Carrier Ethernet unlocks many potential revenue-generating services that telecommunications service providers, known as carriers, must deploy to maintain their competitive position. However, most carriers aren’t ready to convert to a pure Ethernet network due to Ethernet’s lack of native support for link monitoring, fault isolation, and diagnostic testing.
These attributes, which enhance service quality, are native to the plesiochronous digital hierarchy (PDH) and synchronous Sonet/synchronous digital hierarchy (SDH) networks. Over decades, carriers have come to trust PDH and Sonet/SDH networks as dependable platforms for delivering critical services to demanding customers.
Achieving the transparent and efficient transport of native Ethernet frames from network edge to network edge is challenging. And, in the past, overcoming these challenges was a costly endeavor. Near the end of the 1990s, many carriers forklifted some portion of their networks and replaced them with what was then called “Next Generation” Sonet/SDH (NGS) equipment.
The strength of this equipment was the efficient transport of Ethernet and time-domain multiplexed (TDM) services when the infrastructure might approach 100% utilization. Its weakness, though, was the lack of interoperability with legacy systems. Each node that terminated or handed off a service needed to be replaced with a new system. While this stimulated business for equipment makers, replacing legacy nodes was an inefficient use of carriers’ capital. Today, though, using new protocols that enable the reuse of legacy equipment minimizes the overall cost of delivering new carrier Ethernet services.
Before understanding the advantages of the new methodology, it’s important to understand a few details of NGS. When transporting Ethernet, NGS solutions place Generic Framing Protocol (GFP) encapsulated Ethernet frames directly into variable-bandwidth concatenated Sonet/SDH virtual containers, primarily using the methods defined by ITU-T G.707. This transport scheme promised optimal bandwidth usage in a Sonet/SDH link when running near full capacity by allowing a very fine bandwidth granularity for each service on an NGS network.
Many carriers regarded this class of equipment as the ideal technological solution of the time. Yet when terminating or handing off a service, these concatenated virtual containers must be resolved into a physical interface, such as OC-3, STM-1, T1, E1, or DS3.
NGS systems don’t interoperate well with legacy systems because the concatenated virtual containers originating at an NGS node can’t be resolved to a standardized physical interface by a legacy Sonet/SDH system. Since legacy Sonet/SDH systems can’t perform this task, NGS equipment is required at these nodes. In addition, when a legacy network is used to transport a service that originates at an NGS node, typically an entire legacy Sonet/SDH container is allocated to the path. This eliminates the fiber bandwidth efficiency gained by using NGS. In short, NGS systems ignored interoperability with the established transport methods in favor of bandwidth utilization promises that were rarely achieved.
The new approach for efficient transport of Ethernet over Sonet/SDH leverages, rather than deviates from, traditional transport methods. To grasp the importance of this approach, we must start with some fundamentals of legacy Sonet/SDH systems.
All telecommunications equipment depends on protocol processing in silicon and software to perform the bulk of its duties. The basic protocol stack of a legacy Sonet/SDH add-drop multiplexer (ADM) is shown in Stack A of Figure 1. This protocol stack has been used for many years to carry the PDH TDM services, such as leased T1, E1, and DS3 lines.
The T1, E1, and DS3 services are well understood, globally deployed, and trusted. Therefore, it’s understandable that the International Telecommunications Union (ITU) would adopt these PDH technologies as the transport layer for new Ethernet services. Recently, the ITU developed new recommendations for Ethernet transport over single and multiple PDH links. The applicable standards are ITU-T G.7041, G.7042, and G.7043. Collectively, these recommendations are the fundamental building blocks of Ethernet-over-PDH (EoPDH) technology. The protocol stack used in EoPDH equipment is labeled and shown in the top portion of Stack B in Figure 1.
EoPDH is a collection of technologies and new standards that allow carriers to use the extensive existing telecommunications copper infrastructure to provide new Ethernet-centric services. EoPDH standards pave the way for interoperability and the gradual migration of carriers to pure Ethernet networks.
The standardized technologies used in EoPDH include frame encapsulation, mapping, link aggregation, link capacity adjustment, and management messaging. Common practices in EoPDH equipment also include the tagging of traffic for separation into virtual networks, prioritization of user traffic, and a broad range of higher-layer applications. Although EoPDH was created for point-to-point delivery of Ethernet over physical PDH tributaries, when combined with legacy Sonet/SDH, EoPDH becomes an important element and cost-effective tool for Ethernet service delivery.
A new class of Sonet/SDH equipment maps Ethernet frames into virtually concatenated PDH tributaries using the EoPDH standards and then uses traditional mapping techniques to transport the PDH connections over the existing Sonet/SDH network. The protocol stack of this equipment is shown in Stack B of Figure 1.
Because this stack combines EoPDH and PDH-over-Sonet/SDH, the technology is called Ethernet over PDH over Sonet/SDH, or EoPoS. The division between the legacy stack and the EoPDH stack at a protocol layer compatible with standardized PDH technology allows for an optional physical interface. As a result, stack processing can be spread across multiple pieces of equipment.
One advantage of EoPoS technology is that it enables a mixed environment of legacy and new equipment. The real strength of EoPoS is that it leverages the existing infrastructure of systems and knowledge for transporting PDH tributaries over Sonet/SDH networks. Unlike the NGS approach, which attempted to optimize bandwidth at all costs, EoPoS minimizes costs while still efficiently using bandwidth. To understand these advantages, let’s look at an example application.
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