Abstract: Lossy links are one of the fundamental characteristics of wireless sensor networks (WSNs). A large amount of work has been performed on characterizing link properties of 802.15.4 radios, in particular in the 2.4 GHz band. Unfortunately, the 2.4 GHz band has the apparent disadvantage of a crowded spectrum and considerable external interference, e. g., from WiFi, Bluetooth or even microwave ovens. We therefore investigate the performance of radios operating on the alternative 868 MHz frequency band, which is basically noise-free as determined from extensive experiments on a large-scale indoor testbed featuring more than 100 nodes. Although the lack of external interference eases protocol design, our study reveals that — and characterizes to what extent — wireless links in the 868 MHz band still show large variations in performance that must be accounted for.