why does wireless have high latency?

Discussion in 'Electronics & Electrics' started by Luke212, Mar 8, 2015.

  1. Luke212

    Luke212 Member

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    Working with VR headsets, they are all wired. I am wondering why you cant get 1080p 90Hz through wireless? is it latency or bandwidth? im pretty sure its latency. you can get hdmi wifi. but what in particular about the way wireless transmission works does wireless have a high latency? is there a propogation speed lower than light speed? or is it the electronics? and if so cant the electronics just be beefed up?
     
  2. Symon

    Symon Castigat ridendo mores

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    The radio spectrum is a finite resource, so there are plenty of other things using the spectrum than what you are trying to push though. This creates noise, and the more noise the higher the latency.

    If you have a cable connection, then you pretty much have your own private channel to pass your data through.
     
  3. [SweN]

    [SweN] Member

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    the overheads are pretty high as well, especially compared to wired.
     
  4. trackhappy

    trackhappy Member

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    There are lots of factors that go into wireless, much more than wired solutions. As people have said, it comes down to things like throughput/goodput, overheads and the like.

    It's also worth pointing out that collisions are possible with wireless. At least with 802.11, this problem is mitigated somewhat by CSMA/CA, or Carrier Sense Multiple Access/Collision Avoidance.

    From my understanding (and I am open to correction), old style wired networks that used hubs used CSMA/CD (collision detection) which would detect when a collision occurs, tell all devices to stop, wait for a period, and then retransmit, wireless networks try to avoid collisions happening in the first place by affording each host a time slice to transmit it's frames in, every host will wait until it's their turn to transmit, and you'd hope that the other host has finished transmitting before the next host starts. The other hosts listen until that host has stopped transmitting, waits, and attempts transmission.

    I'm not sure how laggy CSMA/CA could make things, but it's there, whereas modern wired switch networks have no collision detection or avoidance at all (collisions are impossible with full duplex switched networks).

    Long story short, wireless is always going to be a bit laggier, it's more or less by design, but it beats not working at all. And Wireless 101 was fun, I wish I had gotten more into it. And I'm probably wrong somewhere... :Paranoid:
     
    Last edited: Mar 8, 2015
  5. callan

    callan Member

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    1: Wireless is half-duplex - IE: only one side can transmit at a time (as opposed to twisted pair ethernet, where traffic can flow in both directions at the same time). The arrangements to negotiate this "turnaround" in directions takes time. Since interactivity by it's nature means that the visual stream is controlled by inputs from the receiver, the protocols involved in managing this turnaround add significant latency. Since the introduction of twisted pair / Structured cabling Ethernet these protocols are simply almost never used anymore in wired connections, as they're almost never needed.

    2: Any time you assemble data into a stream of packets and encrypt them you automatically add latency by it's very function. Any packet of data over wifi has to be assembled, encrypted, error correction added, split into many different channels and transmitted out. Then it has to be received, agglomerated, any corruptions corrected (and if necessary retransmitted!), decrypted, and then passed out. To optimise throughput it's often necessary to wait to see if more data can be packed into the wifi packet before sending it out - adding even more latency.

    3: Also remember that the BAUD rate (signalling rate) of data in WIFI does not equal its BITRATE. Imagine if you will a single-lane Vs a multilane highway.
    Take this analogy: Two roads - one a single lane, the other a dual-lane carriageway.
    Over 1 Km of distance 2 lanes of traffic travelling at 1/2 the speed will still carry the same number of cars per hour (the bitrate), but EACH CAR takes twice as long (the latency).

    Wifi works by using many, many lanes (frequency channels) and clocking-out the data in parallel SLOWLY across some or all of those channels. As a result, although the TOTAL number of bits can be high, the actual time spent signalling them from one end to the other is a lot longer, and just as importantly can vary.

    That's a bit ELI5 (Explain Like I'm 5) - but it hopefully explains things a bit.

    Callan
     
    Last edited: Mar 9, 2015
  6. OP
    OP
    Luke212

    Luke212 Member

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    well all of those points can be addressed trivially. so i am not seeing the problem.
     
  7. Rezin

    Rezin Member

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    Can you do that?
     
  8. Jonchilds

    Jonchilds Member

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    If it's plugged in, it doesn't need a battery.
    If it's already plugged in, it doesn't need more expensive & power hungry Wifi.
    If it's plugged in, it's cheaper.
    If it's plugged in, it's easier to troubleshoot (prudent since most kits are still in development phase).
     
  9. Ghoatman

    Ghoatman Member

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    I use a wireless link over about 700m and get <5ms across it. Using RDP/Teamviewer feels like I'm on the machine locally.

    Edit: Doing a transfer at max speed (10mbytes/sec) increases latency to between 10-30ms.
     
    Last edited: Mar 8, 2015
  10. Supplanter

    Supplanter Member

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    You know what, you're right. It is a giant multinational government conspiracy to keep you dumb by slowing down your ability to look at cats on imgur.
     
  11. Renza

    Renza Member

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    Please elaborate!
     
  12. hi_lectro

    hi_lectro Member

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    I knew it!

    Meow
     
  13. the3coopers

    the3coopers Member

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  14. Symon

    Symon Castigat ridendo mores

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    Well go forth and patent your solutions, and profit!
     
  15. hosh0

    hosh0 Member

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    You can get around the radio spectrum being a finite resource trivially?? With your whiteboxes right? I feel sorry for those who actually tried to help you in this thread, they didn't realise who they were talking to.
     
  16. OP
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    Luke212

    Luke212 Member

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    im trying to pump 1080p@90Hz at short distances. im not asking for infinite bandwidth. once you solve the point to point latency, you can just go wider for bandwidth. video parallelises very well. but i may be overestimating current spectrum and going to higher GHz may be necessary. But everything else is trivial, in that, it is just the design of the network. you can create low latency protocols to solve this issue.

    i think there are some companies that do it now. eg. http://www.amimon.com/ claims zero latency radio.
     
    Last edited: Mar 9, 2015
  17. ShadowBurger

    ShadowBurger Member

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    is a few ms latency really a problem for you?

    The more decent high end access points have the ability to do what you're asking with low latency and zero issues, the question is rather why does your VR headset not have support?

    Look at Steam in-house streaming - it can do it, maybe not at 60hz (can it? I dunno) but if it can't it's not a hardware limitation

    as has been said, the cost/demand ratio for your VR manufacturers to add wireless as a feature just isn't right.
     
  18. elvis

    elvis Old school old fool

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    Sounds like you've found yourself a business opportunity where you can out-do some of the brightest minds on the planet and offer a better product than anyone else.

    You'll be an overnight billionaire!
     
  19. paulvk

    paulvk Member

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    Think about wireless this way > your in a pub its packed with people and you want to talk to the guy across the room you have to wait till is quiet enough for him to hear you
    This is the way it is for wireless.
     
  20. OP
    OP
    Luke212

    Luke212 Member

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    Last edited: Mar 9, 2015

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