9 Replies Latest reply: Nov 8, 2011 10:58 PM by Jimmy P RSS

    understanding csma/ca

    Jimmy P

      Maybe someone can help me understand the concept of csma ca when it comes to sending data.


      Explanation from WikiP:


      "a node wishing to transmit data has to first listen to the channel for a predetermined amount of time to determine whether or not another node is transmitting on the channel within the wireless range. If the channel is sensed "idle," then the node is permitted to begin the transmission process. If the channel is sensed as "busy," the node defers its transmission for a random period of time."


      Lets say the AP is on a fixed channel 8 and is configured to use only 802.11g.


      Node A listens and since there no one on  the channel starts downloading from a FTP server.

      this goes on for a next 10 minutes.


      While this is taking place Node B, C and D would like to download data from the same FTP server using the same AP(Ch8) According to the explanation this is not possible Node B, C and D will have to wait till Node  A finishes download and then start the download on a first come first serve basis. We know this is not true. Nodes A B C D can download at the same time sharing the 802.11g band. What am I missing?

        • 1. Re: understanding csma/ca

          Hello there, what you are missing is that the AP point are half duplex which means that they can send or recieve the data but not both at the same time.

          Think of the AP as a hub and I think you will get the concept about them, if you think of it as a switch you will really get confused a lot.

          • 2. Re: understanding csma/ca

            The timeframe a wireless device occupies the air with a single transmission is very short, and the process (DCF w/CSMA/CA) forces all the devies in the BSS to share pretty well. Example, Node A waits its turn then transmits one Frame. Then it goes silent and waits for the AP to send an ACK. Then, Node B's Back off timer expires and it senses the air as silent and transmits or reserves its air time via the RTS/CTS frames if that option is enabled. Node B transmists its one frame and goes silent and so on and so on. This quick handoff of small amounts of data and the sharing of the RF space is what makes it seem like they are all tranmsitting in parallel. In fact they are only sending small amounts of data very quickly and then going silent while another device reserves a small amount of time for its transmission.  


            I wrote this kinda quick while studying another part so I hope it makes since.

            • 3. Re: understanding csma/ca

              Yes , Phil you are correct.


              Jimmy , you maybe was thinking that node A will occupy a channelfor 10 minutes?


              No! Just for short time slot ( I think standard is for 20 microseconds )

              • 4. Re: understanding csma/ca
                Jimmy P

                It did. Thanks! looks like this works differently at the MAC layer than at the Application layer.

                • 5. Re: understanding csma/ca

                  Hi Jimmy


                  Of course the two PC might be able to hear the AP but not each other so it is forced to use the CTS/RTS mechanism. Also a 11b device can not recognise a g or n so there are lots a magic tricks going on in this environment. The Designing Deploying 802.11n  Networks by Jim Geier is a good read.


                  Regards Conwyn

                  • 6. Re: understanding csma/ca
                    Jimmy P

                    Hi Conwyn,


                    Thanks I will check out that book. As I was going thru the documentation and had a question: If you look at time line method  for sending data it goes something like this Carrier Sensing >DIFS> random backoff  and if the medium is free data is send sucessfully the  recv  station  sends a ack at the end. Where does CS check ie  VCS and PCS fit into all of  this.

                    Maybe some one can answer.





                    • 7. Re: understanding csma/ca

                      Hi Jimmy


                      The books will cover the subject far better than me.


                      Regards Conwyn

                      • 8. Re: understanding csma/ca
                        Scott Morris - CCDE/4xCCIE/2xJNCIE

                        On a wired network, each device can sense voltage change when another device is talking (two talkers = double voltage = collision).  In the air, it's not quite the same.  Two talkers = garbled message = noise.


                        The object in any half-duplex medium is that you can only have one talker at a time and need to figure out a way to avoid otherwise! 


                        There are different methods (as noted above) depending on how congested the airwaves are (or how controlling your admin is!).  But it's all about being able to be understood!


                        Remember, in the airwave idea (unlike physical media) not everyone has the ability of "seeing" everyone else (hidden node problem).



                        • 9. Re: understanding csma/ca
                          Jimmy P

                          Hi Scott,


                          Thanks for your input.