Data traffic in ultra-long-haul WDM networks is usually characterized by large, homogeneous data flows, and metropolitan area WDM networks (MAN) have to deal with dynamic, heterogeneous service requirements. In such WAN and MAN networks, equipment costs increase if separate wavelengths are used for each individual service. Moreover while each wavelength offers a transmission capacity at gigabit per second rates (e.g., OC-48 or OC-192 and on to OC-768 in the future), users may request connections at rates that are lower than the full wavelength capacity. In addition, for networks of practical size, the number of available wavelengths is still lower by a few orders of magnitude than the number of source-to- destination connections that may be active at any given time. Hence, to make the network viable and cost-effective, it must be able to offer subwavelength level services and must be able to pack these services efficiently onto the wavelengths. These subwavelength services, henceforth referred to as low-rate traffic streams, can vary in range from, say, STS-1 (51.84 Mbit/s) capacity up to the full wavelength capacity. Such an act of multiplexing, demultiplexing, and switching of lower-rate traffic streams onto high-capacity lightpaths is referred to as traffic grooming. WDM networks offering such subwavelength low-rate services are referred to as WDM grooming networks. Efficient traffic grooming improves the wavelength utilization and reduces equipment costs.

In WDM grooming networks, each lightpath typically carries many multiplexed lower-rate traffic streams. Optical add–drop multiplexers (OADMs) add/drop the wavelength for which grooming is needed and electronic SONET-ADMs multiplex or demultiplex the traffic streams onto the wavelength.