The design and installation of a fiber-centric equipment room may require more expertise than many broadcasters have on staff.
Whether driven by the industry, by consultants, or by clients, the deployment of IP video routing switches at the heart of media facilities is a rapidly growing trend. One result is that many more projects are converting what used to be largely copper cabled broadcast-centric rack rooms into hybrid datacenter-type infrastructure with traditional broadcast cabling.
The increasing volume of fiber isn’t the only challenge. Now, all of a sudden, we have all these devices with fiber — sometimes on the front, and sometimes on the back — and we need to re-examine how much patching to do and how best to do it. At BeckTV we’ve developed different solutions for these and other challenges that come with designing a central equipment room for scalable fiber infrastructure. Here are some solutions that work well in our installations.
New Fiber Rack Designs
The traditional approach for broadcast integrators in handling fiber infrastructure in rack design has been to use fiber tubs with either cassettes or pass-through plates in them. Options in this case have been to splice or just connect patch cables, or to connect an MTP cable and put a breakout panel on the front of the tub. In designing new equipment rooms involving a lot of fiber, however, we’ve begun to use different types of fiber racks that are custom-made for interconnecting on one side. They occupy a smaller footprint, can sit at the end of a row or against a wall, and feature integrated fiber management.
While fiber can be handled a number of different ways, we’ve learned that it’s best to have an area dedicated to just patch and fiber. Inexpensive options for SFP-connected devices situate these devices near one another and allow for short-distance direct-attach fiber cables, but a more optimal solution certainly would be to run everything from a centralized patch point instead.
Separate Fiber Pathways
Prior to the industry’s embrace of IP, we would have run all equipment room cables either in a cable tray or in an access floor. During the design phase, we were able to draw a rectangle around the equipment room and identify it as the cable tray. Now we carefully design dedicated fiber-runner pathways that distribute fiber around the room with many parts and pieces creating ideal pathways and access spouts into racks. It is our preference then to run traditional copper down into an access floor pathway if possible. (In any case, we insist on planning parallel paths, whether they are side by side, on top of one another, or one on the floor and one above the racks.)
In addition to scaling up the already-familiar fiber runners that live above a rack and spout cables into it, we add cable chases between heavy fiber-cabled racks to pass that cabling down, to isolate and protect it, and to keep it looking pretty. Fiber runners are a little bit more complicated to design in just because there are so many parts and pieces, and so many different ways to accommodate them. Do you want to spout them into a rack or funnel them in? Do you want cabling to go into the front of the rack or the back? Modern equipment room design involves a host of little details like these that need to be taken into consideration.
|Fiber Connector Terminology|
|SFP:||Small form pluggable connector.|
|ST:||Straight tip, a high-performance fiber-optic connector with round ceramic ferrules.|
|SFF:||Small Form Factor.|
|LC:||Lucent Connector. Developed by Lucent Technologies (hence, Lucent Connector). It uses a retaining tab mechanism, similar to a phone or RJ45 connector, and the connector body resembles the square-like shape of SC connector.|
|QSFP:||Quad Small Form-factor Pluggable, a compact, hot-pluggable transceiver.|
|MTP/MPO:||Cables bring together 8, 12 or 24 fibers into a single interface.|
|MTP:||MTP is a brand name for a version of the MPO (Multi-Fiber Push-on) connector. The MTP connector is designed to terminate several fibers—up to 12 strands—in a single ferrule. Because of the high number of fiber strands available in a small connection, MTP assemblies are used for backbone, cross-connect, and breakout applications.|
|JBT:||Junction box terminal.|
Fiber Connections to Devices
Every time we run fiber to a new device, we create a smaller model of what that would look like and then scale it to see if it is a viable approach. Because our engineers have been involved in numerous fiber installations, they share lessons learned regarding the pros and cons of different approaches. With the luxury of experience gained in various projects, it’s easier to avoid common problems that can compromise fiber infrastructure.
Figure 1. Illustration of Corning 8-fiber Uniboot Reverse Polarity MTP-LC assembly. The cable begins as an 8-fiber MTP connector that interfaces with QSFP transceiver and breaks out into duplex Uniboot LC connector that interfaces with a SFP transceiver. This Corning patented design gives the installer the ability to reverse the polarity in the field if needed. However the above illustration uses Universal polarity and would not need to be flipped.
Polarity is one of the aspects of fiber infrastructure that often gets overlooked, especially by people who have never dealt with it before. Most of these fiber devices in the IP routing world are duplex, and that means that there’s usually a transmit and a receive path. If the polarity gets flipped, which can happen in a few different ways, it forces engineers to split fibers apart and flip them around so that their devices will start to talk.
Many facilities today have infrastructure that connects a room down the hall or eight floors away to the rack room. The ties that low voltage cable makes are actually responsible for flipping polarities. Every tie that’s effectively from a rear tub to a rear tub will flip polarity.
In fact, designs at sports stadiums typically don’t use LC connectors for fiber outside the rack room infrastructure because it’s virtually impossible to predict how many polarity flips they are going to go through out at field JBT position. Instead, they will rely on two ST connections — which are easy to flip around — even though the end that lives in the rack room would be an LC duplex. This approach allows facility engineers to be consistent in one place and then just sort of roll with the punches out in the field.
Figure 2. This MTP connector pin-out shows how 12 fibers can be easily terminated in a single connector. Even so, breakouts are sometimes needed. Account for them at the design stage, not at installation.
We’ve worked with IP routers that use all 12 strands of fiber — where the Tx/Rx paths are paired like 1-2, 3-4, 5-6, 7-8, 9-10, and 11-12 — in an MTP connector, but solutions that come with commercial off-the-shelf (COTS) switches work differently. Because they are really data switches, they use only eight fibers and pair them 1-12, 2-11, 3-10, and 4-9. Working with these routing switches, it’s our preference to connect an MTP to the switches QSFP ports, run it to a cassette within a fiber tub, and then break it out into pairs of duplex fiber cabling on the front side of the cassette. Remember that MTP cables (MTP A, MTP B, MTP C) roll in a number of different ways — straight through, full roll, and paired roll — and only with the right version will all connections work.
MTP cables can simplify installation but make testing more difficult. It may be necessary to take down multiple ports to verify that the MTP cable is good. These days, ports can be carrying 6x6 or more, depending on whether it’s a 10 gig or 25 gig system, so testing an MTP might call for taking down a large portion of the system.
Multimode Versus Singlemode
Going multimode can be a big money-saver, with further savings through the use of direct-attach fiber optic cables, but it comes at the cost of cleaner looking installs as well as a potentially more convoluted future installation with the mix of fiber infrastructure type and layout. Ideally, everything is accessible and neat and easy to use, but the cost of optics is significant enough that it might be worth considering multimode where possible. Likewise, varying performance requirements might call for singlemode, which provides more flexibility and distance but comes at a higher cost.
While trying to optimize both cost and performance, some facilities get into a mix-and-match scenario for different areas of infrastructure. In practice, we’re seeing a lot of purely singlemode infrastructure for broadcast media and then multimode for the corporate network and such. Still, the debate over multimode versus singlemode will continue as everyone looks to drive down costs while getting the functionality they need.
In considering bandwidth, it’s not the fiber that matters so much as the SFPs and switches being used. They allow for the high-density cabling essential to ensuring sufficient bandwidth. These SFPs can be 10GbE or 25GbE, or aggregated into QSFPs — basically quad SFPs — so that a 10GbE becomes a 40GbE SFP and a 25GbE becomes a 100GbE SFP, with MPOs breaking out four duplex paths or eight fibers. In designing the facility, planners need to know what the bandwidth is at the core switch because that’s what ultimately will form the layout of infrastructure and the connectors and fibers that should be used.
Every facility’s central equipment room is different and presents unique challenges. Thoughtful integration of fiber can be critical to a successful migration toward IP-based operations — and realization of the technology’s benefits.
Brendan Cline, Director of Engineering, BeckTV
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