01-05-2012 09:13 AM - edited 07-03-2021 09:19 PM
Hi
I have 2 questions about physical layer of wireless lan.
1-) Many WLANs(SSID) use same channel for communication, that means they use same frequency, what prevents the collisions between different wlans but same channel?( I am asking about RF signal collision)
2-) Do rts and cts valid for just one wlan that computer which is generated, belongs to or do rts/cts synchronize all different wlans that use same frequency.
Thanks.
Solved! Go to Solution.
01-05-2012 10:14 AM
That is all correct!
the RTS/CTS is for the Radio not the WLAN.
anybody near you on the channel your AP/modem is using, has to wait until you are done communicating before they can send.
HTH,
Steve
01-05-2012 09:22 AM
1.) Congestion Avoidance. Before the client transmits, it listens to for energy on the frequency it is transmitting on. If it hears energy, it backs off for a time. Then listens again.
http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidance take a look at the diagram on the left
2.) RTS/CTS client sends the RTS, then the AP will send the CTS.
http://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS
HTH,
Steve
01-05-2012 10:04 AM
So RTS / CTS is inter-wlan. Not just valid for only one wlan.
So when my computer starts to communicate with my wireless modem, other computers even belongs to different wlan/ssid must wait for my computer to finish communication if they are on the same channel ??
So computers or APs that belongs to different wlans/ssids effect each others if they are on the same channel.
Right?
01-05-2012 10:14 AM
That is all correct!
the RTS/CTS is for the Radio not the WLAN.
anybody near you on the channel your AP/modem is using, has to wait until you are done communicating before they can send.
HTH,
Steve
01-05-2012 07:46 PM
To add to Steve response. The air time fairness for 802.11 is call CSMA-CA. It uses phyiscal and a virtual carrier sense. And this is by frequency. So if your ap is on channel 6 and my ap is on channel 6 and steves ap is on channel 6 and we are all close. We would all have to share the air space.
Always try and keep 20 db difference between aps on the same channel.
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