# A Novel IEEE 802.15.4e DSME MAC for Wireless Sensor Networks
## 3 The Proposed DSME MAC
### Proposed Dynamic GTS Allocation Scheme  
Group acknowledgement (GACK) plays an important role to provide the transmission opportunity within the same DSME superframe. The failed devices who get the information in GACK can have a chance to transmit data again in the allotted slots within the DSME superframe. In the current IEEE 802.15.4e standard, the coordinator allocates two separate slots within the CFP for this propose. Again the device has to reserve two slots, i.e., GTS and GTSR for each transmission slot in order to avail the retransmission opportunity within the DSME superframe. The next GTSR allotted slot will be used for new data packet of that device, if the transmission is successful in the first GTS allotted slot. Most of the time the devices may not have new data to send in the GTSR allotted slots. Hence, the overall throughput may be reduced significantly under this scenario.  
To overcome the drawbacks of the GTS scheme of IEEE 802.15.4e standard, we design here a new low complexity and reliable GTS allocation scheme. In order to increase the network throughput and to reduce the delay for retransmission, we modify the beacon format by adding a new seven bit field to the beacon frame called GACK as shown in Figure 2. The CFP of one superframe contains seven GTS slots and these seven bits will be one or zero based on the corresponding GTS transmission is success or failed, respectively. Upon receiving the beacon, each device counts the number of bits having value zero among these seven bit field and starts the channel access during CAP after those number of slots. The failed devices find the suitable slot during the CAP based on the received seven bit field and transmit again without following the CSMA/CA procedure. Hence, the delay can be minimized and unnecessary energy consumption due to contention in the CAP can also be reduced.  
As shown in Figure 4, let devices \(S_{1},S_{2}...,S_{7}\) be allotted to GTS slots 1, 2,..., and 7, respectively. It is assumed that devices \(S_{2},S_{3},S_{7}\) fail to transmit the data during their allotted 2 nd, 3 rd and 7 th GTS slots, respectively due to interference or fading or channel error. The coordinator will inform the transmission failure through the GACK field present in the beacon frame as shown in Figure 2. The value in the 2 nd, 3 rd and 7 th bit of the GACK field is 0. The superframe presents in a DSME superframe contains 16 slots out of which one slot is a beacon slot, next eight (1-8) slots are CAP and the rest seven (9-15) slots are CFP. devices those who receive the beacon get the information about these failed transmission by counting the number of zero bits in the GACK field and start the channel access from the 4 th slot of the current CAP. The failed devices \(S_{2},S_{3},S_{7}\) will start retransmission at slot number 1 st, 2 nd and 3 rd, respectively in CAP without contention.  
According to the existing mechanism of IEEE 802.15.4e, failed devices has to wait for the next retransmission slots to be allocated in the GTS. However, as shown in Figure 2, we add a GACK slot in our proposed MAC so that the failed devices neither have to contend for the channel nor have to wait for the next allotted retransmission slots. Besides, the proposed scheme can have the same superframe structure defined by the IEEE 802.15.4e standard.  
Figure 4: Dynamic GTSR allocation for the failed GTS transmission.