Optical fibre in the structured cabling system

Optical fibre carries almost every backbone in a modern building and campus, and a growing share of horizontal cabling in some designs. It carries light instead of electricity, so it is immune to electrical interference, carries no current between buildings, and reaches distances and speeds that copper cannot. This lesson places fibre within the structured cabling model: how it works, the fibre types and cable constructions, the connectors and panels, polarity and the loss budgets that the standards set. Installing, splicing and testing fibre are the subjects of LVT-210, which builds on this overview.

How optical fibre works

An optical fibre is a strand of very pure glass, about the thickness of a human hair. It has two layers: a central core, through which the light travels, and a surrounding cladding with a slightly lower refractive index. Light entering the core at a shallow enough angle strikes the boundary with the cladding and is reflected back, again and again, so it is guided along the fibre even around gentle bends. The cladding of almost all communications fibre is 125 µm in diameter, so connectors and splicing tools fit every type; what differs is the core.

The glass is protected by a plastic coating applied when the fibre is drawn, about 250 µm in diameter, and in most indoor cables by a further tight buffer of about 900 µm that makes each fibre easier to handle and terminate. A fibre's light is attenuated, weakened, slowly along its length by absorption and scattering in the glass, and faster at bends, splices and connectors. Attenuation is lowest at certain wavelengths, called windows, and fibre systems operate at those: 850 nm and 1,300 nm for multimode fibre, 1,310 nm and 1,550 nm for singlemode.

Multimode and singlemode

Multimode fibre has a relatively large core, 50 µm in all current types, which allows light to travel along many paths, or modes, at once. The modes arrive at slightly different times, which spreads each pulse of light and limits the bandwidth over distance. Multimode systems use inexpensive light sources, vertical-cavity lasers at 850 nm, and are used for links of up to a few hundred metres inside buildings and data centres. The ISO grades, which TIA also uses, are OM3 and OM4, the laser-optimized types used in most backbones, and OM5, which adds performance at several wavelengths for short-wave multiplexing. The older OM1 and OM2 types, with 62.5 µm and 50 µm cores, survive in older buildings but are not recommended for new work.

Singlemode fibre has a core of about 9 µm, small enough that light travels essentially in one mode. Without the spreading caused by many modes, its reach is limited by the light source and receiver, not by the fibre, over any distance found in a building or campus. Its transceivers used to cost much more than multimode ones, which kept singlemode on campus and service provider links; that difference has narrowed, and singlemode is now common in building backbones as well. The current grades are OS1a for indoor tight-buffered cable and OS2 for loose-tube and outdoor cable.

The jacket colour of indoor fibre cable identifies the type by long industry convention: aqua for OM3 and OM4, with some manufacturers using a violet called erika for OM4, lime green for OM5 and yellow for singlemode. Owners may use other colours for their own purposes, so the print legend on the jacket, which states the fibre type and count, is the reliable identification.

Fibre cable construction

Fibres are packaged in cables built for where they will go.

• Tight-buffered distribution cable holds 900 µm buffered fibres, typically 2 to 144 of them, with aramid yarn for strength, in one jacket. It is the usual indoor backbone cable: flexible, easy to terminate directly, and available with riser and plenum ratings.
• Breakout cable gives each tight-buffered fibre its own subunit jacket and strength yarn, so each can be routed and handled like a small cable. It is used where fibres must branch to separate equipment.
• Loose-tube cable carries 250 µm fibres loosely in gel-filled or dry water-blocked tubes, which isolate the fibres from stretching and temperature changes. It is the standard outdoor and campus cable. Fibres from loose tubes are usually spliced to factory-made pigtails rather than terminated directly.
• Indoor/outdoor cable combines outdoor water resistance with an indoor fire rating, so a campus cable can run from one building's equipment room to another's without a transition splice at each entrance.
• Armoured cable adds an interlocking aluminium or steel armour for protection against crushing and rodents. Metallic armour must be bonded where it enters a building, as Lesson 3 noted.

Ribbon cable, in which fibres are bonded side by side in flat ribbons of 12, is used where very high fibre counts must be spliced quickly, as in large data centres and service provider networks. Each ribbon is spliced in one operation.

Connectors, panels and polarity

Fibre connectors align two fibre ends so precisely that light passes from one core to the next with little loss, and they hold the ends in a ceramic or metal ferrule polished to a smooth end face. The common types in structured cabling are the LC, a small connector with a ferrule half the diameter of the SC's, used on almost all current transceivers and patch panels; the older and larger SC, still common on service provider equipment; and the MPO, which holds 12, 16 or 24 fibres in one rectangular ferrule for parallel optics and pre-terminated trunk cables in data centres. Most connectors use an ultra physical contact polish, colour-coded blue on singlemode; angled physical contact connectors, with an 8 degree angled end face, are colour-coded green, reflect much less light and are used in video and passive optical networks. An angled connector must never be mated to a flat one.

Fibre cables terminate in enclosures or patch panels in the telecommunications room and equipment room, where the fibres are protected, managed and presented on adapter panels, plates of mating sleeves. Many current installations use pre-terminated cassettes: a factory-made module with MPO connectors on the back and LC adapters on the front, connected by an MPO trunk cable, so that a backbone can be installed and plugged in without field termination.

A fibre link uses two fibres, one in each direction. Polarity is the discipline of making sure that the transmitter at one end always connects to the receiver at the other. TIA-568 defines it with letters: on a duplex connection, each link is wired so that the fibre on position A at one end arrives at position B at the other, a crossover built into the cabling or the cords. For MPO trunks and cassettes, TIA-568 defines several methods, called A, B and C, which differ in where the crossover is made. The rule for a contractor is simple: use one polarity method consistently across a project, document it, and use the cords and cassettes that method requires. Mixed methods are the commonest cause of a new fibre link that tests well but does not light up.

Loss budgets

A fibre link is designed and tested against a loss budget: the maximum total attenuation allowed between the transmitter and the receiver. TIA-568.3 sets the allowances used to calculate the expected loss of an installed link: 0.75 dB for each mated connector pair, 0.3 dB for each splice, and a fibre attenuation per kilometre at each wavelength. Multimode fibre is allowed 3.5 dB per km at 850 nm and 1.5 dB per km at 1,300 nm. Singlemode fibre is allowed 1.0 dB per km for inside-plant cable and 0.5 dB per km for outside-plant cable at 1,310 and 1,550 nm. The application, such as 10GBASE-SR, also has its own maximum channel loss, and the designer checks that the link's calculated loss is below it.

Here n with subscript c is the number of mated connector pairs, n with subscript s the number of splices, L the length in kilometres and the Greek letter alpha the fibre's allowed attenuation per kilometre. The result is a maximum: a well-made link usually measures much less. LVT-210 shows how the measured loss is compared with this calculated limit.

# Calculated maximum loss of backbone fibre links (TIA-568.3 allowances)
CONN, SPLICE = 0.75, 0.3
ALPHA = {("MM", 850): 3.5, ("MM", 1300): 1.5, ("SM", 1310): 1.0, ("SM", 1550): 1.0}
links = [("ER1 to TR 3A", "MM", 0.085, 2, 0),
         ("ER1 to gym", "SM", 0.32, 2, 2),
         ("ER1 to gym", "MM", 0.32, 2, 2)]
for name, kind, km, conns, splices in links:
    for (k, wl), a in ALPHA.items():
        if k != kind:
            continue
        loss = conns * CONN + splices * SPLICE + km * a
        print(f"{name:13} {kind} at {wl} nm: {km * 1000:4.0f} m, {conns} connector pairs, "
              f"{splices} splices -> {loss:.2f} dB")

ER1 to TR 3A  MM at 850 nm:   85 m, 2 connector pairs, 0 splices -> 1.80 dB
ER1 to TR 3A  MM at 1300 nm:   85 m, 2 connector pairs, 0 splices -> 1.63 dB
ER1 to gym    SM at 1310 nm:  320 m, 2 connector pairs, 2 splices -> 2.42 dB
ER1 to gym    SM at 1550 nm:  320 m, 2 connector pairs, 2 splices -> 2.42 dB
ER1 to gym    MM at 850 nm:  320 m, 2 connector pairs, 2 splices -> 3.22 dB
ER1 to gym    MM at 1300 nm:  320 m, 2 connector pairs, 2 splices -> 2.58 dB

Compared with an application's limit, these figures settle design questions quickly. The 10GBASE-SR application over OM4 allows a channel loss of roughly 2.9 dB, so the 85 m (280 ft) riser link at about 1.8 dB has room to spare, while the gymnasium's 320 m (1,050 ft) multimode link, with its two transition splices, comes to about 3.2 dB at 850 nm and would not be reliable. That matches the choice of singlemode for the gymnasium in Lesson 4.

Fibre beyond the backbone

Fibre also reaches beyond the backbone in some designs. Fibre to the zone runs fibre from the telecommunications room to small switches in ceiling enclosures, each serving a zone with short copper links. Passive optical LAN uses the technology of service provider fibre-to-the-home networks inside buildings: singlemode fibre from the equipment room passes through passive optical splitters to small terminals near the users, with no switches in the telecommunications rooms at all. Both reduce the number of copper cables and telecommunications rooms, at the cost of more active equipment near users. Most of Cobalt Line's work still uses copper horizontal cabling, but Maya should recognize these designs on drawings.

Fibre brings two safety rules that apply from the first day on any job, covered in detail in LVT-210. The light in communications fibre is often invisible infrared, and a laser source can damage the eye, so nobody looks into a fibre end or connector, and inspection is done with a video microscope or a scope with a filter. And the glass slivers produced when fibre is cleaved are sharp and almost invisible: they go into a marked disposal container, never onto the floor or the bench, and nobody eats or drinks where fibre is being terminated.