PDF of this tutorialTo investigate the complexity of a photonic network more closely, it is important to examine the structure of a typical point-to-point DWDM link deployed in today’s networks (see Figure 2).
Figure 2. Overview of a DWDM Link
On the left are the various channels—Tx1, Tx2, etc. Each DWDM link may contain as many as 128 transmitters per channel (with more possible in the future). The signal from each channel is directed into the wavelength multiplexer, to be combined and sent onto the optical path, through the optical amplifier, and then on to a demultiplexer. There, the signal is broken into its individual channels and parsed out to each receiving element.
Two general categories of photonics systems are available: open (stand-alone) systems and integrated (terminal) systems. Open systems are characterized by their ability to accept and transport a variety of incoming signals with a range of formats and characteristics. This is achieved by providing signal conversion and wavelength adaptation using, for example, a transponder or a remodulator function. In this way, noncompliant signals can be adapted and transported by the WDM layer (see Figure 3).
Figure 3. Generic Open WDM System Architecture
Integrated systems are more closely integrated within a parent SONET/SDH system. The SONET/SDH system transmitters themselves provide the required signal format (i.e., “colored” light ready for wavelength multiplexing) (see Figure 4).
Figure 4. Generic Integrated WDM System Architecture
Network operators will typically select the architecture suitable for their purposes by considering their particular economics, installed equipment base, desired network evolution, and services to be offered. Depending upon whether the system being implemented will be open or integrated, different optical-signal treatments will be necessary.
In an open system, equipment providers want their DWDM links to work with any manufacturer’s SONET/SDH network. They use open-system transponders or wavelength adapters that handle wavelength shifting or adaptation to given channels on the International Telecommunications Union (ITU–T) grid. Taking the channel through the optical-electrical-optical (O-E-O) conversion allows it to be shifted to a specific channel within this grid.
In an integrated system, the manufacturer can optimize its own transmitters and thus does not have to use the intermediate step of wavelength shifting.
Open WDM systems are generally seen as more flexible—able to transport legacy PDH as well as SDH and other signals at a variety of bit rates and optical power levels. On the other hand, there is a risk of increased cost due to the additional equipment and management functionality.
Integrated WDM systems are sometimes positioned as the more suitable solution for the longer term. Client signals can be directly adapted to the WDM/photonic layer whether the client terminal equipment is an SDH ADM or an IP router.
Regardless of the photonic system architecture employed, rigorous testing is required for accurate characterization of components and conformance with standards.
