## 5 Protocol Comparison
The multi-path protocols as discussed in Section 4.3 are compared in Table 2. These algorithms are mainly aimed at distributing traffic load through the network and enhancing the network lifetime by avoiding network partitioning. These protocols are compared together according to the following criteria:  
* _Lifetime improvement mechanism_: This field shows that which kind of mechanism for lifetime improvement is used in each protocol. As shown in Table 2, all protocols only use multi-path mechanism except MSMRP [42] that employs both multi-path and power control schemes simultaneously. * _Node or link disjoint_: Disjointness is an important property for multi-path protocols. Node or link disjoint protocols try to prevent interference between multiple paths and avoid packet retransmission caused by collision. Those algorithms [42; 43] in which the node-disjoint scheme is used are congestion avoided and thus, having much better performance than link-disjoint multi-path strategies [14]. Braided protocols [41] cannot guarantee the disjointness among the multiple paths. * _Number of paths_: This factor indicates the rate of traffic distribution through the network. Whenever this metric is increased, the possibility of network partitioning will be decreased. The number of paths in some approaches [42] is specified, however in other approaches [15; 43] the number of parallel routes are increased as far as possible to improve the network lifetime. * _Network structure_: Routing algorithms in WSNs are usually classified into three groups as follows: Flat (data-centric), hierarchical and geographic (location-based). Flat networks [15, 42] employ a query-based strategy in order to decrease redundant data transmission through the network and conserve a huge amount of energy. In a hierarchical architecture, nodes with higher energy are chosen as cluster heads and aggregate data from other nodes (_i.e._, cluster member). Both position information and the greedy forwarding techniques are used by geographic routings [14, 41] to establish one or more energy-efficient paths from the source nodes to the sink. * _Application Type_: This factor shows that which kind of mechanisms will be employed to send data to the sink. In time-driven method, the data are sent to the sink continuously by all or special groups of sensor nodes that caused fast energy depletion through the network. In event-driven strategy [14, 41, 43], on the other hand, only the data about an interested event will be forwarded to the sink while in the query-based method [15], the data should be transmitted according to the sink's request. * _QoS_: The routing protocols [14] that apply quality of service criteria (QoS) to the network have to balance data quality and energy consumption. So, the network has to satisfy certain QoS factors such as energy, bandwidth, and delay when delivering data to the sink. * _Network connectivity_: The algorithms proposed in [15, 41, 43] assume that the sensor nodes in the network should have a connected topology while in some others this assumption is not considered. AGEM [14] is an example that makes use of mobile sensors to transmit data packets between disconnected network areas. * _Mobility_: In a static sensor network, the sensors which located at the sink vicinity may die quickly due to transmitting a large number of data packets from the nodes which are far away from the sink. The fast energy depletion around the sink causes the network partitioning and consequently sink isolation phenomenon. Thus, changing the position of neighbors [14] or sink itself is a smart choice to keep connectivity and enhance the network lifetime. * _Location awareness_: Location information is a powerful tool to find the best next hop in routing mechanisms or can be used for determining the next location of mobile nodes in the network. This information can be acquired from GPS directly or calculated on other localization methods.