Cables & Connectors: Specifying the Physical Layer
Every bit travels over a physical medium. Choose the wrong one and your network fails before it starts.
A contractor walks into your new office buildout and asks: Cat5e or Cat6? Single-mode or multimode? RJ45 or LC?
You need to specify every cable, every connector, and every standard — and justify each choice to management.
Each category is rated for a maximum bandwidth and distance over a 100m run.
Cat 5e
1 Gbps / 100m
Minimum for modern installs. Handles 1000BASE-T. Still common in existing cable plants.
Cat 6
1 Gbps / 100m 10 Gbps / 55m
Tighter twist, internal separator. Preferred for new installs. 10G limited to shorter runs.
Cat 6a
10 Gbps / 100m
Augmented Cat6. Full 10G at 100m. Thicker, stiffer. Required for 10GBASE-T at full distance.
Cat 7
10 Gbps / 100m
S/FTP — each pair and the overall cable individually shielded. Not an IEEE standard; less common in North America.
UTP — Unshielded Twisted Pair
No metallic shielding. Relies entirely on the twist to cancel electromagnetic interference (EMI). Cheaper, easier to terminate, most common in offices. Susceptible to interference near motors and fluorescent fixtures.
STP — Shielded Twisted Pair
Foil or braided shield around pairs. Blocks EMI in industrial environments, server rooms, and dense cable trays. Requires proper grounding — an ungrounded shield acts as an antenna and amplifies interference rather than blocking it.
The buildout spec: Cat6a throughout. Runs near HVAC ceiling trays get STP. Standard cubicle drops get UTP Cat6a — less expensive to terminate and sufficient for the environment.
Slide 3 of 10
Fiber Optic — Single-Mode vs Multimode
Fiber carries light instead of electrical signal. The fiber type determines how far that light travels without degradation.
Single-Mode Fiber (SMF)
OS1 / OS2 | Yellow jacket
Tiny 8-10 micron core allows only one light path (mode). Laser source. OS2 reaches 40km+. Used for campus backbone, WAN links, and ISP interconnects. Higher transceiver cost; cable cost is comparable to multimode.
Multimode Fiber (MMF)
OM1 through OM5 | Orange / Aqua jacket
Larger 50-62.5 micron core allows multiple light paths. Modal dispersion limits distance. LED or VCSEL source. OM4 reaches 400m at 10G. Lower transceiver cost. Standard for in-building data center and MDF-to-IDF runs.
OM1 (62.5um)
1G / 275m
Legacy. Orange jacket. Avoid for new installs.
OM3 (50um)
10G / 300m
Aqua jacket. Common in existing data centers.
OM4 (50um)
10G / 400m 40G / 125m
Aqua/violet jacket. Current standard for new builds.
OS2 (SMF)
100G / 10km+
Yellow jacket. Campus backbone and carrier WAN.
IDF-to-MDF runs under 100m: OM4 multimode — cheaper transceivers, sufficient distance.
Connection to the carrier handoff: OS2 single-mode — that run may eventually extend to another building.
Slide 4 of 10
Connector Types — Match the Medium
The connector on the end of the cable tells you the media type, use case, and approximate era of the installation.
Copper
RJ45
8P8C. Standard Ethernet connector. Cat5e through Cat6a. Every NIC, switch port, and patch panel.
Copper
RJ11
6P2C or 6P4C. POTS telephone and DSL modem connections. Physically smaller than RJ45.
Fiber
LC (Lucent)
Small form factor, push-pull latch. Dominant in data centers and SFP/SFP+ transceivers. Most common fiber connector today.
Fiber
SC (Subscriber)
Square push-pull body. Older installs, patch panels, and some GPON equipment. Larger than LC.
Fiber
ST (Straight-Tip)
Bayonet twist-lock. Legacy multimode. Found in older campus buildings and analog security camera systems.
Fiber
MPO / MTP
Multi-fiber push-on. 12 or 24 fibers in one connector. High-density data center pre-terminated trunk assemblies.
Coax
F-Type
Threaded coaxial connector. Cable TV distribution and broadband cable modem (DOCSIS). Screw-on attachment.
Coax
BNC
Bayonet Neill-Concelman. Legacy 10BASE2 Thinnet. Still used in SDI broadcast video and RF test equipment.
Exam tip: LC is the connector inside SFP transceivers on switches. When the question mentions an SFP port, the cable attached to the SFP module will have LC connectors. SC and ST appear on the exam because they exist in legacy field installations.
Slide 5 of 10
802.3 Ethernet Standards — Speed / Medium / Distance
The IEEE 802.3 designation encodes speed, signaling type, and sometimes distance. Learn to decode the naming convention.
100BASE-TX
100 Mbps • Cat5e • 100m
Fast Ethernet. UTP copper. Still active on older desktops, IP cameras, and some IoT devices.
1000BASE-T
1 Gbps • Cat5e+ • 100m
Gigabit over copper. Uses all 4 pairs simultaneously for bidirectional signaling. The current floor standard.
10GBASE-T
10 Gbps • Cat6a • 100m
Requires Cat6a for full 100m. Cat6 limited to 55m. Higher power draw compared to fiber-based 10G.
1000BASE-SX
1 Gbps • MMF • 550m
SX = short-wavelength (850nm). Multimode fiber. LC or SC connectors. Common data center standard.
1000BASE-LX
1 Gbps • SMF • 5km
LX = long-wavelength (1310nm). Single-mode for longer runs. Also usable on OM3/OM4 with a mode conditioning patch cable.
10GBASE-SR / LR / ER
10 Gbps • Fiber
SR = multimode 300m. LR = single-mode 10km. ER = extended single-mode 40km. All use LC connectors via SFP+.
How to decode the name: Speed (100 / 1000 / 10G) + BASE (baseband signaling) + medium/encoding shorthand.
T = twisted pair copper. S = short-wavelength fiber (multimode). L = long-wavelength fiber (single-mode). R = serial encoding variant.
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Wiring Standards — T568A / T568B / Crossover
The pin order on your RJ45 termination determines whether the cable is straight-through, a crossover, or a console cable.
T568A — Pin Order
1White / Green
2Green
3White / Orange
4Blue
5White / Blue
6Orange
7White / Brown
8Brown
T568B — Pin Order (North American standard)
1White / Orange
2Orange
3White / Green
4Blue
5White / Blue
6Green
7White / Brown
8Brown
Straight-Through
Both ends use the same standard (A-A or B-B). Connects unlike devices: host to switch, switch to router. The most common patch cable in any rack.
Crossover
One end T568A, other end T568B. Connects like devices: switch to switch, host to host. Modern switches use Auto-MDIX to detect and adapt automatically — crossover cables rarely needed today.
Rollover / Console
Each pin mirrored to its opposite (pin 1 to pin 8, pin 2 to pin 7). Cisco console cable. Connects a laptop to a router or switch console port for initial configuration.
Slide 7 of 10
Plenum, PoE, and Transceivers
Three specifications the contractor will ask about before placing a single materials order.
Plenum vs PVC
Plenum-rated (CMP) cable uses a low-smoke, fire-retardant jacket. Required by fire code in any air-handling space — above drop ceilings, below raised floors. More expensive. PVC (CMR riser, CMG general) is permitted in conduit and wall runs outside airflow paths. Never install PVC in a plenum space — it releases toxic gases when burned.
PoE — Power over Ethernet
802.3af (PoE): 15.4W — IP phones, basic WAPs.
802.3at (PoE+): 30W — PTZ cameras, enterprise APs.
802.3bt (PoE++): 60-100W — digital signage, thin clients.
Requires Cat5e minimum. Cat6a preferred for high-wattage bundles — the lower resistance reduces heat buildup in tightly bundled cable runs.
SFP / SFP+ / QSFP
SFP (Mini-GBIC): 1 Gbps. Copper or fiber.
SFP+: 10 Gbps. Same physical size, different electrical spec. Standard on 10G switches.
QSFP+: 40 Gbps. Four bonded 10G lanes. Data center aggregation.
QSFP28: 100 Gbps. Spine-leaf 100G links.
Transceivers are hot-swappable — change media type with no downtime.
Buildout final spec: plenum-rated Cat6a (open ceiling throughout). PoE+ switches (802.3at, 30W) for WAPs and cameras.
IDF uplinks: SFP+ modules with OM4 LC fiber. Server room spine links: QSFP28 for 100G headroom.
Slide 8 of 10
Cable Testing — Verify Before You Close the Ceiling
The contractor runs 200 drops. These four tests confirm every termination is correct before walls and ceilings are closed.
Continuity Test
Confirms each wire pair is connected end-to-end with no open circuits (breaks) and no short circuits (wires touching). Basic pass/fail. Every installer has a simple continuity tester. Does not verify electrical performance — only physical connectivity.
Wire Map Test
Tests all 8 pins and confirms correct pin-to-pin mapping. Catches reversed pairs (TX/RX swapped), split pairs (wires from different pairs interleaved — severe crosstalk), transposed pairs, and shorts. Required for certification. Included in most cable qualification testers.
TDR — Time Domain Reflectometry
Sends a pulse down the copper cable and measures the reflected signal. Locates faults — breaks, kinks, improper terminations — by measuring round-trip time. Tells you exactly how far down the cable the fault is. Saves hours of troubleshooting inside walls and ceilings.
OTDR — Optical TDR
Same principle as TDR but uses a light pulse on fiber. Measures backscatter and reflections to locate breaks, bad splices, connectors, and tight bends. Produces a trace graph showing signal loss over the entire run length. Essential for any fiber certification and long-run troubleshooting.
Certification vs qualification: Certification (Fluke DSX series) mathematically proves the cable meets the Cat6a standard. Qualification only confirms a specific application will run — cheaper but less rigorous. For a commercial build, require full certification with printed test reports before final acceptance.
Slide 9 of 10
Scenario — The Full Buildout Specification
The contractor needs a bill of materials. Every segment gets a justified specification.
Workstation & IP Phone Drops
Cat6a UTP Plenum / RJ45 T568B
100m max runs. 1G to desktop, PoE (802.3af) for phones. Plenum jacket required — open ceiling environment. T568B both ends, straight-through.
Wireless Access Points
Cat6a UTP Plenum + PoE+ (802.3at)
Enterprise APs draw up to 25W. 802.3at headroom. Cat6a handles the combined data load and power delivery without thermal buildup in bundled ceiling runs.
IDF-to-MDF Uplinks
OM4 MMF / LC Duplex / SFP+ SR
All runs under 100m between closets. 10GBASE-SR SFP+ transceivers. Lower cost than single-mode. Future-ready for 10G and 40G with QSFP upgrade.
MDF to Carrier Handoff
OS2 SMF / LC / SFP+ LR
Unknown future distance — potential building expansion. Single-mode handles any distance. 10GBASE-LR transceivers. No re-cabling required if the run extends off-campus.
Server Room Switch Uplinks
QSFP28 DAC under 5m / OM4 SR4 longer
Short rack-to-rack: Direct Attach Copper (DAC) cables — no transceiver cost. Longer cross-room runs: OM4 with QSFP28 SR4 modules.
ISP Coaxial Entry
RG-6 / F-Type connectors
DOCSIS broadband from ISP. RG-6 has lower attenuation than RG-59 at cable TV frequencies. F-type at both the wall entry plate and the cable modem.
Slide 10 of 10 | N10-009 Obj 1.3
You Can Specify the Physical Layer
The contractor asks: Cat5e or Cat6? The answer is Cat6a — it supports 1G today and 10G tomorrow at full 100m, and it handles PoE load without heat buildup. Single-mode or multimode? Multimode OM4 for all in-building runs, single-mode OS2 only where the run might extend off-campus. RJ45 or LC? RJ45 for every copper drop, LC inside every SFP transceiver. Every decision is justified by distance, speed, power, fire code, or future capacity.