[newhouseb]

The short but unfulfilling answer is that this is because the DOCSIS standard historically has allocated a much broader frequency range for DL than UL. Unlike other forms of communication (like cell) that can use similar frequency ranges for transmit and receive, DOCSIS tends to slice something like < 70Mhz for UL and >70Mhz to 1Ghz for DL (I'm probably remembering the details incorrectly, trust Google over me!). Switching the frequency ranges often requires different circuitry and therefore different hardware.

I would guess that unlike fiber -- rarely saturated in a consumer context -- people have ~always wanted more than what cable can provide and thus the operators needed to be strategic about the allocation of bandwidth between DL and UL, hence the asymmetry.

[martinald]

Actually most of the capacity problems on cable networks come from upstream congestion; not downstream congestion and this has nearly always been the case. When the upstream gets congested TCP ACKs get dropped and it kills download speed.

Upstream has always been challenging on coax. The first cable modems didn't even use the coax for upstream, instead it used a separate dial up modem.

I'm not entirely sure of the details but even after that DOCSIS has always had to use much lower frequency channels for upstream which are much more limited vs downstream which tends to use higher frequencies. Potentially because it was hard to get high frequency transmitting upstream low cost CPE devices back in the day?

[throwawaaarrgh]

It's about prioritizing downstream since that's 90% of the traffic and what customers complain about the most. No normal customer complains about their upstream cap, but they will freak out over congestion or not having the fastest download speeds

[virtuallynathan]

I didn’t generally see upstream congestion when I worked on Internet QoE at Comcast, downstream was more common. Cable modems do ACK suppression as well to help there.

[whaleofatw2022]

That, also the relatively low frequency range was originally only there for the sake of public access and the like; they only expected a relatively small number of channels on the return path.

[mdasen]

I would say that cell signals mostly can't use the same frequencies for sending and receiving. The newer TDD allocations at 2.5GHz and above that carriers in the US are rolling out today do use the same for transmit and receive, but most of the old frequency ranges are exclusively up or down.

The difference is that cellular licenses generally offered equal bandwidth to up and down (ex. 10MHz down and 10MHz up). That's because they were originally designed for somewhat symmetrical communication: phone conversations where you assume each party is talking (sending) and listening (receiving) reasonably equally. With DOCSIS, it was designed assuming (correctly) that there'd be a lot more download traffic than upload traffic (which still holds true even in the era of remote work).