Resumen de Wireless real-time communication in tunnel-like environments using wireless mesh networks. The wickpro protocol
Jesús Aísa Vicén
Industrial applications have been shifting towards wireless networks in recent years because they present several advantages compared with their wired counterparts: lower deployment cost, mobility support, installation in places where cables may be problematic, and easier reconfiguration. These industrial wireless networks usually must provide real-time communication to meet application requirements. Examples of wireless real-time communication for industrial applications can be found in factory automation and process control, where Radio Frequency wireless communication technologies have been employed to support flexible real-time communication with simple deployment. Likewise, industry is also interested in real-time communication in underground environments, since there are several activities that are carried out in scenarios such as tunnels and mines, including mining, surveillance, intervention, and rescue operations.
Wireless Mesh Networks (WMNs) are promising enablers to achieve wireless real-time communication because they provide a wireless backbone comprised by dedicated routers that is utilized by mobile terminals. However, WMNs also present several challenges: wireless multi-hopping causes inter-flow and intra-flow interferences, and wireless propagation suffers shadowing and multi-path fading.
The IEEE 802.11 standard has been widely used in WMNs due to its low cost and the operation in unlicensed frequency bands. The downside is that its Medium Access Control (MAC) protocol is non-deterministic, and that its communications suffer from the hidden and exposed terminal problems.
This PhD thesis focuses on real-time communication in tunnel-like environments by using WMNs. Particularly, we develop a MAC and network protocol on top of the IEEE 802.11 standard to provide real-time capabilities, so-called WIreless Chain networK Protocol (WICKPro). Two WICKPro versions are designed to provide Firm Real-Time (FRT) or Soft Real-Time (SRT) traffic support: FRT-WICKPro and SRT-WICKPro. We also propose a hand-off algorithm dubbed Double-Threshold Hand-off (DoTHa) to manage mobility in SRT-WICKPro.
WICKPro employs a token-passing scheme to solve the inter-flow and intra-flow interferences as well as the hidden and exposed terminal problems, since this scheme does not allow two nodes to transmit at the same time. This is a reasonable solution for small-scale networks where spatial reuse is impossible or limited. The non-deterministic nature of IEEE 802.11 is faced by combining the token-passing mechanism with a polling approach based on a global cyclic packet schedule. As usual in cyclic scheduling, the hyper-period is divided into minor cycles. FRT-WICKPro triggers the token synchronously and fulfills strictly minor cycles, whereas SRT-WICKPro carries out asynchronous token-passing and lets minor cycles be overrun, thereby decoupling the theoretic and the actual minor cycles. Finally, DoTHa deals with shadowing and multi-path fading. Shadowing is addressed by providing the opportunity of triggering hand-off in the connected and transitional regions of a link, while multi-path fading is neglected for hand-off purposes by smoothing the received signal power.
We tested our proposals in laboratory and field experiments, as well as in simulation. As a case study, we carried out the tele-operation of a mobile robot within two confined environments: the corridors of a building and the Somport tunnel. The Somport tunnel is an old out-of-service railway tunnel that connects Spain and France through the Central Pyrenees. Although autonomous robots are becoming more and more important, technology is not mature enough to manage highly dynamic environments such as reconfigurable manufacturing systems, or to make life-and-death decisions, e.g., after a disaster with radioactivity contamination. Applications that can benefit from mobile robot tele-operation include real-time monitoring and the use of robotized machinery, for example, dumper trucks and tunneling machines, which could be remotely operated to avoid endangering human lives.
