There are at least three companies with xDSL-Solutions:
|Wikipedia||Dokumentation in OpenWrt Wiki||Linux support for xDSL||Notes|
|Broadcom||broadcom.xdsl||✓||✘||✓||✘||about the proprietary driver|
|Ikanos||ikanos.xdsl||✓||✘||✓||✘|| a couple of FRITZ!Boxes are based on the Fusiv Vx180 + IFE-6;
the firmware is Linux-based, ergo there are proprietary drivers
Digital subscriber line is a family of Layer 1 communication protocols. The more prevalent members of the family are: ADSL, ADSL2+ and VDSL2; there are different Annexes: G.992.1#Annex_A and there are different Profiles: VDSL2 Profiles.
To be able to do the DMT (Discrete multitone modulation) fast enough, some DSL-implementations use 2 CPUs, e.g. two MIPS 24Kc Cores, one being solely used for the DMT, other employ one MIPS 34Kc Core (contains support for multi-threading and also some DSP-extensions), and others have some ASIC for this purpose! Explanation: DSPs are fast
Then some AFE (Analog Front-End) is required, this amplifies the signal and also performs the digital-to-analog and analog-to-diginal signal conversions. The AFE is a Mixed-signal integrated circuit, so (probably for manufacturing purposes) most solutions keep the AFE on a distinct Chip, e.g. the Lantiq VRX208 or VRX318 or the MediaTek TC3086 Ikanos IFE-6, etc. Though, the MediaTek RT63260 or the Lantiq VINAX are reported to be a single chip solutions.
For Authentication, Authorization, and Accounting (login) often PPP is used. The most common solution is PPPoE (PPPoA is less common, and there are other solutions). To make things be (or look) a bit more complicated, PPPoE is supported by the Router operating system and by all Desktop operating systems, but not by the Modem and the DSLAM. So the Modem either translates the PPPoE packets into PPPoA packets ("PPPoE to PPPoA") or encapsulates PPPoE packets insides of PPPoA packets ("PPPoEoA"). The Wikipedia visualizes this: "PPPoEoA" or "PPPoE to PPPoA".
The operating system needs to support both parts: Layer 1 and Layer 2.
The protocol TR-069 (Technical Report) / CWMP (CPE WAN Management Protocol) was released in May 2004 (and a couple of amendments to it have been released since then); its original purpose is, to enable the ISP to cost-efficiently manage the configuration of the operating system running on the Customer-premises equipment, that is required to maintain connection. Well actually, it could do more… Hmm…
There is support for tr-069 in OpenWrt (and GNU/Linux in general).