Media selection: copper, multimode and singlemode

The owner's IT director at Harbourview, Ines Kowalczyk, arrives at the media selection meeting with a printout of a vendor's sales sheet that says Category 8 cable is the future, and a question about why the design does not use it everywhere. Daniel has prepared a table of applications, distances and costs. Roz lets him present it. By the end of the meeting Ines agrees that Category 8 belongs in the data room, between servers and switches in the same row, and not in the ceilings of the clinic floors. The reasons are physics, standards and money, and a designer has to be able to explain all three.

What media selection decides

Every link in a cabling plant uses one of a small number of media, and each medium has a range of grades. The designer chooses for each link type: twisted pair of which category and shielding, or optical fibre of which mode and grade. The choice sets which applications the link can carry, how far, at what cost, and for how long. A wrong choice in the horizontal cabling is very expensive to undo, so media selection is one of the decisions an RCDD must justify in writing.

Balanced twisted-pair categories

Balanced twisted-pair cabling is graded by category in the TIA standards and by class in ISO/IEC 11801, where a class describes a channel and a category describes a component. Each category is specified up to a maximum frequency, and its performance limits for insertion loss, crosstalk and return loss are set across that range. Higher categories support higher data rates over the same 100 metre channel.

Category 6A is the workhorse of new horizontal cabling. It carries 10 gigabit Ethernet over the full channel, supports the highest Power over Ethernet levels with less temperature rise than smaller cables, and is what TIA recommends for new installations and for wireless access points. Category 6 is still installed where budgets are tight and 10 gigabit is not expected at the desk. Category 5e is recognized but rarely chosen for new work. Category 8 is a special case: it is designed for data centre links between switches and servers, with a channel of only 30 metres and no more than two connectors. It is not a horizontal office cable, despite marketing that suggests otherwise.

Unshielded and shielded cable

Twisted-pair cable comes unshielded, with an overall foil shield, or with individual foil shields on each pair and an overall braid. The names describe the construction: U/UTP is unshielded, F/UTP has an overall foil, and S/FTP has a braid overall and foil on each pair. Shielding improves immunity to outside noise and reduces alien crosstalk between neighbouring cables, which matters most at Category 6A and above. It also helps in electrically noisy places such as factories, hospitals with imaging equipment and buildings near radio transmitters.

Shielded systems need care. Every component in the channel must be shielded, and the shield must be bonded at the telecommunications room according to the manufacturer's instructions and ANSI/TIA-607, which RCDD-113 covers. A shield that is broken at a connector or not bonded can act as an antenna and make performance worse. Unshielded Category 6A, built with larger cable diameters or special separators to control alien crosstalk, performs well in most office environments. The choice between them is often a matter of owner preference, local practice and the environment, and both are fully recognized.

Optical fibre: multimode and singlemode

Optical fibre carries light in a glass core surrounded by cladding of slightly lower refractive index. Multimode fibre has a relatively large core, 50 micrometres in modern grades, which allows many paths, or modes, of light. That large core works with inexpensive light sources such as vertical-cavity surface-emitting lasers at 850 nanometres, but the different paths spread a pulse in time, which limits how far a given data rate can travel. Singlemode fibre has a core of about 9 micrometres that carries a single mode, avoiding that spreading and supporting very long distances, but it needs more precise and historically more expensive lasers at 1310 or 1550 nanometres.

Within multimode, the grade is mainly about bandwidth at 850 nanometres. OM3 and OM4 differ in effective modal bandwidth, so OM4 reaches farther at the same data rate. OM5 adds bandwidth across a band of short wavelengths, which allows several signals of different colours to share one fibre. Singlemode grades differ mainly in attenuation and in performance at different wavelengths; OS2 is the usual specification for new work. Many singlemode and multimode fibres are now made bend-insensitive, which reduces loss when the fibre is routed through tight spaces.

Reach by application

Fibre reach depends on the application as much as on the fibre. The IEEE 802.3 Ethernet standards specify the maximum distance for each application on each fibre grade. Short-reach multimode applications, used inside buildings and data centres, have reaches of tens to a few hundred metres. Singlemode applications reach kilometres.

The pattern in the chart drives many backbone decisions. A riser that is 90 metres long works on OM4 multimode at 10, 25, 40 and 100 gigabits with common short-reach optics, but a campus link of 400 metres does not work on multimode above 10 gigabits. Data centres often choose multimode for links within a row and singlemode for links between rows and rooms. Because the price of singlemode optics has fallen, some owners now standardize on singlemode everywhere, accepting a small premium in optics to avoid ever having to recable for distance.

Making the decision

Roz uses six questions to choose media for any link type. What applications must it carry now, and what is the most demanding one expected over its life? How long is the longest link? Will it carry power, and how much? What is the electrical environment? What will the connected equipment cost with each medium, including optics and switch ports? And what does the owner already use and support? The answers usually point clearly to one choice. When they do not, the designer presents the options with costs and lets the owner decide, recording the decision in the design narrative.

For Harbourview, the answers produce a short list. Horizontal cabling is unshielded Category 6A everywhere, because it covers every expected application at the desk and in the ceiling, supports high-power PoE and is what the owner's IT team already supports. Backbone is a pair of cables to each room, one OS2 singlemode and one OM4 multimode, to carry today's multimode uplinks while leaving a path to faster singlemode uplinks later. Within the data room, Category 6A patching is used for most equipment, Category 8 for a few high-speed server links within the same row, and OM4 trunks between cabinets. The campus link is OS2 singlemode in an outdoor-rated cable.

Cable construction and jackets

Within each category, cables differ in construction in ways that matter to the designer. Conductor gauge ranges from 22 to 24 AWG for horizontal cable; larger conductors have lower resistance, which reduces loss and PoE heating, but make the cable thicker and stiffer. Category 6A cables come in several outer diameters, and a smaller diameter saves pathway and manager space at some cost in alien crosstalk performance or heat. Some cables use a plastic separator, called a spline, between the pairs to keep them in place. These differences mean that two Category 6A cables can have very different effects on the pathways and rooms that hold them, and the designer should size pathways from the actual cable chosen, not from a generic figure.

Optical fibre cable construction differs too. Tight-buffered cables, where each fibre has its own thick coating, are easy to terminate and are common inside buildings. Loose-tube cables, where fibres float in gel-filled or dry tubes, protect the fibre from temperature and moisture and are common in outside plant. Indoor-outdoor cables combine outdoor durability with an indoor fire rating, so a campus link can enter a building and run to the equipment room without a splice at the entrance. Ribbon cables pack many fibres into flat ribbons for mass fusion splicing in very high counts.

Finally, the jacket must meet the fire rating for the space, as lesson 2 explained, and sometimes other requirements: low smoke zero halogen jackets in some institutional buildings and many international projects, ultraviolet resistance for exposed outdoor runs, and rodent protection where pests are expected. Each requirement narrows the list of products that fit, and the specification must state them all so that the installer cannot substitute a cheaper cable that meets only the category.

Media decisions should also consider the people who will maintain the plant. A college IT team that has worked only with unshielded copper and multimode fibre may need training and new test equipment to support shielded systems or singlemode, and those costs belong in the comparison. Conversely, an owner with an established singlemode practice in its other buildings may find that standardizing on singlemode everywhere simplifies spare parts and skills, even where multimode would have been slightly cheaper.