Credit: this deep dive is adapted, with thanks, from an explanation shared by Reddit user u/PoisonWaffle3, who works in the cable industry. We've lightly edited it for formatting.
There are a lot of reasons upload speeds are generally much lower on cable internet, so this is a deep dive. We'll start with the physical layout, then get into the nitty gritty. Fair warning: this gets pretty technical.
The physical layout
Traditional DOCSIS 3.0 and 3.1 HFC (Hybrid Fiber-Coax) nodes tend to have the following layout: fiber runs to a node, which has four coax legs (branches). Each leg may have 50 to 400 homes connected, depending on how good or how crappy the ISP is. The more homes, the less bandwidth and the worse the experience.
The node can push signal a fair distance down a line to a modem or TV (downstream power), but the modems don't have a lot of transmit power to send data back (upstream), so amplifiers are needed on the lines to boost the upstream data from the modems back to the node. Amps can have one to three outputs, so the layout branches out like, well, branches on a tree. The more amps, the more homes a node can serve, but that creates more points of failure and more noise.
Most good ISPs try to have fewer homes per node so they don't need to "cascade" more than one or two amps deep on any leg. Crappy ISPs tend to go 8 to 10 amps deep, and 20-amps-deep setups do exist (and are absolutely terrible). Keep these amps in mind, they become important later.
Here's a visual of how the spectrum is broken down:
Photo credit to u/PoisonWaffle3 on Reddit.
The new generation: node +0 / fiber deep
The new generation is generally called "node +0" or "fiber deep." The idea is to replace the coax trunk of the tree and the largest branches with fiber, all the way up to where the last amps are, and to replace those amps with nodes (so no amps are needed at all). You end up with very short coax runs. If there is a bad coax line, connector, or fitting, it affects a much smaller number of customers and can be repaired faster. Fewer customers per node also means more bandwidth available for each customer, but that doesn't mean symmetrical speeds yet.
It comes down to spectrum
Cable internet and TV are RF-delivered services, and the DOCSIS specs are specific about which frequencies are used for what. The 3.0, 3.1, and 4.0 specs promise some cool speeds, but you never see them in the real world because of RF noise (generally in the 5 MHz to 110 MHz range), cable TV (which shares the same physical cabling and spectrum), and old modems that people refuse to upgrade.
DOCSIS and cable TV live on coax at RF frequencies from 5 MHz to 1 GHz, with specific ranges having specific purposes. Think of it like radio stations at specific frequencies. DOCSIS 3.0 defined 5 MHz to 65 MHz for upstream (the modem transmitting back to the node, usually through one or more amps), and 85 MHz to 1 GHz for downstream (cable TV and downstream data). Many ISPs put cable TV channels from about 120 MHz up to about 480 MHz, then groups of downstream D3.0 data channels from about 480 MHz to 585 MHz. (These ranges vary a little node to node and city to city, but generally follow the same rough layout.)
That worked great until DOCSIS 3.1 came along and said we can use 5 MHz to 204 MHz for upstream and created 192 MHz-wide OFDM channels for downstream data. It's backwards compatible with old modems, but every amp has to be replaced with one that supports up to 204 MHz.
The spectrum Tetris problem
Let's see where everything fits. We have 200 MHz for upstream data, about 360 MHz for cable TV, 100 MHz for old D3.0 modems that people won't get rid of, and 192 MHz for the new downstream OFDM channels. Factor in some "guard bands" (blank spaces) between each group and we're at about 900 MHz total, so it's a tight fit but it should stay under 1 GHz, right? Not quite.
Remember those amps? Pretty much every cable plant picks up ingress in the FM spectrum (about 80 to 105 MHz), so we have to avoid that entirely. The more homes on a node, the more amps, the more noise, and the more that noise gets amplified. Even if we shuffle things around, we hit equipment incompatibility issues (cable boxes, old modems) and ingress in the spectrum newly allocated for upstream. Even if the outside coax is well maintained, there are too many homes with crappy wiring or loose fittings to make it work reliably. It works in the lab (especially without cable TV), but not in the real world.
The solution: node splits
The fix is node splits and dodging the FM 80 to 105 MHz range on the upstream: push fiber all the way up to the amps and put in nodes, as described above. This is really the only way to make DOCSIS 3.1 work reliably, and it's very expensive. There are tens of thousands of miles of cable to replace with fiber, all either up in the air or buried underground.
A typical "node +0" / "fiber deep" US layout is three DOCSIS 3.0 upstream channels and one D3.1 OFDMA channel, all between 5 MHz and 80 MHz to dodge FM. Cable TV channels still sit from 120 MHz to 480 MHz, but as providers launch IPTV and swap traditional cable boxes for IPTV boxes (all multicast data), that 120 to 480 MHz spectrum opens up for more upstream and downstream data channels. Once all the old D2.0 and D3.0 modems are gone, the legacy channels reserved for them can be swapped for much faster OFDMA (upstream) and OFDM (downstream) channels. Only then can you start to look at multi-gigabit upload and download over DOCSIS, and only with under 100 homes per node.
Why fiber is the endgame
Many providers have stopped building coax networks and moved to EPON fiber-to-the-home for new areas, replacing HFC with fiber as fast as they can. Fiber is already delivering up to 5 Gbps symmetrical to residential customers, with gear easily capable of 10 Gbps and fiber itself ready for 20 and 40 Gbps with equipment upgrades on either end. No RF noise to worry about, and it's rock solid.
For a plain-language primer on the concepts here, see our DOCSIS Basics and Fiber Internet Basics articles. Questions about the right service for a site? Reach out to sales[at]llamanetworks[dot]com.