Synchronous Protocols

As discussed in section 2.2.2, the common design principles of synchronous MAC protocols for wireless sensor networks are: 1) nodes in the network are organized into clusters; 2) the

communication scheme is partitioned into two phases, one is used for the synchronization of the active time of all nodes within the cluster, and the other is used for exchange of data packets.

When mobile nodes accommodate in the network, the scenarios can be classified into two cases: 1) The mobile node roams within a cluster. It can be treated as a static node because it shares the same schedule as its neighbors. 2) The mobile node approaches the border nodes and attempts to cross the border of the two clusters. Communication may be disabled while crossing the border, because the mobile node may go out of the communication range of the current cluster and has not been yet synchronized with the new one. This disconnection, usu- ally up to 2 minutes, is too long and consequently degrades the performance of the protocol in terms of energy1 and latency. Hence, the synchronous mobility management protocol is to find a solution to 1) detect the mobile node that is attempting to cross the cluster border and 2) shorten the synchronization time when the handover occurs.

MS-MAC [PJ04] is the first MAC protocol which supports mobility in wireless sensor net- works. It handles the handover by following two schedules when the mobile node crosses the border of two clusters. At each periodic synchronization phase, each node learns the mobility status of its neighbors by estimating the RSSI value of the received synchronization mes- sages (SYNC). When the mobile node is crossing the cluster border, an active zone is created within the neighborhood of the mobile node to adapt the synchronization period according to the mobility information. The border nodes broadcast their schedules more frequently than in the stationary scenario, thus the mobile node can establish a connection with the new cluster before it disconnects from the old cluster. However, during the handover phase, the border nodes consume more energy due to frequent synchronization.

Inspired by MS-MAC and DS-MAC [YBS+08], the authors developed a mobility-aware, de- lay-sensitive MAC protocol named MD-SMAC [HSFG09]. In addition to the basic operations inherited from MS-MAC and DS-MAC, MD-SMAC makes several modifications to enhance mobility management and power consumption efficiency. In the mobility management as- pect, the mobile node increases its listening frequency to receive the new schedules when it is crossing the cluster border. In the energy consumption aspect, MD-SMAC only allows the mobile node to listen to the schedules of new clusters, and to keep the other nodes in regular operations. After receiving the new schedule, the mobile node decreases the listening fre- quency to its default setup (from 30 seconds to 5 minutes.). At the same time, it withdraws the old schedule and adopts the new one immediately.

To deal with high energy consumption in MS-MAC in random movement and high mobility scenarios, the enhanced mobility-aware S-MAC (EMS-MAC) [ZTBW11] is proposed. The au- thor argued that using RSSI alone to detect mobility is not accurate, due to the unstable nature of RSSI measurement. Even in a stationary network, the RSSI value can fluctuate drastically. These RSSI fluctuations can cause false positive mobility events. To reduce unnecessary han-

1_{the mobile node has to keep awake to listen to the synchronization messages of the new cluster. The active} mode of the radio consumes more energy than the sleeping mode.

dover events, EMS-MAC combines RSSI with link quality indicator (LQI) to predict mobility. Instead of creating an active zone which involves all the border nodes in the neighborhood of the mobile node when the mobile node attempts to cross the border, EMS-MAC introduces a mobility zone which only includes the nodes that detected the mobility of the approaching mobile node. After mobility detection, the border node sends a SYNC message to introduce itself and the mobile node replies to the SYNC packet with the handover request. Then the border node broadcasts the new cluster schedule. Once the mobile node has learned the new schedule, it adopts this schedule as well as the old one to finish the handover process.

CFMA [KA13], collision free mobility adaptive MAC protocol, is another type of synchronous mobility management protocol. Nodes in CFMA are organized in a cluster-tree topology. Each cluster has a head node called coordinator. Other nodes in the cluster are synchronized with the coordinator by periodically transmitted beacons. The coordinators are not only functioned as bridges to the sink node, but also can communicate with each other directly to exchange network information, especially when the mobile node is moving from one cluster to another. Another contribution of CFMA is the idea of allocating predefined backoff value to the nodes which have buffered data to avoid collision at the beginning of each transmission round. The mobile node can overhear these backoff assignment packets. If it detects that the RSSI value of these packets from the neighboring coordinator is continuously increasing, the mobile node requests the current coordinator for a delay value of the neighboring cluster. Once the mo- bile node acquires the delay value from the new cluster coordinator, it can handover to the new cluster with a specific delay. Furthermore, the mobile nodes always have the highest priority when they enter a new cluster, which ensures that they can join the new cluster with minimum latency.

MT-MAC [ZAI+_{14] is the latest developed mobility management MAC protocol in the syn-}

chronous group, which is based on T-MAC [VDL03]. The mobility handling strategy is almost the same as EMS-MAC. The only difference between these two protocols is that MT-MAC inherits an adaptive duty cycling mechanism from T-MAC and supports burst transmission. Alternatively, EMS-MAC uses a fixed duty cycling strategy.