Cost and operational issues of alternative core transport architectures

Many network operators commenced deployment of SONET/SDH (synchronous optical network/synchronous digital hierarchy) transport networks in the early 1990s when digital crossconnect systems (DXCs) capable of switching tens of Gbits of traffic were the most suitable building blocks with which to construct a high speed meshed backbone network. Advances in electronic add-drop multiplexing (ADM) equipment with shared protection functionality has stimulated interest in the prospect of ring deployment in backbone networks. ADM rings have already been deployed extensively in local and metro regions of networks with line speeds of up to STM-4 rate (622 Mbit/s). The availability of high speed electronic products at STM-16 (2.4 Gbit/s) and STM-64 (10 Gbit/s) rates, as well as the emergence of wavelength division multiplexing (WDM) products enabling 40 Gbit/s transmission capacity in a single fibre, suggests there may be a more economic alternative to meshed backbones based on DXCs. This paper compares mesh and ring topologies for backbone transport. It outlines the architectural differences between mesh and ring topologies, addresses the cost tradeoffs, and discusses the operational requirements associated with rings.