In most carriers’ metropolitan networks, services are delivered over interconnected Sonet/SDH rings (). 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 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 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.