Part 2:
With parallel optics, one failed fiber takes down an entire trunk: who can afford that risk? As the use of pre-terminated fiber becomes essential for high performance networks and new topologies in the datacenter, keeping the basic fiber fundamentals in mind will alleviate business, technology and performance issues. Introduction
Since the mid 1990’s, structured fiber cabling has seen many improvements in performance and topologies. As a result, many cabling plants have been upgraded with the latest improvements, possibly multiple times, leading to a desire for a stable future-proofed infrastructure. Major improvements have been enabled by changing to pre-terminated MPO/MTP parallel optical fiber cable trunks. These trunks provide improved optical performance, all while reducing installation time and reducing installation labor cost. Additionally, new topologies employing pre-terminated trunks coupled with fiber cassettes provide a robust future-proofed cabling infrastructure that is ready for easy upgrades to 10G, 40G and 100G application speeds. Foundations of future-proofing
Until the introduction of 40GBASE-SR4 and 100GBASE-SR10 in June 2010, most enterprise communication links utilized a duplex connection comprising separate transmit and receive fibers. Often, these duplex links were run over 12-fiber MPO/MTP trunks that utilized SC or LC fiber cassettes/modules to break out the trunk to the standard duplex interface.
Taking advantage of the large installed base and improvements offered by MPO/MTP trunks, the members of the IEEE standardized on a parallel optical topology for 40GBASE-SR4 and 100GBASE-SR10. The 40G topology defines a 12-fiber MPO/MTP connector to transmit over four parallel fibers and receive over four parallel fibers as illustrated with figure 1. The four unassigned fibers remain “dark”. The 100G application speed employs a similar topology except that it utilizes a total of twenty fibers, including 40G’s “dark” fibers as shown in figure 2. By necessity in a parallel cable one failed fiber takes down the whole trunk.
The obvious advantage of these topologies is straight-forward and low cost upgrade path to move the installed base of MPO/MTP trunks that are coupled with fiber cassettes/modules. The upgrade simply requires removing of the cassette/module and patching the trunk to the transceiver through a 12-fiber patch cable. 
Future-proofing requires pre-emptive steps
True future-proofing can only occur if pre-emptive steps are taken during the initial installation as well as any time the links are disconnected. Even if the initial installation does not utilize all fibers within the trunk, a failure to take proper pre-emptive steps can make the entire trunk unusable. This is because a fault with any one of the 12 fibers within the trunk renders the entire trunk useless for upgrading to 40G or 100G. (1) The first pre-emptive step is to visually inspect each fiber’s end face with an inspection camera. Contamination occurring on fiber end faces may produce damage to the two interfacing fibers. Often this damage is unrecoverable. Caution must be taken when cleaning contaminated multi-fiber connectors to avoid simply moving the contamination from one fiber to another. The correct process is to inspect first, clean and then inspect all fibers again.
(2) The next step required to ensure a future-proofed infrastructure is to certify that the overall optical loss through the trunk meets or exceeds the maximum limits set forth by the appropriate standards. Factory testing does not ensure field reliability. In addition to connector contamination and damage, faults as a result of installation processes, tight bends or breaks may occur. Only a solid test strategy will ensure field reliability and enable upgrading to higher speed applications.
(3) Finally, safeguard the upgradeability of the infrastructure by certifying the optical reflection and loss of each individual connection. Poor connector reflections can cause an increase in the laser’s Relative Intensity Noise. Likewise, individual connectors with high losses can increase noises such as Modal Noise. Higher noise in the network increases error rates which in turn reduces the actual throughput of a network by causing dropped data packets. Summary
Great progress has been made to creating future-proofed optical fiber infrastructures through the use of MPO/MTP pre-terminated fiber trunks. Because the future may require all fibers within the trunk to carry data, this progress can be brought to a halt if simple pre-emptive steps are not taken. There is no substitute for a solid test strategy that includes inspection, overall loss testing, and individual loss and reflection testing. Download PDF» |
