Traffic grooming in optical networks is the technology that allows network operators to transport multiple bit streams, with requirements lower than the bandwidth offered by your transmission hierarchy, across an optical network. Transmission is mainly conducted through dense wave division multiplexing (DWDM). This technology is sometimes known somewhat inappropriately as optical traffic grooming or marketed as sub-lambda traffic grooming. Traffic grooming is the process of grouping many small traffic streams into larger units that can be processed as single entities to lower the cost of add/drop multiplexing in the network. For example, you might want to combine two 1 gigabit per second (Gbps) bit streams into a single 2.5 Gbps bit stream transported in one of the wavelengths of your DWDM network.
This problem was solved traditionally with digital cross-connects (DXCs) or other traffic grooming network elements that combined constant bit rate (CBR) streams into a higher-speed container/frame/envelope, using either SONET/SDH framing or Generic Framing Procedure (GFP; ITU-T specification G.7041). These devices provided time division multiplexing (TDM), giving each subrate bit stream a fixed number of slots in a container transported in a higher-speed bit stream.
The all-optical solutions (including all-optical add-drop multiplexers) that became available in the last few years have reduced the complexity of high-speed optical networks by reducing the number of optical-electrical-optical conversions in the network. Unfortunately, no such advances have been made in traffic grooming technologies. No all-optical traffic grooming products are available on the market, so all traffic grooming is still performed in the electrical domain, as it was in the days when the SONET/SDH transmission technology dominated service provider networks.
Optimizing OTNs without traffic grooming
With all these facts in mind, one has to wonder about the vendor announcements touting concepts from "lambda-level grooming" to "subport-level grooming." These are just marketing terms for a simple fact: Since most traffic has been migrated to IP, it makes no sense to build new networks with TDM grooming equipment. You should optimize your backbone to include only the elements relevant to your services: the switching platform (routers, also known as Layer 3 switches) and the transport platform (WDM).
Real-world dilemmas are always more complex than long-term architectural visions. You should never blindly follow a generic architecture but instead compare the TCO of various alternatives to select the best architecture for your network. The TCO should obviously include acquisition costs and the costs of operating, maintaining and monitoring multiple platforms, potentially acquired from multiple vendors and using different network management systems.
Analyze the services traffic on your network. For example, if you have existing services on your TDM grooming equipment (such as voice, Frame Relay or ATM traffic), don't migrate them to IP just to build a next-generation converged backbone unless the cost-benefit analysis clearly shows a short-term return on your investment. Usually, it's better to keep the old TDM equipment operational as long as you provide the "old-world" services. Don't even try to integrate it with your new network; just give it a separate set of wavelengths on your WDM network.