Described in the ITU-T Recommendation G.709 (2003), OTN adds operations, administration, maintenance, and provisioning (OAM&P) functionality to optical carriers, specifically in a multi-wavelength system such as dense wavelength division multiplexing (DWDM). OTN specifies a digital wrapper, which is a method for encapsulating an existing frame of data, regardless of the native protocol, to create an optical data unit (ODU), similar to that used in SDH/SONET. OTN provides the network management functionality of SDH and SONET, but on a wavelength basis. A digital wrapper, however, is flexible in terms of frame size and allows multiple existing frames of data to be wrapped together into a single entity that can be more efficiently managed through a lesser amount of overhead in a multi-wavelength system.The OTN specification includes framing conventions, nonintrusive performance monitoring, error control, rate adaption, multiplexing mechanisms, ring protection, and network restoration mechanisms operating on a wavelength basis. A key element of a digital wrapper is a Reed-Solomon forward error correction (FEC) mechanism that improves error performance on noisy links. Digital wrappers have been defined for 2.5-, 10-, and 40Gbps SDH/SONET systems. SDH/SONET operation over an OTN involves additional overhead due to encapsulation in digital wrappers.The resulting line rates are defined as optical transport units (OTUs). See also digital, DWDM, error control, FEC, ITU-T, link, multiplexer, ODU, OTN, OTU, rate adaption, ReedSolomon, and wavelength.