Fibre types, modes and cable construction

Before the crew pulls any cable, Hollis has Maya and Teo read the cable data sheets for every job this season. The data sheet tells an installer what the glass is, how the strands are packaged, where the cable may be installed, how hard it may be pulled and how tightly it may be bent. Two cables can both be described as 24-strand singlemode and still need completely different handling, termination and testing. This lesson covers the fibre grades, the ways strands are packaged into cable, and how to choose and read a cable for a job.

Multimode grades: OM1 to OM5

Multimode fibre is graded OM1 to OM5 under ISO/IEC 11801 and the matching TIA fibre specifications. The grade describes the core size and, above all, the modal bandwidth, which decides how far a given Ethernet speed can run. OM1 and OM2 are legacy fibres found in older buildings; new multimode installations use OM3, OM4 or OM5. Each grade has a standard jacket colour for indoor cable, which helps an installer identify fibre that was installed years ago without good records.

The Juniper Point condominium has a 12-strand OM3 riser installed when the building opened. St. Brendan's data room uses OM4 in pre-terminated trunks between racks. The Riverside campus route carries a 12-strand OM4 cable alongside the singlemode, because the school board's existing switches in the gymnasium have multimode optics and the board wanted to keep them for a few years. Sanjay's design notes say the OM4 is for the current equipment and the singlemode is for everything after that.

Singlemode grades: OS1a and OS2

Singlemode fibre in buildings is described as OS1a or OS2. Both use the same glass, a low-water-peak singlemode fibre that has low loss across the whole range from about 1260 to 1625 nm. The difference lies in the cable: OS1a is the designation used for tight-buffered indoor cable, whose maximum cabled attenuation is about 1.0 dB/km, and OS2 for loose-tube and outside plant cable, at about 0.4 dB/km. In practice the glass in either is far better than the cable specification, and most installers see 0.3 to 0.4 dB/km on new singlemode at 1310 nm.

Indoor singlemode cable has a yellow jacket. Outside plant cable is black, whatever the fibre inside, so the strand count and type must be read from the jacket print. A typical print reads something like the manufacturer's name, the cable part number, 24F SM OS2, the rating, the date of manufacture and a length mark repeated every metre. Maya photographs the print on each reel when it is delivered and attaches it to the job record.

Reach: matching fibre to the application

Whether a link works depends on two things: its loss, which the installer controls, and its bandwidth, which the fibre grade and length decide. A link with a low loss can still fail if it is too long for the fibre's bandwidth at that speed. The table in the code below gives typical Ethernet reaches by fibre type. They are the planning values most designers start from; the exact limits come from the Ethernet standard and the transceiver data sheet.

# Typical Ethernet reach by fibre type (planning values, metres)
reach = {
    "10G short-wave":  {"OM3": 300, "OM4": 400, "OM5": 400},
    "40G 4-lane":      {"OM3": 100, "OM4": 150, "OM5": 150},
    "100G 4-lane":     {"OM3": 70,  "OM4": 100, "OM5": 100},
    "400G 8-lane":     {"OM3": 70,  "OM4": 100, "OM5": 100},
    "10G long-reach":  {"OS2": 10000},
    "100G long-reach": {"OS2": 10000},
}
links = [("Riverside gym", 320, "OM4", "10G short-wave"),
         ("Riverside gym", 320, "OS2", "100G long-reach"),
         ("Juniper riser", 74, "OM3", "40G 4-lane"),
         ("St. Brendan's rack row", 38, "OM4", "100G 4-lane")]
FT = 1
for name, length, fibre_type, app in links:
    limit = reach[app].get(fibre_type)
    metres = length * FT
    ok = limit is not None and metres <= limit
    shown = f"{length} m"
    print(f"{name:24s} {shown:>9s} {fibre_type} {app:16s} limit {limit} m: {'OK' if ok else 'TOO LONG'}")

Riverside gym                320 m OM4 10G short-wave   limit 400 m: OK
Riverside gym                320 m OS2 100G long-reach  limit 10000 m: OK
Juniper riser                 74 m OM3 40G 4-lane       limit 100 m: OK
St. Brendan's rack row        38 m OM4 100G 4-lane      limit 100 m: OK

The Juniper Point riser shows why reach matters for upgrades. The building's 12-strand OM3 runs about 74 m (243 ft), well inside the 100 m (330 ft) planning reach for 40 Gb/s on OM3. A 10 Gb/s upgrade on the Riverside OM4 also works. But the OM4 on the campus route could never carry 100 Gb/s with short-wave optics, which is exactly why Sanjay added the singlemode. On fibre, the length that decides an application is the whole channel, from transmitter to receiver, including patch cords and any cross-connects.

How strands are packaged: buffers

The 250 µm coated fibre is too fragile to handle on its own. Cables add protection in one of two ways. In tight-buffered cable, each fibre has a thicker plastic layer applied directly over the coating, bringing it to 900 µm. Tight-buffered strands are stiff enough to be terminated directly with connectors and to be handled in enclosures without extra protection. Indoor distribution cable, riser cable and patch cords are tight-buffered.

In loose-tube cable, the 250 µm fibres lie loosely in a plastic tube, typically 6 or 12 fibres to a tube, often filled with a water-blocking gel or protected with dry water-blocking yarns and tapes. The fibres are slightly longer than the tube, so when the cable stretches or shrinks with temperature, the tube takes the strain and the fibres stay relaxed. Loose-tube cable is the standard for outdoor and campus use, because it handles temperature swings, water and long pulls. Its fibres are too fragile to connectorize directly; they are spliced to pigtails or protected with fan-out kits.

Some cables are rated for both indoor and outdoor use. These indoor/outdoor cables are usually dry loose-tube or tight-buffered designs with a jacket that resists sunlight and water and also meets the fire rating for inside buildings. They let a campus route run from a room in one building to a room in another without a transition splice at the building entrance, as long as the length inside each building and the fire ratings meet the codes, which LVT-330 covers. The Riverside route uses indoor/outdoor loose-tube cable for this reason.

Strength members, armour and jackets

Fibre cables are pulled by their strength members, not by the fibres. The most common strength member is aramid yarn, the same fibre used in protective vests, laid along the cable under the jacket. Outdoor cables often add a central rod of glass-reinforced plastic, which also stops the cable from shrinking in the cold and buckling the fibres. When a cable is pulled, the pulling grip or the knot is made on the aramid yarn or the rod, never on the jacket alone or the tubes.

Armoured cable adds a layer of corrugated steel or aluminium tape, or an interlocking metal armour, for protection from rodents, crushing and direct burial. Metal armour makes the cable conductive, which means it must be bonded and grounded where it enters a building, as LVT-330 covers. Dielectric cables, with no metal at all, are often preferred between buildings for exactly that reason: they carry no lightning or fault current from one building's ground to another's.

Jackets carry the fire rating for indoor cable. In Canada, the ratings are the FT ratings, such as FT6 for plenum spaces and FT4 for risers and general use; in the US, the optical fibre cable types are OFNP for plenum, OFNR for riser and OFN for general use, with conductive versions marked OFCP, OFCR and OFC. Where a cable may go is a code question, decided by the designer and checked by the installer against the jacket print before the pull.

Strand counts and cable types for each job

Strand counts in buildings commonly run in multiples of 6 or 12: 6, 12, 24, 48, 72, 96 and 144. Data centres and large campuses use ribbon cables with hundreds or thousands of strands, where twelve fibres are bonded side by side into a flat ribbon that can be spliced in one operation. Hollis's crew splices single fibres this season, but she shows Maya and Teo a ribbon splicer at the shop so they recognise one.

Checking a delivered reel before the pull

Hollis does not let a reel go into a conduit until it has been checked. A reel can be damaged in shipping, the wrong cable can arrive, and a fault found after the pull costs the whole pull. The receiving check takes a few minutes and has four parts. First, the reel and its packaging are examined for crushed flanges, nail holes or forklift damage, and the cable's outer layers for cuts and kinks. Second, the jacket print is read and photographed: strand count, fibre type, construction, rating and the starting length mark. Third, the factory test sheet that ships with most reels is compared with the print; it lists the attenuation of each strand measured at the factory.

Fourth, for long or costly outdoor pulls, the cable is tested on the reel. The inner end of the cable is usually accessible through a window in the reel flange. Hollis connects an OTDR to one or two strands at the outer end with a bare fibre adapter, or a temporary splice to a pigtail, and checks that the strands are continuous and that the length matches the reel label. A broken fibre on the reel is the manufacturer's problem before the pull and Cobalt Line's problem after it. At Riverside, both reels pass: every strand tested is continuous, and the OTDR length is within a few metres of the reel label once the fibre's extra length in the tubes is allowed for.