## 2 State of the Art  
In [6] authors propose MASN, a new multi-channel allocation protocol for hierarchical ZigBee/IEEE 802.15.4 networks corresponding to realistic situations where the number of nodes is less than 50 with a hierarchical routing and many-to-one transmissions. The main goal is to improve the bandwidth. The unslotted CSMA/CA access method is used. In MASN, assignment of the different channels is centralized and distributed by the coordinator by using a function designed to calculate the channel offset between two successive children. The proposed channel assignment algorithm is inspired by the hierarchical address assignment process used in ZigBee. The performance of MASN is evaluated through simulations using NS2 simulator. MASN is compared with mono-channel, random allocation, and subtree allocation. Results show that the proposed protocol improves the global throughput by a factor between 2 and 5 compared to the single channel, and by an average factor of 2 compared to a random channel allocation or to a simple subtree distribution. The main advantage of this solution is its simplicity of integration in IEEE 802.15.4 devices with light modifications on the MAC layer. On the other hand, authors used a single topology for the simulations which might be the most convenient topology for MASN. Also if a node is performing transmission and reception simultaneously, then it leads to conflict. This case is not studied in MASN. In [7] authors proposed EE-MAC (Energy Efficient hybrid MAC for WSN), a hybrid of TDMA and FDMA MAC protocol with priority technique. It is a centralized protocol that uses a semi-dynamic channel assignment approach for channel allocation. The goal of this protocol is to improve energy efficiency, delay and packet delivery ratio. EE-MAC operates in two phases, a setup phase and a transmission phase. During the first phase, the following operations take place: neighbor discovery, TDMA slot assignment, FDMA slot assignment, local framing, and global synchronization. These operations run only during the set-up phase and every time a change in the topology occurs. During the transmission phase, the period is divided into time intervals. Each frame is equally divided into several fixed time slots. Each slot is divided into schedule subslots and contention subslots. Each cycle starts with scheduled slots followed by contention slots. Nodes use LPL (Low Power Listening) [8] during contention slots and send HELLO messages to the base station. When a node hears a HELLO message sent to the base station by one of its neighbors, it adds that nodes to its neighbor list. The updated neighbor list is included in the next Hello message and sent by that node. Based on Hello messages received from all nodes, the base station performs a breadth first search algorithm in order to assign for each node an appropriate channel as well as a specific time slot. Scheduled node can communicate in a collision free manner. EE-MAC introduces an adaptive priority scheme for channel access. The priority scheme is inspired from IEEE 802.11e [9]. The priority is based on some statistics collected by these nodes during their lifetime which includes forwarding loads, the distance to the base station, and the remaining battery charge. Authors compared the protocol to HyMAC [10] protocol in terms of energy efficiency, packet delivery ratio and end to end delay. The simulation is carried out by varying the number of packets, the number of nodes and by changing the packet size. Simulation results using NS2 simulator showed that EE-MAC provides a significant improvement compared to HyMAC, in energy efficiency, packet delivery ratio, and delay. On the other hand, EE-MAC suffers from the overhead of Hello messages that are exchanged and sent to the base station. In [11], authors proposed MMSN protocol which is a distributed protocol that combines FDMA and CSMA mechanisms. It uses a semi-dynamic channel assignment approach for channel allocation. MMSN uses different strategies for channel assignment.