Dynamic Routing in Mobile Ad Hoc Network

報告者：呂佐鴻指導教授：李鴻璋

What is Ad Hoc? In an ad hoc network, a set of wireless

stations communicate directly with one another without using an AP or any connection to a wired network.

Position-based routing protocol use the geographic position of nodes to make routing decisions.

Location information can be obtained through GPS or some other type of positioning mechanism.

Position-based routing can be divided into two distinct tasks： The location service (discovering the position of th

e destination) The actual routing of data packets (based on locat

ion information)

Position or geographical routing thus allows radios to operate nearly stateless; nodes neither have to store routing tables nor transmit messages to keep routing tables up-to-date.

One potential weakness of this family of location-aware protocols is the dependence on GPS for obtaining one’s location.

There exist three common routing strategies for position-based routing – greedy forwarding, directed flooding, and hierarchical routing.

A node forwards a given packet to one (greedy forwarding) or more (directed flooding) one-hop neighbors that are located closer to the destination than the forwarding node itself.

Distance routing effect algorithm for mobility (DREAM)

DREAM is based on the observation that the greater the distance separating two nodes.

Source and intermediate nodes calculate the direction of destination D and, based on the mobility information about D, associate an angular range with the routing entry to D.

Distance routing effect algorithm for mobility (DREAM)

The direction toward the destination is determined by means of the so-called “expected region”.

A message is forwarded to all neighbors whose direction belongs to the selected range.

Distance routing effect algorithm for mobility (DREAM)

δ

Expectedregion

D

r

S

Figure 2.22 Directed flooding in DREAM

Distance routing effect algorithm for mobility (DREAM)

The neighboring hops repeat this procedure using their view for D’s position.

Nodes in DREAM broadcast position update messages to update the position information by other nodes.

Distance routing effect algorithm for mobility (DREAM)

A node can control the accuracy of its position information available to other nodes by

1) Modifying the frequency with which it sends position updates

2) Indicating how far a position update packet is allowed to travel before being discarded.

Distance routing effect algorithm for mobility (DREAM)

The distance effect is a reasonable paradigm when intermediate hops are allowed to update the position information carried in a packet.

DREAM works well in combination with an all-for-all location service that provides more accurate information close to the destination.

Distance routing effect algorithm for mobility (DREAM)

This reduces the size of expected region and thus the area in which the packet is flooded.

DREAM uses directed flooding to limit the flooding to the direction of the destination.

Directed flooding restricts routing redundancy which may prevent the routing protocol from discovering the shortest paths.

Location-aided routing (LAR) protocol

LAR assumes that data source node knows the location and roaming speed of the destination node.

The LAR protocol basically uses restricted flooding to discover routes.

Specifically, only the hosts in the request zone help forward route-searching packets.

Location-aided routing (LAR) protocol

S initiates a route discovery process, it should include the coordinates of the request zone in the packet.

A receiving host simply needs to compare it own location to the request zone to decide whether or not to rebroadcast the route-searching packet.

Location-aided routing (LAR) protocol

Request zone

A (Xs,Yd+R)

S (Xs,Ys) C (Xd+R,Ys)

B (Xd+R,Yd+R)

D (Xd,Yd)

Expected zone

R=v(t1-t0)

Network space

Figure 2.23 Request and expected zones in the LAR protocol

Location-aided routing (LAR) protocol

When D receives the route request packet, it sends a route reply packet back to S. When S receives the reply, the route is established.

If the route cannot be discovered within a specified time-out period, S can initiate a new route discovery with an expanded request zone.

Location-aided routing (LAR) protocol

One potential weakness of the protocol is that prior knowledge of the destination’s location may not always be available at the source.

The protocol may require mobiles to communicate their location more frequently, or enlist the aid of a distributed location service.

Location-aided routing (LAR) protocol

Furthermore, underlying the location-aided routing protocol is the notion that a route to the destination can be found by search in the general direction of the destination.

Terrain features such as buildings, hills, and foliage are, however, not considered.

Location-aided routing (LAR) protocol

z

Request zone

Expected zone

Network space

Figure 2.24 Route discovery with LAR in the presence of obstructions

C

J

K

D r

S