SNR to download speed on 3G/4G

Discussion in 'Broadband' started by Martin Brown, Dec 18, 2014.

  1. Martin Brown

    Martin Brown Guest


    Can anyone give me ballpark figures for how local SNR on a 3G or 4G
    mobile data link maps to actual measured download speeds?

    The fixed points I have are roughly

    -70dB full speed (subject to contention ratio)

    -90dB 1.5Mbps 3G Three Mifi Huawei

    I plan on doing more experiments and sharing the results.

    I reckon I can get an external aerial with +11dB gain so allowing for
    injection losses a signal at the box of -80dB. I plan on doing the
    experiment but I'd be interested in the range with different kit.
    Recommendations would be very interesting as would kit to avoid.

    My interest is in what speed I might hope to get with a signal level of
    about -80dB and a high end 3G router with an external aerial. And also
    will an "all you can eat" contract fall foul of streaming HD video?

    Basically I am considering abandoning fixed line ADSL in favour of 3G
    "mobile" but used from a fixed location. Some wet days the 1.5M Mifi can
    be faster than the ADSL when junctions get wet and the line noisy. The
    ADSL noise margin is stubbornly stuck at 16dB :(

    Most of the time I get an ADSL sync around 3.6Mbps (lower in winter and
    higher in summer). It is pretty ropey at the moment. Hell will freeze
    over before we get SuperFast Broadband the best cherries have already
    been picked and our village is simply too small to stand a chance.
    Martin Brown, Dec 18, 2014
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  2. Martin Brown

    Graham J Guest

    In my experience 3G performance inside a building is useless.

    I know of routers that accept a 3G dongle into a USB socket - and I can
    see that one of these would work OK if the router and dongle were
    mounted on the roof - but I can't see anybody putting a Vigor 2830 into
    a waterproof box ...

    What would be useful would be a waterproof 3G dongle with an Ethernet
    port, and PoE; that way the dongle could easily be mounted where it
    would receive a signal - up to 100 metres away from the router. Does
    anybody know of such a device?
    Graham J, Dec 19, 2014
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  3. Martin Brown

    Roland Perry Guest

    I have one of those and it's worked fine inside three different houses.
    [Using 3 as the network]. Albeit in one of them I needed to put the
    dongle near the window on a long USB extender cable.
    A better plan if the in-building coverage isn't up to it would be an
    external aerial (possibly a directional one) and a dongle with an aerial
    socket. I've got one of those too, plus an omnidirectional indoor aerial
    which has helped on occasions when I've been getting poor in-train
    coverage to my laptop.
    Roland Perry, Dec 20, 2014
  4. Martin Brown

    Martin Brown Guest

    Strange. I can't measure much difference between signal level inside and
    out provided I stick to rooms with a window in. Extra height gains me a
    lot of signal as the local topography is significantly hilly.

    I can also adjust which base station I can see in favour of a much
    fainter remote -110dB one by standing in just the right place.

    I lose a lot of signal in the core of the house. Some older walls are 3'
    thick stone and they don't let much signal through at all.
    ISTR Solwise do one. It would be nice if there was a cheap 3G dongle
    with an external aerial socket but I don't know of one. You can get
    radiative couplers for external aerials to dongles/Mifis but I am
    sceptical of their efficacy. I am seriously marginal signal levels to
    begin with so if I am to try this I need every last bit of SNR.

    But realistically you want a waterproof aerial on a longish lead and a
    3G dongle with an aerial socket. I have perfected this trick for wifi
    with a flat panel aerial which allows me to run my network out to 300m
    from the base station in the direction I want it to go. Cheap Wifi
    dongles with detachable aerials are much easier to come by.
    Martin Brown, Dec 20, 2014
  5. Martin Brown

    Roland Perry Guest

    Huawei E160, loads of them on eBay.
    Roland Perry, Dec 20, 2014
  6. Martin Brown

    grinch Guest

    3g does not work like that if you are with the signal strength envelope
    of your device you get full link speed. As you get towards the edge the
    link struggles and drops so the throughput drops.

    I work with cisco 887G they fall over at -90dBm but at -80dm you get
    full link speed subject to the usual limitati9ons of a shared link.

    They also support proper external antennas ,but it is
    easier/better/cheaper to have a short antenna lead and longer Ethernet
    rather than the other way round.

    At -90dBm I am surprised it works at all, run wireshark and see if you
    are getting lost of TCP retransmits
    grinch, Dec 20, 2014
  7. Martin Brown

    cl Guest

    Better than a WiFi dongle IMHO (... and yes, I know this is off the
    original subject) are the TP-Link TL-WA7210N and similar devices
    (there's a 5Ghz as well) which are outdoor housings with aerial and
    everything inside powered by POE.

    Aerial cables are lossy.
    cl, Dec 20, 2014
  8. I think you will find that all radio spectrum comms is affected by who
    else is on the channel.

    Like old co-ax ethernet, there is contention and backoff on a busy link.
    Ultimately Shannon/Nyquist gets you. Only so many bits per second for a
    given bandwidth and power-to-noise. How those bits are shared amongst
    competing devices and how close you can get to the maximum bps for the
    channel bandwidth and noise is the black art of modulation schemes, but
    you can't exceed those limits.

    Depends what you mean by 'full link speed' of course.
    The Natural Philosopher, Dec 20, 2014
  9. Martin Brown

    Martin Brown Guest

    Thanks. Worth a punt at that price.
    Martin Brown, Dec 22, 2014
  10. Martin Brown

    Martin Brown Guest

    The great digital cliff! Fun on marginal TDTV signals if you like
    speaker destroying ultrasonic clicks and cubist renditions of the image.
    I suspect it is dropping a lot of packets but I haven't investigated
    fully yet. I am basically interested in knowing whether or not if I can
    get a -80dB signal level will I be close enough to nominally get
    something like the theoretical maximum speed.

    I intend to move around to get slightly higher SNR and establish where
    the threshold for decent performance with my mobile Wifi lies.
    Martin Brown, Dec 22, 2014
  11. Martin Brown

    Martin Brown Guest

    I take your point, but since I live in a very windy location where the
    satellite dish has been known to be hit by flying debris I am not all
    that keen on things outside and out of reach that are less than IP66
    weatherproof as they tend to degrade badly after a year or two.

    Satellite dish has taken damage overnight don't know what but the wind
    is far too strong to contemplate going up to look. Hanging baskets were
    blown to near horizontal this morning and the wind has snapped or rather
    worn through the chains connecting them to the central hook.
    Indeed especially the weedy thin stuff.
    Martin Brown, Dec 22, 2014
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