DWDM Topologies
DWDM Topologies
DWDM Topologies
16
DWDM TOPOLOGIES
16.1
INTRODUCTION
Dense wavelength division multiplexing (DWDM) networks are classified into four
major topological configurations: DWDM point-to-point with or without add-drop
multiplexing network, fully connected mesh network, star network, and DWDM ring
network with OADM nodes and a hub. Each topology has its own requirements and,
based on the application, different optical components may be involved in the respective designs.
In addition, there are hybrid network topologies that may consist of stars and/or
rings that are interconnected with point-to-point links. For example, the Metropolitan
Optical Network project (MONET) is a WDM network developed for and funded by a
number of private companies and by U.S. government agencies. It consists of two subnetworks, one located in New Jersey and one in the Washington, D.C./Maryland area;
the two are interconnected with a long-distance point-to-point optical link.
16.2
POINT-TO-POINT TOPOLOGY
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Part IV
Figure 16.1 A DWDM point-to-point with add-drop multiplexing enables the system to drop and add
channels along its path.
In DWDM, each channel is carried over a specified wavelength (hi) also known
as "optical channel." Different channels may carry different data (e.g., voice, data,
video, data packets) at different bit rates. The transmitter-receiver optical link has
several optical components: fiber(s), optical amplifiers, OADM, optical filters, couplers, laser sources, and modulators and receivers. Each has its own signal-affecting
characteristic, as described in Part II. An end-to-end simplistic view of a DWDM
point-to-point system that includes lasers, an optical multiplexer and demultiplexer,
fibers, optical amplifiers (OA), and an optical add-drop multiplexer is shown in
Figure 16.1.
16.3
RING-CONFIGURED MESH
AND STAR NETWORKS
Chapter 16
DWDM Topologies
199
IP _~~,,~~ lJ~
STM -
...;
Figure 16.2 A DWDM ring network ; the hub station sources and terminates payloads of several
types.
plification (not shown) may be required. The number of nodes is typically less
than the number of wavelengths in the fiber. Figure 16.2 depicts a basic configuration but does not address network survivability or ring fault avoidance.
In the ring topology, the hub station manages channel (wavelength) assignment so that a fully connected network of nodes with OADM is accomplished. The
hub may also provide connectivity with other networks . In addition, an OADM
node may be connected with a multiplexer/demultiplexer where several data
sources are multiplexed. A simple ring topology with a hub and two nodes, A and
B, linked via wavelength Ak is shown in Figure 16.3, where node A also multiplexes several data sources. All data sources are terminated by the corresponding
OADM node (node B), however, since they are on the same channel (and the same
wavelength).
Figure 16.3 In a DWDM ring topology, channel (wavelength) assignment may be managed by the hub
station .
200
Part IV
Downstream
Figure 16.4 View of LUCENT Technologies' project FiberVi sta is illustrative of a DWDM and
CWDM system that delivers all service types to the home. (From LUCENT Technologies,
Bell Labs Technology, vol. 2, no. 2, 1998, p. 13. Reprinted with permission.)
201
16.4
A DWDM HUB
The area of DWDM node and DWDM hub is currently evolving. Thus in this section we attempt to provide stimulating discussion without any effort to provide system solutions.
r
r
Wavelength
manager
Modulators and
TCP/IP
ATM
TCP /IP
ATM
Single-mode fiber
STM
STM
.lI
PHY
Electronic regime
ljii
Transmit direction
Photonic regime
Figure 16.5 The hub (in the transmit direction) receives a variety of traffic types (TCPfIP, ATM,
STM, etc.). Each type is launched into the fiber on a separate wavelength .
16.4.2
Receive Direction
202
Part IV
............... ..................
Detedtors
TCP/IP
ATM
4 '--
-'
1~====~
Single-mode fiber
1+-
STM
.lI!
PHY
Electronic regime
iii ..
Receive direction
Photonic regime
Figure 16.6 The hub (in the receive direction) demultiplexes the optical signal to its component ,
wavelength channels, and it converts each channel to a traffic type, TCP/lP, ATM, STM,
etc.
channel requires its own clock recovery circuitry (only one is shown) because all
channels may be at different bit rates.
16.5
FAULTS
DWDM networks must be able to detect faults on the link or on the ring (broken
fiber, faulty port unit, inoperable node) and to isolate a fault. The objective is to offer continuous transmission (service) or service with the minimum disruption possible, as recommended in the standards. Depending on network topology and architecture, fault avoidance may be accomplished with dual counterrotating rings (in
ring networks), similar to the fiber-distributed data interface (FOOl). When a fault is
detected in a counterrotating ring architecture, the neighboring OADMs avoid the
fault by rerouting traffic via a Ll-turn optical cross-connect (Figure 16.7). When the
system recovers from the fault or the fault is fixed, the ring network returns to its normal (prior to the fault occurrence) state.
Similarly, in point-to-point topology, detected faults will trigger a procedure
that either finds an alternative path or causes alarms . In mesh architecture, faults will
trigger a different path selection procedure that bypasses the fault. One of the outstanding issues for network architects to answer is: When the fault recovers or is
fixed, does the network return to the previous state or does it continue until another
fault is encountered?
203
.'
~.,
....,
Secondary ring
OADM
1\2
1\2
Figure 16.7 DWDM ring networks are capable to detect faults and U-turn traffic via an optical
cross-connect .
It should be pointed out that fault avoidance requires complex optical crossconnect devices that put an additional burden on the power and cost budget of the
ring network. Thus, the burden of the protecting ring should be carefully assessed in
the light of the particular application.
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Part IV
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Part IV
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Chapter 16
DWDM Topologies
207
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