcount of the
connection for his calculations. That's an Information that is
actually impossible to get without the support of the routing-layer.
Since I need to modify the routing-layer anyways now, I'll just modify
it in a way, so that I can read the RSSI-Information of the first hop
too.
Thank you for your help.
Tim Schneider
Am 13.07.2009 um 13:57 schrieb Richard Farina:
Tim Schneider wrote:
Am 13.07.2009 um 08:59 schrieb Richard Farina:
I don't mean to make this a flame so please do not take it that
way. How exactly do you expect to get the RECEIVED Signal
Strength indication for a packet which you are SENDING? If you
send it, you don't receive it...
Just my 0.02
-Rick Farina
Hi Rick,
as far, as I understood, the RSSI-Mechanism also displays the
signal strength of the just sent package. I figured there was some
way the MAC-Layer transported that information, but you're right, I
could be mistaken. Anyways, since I'm trying to implement a TCP
Algorithm, i definitely have a two-way connection. Assuming, that
the route back is going to be the same as it was on the way there
(which basically should be true, based on the Routing algorithms
we're using in our wireless mesh network), I could just use the
RSSI Value of the received ACK.
Any RSSI on a sent packet will be a calculated number not an
observed number and as such it will be completely pointless for any
kind of rate control algorithm. Assuming that the card could
actually receive the packets it is sending (which it can't) then you
would be observing a 24dBm signal (for instance) with 0 patch loss
and your RSSI would be at or above max. Realistically you can
assume that every packet you sent has a max RSSI on your end because
you are the one transmitting it. Now, if you want to know what the
RSSI on the receiver side is that is a completely different
question, one which you cannot answer at all. When you send a packet
you do not have the good fortune to know what RSSI the receiver
got. You can roughly assume based on your RSSI when you get an ACK,
but honestly that isn't even close to correct because every packet
has a very unique experience in the air and you cannot assume things
like the same tx power and rx sensitivity will be the same on both
ends. There is some 802.11 protocol (the letter eludes me at
present) which is in draft form which adds information to packets so
that both ends know each other's RSSI, but I have never seen it
implemented anywhere yet.
I advise you keep the discussion on list as I am by no means an
expert and many people much better at this than I may respond.
-Rick Farina
Regards,
Tim Schneider
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