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.
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.
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.
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.
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.
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).
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.
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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