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External DSL Modems Design Guidelines

Updated: March 10, 2004

This article provides a draft guide for designers of external digital subscriber line (DSL) modems, known as ADSL Termination Unit-Remotes (ATU-Rs), defining principles of operation that will make for standards-based, flexible, and user-friendly systems.

Note: This is a draft version of the design guidelines. Microsoft will use the final version of these guidelines as the basis for future requirements of the Microsoft Windows Logo Program for Hardware.

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Motivation for External ATU-R Devices
General Requirements
Point-to-Point Connection between ATU-R and PC
Shared Connection between ATU-R and Multiple PCs
Other Solutions for ATU-R

Motivation for External ATU-R Devices

In general, an internal ATU-R will be a more cost-effective device for a system manufacturer to include on a DSL-ready PC, because in many cases it can double as a V.34 or V.90 analog modem when DSL service is not available. The availability of power, cooling, and casing inside a PC (or other appliance) can reduce the cost of equipment manufacture.

However, most consumers do not want to "crack the case" of a PC to retrofit an internal device. And when a DSL provider does this, there is a liability for any problems that might subsequently appear on the PC. In addition, a large market is also anticipated for "buy your own" ATU-Rs, where the consumer makes a purchase at a computer store, perhaps using a list of models approved by the Internet service provider.

Therefore, a solid market is anticipated for ATU-Rs that are external to the PC. These need to have the same Plug and Play feel to the user as an internal device.

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General Requirements

External ATU-Rs must provide sufficient uplink buffering such that an operational TCP/IP session can be instantiated and maintained, despite the difference in data rates between in-home and public networks.

The amount of buffering will vary, but must be sufficient to hold multiple full-length Internet Protocol (IP) packets, such that Transmission Control Protocol (TCP) flow control has a chance to react and throttle the source in the event of packet loss. It will be unable to do this if complete IP packets cannot be transmitted successfully.

Other flow control mechanisms may be included, such as the use of Point-to-Point Tunneling Protocol (PPTP) to apply back pressure to a Local Tunnel source.

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Point-to-Point Connection between ATU-R and PC

External DSL modems must adhere to the ADSL Forum recommendation TR12 for support of Internet and other legacy data services. This document specifies null-encapsulated Point-to-Point Protocol (PPP) over asynchronous transfer mode (ATM), known as PPPoA, as the underlying protocol stack for IP service over DSL.

Bridged Ethernet. Many of today's ATU-R devices are simple Ethernet bridges that extend the local area network MAC protocol into the network using bridged RFC 1483. However, this typically requires opening the PC case for installation of a 10BaseT adapter. As such, there is little or no gain in using an external ATU-R. Such dumb bridging devices have had some success in the early days of DSL, but their limitations are becoming increasingly apparent, and their further deployment is not recommended. In addition, these units do not meet the TR12 requirement of PPPoA over DSL.

ATM25. An ATU-R implementing ATM25 to the PC is architecturally superior to the point-to-point Ethernet approach, as it gives the PC access to the Quality of Service (QoS) features of the ATM layer. As for Ethernet, there is a need to open the PC case for installation, with the additional downside of the current high cost of an ATM25 adapter for the PC. However, this increased component cost is compensated by the more advanced capabilities.

ATM25 can support the three different ways of linking your PC to the DSL network:

Bridged Ethernet, using RFC 2684
PPP over Ethernet (PPPoE), using RFC 2684
PPPoA

ATM25 is fully supported in Microsoft Windows XP, Microsoft Windows Server 2003, Microsoft Windows 2000, and Microsoft Windows 98 Second Edition. Many ATM25 adapter vendors also support PPPoA on Windows NT 4.0 and Windows 98 (original release).

As with other point-to-point solutions, the first PC in the home will act as a home gateway for other networked computers or devices.

USB. USB is the preferred architecture for connecting external ATU-Rs. The technology provides excellent Plug and Play capabilities, allows the hosting PC to take full advantage of the PPPoA protocol stack, and avoids the need to open the PC case on the majority of PCs built in recent years.

External ATU-Rs connected using USB should appear to the hosting computer as an NDIS 5.0 (or greater) ATM adapter.

External, USB-based, ATU-Rs can support the three different ways of linking your PC to the DSL network:

Bridged Ethernet, using RFC 2684
PPP over Ethernet (PPPoE), using RFC 2684
PPPoA

The recommended approach is to implement Remote NDIS functionality in the ATU-R. This will avoid the need to load additional software on the host PC. However, those vendors who want to provide their own driver to run on the host can provide an NDIS 5.0 ATM miniport. Such a driver must meet Windows compatibility testing standards for an internal ATM/DSL adapter.

Remote NDIS. In the broadband context, Remote NDIS technology allows the hosting Windows-based PC to access the ATM layer facilities of the ATU-R over an alternative media such as USB. Remote NDIS provides a "virtual ATM adapter" to Windows, allowing the use of functions such as ATM Virtual Channels, ILMI, and UNI Signaling, some or all of which would otherwise have to be provided in the ATU-R.

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Shared Connection between ATU-R and Multiple PCs

Local Tunnel. Some ATU-Rs are designed to support multiple PCs on an Ethernet LAN (or other LAN-like media, for example HomePNA or HomeRF).

These should implement the Local Tunnel architecture described in the Microsoft DSL Architecture white paper. This specifies the use of PPTP or the Layer Two Tunneling Protocol (L2TP) to tunnel traffic between the client PC and an outgoing PPPoA session on the DSL network.

When a local tunnel ATU-R experiences high uplink buffer utilization, it should provide back pressure to the sending client(s) using the flow control mechanisms provided in the tunnel protocol. This may also apply if there is a threat to the QoS contract of other uplink traffic. For this reason, tunnel protocols that do not include back-pressure flow control are not recommended.

IP Address Assignment. The device should attempt to obtain an address from a local DHCP server within the home. If this is not successful, it should either start to act as a DHCP server itself or should automatically configure its own IP unique address from the 169.254.0.0/16 Automatic Private IP Addressing (APIPA) address space.

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