Overhead implementation alternatives
By JOHN EAVES
In the current ITU-T Study Period (1997 to 2000), Study Groups 13 and 15 have undertaken a major work activity to define the characteristics of the future optical transport network (OTN). Study Group 13 has now essentially completed the text of new draft Recommendation G.872 on OTN architectural concepts, and the work of Study Group 15 on OTN implementation aspects is moving toward completion of the first issue of several new ITU-T recommendations. Among them are the new and interrelated draft Recommendations G.709 on OTN signal structures, G.798 on OTN equipment characteristics, and G.959.1 on OTN physical-layer interfaces. At the October 1998 meeting of Study Group 15, alternatives for implementing the optical channel (OCh) overheads were discussed by the experts of the groups responsible for these three new recommendations.
The OCh is the basic unit of optical transmission in the OTN, providing end-to-end transport of continuous bit-rate digital client signals of various types--e.g., Synchronous Optical Network/ Synchronous Digital Hierarchy (sonet/ sdh) signals, and cell-based Asynchronous Transfer Mode. As stated in draft Recommendation G.872, OCh overhead information is required to be added to the client information signal to ensure connection integrity, assessment of signal quality, and transmission defect detection and indication. Several proposals, each with different implementation impacts and with different system-level tradeoffs, have been made for realizing the OCh overheads.
A contribution from Nortel addressed implementation issues for subcarrier modulation of client signals to achieve the OCh overhead. Subcarrier modulation results in a performance degradation of the received client signal in proportion to the modulation depth. This penalty should be kept small, and Nortel assumed that a 1-dB power penalty was acceptable. Nortel`s simulation results for the maximum overhead bit rate using binary phase-shift keying (BPSK) subcarrier modulation of SDH client signals showed that the achievable bit rate for the OCh depended on client signal bit rate and varied from about 1.5 kbits/sec for OC-3/STM-1 to about 100 kbits/sec for OC-192/STM-64.
A contribution from Nippon Telegraph & Telephone stated that the requirements for connection supervision and network-level operations, administration, and maintenance according to draft Recommendation G.872 likely exceed the bandwidth capacity provided by subcarrier modulation of pilot tones but might be sufficient for the connection supervision function. This view was also contained in a joint contribution from Alcatel Italia and ISCTI. Similarly, a contribution from Siemens stated that subcarrier modulation offers only limited bandwidth for the OCh overhead and may result in expensive implementations in case sublayer monitoring is required.
A contribution from Pirelli indicated it was possible to implement the OCh overhead by a time-division multiplexed frame in the optical channel layer as an alternative to the use of modulated pilot tones or the optical supervisory channel.
Last October`s meeting did not resolve the issue of a recommended implementation approach. But the discussion highlighted some critical issues to be addressed for moving forward on this important aspect of OTN characteristics. q