CWDM is an optical technology for transmitting up to 16 channels, each in a separate wavelength or color, over the same fiber strand. The CWDM solutions help enable enter-prises and service providers to increase the bandwidth of an existing Gigabit Ethernet optical infrastructure without adding new fiber strands. Unlike DWDM, which can transmit up to 160 channels on the same fiber by tightly packing them, CWDM technology relies on wider spacing between channels. this design makes CWDM a relatively inexpensive technology for transmitting multiple gigabit-per-second signals on a single fiber strand as compared with DWDM because it can support less-sophisticated, and therefore cheaper, transceiver designs. In the point-to-point configuration shown in Figure 1-1, two rndpoints are directly connected through a fiber link. The ITU has standardized a 20-nm channel-spacing grid for use with CWDM, using the wavelengths between 1310 nm and 1610 nm. Most CWDM systems support eight channels in the 1470-to 1610-nm range. The Fiberstore CWDM Gigabit Interface Converter small form-factor pluggable(SFP) solution allows organizations to add or drop as many as eight channels (Gigabit Ethernet or Fibre Channel) into a pair of single-mode (SM) fiber strands. As a result, the need for additional fiber is minimized. You can create redundant point-to-point links by adding or dropping redundant channels into a second pair of SM fiber strands.

                       Figure 1-1

CWDM Technical Overview

CWDM Multiplexer is achieved thruogh special passive (nonpowered) glass devices known as filters. The filters act as prisms, directing lights from many incoming and outgoing fibers (client ports) to a common transmit and receive trunk pots. Optical multiplexing in a ring with CWDM networks is supported with optical add/drop multiplexers (OADM). OADMs can drop off one or more CWDM wavelengths at a specific location and replace that signal with one or more diferent outbound signals. The Fiberstore CWDM GBIC/SFP solution has two main components: a set of eight different pluggable transceivers (Fiberstore CWDM GBICs and CWDM SFP), and a set of different Fiberstore CWDM passive multiplexers/demultiplexers or OADMs. Both the transceivers and the passive multiplexers are compliant with the CWDM grid defined in the standards. CWDM can be used by enterprises on leased dark fiber to increase capacity (for example, from 1 Gbps to 8 Gbps or 16 Gbps) over metro-area distances. One problem with CWDM is that the wavelengths are not compatible with erbium-doped fiber amplifier (EDFA) technology, which amplifies all light signals within their frequency range. CWDM supports up to a 30 -dB power budget on an SM fiber. This restricts the distances over which CWDM may be used. CWDM supports distances of approximately 60 miles (100km) in a point-to-point topology and about 25 miles (40 km) in a ring topology. in some areas, service providers use CWDM to provide lambda or wavelength services. A lambda service is where a provider manages equipment and multiplexes customer traffic onto one or more wavelengths for a high-speed connection, typically bet ween two or more points.

DWDM Technical Overview

DWDM is a core technology in an optical transport network. The concepts of DWDM are similar to those for CWDM. However, DWDM spaces the wavelengths more tightly, yielding up to 160 channels. The tighter channel spacing in DWDM requires more sophisticated, precise,and therefore more expensive transceiver designs. In a service provider is backbone network, the majority of embedded fiber is standard SM fiber with high dispersion in the 1550-nm wubdiw, DWDM supports 32 or more channels in the narrow band around 1550 nm at 100-GHz spacing, or about 0.8 nm, as illustrated in Figure 1-2. Because of the EDFA compatibility of the wavelenths used, DWDM is also available over much longer distances than CWDM and supports metropolitan-area network (MAN) and WAN applications. In practice, signals can travel for up to 75 miles (120 km) between amplifiers if fiber with EDFA is used. At distances of 375 miles (600 km) to 600 miles (1000 km), the signal must be regenerated.

Figure 1-2

DWDM can be used as a high-speed enterprise WAN connectivity service. Typical DWDM uses include connectivity between sites and data centers for example 1-, 2-, or 4- Gbps Fiber channel; IBMfiber connectivity (FICON) and Enterprise System Connection(ESCON); and Gigabit and 10 Gigabit Ethernet. Protection options include client-side safeguards using rerouting, an optical splitter that allows the signal to go both ways around a ring or line-card-based protection that detects boss of signal and wraps.

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