Topology Control in Wireless Ad Hoc Networks phần 6 potx

The mobile CTR 3 [SantiBlough02]: it is shown through simulation that a relatively modest increase about 21% of the transmitting range with respect to the critical value is sufficient t

The mobile CTR (3)

 [SantiBlough02]: it is shown through simulation that a relatively modest increase (about 21%) of the transmitting range with respect to the critical value is sufficient to ensure full connectivity in case of RWP mobility

 Simulation results also show that the transmitting range can be

considerably reduced (in the order of 35 - 40%) if the requirement for

connectivity is only on 90% of the network operational time (giant

component)

 An analytical result [Santi04]: if we denote with r p the CTR with RWP

mobile networks when the pause time is p>0 and v min = vmax= v, we have

w.h.p If p = 0, then r0 >> Sqrt (ln n / n) w.h.p.

n

n p

v

p

r p

!

ln

521405

0 +

=

CTR in case of stationary and RWP mobile networks

(from [Santi04])

 Remark: note the “threshold phenomenon”: for n ≤ 50, r p when p= 0 is smaller

than the CTR for the stationary case, while when n > 50 the situation is reversed

Non-homogeneous TC

 In case of non-homogeneous TC: more relevant effect of mobility is the message overhead needed to maintain the desired topology

 Overhead depends on the frequency with which the reconfiguration

procedure is executed, which in turn depends on:

– The mobility pattern

– The properties of the topology generated by the protocol

 Example: MST-based vs k-neighbor based TC

– The message overhead needed to build the MST is much larger than that

needed to build the k-neighbors graph

– Given the same mobility pattern, the MST should be reconfigured much more

frequently than the k-neighbors graph k-neighbor based TC is more resilient to mobility than MST-based TC

 In order to be resilient to mobility, a TC protocol should be based on local information only

 Many protocols presented in the literature enjoy this property, but only

some of them have been adapted to explicitly deal with node mobility

– [Li et al.01a]: a reconfiguration protocol for CBTC that deals with mobility is presented

– [RodopluMeng99]: the authors discuss how their protocol can be adapted to the mobile scenario

– MobileGrid [LiuLi02] and LINT [RamanathanRosales-Hain00] k-neighbors

based protocols are explicitly designed to deal with mobility

Mobility: a final observation

 More subtle effect of mobility on k-neighbors based TC protocols:

– Non-uniform node distribution in case of RWP mobility

 This fact should be carefully considered in setting the “optimal value” of k

 In general, we might expect that the “optimal value” of k in presence of

RWP mobility is larger than in the stationary case (similar to the CTR

case)

 How much larger? Open issue

 Considerable body of research devoted to TC in ad hoc networks, but

several aspects have not been carefully investigated yet

 We can classify these “open fields” for research into three areas:

– More realistic (network and energy) models

– More accurate analysis of mobile networks

– Considering the effect of multi-hop data traffic

More realistic network models

 Ad hoc network model used in this presentation is widely accepted, but it is a very idealized model of a real wireless network

 Main limitation of this model: assumption that the radio coverage area is a perfect circle

 In realistic scenarios: radio coverage area influenced by many factors (obstacles,

buildings, existing infrastructure, weather conditions, etc.), and it is hardly regular

 Including too many details in the network model would make it extremely

complicated and scenario dependent

 On the other hand, current network model is maybe too simplistic, at least to

derive quantitative results

 An example of a more realistic wireless channel model could be the

following:

– The occurrence of wireless links between units is probabilistic:

o For instance, we might have a link with probability 1 if δ(u,v) < c, for some value of

c, and with probability p(δ(u,v)) < 1 otherwise

– We might assume that the link probability is a non-increasing function of the

distance

– With this model, the radio coverage area in general is not regular

 A similar model has been proposed in [Farago02]

Open issue: characterize network connectivity under this network model

Impact of interferences

 Another possibility for more realistic models is considering interferences

between nodes

 Preliminary step in this direction: [Dousse et al.03]

– A bi-directional link between u and v exists if the signal to noise ratio at the

receiver is larger than some threshold

– The noise is the sum of the interferences of other nodes and background

noise

– The authors analyze the impact of this wireless link model on network

connectivity

 Further investigation in this direction is needed

 Most of the analytical results presented in the literature assume a

uniform node distribution

 This assumption seems reasonable in some settings, but it is unrealistic

in many scenarios (e.g., RWP mobility)

 Open issues: define “realistic” node distributions, and analyze network

connectivity (and other network properties) using these distributions