Data centre spaces and functional areas

The Harbourview design has reached the point where the small rooms are settled and the big questions remain. The clinic floors, the college classrooms and the offices all depend on a data room on the ground floor, and the college wants that room to do more than hold a few switches. Its IT director, a quiet planner named Elena Varga, has asked whether Harbourview can host the college's backup servers, so that a failure in the main computer room in the existing building 400 m away does not stop teaching. Roz puts the request on the table at the Monday design meeting. 'If we say yes,' she tells Daniel, 'this stops being a telecommunications room with ambitions and becomes a small data centre. That changes the spaces, the cabling, the power, the cooling and the campus link. This course is about all of those things.'

A data centre is a building or part of a building whose main purpose is to house computer, storage and network equipment, together with the power, cooling, cabling and security that keep it running. Large ones fill entire buildings. Small ones, like the room Harbourview will get, may be a single room of a few cabinets. The same design ideas apply to both, scaled to the risk the owner will accept. The data centre cabling standard, ANSI/TIA-942, and the BICSI data centre design standard, BICSI 002, are the main references, and the ISO/IEC 11801 series and CSA's adoption of it cover the same ground internationally. In this lesson Daniel learns the functional areas those standards describe and how to fit them into a real floor plan.

Why data centres get their own design rules

Ordinary buildings are designed around people: offices, classrooms and clinic rooms with a few devices each. Data centres are designed around equipment that runs all day, every day, draws large and steady amounts of power and turns almost all of it into heat. The cabling inside a data centre is denser, changes more often and must be organized so that a technician can add or move a server without disturbing anything else. A cabling mistake in an office affects one desk. A cabling mistake in a data centre can affect every service the organization runs.

Because of this, data centre standards do three things that general building standards do not. They define functional areas, so that the cabling has a predictable structure no matter how large the room is. They tie cabling design to power, cooling and redundancy, because those systems compete for the same space under the floor or overhead. And they describe levels of reliability, often called ratings, tiers or classes, so that the owner and designer can agree on how much redundancy to build in. This lesson covers the first of these. Later lessons cover topologies, airflow, power and availability.

The functional areas

TIA-942 describes a set of spaces, each with a role in the cabling hierarchy. They are functional areas, not necessarily separate rooms. In a large data centre each may be a separate room or a defined zone of a large hall. In a small data centre several may share one room or even one row of cabinets. What matters is that the designer can point to each function and say where it lives.

The entrance room is where the data centre meets the outside world. Service providers bring their cables here, and so do campus backbone cables such as the 400 m link from the college building. In large data centres there may be two or more entrance rooms on different sides of the building, fed by routes that never share a trench, so that one excavator cannot cut every connection. The entrance room must be secure, because provider technicians need access to their own equipment but should not have access to the computer room.

The main distribution area is the heart of the cabling. It holds the main cross-connect, where backbone cables from the entrance room and from each horizontal distribution area meet, and usually the core network equipment. Everything else in the data centre is cabled back to the MDA, so its location sets the length of every backbone cable. Designers try to place it near the centre of the computer room to keep those lengths short. In a small data centre the MDA may be a single row or even a few cabinets at one end of the room.

The horizontal distribution area serves the equipment in its zone. It is the data centre's equivalent of the telecommunications room on an office floor: horizontal cables run from patch panels in the HDA to patch panels in each equipment cabinet. Large data centres have many HDAs, often one per row or per group of rows. Small ones may combine the HDA with the MDA.

The zone distribution area is optional. It is a fixed point between the HDA and the equipment, similar to a consolidation point in an open office. Zone enclosures under the floor or overhead let the designer cable to a zone once and then connect individual cabinets as they are installed. The standard limits what may happen in a ZDA: in general it contains passive connections only, and a channel may have only one ZDA between the HDA and the equipment.

The equipment distribution area is where the work happens: rows of cabinets holding servers, storage and network equipment. Each cabinet usually has patch panels at the top or rear where horizontal cables from the HDA terminate, and short equipment cords connect those panels to the equipment. Some designs place switches in each cabinet, an arrangement covered in the next lesson.

Support spaces

A data centre is more than its computer room. The standards also describe support spaces that the cabling designer must plan for even though they hold little ICT equipment. These include the electrical rooms with switchgear, UPS systems and batteries; the mechanical rooms with chillers, pumps and air handlers; the operations centre where staff monitor the systems; storage and staging rooms where new equipment is unpacked and tested before it enters the computer room; loading docks and corridors wide and strong enough for heavy cabinets; and offices for the people who run the site. Each of these needs its own telecommunications cabling, often from a telecommunications room that serves the support areas.

Two support spaces deserve special mention. The staging room keeps cardboard, packing foam and dust out of the computer room, which protects both the equipment and the fire suppression system. And the battery room, if the UPS uses lead-acid batteries, may need its own ventilation and fire separation. Lithium-ion UPS batteries have different fire risks and may be placed differently. Neither should have ICT cable pathways running through it except to serve equipment in that room.

Locating the data centre

Before any of these spaces can be drawn, the data centre itself has to go somewhere. The standards list site and building considerations that read like a risk assessment. Avoid flood zones and basements below the water table. Avoid rooms below washrooms, kitchens or roof drains, where a leak would land on the equipment. Stay away from large electromagnetic sources such as transformers, elevator machine rooms and radio transmitters. Choose a location with room to grow, with a route for large equipment from the loading dock, and with a structural floor rated for the weight of loaded cabinets and batteries.

Access matters as much as protection. The data centre must be reachable by two separate pathways for service provider and campus cables, by power from the building's main electrical service and by the cooling system's piping. A room in the middle of a building is easy to secure but may be hard to reach with large pipes. A room on an outside wall is easy to reach but may face weather and security risks. Every location is a trade-off, and the designer's job is to make it visible.

How big should it be?

Sizing a data centre starts from the equipment, not the floor plan. The owner's IT team should provide a list of the systems to be housed, with the number of cabinet positions they expect now and in the future. Each cabinet position needs space for the cabinet itself, for the aisles in front and behind, and for its share of the cooling units, power distribution and cable pathways. A rough rule for planning is that a computer room needs about two and a half to three times the floor area of the cabinets themselves once aisles and support equipment inside the room are counted, but the real answer comes from a layout.

Height matters too. Data centres typically need more clear height than offices, to fit overhead cable trays above the cabinets, lighting and fire suppression piping, and sometimes a raised floor below. The standards give minimum clear heights, and the designer should check them against the structure early, because a low ceiling cannot be fixed later.

Raised floor or slab?

Many older data centres were built on raised access floors, with cold air supplied through perforated tiles and cables run under the floor. Many newer ones are built on the structural slab, with cooling delivered by in-row or overhead units and all cabling run overhead in trays. Each approach has defenders. A raised floor hides cables and pipes and makes it easy to add cooling capacity under a row, but under-floor cabling can block airflow and is hard to trace. An overhead design keeps cables visible and accessible and keeps the floor clear for cooling air, but needs careful coordination with lighting and fire protection. At Harbourview, with a modest room and in-row cooling, Roz recommends building on the slab with overhead ladder rack and cable tray.

Security and access

A data centre holds an organization's most valuable information, so access must be controlled. The design should define security zones: public areas, the building's general areas, the data centre support areas, the computer room and, within it, any caged or locked cabinets. Each zone boundary needs an access control point, usually a card reader and sometimes a second factor such as a code or biometric reader. Cameras should cover entrances and aisles. The cabling designer provides pathways and outlets for these systems, which RCDD-114 covered, and should make sure that the entrance room has its own door so that service providers do not walk through the computer room.