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Ethernet moves past 10Gbps

Posted: 01 May 2001 ?? ?Print Version ?Bookmark and Share

Keywords:gigabit ethernet? 10gbps lan? sonet? sdh? dwdm?

Until recently, Ethernet has been confined to the LAN space because of distance and performance limitations inherent in the original protocol. But in the last two years, advances in Ethernet protocols and the emergence of an intelligent optical layer have given birth to an alternative transport architecture that extends the power of Ethernet to metropolitan, regional and long-haul backbone networks. This has led to the use of Gigabit Ethernet in these longer-range networks, and plans are being made to hone 10Gbps Ethernet for them as well.

The inventors of Ethernet more than 25 years ago could not have anticipated the impact of the protocol. It became the dominant LAN technology during the 1990s; by 1999 more than 53 million ports of switched Ethernet were sold. The IEEE has revised the protocol at least four times:

  • From shared to switched.

  • From running over coax to running over fiber.

  • From known distance limitations to the possibility of being limitless.

  • From running at 10Mbps to achieving 10Gbps while maintaining its original frame format.

The deployment of Ethernet service over a MAN can take on many forms because of the limited distances?usually less than 100km?within a city. One option is to deploy Ethernet services across dark fiber without wave-division multiplexing (WDM). Although the IEEE-802.3 standard for Gigabit Ethernet specifies only a 5km distance requirement (1,000Base-LX), by using long-reach optics many switch and router vendors have developed longer-reach Gigabit Ethernet interfaces that can span more than 70km (referred to as 1,000Base-LH by some). By connecting this Gigabit Ethernet equipment over dark fibers, one can build a city-wide Ethernet network to offer Ethernet service to interconnect LANs from enterprise customers.

However, in metro core applications dense wave-division multiplexing (DWDM) is needed because of high traffic demands and fiber exhaustion. Also, traffic has been highly aggregated at the core routers and switches, thus driving the need for Gigabit Ethernet services in the metro core to be supported at a full gigabit rate. These services can be provisioned over the metro core via a number of methods.

Wavelengths can be efficiently shared over the intelligent optical network by packing multiple Gigabit Ethernet streams into a wavelength using Sonet/SDH time-division multiplexing. For example, two Gigabit Ethernet streams can be transported via an OC-48/STM-16 (2.5Gbps) circuit, or eight Gigabit Ethernet streams can be merged onto an oc-192/STM-64 (10Gbps) circuit.

Intelligent optical networking is a separate option that can solve the problems of network scaling and high-speed service delivery. The term refers to the products, technologies and functionality required to construct a flexible and highly scalable optical network architecture for the delivery of public network services.

It combines the functionality of Sonet/SDH, the capacity creation of DWDM and innovative networking software into a new class of optical transport. This approach allows carriers and service providers to transform existing Sonet/SDH infrastructure to offer end-to-end carrier-class Ethernet services at rates up to 1Gbps.

The next logical data rate upgrade after Gigabit Ethernet is 10Gbps. In contrast to previous Ethernet standards, 10Gbps Ethernet is targeting the metropolitan and wide-area networks so that end-to-end Ethernet transport can be realized. Not just an access technology like Ethernet, 10-Gbit Ethernet aims to become the simplest, fastest and most cost-effective backbone network technology.

Prestandard products due

To accelerate the introduction of 10Gbps Ethernet, the 10 Gigabit Ethernet Alliance was formed in March 1999 to foster the development of the technology's standard, IEEE 802.3ae, and promote interoperability among products. The IEEE P802.3ae Task Force recently completed the first draft of the 10-Gigabit Ethernet specification, which is scheduled for ratification in early 2002. However, prestandard products were expected to be available toward the end of 2000.

Several important restrictions apply to 10Gbps Ethernet: The traditional CSMA/CD protocol will not be used, half-duplex operation is not supported and copper wiring will not be an option (at least not for end station links).

The specification includes several major logical elements. In the media-access control (MAC) layer, the specification will preserve the Ethernet frame format, including both minimum and maximum frame sizes. It will also support full-duplex operation only and thus has no inherent distance limitations. Because it is still Ethernet, users can preserve their management tools and architecture.

In addition to traditional LAN connectivity, the standard is being developed with an option for connection across MAN and wan links at an expected data rate compatible with OC192c. The MAC must adapt rates to ensure a data rate of 9.58Gbps by using an open-loop mechanism and appropriately adjusting the interpacket gap. The WAN Interface Sublayer performs Sonet/SDH framing, scrambling and error detection to allow minimal compatibility with the requirement of Sonet/SDH networks. This will extend the benefits of Ethernet technology to the construction of MANs and WANs, and provide cost-effective networks for larger geographic areas using a single technology end to end.

? Albert Crews and Danny Tsang

Sycamore Networks Inc.

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