meaning behind the iwlist output

When I run "iwlist wlan0 scan" I get a display of information for all
the WAPs my wireless card can see. All the displayed information I can
understand except for the last two items, Extra:bcn_int and
Extra:atim. What do these two parameters mean and or indicate?

Cell 01 - Address: xx:xx:xx:xx:xx
ESSID:"xxxxxxx"
Protocol:IEEE 802.11g
Mode:Managed
Frequency:2.462 GHz (Channel 11)
Quality:57/100 Signal level:-59 dBm Noise
level:-96 dBm
Encryption keyn
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/
s
9 Mb/s; 12 Mb/s; 18 Mb/s; 24 Mb/s; 36 Mb/
s
48 Mb/s; 54 Mb/s
Extra:bcn_int=100
Extra:atim=3

Call 02 ...

It's the beacon interval. At each bcn_int interval (miliseconds) the
access point sends keep alive beacon.
atim is not common. I got 3 different wireless adapter and none of
these show this parameter.