Читать книгу Service Level Management in Emerging Environments - Nader Mbarek - Страница 51

In order to guarantee the QoS parameters negotiated in the iSLA service level agreement, we propose a new QoS mechanism for the lowest layer of the IoT architecture, namely the sensing layer. We thus specify a wireless access method called QBAIoT (QoS Based Access method for IoT) (Khalil et al. 2018). This method enables the communication between the IoT objects and the LL-Gw, while adapting the superframe structure of the IEEE 802.15.4 standard and the slotted CSMA/CA mechanism. The adaptation allows the differentiation of traffic in order to meet the QoS requirements of each class.

The QBAIoT method takes into account up to four QoS classes as defined in the iSLA’s qualitative parameters (RTMC, RTNMC, Streaming and NRT). Real-time traffic (RTMC and RTNMC) is very sensitive to delay, Streaming traffic is sensitive to jitter (that is, variation of delays), while NRT traffic is a QoS class that has no QoS constraints. To adapt the structure of the superframe, QBAIoT replaces the single Contention Access Period (CAP), common to all objects, and also replaces the non-contention period (CFP: Contention Free Period) of the classic IEEE 802.15.4 superframe by novel QoS CAPs. Each QoS CAP is specific to a QoS class. Thus, the adapted QBAIoT superframe can contain up to four QoS CAPs. The number of QoS CAPs configured in a LL-Gw depends on the number of QoS classes defined in the iSLA for this gateway. In addition, the inactive part of the classic superframe is eliminated in the context of QBAIoT. This deletion leads to shorter timeframes and enhances the performance of traffic in the Real-Time category. On the other hand, the number of slots available in the superframe is fixed at 16 and the duration of each slot depends on the superframe duration (SD). The SD is calculated based on the value of the Superframe Order (SO). The Beacon Order (BO) is used to calculate the interval for sending beacon frames (Beacon Interval [BI]). In QBAIoT, BO and SO are always equivalent as the inactive period is deleted.

During each QoS CAP in the QBAIoT superframe, only the objects that generate traffic belonging to the corresponding QoS class can compete for access to shared support. Each QoS CAP is assigned a number of slots among the 16 available in the superframe. The configuration of the slots and of the BO/SO values depends on the number of existing QoS classes in the IoT environment under consideration and, in particular, the number of real-time QoS classes. If only one class exists, the classic IEEE 802.15.4 superframe structure will be used, but with no CFP period and no inactive period, as only one QoS CAP exists. In addition, in this case, the values of the BO and SO are set to 14 to minimize the number of beacons sent in the IoT environment. When several QoS classes exist, the BO and SO are initialized to a value of 2 if there is at least one real-time QoS class (RTMC or RTNMC), and to a value of 3 if there is no real-time QoS class. The QBAIoT method is implemented on the LL-Gw (acting as coordinator) as well as on the IoT objects of the sensing layer. Figure 1.4 shows a comparison between the structure of the standard IEEE 802.15.4 and QBAIoT superframes.

Figure 1.4. Comparison of the structure of the IEEE 802.15.4 and the QBAIoT superframes. For a color version of this figure, see www.iste.co.uk/mbarek/service.zip