Page 727 - Introduction to Information Optics

12.4. Applications and Future Development of Optical Networks 71 I

using star, ring, or tree configurations. Finally, optical network units (ONIJ)
at CPE take in light that is sent from the passive splitters, convert it to
specific types of bandwidth such as Ethernet, ATM, and Tl voice and data,
and pass it on to routers, PBXs, switches, and other enterprise-networking
gear. ON Us also incorporate the lasers that send traffic back to the central
office at the command of the OLT. Both SONET MSPP and PON are well
suited for incumbent carriers with a need to stretch their SONET infrastructure
for more data traffic. And DWDM is supported on both platforms for capacity
boosting and for optical-level bandwidth granularity.
The huge traffic demand, coupled with unprecedented capacity and net-
working capability growth in long-haul back and access networks, is pushing
metro networks to grow from < 50 km coverage to a much bigger range of 200
to 400 km. DWDM also is becoming a necessity in metro networks, less for its
capacity boosting capability, than for the transparency and service flexibility
brought forth by wavelength networking. Optical add/drop modules (OADM)
are widely deployed in metro networks as an effective optical way to manipu-
late bandwidth. All this together indicates the beginning of an era of optical
networking.

12.4.3. FUTURE DEVELOPMENT

At the beginning of the 21st century, networking with optical wavelengths
is still at a very early stage. DWDM transmission is in the middle of evolving
into DWDM networking, driven by unprecedented bandwidth demand from
widespread use of the Internet. Advances in optical technology for networking
have so far been trailing market demand. Breakthroughs in the areas of
ultra-long-haul (>3000 km) transmission, optical cross-connect, tunable op-
tical source, optical monitoring, and tunable optical filters will all have a big
impact on the progress of optical networking.
Ultra-long-haul optical transmission is revolutionizing the communica-
tion infrastructure [30]. It not only cuts down backbone transmission cost
and provides transparency by eliminating rate-dependent electrical 3R
along the route, but also facilitates optical layer bandwidth manipulation
through OADM and OXC. Optical amplification is the base for coast-to-
coast ultra long-haul backbone transmission. Conventional optical ampli-
fied systems can only reach < 1000 km distance before electrical 3R is used.
Transmission of DWDM OC-192 channels over 2000 to 5000-km dis-
tances poses major challenges that cannot be met with conventional DWDM
technology [31]. Foremost among these problems are dispersion accum-
ulation, impairments due to optical nonlinearities, accumulation of optical
noise, and optical amplifier gain nonuniformities. At present, Raman am-
plification (both distributed and discrete) [32], super FEC, high-spectral