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Optoelectronics & Communications

Big standard on campus

10 Gigabit Ethernet promises to revise services and network topologies in the LAN, WAN, and MAN.

From oemagazine February 2002
31 February 2002, SPIE Newsroom. DOI: 10.1117/2.5200202.0008

IT managers are mandated to evolve their enterprise networking environments to support process re-engineering, address globalization issues, and serve their customers better--all while managing operational and life-cycle costs in an increasingly complex environment. Today's installed base of campus network infrastructures is seen as an obstacle to meeting these objectives.

Campus networks vary greatly in size and scope, from a couple of workgroups to several hundred workgroups handled by multiple campus switches linked to a wide-area network (WAN) edge device (e.g., a router or an enterprise network switch). Modern campus networking environments also can be overly complex and insufficiently available, which creates barriers when implementing new applications. In all cases, effective in-building and wide-area networking capabilities at campus sites, which have a high concen- tration of employees and functional responsibility, limit the development of networks that can truly meet the need of the enterprise.

One approach to addressing the cost and scalability challenge is Ethernet protocol, which has been a viable campus networking solution for more than 20 years. Ethernet has evolved with the times, to Gigabit Ethernet and most recently to 10 Gigabit Ethernet (10 GE). Although the final standard has not been officially adopted, 10-GE products are now being installed in high-volume networks in which the consolidation of multiple Gigabit Ethernet networks makes sense.

It is important to note that 10 GE is more than simply a faster version of what has gone before. The revamped standard will facilitate new services and topologies: For example, it will allow interconnecting campuses on a single, shared, high-capacity metropolitan ring to seamlessly link all sites on a virtual private network (VPN) under a common Ethernet protocol with no translations between sites.

going to 10 GE

In terms of the open-systems-interconnection (OSI) networking model, 10 GE is a Layer 2 protocol. It does not use the same operating parameters or specification method as SONET but is compatible and within the same operating range. This allows 10 GE to operate over the WAN infrastructure. On the photonic hardware side, the physical media dependent (PMD) sublayer will involve OC-192 (10 Gb/s) components.

When considering a 10-GE strategy, it is important to be aware of the current status of the 10-GE standard and how the standard is likely to evolve and be implemented. In order to take advantage of the 10-Gigabit OC-192c (concatenated) installed base and allow network providers to offer these new services and topologies, the Institute of Electronic and Electrical Engineers (IEEE) 10-GE task force (802.3ae) decided that its definition must be WAN compatible. The task force also recognized that some campus operators might prefer to operate networks at a theoretical maximum throughput--the local area network (LAN) rate. Thus, the team incorporated two physical layer interface (PHY) specifications to the proposed final standard.

The PHY layer includes everything from the medium access controller on down. This layer is divided into the physical code sublayer (PCS), WAN interface sublayer (WIS), physical medium attachment (PMA), and physical medium dependent (PMD). When operating in WAN compatible mode, the PHY engages the WIS; when operating in LAN mode, the PHY bypasses the WIS. The PCS provides the encoding which is 64b/66b for both the LAN and WAN. The WIS layer inserts a SDH/SONET compatible overhead subset and performs SDH/SONET compatible scrambling and management functions.

All networking needs--LAN, metropolitan area network (MAN), and WAN access--can be met with the WAN PHY. This interface ensures compatibility with the installed base of time-division-multiplexed (TDM) and dense-wavelength- division-multiplexed optical networking gear, enabling carriers to leverage existing infrastructures to increase return on investment. The proposed standard has taken into consideration the wide variety and different types of fiber-optic cable installed in networks. The WAN cable types are 10GBASE-SW, 10GBASE-LW, and 10GBASE-EW, which are compatible with WAN 850-nm serial (SW), 1310-nm serial (LW), and 1550-nm serial (EW) transceivers.

The LAN PHY uses similar cabling and transceivers. The two major differences between the LAN PHY and the WAN PHY are the LAN standard's support for multimode fiber and its reduced overhead, which results in a 7% increase in speed. Vendors will eventually support both PHYs in future products by incorporating techniques to convert from one flavor to the other.

The IEEE task force rejected conformance to synchronous-optical-networking (SONET) jitter, stratum clock, and other optical specifications, as these techniques added complexity and costs that were unnecessary and possibly undesirable in the Ethernet environment. The inclusion of a SONET framing sublayer in the WAN PHY, however, allows 10-GE switches and routers to connect to SONET access equipment and use the SONET infrastructure for Layer 1 transport. These links remain asynchronous Ethernet links, presenting Layer 2 Ethernet packets to the SONET infrastructure with just enough SONET management information that the link may be managed as a SONET link.

However, the similarity between SONET and the 10-GE WAN PHY stops there; SONET systems use synchronized, high-accuracy stratum clocks to form a synchronous clock hierarchy. These high-accuracy clocks support regenerators that re-create the signals moving from one SONET segment to the next. On the other hand, WAN-compatible 10 GE remains an asynchronous protocol and operates like any other asynchronous network interface. Therefore, there is no need to support the expensive timing, clocking, and jitter requirements of the synchronous optical network for which every device shares the same precisely aligned stratum clock. So the benefits of transport, speed, and reliability have been retained, while the cost and complexity of SONET has been avoided. As a result, 10 GE is a logical path forward for future networks, providing the industry with a robust Ethernet solution that maintains the simplicity of the Ethernet yet leverages the considerable installed infrastructure while offering superior network reliability.

looking ahead

With the adoption of the 10-GE standard, new solutions will come to the forefront. The 10-GE ring interconnection will be accelerated by Internet access traffic, which is often hubbed to one or two point-of-presence servers (PoPs) in a carrier MAN, creating very high traffic concentrations at these ring nodes. PoPs are beginning to operate on simple, low-cost, Gigabit Ethernet–switched architectures, which make them look more and more like large campus backbones. As 10 GE emerges, Layer 2 Ethernet switches and core routers with these interfaces will address OC-192c capacity requirements and provide scalability and economical connectivity.

The wide adoption of the standard will ensure worldwide multivendor interoperability with the Ethernet installed base while simplifying connection management. With these innovations, coupled with the ability to scale to the full requirements of service providers, optical Ethernet solutions are poised to fundamentally change the economics of the service-provider industry at a time when competition for IP services remains fierce.

All this new technology will affect how campus networks are designed and deployed. In campus and MAN applications, current fiber distributed data interface (FDDI) and 100-Mb/s campus and MAN links running over dedicated fiber can be upgraded to Gigabit Ethernet over distances as large as 50 km and span full MAN reach.

To further increase return on the service provider's investment in infrastructure, long-haul carrier optical networks will use dense wavelength division multiplexing (DWDM). Such networks are capable of offering 160 channels on a single fiber, using a composite optical signal carrying multiple information streams, each signal transmitted on a separate optical wavelength. Each optical signal can carry any protocol, such as SONET or Ethernet, with a bit-rate ranging up to 10 Gb/s.

The path to a finalized and completed standard is quickly drawing to an end. During standards development, a series of drafts (revisions) is proposed and voted on by the members of the IEEE. As of this writing, the first two drafts have been completed. The final vote is due to take place in the first half of 2002. Leading component manufacturers have charged forward, some already offering second-generation products in which both LAN PHY and WAN PHY are available on a single component. This will allow manufacturers to address broad sets of user needs with no additional costs. The first iterations of these products are now available. oe

Marc Bernstein, Rodney Wilson

Marc Bernstein is a senior solutions manager and Rodney Wilson is a technology business leader at Nortel Networks.