A reconfigurable optical add-drop multiplexer (ROADM) is a form of optical add-drop multiplexer that adds the ability to remotely switch traffic from a WDM system at the wavelength layer. This allows individual wavelengths carrying data channels to be added and dropped from a transport fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.
The main advantages of the ROADM are:
The planning of entire bandwidth assignment need not be carried out during initial deployment of a system. The configuration can be done as and when required.
ROADM allows for remote configuration and reconfiguration.
In ROADM, as it is not clear beforehand where a signal can be potentially routed, there is a necessity of power balancing of these signals. ROADMs allow for automatic power balancing.
ROADM functionality originally appeared in long-haul DWDM equipment, but by 2005, it began to appear in metro optical systems because of the need to build out major metropolitan networks in order to deal with the traffic driven by the increasing demand for packet-based services.
The switching or reconfiguration functions of a ROADM can be achieved using a variety of switching technologies including MEMS, Liquid crystal, thermo optic and beam-steering switches in planar waveguide circuits, and tunable optical filter technology.
“ROADM”又称可重构的光分插复用器，是指一种网络元素（或称节点），通过远程的重新配置，可以动态上下业务波长，并且，业务波长的功率也是可以管理的。ROADM的组成模块随应用的不同而不同。不过大多数ROADM都具有这里列出的模块。
ROADM节点的主要功能模块包括：
1、前置和后置光放大器。
2、光业务信道（OSC）的生成和终结。
3、波长下路。
4、波长上路。
5、节点内部聚合信道或单信道的功率监控，可在节点内多处进行。
6、可用/不可用和可选波长的监测，整个节点内的光信号的信噪比监测。
7、上下路和直通波长的功率/衰减控制。
8、色散补偿。
9、前置和后置放大器的增益均衡。
“ROADM”一段时间曾经是通信产业中的一个“热门”词汇。网络节点采用ROADM后，网管系统可以控制某个波长通过这个光节点或者从本地端口下路，终结于这个光节点的光转发器。ROADM系统中，至少可以在两个端口之间交换波长。直通口支持普通WDM信号，这时本地端口可以是任意波长或者某个已知波长。ROADM还有更多的功能，如可以连续控制每一个波长的光功率。
ROADM系统可以给服务提供商们提供一系列的好处：快捷的业务指配、更加自动化的处理、简化的网络规划和施工，更为强大的网络监控能力和网络扩展能力。然而，虽然人们预测它将成各种类型的通信网中的完整的网元，但是ROADM目前仍然处于二线位置。
网络以及业务都需要考虑成本，正是在价格问题上，ROADM没能过关。因此，服务提供商们目前还不愿投钱进去。
