To deploy a unit of IT requires a combination of physical space, power, cooling, connectivity, fire suppression, and lighting. Therefore, the linkage of modularity across subsystem types within the data center is a key concept in modular data center architecture. In principle, a complete set of **balanced and integrated subsystems** could be deployed as a **standard unit of modular data center capacity**. This would clearly be a modular data center architecture. All of the data center subsystems would be linked together into a module. Miniature complete independent data centers could be added at a site over time. This can be said to be the most “pure” form of modular data center architecture. However, while this is a simple idea, it is impractical today, for the following reasons: - It is not practical to manage every module of data center capacity as a separate data center - Some subsystems are much more cost effective when deployed at a larger scale than the desired module size - Achieving redundancy across a set of modules is often much less expensive and more efficient than implementing redundancy in every module - Isolating capacities such as power and cooling to a data center module can create situations of stranded unusable capacity - Module sizes should be chosen to be as small as practical for maximum flexibility, but many device types like generators and chillers are extremely ineffective at small module size. Although linking all of the subsystems together into complete independent data center modules is impractical, a modular data center architecture must have some approach to group the subsystems so they can be deployed in a logical and coherent way. Module linkage is a property of the data center architecture that defines how the deployments of different subsystems relate to each other. As an example of module linkage, consider the deployment of equipment racks and rack power strips. In this simple case we can define a 1 for 1 deployment. Now consider the deployment of PDUs and racks. In this case we might define a rule linking 1 PDU to every 20 racks. We could continue defining relationships such as 1 generator for every 500 racks, one CRAH for every 40 racks, one data acquisition system for every 200 racks, etc. We refer to rules relating these deployments as “linkages” of the modularity of the different subsystems. These linkages may be as simple as rules of deployment, or they can be enforced by pre-engineered and pre-manufactured skids, containers, or “kits” of deployment.