[MLB-WIRELESS] Transfer Rates

[Ben Anderson]

It's easy enough to saturate wired 10Mb ethernet to about 95% with only one station transmitting.  100Mb ethernet is reasonably difficult to saturate to more than about 80%

Collision detection and avoidance is only a problem with wireless ethernet if there's something to collide with...  If you have one station transmitting, most people wind up with about 6-700KBytes/sec (6-7Mbits).  The signal-noise ratio is much higher over the wired phy layer, compared to the wireless phy layer, which would easily explain the drop in efficiency in itself, even without the additional overheads in the 802.11b MAC layer.

In any case, 250-300KB/sec is a bit low for a point-point link.  But after re-reading you don't have point-point - you have an AP.  The packets are being transmitted twice!  Once from the original station to the AP, and then from the AP to the receiving station.  Which explains brilliantly why you're only seeing half the total throughput  you expected to see.

Cheers,
Ben.


  I don't know about the wireless side, but I can tell you that Ethernet uses CSMA/CD (Carrier Sense, Multiple Access, Collision Detection) to modulate the carrier (radio frequency) .
  Only one device can talk at one time per "Carrier" - thus, on Ethernet if you have multiple devices (MA) sharing the carrier and create "two way" communications (ie. pings originating from both ends) - the packets could collide, a pause will occur and they will retry. (/CD) - slowing down your data path.

  THEN, also don't forget, that whilst the CARRIER communicates at "10Mb" - you are encoding with protocols to transport the data ie. 802.3 Frame, Ip Protocol, TCP protocol etc. All of these carry the payload (read: data) that you are sending. 
  I would also guess that if you are also encrypting you should (theoretically) have packet "padding"  to filling up any partially-filled frames - so it will also affect the throughput of the of path (not forgetting the time it takes (latency) to encrypt and decrypt etc)

  Finally, there is Latency. This is the time it takes to get from one side of a device to another. For example, in a router you expect latency, because the router has to strip the Frame (802.11 or 802.3), investigate the IP Addresses and protocol, make a decision and then forward it onto the next interface. All of which takes time - thus affecting the throughput.

  So I would assume that, like Ethernet, in wireless you wouldn't get actual 11Mbs of data passing over a link, just like you will not get "10Mb" of data over an Ethernet link. (In fact, I believe that the throughput of Ethernet is about 3.9Mb)
  I can't tell you what to expect on a wireless link (I don't have those figures) - but look for throughput - it is a better representation.
    -----Original Message-----
    From: Rowan Wainwright-Smith [mailto:Rowan at teleaudit.com]
    Sent: Tuesday, 17 September 2002 9:15
    To: Cameron Donaghey
    Subject: Re: [MLB-WIRELESS] Transfer Rates


    Cameron,

    transfer rates are a little tricky, 'coz there are two ways of measureing them

    1    PURE Bandwidth - how many BITS per second!
    2.   Transfer performance (usually measured in BYTES per second)

    The stats you are getting are pretty reasonable, but you should be able to get about 550(-600)k/sec 


    these cards are 11 Mbit,

    if you work it out, you are getting about 2.5mbit transfer rate (not counting TCP overheads) Bear in mind that these devices are 11mbit TOTAL bandwidth, so if you have two PC's communicating, then you can really only expect up to approx 5Mbit transfer, 3PC's then about 3.5 Mbit & so on.

    Wireless is NOT switched bandwidth like most modern wired networks, but shared (like running a hub instead of a full switch)
     Personally not sure about what effect using an AP is having, but this *could* be reasonable!

    maybe the rest of the list can help you more....

    well done on getting your WLan up & running!

    Rowan

    Cameron Donaghey wrote:

      Hello Everyone,
        I have eventually got my wireless up and running, only on a private level at the moment, and
      yet I am not entirely sure it is running correctly. I have two machines and in each one I have
      a Compaq WL100 card set to infrastructure mode. I also have a linksys Wap11 access point
      which is running as an access point only.
      I have done some testing and I seem to have a max. transfer rate between the two machines
      of about 310kb/s and the machines are only a meter or so apart. I would have assumed that
      being a 11mb/s network that I would be able to achieve more so around 500-1000kb/s rather
      than the 300kb/s max. I am currently achieving. Both card say they have "excellent connections"
      and at 11mb/s.
      My question to you all, is the transfer speed of about 300kb/s typical for 802.11b or should I be
      able to achieve higher rates of transfer? If I should be able to achieve more would anyone have
      any suggestions towards resolving the problem?
        Thanks very much,
           Cameron Donaghey


      
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