The Wavelength Division Multiplexing (WDM) system uses a Multiplexer (MUX) at the transmitter to put multiple optical signals on the fiber along with a DeMultiplexer (DeMUX) at the receiver to split them from the fiber. The signals use different wavelengths. Prior to being multiplexed, source signals might be converted from electrical to optical format, or from optical format to electrical format and back to optical format. There are two types of WDM technologies: CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing).
CWDM Technical Overview
CWDM is definitely an optical technology for transmitting as much as 16 channels, each in a separate wavelength or color, within the same fiber strand. The CWDM solutions help enable enterprises and repair providers to increase the bandwidth of an existing Gigabit Ethernet optical infrastructure without adding new fiber strands.
Unlike DWDM, which could transmit up to 160 channels on a single fiber by tightly packing them, CWDM technology relies on wider spacing between channels. This design makes CWDM a relatively inexpensive technology for transmitting multiple Gbps signals on a single fiber strand compared to DWDM because it supports less-sophisticated, and therefore cheaper, transceiver designs. Within the point-to-point configuration shown within the figure below, two endpoints are directly connected through a fiber link. The ITU has standardized a 20-nm channel-spacing grid for use with CWDM, while using wavelengths between 1310 nm and 1610 nm. Most CWDM systems support eight channels in the 1470- to 1610-nm range. The CWDM GBIC and CWDM SFP solutions allow organizations to include or drop as much as eight channels (Gigabit Ethernet or Fibre Channel) right into a pair of Single-Mode Fiber (SMF) strands. As a result, the need for additional fiber is minimized. You may create redundant point-to-point links with the addition of or dropping redundant channels right into a second set of SMF strands.
CWDM multiplexing is achieved through special passive glass devices known as filters. The filters act as prisms, directing lights from many incoming and outgoing fibers (client ports) to some common transmit and receive trunk port. Optical multiplexing inside a ring with CWDM networks is supported with OADM (Optical Add/Drop Multiplexer). OADMs can drop off one or more CWDM wavelengths in a specific location and replace that signal with one or more different outbound signals.
The CWDM solutions have two primary elements which are a set of eight different pluggable CWDM Transceiver modules, along with a set of different passive CWDM MUX/DeMUX or CWDM OADM modules. Both transceivers and the passive MUXes are compliant with the CWDM grid defined within the ITU-T G.694.2 standards.
CWDM may be used by enterprises on leased dark fiber to improve capacity (e.g., from 1 Gbps to 8 Gbps or 16 Gbps Fibre Channel) over metro-area distances. One problem with CWDM is that the wavelengths are not compatible with EDFA (Erbium-Doped Fiber Amplifier) technology, which amplifies all light signals inside their frequency range.
Note: EDFA technology is beginning to make repeaters obsolete. EDFA is a form of fiber optical amplification that transmits a light signal via a section of erbium-droped fiber and amplifies the signal with a laser pump diode. EDFA can be used in transmitter booster amplifiers, inline repeating amplifiers, and in receiver preamplifiers.
In some areas, providers use CWDM to supply 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 between two or more points.
DWDM Technical Overview
DWDM is 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 modern-day, precise, and for that reason more expensive transceiver designs. Inside a service provider’s backbone network, nearly all embedded fiber is standard SMF (G.652) with high dispersion in the 1550-nm window. DWDM supports 32 or even more channels within the narrow band around 1550 nm at 100-GHz spacing, or about 0.8 nm, as illustrated in the figure below.
Note: Current DWDM cards supports 32 wavelengths.
Because of the EDFA compatibility of the wavelengths used, DWDM is also available over much longer distances than CWDM and supports MAN (Metropolitan Area Network) and WAN (Wide Area Network) applications. In practise, signals can travel for approximately 120 km (75 miles) between amplifiers if fiber with EDFA can be used. At distances of 600 km (375 miles) to 1000 km (600 miles), the signals should be regenerated.
DWDM can be used as a high-speed enterprise WAN connectivity service. Typical DWDM uses include connectivity between sites and knowledge centers for example1-, 2-, or 4-Gbps Fibre Channel; IBM fiber connectivity (FICON) and Enterprise System Connection (ESCON); and Gigabit and 10 Gigabit Ethernet.
The DWDM solutions also have two main components which are the DWDM transceiver modules (e.g., DWDM GBIC, DWDM SFP, etc.) and the DWDM MUX/DeMUX or DWDM OADM modules. Moreover, DWDM can be used in the amplifier applications as well as the transponder and MUX/DeMUX applications.
- Transponder: Receives the input optical signal (from the client-layer SONET/SDH or other signal), converts that signal in to the electrical domain, and retransmits the signal utilizing a 1550-nm band laser.
- Multiplexer: Takes the different 1550-nm band signals and places them onto an SMF. The terminal MUX may or may not also support a local EDFA. An OADM extracts a channel of signal and inserts (replaces it with) an outgoing signal from a site.
- Amplifier: Provides power amplification from the multiwavelength optical signal (e.g., DWDM EDFA).