## 4 The IEEE 802.15.5 WPAN Mesh Standard
Secondly, nodes join the network as children nodes of the MC by means of the transmission of association requests, thus fulfilling tree level 1. In turn, these devices become parents (they have the role of _coordinators_, running in non-beacon mode) of new nodes which are out of the MC's coverage range (e.g., nodes labeled as E, F and G -tree level 2-) but can communicate directly with their parents. Consequently, these parents act as routing nodes between the MC and the new children. This process is repeated until all network devices are associated to a parent, forming the different branches of the logical tree multi-hop topology (tree links) whose root is the MC. Once the network is created, IEEE 802.15.5 executes the second mandatory function, assigning logical addresses to each device. This process is divided into two steps: in the first step, leaf nodes [devices which have no children associated, usually the most remote ones and/or end-devices from the MC, which according to our Figure 3(a) are the nodes labeled as D, H, K, L, M, and N] send their respective parents a message requesting a logical address and additional ones for future use (e.g., new nodes joining the network). In this context, upon joining the network, end-devices dispatch promptly this message. The remaining network nodes trigger a timer to wait for the association of new nodes. Once the timer expires, nodes without associated children become leaf nodes; therefore, they transmit the message asking for the addressing information. When each parent device receives this message from all its children, this device demands its corresponding up-parent the addresses requested by its children and its own address. The process continues for each branch and finalizes when the MC is reached. Then, in the second step, the addresses assignment to every network node starts. The MC is aware of the number of addresses requested by each branch, so this device assigns its children blocks of consecutive 16-bit IEEE logical addresses according to this number. To this aim, the MC sends a control message to each of its children in coverage with the exact number of addresses (block) that they require. After receiving this message, each MC's child extracts its own address from the assigned block (in particular, the first address of the assigned block to identify itself in the network) and, acting as parent, divides the remaining addresses into consecutive sub-blocks which are dispatched to its own children. Again, this process is repeated until the last device(s) of the network (leaf nodes) obtains its address(es). Figure 3(a) illustrates the second mandatory function. In this Figure, we have supposed for a better comprehension of the reader that every device requests only one address, and no additional addressing space is required in step one. Note that the extra space for addressing allows network nodes to start an individual assignment process with new nodes joining the network. This reduces the overhead required in requesting new addresses directly to the MC and enhances the scalability of the network. Thus, when the second step begins, the MC (which is identified by the 16 bit IEEE logical address 0) assigns each of its branches the corresponding blocks (1-9), (10-12) and (13), respectively. Among all of them, we focus the study on the branch corresponding to node B. So, after receiving block (1-9) from the MC, device B retains the first address for itself (1), and assigns the rest of the block as follows: sub-block (2-3) to node E, and sub-block (4-9) to node F. Following this procedure, the address assignment continues until the devices labeled as H, L, M and N obtain their respective addresses. The key of this function is twofold: (1) every device is identified by a unique address; and (2) the use of 16-bit IEEE logical addresses reduces the number of processing tasks and memory usage, facilitating  
the management of the routing tables.