Data Centre Design and Infrastructure
The Design and Infrastructure domain tests whether you can read a facility's physical and electrical topology and state precisely what availability, capacity and redundancy it actually delivers. Questions are usually scenario-shaped: a configuration is described, and you identify its weakest link or the class it satisfies.
Functional spaces and cabling topology
ANSI/TIA-942 organises the data centre into an entrance room (carrier demarcation), a main distribution area (MDA, the core cross-connect), horizontal distribution areas (HDA), equipment distribution areas (EDA, the racks) and optional zone distribution areas (ZDA) in open-plan halls. Knowing which space hosts the core switches and the main cross-connects is a recurring question.
Redundancy and availability
Learn the tier ladder as definitions, not slogans:
- Tier I: single path, no redundant components, about 99.671% availability.
- Tier II: redundant components (N+1) but still one distribution path.
- Tier III: concurrently maintainable - every component can be serviced without shutting down the IT load, about 99.982%.
- Tier IV: fault tolerant, surviving a single worst-case failure, about 99.995%. Availability can be computed as MTBF ÷ (MTBF + MTTR). Redundancy notation: N is the load, N+1 adds one spare unit, 2N provides two complete systems, 2(N+1) provides two complete systems each with a spare.
Power and cooling chain
Typical electrical path: utility → medium-voltage switchgear → transformer → low-voltage switchboard → UPS → PDU → remote power panel or busway → rack PDU. Cooling path: chillers → pumps → CRAH/CRAC units → plenum or ducted supply → hot/cold aisle with containment → return air. PUE = total facility energy ÷ IT energy; it is a ratio, never a percentage.
Worked example
A hall holds 20 racks at 6 kW each, so IT load = 120 kW. Metering shows the facility draws 168 kW at the utility. PUE = 168 ÷ 120 = 1.40. If the design target was 1.30, the shortfall is 0.10 - roughly 12 kW of avoidable overhead - so investigate cooling and UPS losses before adding capacity, because every extra kW of overhead must be designed into power and cooling plant.
Traps that cost marks
- Calling Tier II "dual path": Tier II duplicates components, not the distribution path.
- Confusing Tier III concurrent maintainability with Tier IV fault tolerance.
- Quoting PUE as a percentage, or comparing a design PUE with an annualised one.
- Assuming a raised floor is mandatory; slab-on-grade with contained aisles is valid.
- Treating N+1 and 2N as interchangeable when N is large.