Special Report: Next Generation Internet Applications
The SONET standard supports synchronous framing for data and overhead
octets. Several protocols can be run over SONET, such as Asynchronous Transfer
Mode (ATM) or Internet Protocol (IP). When run over the optical transmission
medium of single-mode fiber, SONET can be utilized as an interfacing protocol
for long distance fiber runs. Typical SONET networks consist of Digital
Cross Connects (DXC), Add/Drop Multiplexers (ADM), and Line Terminating
Equipment (LTE). DXCs are the interconnecting structures of SONET network
rings. These provide the Optical-Electronic-Optical (OEO) conversions, multiplexing
and demultiplexing functions, and programmable mapping for network interconnection
and switching. The ADMs allow for the direct insertion and removal of individual
services onto the same SONET connection.[3] This ability is an improvement
over PDH which requires demultiplexing down to the speed of the inserted
channel. The Line Terminating Equipment allows users to interface with the
SONET network. Such devices could be a telephone switch or network gateway.
SONET also provides for network management and fault detection/traffic redirection.
Figure 1 depicts a typical SONET network. ADM ADM Figure 1: Typical SONET
Network Once a network infrastructure has been deployed, upgrades to increase
capacity can be expensive. Cost effective ways of increasing optical network
capacity could save corporations substantial capital. Wavelength Division
Multiplexing (WDM) entered into the picture and dramatically increased effective
fiber optic bandwidth. WDM, and its Dense Wavelength Division Multiplexing
(DWDM) counterpart, operate under the premise of multiplexing different
optical signals by wavelength on the same physical medium. This allows network
engineers to increase network capacity without extensive and expensive infrastructure
redesign. Similar in functionality to SONET's ADMs, but different in method,
DWDM's Wavelength Cross Connects (WXCs) populate the DWDM ring topology.
WXCs coexist with ADMs and can provide substantial benefit to the infrastructure
when implemented together within the backbone. DWDMs provide wavelength
conversion to allow for optical routing and cross-wavelength traffic redirection.
SONET provides the network management and control protocol to ensure the
integrity of the optical transmission. Thus, a hybrid of SONET and DWDM
network infrastructures provides an effective, efficient, and dependable
backbone architecture. Missing from the DWDM/SONET hybrid is the concept
of high-speed fully optical switching, also known as "photonic switching."
Currently, for network layer switching to occur, a conversion must be done
from the optical domain to the electrical domain. The necessary routing
and switching is performed electrically and the signal is converted back
into the optical domain for transport. This process becomes increasingly
difficult as the speed disparity of optical transport and electronic switch
backplanes increases almost daily. As networks evolve towards all-optical
networks, also known as "transparent networks", protocols must be developed
to ensure manageability and controllability without inefficient OEO conversions.
2.0 ENABLING TECHNOLOGIES FOR OPTICAL NETWORKS Optical networks are composed
of various enabling technologies, merged in harmony to produce high- speed,
reliable, and affordable communications backbones. The benefits of all-optical
networks are vast, yet the technologies necessary to render this dream a
reality are complex. Advancements in semiconductor research, networking
concepts, optical processing techniques, and switching implementations continue
to be investigated. Due to all of these research endeavors, the realization
of all-optical transport networking is cresting the horizon.
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