Meeting the New Challenges of CWDM
Marc Rondeau, Product Specialist, Optical Business Unit
Mario Simard, Senior Product Manager, Optical Business Unit

Network operators are constantly faced with the challenge of optimizing the existing infrastructure of their networks to meet the needs of their customers. The radical increase in bandwidth demand has forced operators to find innovative and inexpensive ways to offer customers more services, thereby increasing revenues. One solution that has proven to be cost-effective is coarse wavelength-division multiplexing (CWDM), as it makes use of existing infrastructures.

The Importance of CWDM

When there is an existing infrastructure, finding an inexpensive way to increase bandwidth is not a simple feat, and unfortunately not even CATV and multiservice operators (MSOs) can escape this reality. In many cases, the fiber reach must be extended closer to the customer to enable split nodes to respond to the residential requirements. In other situations, such as for business requirements, the bandwidth demand cannot be fulfilled by extending the fiber network node; the easiest solution is to use dedicated wavelengths to serve the bandwidth needs. The most inexpensive way to do this is with CWDM channels, which provides customers access to more bandwidth, using the existing infrastructures.

Different variations of CWDM network deployments are available. The most basic configuration is based on a single fiber pair: one fiber is used to transmit and the other to receive. This configuration is known as the eight-channel system, where eight wavelengths (λ) are used to add and drop traffic—from 1470 nm to 1610 nm, with 20 nm spacing between channels.

Several approaches can be used. One is to use a CWDM application in the O band. An alternative solution would be to use a dedicated wavelength for upstream traffic and a dedicated wavelength for downstream traffic using one fiber for all of these signals, which optimizes the number of fibers available. Many other configurations are also possible; for example, a combined passive optical network (PON) and CWDM structure, also referred to as a mixed network.

Using CWDM as a means of increasing the bandwidth to the end customer also brings network characterization challenges, which can be encountered at different stages of the network deployment and maintenance testing process.

CWDM Testing

During the construction or installation phases, tests must be performed on the fiber from the head-end to the end customer and vice versa. Single-ended testing is definitely an advantage, as it optimizes labor resources. In such a case, the goal is to fully characterize the link, including the optical add/drop multiplexer (OADM), and to verify the continuity up to the final connection to the customer. Testing at a standard wavelength will be of no use since 1310 nm or 1550 nm cannot be used for all links—they are filtered out at the multiplexer (MUX) or never reach the intended customer since they are not dropped at the customer.

The question that remains is how can the link be fully characterized from the head-end to the end customer? The answer is to use an optical time-domain reflectometer (OTDR) that is specially designed for CWDM applications. A CWDM OTDR uses the required wavelength for a complete test to verify that the end connection point to the customer is reached and meets the total loss budget. It also ensures that all reflectance and splice/connector losses fall within the specified acceptance threshold and confirms that the add/drop process works properly. By analyzing the OTDR trace, technicians are able to ensure that there is no excess loss caused by a macrobend or any other type of fault.

In Figure 4, the CWDM OTDR trace (in black) illustrates four add/drops on the main fiber at 1470 nm, 1490 nm, 1550 nm and 1570 nm. The gray traces represent the tests performed at each individual wavelength where the fiber from the optical add/drop multiplexer (OADM) to the customer premises can be observed.

In Figure 5, the CWDM OTDR trace at 1550 nm illustrates the end of the fiber at the customer premises. The add/drop (event 2) can be observed at 1570 nm.

The OTDR not only provides the total loss and optical return loss (ORL) of the link, but also identifies and locates splices and connectors, in addition to indicating macrobends or any other faults. Note that the OTDR used to perform such tests needs a short dead zone in order to be able to identify closely spaced events. It must also have enough dynamic range (in combination with accurate central wavelength) to go through the add/drop to reach the appropriate end customer premises. Standard OTDRs have as much as 20 nm tolerance on the central wavelength, making these units unreliable for CWDM applications.

Another challenge arises when the network is live and a new customer is added on or when an existing customer experiences difficulties (maintenance and troubleshooting). In this case, two approaches can be used: CWDM OTDR or out-of-band testing—both with a CWDM power meter.

CWDM Approach vs. Out-of-Band Approach

The CWDM OTDR approach is relatively simple: Before turning on the service, the CWDM OTDR is used from the head-end and at the customer premises to verify the link (splice, connector, add/drop, excess loss and overall loss). The CWDM power meter is then used at the customer premises (when the service is activated) to verify that the power level is under budget. However, if the power level is not under budget, a test with the CWDM OTDR will reveal an anomaly in the link (in some cases the head-end transmission system will need to be turned off).

The out-of-band approach uses an out-of-band OTDR at 1650 nm combined with a CWDM power meter. The technician verifies at the customer premises if the power meets the specifications. If not, the 1650 nm out-of-band test is then performed to detect anomalies on the link. The advantages of performing the 1650 nm out-of-band test is its ability to test without disconnecting at the head-end plus the fact that the 1650 nm is filtered out at the OADM, thereby not affecting the remainder of the network—which prevents damage to the transmitting and receiving equipment.

If a problem is detected during a test, the first step is to verify that the electronics are operating properly. Since only one wavelength is dropped at the customer premises, the technician can identify the wavelength, and can therefore select the appropriate wavelength on the CWDM power meter. If the power level does not meet the specifications, the only troubleshooting tool required is the OTDR.

Additionally, out-of-band testing at 1650 nm can be performed without affecting the rest of the network, which enables the technician to verify and troubleshoot the last part of the link—from the OADM to the end customer.

Conclusion

CWDM testing challenges during construction and installation can be overcome with the use of a CWDM OTDR. Using such a tool will reinforce the confidence in the conformity of the total physical link (head-end to the customer premises), drastically reducing the probability of failure due to the physical infrastructure at the activation phase and eliminating the need to bring in another work crew to fix a problem that could have been found beforehand.

According to Murphy’s Law — based on the principle that one day or another, a problem will occur — it is always best to be prepared. The CWDM OTDR or the out-of-band OTDR and CWDM power meter combo become a valuable line of defense. Troubleshooting with these tools will reduce downtime, helping you reach the ultimate goal; i.e., to maintain and improve the customer experience and the quality of service.