Fiber in the data center: OM4, OM5 and OS2

Jamal already knows how optical fiber works from LVT-210: light guided in a glass core by total internal reflection, multimode fibers with large cores and many paths for the light, single-mode fibers with tiny cores and one path, attenuation in decibels per kilometer and the colors of the jackets. This lesson does not repeat that. It is about which fibers a data center uses, why, and what that means for the links a technician installs. Nadia begins with a question: why does Hall A have orange, aqua and lime-green cords in its panels, Hall B mostly aqua and yellow, and Hall D almost nothing but yellow?

The fiber types in the halls

The table tells a story. When Hall A was built, short server links ran at 10 Gb/s over multimode fiber, because multimode transceivers were far cheaper than single-mode ones. As speeds climbed, the reach of multimode links shrank and single-mode transceivers got cheaper, until for 400 and 800 Gb/s many designers chose single-mode for everything longer than a rack or two. Hall D, designed in 2024, uses single-mode for all of its optical links. Hall A will carry its mixture of fiber types for as long as its customers keep their equipment.

Why multimode reach shrinks

A multimode fiber carries light along many paths of slightly different lengths, so a pulse of light spreads out as it travels. This modal dispersion limits how fast the pulses can follow each other over a given distance, which is described by the fiber's modal bandwidth. OM3 and OM4 have their core's refractive index carefully graded so that the paths arrive almost together, and OM4 is graded more precisely than OM3, which is why it reaches farther at the same speed. At higher speeds, each pulse is shorter, so the same spreading matters more and the reach drops.

Multimode links in data centers use 850 nm vertical-cavity lasers, VCSELs, which are cheap and efficient. Faster multimode links get their speed by running several lanes in parallel, one fiber pair per lane, rather than by running one lane faster. A 400GBASE-SR4 link, for example, uses four lanes of 100 Gb/s on four fibers in each direction. OM5 was designed to carry several wavelengths between 850 and 950 nm on each fiber, so that the lanes can share fibers instead, but it has seen limited use because single-mode took over the longer links.

# Typical maximum reach on multimode and single-mode fiber (meters), for planning only
reach = {
    "10GBASE-SR":   {"OM3": 300, "OM4": 400, "OM5": 400},
    "100GBASE-SR4": {"OM3": 70,  "OM4": 100, "OM5": 100},
    "400GBASE-SR4": {"OM3": 70,  "OM4": 100, "OM5": 100},
    "800GBASE-SR8": {"OM3": 60,  "OM4": 100, "OM5": 100},
    "800GBASE-VR8": {"OM3": 30,  "OM4": 50,  "OM5": 50},
}
single_mode = {"400GBASE-DR4": 500, "800GBASE-DR8": 500, "400GBASE-FR4": 2000, "400GBASE-LR4": 10000}
for app, row in reach.items():
    print(f"{app:14}", "  ".join(f"{k} {v * 3.281:>6.0f} ft" for k, v in row.items()))
for app, v in single_mode.items():
    print(f"{app:14} OS2 {v * 3.281:>6.0f} ft")

10GBASE-SR     OM3    984 ft  OM4   1312 ft  OM5   1312 ft
100GBASE-SR4   OM3    230 ft  OM4    328 ft  OM5    328 ft
400GBASE-SR4   OM3    230 ft  OM4    328 ft  OM5    328 ft
800GBASE-SR8   OM3    197 ft  OM4    328 ft  OM5    328 ft
800GBASE-VR8   OM3     98 ft  OM4    164 ft  OM5    164 ft
400GBASE-DR4   OS2   1640 ft
800GBASE-DR8   OS2   1640 ft
400GBASE-FR4   OS2   6562 ft
400GBASE-LR4   OS2  32810 ft

The reaches are the typical figures from the IEEE Ethernet standards and the transceiver makers; the exact limit for a given link depends on its loss and the number of connections, which Lesson 11 covers. The pattern is what matters: at 10 Gb/s multimode reaches across a hall, at 400 and 800 Gb/s it reaches across a row or two, and single-mode reaches the far end of the campus.

800 Gb/s optics

The IEEE 802.3df amendment, published in 2024, defined Ethernet at 800 Gb/s over eight lanes of 100 Gb/s: 800GBASE-SR8 and VR8 on multimode, DR8 on single-mode to about 1,640 ft and DR8-2 to about 1.2 mi, as well as copper and backplane versions. Some 800 Gb/s optics are instead built as two 400 Gb/s engines in one module, for example 2×400G-FR4, which use two duplex connectors and can talk to two separate 400 Gb/s ports. Work on 200 Gb/s per lane, which will bring 1.6 Tb/s links, continues in the IEEE 802.3dj project, and the first such optics are already appearing in the newest AI equipment.

For Jamal, the details of the electronics matter less than the connector on the front of the module, because that decides what he plugs in:

• Duplex LC: one fiber pair, used by single-wavelength 10 and 25 Gb/s links and by multiplexed links such as 400G-FR4 and LR4.
• MPO-12: twelve positions in one row, of which a 400G-SR4 or DR4 link uses eight, four to transmit and four to receive.
• MPO-16: sixteen positions in one row, the connector chosen for eight-lane 800 Gb/s links such as DR8 and SR8, which use all sixteen.
• Dual MPO-12 or dual duplex connectors on twin-port modules, which carry two independent links.

Corvid's backend in Hall D runs 800 Gb/s links on single-mode fiber with MPO-16 connectors at both ends, and its frontend on 2×400 Gb/s twin-port modules. Lesson 6 is about the multi-fiber connectors.

Single-mode in the data center

Single-mode fiber in data centers is almost always OS2, a low-water-peak fiber with low loss across the 1260 to 1625 nm window. Data center single-mode links use two kinds of optics. Parallel single-mode, such as DR4 and DR8, uses one wavelength near 1310 nm and one fiber per lane per direction, on MPO connectors. Wavelength-multiplexed single-mode, such as FR4 and LR4, puts four wavelengths on one fiber pair with duplex connectors. Parallel optics are cheaper per bit for short links but use more fiber; multiplexed optics save fiber at a higher price per module. Hall D's backend is parallel because its links are short and fiber inside a hall is cheap compared with the optics.

Single-mode connectors in Hall D are angled physical contact, APC: the end face of each ferrule is polished at about 8 degrees so that light reflected at the joint leaves the core instead of returning to the laser. APC connectors are usually green and must never be mated with flat-polished UPC connectors, which are usually blue on single-mode, because the mismatched faces leave an air gap and can be damaged. Many parallel single-mode transceivers require APC MPO connectors; multimode MPO connectors are normally flat-polished. Nadia makes Jamal say the polish out loud every time he picks up a cord in Hall D.

Bend-insensitive fiber and small cables

Data center cables are packed tightly and bent around managers, so nearly all fiber installed since about 2015 is bend-insensitive: multimode fibers with a trench around the core, and single-mode fibers meeting the G.657.A1 or A2 bend classes. They lose much less light in tight bends than older fibers. That does not make bend radius irrelevant. Bend-insensitive fiber still breaks if it is kinked, crushed or pinched by a hook-and-loop strap pulled too tight, and the cable around it still has a minimum bend radius set by its maker. The LVT-210 rules for bend radius still apply; what changes is that a bend that would have cost a dB in an old cable may now cost almost nothing, so a bad bend may not show on a loss test until the fiber cracks.

Data center fiber cables are also thinner than building cables. Patch cords use 0.063 or 0.079 in jackets, uniboot duplex cords put both fibers in one jacket, and high-count trunks use ribbon or rollable ribbon construction to fit hundreds of fibers into a cable the size of a finger.

Estimating the loss of a link

Before a link is installed, a technician should know roughly what its loss will be, so that a test result can be judged. The rule from LVT-210 is that the loss is the fiber's attenuation times its length, plus the loss of each mated connector pair and each splice. Data center links are short, so the connectors dominate.

# Rough loss of three Hall links: fiber plus connections (planning values)
links = [
    ("A-31-03 to A-HDA-31, OM4 at 850 nm", 0.038, 3.5, 2, 0.5),
    ("B-12-09 to B-NET-04, OS2 at 1310 nm", 0.052, 0.5, 2, 0.35),
    ("D-3-07 to D-4-02, OS2 MPO-16", 0.061, 0.5, 2, 0.35),
]
for name, km, db_per_km, pairs, per_pair in links:
    fiber = km * db_per_km
    conn = pairs * per_pair
    total = fiber + conn
    print(f"{name}: fiber {fiber:.2f} dB + {pairs} pairs {conn:.2f} dB = {total:.2f} dB  (length {km * 3281:.0f} ft)")

A-31-03 to A-HDA-31, OM4 at 850 nm: fiber 0.13 dB + 2 pairs 1.00 dB = 1.13 dB  (length 125 ft)
B-12-09 to B-NET-04, OS2 at 1310 nm: fiber 0.03 dB + 2 pairs 0.70 dB = 0.73 dB  (length 171 ft)
D-3-07 to D-4-02, OS2 MPO-16: fiber 0.03 dB + 2 pairs 0.70 dB = 0.73 dB  (length 200 ft)

On every link, the fiber itself contributes a few hundredths of a decibel and the two mated pairs contribute nearly all the loss. That is why connector quality and cleanliness, the subjects of Lessons 6 and 8, matter so much more in a data center than the length of the run.

Fiber counts and future speeds

Cabling is meant to outlast several generations of equipment. Hall A's 2014 multimode plant has already seen servers move from 10 to 25 Gb/s, and some of its customers would now like 100 or 400 Gb/s links that the plant cannot carry far enough. That history is why Stonebridge's newer halls are cabled with single-mode trunks of high fiber count, often more than the first tenant needs. Single-mode fiber has no modal bandwidth limit at data center distances, so the same strands that carry 800 Gb/s today will carry the next generation of optics, and spare strands in each trunk can be lit when a tenant adds links. Nadia's rule of thumb is that the trunks are installed for ten or fifteen years, the cords for three or four and the transceivers for less, so the trunks are the place to be generous.

For technicians, the practical result is that the fiber type of a trunk is a decision made once, at design time, and recorded in the trunk schedule. When a ticket asks for a link of a given speed and length, the technician checks that the trunk serving it is of a type and count that can carry it, and if not, the request goes back to the cabling lead rather than being forced onto the wrong fiber.