Racks are available with either four or two vertical rails. Four-post racks allow for mounting slides to support the equipment at the front and rear. These racks may be open in construction (similar to the traditional open-style two-post racks), or may be enclosed by doors, side panels, or tops. Two-post racks provide just two vertical rails; a piece of equipment can be mounted either via its front panel holes, or close to its center of gravity to minimize load on its front panel. Two-post racks are most often used for telecommunication installations.

Alternating Current (AC) is electrical current that continuously reverses direction of flow, typically in a periodic fashion. In the United States, standard AC power sources reverse direction sixty times each second (called 60 cycles or 60 Hertz AC).

The unintended mixing of cold and hot air.

Asset Management System

Transfer switches allow switching from a primary power source to a secondary or tertiary power source and are employed in some electrical power distribution systems. Most often transfer switches can be seen where emergency power generators are used to back up power from the utility source. The transfer switch allows safely switching from utility power to emergency generator power while maintaining isolation of each source from the other. The switch may be either a manual switch, an automatic switch, or a combination of manual and automatic. In a home during a power outage, for example, the transfer switch allows isolation of the owner's critical circuits (e.g. cooling, refrigerator, lighting) from the utility service, allowing for operation of the generator without back-feeding to the utility, which can damage utility equipment and hurt (or kill) utility workers.

Automatic Transfer Switches continually monitor the incoming utility power. Any anomalies such as voltage sags, brownouts, spikes, or surges will cause the internal circuitry to command a generator to start and will then transfer to the generator when additional switch circuitry determines the generator has the proper voltage and frequency. When utility power returns or no anomalies have occurred for a set time, the transfer switch will then transfer back to utility power and command the generator to turn off, after another specified amount of "cool down" time with no load on the generator.

A transfer switch can be set up to provide power to only critical circuits or entire electrical (sub)panels. Some transfer switches allow for load shedding or prioritization of optional circuits, such as heating and cooling equipment.

A best practice is a technique or methodology that, through experience and research, has proven to reliably lead to a desired result. A commitment to using the best practices in any field is a commitment to using all the knowledge and technology at one's disposal to ensure success. The term is used frequently in the fields of health care, government administration, the education system, project management, hardware and software product development, and elsewhere.

In software development, a best practice is a well-defined method that contributes to a successful step in product development. Throughout the software industry, several best practices are widely followed. Some of the more commonly used are: an iterative development process, requirement management, quality control, and change control.

In IBM PC Compatible computers, the Basic Input/Output System (BIOS), also known as the System BIOS, is a de facto standard defining a firmware interface.

The BIOS is boot firmware, designed to be the first code run by a PC when powered on. The initial function of the BIOS is to identify, test, and initialize system devices such as the video display card, hard disk, floppy disk and other hardware. The BIOS sets the machine hardware into a known state, so that software stored on compatible media can be loaded, executed, and given control of the PC. This process is known as booting, or booting up, which is short for bootstrapping.

BIOS programs are stored on a chip and are built to work with various devices that make up the complementary chipset of the system. They provide a small library of basic input/output functions that can be called to operate and control the peripherals such as the keyboard, text display functions and so forth. In the IBM PC and AT, certain peripheral cards such as hard-drive controllers and video display adapters carried their own BIOS extension ROM, which provided additional functionality. Operating systems and executive software, designed to supersede this basic firmware functionality, will provide replacement software interfaces to applications.

A device mounted in unused U spaces in a rack that restricts bypass airflow, also called blanking or filler plates

British Thermal Unit is a standard of measure for cooling equipment capacity.

Cold aisle containment system that directs cooled air from air conditioning equipment to the inlet side of racks in a highly efficient manner.

Corporate Average Data center Efficiency (CADE) is a metric used to rate the overall energy efficiency of an organization's data centers. CADE was introduced in a joint repot from the Uptime Institute and McKinsey that proposed the metric as a single key performance indicator that could be used to compare the energy consumption of one data center against another. CADE combines measurements of the energy efficiency and utilization of IT equipment and facilities into a single percentage. A higher CADE indicates a more energy efficient data center.

Computational Fluid Dynamics is high-performance computer modeling applied to air flow in data centers for optimal air-conditioning design.

Cubic feet per minute, an airflow volume measurement.

A data center chiller is a cooling system used in a data center to remove heat from one element and deposit it into another element. Chillers are used by industrial facilities to cool the water used in their heating, ventilation and air-conditioning (HVAC) units. Round-the-clock operation of chillers is crucial to data center operation, given the considerable heat produced by many servers operating in close proximity to one another. Without them, temperatures would quickly rise to levels that would corrupt mission-critical data and destroy hardware.

Cloud computing is a general term for anything that involves delivering hosted services over the Internet. These services are broadly divided into three categories: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS). The name cloud computing was inspired by the cloud symbol that's often used to represent the Internet in flow charts and diagrams.

A cloud service has three distinct characteristics that differentiate it from traditional hosting. It is sold on demand, typically by the minute or the hour; it is elastic -- a user can have as much or as little of a service as they want at any given time; and the service is fully managed by the provider (the consumer needs nothing but a personal computer and Internet access). Significant innovations in virtualization and distributed computing, as well as improved access to high-speed Internet and a weak economy, have accelerated interest in cloud computing.

Coefficient of Effectiveness is an Uptime Institute metric based on the Nash-Sutcliffe model efficiency coefficient

A computer room air conditioning (CRAC) unit is a device that monitors and maintains the temperature, air distribution and humidity in a network room or data center. CRAC units are replacing air-conditioning units that were used in the past to cool data centers. According to Industrial Market Trends, mainframes and racks of servers can get as hot as a seven-foot tower of powered toaster ovens, so climate control is an important part of the data center's infrastructure.

There are a variety of ways that the CRAC units can be situated. One CRAC setup that has been successful is the process of cooling air and having it dispensed through an elevated floor. The air rises through the perforated sections, forming cold aisles. The cold air flows through the racks where it picks up heat before exiting from the rear of the racks. The warm exit air forms hot aisles behind the racks, and the hot air returns to the CRAC intakes, which are positioned above the floor.

A Computer Room Air Handler includes only fans and a cooling coil, often using chilled water to remove heat from the data center.

Direct-attached storage (DAS) refers to a digital storage system directly attached to a server or workstation, without a storage network in between.  A typical DAS system is made of a data storage device (for example enclosures holding a number of hard disk drives) connected directly to a computer through a host bus adapter (HBA). Between those two points there is no network device (like hub, switch, or router), and this is the main characteristic of DAS.  The main protocols used for DAS connections are ATA, SATA, SCSI, SAS, and Fibre Channel.

Data Center Infrastructure Efficiency metric shows the power used by a data center’s IT equipment as a percentage of the total power going into the data center. DCiE is the reciprocal of PUE and is expressed as a percentage that improves as it approaches 100%.

How to Determine DCIE:
1. Take a measurement of energy use at or near the facility utility's meter. If the data center is in a mixed-use facility or office building, take a measurement only at the meter that is powering the data center. If the data center is not on a separate utility meter, estimate the amount of power being consumed by the non-data center portion of the building and remove it from the equation.

2. Measure the IT equipment load, which should be measured after power conversion, switching, and conditioning is completed. According to The Green Grid, the most likely measurement point would be at the output of the computer room power distribution units (PDUs). This measurement should represent the total power delivered to the server racks in the data center.

Direct Current (DC) is electrical current that flows only in one direction.

An alternating current that includes spikes, surges or other unwanted fluctuations.  UPS are often used to condition the power.

EIA-310 is a specification for what is often called the “standard rack”. This specification standardizes several important features of 19″ racks, such as the Rack Unit (RU or U), vertical hole spacing, horizontal hole spacing, rack opening, and front panel width. The specification also set tolerances on each of these dimensions.

If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. (The term 'load' may also refer to the power consumed by a circuit; that topic is not discussed here.)

Load affects the performance of circuits that output voltages or currents, such as sensors, voltage sources, and amplifiers. A household's power outlets provide an easy example: they are a voltage source, outputting 120 V AC for example (in North America), with the household's appliances collectively making up the load. When a power-hungry appliance switches on, it dramatically reduces the load impedance, causing the output voltage to drop. This drop is easily observed; for instance, turning on a vacuum cleaner dims the lights.

High frequency waveform EMI interference can be caused by either RFI or EMI caused by interference generated by transmitters, welding devices, SCR driven printers, lightning, etc.

A green data center is a repository for the storage, management, and dissemination of data in which the mechanical, lighting, electrical and computer systems are designed for maximum energy efficiency and minimum environmental impact.

Distortion of the normal line waveform, generally transmitted by nonlinear loads. Switch mode power supplies, variable speed motors and drives, copiers and fax machines are examples of non-linear loads.

To remove and replace a system component while power is on and the system is functional.

The International Electrotechnical Commission (IEC) is a not-for-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology". IEC standards cover a vast range of technologies from power generation, transmission and distribution to home appliances and office equipment, semiconductors, fibre optics, batteries, solar energy, nanotechnology and marine energy as well as many others. The IEC also manages three global conformity assessment systems that certify whether equipment, system or components conform to its International Standards.

KVM over IP devices use a dedicated microcontroller and potentially specialized video capture hardware to capture the video, keyboard, and mouse signals, compress and convert them into packets, and send them over an Ethernet link to a remote console application that unpacks and reconstitutes the dynamic graphical image.  Access to most remote or "KVM" over IP devices today use a web browser but proprietary viewer software can increase performance.

A KVM (keyboard, video, mouse) switch is a hardware device that allows a user to control multiple computers from a single keyboard, video display monitor and mouse. KVM switches are often found in data centers where multiple servers are placed in a single rack. By pressing a button on the kvm switch, the administrator can change control from one server to the next. Using a KVM switch can save the administrator the cost of purchasing a dedicated key-board, monitor and mouse for each computer, save space in the server room and limit cable clutter.

The kW consumption of equipment, typically installed in a rack. Also, the heat level a cooling system is required to remove from the data center environment.

The National Electrical Manufacturers Association or NEMA is a U.S.-based association, which was created on September 1, 1926.  It sets many common standards used in electrical products among its over 400 members, and helps to develop and promote the International Electrotechnical Commission's standards in the U.S.   NEMA defines standards for many commonplace electrical interconnects, as well as outlining manufacturing standards for electrical products, such as various grades of electrical enclosures.

Network operating / operations center.

Non-recurring engineering (NRE) refers to the one-time cost of researching, developing, designing, and testing a new product.

A device used in datacenters to distribute AC power to multiple servers and other equipment. Power Distribution Units (PDUs) range from simple 120v power strips to units that break out 120 volts from 240v and three-phase power. Advanced units are managed remotely via the SNMP management protocol or from a Web browser or other management console, causing outlets to be turned on and off at prescribed times and in a proper sequence for shutting down and powering up equipment.

In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning full) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC) and typically provided in the space between the structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor. In buildings with computer installations, the plenum space is often used to house connecting communication cables. Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas.

Short-term low voltage triggered by the startup of large loads, utility switching, utility equipment failure, lightning and power service that’s too small for the demand.

Power usage effectiveness (PUE) is a metric used to determine the energy efficiency of a data center. PUE is determined by dividing the amount of power entering a data center by the power used to run the computer infrastructure within it. PUE is therefore expressed as a ratio, with overall efficiency improving as the quotient decreases toward 1.

How to Determine PUE
1. Take a measurement of energy use at or near the facility's utility meter. If the data center is in a mixed-use facility or office building, take a measurement only at the meter that is powering the data center. If it is not on a separate utility meter, estimate the amount of power being consumed by the non-data center portion of the building and remove it from the equation.

2. Measure the IT equipment load after power conversion, switching, and conditioning is completed. According to The Green Grid, the most useful measurement point is at the output of the computer room power distribution units (PDUs). This measurement should represent the total power delivered to the server racks in the data center.

According to the Uptime Institute, the typical data center has an average PUE of 2.5. This means that for every 2.5 watts in at the utility meter, only one watt is delivered out to the IT load. Uptime estimates most facilities could achieve 1.6 PUE using the most efficient equipment and best practices.

Variety of device whose special design enables it to be mounted in a standard cabinet, often in a 19” wide opening.

A Rack Unit or U (less commonly, RU) is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack (The dimension refers to the width of the equipment mounting frame in the rack i.e. the width of the equipment that can be mounted inside the rack). One rack unit is 1.75 inches (44.45 mm) high.  The size of a piece of rack mounted equipment is frequently described as a number in "U". For example, one rack unit is often referred to as "1U", 2 rack units as "2U" and so on.

Half-rack units typically describe units that fit in a certain number of rack units, but occupy only half the width of a 19-inch rack (9.5 in or 241 mm). These are commonly used when a piece of equipment does not require full rack width, but may require more than 1U of height. For example, a "4U half-rack" DVCAM deck would occupy 4U (7 in) height × 9.5 in width, and in theory, two 4U half-rack decks could be mounted side by side and occupy the 4U space.

A front panel or filler panel in a rack is not an exact multiple of 1.75 inches (44.45 mm). To allow space between adjacent rack-mounted components, a panel is 1⁄32 inch (0.031 inch or 0.79 mm) less in height than the full number of rack units would imply. Thus, a 1U front panel would be 1.719 inches (43.66 mm) high. If n is number of rack units, the formula for panel height is h = (1.750n − 0.031) inch = (44.45n − 0.79) mm.  The rack unit size is based on a standard rack specification as defined in EIA-310.

Heavy equipment or equipment which is commonly accessed for servicing, for which attaching or detaching at all four corners simultaneously would pose a problem, is often not mounted directly onto the rack but instead is mounted via rails (or slides). A pair of rails is mounted directly onto the rack, and the equipment then slides into the rack along the rails, which support it. When in place, the equipment may also then be bolted to the rack. The rails may also be able to fully support the equipment in a position where it has been slid clear of the rack; this is useful for inspection or maintenance of equipment which will then be slid back into the rack.

Slides or rails for computers and other data processing equipment such as disk arrays or routers often need to be purchased directly from the equipment manufacturer, as there is no standardization on such equipment's thickness (measurement from the side of the rack to the equipment) or means for mounting to the rail.

Raised metal floor, an alternate term for the more commonly used term ‘raised floor’.

Remote Power Panels are typically electrical panels outside of electrical equipment rooms.

A storage area network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries, and optical jukeboxes) to servers in such a way that the devices appear as locally attached to the operating system. A SAN typically is its own network of storage devices that are generally not accessible through the regular network by regular devices. The cost and complexity of SANs has dropped in recent years, resulting in much wider adoption across both enterprise and small to medium sized business environments.

Server Core is a bare-bones installation option for computers running the Windows Server 2008 operating system. The installation results in a server environment that is easy to manage and maintain but offers less functionality than more complex options.

Server Core can be configured for five well-known server roles: file server, DHCP Server, DNS server, Media Services, and Active Directory.  Server Core lacks a Windows shell. There is a command prompt interface and very limited graphical user interface (GUI) functionality.

Simple Network Management Protocol (SNMP) is a UDP-based network protocol. It is used mostly in network management systems to monitor network-attached devices for conditions that warrant administrative attention. SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force (IETF). It consists of a set of standards for network management, including an application layer protocol, a database schema, and a set of data objects.

Storage Utilization (SU) is a measure of how well the available data storage space in an enterprise is used. There are a number of variables that can be used to determine the storage utilization in a system. The relative priorities assigned to each variable can also affect the utilization figure.

Variables that are sometimes used to determine storage utilization include: (1) the total available storage space in the entire system (raw storage), (2) the time-averaged percentage of raw storage used, (3) the percentage of raw storage used at times of peak demand, (4) the available storage space in each storage volume, (5) the time-averaged percentage of storage used in each volume, (6) the peak-demand percentage of storage used in each volume, (7) the average time required to store or retrieve a file of a given size, (8) the time during peak-demand periods required to store or retrieve a file of a given size, (9) the average size of files stored or retrieved, (10) the ease with which data can be shared among the servers in a network, (11) the microprocessor speed at each server, (12) the random-access memory (RAM) capacity at each server, (13) the speed (in megabits or gigabits per second) with which data is transmitted among different servers, (14) the operating systems used by the servers, and (15) the nature and economic value of data handled by the business.

Static transfer switch.   Transfer switches allow switching from a primary power source to a secondary or tertiary power source and are employed in some electrical power distribution systems. Most often transfer switches can be seen where emergency power generators are used to back up power from the utility source. The transfer switch allows safely switching from utility power to emergency generator power while maintaining isolation of each source from the other. The switch may be either a manual switch, an automatic switch, or a combination of manual and automatic.

Total Cost of Ownership (TCO) is a financial estimate designed to help consumers and enterprise managers assess direct and indirect costs commonly related to software or hardware.   A TCO assessment ideally offers a final statement reflecting not only the cost of purchase but all aspects in the further use and maintenance of the equipment, device, or system considered. This includes the costs of training support personnel and the users of the system, costs associated with failure or outage (planned and unplanned), diminished performance incidents (i.e. if users are kept waiting), costs of security breaches (in loss of reputation and recovery costs), costs of disaster preparedness and recovery, floor space, electricity, development expenses, testing infrastructure and expenses, quality assurance, boot image control, marginal incremental growth, decommissioning, e-waste handling, and more.

TIA-942 is a standard developed by the Telecommunications Industry Association (TIA) to define guidelines for planning and building data centers, particularly with regard to cabling systems and network design. The standard deals with both copper and fiber optic media.

The TIA-942 specification references private and public domain data center requirements for applications and procedures such as:

• Network architecture
• Electrical design
• File storage, backup and archiving
• System redundancy
• Network access control and security
• Database management
• Web hosting
• Application hosting
• Content distribution
• Environmental control
• Protection against physical hazards (fire, flood, windstorm)
• Power management

The principal advantages of designing data centers in accordance with TIA-942 include standard nomenclature, failsafe operation, robust protection against natural or human-made disasters, and long-term reliability, expandability and scalability.

As known as brownout, reduced line voltage for an extended period can be caused by an intentional utility voltage reduction to conserve power during peak demand periods or other heavy loads that exceed supply capacity.

An uninterruptible power supply (UPS) is a device that allows your computer to keep running for at least a short time when the primary power source is lost. It also provides protection from power surges. A UPS contains a battery that "kicks in" when the device senses a loss of power from the primary source. If you are using the computer when the UPS notifies you of the power loss, you have time to save any data you are working on and exit gracefully before the secondary power source (the battery) runs out. When all power runs out, any data in your computer's random access memory (RAM) is erased. When power surges occur, a UPS intercepts the surge so that it doesn't damage your computer.

Virtualization is the creation of a virtual (rather than actual) version of something, such as an operating system, a server, a storage device or network resources. Implementing server virtualization, for instance, using VMware or open source competitor Xen, drastically reduces the number of servers required in an enterprise. That translates directly into reduced energy consumption, less maintenance and a smaller data center footprint.