Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Same Channel on Different WLANs and RTS/CTS

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.

1 ACCEPTED SOLUTION

Accepted Solutions

Same Channel on Different WLANs and RTS/CTS

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

HTH, Steve ------------------------------------------------------------------------------------------------ Please remember to rate useful posts, and mark questions as answered
4 REPLIES

Same Channel on Different WLANs and RTS/CTS

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

HTH, Steve ------------------------------------------------------------------------------------------------ Please remember to rate useful posts, and mark questions as answered
New Member

Same Channel on Different WLANs and RTS/CTS

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?

Same Channel on Different WLANs and RTS/CTS

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

HTH, Steve ------------------------------------------------------------------------------------------------ Please remember to rate useful posts, and mark questions as answered

Same Channel on Different WLANs and RTS/CTS

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.

__________________________________________________________________________________________ "Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin ___________________________________________________________
565
Views
5
Helpful
4
Replies