Standards bring more flexibility to SONET/SDH


About three years ago, it became clear that the required telecommunications bandwidth was growing faster than anticipated and that transport of data was becoming more important than voice (see Lightwave, May 1999, p. 43). Information to be sent over the SONET/SDH network is placed into "containers" that are then mapped into the payload area of that particular SONET/SDH frame. The containers are "virtual" in the sense that they are pointed to by a frame but can be interchanged or moved without difficulty. Existing SONET/SDH virtual containers (VC/SPE) do not match the bandwidth of the data channels to be transported. These containers support 2, 45, 155, and 2,500 Mbits/sec, while popular data rates are 10, 100, and 1,000 Mbits/sec. In addition, operators like to be able to sell their bandwidth in small chunks. Standards organizations have taken several successful steps to solve this issue.

First proposed in November 1999, virtual concatenation is now accepted in all standards (i.e., ITU-T Recommendations G.707 and G.783, ETSI EN 300 417-9-1, and ANSI T1.105). Virtual concatenation provides an efficiency of 95-100% by grouping a number of VCs (VC-n-Xv), using the combined payload to match the required bandwidth featuring:

  • No requirements on existing SONET/SDH nodes that transit the VC's part of a VC-n-Xv group.
  • Compensation of differential delay (i.e., no routing constraints for operators).

Although virtual concatenation im proves efficiency, the size of the bandwidth is fixed, and if one of the VCs fails, then the full payload is discarded. Data transport can have a variable requirement for bandwidth. This issue is ad dressed by the Link Capacity Adjustment Scheme (LCAS).

LCAS provides a flexible increase and decrease in the allocated bandwidth. If one of the VCs fails, then the transport continues with a reduced bandwidth. First proposed in June 2000, LCAS is now in ANSI T1.105 and ready for ITU-T as a generic Recommendation: G.lcas. ETSI will follow ITU-T.

Because there are several standards in use for data transport, proposals were emerging for each individual signal to map the signal into SONET/SDH. To avoid an avalanche of mapping standards, it was decided to develop a more generic approach: Generic Framing Procedure (GFP). The first proposal for this approach was made in the July 2000 meeting of T1X1.5, with the objective to get approval for ITU-T Recommendation G.gfp at the ITU-T Study Group 15 meeting last October.

Implementations can use any or all of the standards mentioned here for transporting data over SONET/SDH.

Huub van Helvoort is a systems architect for Lucent Technologies' Optical Networking Group in Huizen, the Netherlands. He can be reached at tel: +31 35 687 4393; fax: +31 35 687 5964; e-mail:

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