WIFI half duplex questions
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Thread: WIFI half duplex questions

  1. #1
    Join Date
    Jun 2011
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    6

    WIFI half duplex questions

    hello,
    i have many questions and i would appreciate it if someone can help me with it

    1- regarding the half duplex nature of WiFi, let say the practical throughput of a 802.11g network is 20mbps. can i use the whole bandwidth to send or receive or the bandwidth is divided into 2 independent portion ? to be more clear .... can i send at 20mbps or receive at 20mbps? or i only use 10mbps to receive and the other 10mbps is reserved for sending


    2- in WiFi, the channel can only be used by a single user, this means that if you want to send something you have to wait. why don't we see this in real world? or it happens so fast we don't feel like waiting

  2. #2
    Join Date
    Feb 2002
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    68
    1- The entire bandwidth is used in each direction.

    2- The wait time is very short.

  3. #3
    Join Date
    Jun 2011
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    6
    thank you so much for answering

    3- there is a Meraki AP(triple radio 2x2 900 mbps throughput, with a fast-ethernet (10/100) port, is that normal ?? i mean shouldn't the 10/100 port cause a throughput bottle neck ?

  4. #4
    Join Date
    Feb 2002
    Posts
    68
    First, there are 2 load balancing 100 mbps full duplex Ethernet ports.
    Second, it is highly doubtful that the throughput from those 3 radios could ever tax the Ethernet bandwidth due to the nature of wireless transmission and the protocols used.
    That 900 mbps maximum throughput is theoretical and can only be achieved under laboratory conditions.

  5. #5
    Join Date
    Jun 2011
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    ok correct me if i am wrong,

    1- "First, there are 2 load balancing 100 mbps full duplex Ethernet ports."
    where does it say that ?

    2-what is the practical throughput of those 3x radios, because i would like to see a comparison between the throughput of the Ethernet and wireless transmission

  6. #6
    Join Date
    Feb 2002
    Posts
    68
    I looked at the specs again. The two Ethernet ports are auto-crossover. Load balancing will occur at the router.

    Wi-Fi throughput is generally about 50% of the signaling rate, never more and often less.
    Signaling rates are dependent on signal quality and signal strength. The rate is variable. The signaling rate will be tailored to the equipment being used. The attached file shows a signaling rate of 270 mbps out of a max of 300 mbps for my 802.11n (5 GHz ch 36 + 40) lab setup because 270 is the maximum my notebook can handle. In other words the speed will depend on the clients.

    Wi-Fi is half-duplex with a lengthy silent period between receiving and transmitting in addition to the delay between sequential transmissions. There is a high potential for collisions and interference. Those cause major delays.

    The three rules of Wi-Fi
    1. It never goes as fast as they say it does
    2. It never goes as far as they say it does
    3. It never sets up as easily as they say it does


    Ethernet has some similar delays, but not as severe. Being full-duplex, there is no delay to turn around the circuit as there is with half-duplex. Propagation delays are also shorter. There is no collision or interference delays with switched Ethernet. The big advantage is that 100 mbps switched Ethernet has a bandwidth of 200 mbps - 100 mbps in each direction.
    Attached Images Attached Images

  7. #7
    Join Date
    Jun 2011
    Posts
    6
    thats a really good explanation...

    correct me if i am wrong

    1- the throughput of the Ethernet is 200mbps (100 mbps each direction).... but i can't use the whole throughput in one direction

    2- wifi, lets say, 802.11n has a theoretical throughput 300 Mbps and the practical is, lets say, 150 Mbps. I can use the 150 Mbps in any direction but i cant download and upload at the same time

    3- the speed is dependent on the client's capability

    question .... the Meraki AP we are talking about has 3 radios, each radio has a theoretical throughput of 300 Mbps and practical of, lets say 150mbps. so in real world, each radio can transmit or receive at speed 150 Mbps ( thats a total of 450 Mbps actual throughput).

    what i find it very strange is how the AP handles all this actual throughput if it is being fed by a fast Ethernet connection (100 Mbps or 200 Mbps throughput)

  8. #8
    Join Date
    May 2011
    Posts
    369
    The multi radios APs do not help much if the Wireless client's cards are single radio units.

    The half duplex is generated because a single radio has to filp/flop between transmit and receive.




    -----------
    CAT5e
    Microsoft, MVP - Networking.

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