## 2 Time Slotted Channel Hopping (TSCH) Concepts  
Time Synchronized Mesh Protocol (TSMP) [14], developed by Dust Networks, is a media access and networking protocol that is designed for low power and low bandwidth reliable communication. TSMP concepts are used in several existing industrial wireless technologies such as WirelessHART, ISA100.11a and IEEE 802.15.4e (TSCH mode). IEEE 802.15.4e (TSCH mode) is a MAC amendment of the existing standard 802.15.4-2006. It enables robust communication through channel hopping. TSCH is based on a time-slotted mechanism, where a schedule dictates on what slot and which channel a node should transmit/receive data to/from a particular neighbor. TSCH does not address routing issues, but leaves this to the upper layers. TSCH divides the wireless channel into time and frequency. Time is divided into superframes, which have a collection of discrete time slots. Figure 1 shows the TSCH matrix (e.g., for the network shown on the left) and illustrates a superframe of 10 slots. The size of a timeslot in TSCH is typically 10 ms. A link is a transaction that occurs within a cell. Link information consists of a superframe ID, source and destination IDs, a slot number referenced to the beginning of the superframe, and a channel offset. The simplest version of a link contains one transmitter and one receiver. The two nodes at either end of the link communicate periodically once in every superframe. If only one transmitter is scheduled, the cell is contention-free (in this paper, we focus on the procedure of constructing the communication schedule in contention-free (not shared) or dedicated/exclusive cell). A slotted CSMA approach can be used if multiple transmitters are scheduled to compete in a shared cell for transmitting to the same device simultaneously. Multiple links can be allocated from one node to another in different cells. For example, in Figure 1 two Tx-links from node A to C are shown. TSCH links hop pseudo-randomly over a set of predefined channels, one packet at a time. Each time a transmission is going to occur on a link, both sides of the link calculate the radio channel of the communication by taking \"_(Absolute Slot Number + Channel offset) % Number of channels_\" and by mapping the result to the related superframe frequency hopping pattern. _Absolute Slot Number_ (ASN) is the number of timeslots since the beginning of the network, _Channel offset_ is the link's channel offset in the matrix of slot-channel, % indicates modular division, and _Number of channels_ is the number of available channels. Figure 2 depicts the specific timing requirement inside a TSCH timeslot. The scheduled communication in a timeslot between two nodes relies on an accurate time synchronization across the network. The network devices should have the same notion of when each timeslot begins and ends. TSCH, unlike the IEEE 802.15.4 that uses the beacon-based synchronization scheme, relies on exchanging timing offset information of the received and sent packets to provide synchronization. Each device should periodically synchronize its network time with at least one of its neighbors that is selected as a time synchronization source. The new device initially becomes synchronized with the network during the joining process and remains synchronized in the node's normal operation during any communication in a timeslot with a time source neighbor. Whenever a node receives an ACK with time correction information or a data packet from its times sources neighbor, it can adjust its clock. The mechanisms for time synchronization are described in [10]. TSMP works based on graph routing schemes. A graph is a routing structure that establishes directed end-to-end connection among devices. Each destination has its own graph, and several sources can share the same graph. Each graph in a network is identified with a unique Graph ID. Figure 3 illustrates the graph routing. In this figure, node 0 uses Graph 1 and 2 to communicate with nodes 43 and 45 respectively.