Wireless real-time networks are a natural step for deployments in industrial automation, automotive, avionics, or robotics targeting features such as improved mobility, reduced wiring costs, and easier more flexible network developments. However, the open transmission medium where wireless networks operate is generally more prone to interference and transmission errors caused by fading. Due to this, real-time communications is in general still provided by wired networks in many of these application fields. At the same time, wired and wireless standards traditionally associated with the consumer electronics application field (e.g., IEEE 802.3 "Ethernet" and IEEE 802.11 “WiFi”) are trying to find their way into industrial automation, automotive, avionics, and robotics use cases, since they provide features like high throughput and cheap hardware. Many times, applications with diverse real-time and reliability requirements have to co-exist, and often in hybrid wired-wireless networks to ensure compatibility with existing systems. Given this scenario, it is essential to provide support for data traffic with requirements ranging from real-time time-triggered and event-driven to non-real-time, and enable high reliability with respect to timing constraints, in the context of hybrid wired-wireless networks. This thesis aims at covering the aforementioned requirements by proposing a medium access control (MAC) solution suitable for wireless communications, with support for real-time traffic with diverse time and reliability requirements. The MAC layer is in charge of providing timely access to the transmission medium, and can be effectively used to increase reliability by means of, e.g., avoiding concurrent transmissions and performing retransmissions. To this end, a set of evaluation criteria is proposed to determine the suitability of a particular MAC method to meet the identified emerging requirements. These criteria include channel access delay, reliability, protocol overhead, capability to integrate with wired networks, and sensitivity to interference from collocated systems. Next, based on these requirements, a MAC protocol with a set of tunable features is proposed, and evaluated in terms of support for data traffic with different loads and distributions, i.e., emanating from different traffic classes, and from different number of senders. The evaluation is made both analytically, by calculating the worst case delay and, with the help of real-time schedulability analysis, determining the effective load required to guarantee real-time deadlines, as well as by means of computer simulations using the INET framework for OMNeT++ to determine the average delay. Finally, the thesis proposes a set of retransmission schemes to be used together with the proposed MAC protocols in order to improve the resistance against interference and transmission errors. For this, a set of interference patterns with different characteristics is proposed and applied in the simulator. The resulting MAC layer solution is designed to be used at the wireless segment of a hybrid wired-wireless network, and is able to schedule data traffic originating from three different classes: time-triggered, rate-constrained and best-effort. To achieve this, an additional collision domain introducing wireless segments is added to the real-time scheduler, as well as support for real-time retransmissions, to enable high reliability while keeping real-time deadlines.
Västerås: Mälardalen University , 2016.