Glossary
Definitions for the network, cabling, and low-voltage terms that come up on site — each one saying what the thing is, then what it actually means when you are standing in front of it. Searchable, cross-linked, and filed by category.
45 terms,
defined properly
Every entry says what the thing is, then what it means when you are standing in front of it. Search filters as you type; the whole list is on the page either way.
- 66 blockCabling
A 50-row punchdown block, split into four columns, with the left and right halves internally bridged. The workhorse of analogue telephone distribution for fifty years.
In practiceStill everywhere in older buildings, still fine for POTS, and not suitable for anything above Cat3. Expect to find them undocumented and half-abandoned.
- 110 blockCabling
The punchdown block that replaced the 66 block for data. Wires lay into a base in colour order and a connecting block is pressed over them, which seats and cuts every conductor at once.
In practiceRated to Cat6 in the right hardware, where a 66 block is not. If you are terminating data on a 66 block in a new install, you are building something that will fail certification.
- Access portSwitching
A switch port carrying exactly one untagged VLAN — what a workstation, printer, or camera plugs into.
In practiceSet the mode explicitly rather than leaving it dynamic. A port left on dynamic auto can be talked into becoming a trunk by whatever is plugged into it.
- ARPAddress Resolution ProtocolAddressing
How a device finds the MAC address behind an IP address on the local segment. It shouts to the broadcast address and the owner answers.
In practiceA stale ARP entry is the reason a device stays unreachable for a few minutes after you swap hardware with the same IP. Clearing it on the gateway is faster than waiting.
- AttenuationCabling
Signal loss along a cable, measured in decibels. It rises with length, with frequency, and with heat.
In practiceThe reason for the 100 metre channel limit. It also rises noticeably in a hot ceiling, which is why a run that certified in February can fail in August.
- Auto-MDI/MDI-XSwitching
The port feature that detects whether the far end has transmit and receive the way round it expects, and swaps internally if not.
In practiceWhy crossover cables stopped mattering. Mandatory on 1000BASE-T, so any gigabit port has it whether or not the datasheet mentions it.
- AutonegotiationSwitching
The exchange at link-up where two ports agree on speed and duplex.
In practiceHard-setting one end and leaving the other on auto is the classic duplex-mismatch fault: the link comes up, works fine on light traffic, and collapses under load. Set both ends or neither.
- BPDU GuardSwitching
A switch feature that shuts a port down the instant it receives a spanning-tree BPDU — the signature of somebody plugging a switch into a desk port.
In practiceBelongs on every access port, paired with PortFast. It converts an all-day outage into one dead port and a log line naming it.
- Bridging clipCabling
A small metal clip that jumps the left and right halves of a 66 block row together, completing the circuit between the incoming pair and the outgoing one.
In practicePulling clips is how you isolate a line without unpunching anything. A missing clip looks identical to a dead pair and is the first thing to check.
- Broadcast domainSwitching
The set of devices that receive each other's broadcast traffic. A switch is one broadcast domain per VLAN; a router terminates it.
In practiceThe practical argument for VLANs. One flat domain across a large site means every device processes every broadcast, which is what makes a big flat network feel slow before it is actually saturated.
- CCA cableCopper-Clad AluminiumCabling
Cable with aluminium conductors under a thin copper skin, sold as a cheap substitute for solid copper.
In practiceIt is not to standard, it will not certify, it is brittle at the punchdown, and it carries PoE badly enough to be a fire concern. If a price looks impossible, this is why.
- ChannelCabling
The complete end-to-end path a signal travels: patch cord, horizontal run, any consolidation point, and the cord at the far end. Limited to 100 metres.
In practiceThat 100 metres is 90 metres of solid horizontal cable and 10 metres of patch cords, not 100 metres of whatever reaches. Certification tests the channel, not the run.
- Cross-connectCabling
A wiring arrangement where incoming and outgoing cable both terminate permanently on blocks or panels, and the connection between them is made with a removable jumper or patch cord.
In practiceThe whole point is that changes happen on the jumper, never on the terminated cable. Re-punching a permanent termination to move a service is how a rack degrades.
- Crossover cableCabling
A patch cord wired T568A on one end and T568B on the other, swapping transmit and receive so two like devices can talk directly.
In practiceEffectively obsolete. Auto-MDI/MDI-X has been standard on gigabit ports for twenty years and sorts it out in hardware. Keep one in the bag for ancient equipment and label it, or it will get patched into a rack and cause a very confusing afternoon.
- Demarcation pointDemarcPractice
Where the carrier's responsibility ends and the building's begins.
In practiceKnowing exactly where it is turns a carrier ticket from an argument into a measurement. If the fault is on their side of it, that is the whole conversation.
- Duplex mismatchSwitching
One end of a link running full duplex while the other runs half. The link is up and passes a ping, but collides constantly under real traffic.
In practicePresents as 'the network is slow', not as an outage. Look for late collisions and FCS errors on the interface counters — those two together are close to diagnostic.
- FCS errorFrame Check SequenceSwitching
A frame that arrived with a checksum that does not match its contents, meaning it was corrupted in transit.
In practiceA rising FCS count on one port is a physical problem: a damaged cable, a bad termination, or a duplex mismatch. It is never a software problem, so do not spend the afternoon in the config.
- Horizontal cablingCabling
The permanently installed run between the telecom room and the work area outlet. Solid conductor, 90 metres maximum.
In practiceSolid core, not stranded — stranded is for patch cords and will not hold in a punchdown. Getting this the wrong way round is a common and expensive mistake on a first install.
- Insertion lossCabling
The certification measurement of attenuation across the link, in decibels, at each tested frequency.
In practiceFailing insertion loss usually means the run is too long or the cable is not what the box said it was. Copper-clad aluminium fails this test almost immediately.
- Keystone jackCabling
The snap-in modular jack that mounts in a faceplate or patch panel opening. Terminates on the back, presents an eight-position socket on the front.
In practiceThe body is usually printed with both A and B colour codes. Pick one for the whole site and never mix them, because the labels are small and someone will follow whichever is nearest.
- MAC addressMedia Access ControlAddressing
The 48-bit hardware address burned into a network interface, used to deliver frames within a single segment.
In practiceThe switch's MAC address table is the fastest way to find which port a device is on. Learning to read it turns 'find this laptop' from a walk into a command.
- Native VLANSwitching
The one VLAN on a trunk whose frames cross untagged.
In practiceLeaving it as VLAN 1 is a standing finding on any security review. Set it to an unused, dead VLAN on every trunk, and make sure both ends agree — a mismatch silently merges two VLANs.
- NEXTNear-End CrosstalkCabling
Signal from one pair bleeding into another at the same end of the cable. The measurement that punishes untwisting.
In practiceThe single most common certification failure, and almost always the technician's fault rather than the cable's: too much jacket stripped, too much twist removed at the termination.
- Patch panelCabling
The rack-mounted array where horizontal runs terminate, presenting each as a front-facing port.
In practicePorts should be labelled to match the outlet at the far end, and the label should survive. A panel labelled in marker is a panel that will be re-toned in three years.
- Permanent linkCabling
The fixed portion of a run — outlet to patch panel — excluding patch cords. Maximum 90 metres.
In practiceThis is what you certify at installation, because it is the part that stays. The channel test comes later with the actual cords in place.
- Plenum cableCMPCabling
Cable with a jacket rated for air-handling spaces — low smoke, flame retardant, and required by code in any ceiling or floor void used for return air.
In practiceRiser-rated cable in a plenum space is a code violation and an inspection failure, and it is invisible until someone reads the jacket print. Check what is already there before you match it.
- PoEPower over EthernetPower
Delivering DC power to a device over the same cable that carries its data. 802.3af supplies up to 15.4 W at the switch, 802.3at up to 30 W, 802.3bt up to 90 W.
In practiceThe number that matters is what the switch can supply across all ports at once, not per port. A 48-port switch almost never has 48 ports' worth of budget.
- PoE classPower
The negotiated power bracket a device requests at link-up, 0 through 8. The switch reserves the whole class, not the actual draw.
In practiceThis is why budgets run out earlier than the arithmetic suggests: a camera drawing 4 W can reserve 15.4 W if it classifies badly, and the switch honours the reservation.
- PortFastSwitching
Puts an access port straight into forwarding at link-up instead of walking it through spanning-tree listening and learning.
In practiceWithout it a workstation waits roughly thirty seconds for a link, which reads to a user as 'the network is broken'. Always pair it with BPDU Guard — PortFast alone removes the protection that delay was providing.
- POTSPlain Old Telephone ServiceCabling
The analogue telephone line: a single pair carrying −48 V DC, with about 90 V AC superimposed while ringing.
In practiceThat ring voltage is why you do not lay a hand across a live 66 block. It will not kill a healthy adult, but it will make you drop what you are holding, which at the top of a ladder is the actual hazard.
- Power budgetPower
The total PoE wattage a switch can deliver across all ports simultaneously, set by its internal supply rather than its port count.
In practiceSize it against class allocation with headroom for the day someone adds four more cameras. Running a PoE switch at its limit means the next device to boot is the one that fails.
- Punchdown toolCabling
The impact tool that presses a conductor into an insulation-displacement contact and, with a cutting blade fitted, trims the tail in the same stroke.
In practiceBlades are not interchangeable: 66 and 110 are different profiles. Cut side faces the waste end — getting that backwards cuts the conductor you meant to keep.
- RJ11Cabling
A six-position, two-conductor telephone connector using the centre pair only. RJ14 loads four positions for two lines, RJ25 loads all six for three.
In practiceThe body is narrower than an eight-position plug. Forcing an RJ11 plug into an RJ45 jack deforms the outer contacts and quietly ruins the jack.
- Solid vs strandedCabling
Solid conductor is a single wire per conductor, used for permanent runs. Stranded is many fine wires, used for patch cords that flex.
In practiceSolid will not survive repeated bending; stranded will not hold reliably in an insulation-displacement contact. Each fails in the other's job, usually months later.
- Spanning Tree ProtocolSTPSwitching
The protocol that finds and blocks redundant paths so a switched network cannot loop. Rapid PVST+ and MSTP are the versions worth running.
In practiceSet your root bridge explicitly. Left to elect itself, the root becomes whichever switch has the lowest MAC address — frequently the oldest, slowest box in the building.
- Split pairPractice
A run where each pin reaches the right pin at the far end, but the conductors are drawn from two different twisted pairs.
In practiceContinuity is perfect, so a cheap tester says the cable is good. Crosstalk is catastrophic, so gigabit will not hold. If a link tests fine and behaves badly, suspect this first.
- T568ACabling
One of the two eight-position colour codes in ANSI/TIA-568. Green pair on positions 1 and 2, orange on 3 and 6.
In practiceSpecified by some US federal and residential work, and the common default outside North America. Backward-compatible with one- and two-line telephone pinouts, which B is not.
- T568BCabling
The other eight-position colour code. Orange pair on positions 1 and 2, green on 3 and 6 — A with those two pairs exchanged, and nothing else changed.
In practiceThe predominant choice in US commercial installations. Neither code performs better; the only wrong answer is using both in one building.
- Tip and ringCabling
The two sides of an analogue telephone pair, named after the tip and ring contacts of the quarter-inch plugs used on manual switchboards.
In practiceTip is the white-striped conductor in twisted pair, ring is the solid. Reversing them on a POTS line usually still works, which is exactly why the mistake propagates.
- Toner and probePractice
A two-part tool: a generator that puts an audio tone on a pair, and an inductive probe that finds it at the other end.
In practiceThe fastest way to identify an unlabelled run. Tone one pair, not the whole bundle, or the signal bleeds onto neighbours and every cable sings.
- TrunkSwitching
A link carrying several VLANs at once, tagging each frame with 802.1Q so the far end can tell them apart.
In practicePrune the allowed list to the VLANs actually needed. 'Allow all' works on day one and becomes the reason a broadcast storm in one VLAN reaches every closet.
- Twist rateCabling
How tightly each pair is twisted. Every pair in the cable uses a different rate, which is what stops them coupling into one another.
In practiceHold the twist to within half an inch of the contact. The untwisted tail you leave at a termination is the single biggest thing you control in a NEXT measurement.
- VLANVirtual LANSwitching
A logical broadcast domain within a switch, letting one physical infrastructure carry several separate networks.
In practiceSegment by what a device is, not where it sits: cameras, voice, workstations, building systems. The camera VLAN is the one that keeps a compromised recorder off the finance network.
- VLAN 1Switching
The default VLAN every port belongs to out of the box, and the default native VLAN on trunks.
In practiceNothing user-facing should live here. It cannot be deleted, it is where every unconfigured port lands, and control-plane protocols use it — so treat it as the VLAN that exists to be empty.
- Wire mapPractice
The certification test that confirms each pin at one end reaches the matching pin at the other, with pairs intact.
In practiceCatches the four classic faults: opens, shorts, reversals, and split pairs. A split pair passes a cheap continuity tester and fails a real one, which is the argument for owning a real one.
The rest of the section
- 01RJ45 Wiring: T568A and T568BBoth colour codes, pin by pin, side by side.
- 02RJ11 Wiring and Phone PinoutsSix positions, three lines, two colour systems.
- 03Keystone Jack TerminationStrip, seat, punch, snap — without failing the certification.
- 0466 Block WiringFifty rows, four columns, and where the bridging clips go.
- 05110 Block WiringLay the pairs in, press the connecting block on.
- 06Writing a Switch ConfigurationFrom factory default to a switch you would hand over.
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