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Wireless data propagation time

Discussion in 'Electronic Design' started by George, Oct 13, 2012.

  1. George

    George Guest

    I need a non-continuous low speed (<5 kbps) wireless data connection to my laptop from a desktop computer. But I'm having difficulty confirming that the data propagation time will be what I need: under 300 msec one-way. Thecircuit length is short, within the same city.

    Does this propagation time require a dialup circuit-switched connection like an old-style voice circuit on which we never hear delay? Or does it haveto be packet-switched nowadays and, if so, is there a way to keep the datapropagation time low?

    I'm assuming I'll use a dongle on the laptop and subscribe to a pay-as-you-go data plan.
     
  2. Ecnerwal

    Ecnerwal Guest

    Or packet-based wires/fibers, which is what wireless connects to.
    And I (at the moment) get 23-42 ms from the US East coast to John's
    company on the West coast. Depending what else is going on, delay can
    exceed (though it rarely does) 1000ms over half a mile.

    Best if you can work with the realities of how networks work (can be,
    often are quick - can be slow, can be lost completely.)

    At the moment, a device on my local net about 1/4 mile away has times
    between 7 and 84ms - based on that particular pair of samples (8 packets
    each), it can be twice as slow to go 1/4 mile as to go 2500 miles.
    Welcome to networks 101.

    Your likely options are to get away from a hard 300ms limit, or to
    figure some way to verify transit times when you are doing "whatever it
    is you need to do" and be able to re-do it if the time slips while you
    are doing it. Most of the time it will probably work, but guaranteeing
    it is not really in the cards via normal networking channels.
     
  3. Jasen Betts

    Jasen Betts Guest

    large buffers are the enemy of latency, they do unfortunately improve
    other throughput benchmarks so many equipment makers include them.

    also look into the QOS bits.

    Can you afford some packet loss?, UDP has lower latency by eschewing error recovery.

    a GSM voice circuit is 9.6Kbps, and low latency, but lost packets are
    not retried. I've heard echo on GSM circuits, but I'm not sure how much
    latency it represents.
     
  4. whit3rd

    whit3rd Guest

    This, in general, is not possible with IP protocols. There is no isochronous layer, no
    way to ensure a clear channel for your requirement. The video folk have workarounds,
    and are trying to come up with a successor to TCP/IP, but the basic scheme is to tolerate
    delays and not to control them.

    This reminds me of an incident a few months ago, when a mis-configured router, somewhere
    in China, was taking much of the worlds internet traffic (not delivering, just advertising
    that it was 'free' and capable of delivering). People noticed the delays.

    <http://arstechnica.com/security/2010/11/how-china-swallowed-15-of-net-traffic-for-18-minutes/>
     
  5. josephkk

    josephkk Guest

    That really depends on how critical limited latency is. If it is absolute
    hard line limit just use circuit switched technology.

    ?-)
     
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