Reprogramming WSNs usually are unattended for a long

Reprogramming WSNs usually are unattended for a long period of time. However, programs on the nodes may need to be changed. Changing parameters Changing functions Fixing bugs Over-the-air reprogramming needed due to accessibility, network size etc.

Reprogramming protocols There have been developed several protocols thus far XNP, MOAP, MNP, Deluge, etc. The protocols can be classified according to their encoding and code dissemination techniques

Encoding techniques Instead of sending the entire code, it could be possible to send only the difference Energy efficient Faster

XNP (Crossbow In-Network Programming) Designed for complete networks, i.e., nodes that are in the communication range of a sink The sink broadcasts the new code Nodes copies the code capsules in external memory Then the sink broadcasts query if there are nodes who have not received the entire code. If necessary, nodes can request unreceived capsules

XNP Drawbacks: Requires bi-directional links between the nodes and the sink If the link between a node and the sink fails, the node may not get the new code

Multihop Over-the-Air Programming (MOAP) Addresses the drawbacks of XNP Updates codes of nodes in a multihop fashion Whenever a node gets a new code, it forwards the code NACK is used to request missing packets

Deluge Like MOAP it is multihop based Uses NACK Unlike MOAP, it divides the code into fixed-size pages, each page is divided into packets Pages are delivered in a sequential order Uses pipelining technique Whenever a node gets a page, it can start to forward it

Multihop Network Programming (MNP) Like Deluge, divides the code into segments Uses pipelining technique Unlike Deluge, it selects one forwarded at a time in a given neighborhood

Reprogramming Problem One of the main functions of network reprogramming is code dissemination. Code is disseminated over the air to reach the selected nodes. Part or entire network should get the new code. Needs to account for transient disconnections

Reprogramming Problem Solution could be code advertisement. Advertise metadata (version number) Idea: we continuously broadcast the metadata. Too expensive In general, message transmission is the main energy consumption function. Moreover sending metadata may cause unnecessary collisions.

Trickle Uses suppression mechanism. A node periodically sends metadata message at time t selected randomly between [t/2, t], where tl  t  th If a node hears the same metadata, increment counter. If the counter is less than K at t time, it broadcasts its metadata. Else if t expires, it doubles the value of t. If a node hears new metadata/code, it sets t to tl. If a node hears older metadata, send updates.

Trickle When there is no new code in the network, Trickle continuous sending metadata at [th/2, th] Therefore, it consumes linear energy as a function of time.

Varuna Main idea: send metadata only when necessary. Tries to improve on Trickle that requires periodic advertisement. May lead to energy reduction. Uses a table to store consistent nodes. If updated node receives data from outdated node, then send metadata, send update, store ID of the node in the table.

Varuna In Trickle, inconsistency is detected either by an updated node or by an outdated node. In Varuna, inconsistency is detected by an updated node. Varuna has been shown to have almost constant energy usage.

Varuna Not good in networks which are event based, or where the communication rate is low. Depends on frequent communication to detect inconsistency quickly. Else, increases completion latency, unnecessary data discarding and usage of resource.