Delivering VPNs over high-speed DSL links

Digital subscriber line (DSL) service offerings are emerging as an attractive alternative to T-1 and frame relay for building virtual private networks (VPN).

Symmetric DSL (SDSL), for example, which operates over a single twisted copper pair, offers the same amount of bandwidth as T-1, at about half the price.

Before DSL, network professionals were confined to building VPNs over the Internet using IP tunneling, or over a carrier's leased line or frame relay services.

Now a new VPN option is entering the fray: ATM over DSL. The ADSL Forum's Technical Report TR-002 defines asymmetric DSL and ATM interworking. ATM was selected by the ADSL Forum as the Layer 2 protocol for ADSL because of its quality-of-service (QoS) support, the security it provides to users, and ATM's ability to support parallel sessions over a single DSL line. ATM over DSL lets users build secure, high-performance VPNs over a low-cost access technology.

ATM supports a rich QoS feature set and the extensive traffic-management capabilities needed to deliver highquality VPNs over DSL. User-definable parameters, such as peak cell rate, sustainable cell rate, minimum cell rate and cell delay variation tolerance, let users define QoS for each application carried over a DSL-based VPN. This ensures optimal application performance.

The Service Network Architecture Group of the ADSL Forum is currently debating two options for an end-to-end Layer 2 architecture for DSL nets: packet-mode for the native delivery of IP and ATM, and regular IP delivery. IP, which is still developing QoS capabilities such as Differentiated Services, a proposed IETF standard, does not appear to be a viable option for QoS. While IP can perform some class-based QoS, which maps multiple traffic flows into a few service categories, IP does not support customizable, user-definable QoS for individual applications such as voice and video. With IP, users can't modify the class of service (CoS) to fit the unique performance characteristics of the application.

ATM's ability to provide QoS for multiple virtual circuits to each location in a VPN allows for the provision of delay-sensitive applications, such as voice and video, over the same DSL link that carries data traffic.

Integrated access devices (IAD) with DSL interfaces multiplex voice and data traffic streams into ATM virtual circuits for transmission over a DSL line. Ensuring QoS for multiple virtual circuits on a DSL local loop requires the central office DSL access system and the IAD to support the constant bit rate (CBR) and variable bit rate (VBR) ATM CoSes.

While most DSL modems support these CoSes, many DSL access multiplexers in central offices support only the unspecified bit rate (UBR) CoS. UBR connections receive only best-effort service and lack QoS guarantees that control transmission characteristics such as cell loss. If there is no bandwidth available to transport UBR cells, they are discarded.

Before building a DSL VPN, users must verify that their DSL service provider supports multiple virtual circuits over a DSL local loop, and that multiple CoSes -_CBR, VBR and UBR - are supported. With multiple CoSes, voice virtual circuits can be configured as either CBR or VBR real-time connections, with limited cell loss and transit delay, in order to guarantee voice quality.

Depending on the service level required, data virtual circuits can be configured as CBR, VBR or UBR connections and share the same DSL line with voice calls. Because virtual circuits carrying voice calls receive a higher priority for bandwidth, the data connections on the same line do not interfere with voice quality.

For telecommuters, network professionals can take advantage of PPP over ATM. RFC 2364, the IETF's recommendation, describes the use of ATM Adaptation Layer 5 for framing PPP-encapsulated packets. PPP over ATM is becoming the most common service architecture for remote access over DSL and is serving as the baseline for vendor interoperability.

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PPP Over AAL5
RFC 2364.

Mitchell is director of product management at Promatory Communications, a broadband access system provider in Fremont, Calif. He can be reached at tmitchell@ promatory.com or (510) 413-4864.