hrough a digital interface with an appropriate language. Even if single-component modules are quite popular as far as switches and similar components are concerned, in the case of the transmitting and receiving interfaces a fur- ther evolution has pushed the system and component vendors to define Multiple-Source Agreements (MSA) for more complex modules comprising both transmitter and receiver. These optical interface modules can be divided into three classes: 1. MSA TX/RX modules 2. Fixed transceivers 3. Pluggable transceivers The so-called MSA modules are generally DWDM-quality interfaces (even if gray and short-haul MSA also exist) that are designed to be directly integrated into the system card. They exit with two fiber pigtails and two connectors SO that they can be connected with the transmission fibers. In addition, fixed transceivers are modules that are mounted on the system card, but they are generally low-quality interfaces, designed more to occupy a small space and dis- sipate less power. Since a couple of years they have been completely substituted by plug gable transceivers. Pluggable transceivers are interfaces, both WDM and gray, that are mounted on a pack- age inserted into the hosting card from the front through a suitable housing while the system is running. The transceiver is recognized and switched on by the system controller, all without interfering with the system working. In this way, when a failure occurs, there is no need of changing the whole card, but it is possible to change only the transceiver. Telecommunication Networks Due to their small form factor, pluggable transceivers cannot host high-performance interfaces, which are still reserved for MSA modules, but miniaturization of lasers and electronics is going on and perhaps the day is not far when pluggable transceivers will substitute MSA modules as well. 5.9.1 MSA Transmitting-Receiving Modules The MSA 300 PIN transponder is the standard interface for high-performance DWDM sys- items, especially if tunable lasers are required. It