Time Synchronized Channel Hopping (TSCH) is a technique known to efficiently combat external interference and multi-path fading. TSCH is at the heart of industrial low-power wireless standards such as WirelessHART and IEEE802.15.4e, and the focus of the current standardization activities at IETF 6TiSCH. In a TSCH network, communicating nodes send successive link-layer frames at different frequencies. The performance of such frequency-agile communication is not well understood. In this paper, we propose an empirical approach. We conduct an experiment to record the connectivity between 350 nodes in a typical office environment, simultaneously on each of the 16 frequencies at 2.4 GHz. Analysis reveals the impact of WiFi interference on the reliability of the IEEE802.15.4 wireless links: even when the WiFi network sits idle, IEEE802.11 beaconing causes a significant number of links to drop from 90% to 70-80% packet delivery ratio. It also reveals the impact of multi-path fading, showing how moving a pair of nodes can cause their link to go from perfect to non-existing. Results show that the quality of each link depends heavily on the communication frequency. The paper shows the effectiveness of channel hopping: by simply changing the communication frequency between successive transmissions, less nodes are required to cover a geographical area. The paper discusses the importance of the frequency to retransmit on. The 5-hour connectivity trace reveals a wealth of information. This paper is only a first step towards a much larger dataset collected over a representative number of real-world deployments.