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If I take any internet connection speed test (e.g. https://www.speedtest.net/) at home then a report is provided which shows different download and upload speeds. Why is this? How is downstream and upstream information handled differently in a manner that would cause this?

My intuition would be that both of these bandwidths would be the same as I would presume they both rely on the same networking infrastructure (routers, cables etc.).

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  • Unfortunately, questions about home networking are explicitly off-topic here.
    – Ron Maupin ♦
    Jul 11, 2020 at 20:19

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In part, the consumer access technologies are optimized for the way most households and small businesses need to use their connectivity.

From a technological perspective, DOCSIS cable modem networks have more downstream bandwidth than upstream because only one device transmits on downstream RF channels (the cable company equipment) so there is no chance of collisions -- when two devices try to transmit at the same time, corrupting each-others' transmissions. Upstream channels have to be shared by many subscribers in a neighborhood.

xDSL networks technically could provide symmetrical downstream & upstream bandwidth, and some do, usually to small business subscribers who have different needs than households.

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  • That's not why DOCSIS (cable) is asymmetric. Cable networks were designed for the one-to-many distribution model (one broadcaster, many listeners) As such, there was no return channel originally. To support "addressable" boxes, a small chunk of spectrum was allocated for a return channel (0 o 42, 54, and now 85 MHz) while the downstream extended to 500+ MHz (and today up in the GHz) So, because there's more downstream than upstream spectrum, download speeds are always higher. xDSL has the same unequal split, mostly due to limited RF transmit power.
    – Ricky
    Jul 12, 2020 at 0:13
  • Your familiarity with DOCSIS networks may be out-dated, Ricky. Cable networks still require much more MHz-per-effective-Mb/s for upstream transmission due to the collision problem, even though they have been re-architected to support far more raw upstream RF capacity. In short, the limitation isn't legacy radio filters; cable companies had 25 years to reconfigure their networks and they've done so.
    – Jeff Wheeler
    Jul 12, 2020 at 0:41
  • Yes, most upstream channels use different encoding, but they still face the same fundamental problem: at most 85MHz to work with. (vs >800 down) OFDM upstream is rare (only seen it once) and is still limited by the available channel space -- 96-192MHz down, 6 up. Despite having decades to "reconfigure their networks", most cable networks still only have a 54MHz return path. However, there has been movement in the last few years to support "full duplex" and other high channel return signalling. (making symmetric gigabit possible) But it's slow going as it's an expensive, large scale project.
    – Ricky
    Jul 12, 2020 at 4:24
  • Also, the reason for 64QAM vs 256QAM is signal quality, not collisions.
    – Ricky
    Jul 12, 2020 at 4:39

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