This is more “home networking” than “homelab,” but I imagine the people here might be familiar with what in talking about.
I’m trying to understand the logic behind ISPs offering asymmetrical connections. From a usage standpoint, the vast majority of traffic goes to the end-user instead of from the end-user. From a technical standpoint, though, it seems like it would be more difficult and more expensive to offer an asymmetrical connection.
While consumers may be connected via fiber, cable, DSL, etc, I assume that the ISP has a number of fiber links to “the internet.” Those links are almost surely some symmetrical standard (maybe 40 or 100Gb). So if they assume that they can support 1000 users at a certain download speed, what is the advantage of limiting the upload? If their incoming trunks can support 1000 users at 100Mb download, shouldn’t it also support 1000 users at 100Mb upload since the trunks themselves are symmetrical?
Limiting the upload speed to a different rate than download seems like it would just add a layer of complexity. I don’t see a financial benefit either; if their links are already saturated for download, reducing upload speed doesn’t help them add additional users. Upload bandwidth doesn’t magically turn into download bandwidth.
Obviously there’s some reason for this, but I can’t think of one.
Historically, last-mile technologies like dial-up, DSL, satellite, and DOCSIS/cable had limitations on their uplink power. That is, the amount of energy they can use to send upload through the medium.
Dial-up and DSL had to comply with rules on telephone equipment, which I believe limited end-user equipment to less power than what the phone company can put onto the wires, premised on the phone company being better positioned to identify and manage interference between different phone lines. Generally, using reduced power reduces signal-to-noise ratio, which means less theoretical and practical bandwidth available for the upstream direction.
Cable has a similar restriction, because cable plants could not permit end-user “back feeding” of the cable system. To make cable modems work, some amount of power must be allowed to travel upstream, but too much would potentially cause interference to other customers. Hence, regulatory restrictions on upstream power. This also matched actual customer usage patterns at the time.
Satellite is more straightforward: satellite dishes on earth are kinda tiny compared to the bus-sized satellite’s antennae. So sending RF up to space is just harder than receiving it.
Whereas fibre has a huge amount of bandwidth, to the point that when new PON standards are written, they don’t even bother reusing the old standard’s allocated wavelength, but define new wavelengths. That way, both old and new services can operate on the fibre during the switchover period. So fibre by-default allocates symmetrical bandwidth, although some PON systems might still be closer to cable’s asymmetry.
But there’s also the backend side of things: if a major ISP only served residential customers, who predominantly have asymmetric traffic patterns, then they will likely have to pay money to peer with other ISPs, because of the disparity. Major ISPs solve this by offering services to data centers, which generally are asymmetric but tilted towards upload. By balancing residential with server customers, the ISP can obtain cheaper or even free peering with other ISPs, because symmetrical traffic would benefit both and improve the network.
This is a really good explanation; thank you!
There is one thing I’m having a hard time understanding, though; I’m going to use my ISP as an example. They primarily serve residential customers and small businesses. They provide VDSL connections, and there isn’t a data center anywhere nearby, so any traffic going over the link to their upstream provider is almost certainly very asymmetrical. Their consumer VDSL service is 40Mb/2Mb, and they own the phone lines (so any restriction on transmit power from the end-user is their own restriction).
To make the math easy, assume they have 1000 customers, and they’re guaranteeing the full 40Mb even at peak times (this is obviously far from true, but it makes the numbers easy). This means that they have at least a 40Gbit link to their upstream provider. They’re using the full 40Gb on one side of the link, and only 2Gbit on the other. I’ve used plenty of fiber SFP+ modules, and I’ve never seen one that supports any kind of asymmetrical connection.
With this scenario, I would think that offering their customers a faster uplink would be free money. Yet for whatever reason, they don’t. I’d even be willing to buy whatever enterprise-grade equipment is on the other end of my 40/2 link to get a symmetrical 40/40; still not an option. Bonded DSL, also not an option.
With so much unused upload bandwidth on the ISP’s part, I would think they’d have some option to upgrade the connection. The only thing I can think is that having to maintain accounts for multiple customers with different service levels costs more than selling some of their unused upload bandwidth.
The routing equipment at the distribution boxes is likely a limit. Both in regards to power consumption and heat production, plus especially with older equipment the total throughput it is capable of.