Electronic Design

Use Ethernet Over PDH In Sonet/SDH Networks

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.

In most carriers’ metropolitan networks, services are delivered over interconnected Sonet/SDH rings (Fig. 2). Although the legacy ADM is diagrammed as a single node in the figure (node C), it represents the bulk of telecom equipment deployed in the field. To place appropriate emphasis on the weight of this factor, the installed base of legacy Sonet/SDH equipment is worth hundreds of billions of dollars.

It’s very important to note that a large portion of this equipment is fully depreciated and only incurs ongoing operating expenses. For a new piece of equipment to decrease total operating costs, the asset’s depreciation expense plus the maintenance expense together must be less than the operating expense of the old, fully depreciated equipment. This single factor makes a strong cost argument for maintaining the operation of legacy Sonet/SDH equipment.

Node A in Figure 2 represents a new piece of equipment using EoPoS technology. In keeping with the principle of interoperability, this equipment typically supports the traditional Ethernet-over-Sonet/SDH (EoS) and NGS protocols. Therefore, Ethernet traffic can flow from the new EoPoS node to the NGS system at node B and from the EoPoS node to the legacy node.

As discussed earlier, the legacy node’s protocol stack doesn’t include the NGS protocol. Because the NGS protocol lacks a physical PDH interface, the legacy node can’t terminate an Ethernet flow sourced from the NGS node. The legacy ADM at node C can transport and hand off the EoPoS flow from Node A. The legacy ADM processes the bottom portion of Stack B in Figure 1 and provides physical PDH connection to a low-cost piece of equipment. A CPE supporting EoPDH processes the top portion of Stack B in Figure 1 and thus fully terminates the EoPoS flow.

When an existing customer converts from a legacy TDM service to an Ethernet service, the incremental cost at the legacy node is only a low-cost piece of equipment compliant with the EoPDH standards, not an expensive NGS Sonet/SDH box. This natural division of protocol processing at the PDH layer is also useful in applications where leased PDH lines are required to reach a customer site where the EoPDH equipment resides.

In addition, when the Sonet/SDH network between nodes A and C consists of a complex web of interconnected legacy equipment, the legacy equipment can manage the component EoPoS flows as if they were simple PDH tributaries. While an ADM is used for this example, carrier Ethernet equipment benefiting from EoPoS technology includes a broad range of equipment types, such as multi-service provisioning platforms (MSPPs), demarcation units, reconfigurable optical ADMs (ROADMs), media gateways, IP DSL access multiplexers (DSLAMs), and microwave radios.

Sonet/SDH equipment enabled with EoPoS technology delivers many of the benefits promised by NGS equipment, while optimizing deployment expense. By using a standardized virtual concatenation method, the bandwidth consumed by a carrier Ethernet service can be dynamically adjusted in increments as small as 1.5 Mbits/s. The ITU-T G.7042 VCAT/LCAS protocol offers dynamic allocation and the flexibility to effectively use all of the Sonet/SDH bandwidth. Carrier Ethernet service subscribers can be allocated the bandwidth they require, with little wasted bandwidth. By making intelligent use of the EoPDH protocols in conjunction with Sonet/SDH equipment, costs can be minimized while transitioning a network to support new carrier Ethernet services.

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