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Question 16/15 - Characteristics of optical systems for terrestrial transport networks

(Continuation of Questions 16 and 19/15)

Background and justification

Fibre optic networks are being deployed at accelerated rates in telecommunication systems worldwide. Structural reforms leading to increased privatization of telecommunications networks create an operating environment requiring optical networking and interconnection among different carriers. Simultaneously, there is occurring a rapid evolution toward a packet-based (IP type) infrastructure supporting integrated, multi-service networks in both metropolitan and backbone environments. These developments are being fuelled by customer demand for ever higher bit rates data services, high-speed Internet access, and other innovative services. It leads to a push for higher bit-rate (Terabit/s) optical transport systems in the intra-office, inter-office and long distance networks of the various network operators.

Along with the continuation of the plesiochronous digital hierarchy (PDH) in some networks and the increasing use of the synchronous digital hierarchy (SDH) worldwide, there is now an interest by network operators to deploy an optical transport network (OTN). The OTN utilizes the technologies of time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) or optical frequency multiplexing (OFM); new technologies such as optical time division multiplexing (OTDM) and other techniques toward an "all optical" networking capability may be expected in the future. Specifications are needed for physical layer interfaces of OTDM and WDM systems, including the OTN, to enable the evolution of the intra-office, inter-office and long distance networks to support the ubiquitous availability of next-generation high-bandwidth services. To the greatest extent possible, these specifications should enable transverse compatibility in a multi-vendor, multi-network-operator environment.

The responsibility under this Question includes the following draft or published Recommendations:

G.664: Optical safety procedures and requirements for optical transport systems

G.955: Digital line systems based on the 1544 kbit/s and the 2048 kbit/s hierarchy on optical fibre cables

G.957: Optical interfaces for equipment and systems relating to the synchronous digital hierarchy

G.691: Optical interfaces for single-channel STM-64, STM-256 and other SDH systems with optical amplifiers

G.692: Optical interfaces for multichannel systems with optical amplifiers

G.959.1: Optical transport network physical layer interfaces


  • What system aspects and physical layer characteristics are necessary and sufficient to enable longitudinally compatible and transversely compatible optical systems in intra-office, inter-office and long distance networks, including evolution to the OTN?
  • What enhancements to existing draft or published Recommendations and what new Recommendations are necessary to describe TDM transport systems having bit rates up to or above 40 Gbit/s, as well as their upgrade by WDM techniques?
  • What systems and physical layer considerations are necessary for optical transport systems optimized for packet data transport, e.g. IP over WDM?
  • What systems and physical layer characteristics are necessary for optical transport systems optimized for metropolitan networks?
  • What modifications are required for existing optical interface specifications to enable them to be compatible with applications beyond those specified in ITU-T, e.g. Ethernet?
  • What enhancements should be made to existing draft or published Recommendations to reflect technological developments?

Study items

Study items to be considered include:

  • General considerations for optical systems based on the PDH, SDH, and OTN using several types of single-mode fibre
  • Elementary statistical and semi-statistical power budget approaches
  • Optical link and optical interface parameters for SDH systems to enable transverse compatibility
  • Optical frequency plan, including opt. supervisory channel wavelength range, for WDM systems
  • Optical aspects of TDM and WDM systems such as:
    • Optical power levels including safety aspects and automatic gain control
    • Dispersion accommodation techniques, mainly passive
    • Polarization mode dispersion system penalty
  • Optical systems optimized for metropolitan networks
  • Modifications to existing optical interface specifications to enable them to be compatible with applications beyond those specified in ITU-T, e.g. Ethernet
  • Clarification and resolution of technical issues in current and draft Recommendations
  • Specifications to enable transverse compatibility in single-channel, multichannel, and OTN optical systems
  • Descriptive methodology for classifying application code structures of optical interfaces to be standardized (e.g. increasingly complicated systems with various dispersion accommodation techniques, line codes, etc.)
  • Characteristics of TDM line systems up to or above 40 Gbit/s and their upgrade by WDM
  • Short-distance systems to bring high-speed services closer to the local access (with Q.2/15), while interfacing with the longer-distance OTN systems
  • Utilization of optical add-drop multiplexers (OADMs) and optical cross-connects (OXCs)
  • Application of forward error correction (FEC) to terrestrial optical PDH, SDH and OTN transmission systems (e.g. to enhance system margin or to relax optical parameter specifications)
  • Optical systems utilizing alternative line codes, including soliton-type techniques (e.g. dispersion-supported RZ or solitons)
  • Use of new types of optical amplifiers with changes in systems wavelengths and/or power levels
  • Additional passive and active dispersion accommodation techniques
  • Enhanced statistical design approaches
  • Characteristics of optical systems optimized for data transport, e.g. IP, SDH, ATM and Ethernet over an OTN
  • Availability/reliability aspects of optical systems

Specific tasks

  • Revisions to Recommendations G.664, G.955, G.957, and G.692 (2003)
  • Develop draft new Recommendation G.dsn on optical system design and engineering considerations for single channel and multichannel terrestrial applications (2003)
  • Enhance Recommendations G.959.1 and G.691 (2003)
  • Develop additional Recommendations or combine existing Recommendations from progress on the above study points


  • Other relevant Questions and Recommendations of SG 15
  • ITU-T SG 13 on SDH, OTN, and data-centric architectures and network performance objectives (e.g., G.8070, draft G.optperf)
  • IEC SC86C on system measurement test methods and on optical amplifier test methods
  • IEEE on optical Ethernet technology
  • IETF (e.g. for WG "IP over Optical" activities)
  • OIF – PLL (e.g. on intra-office interfaces and on CWDM)
  • Other national, regional, and multinational fora and standards bodies, as appropriate


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