Category Archives: CWDM Systems

Differences between CWDM System and DWDM System

As more bandwidth and faster data transmission rates over long distances are needed, users have to rely on more fiber optics. But laying more fibers will cost much. So WDM (Wave Division Multiplexing) technology appeared and solved this problem.

WDM is a technology which transmits multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry the different signals. By using bidirectional communications over a single fiber, network users can realize a multiplication effect in their available fiber’s capacity. The effect of expanding the capacity of the network without laying more fibers makes WDM systems so popular. The concept was first published in 1978. At the very beginning, only two signals could be combined by using WDM systems. But now more signals could be handled and transmission rates get faster.

Currently WDM systems are divided into two different types: CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). CWDM systems typically have 8 channels with 8 wavelengths separated by 20 nm from 1470 nm to 1610 nm. But the channels could also be increased to 16. Typical DWDM systems use 40 channels and provide wavelengths up to 96 with 0.4nm spacing. The channel spacing decides the call of “coarse” and “dense”. There are many differences between these two systems.


Transmission Distance

CWDM systems can’t transmit data over distances as long as DWDM system because the wavelengths are not amplified. Usually CWDM can travel somewhere about 100 miles. Therefore CWDM is limited in its functionality over longer distances. While DWDM systems are set for long haul transmission by keeping the wavelengths tightly packed. They can transmit more data over longer distances with less interference than a CWDM system. So it’s suitable for data transmission over long distances.

Power Requirements

The power requirements for DWDM are significantly higher. For example, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4 W per wavelength. But CWDM uses uncooled laser transmitter and it consumes about 0.5 W of power.


Two main factors like operating and hardware cost cause difference between CWDM and DWDM. The first example has been referred before. CWDM modulation laser is uncooled, but DWDM laser is cooling. Cooling laser using temperature tuning, uncooled laser adopts electronic tuning. The range of temperature distribution is non-uniform in a very wide wavelength, so the temperature tuning is very difficult to realize, which results in high cost. CWDM doesn’t have this problem. Second, for instance, DWDM transceivers are typically four or five times more expensive than CWDM counterparts. That’s mainly because of the lasers. Typical wavelength tolerances for DWDM lasers die are on the order of ±0.1 nm; whereas tolerances for CWDM lasers die are ±2-3 nm. Lower die yields increase the costs of DWDM lasers relative to CWDM lasers. From these two sides, CWDM system is cheaper than DWDM system.

Each kind of WDM system has its advantages and disadvantages. CWDM is very flexible and is good for campus LAN expansion. DWDM has large capacity and is ideal for long distance data transmission. CWDM and DWDM technology continue be to improved. The two have different functions and will complement not replace the other.

An overview of something about CWDM technology

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 solution help enable enterprise and service provides 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 design. In the point-to-point configuration , two endpoints 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 1310nm and 1610 nm. Most Cisco CWDM system support eight channels in the 1470-to 1610-nm range, shorthand for CWDM-SFP-1470 or CWDM-SFP-1550 or other wavelengths. The Cisco CWDM Gigabit Interface Converter(GBIC)/small form-factor pluggable (SFP)solution allow organization to add or drop as many as eight channels (Gigabit Ethernet or Fiber Channel) into a pair of single-mode (SM) fiber strads. 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.


CWDM multiplexing is achieved through special passive (nonpowered) glass devices known as filters. The filter act as prisms, directing lights from many incoming and outgoing fibers (client ports) to a common transmit and receive trunk port. Optical multiplexing in a ring with CWDM network 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 different outbound signals.

The Cisco CWDM GBIC/SFP solution has two main components: a set of eight different pluggable transceivers (Cisco CWDM GBICs and Cisco CWDM SFPs), and a set of different Cisco CWDM passive multiplexers are compliant with the CWDM grid defined in the ITU-T G.694.2 standards.

CWDM can be used by enterprise on leased dark fiber to increase capacity (for example), from 1 Gbps to 8Gbps or 16Gbps) 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 signals within their frequency range.

EDFA technology is beginning to make repeaters obsolete. EDFA is a form of fiber optical amplification that transmits a light signal through a section of erbium-doped fiber and amplifiers the signal with a laser pump diode. EDFA is used in transmitter booster amplifiers, inline repeating, and in receiver preamplifiers.

CWDM supports up to a 30-dB power budget on an SM fiber. This restricts the distances over which CWDM may be used. CWDM may be used. CWDM supports distances of approximately 60 miles (100 km) 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 service. A lambda service is where a provider manage equipment and multiplexes customer traffic onto one or more wavelengths for a high-speed connection, typically between two or more points.

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