Hierarchical Server-based Traffic Scheduling in Ethernet Switches Rui Santos, Paulo Pedreiras IEETA / University of Aveiro Aveiro, Portugal {rsantos, pbrp}@ua.pt Moris Behnam, Thomas Nolte MRTC / M¨ alardalen University V¨ aster˚ as, Sweden {moris.behnam, thomas.nolte}@mdh.se Luis Almeida University of Porto Porto, Portugal lda@fe.up.pt I. I NTRODUCTION Distributed Embedded Real-Time Systems (DERTS) are ever more complex, consisting of an increasing number of nodes with more functionality and handling more data. This scenario is pushing for paradigm changes in the method- ologies to design complex DERTS. Namely, component- oriented design methodologies, exhibiting the composability property are considered an effective means to tackle the growing complexity of DERTS. Server-oriented architectures have proven to be an effective means to enable controlled resource sharing, providing composability among application components as well as between the application software and the hardware platform on which they execute. Real-Time Ethernet-based protocols (RTE), such as PROFINET, EtherCAT, ETHERNET-Powerlink and TTEther- net, are emerging as the de facto communication technologies for wireline-based DERTS, taking advantage of the appealing attributes of the underlying Ethernet protocol (e.g. large band- width, cheap silicon), while removing or reducing the sources of non-determinism arising from its MAC protocol and/or from the standard switched architectures. However, currently available RTE protocols do not allow efficient server-based scheduling policies. Even when available, network partitions are typically static, as in TDMA-based approaches. Moreover, the respect for network partitions is frequently delegated to the end nodes that must execute a specific software layer, preventing the integration of legacy systems and other general purpose systems. Finally, even in the cases in which such a layer can be effectively deployed, the proper operation of the system requires the compliance of all system components to achieve a correct temporal behavior. To overcome these limitations, to manage the bandwidth in a switched Ethernet network, a hierarchical server-based traffic scheduling framework has been developed by the authors [1] [2]. This framework exhibits the following fea- tures: 1) A hierarchical architecture that supports server composition, virtual real-time channels providing temporal isolation (composability in the time domain); 2) Analytical tools for guaranteed real-time behavior; 3) Simple interface to adapt and reconfigure servers during runtime (create, remove and update attributes). The purpose of demonstrating our research prototype (in- cluding hardware) is to illustrate the potential of hierarchical sever-based traffic scheduling on Ethernet switches. In par- ticular we will emphasize composability in the time domain, predictability and reconfigurability of real-time channels. II. HIERARCHICAL SERVER- BASED TRAFFIC SCHEDULING The hierarchical server-based traffic scheduling framework has been presented as a way to provide a composability of real-time channels that handle communications in each output port of an Ethernet switch. This framework can be organized as a hierarchical structure of entities, represented as an inverted tree (Figure 1). Each branch of the tree represents a server that handles a portion of the total bandwidth assigned by the parent server. Therefore, each server (parent) can be connected to several servers (children) that share the bandwidth assigned to the parent. This procedure is recursive, and thus arbitrary server compositions can be designed. The actual data flows are placed at the end of the branches, i.e., the leaves, being connected to the corresponding servers. Figure 1. Server Hierarchy. Many DERTS exhibit requirements that may vary over time. To cope with those evolving requirements efficiently, the hierarchical server framework herein presented supports both server adaptation, i.e., the server attributes such as capacity and period may be adapted online, and hierarchy reconfiguration, i.e. servers may be added, moved to different