Energy conservation in wireless sensor networks has been a primary design objective. However, this constraint has not overshadowed other key objectives like scalability, architecture, latency, routing etc. Several applications of wireless sensor networks are envisioned to handle critical scenarios where data retrieval time is crucial. There are some hard-real-time applications where there is little tolerance for delayed data. We present a survey of some key mechanisms to minimize latency in single-hop and multi-hop wireless sensor networks. This includes sampling time, processing time, propagation time, scheduling, MAC protocols, use of directional antennas, predictions, sleep/wakeup cycles, use of dual-frequency radios and more.
