Figure 7.2 Categories of routing protocols.
incurs delays before actual data transmission can occur, proactive routing protocols establish
routes before they are actually needed. This category of protocols is also often described
as table-driven, because local forwarding decisions are based on the contents of a routing
table that contains a list of destinations, combined with one or more next-hop neighbors
that lead toward these destinations and costs associated with each next hop option. While
table-driven protocols eliminate the route discovery delays, they may be overly aggressive
in that routes are established that may never be needed. Further, the time interval between
route discovery and actual use of the route can be very large, potentially leading to outdated
routes (e.g., a link along the route may have broken in the meantime). Finally, the
cost of establishing a routing table can be significant, for example, in some protocols it
involves propagating a node’s local information (such as its list of neighbors) to all other
nodes in the network. Some protocols exhibit characteristics of both reactive and proactive
protocols and belong to the category of hybrid routing protocols. Finally, routing protocols
also differ in their operation, for example, negotiation-based protocols aim to reduce
redundant data transmissions by relying on the exchange of negotiation messages between
neighboring sensor nodes before actual data transfers occur. The SPIN family of protocols
(Section 7.5) belongs to this category. Multipath-based protocols use multiple routes simultaneously
to achieve higher performance or fault tolerance. Query-based routing protocols
are receiver-initiated, that is, sensor nodes send data in response to queries issued by the
destination node. The goal of QoS-based routing protocols is to satisfy a certain Qualityof-
Service (QoS) metric (or a combination of multiple metrics), such as low latency, low
energy consumption, or low packet loss. Finally, routing protocols also differ in the way
they support in-network data processing. Coherent-based protocols perform only a minimum
amount of processing (e.g., eliminating duplicates, time-stamping) before sensor data
is sent to receivers and data aggregators. However, in noncoherent-based protocols, nodes
may perform significant local processing of the raw data before it is sent to other nodes for
further processing