Advanced wireless networks 4g technologies phần 7 ppsx

JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 CLUSTERING PROTOCOLS 507 availability associated with each neighbor according to either a system default mobility profile or mobility information obtained through the network-interface layer protocol or physical-layer sensing. The precise methodology and the information required for the eval- uation of link availability is described later in this section. Finally, the neighbors, having discovered the unclustered status of the source node, automatically generate and transmit complete cluster topology information, which they have stored locally as a result of participating in the cluster’s intracluster routing protocol. This topology synchronization function is a standard feature of typical proactive routing protocols when a router discovers the activation of a link to a new router. The source node does not immediately send its topology information to any of the neighbors. Link activation A link activation detected by a clustered node that is not an orphan is treated as an intracluster routing event. Hence, the topology update will be disseminated throughout the cluster. Unlike reactive routing that responds after path failure, the dissemination of link activation updates is a key factor to an (c,t) cluster node’s ability to find new (c,t) paths in anticipation of future link failures or the expiration of the timer. Link failure The objective of a node detecting a link failure is to determine if the link failure has caused the loss of any (c,t) paths to destinations in the cluster. A node’s response to a link failure event is twofold. First, each node must update its view of the cluster topology and re-evaluate the path availability to each of the cluster destinations remaining in the node’s routing table. Second, each node forwards information regarding the link failure to the remaining cluster destinations. Expiration of c timer The c timer controls cluster maintenance through periodic execution of the intracluster routing algorithm at each node in a cluster. Using the topology information available at each node, the current link availability information is estimated and maximum availability paths are calculated to each destination node in the cluster. If any of the paths are not (c,t) paths, then the node leaves the cluster. Node deactivation The event of node deactivation encompasses four related events, namely, graceful deacti- vation, sudden failure, cluster disconnection and voluntary departure from the cluster. In general, each of these events triggers a response by the routing protocol. As a result, nodes determine that the node that has deactivated is no longer reachable. 13.5.3.3 Ad hoc mobility model The random ad hoc mobility model used in this section is a continuous-time stochastic process, which characterizes the movement of nodes in a two-dimensional space. Based on JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 508 AD HOC NETWORKS q 2 q 3 R 2 n R 5 n R n (t) R 6 n n' n' R 4 n R 1 n =V 1 n T 1 n q 4 q 6 q q 5 q 1 n n (a) (b) Figure 13.40 Ad hoc mobility model node movement: (a) epoch random mobility vectors; (b) ad hoc mobility model node movement. the random ad hoc mobility model, each node’s movement consists of a sequence of random length intervals called mobility epochs during which a node moves in a constant direction at a constant speed. The speed and direction of each node varies randomly from epoch to epoch. Consequently, during epoch i of duration T i n , node n moves a distance of V i n T i n in a straight line at an angle of θ i n . The number of epochs during an interval of length t is the discrete random process N n (t). Figure 13.40(a) illustrates the movement of the node over six mobility epochs, each of which is characterized by its direction, θ i n , and distance V i n T i n . The mobility profile of node n moving according to the random ad hoc mobility model requires three parameters: λ n ,μ n and σ 2 n . The following list defines these parameters and JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 CLUSTERING PROTOCOLS 509 states the assumptions made in developing this model: (1) The epoch lengths are identically, independently distributed (i.i.d.) exponentially with mean 1/λ n . (2) The direction of the mobile node during each epoch is i.i.d. uniformly distributed over (0, 2π) and remains constant only for the duration of the epoch. (3) The speed during each epoch is an i.i.d. distributed random variable (e.g. i.i.d. normal, i.i.d. uniform) with mean μ n and variance σ 2 n and remains constant only for the duration of the epoch. (4) Speed, direction and epoch length are uncorrelated. (5) Mobility is uncorrelated among the nodes of a network, and links fail independently. Nodes with limited transmission range are assumed to experience frequent random changes in speed and direction with respect to the length of time a link remains active between two nodes. Furthermore, it is assumed that the distributions of each node’s mobility character- istics change slowly relative to the rate of link failure. Consequently, the distribution of the number of mobility epochs is stationary and relatively large while a link is active. Since the epoch lengths are i.i.d. exponentially distributed, N n (t) is a Poisson process with rate λ n . Hence, the expected number of epochs experienced by node n during the interval (0,t) while a link is active is λ n t  = 1. These assumptions reflect a network environment in which there are a large number of heterogeneous nodes operating autonomously in an ad hoc fashion, which conceptually reflects the environment considered in the design of the (c,t) cluster framework. In order to characterize the availability of a link between two nodes over a period of time (t 0 , t 0 + 1), the distribution of the mobility of one node with respect to the other must be determined. To characterize this distribution, it is first necessary to derive the mobility distribution of a single node in isolation. The single node distribution is extended to derive the joint mobility distribution that accounts for the mobility of one node with respect to the other. Using this joint mobility distribution, the link availability distribution is derived. The random mobility vector can be expressed as a random sum of the epoch random mobility vectors R n (t) = N n (t)  i=1 R i n as shown in Figure 13.40(b). Let be R n (t) the resulting random mobility vector of a mobile node which is located at position [X(t 0 ), Y (t 0 )] at time t 0 and moves according to a random ad hoc mobility profile,  λ n ,μ n ,σ 2 n  . The phase of the resultant vector R n (t) is uniformly distributed over (0, 2π) and its magnitude represents the aggregate distance moved by the node and is approximately Raleigh distributed with parameter α n = ( 2t/λ n ) (σ 2 n + μ 2 n ) Pr ( θ n ≤ φ ) = φ/2π, 0 ≤ φ ≤ 2π (13.8) Pr[R n (t) ≤ r] ≈ 1 −exp(−r 2 /α n ), 0 ≤ r ≤∞ (13.9) The derivation ofthesedistributions is an applicationoftheclassictheoryofuniform random phasor sums [80] that applies central limit theorem to a large number of i.i.d. variables. JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 510 AD HOC NETWORKS Joint node mobility Based on the assumption of random link failures, we can consider the mobility of two nodes at a time by fixing the frame of reference of one node with respect to the other. This transformation is accomplished by treating one of the nodes as if it were the base station of a cell, keeping it at a fixed position. For each movement of this node, the other node is translated an equal distance in the opposite direction. So, the vector R m,n (t) = R m (t) − R n (t), representing the equivalent random mobility vector of node m with respect to node n, is obtained by fixing m’s frame of reference to n’s position and moving m relative to that point. Its phase is uniformly distributed over (0, 2π) and its magnitude has Raleigh distribution with parameter α m,n = α m + α n . Random ad hoc link availability If L m,n (t) = 1 denotes an active and L m,n (t) = 0 an inactive link, then for nodes n and m, link availability is defined as A m,n (t) ≡ Pr[L m,n (t 0 + t) = 1 |L m,n (t 0 ) = 1] (13.10) Note that a link is still consideredavailable at timet even if it experienced failuresduring one or more intervals (t i , t j ); t 0 < t i < t j < t 0 + t. By definition, if m lies within the circular region of radius R centered at n, the link between the two nodes is considered to be active. Depending on the initial status and location of nodes m and n, two distinct cases of link availability can be identified. (1) Node activation – node m becomes active at time t 0 , and it is assumed to be at a random location within range of node n. In this case we have A m,n (t) ≈ 1 −  1 2 , 2, −R 2 /α m,n    1 2 , 2, z  = e z/2 [ I 0 (z/2) − I 1 (z/2) ] α m,n = 2t  σ 2 m + μ 2 m λ m + σ 2 n + μ 2 n λ n  (13.11) (2) Link activation: node m moves within range of node n at time t 0 by reaching the boundary defined by R, and it is assumed to be located at a random point around the boundary. In this case we have A m,n (t) = 1 2  1 − I 0 (−2R 2 /α m,n ) exp(−2R 2 /α m,n )  (13.12) Random ad hoc path availability Let P k m,n (t) indicate the status of path k from node n to node m at time t.P k m,n (t) = 1if all the links in the path are active at time t, and P k m,n (t) = 0 if one or more links in the path are inactive at time t. The path availability π k m,n (t) between two nodes n and m at time t ≥ t 0 is given by the following probability π k m,n (t) ≡ Pr  P k m,n (t 0 + t) = 1   P k m,n (t 0 = 1)  =  (i, j)∈k A i, j (t 0 + t) (13.13) JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 CLUSTERING PROTOCOLS 511 If π k m,n (t) is the path availability of path k from node n to node m at time t, then path k is defined as an (c,t) path if and only if π k m,n (t) ≥ c (13.14) Node n and node m are (c,t) available if they are mutually reachable over (c,t) paths. An (c,t) cluster is a set of (c,t) available nodes. This definition states that every node in an (c,t) cluster has a path to every other node in the cluster that will be available at time t 0 + t with a probability ≥ c. Path availability cost calculation The above discussion demonstrates how the link availability can be calculated, thereby providing a link metric that represents a probabilistic measure of path availability. This metric can be used by the routing algorithm in order to construct paths that support a lower bound c on availability of a path over an interval of length t. The availabilities of each of the links along a path are used by the (c,t) cluster protocol to determine if the path is an (c,t) path, and consequently, if a cluster satisfies the (c,t) criteria. In order to support this functionality in an ad hoc network, the routing protocol must maintain and disseminate the following status information for each link: (1) the initial link activation time, t 0 ; (2) the mobility profiles for each of the adjacent nodes  λ i ,μ i ,σ 2 i  , i = m, n; (3) the transmission range of each of the adjacent nodes, R; (4) the event which activated the link: (a) node activation at time t 0 or (b) nodes moving into range of each other at time t 0 . Based on this information, any node in an (c,t) cluster can estimate, at any time τ , the availability of a link at time t + τ . This can be achieved because each node knows the initial link activation time t 0 ; hence, link availability is evaluated over the interval (t 0 , t +τ). Nodes can use conditional probability to evaluate the availability of their own links because they have direct knowledge of such a link’s status at time τ, whereas remote nodes do not. Specifically, for an incident link that activated at time t 0 , a node will evaluate the availability at time t, given that it is available at time τ ≥ t 0 . 13.5.3.4 Performance example A range of node mobility with mean speeds between 5.0 and 25.0 km/h was simulated in McDonald andZnati[74].The speeds during eachmobilityepochwere normally distributed, and the direction was uniformly distributed over (0, 2π). A node activation rate of 250 nodes/h was used. The mean time to node deactivation was 1 h. Nodes were initially randomly activated within a bounded region of 5 ×5 km. Nodes that moved beyond this boundary were no longer considered to be part of the ad hoc network and were effectively deactivated. (c,t) path availability was evaluated using Dijkstra’s algorithm. For each simulation run, data was collected by sampling the network status once per second over an observation interval of 1 h. The first 2 h of each run were discarded to eliminate transient effects, and each simulation was rerun 10 times with new random seeds. JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 512 AD HOC NETWORKS Simulation results for cluster size and cluster survival times are given in Figures 13.40 and and 13.41. Finally logical relationships among MANET network-layer entities is given in Figure 13.42. 13.6 CASHING SCHEMES FOR ROUTING A large class of routing protocols for MANETs, namely reactive protocols, employ some form of caching to reduce the number of route discoveries. The simplest form of caching is based on timeouts associated with cache entries. When an entry is cached, a timer starts. When the timeout elapses, the entry is removed from the cache. Each time the entry is used, the timer restarts. Therefore, the effectiveness of such a scheme depends on the timeout 0 5 10 15 20 25 0 5 10 15 20 25 Mean mobile speed (km/h) t = 1 min t = 5 min 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 0 5 10 15 20 25 Mean mobile speed (km/h) .4 0 5 10 15 20 25 0 5 10 15 20 25 t = 1 min t = 5 min 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 0 5 10 15 20 25 a = 0.4 a = 0.2 Mean cluster size (number-of-nodes) Mean cluster size (number-of-nodes) (a) (b) Figure 13.41 Simulation results: (a) cluster size (R = 1000 m); (b) cluster size (R = 500 m); (c) cluster survival (R = 1000 m); and (d) cluster survival (R = 500 m). (Reproduced by permission of IEEE [74].) JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 0 10 20 30 40 50 60 70 80 90 0 5 10 15 20 25 t = 1 min t = 5 min 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 0,4 = 0,2 0 10 20 30 40 50 60 70 80 90 Mean mobile node speed (km/h) Cluster age (min) Mean mobile node speed (km/h) Cluster age (min) t = 1 min t = 5 min 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 = 0.4 = 0.2 a a (c) (d) Figure 13.41 Continued. Routing protocol MANET Encapsulation Protocol Internet protocol Network-Interface Layer Routing protocol (α,t)-cluster algorithm protocols MANET encapsulation protocol Internet protocol Network-interface layer Routing table Figure 13.42 Logical relationships among MANET network-layer entities. 513 JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 514 AD HOC NETWORKS value associated with a cached route. If the timeout is well-tuned, the protocol performance increases; otherwise, a severe degradation arises as entries are removed either prematurely or too late from the cache. 13.6.1 Cache management A cache scheme is characterized by the following set of design choices that specify cache management in terms of space (cache structure) and time (i.e. when to read/add/ delete a cache entry): store policy, read policy, writing policy and deletion policy. The store policydetermines the structure of the routecache. Recently,two different cache structures were studied [81], namely link cache and path cache, and applied to DSR. In a link cache structure, each individual link in the routes returned in RREP packets is added to a unified graph data structure, managed at each node, that reflects the node’s current view of the network topology. In so doing, new paths can be calculated by merging route information gained from different packets. In the path cache, however, each node stores a set of complete paths starting from itself. The implementation of the latter structure is easier compared with the former, but it does not permit inference of new routes and exploitation of all topology information available at a node. The reading policy determines rules of using a cache entry. Besides the straightforward use from the source node when sending a new message, several other strategies are possible. For example, DSR defines the following policies: r cache reply – an intermediate node can reply to a route request with information stored in its own cache; r salvaging – an intermediate node can use a path from its own cache when a data packet meets a broken link on its source route; r gratuitous reply – a node runs the interface in the promiscuous mode and it listens for packets not directed to itself. If the node has a better route to the destination node of a packet, it sends a gratuitous reply to the source node with this new better route. The writing policy determines when and which information has to be cached. Owing to the broadcast nature of radio transmissions, it is quite easy for a node to learn about new paths by running its radio interface in the promiscuous mode. The main problem for the writing policy is indeed to cache valid paths. Negative caches are a technique proposed in Johnson and Maltz [82] and adapted in Marina and Das [83] to filter the writing of cache entries in DSR out. A node stores negative caches for broken links seen either via the route error control packets or link layer for a period of time of δt s. Within this time interval, the writing of a new route cache that contains a cached broken link is disabled. The deletion policy determines which information has to be removed from the cache and when. Deletion policy is actually the most critical part of a cache scheme. Two kinds of ‘errors’ can occur, owing to an imprecise erasure: (1) early deletion, a cached route is removed when it is still valid; and (2) late deletion, a cached route is not removed even if it is no longer valid. The visible effect of these kinds of errors is a reduction in the packet delivery fraction and an increase in the routing overhead (the total number of overhead packets) [84]. Late deletions create the potential risk of an avalanche effect, especially at high load. If a node replies with a stale route, the incorrect information may be cached by other nodes and, in turn, used as a reply to a discovery.Thus, cache ‘pollution’ can propagate fairly quickly [83]. JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 CASHING SCHEMES FOR ROUTING 515 Caching schemes in DSR All such schemes rely on a local timer-based deletion policy [81, 84]. The only exception has been proposed in Marina and Das [83]. They introduce a reactive caching deletion policy, namely, the wider error notification, that propagates route errors to all the nodes, forcing each node to delete stale entries from its cache. Simulation results reported in References [81, 83] show that performance of a timer- based caching deletion policy is highly affected by the choice of the timeout associated with each entry. In the path cache, for a value of timeout lower than the optimal one (i.e. early deletion), the packet delivery fraction and routing overhead are worse than caching schemes that do not use any timeout. In the link cache, late deletion errors increase the routing overhead while the packet delivery fraction falls sharply. The cache timeout can obviously be tuned dynamically. However, adaptive timer-based deletion policies have their own drawbacks. This policy suffers from late or early deletion errors during the transition time from one optimal timeout value to the successive one. So, the more the network and the data load are variable, the worse the performance will be. To reduce the effect of such imprecise deletions, the adaptive timer-based cache scheme has been combined with the wide error notification deletion technique and studied for DSR in Perkins and Royer [17]. According to such a combined scheme, routes that become stale before their timeout expiration are removed reactively from all the sources using that route. In this combined technique, however, two more points remain unresolved: (1) Owing to the reactive nature of the deletions, if a cache entry is not used, it remains in the cache, even if no longer valid, thus it can be used as a reply to a path discovery and (2) the effect of early deletions cannot be avoided. Caching schemes in ZRP The caching zone with radius k ∗ for a cache leader n is defined as the set of nodes at a distance of at most k ∗ hops from n. An active path is created as a result of the discover phase and it is composed of a set of nodes, referred to as active nodes, forming a path from a source node S to a destination node D. Cache leader nodes are a subset of the active nodes. The key consideration is to avoid the possibility that nodes can cache route information autonomously. Therefore, a cache leader n is the only node that is authorized to advertise route information inside its caching zone which is written into caches. On receiving the advertising message, a node proactively maintains a path to n so that it can be used as the next-hop node to any of the advertised routes. A cache leader is responsible for the validity of the advertised routes. Thus, it monitors such routes and forces each node in its caching zone to remove a route as soon as it becomes stale, so the deletion policy is proactive. Let us note that, if we consider k ∗ = k and each node of a ZRP interzone path as a cache leader, we get the same underlying zone structure of ZRP (this implies that each active node is a cache leader). However, more generally, a cache leader can decide to advertise paths only to those nodes located at a distance k ∗ < k, and not all active nodes need to be cache leaders. Implementation of C-ZRP For simplicity, the implementation assumes: (1) k = k*. (2) All active nodes act as cache leader nodes and vice versa. JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 516 AD HOC NETWORKS (3) Only paths to active nodes are advertised as external routes. (4) Caches are managed using explicit injection/deletion messages. (5) To stop redundant query threads, LT (loop back termination), QD2 (query detec- tion) and ET (early termination) redundant filtering rules are used which have been described earlier in this chapter. When a node S, in Figure 13.43, executes a route request for a node D, an interzone path from S to D is identified. A node B i belonging to an interzone path is an active node for the caching scheme. In Figure 13.43 an interzone path between S and D is formed by nodes b, e, p and t. Thus, an interzone path is also an active path. An interzone path is stored according to a distributed next-hop fashion, where the next-hop node is an active node. B i stores B i+1 as the next-hop active node for all the downstream nodes from B i+2 to B M+1 and B i−1 as the next-hop active node for all the upstream nodes from B 0 to B i−2 . These two Node S 2ab 2ac 1aaIZT 2ab 2ac 1aaIZT 5bD 4bt 3bp 2beEZT 5bD 4bt 3bp 2beEZT 2he 2hl 2fg 2ac 2aS 1hh 1ff 1aaIZT 2he 2hl 2fg 2ac 2aS 1hh 1ff 1aaIZT 4eD 3et 2epEZT 4eD 3et 2epEZT IZP=(ID,S,B2) Node B1 2hb 2mn 2mp 2hl 1mm 1hhIZT 2hb 2mn 2mp 2hl 1mm 1hhIZT 3pD 2pt 2bSEZT 3pD 2pt 2bSEZT 2qt 2me 1qq 1mmIZT 2qt 2me 1qq 1mmIZT 2tD 2eb 3eSEZT 2tD 2eb 3eSEZT 2rD 2qp 1rr 1qqIZT 2rD 2qp 1rr 1qqIZT 2pe 3pb 4pSEZT 2pe 3pb 4pSEZT 2rt 1rrIZT 2rt 1rrIZT 5tS 4tb 3te 2tpEZT 5tS 4tb 3te 2tpEZT IZP=(ID,B1,B3) Node B2 IZP=(ID,B2,B4) Node B3 IZP=(ID,B3,D) Node B4 Node D S c o a f g h i k n m q r D 1 b =B 2 e=B 3 p =B 4 t=B Figure 13.43 An example of values of data structures used in C-ZRP, k = 2. 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SENSOR NETWORKS Table 14.1 Frequency bands available for ISM applications Frequency band Center frequency 676 5– 679 5 kHz 13 553–13 5 67 kHz 26 9 57 27 283 kHz 40.66–40 .70 MHz 433.05–434 .79 MHz 902–928 MHz 2400–2500 MHz 572 5–5 875 MHz 24–24.25 GHz 61–61.5 GHz 122–123 GHz 244–246 GHz 678 0 kHz 13,560 kHz 27, 120 kHz 40.68 MHz 433.92 MHz 915 MHz 2450 MHz 5800 MHz 24.125 GHz 61.25 GHz 122.5 GHz 245 GHz Data Link... require wireless ad hoc networking techniques Although many protocols and algorithms have been proposed for traditional wireless ad hoc networks, as described in Chapter 13, they are not well suited for the unique features and application requirements of sensor networks To illustrate this point, the differences between sensor networks and ad hoc networks (see Chapter 13, and Advanced Wireless Networks: 4G. .. 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[D s (t)+ΔD s (t)] Δ Ω f (a) (b) Figure 13. 47 Token curves. JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 DISTRIBUTED QoS ROUTING 525 13 .7. 7 Forwarding the received tokens 13 .7. 7.1 Candidate neighbors If. route the traffic to another path with a lower cost. 13 .7. 1 Wireless links reliability One element of the cost function will be reliability of the wireless links. The links between the stationary or. survival (R = 500 m). (Reproduced by permission of IEEE [74 ].) JWBK083-13 JWBK083-Glisic February 23, 2006 5:53 Char Count= 0 0 10 20 30 40 50 60 70 80 90 0 5 10 15 20 25 t = 1 min t = 5 min 0 5 10 15 20 25 30 35 40 45 0