Ad Hoc Networks
Routing protocols

The routing protocols are a building block of MANETs and thus lots of routing protocols have been proposed taking into account different metrics for selection of optimal routing path. In general, all routing protocols encompass three basic mechanisms:

- Route discovery – The purpose of this mechanism is to find optimal route between two stations that are exchanging some data packets. The most spread principle is based on so called flooding algorithm when source station transmits RREQ (Route Request) packet. When destination station receives this packet, RREP (Route Response) packet is generated and sent back to source station. The main issue is to find optimal routing path while signalling overhead of routing is minimized.
- Route selection – During routing discovery, several feasible paths could be found. Consequently, the main objective of route selection mechanism is to select the most appropriate routing path. The path selection can be based on many metrics, such as link capacity, the number of hops along the path, link reliability and its stability, etc.
- Route maintenance – When the proper path is selected, it is necessary to guarantee that it will be maintained at least as long as the communication between the stations is occurring. Nevertheless, due to UTs’ mobility, significant changes in network topology can be observed. Thus, the objective of this mechanism is to find new routing path if the previous one is no longer available.

## Classification of routing protocols

Routing protocols can be classified into several groups depending on their principle and applications. The most common routing protocols are “topology based” protocols that select proper path according to changing network topology. The topology based protocols can be further divided into:

- Proactive (also known as “Table driven”) – The main principle of proactive protocols is to periodically update information regarding current network topology. All information is maintained in “tables” of individual nodes, hence we are called them as table driven protocol. The advantage is that when needed, proper path is already known and no delay is introduced by finding optimal routing path. The disadvantage is that significant signalling could be generated in order to maintain up to date information regarding network topology. The proactive protocols are mostly applicably only to small networks with low mobile UEs. The most common proactive protocols are DBF (Distributed Bellman-Ford), DSDV (Distance Source Distance Vector), WRP (Wireless Routing Protocol), or OLSR (Optimized Link State Routing).
- Reactive (also known as “On demand”) – The reactive protocols do not periodically maintain information regarding individual routing paths if no data exchange is in progress. The path selection is activated only if data transmission is about to be initiated. This way, scarce radio resources are saved since the signalling overhead is low. On the other hand, it could be time consuming to find appropriate path when needed and high delay could be introduced. Among reactive protocols belong, for example, DSR (Dynamic Source Routing), AODV (Ad-hoc On Demand Distance Vector), or SSR (Signal Stability Routing).
- Hybrid – These protocols combine the advantages of proactive and reactive protocols. The most common known hybrid protocols are ZRP (Zone Routing Protocol) or ZHLS (Zone-based Hierarchical Link State).

Besides topology based protocols, the proper path can be selected also by means of position-based routing protocols (e.g., DREAM (Distance Routing Effect Algorithm for Mobility)) or by means of power-aware protocols which preserves UEs’ battery life (e.g., CPC (Cluster Power Control)).