4 Replies Latest reply: Jun 21, 2009 7:04 AM by jj RSS

    Wireless question, Full duplex / Half Duplex

    Irish lad

      I have been studing wireless for the CCNA for the past few days, and have a one quick question for you.

      Wireless lan's operate at half duplex, and wired lan's operate at full duplex (when configured). My home  lab consists of a laptop with a wireless connection(b) to my router, and a desktop connected to my router at 100 full Duplex. My query is I don't seem to notice a big difference between my half duplex connection and my 100mb full duplex connection during a download from the net, if only one device is downloading at a time. I would have expected the 100mb full connection to be a lot quicker as it can send and recieve where as my wireless connection can only send or recieve during a download. Both connections always seem to have the same download speed. My net connection is 1mb DSL. If I had more laptops connected via a wireless connection would the wireless suffer more during a download.



      Hope that makes sense!

        • 1. Re: Wireless question, Full duplex / Half Duplex
          Paul Stewart  -  CCIE Security

          There is a ton of theory that could be discussed out of this topic.  I will try to keep my answer simple and to the point.  If you have a 1Mb dsl connection on even a 10Mb wireless connection, the download could theoretically consume close to 10% of your wireless lan segment.  There is a lot more to this like layer 2 encapsulation, remote server speed, internet congestion etc.  However you still have 90% of the bandwidth available to you for outbound traffic.  What is this outbound traffic?  Really, it is mostly requests and acknowledgements.  The actual amount of data here may only be two or three percent of what is incoming.


          So you have the following two scenarios from your land perspective:


          Wired Lan at 100Mb Full Duplex (Switch)


          Incoming internet traffic not to exceed 1Mb (or 1% of the LAN)


          Outgoing internet (incoming to the wired LAN Switch)

          may be something like 30Kb or .03% of the fully available 100Mb.


          Wireless LAN 10Mb for example half duplex.


          Incoming internet traffic not to exceed 1Mb (or 10% of the Wireless LAN).


          Outgoing internet traffic may be something like 30Kb.  With the above 10% used, there should still be 9Mb available.


          As you can see there is still no real bottleneck.


          Now there are other factors and you can look at this into greater detail.  For example discussing time slices and the actual behavior created whit collision detection mechanisms and back off that are required when you do not have full duplex.

          • 2. Re: Wireless question, Full duplex / Half Duplex



            Paul is right, this is a huge subject and can go in all different directions, your wireless clients in your home would also make a big difference.


            802.11b does not understand OFDM so in theory you lose 60% of your bandwidth (so the book says). However, if you are the only person in your home on your wireless network you probably would not notice much difference if any, BUT


            802.11b and 802.11g get affected by your microwave in your kitchen, game machines, Bluetooth, cordless phones etc and other clients (neighbors) because they are also on the same frequency i.e. 2.4 GHz.


            Even if you have only one 802.11b in your home then everyone is using CSMA/CA regardless if they are 802.11b or g. Best way around this is to use 802.11a as it use's a 5 GHz frequency or you could not allow 802.11b clients by forcing the use of OFDM.


            If you are already have CCNA routing and switching thats good, however, if you intend on doing the CCNA wireless before the CCNA routing and switching I would advise against that. That is what I attempted, I passed the CCENT then studied for Wireless and failed it twice, only by a very small margin but its still a fail. I was told that I should have studied CCNA routing and switching first as Cisco assume that you have done that first before attempting anything else. It is the starting block if you like.


            I am now studying CCNA R&S, when I pass the exam I will be back on the wireless.

            • 3. Re: Wireless question, Full duplex / Half Duplex
              Irish lad

              Thanks very much for the speedy replies.  I understand there are a lot of parts to this equation! But your replies have certainly made the fundamentals of this query more clear to me.

              • 4. Re: Wireless question, Full duplex / Half Duplex

                Hi Paul,


                Nice explanation....verymuch informative..