Copper and fiber media selection

Every link in a data center needs a medium: twisted-pair copper, twinaxial copper, multimode fiber or single-mode fiber. The choice depends on the link's length and speed, on the optics or transceivers the network team will buy, on the cost of the whole channel (cable plus transceivers, which usually dominate), on power and heat at the switch, and on how long the cabling must last. Getting it wrong is expensive: a plant that cannot carry the next generation of speeds must be re-cabled while the facility is live.

This lesson reviews how each medium carries signals, the categories and grades defined by the standards, the reach of common Ethernet applications on each, and a method for choosing media for a new facility. DCE-210 introduced optics and the naming of Ethernet interfaces; here the focus is on the cabling plant.

Balanced twisted-pair copper

Twisted-pair cable carries signals as the voltage difference between the two conductors of each pair (balanced transmission). Twisting makes any interference induce nearly equal voltages in both conductors, which cancel at the receiver, and different twist rates in the four pairs reduce crosstalk between them. Performance is specified by category, each tested up to a maximum frequency:

Category 6A is the minimum recommended by TIA-942 for new horizontal copper cabling. It is used for 1 and 10 Gb/s server connections in enterprise halls, and everywhere for out-of-band management networks, building systems, security cameras and access points. Category 8 was designed for switch-to-server links in ToR and EoR layouts, but 25GBASE-T and 40GBASE-T saw little adoption because twinax and fiber do the same job with less power and latency.

Copper cables are built as unshielded (U/UTP), with an overall foil (F/UTP), or with individually foiled pairs (U/FTP or S/FTP). Shielded cables resist interference better and allow tighter bundling, but the shield must be bonded at the patch panel to the telecommunications bonding system (TIA-607), and they are stiffer and larger. Category 6A cable is roughly 6 to 8 mm in diameter, noticeably larger than Category 6, which matters for tray fill (Lesson 8).

Bundling also matters thermally: when copper carries Power over Ethernet (PoE) at the higher power levels of IEEE 802.3bt (up to about 90 W at the source), the current heats the conductors, and large bundles must be derated or split, as with power conductors in an electrical raceway.

Twinaxial copper and active cables

For the shortest high-speed links, inside a cabinet or to the next cabinet, the network team usually buys direct-attach copper (DAC): a twinaxial cable with a transceiver-shaped plug molded on each end. Passive DACs reach about 3 m at 100 Gb/s per lane and less at 200 Gb/s per lane; active electrical cables (AECs) with signal-conditioning chips reach about 5 to 7 m. Active optical cables (AOCs) have optics built into the plugs and a fiber between them, reaching tens of meters.

DACs, AECs and AOCs are not part of the structured cabling plant: they are point-to-point equipment cords, bought with the network equipment and replaced with it. The structured plant must still provide pathways and cable management for them, and the designer must know which links will use them, because a link that leaves the cabinet row usually cannot.

How optical fiber works

An optical fiber is a glass core surrounded by a glass cladding with a slightly lower refractive index. Light launched into the core is guided by total internal reflection at the core-cladding boundary. Both have an outer diameter of 125 µm; the core is 50 µm in data center multimode fiber and about 9 µm in single-mode fiber. A 250 µm (or 200 µm in reduced-diameter designs) acrylate coating protects the glass, and cables add buffers, strength members and jackets.

Two properties limit how far a signal can travel:

• Attenuation: the loss of optical power along the fiber, in dB/km, caused mostly by scattering in the glass. It is lowest at 1550 nm (about 0.2 dB/km in cabled single-mode fiber, although specifications allow more) and higher at 1310 nm and 850 nm.
• Dispersion: the spreading of each light pulse as it travels, which blurs successive bits into each other. In multimode fiber the main cause is modal dispersion: light travels along many paths (modes) of slightly different lengths. Graded-index multimode fiber reduces this by grading the refractive index so that longer paths are faster, and the quality of that grading is measured as the fiber's effective modal bandwidth (EMB) in MHz·km. Single-mode fiber carries only one mode, so modal dispersion disappears, and only much smaller chromatic dispersion remains.

Because multimode fiber has a large core, it can be used with inexpensive vertical-cavity surface-emitting lasers (VCSELs) at 850 nm, which is why multimode optics have historically been cheaper. Single-mode optics need more precise lasers and alignment, but high-volume production for hyperscale data centers has narrowed the price gap, and single-mode reach is far longer.

Fiber grades

The standards define multimode grades by bandwidth and single-mode grades by attenuation and dispersion:

Standard cabling specifications allow higher maximum attenuation than these typical figures. For example, TIA-568.3 allows 3.5 dB/km at 850 nm for multimode cable and, for single-mode, 1.0 dB/km for inside plant cable and 0.5 dB/km for outside plant cable. A loss budget calculated with the standard's maximums is conservative; one using the manufacturer's guaranteed values is tighter but still defensible (Lesson 6).

Bend-insensitive fibers (ITU-T G.657 for single-mode, and bend-insensitive multimode) lose much less light in tight bends. They are now standard in data center cables because the plant is full of small-radius routing in panels and cabinets. Fiber also comes in reduced-coating (200 µm) versions that allow more fibers in a smaller cable.

Reach of common applications

For each Ethernet interface, the IEEE 802.3 standard sets a maximum reach on each fiber grade together with a maximum channel insertion loss. These are the most important numbers for media selection:

Two trends stand out. First, multimode reach has stayed at about 100 m even as speeds rose, because faster signals tolerate less modal dispersion; each new speed has needed more fibers or more wavelengths rather than more reach. Second, single-mode parallel optics such as DR4 (and the 800 Gb/s DR8) have become cheap enough to use for short links too. Most hyperscale and AI facilities now install single-mode fiber almost everywhere, while many enterprise and colocation operators still use OM4 for links within a hall.

Connectors

Fiber connectors align two fiber cores so light passes from one to the other. The main types in data centers are:

• LC: a small 1.25 mm ferrule duplex connector, the standard for two-fiber (duplex) links and for breakout of parallel optics. LC replaced the larger SC connector in most data center uses, although SC remains common in carrier and outside plant equipment.
• MPO (multi-fiber push-on): a rectangular ferrule holding 8, 12, 16 or 24 fibers in rows, used for parallel optics and for trunk cables. MTP is a widely used high-performance brand of MPO. Lesson 6 covers MPO polarity and design in detail.
• Very small form factor (VSFF) connectors such as CS, SN and MDC: duplex connectors much smaller than LC, which allow two or more independent connections in the space of one LC duplex port and fit the breakout ports of 400G and 800G transceivers.

Single-mode connectors also differ by end-face polish. UPC (ultra physical contact) connectors have a domed, flat-to-the-axis end face, usually blue. APC (angled physical contact) connectors are polished at 8°, so any light reflected at the joint is sent into the cladding rather than back toward the laser; they are green and are required by many single-mode optics, including MPO-based DR4. An APC connector must never be mated with a UPC connector: the angled and flat faces meet with an air gap, causing high loss and possible end-face damage.

Total cost of a link

The cable itself is usually a small part of a link's cost. A full comparison includes:

• Transceivers at both ends, which for high-speed links often cost several times the cabling.
• Connectors, panels, cassettes and installation labor, including termination and testing.
• Power: transceivers and copper PHYs draw power continuously, and every watt also costs cooling. 10GBASE-T ports draw several watts each, compared with roughly 1 W for a 10 Gb/s short-reach optic.
• Lifetime: a structured plant should last through several generations of network equipment (often 10 to 15 years), while transceivers are replaced with each generation.

The following lessons go deeper into each medium: Lesson 3 covers copper channel performance and PoE, Lesson 4 the physics of loss, dispersion and wavelengths in fiber, and Lesson 5 the transceivers and breakouts that decide what each link must carry.