Release Date：2006-12-15 Author：Wang Jiaying, director of WDM standards research, Click：
Optical transport technologies have undergone continuous innovation to meet the requirements of the development of NGNs. This also puts new demands on equipment standards and pushes forward the formulation of standards for the performance of optical transport networks.
The development of WDM enables transmission over thousands of kilometres without electric regeneration, and lays a good foundation for wide area routing in automatic switched transport networks. This, together with the commercial viability of reconfigurable wavelength switch and add-drop devices holds great promise for dynamic routing and large capacity transmission.
Main ITU-T standards for metro and long-haul WDM systems
Coarse and dense wavelength division multiplexing (CWDM and DWDM) systems optimised for metro applications have been widely deployed in recent years. ITU Recommendation G. 695 applies to CWDM systems that are installed on single-mode fibre. Recommendations G.698.1 and G.dapp.2 allow operators to deploy DWDM systems from multiple vendors. Recommendation G. 959.1 provides physical layer inter-domain interface specifications for optical networks employing WDM. Recommendation G.696.1 targets longitudinally compatible intra-domain applications in long-haul WDM links. This standard ensures the high performance transmission of WDM signals in the whole C and L bands over a distance of up to 2,000km.
New application codes
The development of new technologies is changing transmission limitations and allowing the development of new recommendations, applications and interfaces.
- Forward error correction and enhanced forward error correction (FEC and EFEC) coding has 6 to 9dB system gain, alleviating optical signal-to-noise ratio (OSNR) limitations.
- The functional blocks and specifications of optical terminal equipment will be changed by dynamic chromatic dispersion and polarized mode dispersion (PMD) compensators, and signal equalisers for transmitters and receivers.
- Various novel modulation techniques such as optical duobinary (ODB), differential phase shift keying (DPSK), chirped modulation, and return-to-zero (RZ) have been developed to overcome OSNR limitations, thus changing the input and output parameters and specifications of optical terminal equipment.
- The application of the dynamic erbium doped fibre amplifier (EDFA) and Raman amplifier with automatic gain spectrum control changes the requirements on optical interfaces, and involves the standardisation of optical performance monitoring.
- It will be necessary to draft new standards or revise the current standards on single-and multi-channel optical interfaces for 40Gbps or faster optical transport systems.
While long-haul transport systems are still developing, it is imperative to reflect in current standards recent worldwide achievements in ultra long-haul transport systems.
Future dynamic WDM system
The fast connection set-up and rerouting of automatic switching optical networks (ASONs) will induce fluctuations of optical power, residual chromatic dispersion, PMD and wavelength. Thus, it is necessary to incorporate adaptability into the optical transmission systems, through various compensation techniques such as dynamic gain equalisation, robust modulation format and dynamic adjustment of the OSNR.
It is necessary to define the transfer functions of the optical network elements to evaluate whether the new connection established after rerouting can handle optical constraints. The ITU draft G. otf provides the basic methodology for describing and evaluating network performance in complicated networking and dynamic routing environments. Performance monitoring which allows preventive maintenance actions and fault identification is described in Recommendation G.697.
Requirements for performance control will accelerate the development of diversified passive/active, hybrid and dynamic optical components. For example, the emergence of dynamic PMD compensators covered in ITU-T G.666 notes that the commercial application of ultra long-haul systems will soon take off. Recommendation G.671 specifies the characteristics of tuneable dispersion compensators, dynamic channel equalisers, optical switches and optical add/drop modules. Draft Recommendation G.667 (G. adc) on adaptive chromatic dispersion compensators has been finished. All of these standardisation activities will promote the commercial application of these dynamic systems.
ZTE drafting standards in key technologies
ZTE has played an active role in developing the standards mentioned above and some of its experts serve as editors in the ITU-T’s key areas of study.
ITU-T Recommendation G.665
ZTE’s proposal to draft a new recommendation (G.665) on optical fibre Raman amplification technology was approved at the ITU-T SG15 meeting in Geneva in October 2003. ZTE was appointed as the editor of the recommendation and submitted the draft of this recommendation in February 2004, following a one-year collaboration between several member companies of the SG15/Q17 group. The final draft was consented and approved at the SG15 meeting in Geneva in November 2004 and published in January 2005.
Recommendation G.665 specifies the generic characteristics of Raman amplifiers and Raman amplified subsystems. Raman amplifiers pave the way for flexible and effective signal transmission in the context of wide area routing and future mesh networks, and have found commercial applications in ultra long-haul and longer span transmission. The recommendation provides a standard for evaluating various Raman amplifiers, which are required for the large-scale application of Raman amplification technology. The timely publication of this recommendation will help facilitate acceptance of the technology and development of the manufacturing supply chain for items as small as a component or a module.
The functional models included in this standard are founded on theory and experimental results. The newly added optical parameters contribute to the unique attributes of Raman amplification as compared with EDFA. These models and parameters are related to ZTE’s patented designs, applications, and to optimum control of Raman amplifiers.
ITU-T Recommendation G.667
ZTE proposed that the ITU should emphasise drafting and publishing standards on adaptive chromatic dispersion compensators at the ITU-T SG15 meeting in Geneva in May 2005. The proposal was adopted while ZTE was asked to draft this standard as an editor. ZTE submitted the first draft in September 2005 and plans to submit the revised and supplemented draft for consent at this November’s SG15 meeting.
Adaptive dispersion compensators (ADC) provide a means for flexible and effective transmission in the context of dynamic routing across future mesh networks. Transmission systems using ADC can increase their adaptability and decrease their design costs. This technology, which will be widely utilised in both metro and long-haul systems, holds special importance for next generation optical transport systems.
Some types of ADC such as optical and electronic dispersion compensators are currently being commercially deployed. After Recommendation G.667 comes into effect, the technology will be further promoted, and the supply chain for components, modules and systems will be developed. This recommendation specifies parameters and definitions for devices which provides adaptive dispersion compensation needed for signal transmission and detection in a system exhibiting dynamic variation of chromatic dispersion.
The functional models included in this standard are based on achievements in design theories and the results of experiments in dynamic dispersion compensation. The newly added optical parameters in ADC devices distinguish them from the static dispersion compensators related to G.671. The models and parameters are used in designs and applications and for optimum control of ZTE’s optical transport products for which the company owns several intellectual property rights.
ZTE is determined to drive forward the application and standardisation of a wide range of technologies and to stay at the forefront of technical innovation with its continued and active participation in the formulation of international telecoms standards.
ZTE plays an active role at the ITU, having been appointed to edit standards relating to optical communications, next generation networks (NGNs) and information security in mobile communications. A member of all three ITU sectors– telecommunication development (ITU-D), telecommunication standardisation (ITU-T) and radiocommunication (ITU-R)–ZTE has hosted several important conferences and participated in more than 50 international standardisation groups, including the Third Generation Partnership Project (3GPP), the CDMA Development Group (CDG), the DSL Forum, and the Network Processing Forum (NPF).