Photonic networks are comprised of two key parts: the core, made up of a high-speed Internet backbone or telephony network (typically 2.5 to 10 Gbits/s), and a metropolitan or access network splitting off from the core through a DWDM element (at rates of 155 to 622 Mbits/s).
Let's examine the structure of a typical DWDM link more closely (see the figure). The various channels (Tx1, Tx2, etc.) are on the left. Each link may contain as many as 128 transmitters or channels (possibly more in the future). Each channel's signal is directed into the wavelength multiplexer. There, it gets combined and sent through the optical path, which includes an optical amplifier. At the end of this path, the combined signal is demultiplexed and parsed out to each receiving element.
Each DWDM network element is composed of submodules, whose performance characteristics all need to be assessed. But higher channel counts and faster per-channel speeds are requiring new kinds of measurements. Testing must be done on the transmitter output, the multichannel input/output interface, the optical path, the optical amplifier, the receiver input, and of course, the entire system.
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