In this article, we will discuss about various data center availability models such as data center Tier Ratings 1-2-3-4, EN 50600 Availability classes (1-4), TIA 942-B Ratings (1-4), 10 Syska Hennessy Criticality Levels, ANSI/BICSI Classes (F0-F4), and N, N+1, 2N, 2(N+1)

Data Center availability modes define the availability of physical infrastructure in case of any failure. It’s quite obvious that some machines may get damage/stop working unexpectedly due to some unexpected incidents. Have you ever thought what will be the impact when this happens to a data center? Well as a computer engineer I would say that I will never imagine this incident since the impact can be nothing but stopping an entire business or partial business that is run on the data center. In some instances, machine damages can’t be predicted or controlled completely by humans. The best thing that we can do to avoid downtime is to minimize the maximum risk factor by having multiple backup plans. Here is the importance of data center availability modes which are the general standards to avoid any downtime.

We will discuss the general standards followed by various data centers to avoid the challenges that may happen to physical infrastructure.

• Uptime Institute Tiers (I-IV)
• EN 50600 Availability classes (1-4)
• TIA 942-B Ratings (1-4)
• 10 Syska Hennessy Criticality Levels
• ANSI/BICSI Classes (F0-F4)
• Others: N, N+1, 2N, 2(N+1)

Based on any one of the above data center standards, your DC design and operation will vary.

Uptime Institute Tiers (I-IV)

Uptime Institute is the IT industry’s most trusted and adopted global standard for the proper design, build and operation of data centers. They evaluate data center infrastructure based on certain criteria and DC availability will be certified as per that. The Tier Classification System provides the data center industry with a consistent method to compare typically unique, customized facilities based on expected site infrastructure performance, or uptime.

Uptime institute tier levels are mainly evaluated for 3 categories of a data center facility and namely Tier Certification Design Documents (TCDD), Tier Certification Constructed Facility (TCCF), Tier Certification Operational Sustainability (TCOS). Below are the various data center standards categorized by Uptime Institute.

Tier 1: A Tier 1 data center have no redundant capacity components (single uplink and servers). It has an expected uptime of 99.671% (28.8 hours of downtime annually).

Tier 2: A Tier 2 data center has a single path for power and cooling and some redundant and backup components. It has an expected uptime of 99.741% (22 hours of downtime annually).

Tier 3: A Tier 3 data center has multiple paths for power, cooling, and systems in place to update and maintain it without taking it offline. It has an expected uptime of 99.982% (1.6 hours of downtime annually). These data centers provide N+1 fault tolerant providing at least 72-hour power outage protection. This is the most adapted Tier levels that you can see throughout the world.

Tier 4: A Tier 4 data center is built to completely fault-tolerant and has redundancy for every component including uplinks, storage, chillers, HVAC systems, servers, etc. Everything is dual-powered. These data centers are 2N+1 fully redundant infrastructure. It has an expected uptime of 99.995% (26.3 minutes of downtime annually).

Have you noticed the letter ‘N’ which is indicating the redundancy capability of data centers? Ever wonder what does it mean?

N is simply the amount required for operation. It represents the capacity that you need to operate. There is no backup system so if the system fails, it fails and downtime is the ultimate result. If you have a flat tire and no spare, you have N.

N+1 represents the amount required for operation plus a backup. It ensures system availability even in the event of a component failure. It is similar to the concept of a spare tire in your car. When you get a flat, you have the ability to swap out the flat with a spare tire. This simply means that you could survive one flat tire.

2N+1 means that you have two times the amount required for operation plus a backup. This means that you have a full-size spare tire plus a temporary spare tire just in case. That means that you could incur two flat tires and still operate.

Typically the uptime institute certifications will look like below,

EN 50600 Availability classes (1-4)

EN 50600 represents the first European standard that uses a holistic approach to make comprehensive specifications for the new construction and operation of a data center. It defines requirements for the construction, power supply, air conditioning & ventilation, cabling, security systems, and defines criteria for the operation of data centers. Created by CENELEC (European Committee for Electrotechnical Standardization), EN 50600 offers various degrees of freedom, and can to a certain extent be understood as a modular system. First and foremost, EN 50600 represents a standard that is applicable during the construction of new data centers. It defines the assessment reports and analyses that are necessary in advance of the design and construction work.

 There are four types of EN 506000 certifications that defines the availability classes. The availability classes of EN 50600 apply to the power supply, the cooling system, and the cabling. The classes differ as follows according to EN 50600-1 (General aspects for design and specifications)

Availability class 1: Low availability. Design without redundancies based on a supply path

Availability class 2: Extended availability. Design with partial redundancies based on a supply path

Availability class 3: High availability. Design with redundant components based on two supply paths (but only one refrigeration supply path)

Availability class 4: Very high availability. Design with system redundancies based on two supply paths (but only one refrigeration supply path)

TIA 942-B Ratings (1-4)

The Telecommunications Industry Association (TIA) is accredited by the American National Standards Institute (ANSI) to develop voluntary, consensus-based industry standards for a wide variety of Information and Communication Technologies.

TIA-942-B will define the various data center uptime standards based on its Design, Site and Ready.

ANSI/TIA-942 describes four Rating levels in which data centers can be classified. Below are the high-level descriptions of each Rating level.

Rated-1: Basic site Infrastructure

A data center that has single capacity components and a single, non-redundant distribution path serving the computer equipment. It has limited protection against physical events.

Rated-2: Redundant Capacity Component Site Infrastructure
A data center that has redundant capacity components and a single, non-redundant distribution path serving the computer equipment.  It has improved protection against physical events.

Rated-3: Concurrently Maintainable Site Infrastructure
A data center that has redundant capacity components and multiple independent distribution paths serving the computer equipment. Typically, only one distribution path serves the computer equipment at any time. The site is concurrently maintainable which means that each and every capacity component including elements that are part of the distribution path, can be removed/replaced/serviced on a planned basis without disrupting the ICT capabilities to the End-User. It has protection against most physical events.

Rated-4: Fault Tolerant Site Infrastructure
A data center that has redundant capacity components and multiple independent distribution paths serving the computer equipment which all are active. The data center allows concurrent maintainability and one (1) fault anywhere in the installation without causing downtime. It has protection against almost all physical events.

Typically the TIA 942-B certifications will look like below,

Note: TIA does not issue data center certificates. Any certified 3rd party auditing (e.g. Datwyler & EPI) will evaluate the site as per TIA 942 B standards and provide this.

10 Syska Hennessy Criticality Levels

ENR has ranked Syska Hennessy Group as one of the world’s top five in data-center engineering design for the last ten years. Syska’s Criticality Level concept is comparable in parts to data center “tier” levels used in the industry. These criticality or tier levels differentiate expected availability and reliability between sites that are designed, constructed, commissioned, maintained, and operated at different budgets and priority levels. Whereas many earlier published tier classifications focused on power to the critical load, Syska’s Criticality Levels address everything that is important for the availability of a critical facility. The best way to understand these 10 levels of the standard is to compare with the uptime institutes tier levels. Let us understand that as below when comparing,

Syska Criticality	Uptime institute	Estimated % of availability	Expected Annual downtime.
C1	Tier 1	98	20–40 hours
C2	Tier 2	99	10–25 hours
C3	Tier 3	99.9	1-15 hours
C4	Tier 4	99.99	0.25–1 hours
C5		99.999	1–20 minutes
C6		99.9999	1–15 hours
C7		99.99999
C8		99.999999
C9		99.9999999
C10		99.99999999

If you would like to understand more about Syska Hennessy standards read through here

PS: ENR specified on the above notes is denoted for Engineering News-Record which is an American weekly magazine that provides news, analysis, data, and opinion for the construction industry worldwide.

ANSI/BICSI Classes (F0-F4)

BICSI 002-2019 (Building Industry Consulting Service International) standard is written to be used as a standard for Data Centre Design and Implementation. Written by industry professionals from all major disciplines, this standard not only lists what a data center requires, but also provides ample recommendations on the best methods of implementing a design to fulfill your specific needs. These standards are used in conjunction with other multiple standards or publications such as ,

– TIA: 942, 568C, 569B,

– ISO/IEC:11801, 24764

– CENELEC: EN 50173, 50174

– NFPA: 70 (NEC®), 75

– ASHRAE: Datacom and Data Centre

– IEEE: 493, 1100 (Gold and Emerald Books) – EN 50600

There are 5 availability classes defined as per BICSI 002-2019 and those can be defined as below,

Availability Class 0 : The objective of Class 0 is to support the basic requirements of the IT functions without supplementary equipment. Capital cost avoidance is the major driver. There is a high risk of downtime because of planned and unplanned events.

Availability Class 1 : The objective of Class 1 is to support the basic requirements of the IT functions. There is a high risk of downtime because of planned and unplanned events. However, in Class 1 data centers, remedial maintenance can be performed during nonscheduled hours.

Availability Class 2: The objective of Class 2 is to provide a level of reliability higher than that defined in Class 1 to reduce the risk of downtime because of component failure. In Class 2 data centers, there is a moderate risk of downtime as a result of planned and unplanned events. Maintenance activities can typically be performed during unscheduled hours.

Availability Class 3: The objective of Class 3 is to provide additional reliability and maintainability to reduce the risk of downtime because of natural disasters, human-driven disasters, planned maintenance, and repair activities. Maintenance and repair activities will typically need to be performed during full production time with no opportunity for curtailed operations.

Availability Class 4: The objective of Class 4 is to eliminate downtime through the application of all tactics to provide continuous operation regardless of planned or unplanned activities. All recognizable single points of failure are eliminated.

Below table is the simple representation of these fictional availability classes,

If you would like to understand more about BICSI Classes read through here

Others: N, N+1, 2N, 2(N+1)

After all, the above are international standards that can be followed to achieve the highest redundancy and reliability for a data center infrastructure. These standard certifications are proof to our clients that the facility achievements that you are following. The periodic auditing from certification authority will make sure that your facilities are most efficient at a frequent interval too. But what is the case when you want to have your own small data centers that don’t require someone else surety for your facility? You have literally a small scale data center? Yes, you can have this(the least case scenario that it will be followed on these days).

All that you need is to have your own physical infrastructure(mainly power, cooling, and connectivity)  that can be defined in terms of N, N+1, 2N, 2N+1.

The concept is the same as the things that we discussed in uptime institute Tier levels.

N is simply the amount required for operation. For instance, a data center server may require one power supply operating at specific power ratings to keep the server running at optimal availability conditions.

N+X refers to a redundant system that contains X number of spare components to act as an independent backup when the appropriate component fails to operate as intended. N+1 means that only one backup component is available to complement the N original component(s). N+2 means that two backup components are available to ensure additional resilience.

2N means that you have two times the amount required for operation. You have two units of equal size, capabilities, and capacity. 2N+1 means that you have two times the amount required for operation plus a backup.

Knowledge Credits: Syska Criticality Levels, Uptime institute

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