Many people may be confused or don’t necessarily understand the “tier” of their existing data center provider. However it is a topic at the core of outsourced IT services. Direct or indirect clients of data centers have made a conscious decision to outsource their IT needs for a variety of reasons, but the cornerstones of that decision should be security of data and availability of that data. Physical security, including man traps, carded access, bio-metric access control and CCTV, is a commodity and should be a given for any data center.
Data availability is effectively dependent upon  power (both racks and cooling) and  connectivity. Fundamentally, data availability is maximized when your data center can power and connect your racks to 2 separate power feeds and physical distribution of that power to your racks, whether the external power grid is working or not, and 2 physically separate fiber loops (in case of an outage in one loop). Basically, the level of full redundancy is the key attribute.
The basic equipment in this power string include very large generators with substantial fuel capacity, delivering power to:  Uninterruptible Power Supply / Backup Power (UPS) and in turn to Power Distribution units (PDU’s), and  very large air conditioners.
My research around tiering definitions (and I discovered many!) indicates that:
- A data center built to a tier II standard will typically only offer a single path for power, with some redundant equipment, but no concurrent maintainability, and
- A data center built to a tier III standard will typically offer 2 power / equipment paths, and is concurrently maintainable, meaning one of the paths (Generators through to PDU’s) can go down or be taken down for maintenance while maintaining power, cooling and connectivity to your racks, substantially decreasing potentially catastrophic downtime, and minimizing the possible loss of data.
You will see or hear many claims as to the tier level of any given data center but it would appear to really come down to the aforementioned redundancy. To clear up this confusion, I would suggest that best practice when choosing a data center (or when your IT provider chooses one for you), would be to ask to see a map of their power, cooling, connectivity / network infrastructure. Price is important but generally speaking you get what you pay for due to the high capital cost of a data center built to a tier III standard. We believe price should always be the third consideration behind data availability and security.