RTS/CTS -unicast or broadcast + Random backoff time

Unanswered Question
Oct 10th, 2008

Hi everybody,

Iam reading through the frame handshake in WiFi. Also went through some of the good articles in this forum.

I understand that to avoid collision, RST/CTS handshake is used in the wireless Air medium.

I have attached the RTS-CTS handshake and I would like to know if these frames are unicast or broadcast.

Since they are sent from a specific source say Wifi Client to a specifc destination say Wifi AP, i assume the RTS and CTS signals to be unicast but i dont understand then how the other wifi stations/clients will listen/get the duration (NAV)

Also regarding the backoff time interval, I would like to know how this time is decided by the waiting clients? Say client A and Client B are waiting for the medium to be free. Both get the NAV duration as client C is using the medium. Now after the NAV time is complete, the clients A and B will wait for a backoff time.

>how much is this random time?

>Can both set the same backoff time say 1? >Will the clients with min backoff time will get to check the medium first.

>What is the max time each client will get to use the medium? What if Cient C using the medium has lots of data to send. Will clients A and B have to wait till Client C finishes? OR is there a max time duration.

Pl advise.

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amritpatek Thu, 10/16/2008 - 14:29

Whenever a wireless client wants to send a data packet to the access point, it actually transmits a four-packet sequence called the RTS-CTS-DATA-ACK packet sequence. Each of the four 802.11 frames carries a NAV field that indicates the number of microseconds that the channel is reserved for by a wireless client. During the RTS/CTS handshake between the wireless client and access point, the wireless client sends a small RTS frame that includes a NAV interval large enough to complete the entire sequence. This includes the CTS frame, the data frame, and the subsequent acknowledgment frame from the access point.

When the wireless client transmits its RTS packet with the NAV set, the transmitted value is used to set the NAV timers on all other wireless clients associated to the access point. The access point replies to the RTS packet from the client with a CTS packet that contains a new NAV value updated to account for the time already elapsed during the packet sequence. After the CTS packet is sent, every wireless client that can receive from the access point has updated their NAV timer and defers all transmissions until their NAV timer reaches 0. This keeps the channel free for the wireless client to complete the process of transmitting a packet to the access point.

robinjellum Thu, 10/16/2008 - 15:00

The RTS / CTS packets are sent without BSSID info and without to DS / from DS designation (Ad Hoc) so that all devices on channel that can hear the packets will respect the NAV.

Queen245_goa Mon, 10/20/2008 - 02:29

Thanks for your replies.

The RTS/CTS packets have transmitter address and receiver address in the frame sent.

While looking through wireshark, i notice that for beacons frames the type shows as braodcast while the same is not for the RTS/CTS frames.

Can someone help me know if RTS/CTS is broadcast or not?

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