OTDR: Oughta Try Doin’ it Right
An article on the topic of optical time-domain reflectometers (OTDRs) that appeared in the April issue of this magazine correctly noted...
An article on the topic of optical time-domain reflectometers (OTDRs) that appeared in the April issue of this magazine correctly noted, “Bad connectors and too tight bends can be masked by other good connections” when an optical-loss test set (OLTS) is used for fiber testing (see “OTDRs make inside move to premises fiber testing,” page 46).
One point missing from the article is network reliability.
OTDR testing of a premises network results in verification of maximum possible reliability (or not). This is the prime benefit of OTDR certification: Examination of each connector pair, splice, and cable segment, with automatic analysis, lets the installer prove that each part of each link is properly and reliably installed.
On the other hand, the OLTS result provides absolutely no verification of reliability, as indicated by the quote above. In addition, the OLTS, if programmed to the TIA/EIA-568-B values, will pass a link with reduced reliability. (Note that I provide consulting to attorneys in lawsuits. Were I to be on the witness stand, I would have to state that the OLTS provides no proof of installation reliability while the OTDR can provide such proof. In rebuttal, I might state that the OLTS provides an inference, but not proof of, installed reliability.)
Here are three examples from our installation training programs:
• The multimode 3M Hot Melt connector is rated at a typical loss of 0.3 dB/pair and a maximum loss of 0.75 dB/pair. We have installed or supervised the installation of approximately 12,000 Hot Melts. Whenever the loss exceeds 0.4 dB/pair, there is visible damage on the core. Such damage reduces the additional damage the connector can withstand before its loss becomes unacceptable. The OLTS will not detect such damage. With a sufficiently short dead zone, an OTDR will.
• TIA/EIA-568-B allows a maximum splice loss of 0.3 dB. During more than 2,000 splices made with six different fusion splicers and two different mechanical splices, we never see more than 0.15 dB unless the installer makes an error. As a general statement, installer errors reduce the reliability of a fiber link. The OTDR will detect this increased splice loss. The OLTS will not detect the difference between a 0.05-dB and a 0.15-dB splice.
• We deliberately violated the bend radius of a 3-mm cable. The cable survived for nine months as part of a troubleshooting hands-on activity. During the tenth month, the cable broke in the area of the violation. An OLTS would not detect this condition. The OTDR can.
Also in that article from the April issue, the focus on the cost of an OTDR is misdirected. The cost of the OTDR, while important to the installation organization, is not the key issue. The issue is, and should be, focused both on the reduction of reliability that can occur without an OTDR test and on the cost per test of each link.
The cost to rent an OTDR for a week is about $1,000. Let’s be conservative and say it costs $2,000. During that week, we assume testing of a hierarchical star vertical riser backbone of 36 fibers to each of 20 stories. That’s a total of 720 fibers for a cost of $2,000-less than $3 per fiber-plus the labor cost. At most, the labor cost will be several hours of testing time.
The benefit of this testing is verification reliability. The cost to repair a link that is defective from installation and not revealed by an OTDR will be a relatively expensive repair call.
Let’s examine this repair-call cost. The owner will want the repair for free. The installer will want to charge for the call. With an OTDR test, there will be evidence of proper installation, meaning the installer will be able to charge for the call because the problem arose from misuse of the cable system. Without that evidence, the owner will be unhappy about being charged, and the installer may be forced (by a desire to keep the customer satisfied) to provide the repair without charge.
Looking at cost another way, a professional fiber installation firm can purchase an OTDR. Arbitrarily, let’s assume the cost is $12,000. Let’s also assume the firm will install 12,000 connectors within two years. Twelve-thousand connectors is 3,000 full-duplex links in a vertical riser backbone. If the installer does indeed install those 3,000 links, at an amortized cost of $4 per link, the investment is recovered in two years. Not a bad return-on-investment.
In summary, it is in the best interest of both the installer and the owner of a fiber network to require OTDR testing.
ERIC R. PEARSON, CPC, CFOS, is president of Pearson Technologies Inc. (www.ptnowire.com).