## 3 Time-Slotted Channel Hopping
### Component Structure and Operation  
The IEEE 802.15.4e TSCH does not define a component structure and management information base (MIB) organization, but only the overall MAC layer behavior, command-based API (application programming interface), message formats and the relevant personal area network information base (PIB) elements that are used by the MAC. The IETF 6 TiSCH working group is defining the management interface and over-the-air messages to operate and configure the schedule of the TSCH MAC layer. [PERSON] _et al_. [15] define the minimal configuration of such a MAC layer to operate under a static schedule and the best effort routes guided by the routing protocol for low-power lossy networks (RPL) [19]. The component structure is open to implementers. In order to leverage the introduction of the technology, in this article, a component structure is defined. Figure 1 depicts a suitable component structure for the IEEE 802.15.4e implementation. The low MAC components are formed by a IEEE 802.15.4e module that is in charge of executing the state machine that guides the activities carried in a time slot, and the IEEE 802.15.4 component is used to manage headers and decouple time slot activities from header parsing. At the upper MAC layer, a 6 top component is in charge of managing the schedule, the neighbor table, the queues and the monitoring functions of the MAC layer.  
Time slot timing is not defined in IEEE 802.15.4e. This is left open to implementers and can be configured dynamically by means of information elements (IEs) carried on enhanced beacon (EB) packets that are used to bootstrap the network. Determining those parameters is of utmost importance, as timing is critical in TSCH and the tuning of the MAC layers timing is time consuming. Table 1 presents a suitable timing for the low MAC layer as described by [15].  
The presented component structure and timing is aligned with our previous work at the OpenWSN project [18], which constitutes the only existing open-source implementation of IEEE 802.15.4e available at the time of writing this article. The default MAC layer timin  
Figure 1: Component-based medium access control (MAC) organization.  
experimenting with different platforms, including very old TelosB motes running at 4 MHz that required several milliseconds to handle serial port activity within a time slot. The separation of lower MAC from higher MAC is of utmost importance to decouple time slot intervals from higher layer traffic demand. In addition, the higher MAC layer is responsible for the management and frame control, while the lower MAC executes specific actions to send and/or receive packets, regardless of their type (data, control, _etc._). The OpenWSN project supports a wide range of microcontrollers, including the Texas Instruments MSP430 family, the Freescale Kinetis family and the ST Microelectronics STM32F1 family, as well as radio transceivers, including the Texas Instruments CC2520 and the Atmel AT86 RF23x. The latest addition is the Texas Instruments CC2538 SoC (system on chip), which includes an ARM Cortex M3 microcontroller and a radio transceiver compatible with the IEEE 802.15.4-2006 standard.