Other Key Terms
Other terms need to be defined and clarified in order for readers to understand the other chapters in this book. For the first few times I use these terms, I will refer you back to these definitions, or repeat them. Unfortunately, the industry is not consistent in the use of some of these terms; some varying interpretations are explained below.
Fiber link set: This term refers to all the fibers (if there are more than one) connecting two adjacent XCs or other fiber nodes. The link set may consist of scores of individual optical fibers and hundreds of wavelengths.
Edge, ingress, egress nodes: These terms refer to the placement of the XCs at the boundaries of the network. The term edge encompasses both ingress and egress. Ingress obviously means the XC sending traffic into the network, and egress is the node sending traffic out of the network.
Interior, transit, or core nodes: These three terms refer to an XC that is located inside the optical network and communicates with other XCs for internal network operations or with the edge nodes for communications (perhaps) outside the network.
Optical switched path (OSP): The optical path between two adjacent optical nodes. The OSP is one logical channel of a fiber link set.
Lightpath and trail: The term lightpath defines an end-to-end optical path through one or more optical nodes or networks to the end users. This term is also used in some literature to identify the optical path between two adjacent nodes, so it must be interpreted in the context of its use. Also, some literature uses the terms lightpath and trail synonymously.
Label switched path (LSP): The end-to-end MPLS path across one or more MPLS nodes (and perhaps optical as well) networks to the end users.
Another Look at the Optical Node
Figure 111 shows a more detailed view of the optical network node and its components [NORT99b]. This example shows the light signal transmitted from the left side to the right side of the figure. The events for the operation are explained below, with references to the chapters in this book that provide more detailed explanations.
Figure 111 Optical components in more detail [NORT99b].
First, laser devices generate light pulses tuned to specific and precise wavelengths, such as 1533 or 1557 nanometers (nm). Lasers are explained in Chapter 3.
Next, the optical modulators accept the electrical signal (an incoming bit stream), and convert it to an optical signal. In addition to the conversion, the modulator uses the incoming bit stream to make decisions about turning the light stream on and off to represent the digital 1s and 0s of the incoming stream. Chapters 3 and 4 provide more information on this process.
The multiplexer (MUX) combines different TDM slots or WDM wavelengths together. Chapters 5, 6, and 7 explain multiplexing in considerable detail.
The signal is passed to an optical post-amplifier (Post-Amp). This amplifier boosts the strength of the power of the signal before it is sent onto the fiber. See Chapters 3 and 7 for more information on amplifiers.
On the fiber, a dispersion compensation unit (not shown in Figure 111) corrects the dispersion of the signal as it travels through the fiber. As explained in more detail in Chapters 3 and 7, dispersion is the spreading of the light pulses as they travel down the fiber, which can cause interaction (and distortion) between adjacent pulses.
As the signal travels down the fiber, it loses its strength. Therefore, the signal power is periodically boosted with an amplifier (Line Amp) to compensate for these losses, again as explained in Chapters 3 and 7.
There may be an XC on the link to switch the signals to the correct destination. The manner in which the signals are relayed through a cross-connect is one of keen interest in the industry and is examined in Chapters 8, 9, 10, 12, and 14.
At the final receiver, optical pre-amplifiers (Pre-Amp) boost the strength of the signal once again (Chapters 3 and 7).
A demultiplexer separates the multiple wavelengths (Chapters 5, 6, and 7).
Optical photodetectors convert the optical wavelengths into an electronic bit stream (Chapter 3).