[orbit-user] Follow up on: RSSI per received packet using Atheros

[Rodney Berriman]

thanks for your replies Kishore and Gautam.

 
We have tried to catch the RSSI in Prism2 header, having first ensured:
/proc/sys/net/ath0/dev_type = 802     as per 
http://madwifi.org/wiki/UserDocs/MonitorModeInterface

 then run command:
    tcpdump -i ath0 -vvv -s 0 -y PRISM_HEADER -e

 Typical result for a ping request packet is:

23:10:44.580785 314us DA:00:0b:6b:0a:83:78 SA:00:0b:6b:0a:83:21 
BSSID:02:0b:6b:0a:83:7e LLC, dsap SNAP (0xaa), ssap SNAP (0xaa), cmd 
0x03, IP (tos 0x0, ttl  64, id 0, offset 0, flags [DF], length: 84) 
192.168.200.109 > 192.168.200.126: icmp 64: echo request seq 5379

tcpdump version info:
   tcpdump version 3.8.3
   libpcap version 0.9.4

These are quite up to date, but I'm wondering if libpcap has the 
necessary decoder for all the fields of prism2?

There was no warning about libpcap being unable to decode, such as does 
appear if I change the format to  804 "Atheros descriptors".


Now looking through the madwifi source code to see if RSSI is hiding in 
the binary format.
Not seeing any reference to Prism2 header in the code, but maybe 
IEEE802_11 RX stats is relevant.
Is prism2 an extension of IEEE802_11 header?

/* not all hardware provides this information for frames received */  
(that might be significant!)
struct ieee80211_rx_stats {
  u32 mac_time;
  s8 rssi;   /* what we want */
  ...

Any pointers most welcome,

Rodney
 

Networks Systems Group

NICTA, Sydney


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