BS ISO 20211:2015 pdf download

BS ISO 20211:2015 pdf download.Space data and information transfer systems — Spacecraft Onboard Interface Services — Device Access Service
1.5.1.2 Definitions from the Open Systems Interconnection (OSI) Basic Reference Model
This document is defined using the style established by the Open Systems Interconnection (OSI) Basic Reference Model (reference [D2]). This model provides a common framework for the development of standards in the field of systems interconnection. The following terms used in this Recommended Practice are adapted from definitions given in reference [D2]: (N)-layer: A subdivision of the architecture, constituted by subsystems of the same rank (N). (N)-protocol: A set of rules and formats (semantic and syntactic) which determines the communication behaviour of (N)-entities in the performance of (N)- functions. (N)-protocol-data-unit: A unit of data specified in an (N)-protocol and consisting of (N)- protocol-controlinformation and possibly (N)-user-data. (N)-service: Capability of a layer, and the layers beneath it (service providers), provided to the service users at the boundary between the service providers and the service users. (N)-service-access-point, (N)-SAP: The point at which (N)-services are provided by an (N)- entity to an (N+l)-entity. Within the spacecraft, a SOIS User SAP. As a minimum it locates a data system and an application within that data system.
The DAS provides a standard interface between onboard software applications and flight hardware such as sensors and actuators. The basic concept underlying the service is that the software application is able to access hardware devices independently of the precise physical location of the device, and without requiring detailed knowledge of the electrical interface to the device. A standard interface makes it easier to develop the onboard software, enables configuration changes in the spacecraft design to be easily tolerated, and increases the re-use potential of the software. To acquire a value (i.e., data) from a device, an application provides a physical device identifier and a value identifier. The service resolves the physical device identifier in order to determine the Device-specific Access Protocol (DAP) and an Underlying Transport (UT) service. The service maps information associated with the request onto the parameters of the protocol or the underlying service’s service-access-point, e.g., destination address and QoS parameters. The service then uses the DAP to transfer the value from the device and returns the acquired value. The logical relationship between the service, the DAPs and the UT service-access-points is illustrated in figure 2-3.