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Optoelectronics & Communications

Ethernet moves into the metro

Once strictly a data protocol, optical Ethernet is finding applications in the metropolitan area network.

From oemagazine August 2002
31 August 2002, SPIE Newsroom. DOI: 10.1117/2.5200208.0003

Initially designed for premises network communications, optical Ethernet is poised to move into the metropolitan area network (MAN). Enterprise offices are connected internally with 100-Mb/s and Gigabit Ethernet connections, while data rides between cities over 10 Gb/s dense wavelength division multiplexing (DWDM) networks. Unfortunately, the MAN that connects the Gigabit enterprise office to the terabit network core often runs on time-division multiplexed (TDM) circuits at speeds of 64 Kb/s to 1.544 Mb/s, which creates a bottleneck.

Service providers are equally handicapped. Local exchange carriers report tens of thousands of held service orders for private-line circuits at T1 and higher data rates and recognize the complexity of selling, managing, and provisioning TDM circuits as a root cause of high operating cost and poor service response. Furthermore, advanced service offerings such as Internet protocol virtual private networks (VPNs), Internet access, asynchronous transfer mode (ATM), and frame relay ride on traditional TDM transport services and are constrained by limited high-cost metro-area bandwidth.

reasons behind the shift

Data has surpassed voice as the majority of traffic in today's metro networks. Service providers and carriers are looking to build a new infrastructure optimized to carry data traffic or update their existing infrastructure. Ethernet is well-suited to carrying next-generation service offerings due to its compatibility with typical bursty data traffic. Using the same protocol with similar functionality as in a local area network (LAN) translates into simplicity. More than 90% of end points in today's Internet are Ethernet. Using Ethernet in the metro network allows users to maintain the same service interfaces and potential transport as the enterprise uses today in their LAN, eliminating the need for protocol translations such as Ethernet to ATM.

The overriding reason for the success of Ethernet in the enterprise network has been cost-effectiveness. Cost is a major concern in the MAN space. If the metro network can leverage components of wide area network (WAN)-deployed Ethernet, it can achieve the same economies as the enterprise space. A recent study conducted by Network Strategy Partners (Boston, MA) found that medium-sized enterprise customers could cut telecommunications expenses more than 50% on Internet access and private-data services using Ethernet in the metro. The approach offers scalability from 1 Mb/s to 1 Gb/s in granular 1-Mb/s increments. This eliminates overprovisioning of fixed TDM bandwidth increments to accommodate intermediate speeds and avoids complex midstream architectural transitions to satisfy high-capacity requirements.

Service turn-up and scalability are more economical for metro Ethernet than for traditional metro data services. Conventional services can require initial capital outlay for a WAN router, a frame-relay access device, or an ATM access device. In a metro Ethernet system, these services are delivered over an Ethernet interface, so networks require only an Ethernet port on an economical layer 2/3 LAN switch (existing or newly procured).

Enterprises transitioning to Ethernet services from traditional alternatives can leverage Ethernet ports on their existing WAN access devices, which are typically 25% to 40% less expensive per Mb/s of bandwidth than TDM, frame-relay, and ATM ports and up to 10 times less expensive than high-speed synchronous optical networking (SONET) interfaces. Recent studies conducted by Gartner Group (Stamford, CT) and Yankee Group (Boston, MA) project that Ethernet switching costs will continue to decrease approximately 30% year over year. Additionally, software-provisioned rate-limiting techniques allow enterprises to provision/deliver service options from 1 Mb/s to 1 Gb/s from a single Gigabit Ethernet port, meaning that upgrading service does not require purchasing and swapping out expensive interface cards or platforms.

metro Ethernet standards

In order to make Ethernet successful in metro networks, it indeed needs to evolve to meet requirements of service providers and carriers. First, Ethernet services must be defined so that service providers can offer a set of services over any metro Ethernet infrastructure. Second, Ethernet itself must be enhanced to "carrier-class" levels.

No vendor can do it alone. It calls for standardization across the industry to provide interoperable equipment to deliver the above. The Metro Ethernet Forum (MEF), a consortium of equipment manufacturers, was created in June 2001 to fulfill this promise. The MEF's primary priorities are first to define Ethernet services for metro transport networks. Such services would be delivered over native Ethernet-based metro networks and could also be supported by other transport technologies. Second, the MEF is defining carrier-class Ethernet-based metro transport technologies by specifying architecture, protocols, and management required for such networks.

Since its inception, the MEF has developed metro Ethernet services definitions to enable standardized service offerings from service providers to end-customers and metro Ethernet protection that enables sub-50-ms network restoration and hitless protection capabilities to enable support of TDM and other time-sensitive applications over Ethernet. They have also established quality of service (QoS) specifications to enable guaranteed service level agreements over Ethernet-based networks; a user network interface standard for how services will be provisioned, monitored, and communicated between user and network interface (including loop-back capabilities, link monitoring, and remote fault detection); and end-to-end operation, administration, and maintenance (OAM), element management systems (EMSs), and network management services (NMSs) over Ethernet to enable network manageability.

The MEF divides its efforts into four major areas: protocols and transport, service, management, and architecture. To meet requirements, metro Ethernet will need application-specific protocols and transport mechanisms. The Metro Ethernet Protection Requirement Document defines service providers' requirements for deploying Ethernet-based metro networks. It is in the straw-ballot stage. Protection Framework 1.0 describes a model and framework to deliver network-protection services, including hitless end-to-end network protection with sub-50-ms recovery time over metro Ethernet networks. The document is in the draft stage. The QoS Framework details a QoS network reference model that includes the metro Ethernet networks (MEN) architecture and how QoS functional model/mechanisms can be applied at MEN to achieve required performance characteristics for any MEF Ethernet services. This document is also in the draft stage.

The services area is chartered to define specific service offerings at the request of service providers, the primary metro Ethernet client. Committees in the services group are working to define Ethernet virtual private line services (EVPLS), Ethernet virtual private LAN services (EVPLnS), and circuit emulation service (CES). EVPLS refers to point-to-point services that can replace today's private lines and frame-relay connections with Ethernet technology. EVPLnS allows connection of multiple sites as if they were on a single Ethernet, and CES allows the transport of time-division circuits over a metro Ethernet network. The group is also working to define the mechanisms of traffic and performance parameters, including how to scale bandwidth up and down over a metro Ethernet network.

Meanwhile, the architecture area is working to define the framework between end user and service provider, and the management area is defining requirements and information models describing OAM, EMSs, and NMSs

Optical Ethernet combines the distinct benefits of Ethernet ubiquity and its performance (scalability, low cost, flexibility, and simplicity), therefore enabling delivery of the next generation of a broad set of metro services with the same carrier-class features available in traditional SONET/SDH-based solutions, including sub-50-ms protection, TDM support, and guaranteed SLAs. The Metro Ethernet Forum is at work to make Optical Ethernet metro services and metro transport reality. oe

Nan Chen
Nan Chen is president of the Metro Ethernet Forum and director of product marketing at Atrica Inc. in Santa Clara, CA.