Typical CWDM Optical Elements and Features

CWDM VS DWDM differ noticeably in the spacing between adjacent wavelengths. DWDM packs many channels into a small usable spectrum, spacing them 1 to 2 nm apart; DWDM systems support a high channel count, but also require expensive cooling equipment and independent lasers and modulators to ensure that adjacent channels do not interfere. CWDM systems, on the other hand, use 10 to 25 nm spacing, with 1300 or 850 nm lasers that drift less than 0.1 nm/c. This low drift eliminates the need for cooling equipment, which, in turn, reduces the total system cost. As a result, CWDM systems support less total bandwidth than DWDM systems, but with 8 to 16 channels, each operating between 155 Mbps and 3.125 Gbps to over 100 Gbps. Typical systems support eight wavelengths, data rates up to 2.5 Gbps per wavelength, and distances up to 50 km.

CWDM uses lasers with a wide channel CWDM wavelength spacing. In contrast, DWDM, which is widely used in long-haul networks and some metro core networks (particularly those with large diameters), uses lasers with much narrower wavelength spacing, typically 0.8 or 0.4 nm. The wide channel spacing of CWDM means a lower system cost can be achieved. This lower equipment cost is the result of a lower optical CWDM mux/demux cost (due to wider tolerance on the wavelength stability and bandwidth).

CWDM represents significant costs savings-from 25% to 50% at the component level over DWDM, both for equipment OEMs and service provides. CWDM products cost about 3500 dollars per wavelength. Traditional CWDM only scale to about eight wavelengths, but for metro access applications, this may be adaquare. Also, mux demux manufacturer china have found ways to combine CWDM with its regular DWDM blades that allow the systems to scale up to 20 wavelengths. CWDM system architecture can benefit the metro access market because it takes advantage of the inherent natural properties of the optical devices and eliminates the need to artificially control the component characteristics.

8 channels, 1RU Rack Mount, Duplex, CWDM Mux  Demux

The typical CWDM optical elements are as follows:

CWDM Uncooled Coaxial Lasers: Distributed-feedback multiquantum well (DFB/MQW) lasers are often used in CWDM systems. These lasers typically come in eight wavelengths and feature a 13 nm bandwidth. Wavelength drift is typically only 5 nm under normal office conditions (say, with a 50℃ total temperature delta), making temperature compensation unnecessary. For additional cost savings, the lasers do not require external gratings or other filters to achieve CWDM operation. They are available with or without an integral isolator.

CWDM Transmitters/Receives: OC-48 CWDM transmitters typically use an uncooled DFB laser diode and are pigtailed devices in a standard 24-pin DIP package. Six to eight channels are supported (six channels: 1510 to 1610 nm; two additional channels are located at 1470 and at 1490 nm.) The OC-48 receiver typically uses an APD photodetector, has a built-in DC-DC converter, and employs a PLL for clock recovery. Transmission distances of up to 50 km are achievable with these modules.

CWDM Multiplexers/Demultiplexer: These come in 4 or 8 channel module, just 4 channel CWDM Multiplexer or 8 channel CWDM Multiplexer, typically use thin -film filters optimized for CWDM applications, with filtering bands matched t other CWDM wavelengths. Filters need to feature low insertion loss and high isolation between adjacent channels.

CWDM Optical ADD/Drop Modules (OADMS): These are available in various configurations with one, two, or four add and drop channels using the same thin-film filters as the CWDM mux and demux modules.

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Talking about the comparison between CWDM and DWDM

CWDM, just as DWDM, use multiple light wavelengths to transmit signals over a single optical fiber. However, there are still some differences betwwen the two techologies in many ways.

CWDM uses a 20-nm wavelength spacing that is much wider than the 0.4 nm for DWDM. The wider wavelength spacing in CWDM means lower product development costs. This is one reason why CWDM is less costly than DWDM.

Most CWDM devices operate in the 1470-nm to 1610-nm range. The frequency grid for DWDM and the wavelength grid for CWDM systems are defined by the international telecommunication union (ITU) standard G.694.1 and G.694.2, respectively.

CWDM provides a maximum of 8 lambadas between two CWDM multiplexers over a single fiber pair as compared to DWDM Multiplexers, which support up to 32 lambdas (based on 0.8-nm or 100-GHz wavelength spacing) over a single fiber pair. some long-haul DWDM systems can support up to 160 lambdas per fiber pair.

Each CWDM channels uses a specialized gigabit interface converter (GBIC) or small form-factor pluggable (SFP) transceivers are commonly known as colored GBIC and SFP. Each CWDM channels uses a different “color” GBIC or SFP because each lambda represents a different color in the spectrum. In this case, the native GBIC or SFP in the client devices are substituted with a colored GBIC or SFP.

CWDM multiplexers are usually passive (i.e, not powered) devices containing a very accurate prism to multiplex eight separate wavelengths of light along a single fiber pair. And passive CWDM devices cannot generate or repeat optical signals.

No amplification is possible with CWDM because CWDM uses wavelengths that cannot be amplified with EDFA amplifiers. Therefore, the maximum distance for a CWDM link is approximately 100 km.

The Cisco ONS 15501 EDFA, which has a wavelength range of 1530 nm to 1563 nm, can only amplify two signals (1530 nm and 1550 nm) out of the eight signals that are multiplexed onto the fiber pair.

CWDM provides an alternative solution to DWDM for low-latency and high-bandwidth requirements associated with synchronous replication application. However, DWDM is more scalable than CWDM. DWDM also has a longer distance capacity than CWDM because DWDM can be amplified. The main benefit of CWDM is its low cost. It is a cheaper solution than DWDM. In other words, CWDM is optimized for cost, while DWDM is optimized for bandwidth. For enterprises that have access to dark fiber and have only limited scalability requirement, CWDM is a relatively inexpensive way to achieve low-latency and high-bandwidth interconnections between DCs. The CWDM implementation also results in less complex installation, configuration, and operation as compared to DWDM.

CWDM can be deployed in point-to-point, linear, or fiber protected ring topologies, It is limited to a distance of up to 120 km for Gigabit Ethernet and 100 km for 2-G FC in a point -to-point topology. It is typically used only for extension of the FC fabric in a metro or campus application. As CWDM carries only eight lambdas on a single fiber pair, there are limits to the number of possible drops and the number of sites that can be interconnected. A ring or linear topology reduces the distance depending on the number of OADMs traversed by the CWDM channels because each CWDM OADM introduces additional power loss in the network.


CWDM can also be used to enable multiple ISL connections between the switches over a single fiber since it requires less fiber for interconnecting two metro sites. The same benefit applies to port channel implementation between the switches.

In short, DWDM is a solution that provides a higher number of connections and longer reach, or extension, at a much higher cost while CWDM is a more cost-effective solution for metro or campus solutions where the distance is limited.

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Article Source: http://www.fiber-optical-networking.com/




CWDM DWDM Networking Solutions

Wavelength division multiplexing is a cost effective and efficient way for expanding the fiber optic transmission capacity, because it allows using current electronics and current fibers and simply shares fibers by transmitting different channels at different color (wavelength) of light.

Wavelength Division Multiplexing, WDM is a technique that multiplexing several signals over a single fiber optic cables by optical carriers of different wavelength, using light from a laser or a LED. According to the number of wavelengths it supports, there are Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

CWDM was introduced as a low-cost approach to increasing bandwidth utilization of the fiber infrastructure. By using several wavelengths/colors of the light, 18 channels are viable and defined in the ITU-T standard G.694.2. CWDM systems typically provide 8 wavelengths, separated by 20nm, from 1470nm to 1610nm.

Benefits of CWDM
Passive equipment that uses no electrical power
Extended Temperature Range (0-70C)
Much lower cost per channel than DWDM
Scalability to grow fiber capacity with little or no increased cost
Protocol Transparent
Simple to install and use

Drawbacks of CWDM
16 channels may not be enough
Passive equipment offers no management capacities

DWDM packing WDM channels denser than in CWDM systems, 100 GHz spacing (approx. 0.8nm), more channels and higher capacity can be achieved using DWDM. IUT-T recommendation G.694.1 defines the DWDM channels spectrum. DWDM comes in two different versions: an active solution and a passive solution. An active solution is going to require wavelength management and it a good fit for applications involving more than 32 lines over the same fiber. In most cases, passive DWDM is looked at as a more realistic alternative to active DWDM.

Benefits of DWDM
Up to 32 channels can be done passively
Up to 160 channels with an active solution
Active solutions typically involve optical amplifiers to achieve longer distances

Drawbacks of DWDM
DWDM is very expensive
Active solutions require a lot of set-up and maintenance expense
“Passive” DWDM solution still requires power

Optical Add/Drop Multiplexing (OADM)
By optical add/drop multiplexing techniques, wavelength channels may be added and dropped at intermediate nodes using passive optical components only. Optical Add/Drop Multiplexers are used in WDM Systems for multiplexing and routing fiber optic signals. They can multiplex several low-bandwidth streams of data into a single light beam, and simultaneously, it can drop or remove other low-bandwidth signals from the stream of data and direct them to other network routers. There are CWDM OADM and DWDM OADM.

FiberStore offer a wide range of WDM optical networking products that allow transport of any mix of service from 2Mbps up to 200Gbps. Our highly reliable WDM/CWDM/DWDM products include CWDM multiplexers and demultiplexer, DWDM Multiplexers and demultiplexers, CWDM & DWDM Optical Add-drop Multiplexer, Filter WDM modules, CATV amplifier, OEO converters as well as many other most demanding CWDM DWDM networking infrastructure equipment.