Topology Control in Wireless Ad Hoc Networks phần 6 potx

5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica The mobile CTR (3) Mobility: 6/10  [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 = v max = v, we have w.h.p. If p = 0, then r 0 >> Sqrt (ln n / n) w.h.p. n n p v p r p ! ln 521405.0 + = 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica The mobile CTR (4) Mobility: 7/10 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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Non-homogeneous TC Mobility: 8/10  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Mobile TC protocols Mobility: 9/10  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Mobility: a final observation Mobility: 10/10  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Open issues Open issues: 1/11  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica More realistic network models Open issues: 2/11  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica A more realistic network model Open issues: 3/11  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Impact of interferences Open issues: 4/11  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 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica More realistic node distributions Open issues: 5/11  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 . issue 5 th ACM MobiHoc – Tokyo, May 24, 2004 Istituto di Informatica e Telem atica Open issues Open issues: 1/11  Considerable body of research devoted to TC in ad hoc networks, but several. 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. 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