Open System Interconnection Reference Model - OSI Model

The OSI Reference Model is founded on a suggestion developed by the International Organization for Standardization (ISO). The model is known as ISO OSI (Open Systems Interconnection) Reference Model because it relates with connecting open systems – that is, systems that are open for communication with other systems. It was developed in 1984. It defines a networking framework for implementing protocols in seven layers.
Layers in the OSI model are ordered from lowest level to highest.The stack contains seven layers in two groups:The layers are stacked this way:
* Application
* Presentation
* Session
* Transport
* Network
* Data Link
* Physical
The upper layers of OSI model are application, presentation and session layer. The software in these layers performs application-specific functions like data formatting, encryption, and connection management.
The lower layers are transport, network, data link and physical layer. These layers provide more primitive network-specific functions like routing, addressing,and flow control.

Benefits of OSI model

- Helps users understand how hardware and software elements function together.
- OSI is independent of country.
- OSI is independent of the operating system.
- Makes troubleshooting easier by separating networks into manageable pieces.
- Helps users understand new technologies as they are developed.

Cleanroom Software Engineering - Certification

The verification and testing techniques leads to software components that can be certified. Certification implies that the reliability can be specified for each component. The potential impact of certifiable software components goes far beyond a single cleanroom project. Reusable software components can be stored along with their usage scenarios, program stimuli, and probability distributions.

The certification approach involves five steps :
1. Usage scenarios must be created.
2. A usage profile is specified.
3. Test cases are generated from the profile.
4. Tests are executed and failure data are recorded and analyzed.
5. Reliability is computed and certified.

Cleanroom Certification Models :
- Sampling model : Software testing executes m random test cases and is certified if no failures or a specified numbers of failures occur. The value of m is derived mathematically to ensure that required reliability is achieved.
- Component model : A system composed of n components is to be certified. The component model enables the analyst to determine the probability that component i will fail prior to completion.
- Certification model : The overall reliability of the system is projected and certified.