Mobile data collector strategy for delay-sensitive applications over wireless sensor networks

1. Mobile data collector strategy for delay-sensitive applications over wireless sensor networks 无线传感器网络中的移动数据收集策略

2. 摘要 需要快速响应的应用，如对突发事件的处理对无线传感器网络提出了挑战。路由协议必须提供快速和可靠的技术实现数据的传输。无线传感器网络中大多数的路由策略都是采用静态节点收集整个网络重的数据。这种方法会导致汇聚节点周围区域很高的通信负担。在汇聚节点周围的节点也会比网络中的其他节点又更高的访问频率。因此，这些节点会消耗更多的能量，尤其是在规模较大的网络中，会出现严重的拥塞现象。本文提出了移动数据收集节点来解决上述问题。接收到信号的节点加入MDC簇，并且更新路由信息，来中继数据包到MDC。传感器节点用信号强度实现一个简单但高效的换手。

3. Applications that require fast response time such as emergency preparedness and hostile environment surveillance pose challenging obstacles to wireless sensor network (WSN) protocols. A routing protocol must provide fast and reliable techniques for data propagation.Most routing solutions for WSNs utilize static sinks to collect data from the entire network. This approach results in high traffic load in the sink’s vicinity. The nodes located near the sink will be more requested than other nodes in the network. Therefore, these nodes will consume more energy and face high congestion in a large scale network. In this paper, we propose a solution to the problem of deploying mobile data collectors in order to alleviate the high traffic load and resulting bottleneck in a sink’s vicinity caused by static approaches. Our proposed MDC/PEQ protocol employs mobile data collectors (MDCs) that broadcast beacons periodically. Sensor nodes that receive the beacon will join the MDC’s cluster and update their routing information in order relay data packets to the MDC. Sensor nodes use the signal strength of the beacon in order to perform a simple but efficient route re-configuration (handoff).

4. 1.简介 无线传感器网络可以看成是很多小的无线设备的集合，这些小的无线设备可以自组织在一个自组织网络中，他们可以感知在他们感知范围内的环境，但是他们的能量，处理能力和通信能力都非常有限，在采集数据后，传感器节点需要敬爱那个数据传送给基站，基站通过程序处理这些数据。然而无线传感器网络一般都没有很完善的组织机制，所以传感器节点必须自己组织自己以产生到达汇聚节点的路由。因此，WSN多采用多跳距的数据传播方法来中继数据到静态的汇聚节点。 Wireless sensor networks (WSNs) can be seen as a large collection of small wireless devices that can organize themselves in an ad hoc network capable of sensing environmental conditions within their range and have constrained energy, processing and communication resources. After the sensing phase, a sensor node need to transmit the data to a base station, where an application will process the data. However, a wireless sensor network usually lacks infrastructure and sensor nodes must organize themselves in order to create routes that lead to a sink. Therefore, WSNs perform multihop data propagation in order to relay data to a static base station (or data sink).

5. 在大规模的WSN中，传感器节点可以用一种基于簇的方法。该方法中簇首负责从簇内的其他节点收集数据。分层的或基于簇的路由在提供网络的可扩展性和提高能量性能中被广泛应用。由于簇首节点的能量消耗较多，经常会出现能两耗尽的问题，因此我们采用随即选用簇首的方法，簇内的每一个节点都可以充当簇首，以实现能耗的均衡。在大规模的WSN中，传感器节点可以用一种基于簇的方法。该方法中簇首负责从簇内的其他节点收集数据。分层的或基于簇的路由在提供网络的可扩展性和提高能量性能中被广泛应用。由于簇首节点的能量消耗较多，经常会出现能两耗尽的问题，因此我们采用随即选用簇首的方法，簇内的每一个节点都可以充当簇首，以实现能耗的均衡。 In large scale WSNs, sensor nodes may use a clusterbased approach in which a cluster-head is responsible for collecting data from its cluster nodes. Hierarchical or cluster- based routing is a well-known approach that aims at providing scalability and energy efficiency in WSNs. A randomized selection of cluster-heads is usually used as a solution to the problem of energy depletion of cluster-head nodes [13]. In this technique, any sensor node can become a cluster-head, and the task of being a cluster-head is alternated periodically in order to provide load balancing.

6. 本文提出了一种低延时，高可靠性的移动数据收集策略用于实现WSN在对延时很敏感的应用中。采用MDC/PEQ protocol employs mobile data collectors (MDCs)间隔地发出一个信标。收到该信标的节点会加入到MCD的簇内，并且更新各自的路由信息来中继数据包到对应的,MDC。传感器节点用该信标的信号强度来实现高效的换手。我们的方法由于减少了数据包传输所经过的跳距数目，因而缓解了数据量过载的现象，将少了延时，提高了传输的可靠性。 In this paper, we propose a low-latency and reliable mobile data gathering solution for delay-sensitive applications for WSNs. Our proposed MDC/PEQ protocol employs mobile data collectors (MDCs) that broadcast beacons periodically. Sensor nodes that receive the beacon will join the MCD’s cluster and update their routing information in order to relay data packets to the corresponding MDC. Sensor nodes use the signal strength of the beacon in order to perform a simple but efficient route re-configuration (handoff). Our strategy contributes to reducing packet delivery delay and increasing reliability with little or no overhead by reducing the number of hops a data packet have to traverse.

7. 2.相关工作 如图所示。该方法中只采用一个移动的节点，并且沿着直线前进。前进过程中，不断的向周围节点发送信息，该信息中包含了移动节点感兴趣的信息，当节点接收到信息后，就和自己采集到的相对比，当匹配上时，就将该数据发给移动的节点，如果不匹配，就会将移动节点发送的信息转发给下一个相邻节点。但是这种方法中移动节点很同意移出传感器节点的范围，从而造成数据包的丢失。因此通过采用移动节点发回的确认消息来保证数据包的正确接收。

8. 3.无线传感器网络模型 考虑在区域A中采用N个传感器节点。节点被随即的分散在区域A中，还有一个汇聚节点，负责数据的收集，在传感器网络中充当一个网关的角色。传感器节点用无线电波与和它相聚以跳距的节点通信。每一个节点包括汇聚节点都被当作是静止的，因此采用多跳距的方式传输数据，这是静态节点的传感器网络的典型代表。 我们提出的CPEQ/PEQ，采用几个动态节点来扩张传统的网络，并将其作为基本的路由策略。PEQ路由协议用一个汇聚节点初始化路由，发送第一个感兴趣的信息。该信息中携带了该汇聚节点的ID，和跳距范围h，h代表了源节点的跳距层次。汇聚节点先广播INIT_SETUP ，并令h=1.每一个传感器节点都携带自己的跳距级hs,代表了该节点距汇聚节点的距离，和一张路由表其中包含汇聚接待你的ID和目标节点的地址，所谓的目标节点即是要达到汇聚节点要经过的节点。在接收到INIT_SETUP信息后，传感器节点将h与自己的hs想比较，来决定是否要更新路由信息，发送信息或是将将数据包丢掉。如果h<hs，节点就会更新跳距级，令hs=h，根据INIT_SETUP中的信息更新目的地址和设置汇聚节点的ID。并且令h=h+1，然后广播INIT_SETUP。其他情况下，传感器节点只是简单的将数据包丢失。在整个过程的结尾，就形成了一个属性的网络。其中汇聚节点是跟接节点，并且配置了其余节点的跳距级别。

9. 4.移动数据收集策略 想监测区域A引入M个移动数据收集节点，以最大为v的速度移动，并且可以与其他的传感器节电通信。本方法，MDC节点根据两种不同的移动模式随即的移动，一个是Random Walk Mobility Model和Boundless Simulation Area Mobility Model (BSMM)。在Random Walk Mobility中，节点随机的移动，并且他的移动与其他节点无关。BSMM是一个基于组的移动模式，他代表了一个较为平滑的移动方案。本方法中每一个MDC都是一个簇首。先开始一个CPEQ簇的确认过程，这样所有的加入该簇的节点都回获得如何到达MDC节点的合适的路由信息。我们的目标是研究无线传感器网络在对反应时间敏感的情况下的应用，所以我们采用了一个混合策略，即引入一个静态节点。这样，传感器节点不许哟啊等到MDC接近之后才可以中继数据。每一个节点保持一个跳距级别hm，代表与距离最近的MDC的距离。节点可以根据hm和hs决定将数据发送到哪一个节点（移动节点或静态节点）。

10. 4.1 簇的确认 MDC/PEQ的簇的确认过程与CPEQ相同，只是在非移动节点，或汇聚节点接收到信标信号后，先检测信号强度SS，然后在更新路由信息。 4.2路由的保持和换手 每一个移动数据收集点MDC每隔Tb秒广播一个BEACON数据包。该数据包包含了MDC的ID，生存时间TTL，和跳距h。图三描述了节点接收到BEACON数据包后的数据流图。我们根据节点的通信范围R，确定接收阈值Rxthresh。根据信号的强度，节点会根据以下的规则做出反应。 (1) if (SSP Rxthresh): the node is receiving strong signal from the MDC. After that, the node detects if the sender of the BEACON is its current MDC (based on MDC ID and MDC IDcurr): 节点接收MDC发送的强信号，然后节点检测发送BEACON的节点是否是当前的MDC，这是根据MDC的ID号来判断的。

11. (1.a) (MDC ID==MDC IDcurr): the BEACON was sent from the node’s current MDC, thereby the node does not need to perform any routing changes. It will only update (SScurr = SS), in which SScurr represents the most up-to-date signal strength received from its currentMDC, and SS is the signal strength measured from the received BEACON message. The SScurr value is kept by the node and it will be used when replying to nodes that might request a route to an MDC; 如果MDC ID==MDC Idcurr，则BEACON是又当前的MDC发送的，因此节点不需要改动路由信息，只需要令SScurr = SS，Sscurr代表了从当前MDC接收到的最新的信号强度，SS是从BEACON信息中得到的信号强度。Sscurr的值会被节点保留可以被用来响应其他的需要通过该节点到MDC的节点。

12. (1.b) (MDC ID =！ MDC IDcurr): if the beacon was sent from a different MDC, the node silently drops the packet because it is already in a cluster. In case the node does not belong to any cluster, i.e., by checking if (hm==_1)), or in the case the node already belongs to a cluster but (hm > h), which means that there is a shorter path to an MDC, the node joins the MDC’s cluster by updating its routing table and state variables as describe in Fig. 3. Thereafter, the node broadcasts a CLU_CFG message to its neighbors if TTL has not reached 0. 当MDC ID不等于MDC Idcurr ，如果BEACON是从另一个不同的MDC发送过来的，节点就丢包，因为它已经属于一个簇了。加入节点不属于任何一个簇，通过检查hm是否等于-1，或者节点已经属于一个簇，但是hm>h，这意味着存在一个更短的路径到达一个MDC，节点就加入这个新的MDC簇，并且更新路由表，按照表三来设置其参数。然后，节点广播一个CLU_CFG信息给他的的邻节点，如果生存期TTL还没有到0 。

13. (2) if (SS < Rxthresh): the node is within the communication range of an MDC, but the signal strength is below the allowed threshold. This can have two meanings: the MDC is moving away from the node; or the MDC has just entered the node’s communication range. Therefore, the node must find out the context of this BEACON message: 如果SS < Rxthresh 节点在MDC的通信范围你，但是信号强度低于允许的阈值。这又两种意思：MDC正在远离该节点，或者MDC刚刚进入该节点的通信区域。因此，节点必须BEACON的相关信息：

14. (2.a) (MDC ID==MDC IDcurr): the originator of the beacon was the node’s current MDC, but the node is receiving low signal strength. Therefore, the node must update its routing information and start looking for another MDC. The node removes the routing entry for that MDC and updates its parameters. Usually, if the MDC is moving out of the node’s range, the node will probably find out that a neighbor has a route to that same MDC. The process of finding another route to an MDC is initiated by the node broadcasting a Low Signal Strength message LSS. An LSS message also informs other nodes that its originator node does not have a route to that specific MDC anymore. The LSS message is sent only after there is a change in the node’s routing information; BEACON信息的发出者是当前的MDC，但是节点收到的信号强度很低。因此，该节点必须更新路由信息，开始寻找一个新的MDC。节点将当前MDC的路由移除，更新它的参数。经常，如果一个MDC正在移出一个节点的通信区域时，节点就很有可能发现它相邻的节点的路由也是到达相同的MDC，寻找新的MDC路由的过程如下：节点先广播一个低信号强度的信息LSS，该LSS消息通知其他节点，该节点已经不含有到达原MDC的路由。该LSS消息只有在路由信息改变后才会别发出。

15. (2.b) (MDC ID！=MDC IDcurr): a beacon with low signal strength was sent from a differentMDC than the node’s current one, thereby no routing maintenance is necessary. The node silently drops the packet. MDC ID！=MDC Idcurr，代表BEACON信号强度很低的原因是，该信号是由其他的给该传感器节点的当前MDC发出的，因此没有必要保持路由，所以节点将数据包丢掉。 After a sensor node receives a BEACON message, it might send a CLU_CFG message (see rule (1.b)). A sensor node that receives a CLU_CFG message decides whether to join the cluster, as depicted in Fig. 4. For that, the node will check if it already belongs to a cluster or if it only needs to update its routing information. The sensor node is able to determine what to do next by checking its hop level hm according to the following rules: 在一个节点接收到BEACON时，它可能发送一个CLU_CFG 消息。收到该消息的节点要决定是够加入该簇，如图4所示。节点要检查它是够已经属于该簇或者需要更新路由信息。传感器节点能够决定接下来做什么，是根据检测它自己的hm来判断的：

16. (3) if (hm==_1): the node does not have any MDC in its routing table, thereby it joins the cluster and creates an entry by setting up the address of the CLU_CFG message originator as the destination to the MDC in its routing table; 如果hm=-1 节点没有达到任何MDC的路由表，因此它加入一个簇，并将发给其CLU_CFG消息的节点作为它的达到MDC的目的地址，并将其加入路由表。 (4) if (hm > h): the node has a route to an MDC, but there is a shorter path to reach the same or another MDC. The sensor node updates its routing information and its parameters. Thereafter, the node forwards CLU_CFG if TTL has not reached 0; 节点又达到一个MDC的路由，但是有一条更短的路径到达同一个MDC或另外一个MDC。传感器节点更新路由信息和它的参数，然后，发送一个CLU_CFG,如果TTL还没有到0. (5) Otherwise: the node has already the shortest path to an MDC. It silently drops the packet. 其余的情况，节点已经有了一个到达MDC的最短路径，则他据丢掉数据包 。

17. 当收到BEACON时，并且检测到该信号强度SS低于Rxthresh，传感器节点就广播一个LSS消息，通知周围节点，如规则（2.a所示）。当收到BEACON时，并且检测到该信号强度SS低于Rxthresh，传感器节点就广播一个LSS消息，通知周围节点，如规则（2.a所示）。 When a BEACON is received and the measured signal strength (SS) is below the reception threshold Rxthresh, the sensor node might broadcast a low signal strength (LSS) notification (see rule (2.a)). 当传感器节点收到BEACON时，它就设置定时器的定时时间为Tbeacon，用来接收下一个BEACON。如果在定时器超时时，仍没有收到BEACON，传感器节点就需要寻找另一条路径，这是它就发送一个LSS消息，其他节点一旦接收到LSS消息，就判断发出该消息的节点是否是其路由表中的通往MDC的目的地址。然后决定是否需要更新路由信息。如图5所示，判断过程要遵循以下几条标准： When a sensor node receives a BEACON message, it sets a timeout timer Tbeacon for receiving the next BEACON. If this timeout expires, the sensor node need to find another route by sending an LSS message. Upon receiving an LSS message, the node verifies if the source node is in its routing table as a destination to reach an MDC, so as to decide if it should update its routing information, as shown in Fig. 5, according to the following rules:

19. 如果Next_hop==LSS_src，节点检查路由表，发现发出LSS的源节点是其到达MDC所要经过的节点，但此时该节点以不可用，在这种情况下，节点必须更新自己的路由表，将源节点移除，并且按照图5所示来设置相关的参数。如果Next_hop==LSS_src，节点检查路由表，发现发出LSS的源节点是其到达MDC所要经过的节点，但此时该节点以不可用，在这种情况下，节点必须更新自己的路由表，将源节点移除，并且按照图5所示来设置相关的参数。 (6) if (Next_hop==LSS_src): the node checks its routing table and realizes that the LSS source is a route to the MDC, i.e., the route the node was using to reach an MDC in no longer valid. In this case, the node must update its routing table by removing the entry and resetting its parameters according to Fig. 5;

20. (7)如多Next_hop==_1，表明该节点没有通往MDC的路由，因此它只是将数据包丢弃即可。(7)如多Next_hop==_1，表明该节点没有通往MDC的路由，因此它只是将数据包丢弃即可。 (7) if (Next_hop==_1): the node does not have an entry in its routing table that leads to an MDC. Therefore, it silently drops the packet; 其他情况：节点确实又达到某个MDC的路径。在这种情况下，节点回应一个LSS_REPLY消息给发出LSS的源节点。当源节点收到LSS_REPLY时，该节点就增加源节点到它通往MDC的路由表中。 (8) Otherwise: the node does have an entry in its routing table that leads to an MDC. In this case, the node replies with an LSS_REPLY message to the LSS source node. When the node receives an LSS_REPLY, it will retrieve and add the source address as a route to a new MDC in its routing table. An LSS packet carries the ID of the MDC that originated the BEACON, thereby if the same MDC receives an LSS it will not reply in order to avoid message loops.

21. (7)如多Next_hop==_1，表明该节点没有通往MDC的路由，因此它只是将数据包丢弃即可。(7)如多Next_hop==_1，表明该节点没有通往MDC的路由，因此它只是将数据包丢弃即可。 (7) if (Next_hop==_1): the node does not have an entry in its routing table that leads to an MDC. Therefore, it silently drops the packet; 其他情况：节点确实又达到某个MDC的路径。在这种情况下，节点回应一个LSS_REPLY消息给发出LSS的源节点。当源节点收到LSS_REPLY时，该节点就增加源节点到它通往MDC的路由表中。 (8) Otherwise: the node does have an entry in its routing table that leads to an MDC. In this case, the node replies with an LSS_REPLY message to the LSS source node. When the node receives an LSS_REPLY, it will retrieve and add the source address as a route to a new MDC in its routing table. An LSS packet carries the ID of the MDC that originated the BEACON, thereby if the same MDC receives an LSS it will not reply in order to avoid message loops.

22. 4.3 数据传输 如前面讨论的，我们采用一个混合方法，即在含有移动节点的传感器网路中，加入一个动态节点，传感器节点可以将数据发送到动态节点MDC，也可以发送到该静态的汇聚节点。我们关注的是对延时很敏感，需要及时相应的应用。如果只采用MDC，则一个传感器节点不读不对收到的数据进行缓冲，直到MDC移动到该节点的附近，因此这回产生很高数据包延时。还有一种解决方法是不论MDC是否远离该传感器节点，传感器节点都保持该路由信息，但这种不可扩展的方法会产生很高的信息重叠，因为很多节点随着MDC的移动都要更新他们的路由信息。本文的方法是，如果一个传感器节点不再簇的范围内，即如果它的hm > TTL，在一个MDC移动后，它必须从它的路由表中移除该路径信息。这样，我们的方法不会造成信息的重叠，在MDC的移动过程中

23. As discussed earlier, we use a hybrid approach in which data can be relayed to a mobile data collector as well as to a static sink. We focus in delay-sensitive applications that require fast response time and reliability. If only MDCs were used, a sensor node would have to buffer its sensed data until an MDC was nearby, thereby introducing high packet delivery delay. Another solution would be the crea- tion and maintenance of routes to an MDC, even if it goes far away from the source node. This unscalable approach would generate high message overhead because several nodes would have to update their routing information frequently. In our solution, if a sensor node turns to be out of the “‘cluster’s range”, i.e., if its hop level hm > TTL after an MDCs move, it must remove the route to the MDC from its routing table. This way, our approach does not leave a “bread crumb trail” while an MDC moves along its path. Hence, future route changes are not disseminated through a large number of nodes, but kept locally to a cluster.

24. 每一个传感器节点都有两个跳距级hm和hs，分别代表了对应于MDC的簇内跳距级和对应于静态节点的跳距级。当节点又数据包需要发送时，可能是其自身产生的也可能是别的节点发送过来的，它就要决定用哪一条路径开发送该数据包，该决定是根据跳距级来判断的。目前的MDC/PEQ协议，传感器节点是选择最小的跳距作为其选择路径的标准。例如，假设节点A又数据要发送，但是它不属于任何一个簇，A节点就检查自己的跳距级，确定它没有到达一个MDC的路径，因此，节点用到达该静态节点的路径实现对该数据的中继。如果又某个中间节点又道道某个MDC的路由，该中间节点就转发该数据到达它所属的MDC.通过这种方法，来自于远距离的不属于任何簇的节点的数据就被某个接单解释并转发到最近的MDC。该方法显著了延时，缓解了静态的汇聚节点周围节点的负担。每一个传感器节点都有两个跳距级hm和hs，分别代表了对应于MDC的簇内跳距级和对应于静态节点的跳距级。当节点又数据包需要发送时，可能是其自身产生的也可能是别的节点发送过来的，它就要决定用哪一条路径开发送该数据包，该决定是根据跳距级来判断的。目前的MDC/PEQ协议，传感器节点是选择最小的跳距作为其选择路径的标准。例如，假设节点A又数据要发送，但是它不属于任何一个簇，A节点就检查自己的跳距级，确定它没有到达一个MDC的路径，因此，节点用到达该静态节点的路径实现对该数据的中继。如果又某个中间节点又道道某个MDC的路由，该中间节点就转发该数据到达它所属的MDC.通过这种方法，来自于远距离的不属于任何簇的节点的数据就被某个接单解释并转发到最近的MDC。该方法显著了延时，缓解了静态的汇聚节点周围节点的负担。

25. Each sensor node holds two hop levels: hm, that represents the hop level regarding an MDC; and hs, the hop level with respect to the static sink. When a node has a DATA packet ready to be transmitted, either received from another node or generated by the node itself, it decides what route should be used. This decision is based on the hop level. In the current version of the MDC/PEQ protocol, a sensor node chooses the route with the smallest hop level, or it chooses the static sink as its destination if hm = hs. For instance, suppose that the sensor node A has DATA to transmit and does not belong to any cluster. The node A checks its hop levels and finds out that it does not have a hop level entry to an MDC. Hence, the node uses the route to the sink in order to relay its DATA. If an intermediate sensor node has a route to an MDC, it will forward DATA through the route leading to its MDC. By this, DATA coming from distant nodes that do not belong to any cluster might be intercepted by a node that belongs to a cluster in its way to the sink. The intercepted data will be forwarded to the nearest MDC. This approach contributes to the main objectives of the protocol presented in this paper that are to reduce delay and relief the burden of the nodes closer to the sink.

26. 5.仿真试验 我们将100个传感器节点分散在150*150的区域内。所有的节点都能够噶送数据到静态的汇聚节点。该接单被放置在区域的边界。所有节点都有一个R=25米的无线电波的传播范围。我们设置每个节点的Rxthresh，以达到最大的接收距离R=20米。其中随即的选择了50个源节点每一个源节点每秒钟产生两个数据包。每个数据包的大小是32个字节。移动节点MDC的数据是5个，生存周期是TTL=2。每个MDC每秒钟发送2个BEACON，并且以5米/s的速度匀速运动。传感器节点的初始参数设置如下hm = _1, h = _1, MDC IDcurr = _1,，并且节点以一个空的路由表开始仿真过程。仿真参数的完整情况如表1所示.

28. 我们主要考察WSN在以下几个方面的性能：数据包延时，能量消耗，汇聚节点的临节点的传输负载。我们主要考察WSN在以下几个方面的性能：数据包延时，能量消耗，汇聚节点的临节点的传输负载。 1.数据包的平均传输延时：端对端的数据包传送延时，即从源节点到目的节点（MDC或汇聚节点） 2.平均数据包传输率：即成功的发送到MDC的数据包的比例。 3.平均能耗：即每个节点的能耗 4.每个节点的平均传输数据量 _ average packet delivery delay: the end-to-end packet delay measured from the source node to the destination (sink or MDC); _ average packet delivery ratio: the ratio of packets received successfully at the sink or MDCs; _ average energy consumption: the energy dissipated per node; _ average transmissions per node: to evaluate the transmission overhead of our protocol;

29. 5.2仿真结果 表2是对多组仿真试验结果的汇总表。 如图6所示，由于MDC的引入，几乎使数据包的传输延时减少了一半。

30. 图7 (a)表明，在跳距数目很小时，MDC/PEQ相对于PEQ显著的增加了传送数据包的数目。 图7 (b)表明，MDC接收数据包是数据包总量的80%，汇聚节点20%。 在MDC/PEQ’s的两种模型中，BRAM中的MDC比Random Walk中的MDC移除节点范围的频率更高一些，所以更改路由信息更频繁。

31. 图8表明MDC/PEQ比PEQ的数据成功传送的比例更高。这是因为它减少了跳距的数目，因而降低了冲突，提高了数据传送率。图8表明MDC/PEQ比PEQ的数据成功传送的比例更高。这是因为它减少了跳距的数目，因而降低了冲突，提高了数据传送率。

32. 图9描述了每个节点的能耗情况。同样是因为减少了跳距的数目使得碰撞和拥塞的现象得到缓解，从而降低了能耗。图9描述了每个节点的能耗情况。同样是因为减少了跳距的数目使得碰撞和拥塞的现象得到缓解，从而降低了能耗。

33. 接下来进行第二部分的仿真试验，目的是考察MDC/PEQ的可扩展性。我们只改变MDC的数目和其运动速度，其余的参数的设置和第一部分相同。在该实验中我们采用的是BRAM的运动模型。接下来进行第二部分的仿真试验，目的是考察MDC/PEQ的可扩展性。我们只改变MDC的数目和其运动速度，其余的参数的设置和第一部分相同。在该实验中我们采用的是BRAM的运动模型。 图10(a)表明，引入一个MDC，可以明显的减少数据传输的延时，随着MDC数目的增加，延时会被进一步的减少。MDC数目的增多也使得更多的源节点可以将数据包发送到离他最近的簇首MDC，而不用经过汇聚节点。 传输速率的增加显然能够减少延时，但相应的也会增加丢失数据包的比例。图10（b）表明当以10米每秒的速度移动时，数据包的成功传输率下降到80%。

34. 另一个分析MDC/PEQ性能的重要标准是，MDC每发一个BEACON，路由信息的变更数目。如图11所示，我们将速度从1增加到10，显然速度的增加会是路径的变更次数显著增加。另一个分析MDC/PEQ性能的重要标准是，MDC每发一个BEACON，路由信息的变更数目。如图11所示，我们将速度从1增加到10，显然速度的增加会是路径的变更次数显著增加。

35. 图12表明 速率的增加会使得能耗相应的增加，原因是增加的信息交换的次数，因而消耗了更多的能量。

36. 结论