Internet DRAFT - draft-watteyne-6tisch-tsch
draft-watteyne-6tisch-tsch
6TiSCH T. Watteyne, Ed.
Internet-Draft Linear Technology
Intended status: Informational MR. Palattella
Expires: April 23, 2014 University of Luxembourg
LA. Grieco
Politecnico di Bari
October 20, 2013
Using IEEE802.15.4e TSCH in an LLN context:
Overview, Problem Statement and Goals
draft-watteyne-6tisch-tsch-00
Abstract
This document describes the environment, problem statement, and goals
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
only.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 4
3. Problems and Goals . . . . . . . . . . . . . . . . . . . . . 5
3.1. Network Formation . . . . . . . . . . . . . . . . . . . . 6
3.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 6
3.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . 7
3.4. Routing and Timing Parents . . . . . . . . . . . . . . . 7
3.5. Resource Management . . . . . . . . . . . . . . . . . . . 7
3.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . 8
3.7. Deterministic Behavior . . . . . . . . . . . . . . . . . 8
3.8. Scheduling Mechanisms . . . . . . . . . . . . . . . . . . 8
3.9. Secure Communication . . . . . . . . . . . . . . . . . . 9
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . 9
5.2. Informative References . . . . . . . . . . . . . . . . . 9
5.3. External Informative References . . . . . . . . . . . . . 12
Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 14
A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 14
A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . 15
A.3. Node TSCH Schedule . . . . . . . . . . . . . . . . . . . 15
A.4. Cells and Bundles . . . . . . . . . . . . . . . . . . . . 15
A.5. Dedicated vs. Shared Cells . . . . . . . . . . . . . . . 16
A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . 16
A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 16
A.8. Time Synchronization . . . . . . . . . . . . . . . . . . 17
A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 18
A.10. Network TSCH Schedule . . . . . . . . . . . . . . . . . . 18
A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . 18
A.12. Information Elements . . . . . . . . . . . . . . . . . . 19
A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 19
Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 19
B.1. Collision Free Communication . . . . . . . . . . . . . . 19
B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 19
B.3. Cost of (continuous) Synchronization . . . . . . . . . . 20
B.4. Topology Stability . . . . . . . . . . . . . . . . . . . 20
B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
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The IEEE802.15.4e standard [IEEE802154e] was published in 2012 as an
amendment to the Medium Access Control (MAC) protocol defined by the
IEEE802.15.4-2011 [IEEE802154] standard. The Timeslotted Channel
Hopping (TSCH) mode of IEEE802.15.4e is the object of this document.
This document describes the main issues arising from the adoption of
the IEEE802.15.4e TSCH in the LLN context, following the terminology
defined in [I-D.palattella-6tisch-terminology].
TSCH was designed to "allow IEEE802.15.4 devices to support a wide
range of industrial applications" [IEEE802154e]. At its core is a
medium access technique which uses time synchronization to achieve
ultra low-power operation and channel hopping to enable high
reliability. This is very different from the "legacy" IEEE802.15.4
MAC protocol, and is therefore better described as a "redesign".
TSCH does not amend the physical layer; i.e., it can operate on any
IEEE802.15.4-compliant hardware.
IEEE802.15.4e can be seen as the latest generation of ultra-lower
power and reliable networking solutions for LLNs. [RFC5673]
discusses industrial applications, and highlights the harsh operating
conditions as well as the stringent reliability, availability, and
security requirements for an LLN to operate in an industrial
environment. Commercial networking solutions are available today in
which motes consume 10's of micro-amps on average [CurrentCalculator]
with end-to-end packet delivery ratios over 99.999%
[doherty07channel].
IEEE802.15.4e TSCH focuses on the MAC layer only. This clean
layering allows for TSCH to fit under an IPv6 enabled protocol stack
for LLNs, running 6LoWPAN [RFC6282], RPL [RFC6550] and CoAP
[I-D.ietf-core-coap].
Bringing industrial-like performance into the LLN stack developed by
the 6LoWPAN, ROLL and CORE working groups opens up new application
domains for these networks. Sensors deployed in smart cities
[RFC5548] will be able to be installed for years without needing
battery replacement. "Umbrella" networks will interconnect smart
elements from different entities in smart buildings [RFC5867]. Peel-
and-stick switches will obsolete the need for costly conduits for
lighting solutions in smart homes [RFC5826].
While [IEEE802154e] defines the mechanisms for a TSCH mote to
communicate, it does not define the policies to build and maintain
the communication schedule, match that schedule to the multi-hop
paths maintained by RPL, adapt the resources allocated between
neighbor nodes to the data traffic flows, enforce a differentiated
treatment for data generated at the application layer and signalling
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messages needed by 6LoWPAN and RPL to discover neighbors, react to
topology changes, self-configure IP addresses, or manage keying
material.
In other words, IEEE802.15.4e TSCH is designed to allow optimizations
and strong customizations, simplifying the merging of TSCH with a
protocol stack based on IPv6, 6LoWPAN, and RPL.
2. TSCH in the LLN Context
In many cases, to map the services required by the IP layer to the
services provided by the link layer, an adaptation layer is used
[palattella12standardized]. The 6LoWPAN working group started
working in 2007 on specifications for transmitting IPv6 packets over
IEEE802.15.4 networks [RFC4919]. Typically, low-power WPANs are
characterized by small packet sizes, support for addresses with
different lengths, low bandwidth, star and mesh topologies, battery
powered devices, low cost, large number of devices, unknown node
positions, high unreliability, and periods during which communication
interfaces are turned off to save energy. Given these features, it
is clear that the adoption of IPv6 on top of a Low-Power WPAN is not
straightforward, but poses strong requirements for the optimization
of this adaptation layer. For instance, due to the IPv6 default
minimum MTU size (1280 bytes), an un-fragmented IPv6 packet is too
large to fit in an IEEE802.15.4 frame. Moreover, the overhead due to
the 40-byte long IPv6 header wastes the scarce bandwidth available at
the PHY layer [RFC4944]. For these reasons, the 6LoWPAN working
group has defined an effective adaptation layer [RFC6568]. Further
issues encompass the auto-configuration of IPv6 addresses
[RFC2464][RFC6755], the compliance with the recommendation on
supporting link-layer subnet broadcast in shared networks [RFC3819],
the reduction of routing and management overhead [RFC6606], the
adoption of lightweight application protocols (or novel data encoding
techniques), and the support for security mechanisms (confidentiality
and integrity protection, device bootstrapping, key establishment,
and management).
These features can run on top of TSCH. There are, however, important
issues to solve, as highlighted in Section 3.
Routing issues are challenging for 6LoWPAN, given the low-power and
lossy radio-links, the battery-powered nodes, the multi-hop mesh
topologies, and the frequent topology changes due to mobility.
Successful solutions take into account the specific application
requirements, along with IPv6 behavior and 6LoWPAN mechanisms
[palattella12standardized]. The ROLL working group has defined RPL
in [RFC6550]. RPL can support a wide variety of link layers,
including ones that are constrained, potentially lossy, or typically
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utilized in conjunction with host or router devices with very limited
resources, as in building/home automation [RFC5867][RFC5826],
industrial environments [RFC5673], and urban applications [RFC5548].
RPL is able to quickly build up network routes, distribute routing
knowledge among nodes, and adapt to a changing topology. In a
typical setting, motes are connected through multi-hop paths to a
small set of root devices, which are usually responsible for data
collection and coordination. For each of them, a Destination
Oriented Directed Acyclic Graph (DODAG) is created by accounting for
link costs, node attributes/status information, and an Objective
Function, which maps the optimization requirements of the target
scenario. The topology is set up based on a Rank metric, which
encodes the distance of each node with respect to its reference root,
as specified by the Objective Function. Regardless of the way it is
computed, the Rank monotonically decreases along the DODAG towards
the destination, building a gradient. RPL encompasses different
kinds of traffic and signalling information. Multipoint-to-Point
(MP2P) is the dominant traffic in LLN applications. Data is routed
towards nodes with some application relevance, such as the LLN
gateway to the larger Internet, or to the core of private IP
networks. In general, these destinations are the DODAG roots and act
as data collection points for distributed monitoring applications.
Point-to-Multipoint (P2MP) data streams are used for actuation
purposes, where messages are sent from DODAG roots to destination
nodes. Point-to-Point (P2P) traffic allows communication between two
devices belonging to the same LLN, such as a sensor and an actuator.
A packet flows from the source to the common ancestor of those two
communicating devices, then downward towards the destination. RPL
therefore has to discover both upward routes (i.e. from nodes to
DODAG roots) in order to enable MP2P and P2P flows, and downward
routes (i.e. from DODAG roots to nodes) to support P2MP and P2P
traffic.
Section 3 highlights the challenges that need to be addressed to use
RPL on top of TSCH.
Several open-source initiatives have emerged around TSCH. The
OpenWSN project [OpenWSN][OpenWSNETT] is an open-source
implementation of a standards-based protocol stack, which aims at
evaluating the applicability of TSCH to different applications. This
implementation was used as the foundation for an IP for Smart Objects
Alliance (IPSO) [IPSO] interoperability event in 2011. In the
absence of a standardized scheduling mechanism for TSCH, a "slotted
Aloha" schedule was used.
3. Problems and Goals
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As highlighted in Appendix A, TSCH is different for traditional low-
power MAC protocols because of its scheduled nature. TSCH defines
the mechanisms to execute a communication schedule, yet it is the
entity that sets up that schedule which controls the topology of the
network. This scheduling entity also controls the resources
allocated to each link in that topology.
How this entity should operate is out of scope of TSCH. The
remainder of this section highlights the problems this entity needs
to address. For simplicity, we will refer to this entity by the
generic name "6TiSCH". Note that the 6top sublayer, currently being
defined in [I-D.wang-6tsch-6top], can be seen as an embodiment of
this generic "6TiSCH".
Some of the issues 6TiSCH needs to target might overlap with the
scope of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this
case, it is entailed that 6TiSCH will profit from the services
provided by other protocols to pursue these objectives.
3.1. Network Formation
6TiSCH needs to control the way the network is formed, including how
new motes join, and how already joined motes advertise the presence
of the network. 6TiSCH needs to:
1. Define the Information Elements to include in the Enhanced
Beacons advertising the presence of the network.
2. For a new mote, define rules to process and filter received
Enhanced Beacons.
3. Define the joining procedure. This includes a mechanism to
assign a unique 16-bit address to a mote, and the management of
initial keying material.
4. Define a mechanism to secure the joining process and the
subsequent optional process of scheduling more communication
links.
3.2. Network Maintenance
Once a network is formed, 6TiSCH needs to maintain the network's
health, allowing for motes to stay synchronized. 6TiSCH needs to:
1. Manage each mote's time source neighbor.
2. Define a mechanism for a mote to update the join priority it
announces in its Enhanced Beacon.
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3. Schedule transmissions of Enhanced Beacons to advertise the
presence of the network.
3.3. Multi-Hop Topology
RPL, given a weighted connectivity graph, determines multi-hop
routes. 6TiSCH needs to:
1. Define a mechanism to gather topological information, which it
can then feed to RPL.
2. Ensure that the TSCH schedule contains links along the multi-hop
routes identified by RPL.
3. Where applicable, maintain independent sets of links to transport
independent flows of data.
3.4. Routing and Timing Parents
At all times, a TSCH mote needs to have a time source neighbor it can
synchronize to. 6TiSCH therefore needs to assign a time source
neighbor to allow for correct operation of the TSCH network. A time
source neighbors could, or not, be taken from the RPL routing parent
set.
3.5. Resource Management
A link in a TSCH schedule is a "unit" of resource. The number of
links to assign between neighbor motes needs to be appropriate for
the size of the traffic flow. 6TiSCH needs to:
1. Define rules on when to create or delete a slotframe.
2. Define rules to determine the length of a slotframe, and the
trigger to modify the length of a slotframe.
3. Define rules on when to add or delete links in a particular
slotframe.
4. Define a mechanism for neighbor nodes to exchange information
about their schedule and, if applicable, negotiate the addition/
deletion of links.
5. Allow for an entity (e.g., a set of devices, a distributed
protocol, a PCE, etc.) to take control of the schedule.
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6. Define a set of metrics to evaluate the trade-off between
latency, bandwidth and energy consumption achieved by a
particular schedule.
3.6. Dataflow Control
TSCH defines mechanisms for a mote to signal it cannot accept an
incoming packet. It does not, however, define the policy which
determines when to stop accepting packets. 6TiSCH needs to:
1. Define a queueing policy for incoming and outgoing packets.
2. Manage the buffer space, and indicate to TSCH when to stop
accepting incoming packets.
3. Handle transmissions that have failed. A transmission is
declared failed when TSCH has retransmitted the packet multiple
times, without receiving an acknowledgement. This covers both
dedicated and shared links.
3.7. Deterministic Behavior
As highlighted in [RFC5673], in some applications, data is generated
periodically and has a well understood data bandwidth requirement,
which is deterministic and predictable. 6TiSCH needs to:
1. Ensure timely delivery of such data.
2. Provide a mechanism for such deterministic flows to coexist with
bursty or infrequent traffic flows of different priorities.
3.8. Scheduling Mechanisms
Several scheduling mechanisms can be envisioned, and possibly coexist
in the same network. For example,
[I-D.phinney-roll-rpl-industrial-applicability] describe how the
allocation of bandwidth can be optimized by an external Path
Computation Element (PCE). Alternatively, two neighbor nodes can
adapt the number of cells autonomously by monitoring the amount of
traffic, and negotiating the allocation to extra cell when needed.
This mechanism can be used to establish multi-hop paths in a fashion
similar to RSVP. 6TiSCH needs to:
1. Provide a mechanism for two 6TiSCH devices to negotiate the
allocation and deallocation of cells between them.
2. Provide a mechanism for device to monitor and manage the 6TiSCH
capabilities of a node several hops away.
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3. Define an mechanism for these different scheduling mechanisms to
coexist in the same network.
3.9. Secure Communication
Given some keying material, TSCH defines mechanisms to encrypt and
authenticate MAC frames. It does not define how this keying material
is generated. 6TiSCH needs to:
1. Define the keying material and authentication mechanism needed by
a new mote to join an existing network.
2. Define a mechanism to allow for the secure transfer of
application data between neighbor motes.
3. Define a mechanism to allow for the secure transfer of signalling
data between motes and 6TiSCH.
4. Acknowledgements
Special thanks to Jonathan Simon for his review and valuable
comments. Thanks to the IoT6 European Project (STREP) of the 7th
Framework Program (Grant 288445).
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
5.2. Informative References
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals", RFC
4919, August 2007.
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[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks", RFC
5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6568] Kim, E., Kaspar, D., and JP. Vasseur, "Design and
Application Spaces for IPv6 over Low-Power Wireless
Personal Area Networks (6LoWPANs)", RFC 6568, April 2012.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing", RFC
6606, May 2012.
[RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
for OAuth", RFC 6755, October 2012.
[I-D.wang-6tsch-6top]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TSCH
Operation Sublayer (6top)", draft-wang-6tsch-6top-00 (work
in progress), July 2013.
[I-D.palattella-6tisch-terminology]
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Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terminology in IPv6 over the TSCH mode of IEEE
802.15.4e", draft-palattella-6tisch-terminology-00 (work
in progress), October 2013.
[I-D.thubert-roll-forwarding-frags]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", draft-thubert-roll-forwarding-frags-02 (work in
progress), September 2013.
[I-D.tsao-roll-security-framework]
Tsao, T., Alexander, R., Daza, V., and A. Lozano, "A
Security Framework for Routing over Low Power and Lossy
Networks", draft-tsao-roll-security-framework-02 (work in
progress), March 2010.
[I-D.thubert-roll-asymlink]
Thubert, P., "RPL adaptation for asymmetrical links",
draft-thubert-roll-asymlink-02 (work in progress),
December 2011.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terms used in Ruting for Low power And Lossy
Networks", draft-ietf-roll-terminology-13 (work in
progress), October 2013.
[I-D.ietf-roll-p2p-rpl]
Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J.
Martocci, "Reactive Discovery of Point-to-Point Routes in
Low Power and Lossy Networks", draft-ietf-roll-p2p-rpl-17
(work in progress), March 2013.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Protocol for Low power
and Lossy Networks (MPL)", draft-ietf-roll-trickle-
mcast-05 (work in progress), August 2013.
[I-D.thubert-6lowpan-backbone-router]
Thubert, P., "6LoWPAN Backbone Router", draft-thubert-
6lowpan-backbone-router-03 (work in progress), February
2013.
[I-D.sarikaya-core-sbootstrapping]
Sarikaya, B., Ohba, Y., Moskowitz, R., Cao, Z., and R.
Cragie, "Security Bootstrapping Solution for Resource-
Constrained Devices", draft-sarikaya-core-
sbootstrapping-04 (work in progress), April 2012.
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[I-D.gilger-smart-object-security-workshop]
Gilger, J. and H. Tschofenig, "Report from the 'Smart
Object Security Workshop', 23rd March 2012, Paris,
France", draft-gilger-smart-object-security-workshop-00
(work in progress), October 2012.
[I-D.phinney-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P., and R. Assimiti, "RPL
applicability in industrial networks", draft-phinney-roll-
rpl-industrial-applicability-02 (work in progress),
February 2013.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
5.3. External Informative References
[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendament 1: MAC sublayer", April
2012.
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", June 2011.
[OpenWSN] , "Berkeley's OpenWSN Project Homepage", ,
<http://www.openwsn.org/>.
[OpenWSNETT]
Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
a standards-based low-power wireless development
environment", Transactions on Emerging Telecommunications
Technologies 2012, August 2012, <http://
onlinelibrary.wiley.com/doi/10.1002/ett.2558/abstract>.
[IPSO] , "IP for Smart Objects Alliance Homepage", ,
<http://www.ipso-alliance.org/>.
[CurrentCalculator]
Linear Technology, "Application Note: Using the Current
Calculator to Estimate Mote Power", August 2012, <http://
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cds.linear.com/docs/en/application-note/Application_Note_-
_Using_the_Current_Calculator_to_Estimate_Mote_Power.pdf>.
[doherty07channel]
Doherty, L., Lindsay, W., and J. Simon, "Channel-Specific
Wireless Sensor Network Path Data", IEEE International
Conference on Computer Communications and Networks (ICCCN)
2008, 2007.
[tinka10decentralized]
Tinka, A., Watteyne, T., and K. Pister, "A Decentralized
Scheduling Algorithm for Time Synchronized Channel
Hopping", Ad Hoc Networks 2010, 2010, < http://
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TSMP%20DSN08.pdf>.
[watteyne09reliability]
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[kerkez09feasibility]
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[TASA-SENSORS]
Palattella, MR., Accettura, N., Dohler, M., Grieco, LA.,
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Appendix A. TSCH Protocol Highlights
This appendix gives an overview of the key features of the
IEEE802.15.4e Timeslotted Channel Hopping (TSCH) amendment. It makes
no attempt at repeating the standard, but rather focuses on the
following:
o Concepts which are sufficiently different from traditional
IEEE802.15.4 networking that they may need to be defined and
presented precisely.
o Techniques and ideas which are part of IEEE802.15.4e and which
might be useful for the work of the 6TiSCH WG.
A.1. Timeslots
All motes in a TSCH network are synchronized. Time is sliced up into
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
acknowledgement (ACK) frame indicating successful reception.
The duration of a timeslot is not defined by the standard. With
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
shorter ACK takes about 1ms. With a 10ms slot (a typical duration),
this leaves 5ms to radio turnaround, packet processing and security
operations.
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A.2. Slotframes
Timeslots are grouped into one of more slotframes. A slotframe
continuously repeats over time. TSCH does not impose a slotframe
size. Depending on the application needs, these can range from 10s
to 1000s of timeslots. The shorter the slotframe, the more often a
timeslot repeats, resulting in more available bandwidth, but also in
a higher power consumption.
A.3. Node TSCH Schedule
A TSCH schedule instructs each mote what to do in each timeslot:
transmit, receive or sleep. The schedule indicates, for each
scheduled (transmit or receive) cell a channelOffset and the address
of the neighbor to communicate with.
Once a mote obtains its schedule, it executes it:
o For each transmit cell, the mote checks whether there is a packet
in the outgoing buffer which matches the neighbor written in the
schedule information for that timeslot. If there is none, the
mote keeps its radio off for the duration of the timeslot. If
there is one, the mote can ask for the neighbor to acknowledge it,
in which case it has to listen for the acknowledgement after
transmitting.
o For each receive cell, the mote listens for possible incoming
packets. If none is received after some listening period, it
shuts down its radio. If a packet is received, addressed to the
mote, and passes security checks, the mote can send back an
acknowledgement.
How the schedule is built, updated and maintained, and by which
entity, is outside of the scope of the IEEE802.15.4e standard.
A.4. Cells and Bundles
Assuming the schedule is well built, if mote A is scheduled to
transmit to mote B at slotOffset 5 and channelOffset 11, mote B will
be scheduled to receive from mote A at the same slotOffset and
channelOffset.
A single element of the schedule characterized by a slotOffset and
channelOffset, and reserved for mote A to transmit to mote B (or for
mote B to receive from mote A) within a given slotframe, is called a
"scheduled cell".
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If there is a lot of data flowing from mote A to mote B, the schedule
might contain multiple cells from A to B, at different times.
Multiple cells scheduled to the same neighbor can be equivalent, i.e.
the MAC layer sends the packet on whichever of these cells happens to
show up 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 the slotframe repeats over time (and the length of the
slotframe is typically constant), each cell gives a "quantum" of
bandwidth to a given neighbor. Modifying the number of equivalent
cells in a bundle modifies the amount of resources allocated between
two neighbors.
A.5. Dedicated vs. Shared Cells
By default, each scheduled transmit cell within the TSCH schedule is
dedicated, i.e., reserved only for mote A to transmit to mote B.
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
frequency. To avoid contention, TSCH defines a back-off algorithm
for shared cells.
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
output buffer, or listens otherwise. Marking a cell as
[transmit,shared,receive] results in slotted-Aloha behavior.
A.6. Absolute Slot Number
TSCH defines a timeslot counter called Absolute Slot Number (ASN).
When a new network is created, the ASN is initialized to 0; from then
on, it increments by 1 at each timeslot. In detail:
ASN = (k*S+t)
where k is the slotframe cycle (i.e., the number of slotframe
repetitions since the network was started), S the slotframe size and
t the slotOffset. A mote learns the current ASN when it joins the
network. Since motes are synchronized, they all know the current
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
value depends on the duration of a timeslot) without wrapping. The
ASN is used to calculate the frequency to communicate on, and can be
used for security-related operations.
A.7. Channel Hopping
For each scheduled cell, the schedule specifies a slotOffset and a
channelOffset. In a well-built schedule, when mote A has a transmit
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cell to mote B on channelOffset 5, mote B has a receive cell from
mote A on the same channelOffset. The channelOffset is translated by
both nodes into a frequency using the following function:
frequency = F {(ASN + channelOffset) mod nFreq}
The function F consists of a look-up table containing the set of
available channels. The value nFreq (the number of available
frequencies) is the size of this look-up table. There are as many
channelOffset values as there are frequencies available (e.g. 16 when
using IEEE802.15.4-compliant radios at 2.4GHz, when all channels are
used). Since both motes have the same channelOffset written in their
schedule for that scheduled cell, and the same ASN counter, they
compute the same frequency. At the next iteration (cycle) of the
slotframe, however, while the channelOffset is the same, the ASN has
changed, resulting in the computation of a different frequency.
This results in "channel hopping": even with a static schedule, pairs
of neighbors "hop" between the different frequencies when
communicating. Channel hopping is a technique known to efficiently
combat multi-path fading and external interference.
A.8. Time Synchronization
Because of the slotted nature of communication in a TSCH network,
motes have to maintain tight synchronization. All motes are assumed
to be equipped with clocks to keep track of time. Yet, because
clocks in different motes drift with respect to one another, neighbor
motes need to periodically re-synchronize.
Each mote needs to periodically synchronize its network clock to
another mote, and it also provides its network time to its neighbors.
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
schedule which of neighbor is its "time source neighbor". While
setting the time source neighbor, it is important to avoid
synchronization loops, which could result in the formation of
independent clusters of motes.
TSCH adds timing information in all packets that are exchanged (both
data and ACK frames). This means that neighbor motes can
resynchronize to one another whenever they exchange data. In detail,
in the IEEE 802.15.4e standard two methods are defined for allowing a
device to synchronize in a TSCH network: (i) Acknowledgement-Based
and (ii) Frame-Based synchronization. In both cases, the receiver
calculates the difference in time between the expected time of frame
arrival and its actual arrival. In Acknowledgement-Based
synchronization, the receiver provides such information to the sender
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mote in its acknowledgement. Thus, in this case, it is the sender
mote that synchronizes to the clock of the receiver. In Frame-Based
synchronization, the receiver uses the computed delta for adjusting
its own clock. Therefore, it is the receiver mote that synchronizes
to the clock of the sender.
Different synchronization policies are possible. Motes can keep
synchronization exclusively by exchanging EBs. Motes can also keep
synchronized by periodically sending valid frames to a time source
neighbor and use the acknowledgement to resynchronize. Both method
(or a combination thereof) are valid synchronization policies; which
one to use depends on network requirements.
A.9. Power Consumption
There are only a handful of activities a mote can perform during a
timeslot: transmit, receive, or sleep. Each of these operations has
some energy cost associated to them, the exact value depending on the
the hardware used. Given the schedule of a mote, it is
straightforward to calculate the expected average power consumption
of that mote.
A.10. Network TSCH Schedule
The schedule defines entirely the synchronization and communication
between motes. By adding/removing cells between neighbors, one can
adapt a schedule to the needs of the application. Intuitive examples
are:
o Make the schedule "sparse" for applications where motes need to
consume as little energy as possible, at the price of reduced
bandwidth.
o Make the schedule "dense" for applications where motes generate a
lot of data, at the price of increased power consumption.
o Add more cells along a multi-hop route over which many packets
flow.
A.11. Join Process
Motes already part of the network can periodically send Enhanced
Beacon (EB) frames to announce the presence the network. These
contain information about the size of the timeslot used in the
network, the current ASN, information about the slotframes and
timeslots the beaconing mote is listening on, and a 1-byte join
priority. Because of the channel hopping nature of TSCH, these EB
frames are sent on all frequencies.
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A mote wishing to join the network listens for EBs. Using the ASN
and the other timing information of the EB, the new mote synchronizes
to the network. Using the slotframe and link information from the
EB, it knows how to contact the network.
The IEEE802.15.4e TSCH standard does not define the steps beyond this
network "bootstrap".
A.12. Information Elements
TSCH introduces the concept of Information Elements (IEs). An
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
for TSCH (e.g., the ASN in the EB is contained in an IE), and an
unmanaged range is available for extensions.
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
therefore typically invisible to the next higher layer, and Payload
IEs, which are passed untouched to the next higher layer, possibly
followed by regular payload. Payload IEs can therefore be used for
the next higher layers of two neighbor motes to exchange information.
A.13. Extensibility
The TSCH standard is designed to be extensible. It introduces the
mechanisms as "building block" (e.g., cells, bundles, slotframes,
etc.), but leaves entire freedom to the upper layer to assemble
those. The MAC protocol can be extended by defining new Header IEs.
An intermediate layer can be defined to manage the MAC layer by
defining new Payload IEs.
Appendix B. TSCH Gotchas
This section lists features of TSCH which we believe are important
and beneficial to the work of 6TiSCH.
B.1. Collision Free Communication
TSCH allows one to design a schedule which yields collision-free
communication. This is done by building the schedule with dedicated
cells in such a way that at most one node can communicate with a
specific neighbor in each slotOffset/channelOffset cell. Multiple
pairs of neighbor motes can exchange data at the same time, but on
different frequencies.
B.2. Multi-Channel vs. Channel Hopping
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A TSCH schedule looks like a matrix of width "slotframe size", S, and
of height "number of frequencies", nFreq. For a scheduling
algorithm, these can be considered atomic "units" to schedule. In
particular, because of the channel hopping nature of TSCH, the
scheduling algorithm should not worry about the actual frequency
communication happens on, since it changes at each slotframe
iteration.
B.3. Cost of (continuous) Synchronization
When there is traffic in the network, motes which are communicating
implicitly re-synchronize using the data frames they exchange. In
the absence of data traffic, motes are required to synchronize to
their time source neighbor(s) periodically not to drift in time. If
they have not been communicating for some time (typically 30s), motes
can exchange an dummy data frame to re-synchronize. The frequency at
which such messages need to be transmitted depends on the stability
of the clock source, and on how "early" each mote starts listening
for data (the "guard time"). Theoretically, with a 10ppm clock and a
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
cycle needed to keep synchronized of 5ms/100s=0.005%. While TSCH does
requires motes to resynchronize periodically, the cost of doing so is
very low.
B.4. Topology Stability
The channel hopping nature of TSCH causes links to be very "stable".
Wireless phenomena such as multi-path fading and external
interference impact a wireless link between two motes differently on
each frequency. If a transmission from mote A to mote B fails,
retransmitting on a different frequency has a higher likelihood of
succeeding that retransmitting on the same frequency. As a result,
even when some frequencies are "behaving bad", channel hopping
"smoothens" the contribution of each frequency, resulting in more
stable links, and therefore a more stable topology.
B.5. Multiple Concurrent Slotframes
The TSCH standard allows for multiple slotframes to coexist in a
mote's schedule. It is possible that at some timeslot, a mote has
multiple activities scheduled (e.g. transmit to mote B on slotframe
2, receive from mote C on slotframe 1). To handle this situation,
the TSCH standard defines the following precedence rules:
1. Transmissions take precedence over receptions;
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2. Lower slotframe identifiers take precedence over higher slotframe
identifiers.
In the example above, the mote would transmit to mote B on slotframe
2.
Authors' Addresses
Thomas Watteyne (editor)
Linear Technology
30695 Huntwood Avenue
Hayward, CA 94544
USA
Phone: +1 (510) 400-2978
Email: twatteyne@linear.com
Maria Rita Palattella
University of Luxembourg
Interdisciplinary Centre for Security, Reliability and Trust
4, rue Alphonse Weicker
Luxembourg L-2721
LUXEMBOURG
Phone: +352 46 66 44 5841
Email: maria-rita.palattella@uni.lu
Luigi Alfredo Grieco
Politecnico di Bari
Department of Electrical and Information Engineering
Via Orabona 4
Bari 70125
Italy
Phone: +39 08 05 96 3911
Email: a.grieco@poliba.it
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