As sensor nodes of Wireless Senor Networks (WSN) need software maintenance and function expansion after being deployed, remote code updating has become an indispensable service. Based on the analysis of the code dissemination protocols like MOAP, Deluge and Ripple, a new multihop code update mechanism called Air_Update was proposed, which was reliable, energy efficient, and of low storage cost and low latency. In order to ensure the correctness and integrity of the loaded code image, as well as the availability of node after abnormal reprogramming, Bootloader had been redesigned. Simultaneously, such mechanisms as subset-to-subset dissemination protocol, unicast retransmission request strategy and sliding window were adopted to reduce the traffic, the usage of SRAM and EEPROM, the energy consumption, and to prolong the network lifetime. At last, the correctness and effectiveness of design were verified in testbeds.
