Cost-effective Capacity Growth and Investment Protection — Hybrid DWDM/CWDM

Introduction of Hybrid DWDM-CWDM

CWDM is an excellent, cost-effective, first step solution for scaling metro networks. Low cost hybrid DWDM-CWDM modules can support up to 8 channels at 2.5 Gbps. This is sufficient for many networks in the metro space. If capacity needs grow beyond 8 channels, these modules can be used to merge DWDM and CWDM traffic seamlessly at the optical layer. This allows carriers to add many channels to networks originally designed for the more limited CWDM capacity and reach.

Hybrid DWDM-CWDM technology delivers true pay-as- you-grow capacity growth and investment protection. It offers a simple, plug-and-play option for creating hybrid systems of DWDM channels interleaved with existing CWDM channel plans.


Advantages of Hybrid DWDM-CWDM

The major advantages of hybrid DWDM-CWDM for carriers are as following:

  • Reduced Cost: CWDM has a significant cost advantage over DWDM due to the lower cost of lasers and the filters used in CWDM modules (CWDM MUX, CWDM OADM etc.). Coarse channel spacing allows more tolerance for channel deviations or wavelength deviations. Therefore, CWDM filters and transmitters are easier, and cheaper, to manufacture. This cost saving becomes quite significant for large deployments.
  • Pay-As-You-Grow: Adding new channels one at a time allows for on-demand service introduction with minimal initial investment, a critical feature in times of reduced OPEX and CAPEX spending.
  • Investment Protection: Although 8 channels may be enough in an initial deployment, it’s important to have an upgrade path to avoid a forklift upgrade to DWDM when growth in demand finally requires significant new capacity. Given the DWDM over CWDM upgrade capability, carriers no longer have to choose between CWDM and DWDM—both options can be deployed simultaneously or as part of a planned future, or incremental, upgrade. Hybrid DWDM-CWDM modules can be used in either the DWDM systems or in the CWDM systems. Current capital investment can always be used in the upgraded network.

Theory of DWDM/CWDM Hybridization

The CWDM frequency grid consists of 16 channels spaced at 20 nm intervals. The eight most commonly used channels are: 1470 nm, 1490 nm, 1510 nm, 1530 nm, 1550 nm 1570 nm, 1590 nm and 1610 nm. Within the pass band of these channels there exists the capacity to add twenty-five 100 GHz spaced DWDM channels under the 1530 nm envelope and twenty-five more under the 1550 nm envelope if the filter is properly designed. The theoretical availability of DWDM channels in the 1530 nm and 1550 nm pass-band is shown in the table below.

Theoretical DWDM Channels in the 1530 and 1550 nm CWDM Pass-Band


Practical Application of DWDM/CWDM Hybridization

In practice, adding another 25 DWDM channels in the pass-band of both the 1530 nm and 1550 nm CWDM channels is not achievable because the optical filters are not perfect square functions. The actual filter profile affects the number of channels which can be accommodated. However, actual DWDM filter technology does allow 38 additional channels to clear the CWDM archway as shown in the table below.

Actual DWDM Channels in the 1530 and 1550 nm CWDM Pass-Band

The system impact to adding these channels is equivalent to adding the component in line with existing CWDM equipment. The insertion losses add linearly. Here is a figure that shows the infrastructure in a fully populated CWDM system.

Infrastructure of Hybrid DWDM CWDM System

To add more channels to MUX side of this network, one would plug in a DWDM MUX with the appropriate channels to fall under the pass-band of the existing CWDM filters. The figure below shows the infrastructure of a CWDM system upgraded with 38 additional 100 GHz spaced DWDM channels.

44-Channel Hybrid DWDM-CWDM System

The number of channels present in this hybrid system is 38 DWDM channels plus the existing 6 CWDM channels for a total of 44. The equipment required to go from the first architecture to the second are 2 DWDM multiplexers and demultiplexers, as well as the additional transmitter and receiver pairs required. The additional loss incurred by the upgrade is equal to the additional loss of the DWDM elements and the additional connection points.

Several network types could take advantage of the hybrid architecture. For example, one could increase the capacity of an existing ring by deploying all of the elements above at each node. Or, one could allow DWDM traffic to overlay an existing CWDM network at a pre-determined crossover point.

The two networks would be configured in such a way to allow the DWDM traffic to travel across the CWDM ring. All of the nodes where the DWDM traffic would travel on the CWDM ring would require the DWDM multiplexer and demultiplexer pairs (shown as below).

Hybrid CWDM-DWDM Rings

Another application for the DWDM channels is for long reach links in CWDM rings. If a certain span exists in a CWDM network with a large distance between regenerators, e.g. 100 km, DWDM channels can be used in place of CWDM ones to overcome this distance. The figure below shows a hybrid DWDM-CWDM mixed node.

CWDM-DWDM Mixed Node

System Impact

The added components on the CWDM ring will decrease the link budget for each span by the amount of insertion loss for each new component. The use of high isolation optical filters for the DWDM channels will ensure that cross talk is minimized between closely spaced channels. In the case of very high channel counts, non-linear effects should be taken into consideration. These include self phase modulation and Four-Wave Mixing (FWM).

The lasers used in DWDM networks have a much narrower line width than lasers used in CWDM. As a result the DWDM signals will typically have farther reach, and will undergo less pulse broadening due to chromatic dispersion. However they also lie within the operating range of Erbium-Doped Fiber Amplifier (EDFA). This means that DWDM signals can go un-regenerated for large distances. This limit is reached at the transmitter’s dispersion limit.

Receiver technology is independent of the optical signal present. The same receiver can be used to resolve a CWDM signal as well as a DWDM signal. The InGaAs material used to convert the optical signal into an electrical one has an operating range that includes both wavelength schemes. In the case of a 3R receiver, the receiver should be chosen such that it is compatible with the transmitter’s data rate.

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Fiberstore Coarse Wavelength Division Multiplexing Devices

CWDM mux/demux is a flexible solution that enables operators make full use of available fiber bandwidth in local loop and enterprise architectures. The wavelengths used with CWDM implementations are defined by the ITU-T G.694.2, listing 18 wavelengths from 1270nm to 1610nm with 20nm increased. CWDM solution takes the most important advantage of low price which is typically 1/3rd lower than the equivalent DWDM optics. FS.COM introduces its new generation of coarse wavelength-division multiplexing (CWDM) devices boasting increased functions and improved performance to extend the reach of CWDM metropolitan networks. The following text will introduce CWDM Mux/Demux, CWDM OADM, and optical port configuration used in CWDM network.


The CWDM Mux/Demux in FS.COM is a universal device capable of combining up to 18 optical signals into a fiber pair or 9 optical signals into a single fiber. It is designed to support a broad range of architectures, ranging from scalable point-to-point links to two fiber-protected rings.

Besides, FS.COM CWDM Mux/Demux is a passive device which allows for any protocol to be transported over the link, as long as it is at a specific wavelength (i.e. T1 over fiber at 1570nm transported alongside 10Gbps Ethernet at 1590nm). This allows for long-term future proofing of the networking infrastructure because the multiplexers simply refract light at any network speed, regardless of the protocol being deployed. The following image shows FS.COM 8 Channels 1470-1610nm Dual Fiber CWDM Mux Demux.

8 Channels 1470-1610nm Dual Fiber CWDM Mux Demux

FS.COM CWDM Mux/Demux With Different Optional Port Configurations

FS.COM also provides CWDM Mux/Demux with different optional port configurations such as, express port, monitor port, 1310nm pass band port and 1550nm port for these multiplexers according to customer choice.

  • Monitor Port: Our CWDM Mux/Demux is optional to equip with monitor port that allows our customer connect optical meter or OSA to monitor and troubleshoot the network. It is simple to operate. Add the monitor port to an existing, multiplexed link. A small sample, of each signal, is “leaked” to the outputs, then connect measurement/monitoring equipment, such as power meters or network analyzers, to the module outputs. When finished monitoring, disconnect the instruments. The network is left undisturbed. (Monitor port tap percentage is 5% as default.)
  • Expansion/Express Port: The Expansion Port (EXP) enables the cascading of two CWDM Mux/Demux modules, doubling the channel capacity on the common fiber link. For example, two 4-Channel MUX/DEMUX modules can be cascaded to create an 8-Channel fiber common link. (Express port isolation is 15dB as default.)
  • 1310 Pass Band Port: The 1310 pass band port allows a legacy 1310nm signal to pass through the CWDM MUX DEMUX module. The port can be used to combine an existing legacy 1310nm network with CWDM channels, allowing the CWDM channels to be overlaid on the same fiber pair as the existing 1310nm network. (Note: When you choose 1310nm pass band port, the CWDM 1310nm wavelength channel is NOT available on the CWDM MUX modules.) Besides, the 1310nm port can be used in this way as an optical supervisory channel (OSC) and its range is 1270nm-1350nm (1310nm±40nm). (Note: When you choose 1310nm pass band as an OSC, the available range of wavelength is 1370nm~1610nm on the CWDM MUX modules.)
  • 1550 pass band port: The 1550 pass band port allows a legacy 1550nm signal to pass through the CWDM Mux/Demux module. The 1550nm port can also be used in this way as an optical supervisory channel (OSC) and its range is 1510nm-1590nm (1550nm±40nm). When you choose 1550nm pass band as an OSC, the available range of wavelength is 1270nm~1490nm on the CWDM MUX modules.

Note: that standard (or native) 1310nm and 1550nm wavelengths are not the same as CWDM 1310nm and CWDM 1550nm wavelengths. The center wavelength tolerances for legacy 1310nm and 1550nm are much wider than the CWDM equivalents, and therefore not precise enough to run through CWDM filters. When implementing a CWDM network, a standard wavelength can be converted to CWDM wavelength, or a CWDM Mux with a pass band port can overlay the standard wavelength onto the CWDM common link. A pass band port is an additional channel port on a CWDM MUX that allows a legacy 1310nm or 1550nm signal to pass through the network within a reserved band. The legacy device is connected directly to the pass band port via fiber cabling. Standard wavelengths can be converted to CWDM wavelengths using CWDM Small Form Pluggable (SFP) transceivers, transponders, and media converters that support SFPs.


Since adding new fiber optic cables for signal transmission of the devices would cost too much, IT managers would turn to use OADM in CWDM network, which can couple two or more wavelengths into a single fiber as well as the reverse process, saving a lot of money and installation time when they want to add or drop signal on a single fiber. FS.COM provides a wide selection of CWDM OADM which can add or drop fiber count of 1, 2 and 4. And these OADMs can be categorized into three type with different package form factors: plug-in module, pigtailed ABS box and rack mount chassis. The plug-in modules can be installed in empty rack enclosures. Three CWDM OADM types with different package form factors are shown below.

plug-in module, pigtailed ABS box and rack mount chassis


CWDM is a popular technology which can provide cost-effective solutions for users to upgrade their network using the least fiber strands. FS.COM provides a series of devices used in CWDM network, like CWDM Mux/Demux with different optical port configurations, CWDM OADMs, CWDM transceiver modules, etc. For any requirement, please visit FS.COM.