I have this problem
the “device” is router in repeater mode D-link dir300 with dd-wrt firmware…
dont know why., mikrotik all time disconnect d-link , and connect it fast back…
what’s in the registration table? maybe signal is very bad. also enable wireless debug logs:
http://wiki.mikrotik.com/wiki/Manual:Wireless_Debug_Logs
reg table
http://www.hellbani.com/signal.JPG
with debug ., now put rule..
http://www.hellbani.com/debuglog.JPG
on the other side is D-link DIR300 ., with firmware of dd-wrt ., but before that was " airlive 5460 " had same problems 
but with " linksys - tomato firmware " that DIR300 works good… not disconnecting
what device is on the other side? could be that device which is having problems. also check debug logs like I suggested
or maybe that isnt big problem ? ., its only 1 sec… on arilive was 10 sec or more… and that was big problem 
Take a look at these threads:
http://forum.mikrotik.com/t/nlos-5ghz/44270/1
http://forum.mikrotik.com/t/5-0rc9-lost-connection-medium-access-timeout/44770/1
http://forum.mikrotik.com/t/desperately-seeking-elucidation/44545/1
I think it is trying to frequency hop (roam). Thanks, Hedy! 
http://en.wikipedia.org/wiki/Hedy_Lamarr
ok.,I putted in access list and nothing helped
http://www.hellbani.com/stilldc.JPG
but., with other wireless computer not that kind of problems…
maybe is it problem., because my DIR300 is in repeater mode ? (that device who have disconnects )
my Mikrotik is ap ., called " ulica "
my DIR300 is repeater on Mikrotik
one more thing… my Dir300 is on " 192.168.10.243 " ., mikrotik is on " 192.168.10.254 "
when in winbox I ping all time " 192.168.10.243 ( dir300) , then dir300 not disconnect
Be careful with this one! I am talking about the station. That is the connect-list, not the access-list. The access-list is only for the bridge or ap-bridge.
This is my last response to this:
http://forum.mikrotik.com/t/5-0rc9-lost-connection-medium-access-timeout/44770/1
The station router is going to try to roam (frequency hop) if the signal on that frequency gets busy, not weak!
ok., tried with connect list , tried with WDS ., not working… still disconnect every cca 30 - 50 sec…
only time when not disconnect is when I ping all time that router., or when I establish connection with that IP address all time…
as soon connection is over ( ping or download something ) , mikrotik start to disconnect that device ( router in repeater mode )
for wds I used this idea ( http://www.dd-wrt.com/wiki/index.php/WDS_Linked_router_network#DD-WRT_to_MTik_WDS )
any more ideas ?
hmmm… suddenly wds start to work ( I think ) ., over 10 min still not disconnect and recconnect 
I am taking a guess from testing that “default-authentication=no” means “frequency-hopping=disabled”.
If anyone has different results, I would like to know about it. Maybe there is more to it than that. ??
channels are fixed to 7 (2442 MHz) on all my network
but ok… now when I put wds in dd-wrt(dir300),and mikrotik automatic accept that and create wds, now only 2-3 times disconnect at day from mikrotik.. so thats its ok for me
The ap-bridge is “fixed” at channel 7.
The station will follow the ssid of the ap-bridge to any frequency in that band, even if set to channel 7.
If you change the ap-bridge to channel 4, I bet the station will follow without resetting the frequency in the station.
station will not follow., in ddwrt ., you must configure right channel ., right mode and all., to connect.
like I say., I had that problem with any repeater that I tryed connect to mikrotik. airlive 5460 ., same problem in client or repeater mode…
ok ., here is all configuration inside ;
mikrotik :
www.hellbani.com/mikrotik/mikrotik.html
ddwrt :
I am unfamiliar with dd-wrt now. I used it for a while. I wrote a C code modification for Chillispot for them.
Here is a thread that has the AP and station view IF both are Mikrotik.
http://forum.mikrotik.com/t/nlos-5ghz/44270/1
You can see that the station is not leaving (on both routers) because of bad signal, or one would say
“lost connection: extensive data loss”.
I am not certain the Mikrotik AP is doing what it should. It may not be sending beacons often enough. In that case, maybe Mikrotik should look into it. All I know is the “default-authentication=no” on the station fixed it for me, but both mine are Mikrotik units. ??
ADD: OK, I went to dd-wrt. This would be the equivalent setup there. It is the wireless setup in Client mode.
On that same tab set the Wireless Network Name (SSID) to exactly the same SSID as your primary router you will be connecting to over wireless. Make sure spelling and capitalization match. Note: If there are multiple routers broadcasting the same SSID, and you would like to connect to a specific one (IE, the primary router, instead of the repeater), enable MAC Filtering on the Wireless Tab, and add the MAC Address of the correct Access Point. The MAC Address can be found on the “Site Survey” page, linked from the “Wireless” Sub-tab of the “Status” Tab. - Conjur
I see no reference to a frequency setting on the client router.
Here is a link to the whole thing:
http://www.dd-wrt.com/wiki/index.php/Client_Mode
http://www.dd-wrt.com/wiki/index.php/Repeater_Bridge
I have repeater mode., so must be same channel
but., problem is this., when I put any other router on my side (linksys,siemens,alirive,thomson,d-link,us-robotics ) and ( disconnect mikrotik as primari router), there is no problem with repeater mode…
because of that., I think problem is somewhere inside of mikrotik…
one more funny thing ., the dealer of mikrotik software in my country (Croatia) dont know anything how to help me LOL., here are no help 15 days when you buy software 
I think it is about time to ask Mikrotik support about it, so I did. I will let you know what they say.
thanks very much ! ., I am not so good with english., so this help me alot
I heard from support. They seem certain that “default-authentication” has nothing to do with the beacons and disconnects. Maybe they are correct. Could it be that all the noise or interference disappeared the exact time I set “default-authentication=no” almost 6 weeks ago?
They have asked me to set “default-authentication=yes” and see if the problem reappears, so I have. I do not expect anything in the log until this afternoon if the disconnects start again. The disconnects were only happening when that backhaul radio gets busy.
I will let you know what happens.
ya.. similar problems to me
with " default-authentication=yes " or " no " same thing… with WDS or not same thing… with all options I tried , problem is still here .. only thing to aviod problem with disconnect is to put another router ( not mikrotik )
Here is the update: I tried setting “default-authentication=yes”, and the disconnects did not return. ??
So, after weeks of troubleshooting, changing frequencies, radios, cables and antennas, it was apparently just an incredible coincidence that the noise/interference disappeared at the exact time I changed the default-authentication.
Since nobody else has reported any success with this, I recommend doing the same thing I did. Spend a few hundred dollars on new radios, cables and antennas. Then wait…
ADD: Just one more thing. I was informed by support that beacons are always sent. That is not exactly true. This is from the dd-wrt website:
NOTE There are no special rules for sending beacons, and they must be sent using the mandatory 802.11 carrier sense multiple access / collision avoidance (CSMA/CA) algorithm. If another station is sending a frame when the beacon is to be sent, then the access point (or NIC in an ad hoc network) must wait. As a result, > the actual time between beacons may be longer than the beacon interval> . Clients, however, compensate for this inaccuracy by utilizing the timestamp found within the beacon packet information.
http://www.dd-wrt.com/wiki/index.php/Advanced_wireless_settings#Beacon_Interval