History of transport networks

Transport networks, the underlying optical fiber-based layer of telecommunications networks, have evolved from Digital cross connect system (DCS)-based mesh architectures in the 1980s, to SONET/SDH (Synchronous Optical Networking/Synchronous Digital Hierarchy) ring architectures in the 1990s. In DCS-based mesh architectures, telecommunications carriers deployed restoration systems for DS3 circuits such as at&t FASTAR (FAST Automatic Restoration)[1] and MCI Real Time Restoration (RTR), restoring circuits in minutes after a network failure. In SONET/SDH rings, carriers implemented ring protection such as SONET Universal Path Switched Ring (UPSR) (also called Sub-Network Connection Protection (SCNP) in SDH networks) or SONET Bidirectional Line Switched Ring (BLSR)[4] (also called Multiplex Section - Shared Protection Ring (MS-SPRing) in SDH networks), protecting against and recovering from a network failure in 50 msecs or less[5], a significant improvement over the recovery time supported in DCS-based mesh restoration, and a key driver for the deployment of SONET/SDH ring-based protection.

There have been attempts at improving and/or evolving traditional ring architectures to overcome some of its limitations, with trans-oceanic ring architecture (ITU-T Rec. G.841), “P-cycles” protection, next-generation SONET/SDH equipment that can handle multiple rings, or have the ability to not close the working or protection ring side, or to share protection capacity among rings (e.g., with Virtual Line Switched Ring (VLSR).

Technological advancements in optical transport switches in the first decade of the 21st century, along with continuous deployment of dense wavelength-division multiplexing (DWDM) systems, have led telecommunications service providers to replace their SONET ring architectures by mesh-based architectures for new traffic. The new optical mesh networks support the same fast recovery previously available in ring networks while achieving better capacity efficiency and resulting in lower capital cost. Such fast recovery (in the tens to hundreds of msecs) in case of failures (e.g., network link or node failure) is achieved through the intelligence embedded in these new optical transport equipment, which allows recovery to be automatic and handled within the network itself as part of the network control plane, without relying on an external network management system.

July 18, 2011

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