ITU-T addresses OTN

At last October's meeting of the International Tele communication Union (ITU-T) Study Group 15 in Geneva, Working Party 3/15, chaired by Steve Trowbridge of Lucent Technologies, reached consent on several key Recommendations for the Optical Transport Network (OTN). The OTN is the next generation of transport network after SDH and SONET. It is a new transport networking layer using DWDM, which results in terabits-per-second capacity per fiber; allows wavelength-level switching; provides for enhanced operations, administration, and maintenance, and network functionality (compared to SDH) for all services; and provides for service transparency for clients.

The OTN transport includes multiple optical channels, with nominally one channel per wavelength. Each optical channel transports a digital frame structure, which has payload and overhead areas. The paths transported by the optical channels can have rates of 2.5, 10, or 40 Gbits/sec (i.e., the granularity of the paths is relatively large). The clients (payloads) are mapped into this frame either asynchronously or bit-synchronously.

The ITU-T Recommendations covering SDH refer to "Option 1" and "Option 2" networks. The former are SDH networks optimized for transporting Plesiochronous Digital Hierarchy (PDH) clients of the 2,048-kbit/sec hierarchy. The latter are SDH networks optimized for transporting PDH clients of the 1,544-kbit/sec hierarchy; they are, in effect, SONET networks.

In developing OTN, it was desired that it be transparent for the respective services that are transported (e.g., SONET/SDH, ATM, IP, and 1- and 10- Gigabit Ethernet). That means there should be a single option for OTN, with a single set of requirements that apply for both Option 1 and Option 2 clients. The OTN layer is not required to be synchronized, and its physical layer is not required to transport synchronization. Instead, synchronization is transported via the SDH or SONET clients.

ITU-T Recommendations supporting OTN, which reached consent last October, include G.872, Architecture of Optical Transport Networks; G.709/ Y.1331, Amendment 1, Interfaces for the OTN; G.798, Characteristics of OTN Hierarchy Equipment Functional Blocks; and G.8251, Control of Jitter and Wander within the OTN.

Geoff Garner is a distinguished member of technical staff at Lucent Technologies and the editor of the new ITU-T Recommendation G.8251. He can be reached at tel: 732-949-0374; fax: 732-949-3210; e-mail:

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