Staged CFP Testing

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Unlike previous pluggable transceiver modules there is a real requirement to test the CFP in a staging area prior to deployment.

Previous devices such as the SFP or XFP currently are available with commodity pricing from almost $50 to a a few thousand dollars depending on the required interface support. In comparison the CFP can cost in the region of tens of thousands of dollars and is a much more complex device featuring individual lasers, gearbox and optical multipliers in a single device instead of the relatively simple SFP and XFP modules.

The CFP modules typically also are found to have a shorter warranty than other optical transceivers and network equipment. It is therefore essential that any CFP received by a NSP (Network Service Provider) must be functionally tested as soon as it is received in-house. In the case of maintaining spares stock it will simply not be possible to maintain a large amount of spares due to the cost. It could also be the case that by the time a CFP is removed from spares stock to be deployed the warranty period may have expired, if the CFP is then found to be faulty this would represent a large cost to the NSP. Performing a functional test on the CFP in isolation can therefore provide a good degree of protection against the cost of faulty CFP transceivers.

Due to the complexity of the CFP device it should no longer be considered a simple transceiver and more as a subsystem of the network device and there for be tested in a similar manner to network equipment.

There are a few key tests that should be performed on the CFP.

  • Bit error rate test
  • Skew test
  • Optical power measurement
  • Receiver sensitivity
  • Receiver pulling range test

These are described in more detail below;

  1. The bit error rate test can be performed in a simple looped back mode, however it is important that each supported bit rate should be supported. In the case of dual rate CFPs both the 103Gb/s Ethernet and the OTU4 OTN rate should be tested independently.
  2. While the CFP itself does not have any active function in terms of skew handling it does however represent a source of skew in the 100G link. Therefore measuring the skew introduced in a loopback situation is important as a way of ensuring the CFP does not introduce excessive amount of skew. In this case whilst performing a loopack BER test the skew should be measured and should be at a minimal level.
  3. As a multi-lane interface the CFP will include 4 or 10 lasers within the CFP module. Each laser must of course be working correctly. In addition if it is a WDM based CFP such as the LR4 or LR10 CFP then the optical mux also must be operating correctly. Measuring the optical power for each lane individually is therefore essential. If any of the lasers output or the optical mux is faulty then measuring the optical power for each lane would prove this. Measuring only the aggregate power measurement from the CFP would not be able to prove the device.
  4. Different CFPs will feature differing minimal input powers. Each CFP will need to be tested to ensure that it is able to operate error-free at this lower limit. The receiver sensitivity measurement can be performed by putting the module into a loopback with an inline optical attenuator. A BER test should then be performed with the amount of attenuation slowly increased until errors are detected. At that point the input power measurement should be taken and compared to the stated receiver sensitivity limit.
  5. While each protocol the CFP supports will have a nominal frequency each will also have a frequency range that it must operate over error free. The CFP should be tested to ensure that it can operate over this entire range. In order to test this the CFP module should be placed in a loopback and be tested with a BER test. Once it is observed to be error free the signal being sent to the CAUI interface should have a frequency offset applied to it at both extremes of the frequency range for that protocol. The CFP should still operate error free in both conditions.

Performing these relatively simple tests can prove the functionality of the CFP in a staging area allowing an NSP to deploy the device to the network with confidence that it will be fully operational and prevent costly out of warranty failure cases.

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