Key Optical Nodes

We now leave the subject of the optical network marketplace, and focus our attention on the nodes (machines)
that comprise the network. Figure 1–10 shows the key nodes in an optical network. The topology is a ring, but
the topology can be set up either as a ring, a point-to-point, multipoint, or meshed system. In most large
networks, the ring is a dual ring, operating with two or more optical fibers. The structure of the dual ring
topology permits the network to recover automatically from failures on the optical links and in the link/node
interfaces. This is known as a self-healing ring and is explained in later chapters.

Figure 1–10 Key nodes in the optical network.

End-user devices operating on LANs and digital transport systems (such as DS1, E1, etc.) are attached to the
network through a service adapter. This service adapter is also called an access node, a terminal, or a terminal
multiplexer. This node is responsible for supporting the end-user interface by sending and receiving traffic from
LANs, DS1, DS3, E1, ATM nodes, etc. It is really a concentrator at the sending site because it consolidates
multiple user traffic into a payload envelope for transport onto the optical network. It performs a
complementary, yet opposite, service at the receiving site.

The user signals, such as T1, E1, ATM cells, etc., are called tributaries. The tributaries are converted (mapped)
into a standard format called the synchronous transport signal (STS), which is the basic building block of the
optical multiplexing hierarchy. The STS signal is an electrical signal. The notation STS-n means that the
service adapter can multiplex the STS signal into higher integer multiples of the base rate, The STS signals are
converted into optical signals by the terminal adapter and are then called OC (optical carrier) signals.

The terminal/service adapter can be implemented as the end-user interface machine, or as an add-drop
multiplexer (ADM). The ADM implementation multiplexes various STS input streams onto optical fiber
channels. OC-n streams are demultiplexed as well as multiplexed with the ADM.

The term add-drop means that the machine can add or drop payload onto one of the fiber links. Remaining
traffic that is not dropped passes straight through the multiplexer without additional processing.

The cross-connect (CS) machine usually acts as a hub in the optical network. It can not only add and drop
payload, but it can also operate with different carrier rates, such as DS1, OC-n, E1, etc. The cross-connect can
make two-way cross-connections between the payload and can consolidate and separate different types of
payloads. For example, the cross-connect can consolidate multiple low bit-rate tributaries into higher bit-rate
tributaries, and vice versa. This operation is known as grooming.

Key Terms for the Cross-connect

The convention in this book is to use three terms to describe the optical cross-connect. There is a spate of
terms to describe a cross-connect. I counted six terms in one paper alone. To make sure there is no ambiguity
about the optical cross-connect in this book, the following terms are used:

 Optical/Electrical cross-connect (OXC): Receives optical signals, converts them to electrical signals,
makes routing/switching and/or ADM decisions, then converts the electrical signals back to optical signals
for transmission. These operations are also noted as O/E/O. This technique is also called an opaque
operation.

 Photonic cross-connect (PXC): Performs the functions of the OXC, but performs all operations on optical
signals. These operations are also noted as O/O/O, and are also called transparent operations.

 Cross-connect (XC): A more generic term, used when it is not necessary to distinguish between the OXC
or the PXC.

 Switch: Some recent literature distinguishes between a cross-connect and a switch. This literature states
that cross-connect is an outdated term! Well, the term switch has also been around for quite a while.
Anyway, the book uses the terms cross-connect and switch synonymously.

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 1–11 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 1–11 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 1–11) 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).

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 1–11 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 1–11 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 1–11) 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).