ver the last few years we have witnessed wide deployment of point-to-point wavelength-division multiplexing (WDM) transmission technology in the Internet infrastructure. The corresponding massive increase in bandwidth due to WDM has heightened the need for faster switching at the core of the network. At the same time, there has been a growing effort to augment the Internet Protocol to support different levels of quality of service (QoS). Label switching routers (LSRs) running multiprotocol label switching (MPLS) are being deployed to address the dual issues of faster switching and QoS support. On one hand, LSRs simplify the forwarding function, thereby making it possible to operate at higher data rates. On the other hand, MPLS enables the Internet architecture, built on the connectionless Internet Protocol, to behave in a connection-oriented fashion that is more conducive to QoS and traffic engineering. The rapid advancement and evolution of optical technologies makes it possible to move beyond point-to-point WDM transmission systems to an all-optical backbone network consisting of optical crossconnects (OXCs). Each OXC can switch the optical signal coming in on a wavelength of an input fiber link to the same wavelength in an output fiber link. An OXC may also be equipped with converters that permit it to switch the optical signal on an incoming wavelength of an input fiber to some other wavelength on an output fiber link. The main mechanism of transport in such a network is the lightpath, which is a communication channel established between two LSRs (or other edge devices) over the network of OXCs, and may span a number of fiber links (physical hops). A lightpath is a generalization of the MPLS concept of a label-switched path (LSP) with the wavelength color corresponding to the label and the crossconnect matrix corresponding to the label forwarding table. Optical networks in which OXCs provide the switching functionality are currently the subject of intensive research and development efforts. The Internet Engineering Task Force (IETF) is investigating the use of generalized MPLS (GMPLS) to set up and tear down lightpaths; a number of existing protocols are also being extended to interwork with GMPLS, including OSPF, RSVP, and CR-LDP. GMPLS is an extension of MPLS that supports multiple types of switching, including switching based on wavelengths, usually referred to as multiprotocol lambda switching (MPλS). With GMPLS, the OXC backbone and network of LSRs will share common functionality in the control plane, making it possible to seamlessly integrate OXC backbones within the overall Internet infrastructure. Optical networks have also been the subject of extensive research investigating issues such as virtual topology design, call blocking performance, protection and restoration routing algorithms and wavelength allocation policies, and the effect of wavelength conversion, among others. With the deployment of commercial WDM systems, it has become apparent that the cost of network components, especially line terminating equipment (LTE), is one of the dominant costs in building optical networks, and is a more meaningful metric to optimize than, say, the number of wavelengths. Furthermore, with currently available optical technology, the data rate of each wavelength is on the order of 2.5-10 Gb/s, while channels operating at 40 Gb/s will be commercially available in the near future. In order to utilize efficiently this capacity, a number of independent lower-rate traffic streams must be multiplexed into a single lightpath. These observations give rise to the concept of traffic grooming, which refers to the techniques used to combine lower-speed components onto available wavelengths in order to meet network design goals such as cost minimization. Traffic grooming has received considerable attention recently, and, given the widespread use of SONET networks, it is not surprising that most of the early work has focused on ring topologies. The objective in all the studies is to minimize the network cost either directly, for example, by minimizing the amount of SONET add-drop multiplexers (ADMs), or indirectly, for example, by minimizing electronic switching costs. O O Traffic grooming refers to techniques used to combine low-speed traffic streams onto highspeed wavelengths in order to minimize the networkwide cost in terms of line terminating equipment and/or electronic switching. Such techniques become increasingly important for emerging network technologies, including SONET/WDM rings and MPLS/MPλS backbones, for which traffic grooming is essential. In this article we formally define the traffic grooming problem, and we provide a general formulation that captures the features of a wide range of problem variants. We then present a comprehensive comparative survey of the literature that unveils the significant amount of research on this subject (the traffic grooming past). We also offer a broad set of ambitious research directions (the traffic grooming future) that are motivated by the exciting new challenges arising with the advent of MPλS technology.
For a survey of advances in unicast traffic grooming, the reader is referred to REF .
Traffic grooming in WDM networks: past and future
IEEE Network
Computer Science