By Maarten Vissers
In 1996, the International Telecom munication Union (ITU-T) identified the optical transport network (OTN) as the next-generation transmission hierarchy to support the growing needs for bandwidth. The gigabit-per-second SDH/SONET line signals and associated megabit-per-second path signals had to be extended into terabit-per-second line capacity and associated gigabit-per-second path signals. By means of wavelength-division multiplexing (WDM), line capacity is brought into the terabit-per-second range. By means of optical crossconnects (OXCs), path signals are brought into the gigabit-per-second range.
In the years that followed, WDM point-to-point line systems were introduced into the transport networks, carrying client signals directly on the wavelengths. The step to convert the WDM "network" into an OTN was hindered during the first few years by a lack of the technology necessary to create an optical path layer that would comply with the requirements in ITU-T Recommendation G.872 (February 1999).
With the introduction of the "digital wrapper" concept by Lucent Tech nologies in 1999, a breakthrough was reached, and major progress has been made in the definition of this so-called optical-channel (OCh) path layer in the first half of 2000. Draft ITU-T Re commendation G.709 ("Network Node Interface for the OTN") was determined in ITU-T's SG15 meeting last April and is expected to be approved in its next meeting in February 2001. It defines the OCh signal with bit rates of about 2.5, 10, and 40 Gbits/sec.
The OCh signal consists of two parts: the optical-channel data unit of level k (ODUk, where k=1,2,3) and the optical-channel transport unit of level k (OTUk). The ODUk is a digital framed signal consisting of four rows by 3,824 columns, with the first 16 columns dedicated to ODUk overhead. The ODUk is a network-wide transport entity, which is able to transport client signals, for example, STM-N, IP, Ethernet, and ATM in its payload.
The OTUk adapts the ODUk for transport over a specific line system. Some OTUk signals will be standardized to support the interconnection of two networks of different operators and/or subnetworks of different vendors. Other OTUk signals are proprietary and will be deployed in vendor-specific subnetworks only. The OTN will therefore consist of vendor- and/or operator-specific OTN subnetworks, interconnected via standard optical-transport module (OTM0, OTM-n) interdomain interfaces (IrDI).
The ODUk signal provides monitoring capabilities for every stakeholder in the transport domain: customer, service provider, and network operators. The customers own the OCh endpoints (and their monitoring capabilities), and service providers own the OCh leased circuits for which the network operators provide the OCh connections. Eight levels of nested connection monitoring are defined for this purpose in G.709, and these can be applied also for protected domain monitoring, testing, and optical-link connection monitoring. With the optical-channel layer definition, the stack of transport layers for gigabit-per-second data services is minimized to a single layer on top of WDM.
Maarten Vissers is a network architect and cross-product systems engineer at Lucent Technologies, working in the Nether lands. He is a driving force in the definition of the optical transport network's signal formats, overhead, equipment, protection switching, and maintenance specifications and has a leading role in these areas in ITU-T Study Group 15. He can be contacted at tel: +31 35 687 4270, fax:+31 35 687 5976, e-mail: firstname.lastname@example.org.