Systems Engineering Interfaces: A Model Based Approach

1 Systems Engineering Interfaces: A Model Based Approach Elyse Fosse, Christopher L. Delp Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive Pasadena, CA 91109. Abstract The Engineering of interfaces is a critical function of as existing industry standards for specific kinds of interfaces. the discipline of Systems Engineering . Included in interface SysML[1] provides a precise Model Based representation for Engineering are instances of interaction. Interfaces provide the specifying the interfaces of parts and integration between specifications of the relevant properties of a system or com- parts through those interfaces. The language also allows for ponent that can be connected to other Systems or components domain-specific semantics to be applied as an extension to the while instances of interaction are identified in order to specify the actual integration to other Systems or components.

2 Current basic modeling capability of SysML. Systems Engineering practices rely on a variety of documents and diagrams to describe interface specifications and instances The interface Engineering work performed by the Opera- of interaction. The SysML[1] specification provides a precise tions Revitalization (Ops Rev) Task[2], sponsored by the Model Based representation for interfaces and interface instance Multimission Ground Systems and Service Office of NASA, integration. This paper will describe interface Engineering utilized a Model Based Approach to represent the interfaces as implemented by the Operations Revitalization Task using and interactions required for a Mission Operations system SysML, starting with a generic case and culminating with a (MOS). Mission Operations Systems Engineering concerns focus on a Flight system to Ground Interaction. The reusability with interfaces begin with the Flight-Ground interface .

3 The of the interface Engineering Approach presented as well as its extensibility to more complex interfaces and interactions will be interface focuses on the interaction between the Flight Sys- shown. Model -derived tables will support the case studies shown tem and the Ground system . Within the Ground system the and are examples of Model - Based documentation products. MOS must interface with ground stations, networks, and a host of organizations. Within the MOS, Mission Services, Operations Roles, and Software must interface with each other. TABLE OF C ONTENTS. This paper will describe the key Viewpoints used to define 1 I NTRODUCTION .. 1 the patterns for modeling MOS interfaces. These Viewpoints 2 I NTERFACE PATTERNS AND P OINTS OF V IEW .. 1 will then be complemented by examples from the MOS 3 M ODELING I NTERFACES WITH S YS ML .. 2 Architecture [2]. Beginning with the Flight-Ground interface , 4 O PERATIONS R EVITALIZATION I NTERFACE examples from these models will be used to illustrate specifi- E NGINEERING.

4 6 cation of Interfaces, Interactions between Interfaces, and the limitations and operational contexts of those interactions. 5 S UMMARY .. 8. ACKNOWLEDGMENTS .. 8. R EFERENCES .. 8 2. I NTERFACE PATTERNS AND P OINTS OF. B IOGRAPHY .. 8 V IEW. Ops Revitalization is building a Model of a Mission Opera- tions system (MOS). The interfaces for Mission Operations 1. I NTRODUCTION Systems cover a broad range of Systems , software, hardware, The topic of Interfaces is at the heart of the multi-disciplinary and human interactions. In order to Model this broad range nature of Systems Engineering . This area covers what is of interfaces and interactions, it is useful to describe the necessary in order to connect the individual pieces of the system from different points of view [3]. Ops Revitalization system together into a working system . To accomplish the has developed the Mission Service Architecture Framework connection the relevant properties and behavior of each part (MSAF) [4].

5 The MSAF defines the atomic interface en- of the system must be specified. It is also necessary to specify gineering components (Table 1) as well as a set of patterns the particular connections between each part and the nature for modeling this broad set of integrated Systems and other of those connections in terms of limitations, protocols, and patterns in the MOS. These basic Views can be used to various operational conditions or scenarios. describe the different interfaces related to the MOS. The describing and specifying of interfaces in a general way Table 2 identifies the Viewpoints that address important con- is a challenging problem. Current Systems Engineering cerns with respect to the topic of interfaces. Part interface practices rely on a variety of documents and diagrams to Viewpoints of the system describe what interfaces the part describe interface specifications for different Systems as well presents.

6 The interface Layered Viewpoints describe the interfaces in a recursive manner and are a special case of 978-1-4577-0557-1/12/$ c 2012 IEEE. the Part interface Viewpoint. In Systems where software is c 2012 California Institute of Technology. Government sponsorship ac- part of the system , it is common to have logical layering knowledged of software interfaces on top of hardware. interface Speci- 1 IEEEAC Paper #2562, Version 2, Updated 5/1/2013. fication Viewpoints describe the individual specifications of 1. Table 1. interface Engineering Definitions Term Definition interface The system boundary that is presented by a system for interaction with other Systems . interface Specification Describes the nature of the boundary presented by a system or component in terms of properties and functionality. Interaction An instance of an operational entity ( system , organization, or services) interface .

7 The interaction can connect to other operational entities according to its Interaction Specification. Interaction Specification Describes how an operational entity ( system , organization, or service) can effect another operational entity when a connection exists. Table 2. Viewpoints Viewpoint Purpose Concern Part interface Identify Interfaces for a given Part. What are the interfaces for a given Part? Layered Part interface Specify layering of interfaces such as What is the structure of the different in- application, protocol and data layers. formation aspects on the interface ? interface Specification Specify a given interface in terms of What is the detailed set of functions and functions and properties of that interface . properties of a given interface ? interface Connection Specify the integration of 2 or more What Parts are connected to each other? Parts through their respective Interfaces in terms of the specific conditions and function occurrences that define the inte- gration according to the interface Speci- fication.

8 interface Object Flow Specify how objects (materials, informa- What are the flows between parts of the tion) flow across a given integration of system ? interfaces for a set of Parts. interface Function Occurrence Specification for behavioral interaction How do functions occur between Parts of across interfaces. the system ? Performance and Limitations on In- Specify constraints on interfaces such What are the expectations and limits of terfaces as policies, agreements and performance the given integration? constraints. each interface . The Part interface , Layered Part interface , 3. M ODELING I NTERFACES WITH S YS ML. and interface Specification Viewpoints describe the part of The SysML specification provides a precise Model - Based the system in terms of interfaces without explaining how they representation for interfaces and interactions. interface spec- will be integrated.

9 Providing these views of the system allows the Systems Engineer to evaluate and analyze parts Based on ifications are described using SysML flow ports, operations, and signals which correlate to the system or component their interfaces separately from how they will be integrated. interface properties, as shown in Table 3. The remaining set of Viewpoints in Table 2 refer to how Inter- faces may be instantiated into Interactions in order to describe Table 3. interface Specifications To SysML. context-specific integrations and behavior. interface Connec- tion Viewpoints identify what parts will be integrated with each other and how they will behave when they are integrated. interface Property SysML Property interface Object Flow Viewpoints describe how objects are allowed to flow across the interfaces of the integrated parts in Functions Operations the context with which the parts are integrated in.

10 The objects Signals Signals could be physical objects, like shipping a spacecraft to Cape Information I/O Flow Ports Canaveral, or Radio Waves in deep space. The objects could also be logical like data moving through cables. interface Materials I/O Flow Ports Function Occurrence Viewpoints show behavior interaction between parts. From this vantage point the system can be described in terms of the occurrence of functions and events A simple system will be used to describe the Ops Rev Tasks's among the interfaces of components. interface Constraints SysML interface Engineering implementation. The following Viewpoints describe how the interface is constrained. This figures and tables are views into the SysML Model describing could be in the form of expected performance or operational the system shown in Figure 1, which is comprised of two limitations. Together, these Viewpoints describe integration Systems , A and B.