Ran across an interesting graphic in a new book recently. It single-handedly placed dependability in its proper context. It is an umbrella term that includes most of what we commonly think of as reliability and the other ‘ilities.’ It encompasses the various connotations of dependable and reliable that are conveyed during common use. And, the term dependability permits the overarching context for defining very clearly the various elements and connotations specific for the component, product, system, or network.
Here’s the image as it appears in Thomas Van Hardeveld and David Kiang’s book titled, Practical Application of Dependability Engineering, ASME Press, 2012, page 3.
The definition of dependability is the ability to perform as and when desired. Quite simple actually.
As can be seen the graphic, this includes a product like a decorative case for a phone. One characteristic is the case may reliably survive the elements while covering the case for five years. The case in not maintainable doesn’t require support, and while providing a credible faux diamond encrusting, was not intended to provide the phone security from wireless intrusion.
Another application may be the entire communication system supporting that decorative encased phone. This would include the cell towers, trunk lines, fiber optic, call routing, satellites, sales and support centers. It would include specifications under every block of the image, from individual component reliability to billing record security.
To create a dependable system consider the essential delivery elements to meet the needs of the beneficiary of the system. If it’s something as simple as a rhinestone studded phone case, one may expect the stone to remain affixed during normal use. If we’d like to place a call (already having interacted with many parts of the system and network to enable a call), we depend on the communications network to work as we place that call.
As a consumer, we may not know the source of the materials or types of adhesive used for the phone case. Likewise, when placing a call, we may not even know the location of the cell tower, the route the call is taking nor bandwidth consumption. As the consumer, we are blissfully unaware of many of the details that permit the system to work as expected.
Understanding the details is the role of designers. I use the term designers to include the entire team of individuals from concept to production. Once the system is deployed, it may require another team to maintain and service the system. The decisions made during design significantly impact all aspects of system dependability. If the wrong adhesive is used, the rhinestone may fall off upon opening the shipping package. If the wrong decisions are made about connection protocols or call handling, the call home may not be possible.
We count of systems every day, whether the attachment of an ax to its handle or the traffic signal system on the way to the store. We count on the team of designers to create a dependable system.
The design of the system includes the consideration of dependability attributes along with decisions related to business or organizational goals. Establishing a dependable system may include explicit reliability and security objectives which are set within the context of the designing organization. The remaining dependability attributes may not apply are not considered vital to the delivery of the dependable system. Clearly stating and measuring the specific objectives a management tool and one means to ensure achieving the overall desired dependability.