optical restoration, and allow them to establish QoS levels associated with restora- tion. (Optical switches are discussed later in this chapter.) OADMs Next-generation network services must be easily reconfigurable, and they must support real-time provisioning. Demultiplexing all the wavelengths at each node is costly, it introduces delay, and it reduces the distance over which a signal can travel. OADMs, as shown in Figure 11.5, work much more inexpensively than Pass-Through Traffic Metro Metro Dropped- Off Traffic Site A Site C Gigabet Ethernet Switch Site B Figure 11.5 Optical add/drop multiplexing Chapter 11 Optical Networking demultiplexing all the wavelengths because they simplify the process-they elimi- nate the costly electronics used to convert between light and electricity. Most OADMs use special filters to extract the wavelengths that need to be dropped off at a given location. For most vendors, the wavelength is fixed, SO at the time of con- figuration, the carrier designates the individual wavelengths to be dropped at each location. The ROADM is a fairly new variety of add/drop mux (ADM). This software- based enabler adds the capability to switch between different networks. ROADMs also make it easier for service providers to separate, add, and drop traffic being transported over optical rings to and from customers. One of the distinguishing features of an ROADM compared to a standard OADM is its ability to switch traffic at both the SDH/SONET and wavelength layers. The function of an ROADM has been available in DWDM systems for a short time (since 2005), but the most significant advance is its presence in optical equip- ment designed for MANs. Driven by the need for more bandwidth to support grow- ing packet data traffic as well as the emergence of triple-play services (i.e., voice, data, and video/TV) that telcos and MSOs are intent on providing, today more attention is being focused on the MAN domain, and many optical technologies have been introduced specifically for this market, including coarse