In order to support increasing data traffic levels, equipment developers must build systems that map Ethernet packets over Sonet/SDH links. In this two-part series we'll lay out the encapsulation techniques required to make Ethernet-over-Sonet come to life. Today's incumbent service providers are seeking to accommodate increasing demands for bandwidth, security and service level management resulting from the proliferation of LAN-based applications. According to RHK, 2.4 million metro Ethernet ports distributed among 10/100/1000 Mbps rates are expected to be deployed by 2006. These Ethernet ports will primarily be used to support these emerging applications: • Broadband Internet access for e-mail and web-based content • Packet-based video conferencing
• Tunneled virtual private networks (VPNs)
• E-commerce hosting for outsourced applications services such as customer relationship management (CRM) Storage area networking (SAN) for outsourced access/management of server content including disaster recovery services The big issue carriers face is how to handle Ethernet traffic. Depending on the cost, distance, bandwidth, and traffic management requirements of the WAN/MAN application, several metro approaches may be used to transport Ethernet data traffic. These include direct mapping of Ethernet over wavelengths (EoW), Ethernet over Sonet/SDH (EoS), optical Ethernet (i.e. native Ethernet over fiber for long haul, 1000BaseLX), or implementing an Ethernet in the first mile (EFM) solution over copper or fiber. Of all these solutions, EoS is the one gaining the most ground with today's developers. In this two-part series, we'll lay out some of the key requirements for mapping Ethernet frames over Sonet/SDH links. In part 1, we'll look at the advantages EoS brings to an equipment designer. We'll also begin a discussion on encapsulation techniques, looking at virtual concatenation and the link capacity adjustment scheme (LCAS). The later part will follow with a discussion on the generic framing protocol (GFP) and the link access procedure for SDH (LAPS) EoS--Why it Should Win
EoS collectively represents a group of industry standard specifications that have been developed for optimal transport of Ethernet through battle-tested circuit-switched topologies. It embodies several related technologies and can be implemented using techniques such as virtual concatenation (VC), which is a recent extension to the ITU-T G.707 SDH standards. Together these specifications account for the mapping, aligning, sequencing and delay compensation of the individual channels. The result is a very cost-effective way to provide flexible bandwidth in small to large increments with a rate adaptation benefit. Historically, in order to compensate for transport inefficiencies that existed between Ethernet and Sonet, service providers would over-provision their circuits to ensure reliable delivery of data. This traditionally made use of standard contiguous concatenation along with packet-over-Sonet/SDH (POS) and was fixed in terms of bandwidth granularity. POS, the transmission of IP data over Sonet frames via PPP, has certain real advantages but in the past required that bandwidth be predetermined in a rigid and constrained manner. For example, a Gigabit Ethernet stream in POS would need a full OC-48 pipe with standard contiguous concatenation. Service providers have dealt with the resulting cost penalty by deploying extra router interfaces to support mesh and aggregation of links. On occasion they employ redundant routers to meet protection and load balancing requirements. EoS takes a different approach to the problem. Unlike POS, which calls for rigid bandwidth requirements, EoS allows bandwidth to be shared among several Ethernet ports Figure 1. Using EoS in combination with VC, a Gigabit Ethernet channel can be built (STS-1-24C ot 24 STS-1's concatenated) while the unused portion of the OC-48 bandwidth can be...
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