Topology Control in Wireless Ad Hoc Networks phần 5 ppsx

The CBTC protocol 2 The CBTC protocol produces a connected communication graph if ρ ≤ 2π/3  The obtained communication graph is made symmetric by adding the reverse edge to every unidi

The CBTC protocol (2)

 The CBTC protocol produces a connected communication graph if

ρ ≤ 2π/3

 The obtained communication graph is made symmetric by adding the

reverse edge to every unidirectional link

 A set of optimizations are also proposed, that prune energy-inefficient

edges while not impairing connectivity and symmetry

 Drawback: directional information required

 Several variants of CBTC have been introduced [Bahramgiri et

al.02][Huang et al.02]

Neighbor-based TC

 Other class of TC protocols based on the k-neighbors graph, i.e the graph in

which every node is connected to its k closest neighbors

 First neighbor-based TC protocol: the LINT protocol of

[RamanathanRosales-Hain00]:

– Basic idea: try to keep the number of neighbors of every node within a low and high

threshold centered around an “optimal value”

– Number of neighbors estimated overhearing control and data messages

– Drawbacks: the “optimal value” is not characterized; the estimation of the number of

neighbors might be inaccurate (silent neighbors are not detected); connectivity is not guaranteed

 KNeigh [Blough et al.03a]:

– Goal: maintain the number of physical neighbors equal to (or slightly below) k

– The graph produced is symmetric

– On the average, it is 20% more energy-efficient than CBTC

– Drawback: based on distance estimation; connectivity only w.h.p.

The optimal value of k

Optimal value of k for increasing n

0 1 2 3 4 5 6 7 8 9 10

n

Optimal value of k for increasing values of n

(from [Blough et al 03])

Remark: setting k = 9 guarantees connectivity w.h.p for values of n ranging

from 50 to 500

Sample topologies

Sample topologies generated in case of CTR topology control (left), and after

KNeigh (center) and CBTC (right) execution The number of nodes is n = 100

(from [Blough et al 03])

The XTC protocol

 XTC is a very recent protocol by the same author of CBTC

[WattenhoferZollinger04]

 Basic idea (similar to KNeigh):

– at the beginning, every node orders its neighbors (set of nodes in the

maximum transmitting range) according to some criterion (e.g., link quality)

– then, every node transmits its order at maximum power

– based on its own order, and on the orders of its neighbors, every node

determines the set of “logical” links according to a simple rule

 XTC always produces a connected communication graph (provided the

original graph is connected)

 Drawback: no upper bound on the number of physical neighbors

Mobile networks

 Which is the impact of mobility on TC?

protocol must be re-executed periodically in response to node mobility

the “message efficiency” of the protocol is fundamental: protocols that exchange few messages to maintain the topology are needed

Mobility models

 Impact of mobility on TC depends on the mobility pattern

 Mobility models:

community Every node chooses uniformly at random a destination in [0,1] 2 , and moves

towards it along a straight line with velocity chosen at random in [v min ,v max] When it

reaches the destination, it rests for a time t pause, then it starts moving according to the same rule

[, and velocity chosen at random in [v min ,v max] After a randomly chosen time, the node chooses a new direction and velocity

in a disk centered around the current node position

RWP and Random Direction Mobility

RWP mobility (left) and Random Direction mobility (right) In case of RWP

mobility, nodes tend to cross the center of the deployment region (border effect)

The mobile CTR

 With homogeneous topology control, message overhead is not an issue, since the nodes’ transmitting range is set at the design stage and cannot

be change dynamically

 However, the node spatial distribution generated by the mobility pattern could be an issue

 For instance, it is known that the RWP model generates non-uniform

node spatial distribution [Bettstetter et al.03]

 On the other hand, the node distribution generated by random direction and Brownian mobility is very close to uniform [Blough et al.02b]

The mobile CTR (2)

Node distribution generated by the RWP model with different values of the pause time

(from [Blough et al.02b])

Remark: the fact that the node spatial distribution generated by RWP

mobility is not uniform should be carefully considered when simulating

mobile ad hoc networks