In recent years an enormous demand for bandwidth has been generated by the deployment of modern forms of telecommunication, in particular the Internet and its main application, the World Wide Web, as well as private networks of diverse enterprises and organizations, without forgetting wireless communications, in particular mobile telephones, which in practice requires powerful terrestrial infrastructures if the expectations of customers are not to be disappointed. To face up to this demand, those responsible for providing the networks necessary for the deployment of these new forms of communication have rapidly had to convert to transporting signals carrying information in optical form, in the first place to benefit from the low cost of optical fibers, and in the second place to benefit from the very high bit rates that can be achieved without it being necessary to regenerate the signal, and despite transmission distances that can be measured in kilometers, tens of kilometers, and even hundreds of kilometers. This is because attenuation is very low, especially in monomode fibers, compared to what can be achieved with electrical transmission on copper wires, for example. Moreover, optical transmission avoids all the problems associated with electromagnetic interference, which necessitate costly protection circuits and can lead to frequent transmission errors.
The wavelength division multiplexing (WDM) technique has enabled transmission in the same fiber of a plurality of signals carried by respective different wavelengths, generally in a wavelength window situated around 1550 nm. This increases the number of independent transmission channels associated with the respective carrier wavelengths. In other words, transmitting light waves with different colors makes better use of the portion of the bandwidth of a single fiber. The dense WDM (DWDM) technique, which soon succeeded the WDM technique, can multiplex hundreds of channels, or even more channels.
The proliferation of high added value services has led to the requirement to monitor the quality of service (QoS) offered.
This requires the creation and transmission in the network of control data that can subsequently be detected and analyzed.
A first solution is to create control data on a channel-by-channel basis by separately overmodulating the signal of each channel with control data.
A second solution consists in using a dedicated channel to transmit control data.
However, and especially if the DWDM technique is used, the data to be transmitted over an optical transmission network is organized into bands of wavelengths, i.e. bands of channels, and the various nodes of an optical transmission network communicate by means of entire bands of channels. Thus the requirement to monitor the QoS is manifested on a band-by-band basis. The two solutions referred to above then cause problems.
The first solution involves overmodulating each channel in the same band, and detecting the control data imposes demultiplexing in order to extract the overmodulation from any one channel.
The second solution implies using a dedicated channel for each band to be monitored, and also leads to a very high cost.