How does VDSL training/link speed negotiation work?

[Halcyon]

I've been studying the logs and admin pages of the modem/router that my ISP supplied (it's my version of "don't turn it on, take it apart") and I'm curious as to how it actually determines link speed. I'm paying extra for a 25Mbps connection, but how does the node/exchange actually "know" that? With VDSL I'm not required to enter a username or password, so how does the node etc... know that "this line and this modem relates to this ISP and this account"?

Obviously during training it assesses signal-to-noise ratios and determines what gains to apply for specific line conditions to arrive at a specific uplink/downlink speed. But how does the ISP limit the connection speed? I assume that all the negotiation is done between the modem and the node somewhere in the street or the local exchange, from there, it's just a backhaul link back to whatever ISP I'm connected to.

My modem is VDSL2 capable, so what's stopping it from negotiating with the node at the maximum speed that VDSL2 is capable of (i.e.: 100/100 or 300/100 Mbps depending on the version)?

[Kilrah]

This is usually done either by MAC address matching or more complex provisioning (a profile is loaded into the modem's non-volatile storage through some secured means). The node has a connection to a billing server, when a modem connects it checks what account it belongs to and applies the correct speed limits.

This is not really "speed negotiation" though, the actual "over media" link speed that this would refer to is independent from billing and would always be above what payload throughput is limited to, can be significantly so.

[Someone]

In some of the shared bandwidth delivery systems (cable and wireless for instance) the ISP shaped the incoming/outgoing bandwidth from their connection, you could see much higher bandwidth from their internal servers or DNS or internal peer to peer

[rstofer]

I'm pretty sure Comcast just uses the MAC address of the cable box.  Given a box with adequate capability, I can buy any grade of service I can afford up to the capability of the box.  Just call and give them my credit card number.  Right now, I'm getting 300 Mbps.

In some ways, network bandwidth is a joke.  Sure, I can receive data at a pretty good clip but the senders can't send data to all their connections at my data rate.

[Halcyon]

I called my ISP and asked (basically said, if I was to connect a different modem, how does the authentication work). They are going to call me back after they speak to one of the higher-up guys. It'd be a little silly to go off the MAC address of the modem. I'm thinking it probably goes off the address of the port at the node so no matter what modem the end user connects to the network, it's always going through that port in the street.

[Kilrah]

I'm thinking it probably goes off the address of the port at the node so no matter what modem the end user connects to the network, it's always going through that port in the street.

That's usually not how it works. You should be able to take your modem and go to a friend who has the same provider, cannect there and you'll have the connection that matches your subscription (as long as his line is capable of it of course).
Tying the billing to a particular drop would be a nightmare to maintain through network changes and upgrades, you typically want billing/service to be as independent from infrastructure as possible.

The answer you'll likely get is "you can't use another modem than the one we supply you with".

I recently upgraded from 40Mbps to 100, and it was just a matter of going on the website and selecting the option, 3 minutes later my modem rebooted automatically and I had my 100Mbps.

[Halcyon]

I'm thinking it probably goes off the address of the port at the node so no matter what modem the end user connects to the network, it's always going through that port in the street.

That's usually not how it works. You should be able to take your modem and go to a friend who has the same provider, cannect there and you'll have the connection that matches your subscription (as long as his line is capable of it of course).

That scenario seems out of the ordinary to me. Who takes their gear to their mate's place solely for billing purposes? You might be thinking of ADSL where you actually enter a username/password in the PPPoE settings. VDSL is different, at least in AU, where no username and password is required. Nor is the ISP reliant on the MAC address of the modem they supply (like on the old Cable systems that used DOCSIS).

I spoke to a guy this evening at Belong (my ISP) who said I was free to connect any modem I wanted to at my place without notifying them as long as it supports VDSL2. More often than not, more experienced users will connect their own equipment as opposed to the free modem/router the ISP supplies.

To me that suggests the port at the node/exchange is "hard wired" to their billing servers. It makes sense as the CPE (Customer Premise Equipment) is far more likely to change than anything at the node/exchange end.

[Don Hills]

Dial-up and ADSL connections used a username (usually name@realm, e.g. fred@big.isp.net) and a password. The info was sent to a Radius server which returned the tunnel endpoint for the ISP in the request. The PPP tunnel was set up between the telco's BRAS and the ISP. The ISP's Radius server then performed the actual subscriber authentication / authorisation. The ISP you were tunneled to depended on the user/realm set in the modem, so it would still connect to your ISP if you took it to a friend's place.
Current ADSL / VDSL are now port authenticated - the port on the ISAM is linked to your ISP account. That is, the port is always tunneled to your ISP. The ISP may still require user / password to be set in your modem.

[Don Hills]

Regarding the connection speed, this is intially set by how fast the modem and ISAM / DSLAM can synchronise.
https://en.wikipedia.org/wiki/Asymmetric_digital_subscriber_line
From there, the up and down speeds are determined by the service plan the subscriber has purchased. Queues and policers are applied to the subscriber's traffic, usually at the BRAS or the first internet router.
https://en.wikipedia.org/wiki/Broadband_remote_access_server