xDSL (Digital subscriber line)

There are at least three companies with xDSL-Solutions:

2009-Aug-24: Conexant sells its Broadband Access Product Line

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.

• Layer1: In case some DSP-is used for the DMT-part, software needs to be written, supporting this Instruction set (e.g. the Texas Instruments TMS320). No big deal, given the programmer has the necessary documentation, the knowledge and ability and the time for the work.

AFAIR DSL is Layer 1 only and does not define a Layer 2. As Layer 2 communication protocols either ATM or Ethernet (the Layer2 parts) is employed. Some VDSL2-ISPs are reported to employ this already.

• Layer2: there is some support for ATM in Linux. But it is not very good ;-(
  • http://linux-atm.sourceforge.net/
  • The Layer2-part of the Ethernet protocol family on the other hand is very well supported in the Linux kernel! So for the few cases, where the ISP abandoned ATM in favor of Ethernet, it should be much simpler to obtain fully working fully stable deep shit Linux support!
• sourceforge.net: ATM drivers for PCI ADSL/ADSL2+ modems (solos)

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).