# A Novel IEEE 802.15.4e DSME MAC for Wireless Sensor Networks
## 3 The Proposed DSME MAC
### A New Association Scheme  
In this section, a new scheme of beaconing by the PAN coordinator is designed for the fast association of the devices with it. As per the IEEE 802.15.4e standard, the numbers of superframes, multisuperframes present in a DSME beacon interval are \(2^{BO-SO}\) and \(2^{BO-MO}\), where \(BO\), \(SO\), \(MO\) are \(macBeaconOrder\), \(macSuperframeOrder\) and \(macMultisuperframeOrder\) respectively. As shown in Figure 1, if the coordinator chooses \(BO=6\), \(MO=5\) and \(SO=4\), the number of superframes  
Figure 1: DSME superframe structure with BO = 6, MO = 5, SO = 4. \(2^{\text{BO}-SO}\) is 4. Each superframe has one exclusive beacon slot and therefore the total numbers of available beacon slots in the DSME superframe are \(2^{\text{BO}-SO}\). According to the current standard, coordinator transmits a beacon only at the start of DSME superframe in one selected channel. In order to reduce the discovery/association delay, we propose that the coordinator should broadcast the beacon in different channels during beacon slots. Let \(CH=\{CH_{1},CH_{2},...CH_{k}\}\) be the set of channels and \(SUP=\{SF_{1},SF_{2},...SF_{2^{\text{BO}-SO}}\}\) be the set of superframes present in the DSME beacon interval. According to our proposed scheme, the coordinator broadcasts the beacon in the \(i\)th channel, i.e., in \(CH_{i}\) and \(j\)th superframe, i.e., in \(SF_{j}\). \[(j+BN*2^{\text{BO}-SO})\equiv i\text{ }MOD\text{ }k\text{ }for\text{ }1\leq i \leq k\text{ }and\text{ }1\leq j\leq 2^{\text{BO}-\text{SO}} \tag{1}\]  
where the variable \(BN\) represents the beacon sequence number. When a new device enters into the network and wants to communicate, it should be associated with the PAN coordinator. To perform association, the device must perform the channel scanning to receive a beacon broadcast by the coordinator. The device keeps its radio ON during the scanning until it receives a beacon. There are two types of scanning procedures (active and passive) suggested in the IEEE 802.15.4e standard. In active scanning, devices have to send beacon request command in the chosen channel, which is an energy consuming process. In passive scanning, devices have to listen to the chosen channel until they locate the coordinator. However, passive scanning to discovery the coordinator increases the delay. Since, power consumption is a major constraint in WSN, we focus on passive scanning scheme to save the power. In our proposed method, the new device that wants to join the network should choose a random channel and perform the passive scan, i.e., it waits for the beacon in that channel. The device definitely gets the beacon as the coordinator broadcasts the beacon in every channel. Upon receiving the beacon frame, the devices send the association request frame using CSMA/CA and the coordinator sends back the association response frame, if it has received the association request successfully. Otherwise, the devices repeat the association procedure as suggested in the IEEE 802.15.4e standard. In our proposed scheme, un-associated devices can chose one random channel and perform the passive scan. Hence, our scheme can reduce the busy channel conditions and collisions when un-associated devices are increase. However, according to the IEEE 802.15.4e, all un-associated devices can locate the coordinator simultaneously through the passive scan and all of them can have access to the channel simultaneously. This procedure can increase the delay for the discovery of the coordinator and association with it. On the other hand, our procedure can significantly reduce the delay for discovering the coordinator and association with it. The detailed procedure of our proposed schemes are given in Algorithm 1 and Algorithm 2 for the coordinator and devices, respectively. ```
0: Set of channels \(CH=\{CH_{i}\ |\ 1\leq i\leq k\}\), beacon order (\(BO\)), superframe order (\(SO\)) and beacon sequence number (\(BN\)). 0: Beacon broadcast in every channel.