Bandwidth degradation: WDS vs. WDS and AP

Discussion in 'Tomato Firmware' started by Edrikk, Aug 6, 2008.

  1. Edrikk

    Edrikk Network Guru Member

    Hi everyone,

    I'm not sure if anyone has tested this out, but a Google search did not give out much information.

    I know that WDS+AP mode results in a 50% reduction in wireless bandwidth that's available. However, my question is, in "WDS only" mode, is this 50% loss also going to be achieved?

    Thanks in advance,
  2. fyellin

    fyellin LI Guru Member

    The real question is how many wireless hops are there. If you are wired into your WDS router, which is then talking wirelessly to your main router, you will go full speed. If you are talking wirelessly to your WDS router, which is then talking wirelessly to the main router, you go half speed. Three hops is 1/3 speed.

    Obviously, these numbers are only approximations.
  3. Kiwi8

    Kiwi8 LI Guru Member

    It is not definitely that WDS+AP results in 50% reduction, nor the "WDS only" mode does not result in 50% reduction. It all depends on whether there is retransmission being made.

    For example, if u have a WDS only network where Router A is linked to Router B, and Router B being linked to Router C, communication between computers connected to Router A and Router C respectively, will have 50% reduction, due to the retransmission being made by Router B.
  4. Kiwi8

    Kiwi8 LI Guru Member

    Exactly what I wanted to say.
  5. Edrikk

    Edrikk Network Guru Member


    Thank you both!

    Yes, I was thinking of having:

    DSL Modem <-wire-> Router WDS/AP <-wireless-> Router WDS <-wire-> client(s)

    The WDS/AP would serve all wireless clients and the clients that are connected to it via hardwire. The WDS would connect the two routers, where the WDS router would have one or more wired clients into it.

    So it sounds like I don't lose any performance here due to WDS!

    Thanks again!
  6. Kiwi8

    Kiwi8 LI Guru Member

    Not entirely accurate. If your wireless client needs to communicate with a wired client that is connected to your 2nd WDS router, there will be a halving of bandwidth.
  7. fred3

    fred3 Network Guru Member

    I know this is an old thread...
    First: WHAT wireless client? I don't see one in the description.

    Secondly, it seems to me that there's a reasonable way to figure this out:

    1) Consider that every "radio" has to both transmit and receive. It can't do both at once and certainly not on the same channel.

    2) *Every* radio has to both transmit and receive unless the system is different than "normal". (Strange would be: there is one channel dedicated for receiving and one channel dedicated for transmitting .. a form of "dual band" operation). If a channel bandwidth (relative to Mbps is "B") then one channel can carry B Mbps and we don't have to care about whether it's in transmission or reception.

    3) Most internet traffic is one-way. So let's just assume that that situation rules. In this sense all of the bandwidth at the client is used in reception at B Mbps.

    4) When there's a wireless "hop", in such one-way communication, the intervening router has to:
    a) receive data for half the time at B Mbps
    b) transmit that same data for half the time at B Mbps.
    (If it were to receive data for longer than half the time then it wouldn't be able to get rid of the data and would eventually overflow. If it were to transmit data for longer than half the time then it would run out of data and suffer down time. So, the optimum time is 1/2 the time for each transmitting and receiving .. all other things being equal).
    If the transmit and receive times are 1/2 each then the amount of data that can be both received and thereafter transmitted is B/2.

    So, IF there's a wireless client, out on the end of the chain, then that client will at best get B/2.

    And, if there's a wireless client (or clients) connected to the first wireless router in the chain then *some* of the bandwidth (or maybe "time" is a better term here) will be used up serving that client. This is no different than a router serving many clients at once - which is something everyone is used to - and is the case for either wired or wireless communications (with wireless generally having less total available bandwidth so the sharing is more noticeable). This works better than one might casually think because each client only uses the available bandwidth for short periods of time and provides time-sharing of the resource.

    If you consider that the radio transmit/receive is the limiting factor and it's more about *time* than actual *bandwidth* then perhaps it's easier to understand.
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