Convergence in optical transport networks

Optical transport networks interconnect switches and routers in the network core, or connect high-speed, typically business/enterprise, users to the network (access). These networks are almost entirely based on the Synchronous Digital Hierarchy (SDH) standards today (or the equivalent Synchronous Optical Network (SONET) standard in North America).Traditionally, SDH has been the sole transport network and thus has carried all of the telecommunication service provider's traffic, including voice and data. So, SDH has, in a sense, provided “converged” transport, all along. However, the interfaces provided by SDH equipment for the first 10 years or so (since about 1990 to 2002) were restricted to the rates used by the older plesiochronous digital hierarchy (PDH) systems, namely, 2/34/140 Mbps, or the standard SDH rates, 155/622/2488 Mbps. Data typically originates within an enterprise local area network (LAN), invariably Ethernet at 10, 100 or 1000 Mbps, and had to adapted to a “standard” PDH or SDH data rate,...