A modeling pattern for layered system interfaces

Communications between systems is often initially represented at a single, high level of abstraction, a link between components. During design evolution it is usually necessary to elaborate the interface model, defining it from several different, related viewpoints and levels of abstraction. This paper presents a pattern to model such multi-layered interface architectures simply and efficiently, in a way that supports expression of technical complexity, interfaces and behavior, and analysis of complexity. Each viewpoint and layer of abstraction has its own properties and behaviors. System elements are logically connected both horizontally along the communication path, and vertically across the different layers of protocols. The performance of upper layers depends on the performance of lower layers, yet the implementation of lower layers is intentionally opaque to upper layers. Upper layers are hidden from lower layers except as sources and sinks of data. The system elements may not be linked directly at each horizontal layer but only via a communication path, and end-to-end communications may depend on intermediate components that are hidden from them, but may need to be shown in certain views and analyzed for certain purposes. This architectural model pattern uses methods described in ISO 42010 [1], Recommended Practice for Architectural Description of Software-intensive Systems and CCSDS 311.0-M-1 [2], Reference Architecture for Space Data Systems (RASDS). A set of useful viewpoints and views are presented, along with the associated modeling representations, stakeholders and concerns. These viewpoints, views, and concerns then inform the modeling pattern. This pattern permits viewing the system from several different perspectives and at different layers of abstraction. An external viewpoint treats the systems of interest as black boxes and focuses on the applications view, another view exposes the details of the connections and other components between the black boxes. An internal view focuses on the implementation within the systems of interest, either showing external interface bindings and specific standards that define the communication stack profile or at the level of internal behavior. Orthogonally, a horizontal view isolates a single layer and a vertical viewpoint shows all layers at a single interface point between the systems of interest. Each of these views can in turn be described from both behavioral and structural viewpoints. Introduction Specifying and managing interfaces is at the heart of systems engineering. Well defined interfaces and interoperable protocols allow a designer the freedom to implement the internals of their system independently of the other systems to which it may be connected, and to make their systems more easily re-usable in different contexts. But simple interfaces from one point of view may actually involve multiple levels of complexity. For example, a USB “thumb drive” is a ubiquitous and simple appearing device, but the document for the USB 3.1 specification alone consists of a primary document and several supplements totaling over one thousand pages that cover physical, logical, service and behavioral specifications. With increasing use of model-based systems engineering (MBSE) as a tool to help manage complexity in systems engineering, there is a need for a focused method to model interface complexity. The most common language for MBSE is the Systems Modeling Language (SysML) [3], which is based on the Unified Modeling Language (UML) [4]. For the system engineer an MBSE tool combined with modeling patterns and methodology will help form model information into a “single source of truth”, support static and dynamic analysis of the models, and interface with external modeling and analysis tools. This paper is focused on a pattern for modeling layered interfaces, and making this pattern available for reuse by other systems engineers. It is common in systems engineering to represent a single item at different levels of abstraction. In its simplest form, an item may be represented both logically and physically, but in different views. An archetype for such multi-layer abstraction of interfaces is the pattern for describing protocol stacks for computer networks, such as TCP/IP, using the ISO / OSI Basic Reference Model (OSI BRM) [5]. This concept has been adopted and generalized and applied to systems engineering to allow multiple simultaneous levels of abstraction, or layers. We have defined an interface modeling pattern using SysML that provides a method for modeling interfaces at different levels of abstraction and detail. This pattern is intended as a framework within which users may incorporate as much or as little detail as is needed for their problem domain. That detail may include both structural and behavioral aspects, and is intended to manage the complexity and give a representation of the entire interface. The benefit of this approach is to enable an integrated description and analysis of system interfaces, either statically, dynamically, or both. This paper presents some key viewpoints, views, and related concerns, leveraging ISO 42010 and RASDS specification of architecture description, and shows how they may be applied to interfaces modeled with our pattern. Describing interfaces is standard practice in systems engineering, but in the case of MBSE a clearly defined modeling practices has been shown to be of benefit. We will present some example applications of the pattern. And, finally, we will outline some future work that could be done to adapt and apply this interface modeling pattern to other engineering domains. Basic Concepts A basic, familiar, example of two interacting systems is shown in Fig. 1. This figure shows a PDF file being transferred from a sender, perhaps the author of a paper, to a receiver, perhaps the editor of a technical conference. This first view shows two elements, the sender and receiver, which are transferring a file. It says nothing about the manner in which the file is exchanged. There must be some interfaces and underlying protocols, but these are abstracted away in this applications view. Providing the means for describing the mechanism of the transfer is the focus of the rest of this paper. The OSI BRM standardizes the concepts of a layered communications architecture. This forms the basis for the way that we construct our layered systems interfaces. All the figures in this section comprise a single, unified Sender