draft-ietf-6tisch-tsch-02.txt   draft-ietf-6tisch-tsch-03.txt 
6TiSCH T. Watteyne, Ed. 6TiSCH T. Watteyne, Ed.
Internet-Draft Linear Technology Internet-Draft Linear Technology
Intended status: Informational MR. Palattella Intended status: Informational MR. Palattella
Expires: April 20, 2015 University of Luxembourg Expires: April 30, 2015 University of Luxembourg
LA. Grieco LA. Grieco
Politecnico di Bari Politecnico di Bari
October 17, 2014 October 27, 2014
Using IEEE802.15.4e TSCH in an IoT context: Using IEEE802.15.4e TSCH in an IoT context:
Overview, Problem Statement and Goals Overview, Problem Statement and Goals
draft-ietf-6tisch-tsch-02 draft-ietf-6tisch-tsch-03
Abstract Abstract
This document describes the environment, problem statement, and goals This document describes the environment, problem statement, and goals
for using the IEEE802.15.4e TSCH MAC protocol in the context of LLNs. for using the IEEE802.15.4e TSCH MAC protocol in the context of LLNs.
The set of goals enumerated in this document form an initial set The set of goals enumerated in this document form an initial set
only. only.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2015. This Internet-Draft will expire on April 30, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 4 3. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 4
4. Problems and Goals . . . . . . . . . . . . . . . . . . . . . 6 4. Problems and Goals . . . . . . . . . . . . . . . . . . . . . 6
4.1. Network Formation . . . . . . . . . . . . . . . . . . . . 6 4.1. Network Formation . . . . . . . . . . . . . . . . . . . . 7
4.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 7 4.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 7
4.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . 7 4.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . 7
4.4. Routing and Timing Parents . . . . . . . . . . . . . . . 7 4.4. Routing and Timing Parents . . . . . . . . . . . . . . . 8
4.5. Resource Management . . . . . . . . . . . . . . . . . . . 8 4.5. Resource Management . . . . . . . . . . . . . . . . . . . 8
4.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . 8 4.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . 8
4.7. Deterministic Behavior . . . . . . . . . . . . . . . . . 8 4.7. Deterministic Behavior . . . . . . . . . . . . . . . . . 8
4.8. Scheduling Mechanisms . . . . . . . . . . . . . . . . . . 8 4.8. Scheduling Mechanisms . . . . . . . . . . . . . . . . . . 9
4.9. Secure Communication . . . . . . . . . . . . . . . . . . 9 4.9. Secure Communication . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . 10
8.3. External Informative References . . . . . . . . . . . . . 13 8.3. External Informative References . . . . . . . . . . . . . 13
Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 15 Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 16
A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 15 A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 16
A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . 16 A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . 16
A.3. Node TSCH Schedule . . . . . . . . . . . . . . . . . . . 16 A.3. Node TSCH Schedule . . . . . . . . . . . . . . . . . . . 16
A.4. Cells and Bundles . . . . . . . . . . . . . . . . . . . . 16 A.4. Cells and Bundles . . . . . . . . . . . . . . . . . . . . 17
A.5. Dedicated vs. Shared Cells . . . . . . . . . . . . . . . 17 A.5. Dedicated vs. Shared Cells . . . . . . . . . . . . . . . 17
A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . 17 A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . 18
A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 18 A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 18
A.8. Time Synchronization . . . . . . . . . . . . . . . . . . 18 A.8. Time Synchronization . . . . . . . . . . . . . . . . . . 19
A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 19 A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 19
A.10. Network TSCH Schedule . . . . . . . . . . . . . . . . . . 19 A.10. Network TSCH Schedule . . . . . . . . . . . . . . . . . . 20
A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . 20 A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . 20
A.12. Information Elements . . . . . . . . . . . . . . . . . . 20 A.12. Information Elements . . . . . . . . . . . . . . . . . . 21
A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 20 A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 21
Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 21 Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 21
B.1. Collision Free Communication . . . . . . . . . . . . . . 21 B.1. Collision Free Communication . . . . . . . . . . . . . . 21
B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 21 B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 21
B.3. Cost of (continuous) Synchronization . . . . . . . . . . 21 B.3. Cost of (continuous) Synchronization . . . . . . . . . . 22
B.4. Topology Stability . . . . . . . . . . . . . . . . . . . 21 B.4. Topology Stability . . . . . . . . . . . . . . . . . . . 22
B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . 22 B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
IEEE802.15.4e [IEEE802154e] was published in 2012 as an amendment to IEEE802.15.4e [IEEE802154e] was published in 2012 as an amendment to
the Medium Access Control (MAC) protocol defined by the the Medium Access Control (MAC) protocol defined by the
IEEE802.15.4-2011 [IEEE802154] standard. IEEE802.15.4e will be IEEE802.15.4-2011 [IEEE802154] standard. IEEE802.15.4e will be
rolled into the next revision of IEEE802.15.4, scheduled to be rolled into the next revision of IEEE802.15.4, scheduled to be
published in 2015. The Timeslotted Channel Hopping (TSCH) mode of published in 2015. The Timeslotted Channel Hopping (TSCH) mode of
IEEE802.15.4e is the object of this document. IEEE802.15.4e is the object of this document.
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IEEE802.15.4e is the latest generation of ultra-lower power and IEEE802.15.4e is the latest generation of ultra-lower power and
reliable networking solutions for LLNs. [RFC5673] discusses reliable networking solutions for LLNs. [RFC5673] discusses
industrial applications, and highlights the harsh operating industrial applications, and highlights the harsh operating
conditions as well as the stringent reliability, availability, and conditions as well as the stringent reliability, availability, and
security requirements for an LLN to operate in an industrial security requirements for an LLN to operate in an industrial
environment. In these environments, vast deployment environments environment. In these environments, vast deployment environments
with large (metallic) equipment cause multi-path fading and with large (metallic) equipment cause multi-path fading and
interference to thwart any attempt of a single-channel solution to be interference to thwart any attempt of a single-channel solution to be
reliable; the channel agility of TSCH is the key to its ultra high reliable; the channel agility of TSCH is the key to its ultra high
reliability. Commercial networking solutions are available today in reliability. Commercial networking solutions are available today in
which motes consume 10's of micro-amps on average [CurrentCalculator] which nodes consume 10's of micro-amps on average [CurrentCalculator]
with end-to-end packet delivery ratios over 99.999% with end-to-end packet delivery ratios over 99.999%
[doherty07channel]. [doherty07channel].
IEEE802.15.4e has been designed for low-power constrained devices,
often called "motes". Several terms are used in the IETF to refer to
those devices, including "LLN nodes" [RFC7102] and "constrained
nodes" [RFC7228]. In this document, we use the generic (and shorter)
term "node", used as a synonym for "LLN node", "constrained node" or
"mote".
Bringing industrial-like performance into the LLN stack developed by Bringing industrial-like performance into the LLN stack developed by
Internet of Things (IoT) related IETF working groups such as 6Lo, Internet of Things (IoT) related IETF working groups such as 6Lo,
ROLL and CoRE opens up new application domains for these networks. ROLL and CoRE opens up new application domains for these networks.
Sensors deployed in smart cities [RFC5548] will be able to be Sensors deployed in smart cities [RFC5548] will be able to be
installed for years without needing battery replacement. "Umbrella" installed for years without needing battery replacement. "Umbrella"
networks will interconnect smart elements from different entities in networks will interconnect smart elements from different entities in
smart buildings [RFC5867]. Peel-and-stick switches will obsolete the smart buildings [RFC5867]. Peel-and-stick switches will obsolete the
need for costly conduits for lighting solutions in smart homes need for costly conduits for lighting solutions in smart homes
[RFC5826]. [RFC5826].
IEEE802.15.4e TSCH focuses on the MAC layer only. This clean IEEE802.15.4e TSCH focuses on the MAC layer only. This clean
layering allows for TSCH to fit under an IPv6 enabled protocol stack layering allows for TSCH to fit under an IPv6 enabled protocol stack
for LLNs, running 6LoWPAN [RFC6282], IPv6 Routing Protocol for Low for LLNs, running 6LoWPAN [RFC6282], IPv6 Routing Protocol for Low
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IEEE802.15.4e TSCH focuses on the MAC layer only. This clean IEEE802.15.4e TSCH focuses on the MAC layer only. This clean
layering allows for TSCH to fit under an IPv6 enabled protocol stack layering allows for TSCH to fit under an IPv6 enabled protocol stack
for LLNs, running 6LoWPAN [RFC6282], IPv6 Routing Protocol for Low for LLNs, running 6LoWPAN [RFC6282], IPv6 Routing Protocol for Low
power and Lossy Networks (RPL) [RFC6550] and the Constrained power and Lossy Networks (RPL) [RFC6550] and the Constrained
Application Protocol (CoAP) [RFC7252]. What is missing is a Logical Application Protocol (CoAP) [RFC7252]. What is missing is a Logical
Link Control (LLC) layer between the IP abstraction of a link and the Link Control (LLC) layer between the IP abstraction of a link and the
TSCH MAC, which is in charge of scheduling a timeslot for a given TSCH MAC, which is in charge of scheduling a timeslot for a given
packet coming down the stack from the upper layer. packet coming down the stack from the upper layer.
While [IEEE802154e] defines the mechanisms for a TSCH mote to While [IEEE802154e] defines the mechanisms for a TSCH node to
communicate, it does not define the policies to build and maintain communicate, it does not define the policies to build and maintain
the communication schedule, match that schedule to the multi-hop the communication schedule, match that schedule to the multi-hop
paths maintained by RPL, adapt the resources allocated between paths maintained by RPL, adapt the resources allocated between
neighbor nodes to the data traffic flows, enforce a differentiated neighbor nodes to the data traffic flows, enforce a differentiated
treatment for data generated at the application layer and signaling treatment for data generated at the application layer and signaling
messages needed by 6LoWPAN and RPL to discover neighbors, react to messages needed by 6LoWPAN and RPL to discover neighbors, react to
topology changes, self-configure IP addresses, or manage keying topology changes, self-configure IP addresses, or manage keying
material. material.
In other words, IEEE802.15.4e TSCH is designed to allow optimizations In other words, IEEE802.15.4e TSCH is designed to allow optimizations
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Successful solutions take into account the specific application Successful solutions take into account the specific application
requirements, along with IPv6 behavior and 6LoWPAN mechanisms requirements, along with IPv6 behavior and 6LoWPAN mechanisms
[palattella12standardized]. The ROLL working group has defined RPL [palattella12standardized]. The ROLL working group has defined RPL
in [RFC6550]. RPL can support a wide variety of link layers, in [RFC6550]. RPL can support a wide variety of link layers,
including ones that are constrained, potentially lossy, or typically including ones that are constrained, potentially lossy, or typically
utilized in conjunction with host or router devices with very limited utilized in conjunction with host or router devices with very limited
resources, as in building/home automation [RFC5867][RFC5826], resources, as in building/home automation [RFC5867][RFC5826],
industrial environments [RFC5673], and urban applications [RFC5548]. industrial environments [RFC5673], and urban applications [RFC5548].
RPL is able to quickly build up network routes, distribute routing RPL is able to quickly build up network routes, distribute routing
knowledge among nodes, and adapt to a changing topology. In a knowledge among nodes, and adapt to a changing topology. In a
typical setting, motes are connected through multi-hop paths to a typical setting, nodes are connected through multi-hop paths to a
small set of root devices, which are usually responsible for data small set of root devices, which are usually responsible for data
collection and coordination. For each of them, a Destination collection and coordination. For each of them, a Destination
Oriented Directed Acyclic Graph (DODAG) is created by accounting for Oriented Directed Acyclic Graph (DODAG) is created by accounting for
link costs, node attributes/status information, and an Objective link costs, node attributes/status information, and an Objective
Function, which maps the optimization requirements of the target Function, which maps the optimization requirements of the target
scenario. scenario.
The topology is set up based on a Rank metric, which encodes the The topology is set up based on a Rank metric, which encodes the
distance of each node with respect to its reference root, as distance of each node with respect to its reference root, as
specified by the Objective Function. Regardless of the way it is specified by the Objective Function. Regardless of the way it is
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generic "LLC". generic "LLC".
Some of the issues the LLC needs to target might overlap with the Some of the issues the LLC needs to target might overlap with the
scope of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this scope of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this
case, it is entailed that the LLC will profit from the services case, it is entailed that the LLC will profit from the services
provided by other protocols to pursue these objectives. provided by other protocols to pursue these objectives.
4.1. Network Formation 4.1. Network Formation
The LLC needs to control the way the network is formed, including how The LLC needs to control the way the network is formed, including how
new motes join, and how already joined motes advertise the presence new nodes join, and how already joined nodes advertise the presence
of the network. The LLC needs to: of the network. The LLC needs to:
1. Define the Information Elements included in the Enhanced Beacons 1. Define the Information Elements included in the Enhanced Beacons
advertising the presence of the network. advertising the presence of the network.
2. For a new mote, define rules to process and filter received 2. For a new node, define rules to process and filter received
Enhanced Beacons. Enhanced Beacons.
3. Define the joining procedure. This might include a mechanism to 3. Define the joining procedure. This might include a mechanism to
assign a unique 16-bit address to a mote, and the management of assign a unique 16-bit address to a node, and the management of
initial keying material. initial keying material.
4. Define a mechanism to secure the joining process and the 4. Define a mechanism to secure the joining process and the
subsequent optional process of scheduling more communication subsequent optional process of scheduling more communication
cells. cells.
4.2. Network Maintenance 4.2. Network Maintenance
Once a network is formed, the LLC needs to maintain the network's Once a network is formed, the LLC needs to maintain the network's
health, allowing for motes to stay synchronized. The LLC needs to: health, allowing for nodes to stay synchronized. The LLC needs to:
1. Manage each mote's time source neighbor. 1. Manage each node's time source neighbor.
2. Define a mechanism for a mote to update the join priority it 2. Define a mechanism for a node to update the join priority it
announces in its Enhanced Beacon. announces in its Enhanced Beacon.
3. Schedule transmissions of Enhanced Beacons to advertise the 3. Schedule transmissions of Enhanced Beacons to advertise the
presence of the network. presence of the network.
4.3. Multi-Hop Topology 4.3. Multi-Hop Topology
RPL, given a weighted connectivity graph, determines multi-hop RPL, given a weighted connectivity graph, determines multi-hop
routes. The LLC needs to: routes. The LLC needs to:
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can then feed to RPL. can then feed to RPL.
2. Ensure that the TSCH schedule contains cells along the multi-hop 2. Ensure that the TSCH schedule contains cells along the multi-hop
routes identified by RPL. routes identified by RPL.
3. Where applicable, maintain independent sets of cells to transport 3. Where applicable, maintain independent sets of cells to transport
independent flows of data. independent flows of data.
4.4. Routing and Timing Parents 4.4. Routing and Timing Parents
At all times, a TSCH mote needs to have a time source neighbor it can At all times, a TSCH node needs to have a time source neighbor it can
synchronize to. The LLC therefore needs to assign a time source synchronize to. The LLC therefore needs to assign a time source
neighbor to allow for correct operation of the TSCH network. A time neighbor to allow for correct operation of the TSCH network. A time
source neighbors could, or not, be taken from the RPL routing parent source neighbors could, or not, be taken from the RPL routing parent
set. set.
4.5. Resource Management 4.5. Resource Management
A cell in a TSCH schedule is an atomic "unit" of resource. The A cell in a TSCH schedule is an atomic "unit" of resource. The
number of cells to assign between neighbor motes needs to be number of cells to assign between neighbor nodes needs to be
appropriate for the size of the traffic flow. The LLC needs to: appropriate for the size of the traffic flow. The LLC needs to:
1. Define a mechanism for neighbor nodes to exchange information 1. Define a mechanism for neighbor nodes to exchange information
about their schedule and, if applicable, negotiate the addition/ about their schedule and, if applicable, negotiate the addition/
deletion of cells. deletion of cells.
2. Allow for an entity (e.g., a set of devices, a distributed 2. Allow for an entity (e.g., a set of devices, a distributed
protocol, a PCE, etc.) to take control of the schedule. protocol, a PCE, etc.) to take control of the schedule.
4.6. Dataflow Control 4.6. Dataflow Control
TSCH defines mechanisms for a mote to signal it cannot accept an TSCH defines mechanisms for a node to signal it cannot accept an
incoming packet. It does not, however, define the policy which incoming packet. It does not, however, define the policy which
determines when to stop accepting packets. The LLC needs to: determines when to stop accepting packets. The LLC needs to:
1. Define a queuing policy for incoming and outgoing packets. 1. Define a queuing policy for incoming and outgoing packets.
2. Manage the buffer space, and indicate to TSCH when to stop 2. Manage the buffer space, and indicate to TSCH when to stop
accepting incoming packets. accepting incoming packets.
3. Handle transmissions that have failed. A transmission is 3. Handle transmissions that have failed. A transmission is
declared failed when TSCH has retransmitted the packet multiple declared failed when TSCH has retransmitted the packet multiple
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3. Define an mechanism for these different scheduling mechanisms to 3. Define an mechanism for these different scheduling mechanisms to
coexist in the same network. coexist in the same network.
4.9. Secure Communication 4.9. Secure Communication
Given some keying material, TSCH defines mechanisms to encrypt and Given some keying material, TSCH defines mechanisms to encrypt and
authenticate MAC frames. It does not define how this keying material authenticate MAC frames. It does not define how this keying material
is generated. The LLC needs to: is generated. The LLC needs to:
1. Define the keying material and authentication mechanism needed by 1. Define the keying material and authentication mechanism needed by
a new mote to join an existing network. a new node to join an existing network.
2. Define a mechanism to allow for the secure transfer of 2. Define a mechanism to allow for the secure transfer of
application data between neighbor motes. application data between neighbor nodes.
3. Define a mechanism to allow for the secure transfer of signaling 3. Define a mechanism to allow for the secure transfer of signaling
data between motes and the LLC. data between nodes and the LLC.
5. IANA Considerations 5. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
6. Security Considerations 6. Security Considerations
This memo is an informational overview of existing standards, and This memo is an informational overview of existing standards, and
does define any new mechanisms or protocols. does define any new mechanisms or protocols.
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8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References 8.2. Informative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014. Application Protocol (CoAP)", RFC 7252, June 2014.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, May 2014.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, January 2014.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing", RFC Wireless Personal Area Network (6LoWPAN) Routing", RFC
6606, May 2012. 6606, May 2012.
[RFC6568] Kim, E., Kaspar, D., and JP. Vasseur, "Design and [RFC6568] Kim, E., Kaspar, D., and JP. Vasseur, "Design and
Application Spaces for IPv6 over Low-Power Wireless Application Spaces for IPv6 over Low-Power Wireless
Personal Area Networks (6LoWPANs)", RFC 6568, April 2012. Personal Area Networks (6LoWPANs)", RFC 6568, April 2012.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., [RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
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[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004. RFC 3819, July 2004.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998. (IPv6) Specification", RFC 2460, December 1998.
[I-D.ietf-6tisch-tsch] [I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M., and L. Grieco, "Using Watteyne, T., Palattella, M., and L. Grieco, "Using
IEEE802.15.4e TSCH in an LLN context: Overview, Problem IEEE802.15.4e TSCH in an IoT context: Overview, Problem
Statement and Goals", draft-ietf-6tisch-tsch-01 (work in Statement and Goals", draft-ietf-6tisch-tsch-02 (work in
progress), July 2014. progress), October 2014.
[I-D.ietf-6tisch-architecture] [I-D.ietf-6tisch-architecture]
Thubert, P., Watteyne, T., and R. Assimiti, "An Thubert, P., Watteyne, T., and R. Assimiti, "An
Architecture for IPv6 over the TSCH mode of IEEE Architecture for IPv6 over the TSCH mode of IEEE
802.15.4e", draft-ietf-6tisch-architecture-03 (work in 802.15.4e", draft-ietf-6tisch-architecture-03 (work in
progress), July 2014. progress), July 2014.
[I-D.ietf-6tisch-terminology] [I-D.ietf-6tisch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang, Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terminology in IPv6 over the TSCH mode of IEEE "Terminology in IPv6 over the TSCH mode of IEEE
802.15.4e", draft-ietf-6tisch-terminology-02 (work in 802.15.4e", draft-ietf-6tisch-terminology-02 (work in
progress), July 2014. progress), July 2014.
[I-D.ietf-6tisch-minimal] [I-D.ietf-6tisch-minimal]
Vilajosana, X. and K. Pister, "Minimal 6TiSCH Vilajosana, X. and K. Pister, "Minimal 6TiSCH
Configuration", draft-ietf-6tisch-minimal-02 (work in Configuration", draft-ietf-6tisch-minimal-03 (work in
progress), July 2014. progress), October 2014.
[I-D.ietf-6tisch-6top-interface] [I-D.ietf-6tisch-6top-interface]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Interface", draft-ietf-6tisch- Operation Sublayer (6top) Interface", draft-ietf-6tisch-
6top-interface-01 (work in progress), July 2014. 6top-interface-01 (work in progress), July 2014.
[I-D.wang-6tisch-6top-sublayer] [I-D.wang-6tisch-6top-sublayer]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top)", draft-wang-6tisch-6top- Operation Sublayer (6top)", draft-wang-6tisch-6top-
sublayer-01 (work in progress), July 2014. sublayer-01 (work in progress), July 2014.
skipping to change at page 15, line 49 skipping to change at page 16, line 21
o Concepts which are sufficiently different from traditional o Concepts which are sufficiently different from traditional
IEEE802.15.4 networking that they may need to be defined and IEEE802.15.4 networking that they may need to be defined and
presented precisely. presented precisely.
o Techniques and ideas which are part of IEEE802.15.4e and which o Techniques and ideas which are part of IEEE802.15.4e and which
might be useful for the work of the 6TiSCH WG. might be useful for the work of the 6TiSCH WG.
A.1. Timeslots A.1. Timeslots
All motes in a TSCH network are synchronized. Time is sliced up into All nodes in a TSCH network are synchronized. Time is sliced up into
timeslots. A timeslot is long enough for a MAC frame of maximum size timeslots. A timeslot is long enough for a MAC frame of maximum size
to be sent from mote A to mote B, and for mote B to reply with an to be sent from node A to node B, and for node B to reply with an
acknowledgment (ACK) frame indicating successful reception. acknowledgment (ACK) frame indicating successful reception.
The duration of a timeslot is not defined by the standard. With The duration of a timeslot is not defined by the standard. With
IEEE802.15.4-compliant radios operating in the 2.4GHz frequency band, IEEE802.15.4-compliant radios operating in the 2.4GHz frequency band,
a maximum-length frame of 127 bytes takes about 4ms to transmit; a a maximum-length frame of 127 bytes takes about 4ms to transmit; a
shorter ACK takes about 1ms. With a 10ms slot (a typical duration), shorter ACK takes about 1ms. With a 10ms slot (a typical duration),
this leaves 5ms to radio turnaround, packet processing and security this leaves 5ms to radio turnaround, packet processing and security
operations. operations.
A.2. Slotframes A.2. Slotframes
Timeslots are grouped into one of more slotframes. A slotframe Timeslots are grouped into one of more slotframes. A slotframe
continuously repeats over time. TSCH does not impose a slotframe continuously repeats over time. TSCH does not impose a slotframe
size. Depending on the application needs, these can range from 10s size. Depending on the application needs, these can range from 10s
to 1000s of timeslots. The shorter the slotframe, the more often a to 1000s of timeslots. The shorter the slotframe, the more often a
timeslot repeats, resulting in more available bandwidth, but also in timeslot repeats, resulting in more available bandwidth, but also in
a higher power consumption. a higher power consumption.
A.3. Node TSCH Schedule A.3. Node TSCH Schedule
A TSCH schedule instructs each mote what to do in each timeslot: A TSCH schedule instructs each node what to do in each timeslot:
transmit, receive or sleep. The schedule indicates, for each transmit, receive or sleep. The schedule indicates, for each
scheduled (transmit or receive) cell, a channelOffset and the address scheduled (transmit or receive) cell, a channelOffset and the address
of the neighbor to communicate with. of the neighbor to communicate with.
Once a mote obtains its schedule, it executes it: Once a node obtains its schedule, it executes it:
o For each transmit cell, the mote checks whether there is a packet o For each transmit cell, the node checks whether there is a packet
in the outgoing buffer which matches the neighbor written in the in the outgoing buffer which matches the neighbor written in the
schedule information for that timeslot. If there is none, the schedule information for that timeslot. If there is none, the
mote keeps its radio off for the duration of the timeslot. If node keeps its radio off for the duration of the timeslot. If
there is one, the mote can ask for the neighbor to acknowledge it, there is one, the node can ask for the neighbor to acknowledge it,
in which case it has to listen for the acknowledgment after in which case it has to listen for the acknowledgment after
transmitting. transmitting.
o For each receive cell, the mote listens for possible incoming o For each receive cell, the node listens for possible incoming
packets. If none is received after some listening period, it packets. If none is received after some listening period, it
shuts down its radio. If a packet is received, addressed to the shuts down its radio. If a packet is received, addressed to the
mote, and passes security checks, the mote can send back an node, and passes security checks, the node can send back an
acknowledgment. acknowledgment.
How the schedule is built, updated and maintained, and by which How the schedule is built, updated and maintained, and by which
entity, is outside of the scope of the IEEE802.15.4e standard. entity, is outside of the scope of the IEEE802.15.4e standard.
A.4. Cells and Bundles A.4. Cells and Bundles
Assuming the schedule is well built, if mote A is scheduled to Assuming the schedule is well built, if node A is scheduled to
transmit to mote B at slotOffset 5 and channelOffset 11, mote B will transmit to node B at slotOffset 5 and channelOffset 11, node B will
be scheduled to receive from mote A at the same slotOffset and be scheduled to receive from node A at the same slotOffset and
channelOffset. channelOffset.
A single element of the schedule characterized by a slotOffset and A single element of the schedule characterized by a slotOffset and
channelOffset, and reserved for mote A to transmit to mote B (or for channelOffset, and reserved for node A to transmit to node B (or for
mote B to receive from mote A) within a given slotframe, is called a node B to receive from node A) within a given slotframe, is called a
"scheduled cell". "scheduled cell".
If there is a lot of data flowing from mote A to mote B, the schedule If there is a lot of data flowing from node A to node B, the schedule
might contain multiple cells from A to B, at different times. might contain multiple cells from A to B, at different times.
Multiple cells scheduled to the same neighbor can be equivalent, i.e. Multiple cells scheduled to the same neighbor can be equivalent, i.e.
the MAC layer sends the packet on whichever of these cells shows up the MAC layer sends the packet on whichever of these cells shows up
first after the packet was put in the MAC queue. The union of all first after the packet was put in the MAC queue. The union of all
cells between two neighbors, A and B, is called a "bundle". Since cells between two neighbors, A and B, is called a "bundle". Since
the slotframe repeats over time (and the length of the slotframe is the slotframe repeats over time (and the length of the slotframe is
typically constant), each cell gives a "quantum" of bandwidth to a typically constant), each cell gives a "quantum" of bandwidth to a
given neighbor. Modifying the number of equivalent cells in a bundle given neighbor. Modifying the number of equivalent cells in a bundle
modifies the amount of resources allocated between two neighbors. modifies the amount of resources allocated between two neighbors.
A.5. Dedicated vs. Shared Cells A.5. Dedicated vs. Shared Cells
By default, each scheduled transmit cell within the TSCH schedule is By default, each scheduled transmit cell within the TSCH schedule is
dedicated, i.e., reserved only for mote A to transmit to mote B. dedicated, i.e., reserved only for node A to transmit to node B.
IEEE802.15.4e allows also to mark a cell as shared. In a shared IEEE802.15.4e allows also to mark a cell as shared. In a shared
cell, multiple motes can transmit at the same time, on the same cell, multiple nodes can transmit at the same time, on the same
frequency. To avoid contention, TSCH defines a back-off algorithm frequency. To avoid contention, TSCH defines a back-off algorithm
for shared cells. for shared cells.
A scheduled cell can be marked as both transmitting and receiving. A scheduled cell can be marked as both transmitting and receiving.
In this case, a mote transmits if it has an appropriate packet in its In this case, a node transmits if it has an appropriate packet in its
output buffer, or listens otherwise. Marking a cell as output buffer, or listens otherwise. Marking a cell as
[transmit,receive,shared] results in slotted-Aloha behavior. [transmit,receive,shared] results in slotted-Aloha behavior.
A.6. Absolute Slot Number A.6. Absolute Slot Number
TSCH defines a timeslot counter called Absolute Slot Number (ASN). TSCH defines a timeslot counter called Absolute Slot Number (ASN).
When a new network is created, the ASN is initialized to 0; from then When a new network is created, the ASN is initialized to 0; from then
on, it increments by 1 at each timeslot. In detail: on, it increments by 1 at each timeslot. In detail:
ASN = (k*S+t) ASN = (k*S+t)
where k is the slotframe cycle (i.e., the number of slotframe where k is the slotframe cycle (i.e., the number of slotframe
repetitions since the network was started), S the slotframe size and repetitions since the network was started), S the slotframe size and
t the slotOffset. A mote learns the current ASN when it joins the t the slotOffset. A node learns the current ASN when it joins the
network. Since motes are synchronized, they all know the current network. Since nodes are synchronized, they all know the current
value of the ASN, at any time. The ASN is encoded as a 5-byte value of the ASN, at any time. The ASN is encoded as a 5-byte
number: this allows it to increment for hundreds of years (the exact number: this allows it to increment for hundreds of years (the exact
value depends on the duration of a timeslot) without wrapping over. value depends on the duration of a timeslot) without wrapping over.
The ASN is used to calculate the frequency to communicate on, and can The ASN is used to calculate the frequency to communicate on, and can
be used for security-related operations. be used for security-related operations.
A.7. Channel Hopping A.7. Channel Hopping
For each scheduled cell, the schedule specifies a slotOffset and a For each scheduled cell, the schedule specifies a slotOffset and a
channelOffset. In a well-built schedule, when mote A has a transmit channelOffset. In a well-built schedule, when node A has a transmit
cell to mote B on channelOffset 5, mote B has a receive cell from cell to node B on channelOffset 5, node B has a receive cell from
mote A on the same channelOffset. The channelOffset is translated by node A on the same channelOffset. The channelOffset is translated by
both nodes into a frequency using the following function: both nodes into a frequency using the following function:
frequency = F {(ASN + channelOffset) mod nFreq} frequency = F {(ASN + channelOffset) mod nFreq}
The function F consists of a look-up table containing the set of The function F consists of a look-up table containing the set of
available channels. The value nFreq (the number of available available channels. The value nFreq (the number of available
frequencies) is the size of this look-up table. There are as many frequencies) is the size of this look-up table. There are as many
channelOffset values as there are frequencies available (e.g. 16 when channelOffset values as there are frequencies available (e.g. 16 when
using IEEE802.15.4-compliant radios at 2.4GHz, when all channels are using IEEE802.15.4-compliant radios at 2.4GHz, when all channels are
used). Since both motes have the same channelOffset written in their used). Since both nodes have the same channelOffset written in their
schedule for that scheduled cell, and the same ASN counter, they schedule for that scheduled cell, and the same ASN counter, they
compute the same frequency. At the next iteration (cycle) of the compute the same frequency. At the next iteration (cycle) of the
slotframe, however, while the channelOffset is the same, the ASN has slotframe, however, while the channelOffset is the same, the ASN has
changed, resulting in the computation of a different frequency. changed, resulting in the computation of a different frequency.
This results in "channel hopping": even with a static schedule, pairs This results in "channel hopping": even with a static schedule, pairs
of neighbors "hop" between the different frequencies when of neighbors "hop" between the different frequencies when
communicating. A way of ensuring communication happens on all communicating. A way of ensuring communication happens on all
available frequencies is to set the number of timeslots in a available frequencies is to set the number of timeslots in a
slotframe to a prime number. Channel hopping is a technique known to slotframe to a prime number. Channel hopping is a technique known to
efficiently combat multi-path fading and external interference. efficiently combat multi-path fading and external interference.
A.8. Time Synchronization A.8. Time Synchronization
Because of the slotted nature of communication in a TSCH network, Because of the slotted nature of communication in a TSCH network,
motes have to maintain tight synchronization. All motes are assumed nodes have to maintain tight synchronization. All nodes are assumed
to be equipped with clocks to keep track of time. Yet, because to be equipped with clocks to keep track of time. Yet, because
clocks in different motes drift with respect to one another, neighbor clocks in different nodes drift with respect to one another, neighbor
motes need to periodically re-synchronize. nodes need to periodically re-synchronize.
Each mote needs to periodically synchronize its network clock to Each node needs to periodically synchronize its network clock to
another mote, and it also provides its network time to its neighbors. another node, and it also provides its network time to its neighbors.
It is up to the entity that manages the schedule to assign an It is up to the entity that manages the schedule to assign an
adequate time source neighbor to each mote, i.e., to indicate in the adequate time source neighbor to each node, i.e., to indicate in the
schedule which of neighbor is its "time source neighbor". While schedule which of neighbor is its "time source neighbor". While
setting the time source neighbor, it is important to avoid setting the time source neighbor, it is important to avoid
synchronization loops, which could result in the formation of synchronization loops, which could result in the formation of
independent clusters of synchronized motes. independent clusters of synchronized nodes.
TSCH adds timing information in all packets that are exchanged (both TSCH adds timing information in all packets that are exchanged (both
data and ACK frames). This means that neighbor motes can data and ACK frames). This means that neighbor nodes can
resynchronize to one another whenever they exchange data. In detail, resynchronize to one another whenever they exchange data. In detail,
two methods are defined in IEEE802.15.4e-2012 for allowing a device two methods are defined in IEEE802.15.4e-2012 for allowing a device
to synchronize in a TSCH network: (i) Acknowledgment-Based and (ii) to synchronize in a TSCH network: (i) Acknowledgment-Based and (ii)
Frame-Based synchronization. In both cases, the receiver calculates Frame-Based synchronization. In both cases, the receiver calculates
the difference in time between the expected time of frame arrival and the difference in time between the expected time of frame arrival and
its actual arrival. In Acknowledgment-Based synchronization, the its actual arrival. In Acknowledgment-Based synchronization, the
receiver provides such information to the sender mote in its receiver provides such information to the sender node in its
acknowledgment. In this case, it is the sender mote that acknowledgment. In this case, it is the sender node that
synchronizes to the clock of the receiver. In Frame-Based synchronizes to the clock of the receiver. In Frame-Based
synchronization, the receiver uses the computed delta for adjusting synchronization, the receiver uses the computed delta for adjusting
its own clock. In this case, it is the receiver mote that its own clock. In this case, it is the receiver node that
synchronizes to the clock of the sender. synchronizes to the clock of the sender.
Different synchronization policies are possible. Motes can keep Different synchronization policies are possible. Nodes can keep
synchronization exclusively by exchanging EBs. Motes can also keep synchronization exclusively by exchanging EBs. Nodes can also keep
synchronized by periodically sending valid frames to a time source synchronized by periodically sending valid frames to a time source
neighbor and use the acknowledgment to resynchronize. Both method neighbor and use the acknowledgment to resynchronize. Both method
(or a combination thereof) are valid synchronization policies; which (or a combination thereof) are valid synchronization policies; which
one to use depends on network requirements. one to use depends on network requirements.
A.9. Power Consumption A.9. Power Consumption
There are only a handful of activities a mote can perform during a There are only a handful of activities a node can perform during a
timeslot: transmit, receive, or sleep. Each of these operations has timeslot: transmit, receive, or sleep. Each of these operations has
some energy cost associated to them, the exact value depends on the some energy cost associated to them, the exact value depends on the
the hardware used. Given the schedule of a mote, it is the hardware used. Given the schedule of a node, it is
straightforward to calculate the expected average power consumption straightforward to calculate the expected average power consumption
of that mote. of that node.
A.10. Network TSCH Schedule A.10. Network TSCH Schedule
The schedule entirely defines the synchronization and communication The schedule entirely defines the synchronization and communication
between motes. By adding/removing cells between neighbors, one can between nodes. By adding/removing cells between neighbors, one can
adapt a schedule to the needs of the application. Intuitive examples adapt a schedule to the needs of the application. Intuitive examples
are: are:
o Make the schedule "sparse" for applications where motes need to o Make the schedule "sparse" for applications where nodes need to
consume as little energy as possible, at the price of reduced consume as little energy as possible, at the price of reduced
bandwidth. bandwidth.
o Make the schedule "dense" for applications where motes generate a o Make the schedule "dense" for applications where nodes generate a
lot of data, at the price of increased power consumption. lot of data, at the price of increased power consumption.
o Add more cells along a multi-hop route over which many packets o Add more cells along a multi-hop route over which many packets
flow. flow.
A.11. Join Process A.11. Join Process
Motes already part of the network can periodically send Enhanced Nodes already part of the network can periodically send Enhanced
Beacon (EB) frames to announce the presence of the network. These Beacon (EB) frames to announce the presence of the network. These
contain information about the size of the timeslot used in the contain information about the size of the timeslot used in the
network, the current ASN, information about the slotframes and network, the current ASN, information about the slotframes and
timeslots the beaconing mote is listening on, and a 1-byte join timeslots the beaconing node is listening on, and a 1-byte join
priority. Even if a node is configured to send all EB frames on the priority. The join priority field gives information to make a better
same channel offset, because of the channel hopping nature of TSCH decision of which node to join. Even if a node is configured to send
described in Appendix A.7, this channel offset translates into a all EB frames on the same channel offset, because of the channel
different frequency at different slotframe cycles. As a result, EB hopping nature of TSCH described in Appendix A.7, this channel offset
frames are sent on all frequencies. translates into a different frequency at different slotframe cycles.
As a result, EB frames are sent on all frequencies.
A mote wishing to join the network listens for EBs. Since EBs are A node wishing to join the network listens for EBs. Since EBs are
sent on all frequencies, the joining node can listen on any frequency sent on all frequencies, the joining node can listen on any frequency
until it hears an EB. What frequency it listens on, and whether it until it hears an EB. What frequency it listens on is
slowly changes frequency during this joining period is implementation-specific. Once it has received one or more EBs, the
implementation-specific. Using the ASN and the other timing new node enables the TSCH mode and uses the ASN and the other timing
information of the EB, the new mote synchronizes to the network. information from the EB to synchronize to the network. Using the
Using the slotframe and cell information from the EB, it knows how to slotframe and cell information from the EB, it knows how to contact
contact other nodes in the network. other nodes in the network.
The IEEE802.15.4e TSCH standard does not define the steps beyond this The IEEE802.15.4e TSCH standard does not define the steps beyond this
network "bootstrap". network "bootstrap".
A.12. Information Elements A.12. Information Elements
TSCH introduces the concept of Information Elements (IEs). An TSCH introduces the concept of Information Elements (IEs). An
information element is a list of Type-Length-Value containers placed information element is a list of Type-Length-Value containers placed
at the end of the MAC header. A small number of types are defined at the end of the MAC header. A small number of types are defined
for TSCH (e.g., the ASN in the EB is contained in an IE), and an for TSCH (e.g., the ASN in the EB is contained in an IE), and an
unmanaged range is available for extensions. unmanaged range is available for extensions.
A data bit in the MAC header indicates whether the frame contains A data bit in the MAC header indicates whether the frame contains
IEs. IEs are grouped into Header IEs, consumed by the MAC layer and IEs. IEs are grouped into Header IEs, consumed by the MAC layer and
therefore typically invisible to the next higher layer, and Payload therefore typically invisible to the next higher layer, and Payload
IEs, which are passed untouched to the next higher layer, possibly IEs, which are passed untouched to the next higher layer, possibly
followed by regular payload. Payload IEs can therefore be used for followed by regular payload. Payload IEs can therefore be used for
the next higher layers of two neighbor motes to exchange information. the next higher layers of two neighbor nodes to exchange information.
A.13. Extensibility A.13. Extensibility
The TSCH standard is designed to be extensible. It introduces the The TSCH standard is designed to be extensible. It introduces the
mechanisms as "building block" (e.g., cells, bundles, slotframes, mechanisms as "building block" (e.g., cells, bundles, slotframes,
etc.), but leaves entire freedom to the upper layer to assemble etc.), but leaves entire freedom to the upper layer to assemble
those. The MAC protocol can be extended by defining new Header IEs. those. The MAC protocol can be extended by defining new Header IEs.
An intermediate layer can be defined to manage the MAC layer by An intermediate layer can be defined to manage the MAC layer by
defining new Payload IEs. defining new Payload IEs.
skipping to change at page 21, line 16 skipping to change at page 21, line 40
This section lists features of TSCH which we believe are important This section lists features of TSCH which we believe are important
and beneficial to the work of 6TiSCH. and beneficial to the work of 6TiSCH.
B.1. Collision Free Communication B.1. Collision Free Communication
TSCH allows one to design a schedule which yields collision-free TSCH allows one to design a schedule which yields collision-free
communication. This is done by building the schedule with dedicated communication. This is done by building the schedule with dedicated
cells in such a way that at most one node communicates with a cells in such a way that at most one node communicates with a
specific neighbor in each slotOffset/channelOffset cell. Multiple specific neighbor in each slotOffset/channelOffset cell. Multiple
pairs of neighbor motes can exchange data at the same time, but on pairs of neighbor nodes can exchange data at the same time, but on
different frequencies. different frequencies.
B.2. Multi-Channel vs. Channel Hopping B.2. Multi-Channel vs. Channel Hopping
A TSCH schedule looks like a matrix of width "slotframe size", S, and A TSCH schedule looks like a matrix of width "slotframe size", S, and
of height "number of frequencies", nFreq. For a scheduling of height "number of frequencies", nFreq. For a scheduling
algorithm, these can be considered atomic "units" to schedule. In algorithm, these can be considered atomic "units" to schedule. In
particular, because of the channel hopping nature of TSCH, the particular, because of the channel hopping nature of TSCH, the
scheduling algorithm should not worry about the actual frequency scheduling algorithm should not worry about the actual frequency
communication happens on, since it changes at each slotframe communication happens on, since it changes at each slotframe
iteration. iteration.
B.3. Cost of (continuous) Synchronization B.3. Cost of (continuous) Synchronization
When there is traffic in the network, motes which are communicating When there is traffic in the network, nodes which are communicating
implicitly re-synchronize using the data frames they exchange. In implicitly re-synchronize using the data frames they exchange. In
the absence of data traffic, motes are required to synchronize to the absence of data traffic, nodes are required to synchronize to
their time source neighbor(s) periodically not to drift in time. If their time source neighbor(s) periodically not to drift in time. If
they have not been communicating for some time (typically 30s), motes they have not been communicating for some time (typically 30s), nodes
can exchange an dummy data frame to re-synchronize. The frequency at can exchange an dummy data frame to re-synchronize. The frequency at
which such messages need to be transmitted depends on the stability which such messages need to be transmitted depends on the stability
of the clock source, and on how "early" each mote starts listening of the clock source, and on how "early" each node starts listening
for data (the "guard time"). Theoretically, with a 10ppm clock and a for data (the "guard time"). Theoretically, with a 10ppm clock and a
1ms guard time, this period can be 100s. Assuming this exchange 1ms guard time, this period can be 100s. Assuming this exchange
causes the mote's radio to be on for 5ms, this yields a radio duty causes the node's radio to be on for 5ms, this yields a radio duty
cycle needed to keep synchronized of 5ms/100s=0.005%. While TSCH does cycle needed to keep synchronized of 5ms/100s=0.005%. While TSCH does
requires motes to resynchronize periodically, the cost of doing so is requires nodes to resynchronize periodically, the cost of doing so is
very low. very low.
B.4. Topology Stability B.4. Topology Stability
The channel hopping nature of TSCH causes links to be very "stable". The channel hopping nature of TSCH causes links to be very "stable".
Wireless phenomena such as multi-path fading and external Wireless phenomena such as multi-path fading and external
interference impact a wireless link between two motes differently on interference impact a wireless link between two nodes differently on
each frequency. If a transmission from mote A to mote B fails, each frequency. If a transmission from node A to node B fails,
retransmitting on a different frequency has a higher likelihood of retransmitting on a different frequency has a higher likelihood of
succeeding that retransmitting on the same frequency. As a result, succeeding that retransmitting on the same frequency. As a result,
even when some frequencies are "behaving bad", channel hopping even when some frequencies are "behaving bad", channel hopping
"smoothens" the contribution of each frequency, resulting in more "smoothens" the contribution of each frequency, resulting in more
stable links, and therefore a more stable topology. stable links, and therefore a more stable topology.
B.5. Multiple Concurrent Slotframes B.5. Multiple Concurrent Slotframes
The TSCH standard allows for multiple slotframes to coexist in a The TSCH standard allows for multiple slotframes to coexist in a
mote's schedule. It is possible that, at some timeslot, a mote has node's schedule. It is possible that, at some timeslot, a node has
multiple activities scheduled (e.g. transmit to mote B on slotframe multiple activities scheduled (e.g. transmit to node B on slotframe
2, receive from mote C on slotframe 1). To handle this situation, 2, receive from node C on slotframe 1). To handle this situation,
the TSCH standard defines the following precedence rules: the TSCH standard defines the following precedence rules:
1. Transmissions take precedence over receptions; 1. Transmissions take precedence over receptions;
2. Lower slotframe identifiers take precedence over higher slotframe 2. Lower slotframe identifiers take precedence over higher slotframe
identifiers. identifiers.
In the example above, the mote would transmit to mote B on slotframe In the example above, the node would transmit to node B on slotframe
2. 2.
Authors' Addresses Authors' Addresses
Thomas Watteyne (editor) Thomas Watteyne (editor)
Linear Technology Linear Technology
30695 Huntwood Avenue 30695 Huntwood Avenue
Hayward, CA 94544 Hayward, CA 94544
USA USA
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