14.3 Ethernet Directly Over DSL

Fiber-based Gigabit Ethernet capable of supporting public metropolitan area networks is an emerging technology for supporting a carrier's access network. As these networks are deployed, DSL interfaces become an obvious choice for providing customer access to these new networks. Figure 14.7 gives a high-level overview of the use of DSL to provide access to such a network.

Figure 14.7. An overview of DSL used to access a metropolitan Ethernet.

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Table 14.7. PPP over Ethernet Qualities

Criteria	PPPoE Qualities
Payload efficiency	PPPoE adds a 6-byte header beyond those that are required for AAL5, Ethernet, and PPP. As Ethernet is carried over the ATM network, the Ethernet overhead is added to the end-to-end communication.
Session multiplexing	Multiple PPP sessions can be multiplexed over the Ethernet LAN and over a single ATM VC.
Multiprotocol support	PPP allows the support of multiple protocols over each session. Each PDU contains a protocol ID to identify the payload type. Additionally, because Ethernet is extended over the WAN (using RFC 2684), protocols other than PPP can be supported on the same ATM VC simultaneously with multiple PPPoE sessions.
Autoconfiguration and management	The autoconfiguration functionality provided by ATM, PPP, and the IP stack are available to support autoconfiguration.
Service selection	The ability to negotiate an individual PPP session from each end point on the LAN allows each user to access a particular service on the far end of the network.
Security and AAA	The security functions provided by PPP are available when PPPoE is used.
Standards	Defined in IETF RFC 2516 [7]. This is an informational RFC that indicates common industry practice.

Gigabit Ethernet has compelling attractions as a core transport for metropolitan access networks:

Most LANs at the customer's premises use Ethernet. Extending these Ethernets LANs directly over the metropolitan access network provides a simple architecture.
The development of routers capable of routing packets delivered at gigabit and higher rates (line speed routers) makes development of these networks relatively straightforward.
The capital cost for metropolitan Ethernet architectures is likely to be lower than comparable architectures based on ATM or IP over SONET.
It is claimed that management of these networks will be simpler than the alternatives.

As these networks are deployed, the use of DSL to provide the "last mile" of access to many customers becomes a virtual necessity.

Fiber capable of supporting Gigabit Ethernet directly to a customer's site is economic only for the largest of customers of a metropolitan Ethernet. Other customers require the inexpensive but lower bandwidth interfaces provided by DSL technologies.
Only DSL, with its use of twisted pair copper , provides the ubiquitous access required to reach all possible users.
Although attempts are being made to standardize a specific Ethernet PHY (the IEEE "Ethernet in the last mile" effort) over telephone loop quality twisted pairs, DSL standards and implementations are already mature and capable of supporting Ethernet directly on the DSL PHY. Although additional standards work is required to define the direct support of Ethernet on DSL interfaces, this work is incremental and would be based on the foundation of existing DSL physical layer standards.

Table 14.8. Ethernet Directly over DSL

Criteria	Ethernet Directly over DSL
Payload efficiency	There should be little or no additional overhead beyond those already required for Ethernet and the protocols above it in the stack.
Session multiplexing	Ethernet is capable of supporting sessions via the use of PPPoE.
Multiprotocol support	Because the Ethernet is extended over the WAN, any stack capable of being carried over Ethernet can be transported.
Autoconfiguration and management	Ethernet is largely self-configuring .
Service selection	The service selection functions of protocols carried over Ethernet are available.
Security and AAA	Security is a function of the higher layers of the stack.
Standards	Ethernet is defined in the IEEE 802 series of standards. The specifics of adaptation of Ethernet to the various DSL technologies need to be standardized.

Ethernet directly over DSL has the qualities shown in Table 14.8 when compared with some of the criteria from DSL Forum TR-43.

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