Domain 2 of 5 · Chapter 4 of 4

Physical Installations

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Included in this chapter:

  • Structured cabling: demarc, MDF, IDF, cross-connects
  • Power budgeting: PoE standards and per-switch budget
  • Power continuity: UPS, PDU, circuits, generators
  • Environment: HVAC, humidity, airflow, fire suppression

PoE standards: per-port and per-device power

Property802.3af (PoE)802.3at (PoE+)802.3bt Type 3 (PoE++)802.3bt Type 4 (PoE++)
Max power at the switch port (PSE)15.4 W30 W60 W90 W
Max power at the device (PD)12.95 W25.5 W~51 W~71.3 W
Wire pairs used2 of 42 of 44 (4PPoE)4 (4PPoE)
Typical loadsVoIP phone, basic APPTZ camera, Wi-Fi 5/6 APMulti-radio AP, video phoneHigh-density AP, thin client, display

Decision tree

What does therequirement target?Power a deviceSurvive outageConnectivityProtect roomWithin switchPoE budget?How long mustit ride through?Structured cablingMDF/IDF, patch panelsHVAC +clean agentYesNoPoEaf / at / bt by drawInjector ormains powerSeconds-minHours+UPSbattery ride-throughGeneratorUPS bridges startAlways: feed racks via PDU on a rated circuit; dual PSUs on critical gearsize circuit in VA; plan to ~80% of breaker amps
Cheat sheet

Cheat sheet

  • The demarc is the provider-vs-customer boundary
  • MDF is the building's main wiring room; IDFs serve the floors
  • Backbone cabling links MDF to IDF; horizontal links IDF to the jack
  • Patch panels make moves/adds/changes a patch-cord swap
  • A cross-connect joins two cabling segments
  • PoE delivers power and data over one Ethernet cable
  • Know the PoE wattages: af 15.4 / at 30 / bt 60 or 90
  • 802.3bt (PoE++) energizes all four pairs
  • A switch's PoE budget is a shared pool, not per-port
  • A UPS rides out short gaps; a generator covers the long haul
  • A PDU distributes rack power; size the circuit in VA
  • Redundant (dual) power supplies survive losing one feed
  • HVAC holds temperature and humidity in the gear's range
  • Humidity cuts both ways: too dry static, too humid corrosion
  • Hot-aisle/cold-aisle keeps supply and exhaust air from mixing
  • Use clean-agent fire suppression over water for live gear
  • Twisted-pair Ethernet is limited to a 100 m channel
  • One rack unit (1U) is 1.75 inches, so device height divided by 1.75 gives U
  • Mount the heaviest gear at the bottom of the rack to lower the center of gravity
  • Verify the rack and floor load rating before mounting any equipment
  • A lockable enclosed rack with locking doors restricts physical access to gear
  • Bond each rack to the grounding system to give fault current a safe path
  • Blanking panels seal empty rack U so hot exhaust can't recirculate to intakes
  • Horizontal and vertical cable managers route patch cables clear of airflow
  • When a clean-agent suppression system discharges, HVAC must shut down
  • Monitor cold-aisle intake temperature against the recommended data-center range

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References

  1. IEEE 802.3 Ethernet Working Group (PoE generations + 100 m channel) Whitepaper
  2. IEEE 802.3bt-2018: DTE Power via MDI over 4-Pair (PoE Type 3/Type 4) Whitepaper