Convergence in Optical Transport Networks

Abstract

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, typically using a router within the enterprise. (This application of a router is so prevalent that the PDH interface on a router is called the wide- area network, WAN, interface.)The first step that the SDH standards, and equipment vendors, took to support a data-friendly network was to provide Ethernet interfaces (10/100/1000 Mbps) on the SDH equipment in addition to PDH and SDH interfaces. The provision of Ethernet interfaces on SDH equipment is based on the Generic framing Procedure (GFP), Virtual Concatenation (VCAT) and Link Capacity Adjustment Scheme (LCAS) standards that were published and refined in 2001–03, and which are collectively referred to as the “Ethernet-over-SDH/SONET” or EoS standards. “Next-generation” SDH equipment supporting these EoS standards has been deployed since early 2003; Tejas' first shipment of standards-based EoS equipment to carry commercial traffic was in January 2003.