Connecting hardware, cross-connects and interconnects

Ines, the IT director, has one firm request for the Harbourview telecommunications rooms: her network staff must be able to make every change without touching the switches. In her last building, every move meant reaching into a tangle of cords plugged directly into switch ports, and twice a technician unplugged the wrong cord and took down a clinic. Roz tells Daniel that this request is really a question about connecting hardware and the choice between an interconnect and a cross-connect. Both are allowed by the standards. They lead to very different rooms.

What connecting hardware does

Connecting hardware is everything that terminates cable and lets it be joined to other cable or to equipment: jacks and outlets, patch panels, punch-down blocks, fibre adapters, connectors and splice enclosures. It must meet the same category or grade as the cable, maintain the cable's twist and shielding as closely as possible at the termination, and be installed to the manufacturer's instructions. Most cabling failures in the field are not in the cable at all but at terminations, where pairs were untwisted too far, a shield was not bonded or a fibre end face was dirty.

Twisted-pair connectors and pin assignments

Horizontal twisted-pair cable terminates on 8-position modular jacks at the outlet and on patch panels or punch-down blocks in the telecommunications room. The jacks use insulation displacement contacts, which cut through the insulation as the conductor is pressed in. Each jack is wired to one of two pin assignments in ANSI/TIA-568.2, called T568A and T568B, which differ only in swapping the positions of two pairs. Either works, but the same assignment must be used at both ends of every link and should be used throughout a building. Many owners and government agencies specify T568A; some older commercial practice favoured T568B.

Untwisting must be kept to a minimum at every termination. Each pair should remain twisted as close to the contacts as the hardware allows, because untwisted conductors add crosstalk. Manufacturers' instructions give the limit, usually a few millimetres for Category 6A. The cable jacket should end close to the termination, and the cable should not be pulled tight against the back of the jack. These details are the difference between a link that passes certification with margin and one that fails at the highest frequencies.

Patch panels and blocks

In the telecommunications room, horizontal cables are terminated on patch panels, usually with 24 or 48 ports in one or two rack units, or on punch-down blocks for some voice and building system circuits. Patch panels can be flat or angled, and angled panels can reduce the need for horizontal cable managers. Modular panels accept individual jacks, so that a mix of shielded and unshielded, copper and fibre can share one panel. High-density panels fit more ports per rack unit at the cost of harder access. The panel choice affects rack space, cord routing and labelling, which lesson 9 covers.

Optical connectors

Optical fibre connectors align the cores of two fibres so precisely that light passes from one to the other with little loss. The common connector in new buildings is the LC, a small form-factor connector that usually comes in duplex pairs, one fibre for transmit and one for receive. The larger SC connector is still common in service provider equipment and older plants. Multi-fibre push-on connectors, often called MPO and in one brand MTP, hold 8, 12, 16 or 24 fibres in a single ferrule and are used for parallel optics and pre-terminated trunks.

Fibre connectors have two kinds of end-face polish. Ultra physical contact polish is used on most multimode and many singlemode connectors. Angled physical contact, with the end face polished at 8 degrees and usually marked with a green body, reduces reflection and is used in singlemode systems that are sensitive to back-reflection, such as video overlays and some passive optical networks. An angled connector must never be mated to a flat one; the air gap causes high loss and can damage the end faces. Every connector must be inspected and cleaned before mating, because one particle of dust on a 9 micrometre core can block much of the light.

Fibre polarity

A fibre link must connect each transmitter to a receiver at the other end. That sounds trivial, but with duplex connectors, multi-fibre trunks and adapters in between, it is easy to end up connecting transmit to transmit. ANSI/TIA-568.3 defines polarity methods so that designers and installers get it right. For duplex links, the rule is simple: the link is wired A to B, so that the fibre in position A at one end arrives at position B at the other. For links built with multi-fibre trunks, the standard describes methods A, B and C, which differ in how fibres are arranged inside the trunk and which kind of cord or cassette is used at each end. The designer chooses one method for the whole project and specifies the trunks, cassettes and cords to match it.

Interconnects and cross-connects

In a telecommunications room, the horizontal cabling must reach the active equipment somehow. In an interconnect, a patch cord runs directly from each switch port to the patch panel port where the horizontal cable is terminated. There is one connection between the switch and the horizontal cable. In a cross-connect, each switch port is first cabled permanently to its own patch panel, which mirrors the switch, and changes are made by patch cords between the mirrored panel and the horizontal panel. There are two connections between the switch and the horizontal cable.

Each approach has advantages. An interconnect uses less rack space, fewer panels and fewer cords, has one less connection in the channel and costs less. It is the common choice in smaller rooms. A cross-connect keeps all patching away from the switches, so that staff never unplug a cord from active equipment. It allows the switches to sit in a separate cabinet or row, which is helpful in data rooms and high-density rooms, and it makes the patch field a complete map of the room. Its cost is more panels, more space and one more connection, which matters for loss budgets in high-speed fibre links.

Ines's request leads to a cross-connect in the equipment room and the two largest telecommunications rooms, where switches and patch fields are in separate racks, and interconnects in the smaller rooms, where space is tight. Each choice is recorded in the design narrative, and each fibre loss budget for the cross-connected links is checked with the extra connector pair included.

Splices and enclosures

Optical fibre can be joined by fusion splicing, which melts two fibre ends together with an electric arc, or by mechanical splicing, which aligns them in a sleeve with index-matching gel. Fusion splices have lower loss and reflection and are the standard for permanent joins, especially in outside plant and for pigtails that terminate field cable. Splices live in splice trays inside enclosures, which also hold adapter panels and manage slack fibre. The designer specifies enclosure capacity for the strand count plus growth, and allows room in the rack or on the wall for it.

Hardware for building systems

Building systems often need connecting hardware that differs from the usual desk outlet. A wireless access point or camera in the ceiling may connect through a surface-mount box with a jack, or directly to a field-terminated plug on the end of the horizontal cable, an arrangement known as a modular plug terminated link. ANSI/TIA-568.2 recognizes this arrangement and gives it a test configuration, so that a link ending in a plug at the device can be certified. It saves an outlet and a cord in places where neither adds value, but it must be planned and tested correctly, and the plug must be of a type suited to field termination.

Some building automation and industrial devices use connectors other than the 8-position modular jack, such as the M12 circular connector, which seals against dust and water. Single pair Ethernet, covered in lesson 11, uses its own small connectors. Where these appear, the designer specifies them by device type, often with a short adapter cord from a standard outlet, and records the connector in the outlet schedule so that spares can be stocked and the right test adapters brought to commissioning.

In every case, the connecting hardware must be compatible with the cable it terminates. Insulation displacement contacts are designed for a range of conductor diameters and insulation thicknesses; a cable at the edge of that range may terminate poorly. Shielded jacks must match the shielding of the cable. The safest course is to specify cable and connecting hardware from one tested system, and to require the installer to follow the system manufacturer's instructions and training.

Connecting hardware also needs to be accessible after installation. Panels mounted too high, too low or behind equipment are hard to patch and easy to damage. Wall-mounted fibre enclosures need clear space in front of them to open the door and work on the splice trays. And outlets in ceilings must be reachable through an access panel or a removable tile, because a hidden outlet cannot be tested or repaired.