Fiabilidade

The first two nodes of what would become the ARPANET were interconnected between UCLA and SRI International in Menlo Park, California, on October 29, 1969.

The first description of the TCP protocols were published a paper in May 1974.

Use of the term "Internet" to describe a single global TCP/IP network originated in December 1974.

The first TCP/IP-wide area network was made operational by January 1, 1983.

"Internet," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Internet&oldid=159258860 (accessed September 21, 2007).


Nesta data:

Date: Mon, 16 Jul 2007 12:00
From: "***" <**@***>
To: geral@***
Subject: [geral] Interrupção do serviço de correio electrónico

Estimado utilizador,

Na madrugada de sexta-feira, dia 13, o servidor onde são armazenadas as mensagens de correio electrónico dos utilizadores da *** teve um problema num dos seus componentes. A equipe do ** começou a desenvolver esforços para a sua resolução às 8.30 do mesmo dia.

Foi necessário proceder à substituição do referido servidor e restaurar toda a configuração do sistema. Essa operação foi muito demorada, levando a que o sistema estivesse inoperacional até à tarde de Sábado, apesar da quase ininterrupta disponibilidade dos responsáveis pelo serviço de mail.

Informamos que durante todo este processo não foi perdida nenhuma mensagem e que todas as que remetentes exteriores enviaram com destino à ***, durante este período, ficaram armazenadas temporáriamente noutro local, tendo posteriormente sido encaminhadas para os destinatários; o envio de mensagens para o exterior da *** não foi afectado, tendo estas sido entregues de forma normal.

Lamentando esta ocorrência, o ** continua a melhorar os mecanismos de salvaguarda de informação e a redundância de equipamento de maior importância, procurando garantir uma conectividade interna e externa tão próxima dos 100% quanto possível.

Com os melhores cumprimentos

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***
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Reliability is a probability. This means that there is always some chance for failure. Reliability engineering is concerned with meeting the specified probability of success, at a specified statistical confidence level.

Reliability is predicated on "intended function:" Generally, this is taken to mean operation without failure. However, even if no individual part of the system fails, but the system as a whole does not do what was intended, then it is still charged against the system reliability. The system requirements specification is the criterion against which reliability is measured.

Reliability design begins with the development of a model. Reliability models use block diagrams and fault trees to provide a graphical means of evaluating the relationships between different parts of the system. These models incorporate predictions based on parts-count failure rates taken from historical data. While the predictions are often not accurate in an absolute sense, they are valuable to assess relative differences in design alternatives.

One of the most important design techniques is redundancy. This means that if one part of the system fails, there is an alternate success path, such as a backup system. An automobile brake light might use two light bulbs. If one bulb fails, the brake light still operates using the other bulb. Redundancy significantly increases system reliability, and is often the only viable means of doing so. However, redundancy is difficult and expensive, and is therefore limited to critical parts of the system. Another common design technique is component derating: Selecting components whose tolerance significantly exceeds the expected stress, as using a heavier gauge wire that exceeds the normal specification for the expected electrical current.

"Reliability engineering," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Reliability_engineering&oldid=159261656 (accessed September 21, 2007).