Page 308 - Optical Communications Essentials

Optical Networks

298 Chapter Seventeen

trunk ring for n wavelengths (e.g., one could have n 16). The different wave-
length outputs from each OC-192 transmitter are passed first through a vari-
able optical attenuator (VOA) to equalize the output powers. These then are fed
into a wavelength multiplexer, possibly amplified by a posttransmitter optical
amplifier, and sent out over the transmission fiber. Additional optical amplifiers
might be located at intermediate points and/or at the receiving end.

17.3. Optical Ethernet
Ethernet is deployed widely in local-area networks, since it is known for its
robustness and low cost. Standards-compliant interfaces are available on
numerous devices running at line rates ranging from 10Mbps to 10Gbps.
Therefore Ethernet has matured to become the LAN technology of choice with
the best price and performance characteristics.
Ethernet also is being used in metropolitan-area networks and is extending
into wide-area networks (see ITU-T Recommendation G.985). In these environ-
ments Ethernet can increase network capacity cost-effectively and has the abil-
ity to offer a wide range of services in a simple, scalable, and flexible manner.
When used in a MAN, Ethernet is referred to as Metro Ethernet. In enterprise
applications, Metro Ethernet is used for interfacing to the public Internet and
for connectivity between geographically separate corporate sites. The latter
application extends the functionality and reach of corporate networks.
By using optical fiber transmission lines in MAN and WAN environments,
Ethernet provides a low-cost, high-performance networking solution that can
span distances up to at least 70km. Ethernet over fiber is deployed mainly in a
point-to-point or mesh network topology. A high degree of scalability is possible
through the use of CWDM or DWDM, since capacity can be increased either by
raising the bit rate and/or by adding more wavelengths. In addition, with WDM
users can lease wavelengths with varying bandwidth and protocol characteris-
tics on a temporary or time-of-day basis. For example, server farms or informa-
tion storage systems can furnish users with additional bandwidth at specific
times of day when standard traffic normally is low. The combined flexibility of
Ethernet and WDM on the optical layer allows a rapid activation of such ser-
vices.

17.4. IP over WDM
The movement in the telecommunications industry toward a greater use of IP
is resulting in a dramatic complexity reduction of multiprotocol routing in net-
works. The popularity of IP is that it has widespread use in enterprise networks
and the Internet, it is understood more than any other protocol, gateways for
non-IP applications exist, and protocol stacks are available at both the IP and
higher levels (e.g., TCP).
As shown in Fig. 17.16, the network layering of a typical wide-area network
carries IP on top of ATM, ATM on top of SONET/SDH, and SONET/SDH on top

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