Page 217 - Optical Switching And Networking Handbook

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202 Chapter 8

6. The SAR is passed to the ATM layer, where the 5-byte header is
attached, creating the cell.

7. The 53-byte cells are mapped onto a SONET frame in a
horizontal format.

8. The SONET frame is transmitted across the fiber link.

What about the Metropolitan-Area

Networks?

Manufacturers are now addressing the metropolitan-area optical-
transport architecture based on dedicated optical networks connect-
ing wavelength-routed and traditional IP systems. New products are
appearing with DWDM, adding dynamic add-drop functions and
using broadband active optical amplifiers, when necessary, to extend
system reach or cut the number of optoelectronic conversions within
the metropolitan region.
Passive DWDM was limited in metropolitan areas compared with
the success of long-haul systems. This is due mainly to the LECs
(ILECs and CLECs) being separated from the protection switching
and provisioning capabilities in a DWDM environment. The newer
systems were designed to overcome these limitations by providing
either protected or unprotected channels.The optical subsystem uses
1,310-nanometer (nm) short-reach lasers. Transceivers can be con-
figured via software for OC-3, OC-12, or OC-48 rates.
Essentially, two choices are available for community networks in
the metropolitan areas: lease or build. Most large corporations with
metropolitan-area networking needs lease services from carriers.
Many carriers have built their own highly resilient SONET rings in
urban areas to serve various customers with leased lines connecting
to that ring. Common services include T-1,T-3, Frame Relay, and OC-
3 dedicated lines. This is shown in Figure 8-15, where the configura-
tion uses a SONET architecture.
A real need exists among local communities, governments, and
school districts today to obtain cost-effective high-speed communica-
tions access to enable new multimedia applications such as distance