The constrained physical and link layer technologies are,
instead, generally characterized by low energy consumption and
relatively low transfer rates, typically smaller than 1 Mbit/s. The
more prominent solutions in this category are IEEE 802.15.4
[27], [28] Bluetooth and Bluetooth Low Energy,8 IEEE 802.11
Low Power, PLC [29], NFC and RFID [30]. These links usually
exhibit long latencies, mainly due to two factors: 1) the intrinsically low transmission rate at the physical layer and 2) the power-saving policies implemented by the nodes to save energy, which
usually involve duty cycling with short active periods.