# Time Slotted Channel Hopping

International standard IEEE 802.15.4 September 2015 Industrial wireless sensor networks

Time Slotted Channel Hopping or Time Synchronized Channel Hopping (TSCH) is a channel access method for shared-medium networks.

TSCH is used by Low-Power devices to communicate using a wireless link. It is designed for low-power and lossy networks (LLNs) and aims at providing a reliable Media access control layer.

TSCH can be seen as a combination of Time division multiple access and Frequency-division multiple access mechanisms as it uses diversity in time and frequency to provide reliability to the upper network layers.

The TSCH mode was introduced in 2012 as an amendment (IEEE 802.15.4e) to the Medium Access Control (MAC) portion of the IEEE 802.15.4 standard. The amendment was rolled into the IEEE 802.15.4 in 2015.

## Description

A TSCH slotframe on the 2.4GHz band. Each color represents a layer 2 (MAC) link between two devices.

Wireless communications are often referred as unreliable due to the unpredictability of the wireless medium. While wireless communications bring many advantages (e.g no wires maintenance, costs reduction ...), the lack of reliability slows down the adoption of wireless networks technologies.

TSCH aims at reducing the impact of the wireless medium unpredictability to enable the use of reliable low-power wireless networks. It is very good at saving the nodes' energy because each node shares a schedule, allowing it to know in advance when to turn on or off its radio.[1]

The IEEE 802.15.4 standard uses different frequency bands, and each frequency band is separated in channels. In TSCH, communications are done using those different channels and at different times. However, this standard does not define how to build and maintain the communication schedule. Many works have been proposed to organize the schedule in a centralized[2] or distributed[3][4] way.

## Channel Hopping

Let chOf be the channel offset, assigned to a given link. The channel offset, chOf, is translated to a frequency f (i.e. a real channel) using:

${\displaystyle f=F{(ASN+chOf)\,{\bmod {\,}}n_{ch}}}$

where ASN is the Absolute Slot Number, i.e. the total number of slots that elapsed since the network was deployed. The ASN is incremented at each slot and shared by all devices in the network.

Multipath propagation can create internal destructive interferences of a wireless signal known as multipath fading. This phenomenon can be overcome by shifting the location of the communicating nodes or by switching the communication carrier frequency.

The channel hopping mechanism of TSCH allows to overcome the impact of multipath fading by changing the communication carrier frequency for every transmission[5][6]

## Implementations

TSCH is implemented in simulation or on real hardware.

Simulation:

Firmware:

## 6TiSCH

TSCH is one of the key elements of the 6TiSCH[7] stack[8] as part of the IEEE802.15.4-2015 standard.

## Uses

Due to its low power consumption and reliability, TSCH (or its previous versions) is mainly used in Low-Power Wireless Sensor Networks.

Companies are using it in their wireless sensor networks such as Linear Technology[9][10] and Emerson[11]

## References

1. ^ "Performance Analysis of IEEE 802.15.4e Time Slotted Channel Hopping for Low-Rate Wireless Networks" (PDF). KSII Transactions on Internet and Information Systems. 7 (1): 15. 2013. doi:10.3837/tiis.2013.01.001.
2. ^ Palattella, M. R.; Accettura, N.; Dohler, M.; Grieco, L. A.; Boggia, G. (2012-09-01). "Traffic Aware Scheduling Algorithm for reliable low-power multi-hop IEEE 802.15.4e networks". 2012 IEEE 23rd International Symposium on Personal, Indoor and Mobile Radio Communications - (PIMRC): 327–332. doi:10.1109/PIMRC.2012.6362805. ISBN 978-1-4673-2569-1. S2CID 26494795.
3. ^ Muraoka, K.; Watteyne, T.; Accettura, N.; Vilajosana, X.; Pister, K. S. J. (2016-08-01). "Simple Distributed Scheduling With Collision Detection in TSCH Networks" (PDF). IEEE Sensors Journal. 16 (15): 5848–5849. Bibcode:2016ISenJ..16.5848M. doi:10.1109/JSEN.2016.2572961. ISSN 1530-437X. S2CID 13636138.
4. ^ Accettura, N.; Palattella, M. R.; Boggia, G.; Grieco, L. A.; Dohler, M. (2013-06-01). "Decentralized Traffic Aware Scheduling for multi-hop Low power Lossy Networks in the Internet of Things". 2013 IEEE 14th International Symposium on "A World of Wireless, Mobile and Multimedia Networks" (WoWMoM): 1–6. doi:10.1109/WoWMoM.2013.6583485. ISBN 978-1-4673-5827-9. S2CID 34338607.
5. ^ Watteyne, Thomas; Mehta, Ankur; Pister, Kris (2009-01-01). "Reliability Through Frequency Diversity: Why Channel Hopping Makes Sense". Proceedings of the 6th ACM Symposium on Performance Evaluation of Wireless Ad Hoc, Sensor, and Ubiquitous Networks. PE-WASUN '09. New York, NY, USA: ACM: 116–123. doi:10.1145/1641876.1641898. ISBN 9781605586182. S2CID 2434303.
6. ^ Watteyne, T.; Lanzisera, S.; Mehta, A.; Pister, K. S. J. (2010-05-01). "Mitigating Multipath Fading through Channel Hopping in Wireless Sensor Networks". 2010 IEEE International Conference on Communications: 1–5. doi:10.1109/ICC.2010.5502548. ISBN 978-1-4244-6402-9. S2CID 7905710.
7. ^ "IPv6 over the TSCH mode of IEEE 802.15.4e (6tisch) -". datatracker.ietf.org. Retrieved 2017-07-08.
8. ^ The Internet of Things IoT Inc Business Channel (2015-04-23), Bridging OT and IT with 6TiSCH in the Internet of Things, retrieved 2017-07-08
9. ^ "White Paper - Reliable, Low Power Wireless Sensor Networks" (PDF). `{{cite journal}}`: Cite journal requires `|journal=` (help)
10. ^ dustnetworks (2011-06-20), Dust Networks: SmartMesh IP Introduction, Kris Pister, retrieved 2017-07-08
11. ^ "A Comparison of WirelessHART™ and ISA100.11a" (PDF). `{{cite journal}}`: Cite journal requires `|journal=` (help)