6TSCH T. Watteyne, Ed.
Internet-Draft Linear Technology
Intended status: Informational February 21, 2013
Expires: August 25, 2013
Using IEEE802.15.4e TSCH in an LLN context:
Overview, Problem Statement and Goals
draft-watteyne-6tsch-tsch-lln-context-01
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 5
3. Problems and Goals . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Network Formation . . . . . . . . . . . . . . . . . . . . 7
3.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 7
3.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . . 8
3.4. Routing and Timing Parents . . . . . . . . . . . . . . . . 8
3.5. Resource Management . . . . . . . . . . . . . . . . . . . 8
3.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . . 9
3.7. Deterministic Behavior . . . . . . . . . . . . . . . . . . 9
3.8. Path Computation Engine . . . . . . . . . . . . . . . . . 9
3.9. Secure Communication . . . . . . . . . . . . . . . . . . . 10
4. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 13
6.3. External Informative References . . . . . . . . . . . . . 15
Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 19
A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 19
A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . . 19
A.3. Mote Communication Schedule . . . . . . . . . . . . . . . 19
A.4. Links and Paths . . . . . . . . . . . . . . . . . . . . . 20
A.5. Dedicated vs. Shared Slots . . . . . . . . . . . . . . . . 20
A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . . 21
A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 21
A.8. Time Synchronization . . . . . . . . . . . . . . . . . . . 22
A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 23
A.10. Network Communication Schedule . . . . . . . . . . . . . . 23
A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . . 23
A.12. Information Elements . . . . . . . . . . . . . . . . . . . 24
A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 24
Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 25
B.1. Collision Free Communication . . . . . . . . . . . . . . . 25
B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 25
B.3. Cost of (continuous) Synchronization . . . . . . . . . . . 25
B.4. Topology Stability . . . . . . . . . . . . . . . . . . . . 26
B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
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.
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. Its core
technology is similar to the one used in industrial networking
technologies such as WirelessHART [WHART] or ISA100.11a [ISA100].
These protocol solutions have been targeted essentially at the
industrial market. WirelessHART is for example the wireless
extension of HART, a long standing protocol suite for networking
industrial equipment.
[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. Industrial protocols such as WirelessHART
satisfy those requirements, and with tens of thousands of networks
deployed [Emerson], these types of networks have a large impact on
industrial applications. 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 builds on the same foundations as WirelessHART, and
therefore exhibits similar performance. Yet, unlike an industrial
protocol which is, by nature, application-specific, 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
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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 signaling
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.
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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 supplied 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
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
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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 signaling 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 fully 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] iteroperability event in 2011. In the
absence of a standardized scheduling mechanism for TSCH, a "slotted
Aloha" schedule was used.
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3. Problems and Goals
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, but it is the
entity that sets up that schedule which controls the topology of the
network, and 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 "6TSCH", without loss of generality. In particular, we
do not assume the nature of 6TSCH, whether an adaptation layer, a
distributed reservation protocol, a centralized path computation
engine, or any combination thereof.
Some of the issues 6TSCH need to target might overlap with the scope
of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this case, it
is entailed that 6TSCH will profit from the services provided by
other protocols to pursue these objectives.
3.1. Network Formation
6TSCH 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. 6TSCH 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. This includes a mechanism to select the best
mote through which to join the network.
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, 6TSCH needs to maintain the network's
health, allowing for motes to stay synchronized. 6TSCH needs to:
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1. Manage each mote's time source neighbor(s).
2. Define a mechanism for a mote to update the join priority it
announces in its Enhanced Beacon.
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. 6TSCH 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 at least one time source
neighbor it can synchronize to. 6TSCH therefore needs to assign time
source neighbors to allow for correct operation of the TSCH network.
These time source neighbors could, or not, be related to RPL time
parents.
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. 6TSCH 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.
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5. Allow for a (possibly centralized) entity to take full control
over the schedule.
6. Define a set of metrics to evaluate the tradeoff 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. 6TSCH need 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 acknowledgment. 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. 6TSCH need 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. Path Computation Engine
As highlighted in [I-D.phinney-roll-rpl-industrial-applicability],
bandwidth allocation and multi-hop routes can be optimized by an
external Path Computation Engine (PCE). 6TSCH need to:
1. Provide a mechanism for an external PCE to be able to control the
entire schedule of the network, including the slotframes, links
and time source neighbor assignment.
2. Define a optional mechanism for the schedule managed by this PCE
to coexist with scheduling elements (slotframes, links) managed
up by a different mechanism such as a distribute scheduling
algorithm.
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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. 6TSCH need 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 signaling
data between motes and 6TSCH.
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4. Contributors
Maria Rita Palattella
SnT/University of Luxembourg
maria-rita.palattella@uni.lu
Luigi Alfredo Grieco
Politecnico di Bari
a.grieco@poliba.it
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5. Acknowledgements
Special thanks to Jonathan Simon for his review and valuable
comments.
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. 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.
[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.
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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.thubert-roll-forwarding-frags]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", draft-thubert-roll-forwarding-frags-00 (work in
progress), March 2012.
[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., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-11 (work in
progress), February 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-16
(work in progress), February 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-03 (work in progress),
January 2013.
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[I-D.thubert-6lowpan-backbone-router]
Thubert, P., "6LoWPAN Backbone Router",
draft-thubert-6lowpan-backbone-router-02 (work in
progress), June 2010.
[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.
[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-01 (work
in progress), October 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-13 (work in progress), December 2012.
6.3. 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.
[WHART] www.hartcomm.org, "Highway Addressable Remote Transducer,
a group of specifications for industrial process and
control devices administered by the HART Foundation".
[ISA100] ISA, "ISA100, Wireless Systems for Automation", May 2008,
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< http://www.isa.org/Community/
SP100WirelessSystemsforAutomation>.
[Emerson] Emerson Process Management, "Emerson Process Management
Smart Wireless Homepage", < http://
www2.emersonprocess.com/en-US/plantweb/wireless/Pages/
WirelessHomePage-Flash.aspx>.
[OpenWSN] "Berkeley's OpenWSN Project Homepage",
.
[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, .
[IPSO] "IP for Smart Objects Alliance Homepage",
.
[CurrentCalculator]
Linear Technology, "Application Note: Using the Current
Calculator to Estimate Mote Power", August 2012, .
[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.
[TSMP] Pister, K. and L. Doherty, "TSMP: Time Synchronized Mesh
Prootocol", International Symposium on Distributed Sensor
Networks (DSN) 2008, Nov. 2008, < http://
robotics.eecs.berkeley.edu/~pister/publications/2008/
TSMP%20DSN08.pdf>.
[tinka10decentralized]
Tinka, A., Watteyne, T., and K. Pister, "A Decentralized
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robotics.eecs.berkeley.edu/~pister/publications/2008/
TSMP%20DSN08.pdf>.
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[watteyne09reliability]
Watteyne, T., Mehta, A., and K. Pister, "Reliability
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Palattella, MR., Accettura, N., Dohler, M., Grieco, LA.,
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120531-submitted-tasa-25511.pdf>.
[TASA-SENSORS]
Palattella, MR., Accettura, N., Dohler, M., Grieco, LA.,
and G. Boggia, "Traffic-Aware Time-Critical Scheduling In
Heavily Duty-Cycled IEEE 802.15.4e For An Industrial IoT",
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[TASA-WCNC]
Accettura, N., Palattella, MR., Dohler, M., Grieco, LA.,
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[palattella12standardized]
Palattella, MR., Accettura, N., Vilajosana, X., Watteyne,
T., Grieco, LA., Boggia, G., and M. Dohler, "Standardized
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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 6TSCH.
A.1. Timeslots
All motes in a TSCH network are synchronized. Time is sliced up into
time slots. A time slot 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 acknowledgment (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.
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. Mote Communication Schedule
A communication schedule instructs each mote what to do in each slot:
transmit, receive or sleep. The schedule indicates, for each active
(transmit or receive) timeslot a channelOffset and the address of the
neighbor to communicate with.
Once a mote obtains its schedule, it executes it:
o For each transmit slot, the mote checks whether there is a packet
in the outgoing buffer which matches the neighbor written in the
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schedule information for that slot. If there is none, the mote
keeps its radio off for the duration of the slot. If there is
one, the mote can ask for the neighbor to acknowledge it, in which
case it has to listen for the acknowledgment after transmitting.
o For each receive slot, 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
aknowledgment.
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. Links and Paths
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 slot of the schedule (i.e., a single cell of the grid),
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),
is called a "link".
If there is a lot of data flowing from mote A to mote B, the schedule
might contain multiple slots from A to B, at different times.
Multiple links scheduled to the same neighbor are typically
equivalent, i.e. the MAC layer sends the packet on whichever of these
links happens to show up first after the packet was put in the MAC
queue. The union of all links between two neighbors, A and B, is
called a "path". Since the slotframe repeats over time (and the
length of the slotframe is typically constant), each link gives a
"quantum" of bandwidth to a given neighbor. Modifying the number of
links in a path modifies the amount of resources allocated between
two neighbors.
A.5. Dedicated vs. Shared Slots
By default, each transmit timeslot 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 slot as shared. In a shared
slot, multiple motes can transmit at the same time, on the same
fequency. To avoid contention, TSCH defines a back-off algorithm for
shared slots.
A slot can be marked as both transmitting and receiving. In this
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case, a mote transmits if it has an appropriate packet in its output
buffer, or listens otherwise. Marking a slot 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
occurences over time), 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, and 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 (i) to
calculate the frequency to communicate on, jointly with the
channelOffset, (ii) to build unique security nonce counters used by
CCM*.
A.7. Channel Hopping
For each active slot, the schedule specifies a slotOffset and a
channelOffset. In a well-built schedule, when mote A has a transmit
slot to mote B on channelOffset 5, mote B has a receive slot 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 timeslot, and the same ASN counter since they are
synchronized, they compute the same frequency. At the next iteration
(cycle) of the slotframe, however, the channelOffset will be the
same, but the ASN will have changed, resulting in the computation of
a different frequency. If the slotframe size, S (used for computing
ASN), and the number of channel offsets, nFreq, are relatively prime,
the translation function ensures that each link rotates through k
available channels over k slotframe cycles. This results in "channel
hopping": even with a static schedule, pairs of neighbors "hop"
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between the different frequencies when communicating.
The look-up table F can be built to contain only a subset of all
available channels. This results in frequency "blacklisting".
Channel hopping is a technique known to efficiently combat multi-path
fading and external interference. This results in a TSCH network
having a more stable topology than if only a single channel were used
for the entire network.
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 (32kHz crystal
oscillators are typically used). Yet, because clocks in different
motes drift with respect to one another, neighbor motes need to
periodically re-synchronize.
In detail, each mote periodically synchronizes its network clock to
at least one other mote, and it also provides its network time to its
neighbors. It is up to the entity that manages the schedule to
assign adequate time source neighbor(s) to each mote, i.e., to
indicate in the schedule which of its neighbor(s) are its "time
source neighbors". While setting the time source neighbor, it is
important to avoid synchronization loops, which could result in the
formation of independent clusters of motes.
Typically, in a IEEE802.15.4e TSCH network, time propagates outwards
from the PAN coordinator (i.e., the root node). But the direction of
time propagation is independent of data flow in the network. A new
mote joining a TSCH network, because it does not have a schedule yet,
maintains time synchronization, using the information carried by the
Enhanced Beacons (EBs), sent by the advertising 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) Acknowledgment-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 Acknowledgment-Based
synchronization, the receiver provides such information to the sender
mote in its acknowledgment. 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
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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 time source
neighbors to 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
straighforward to calculate the expected average power consumption of
that mote.
A.10. Network Communication Schedule
The schedule defines entirely the synchronization and communication
between motes. By adding/removing links 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 links 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. This join priority corresponds to the number of hops
separating the mote sending the EB, and the PAN coordinator. Because
of the channel hopping nature of TSCH, these EB frames are sent on
all frequencies.
A mote wishing to join the network listens on some frequency for EBs.
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It can wait to receive multiple, and can use the join priority in
those EBs to identify the best mote through which to join the
network. 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 mote it
just joined.
The TSCH standard does not define the steps beyond this "kickstart".
These steps can include a security handshake and the addition of more
communication links to the new mote's schedule.
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. links, 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.
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Appendix B. TSCH Gotchas
This section lists features of TSCH which we believe are important
and beneficial to the work of 6TSCH.
B.1. Collision Free Communication
TSCH allows one to easily design a schedule which yields collision-
free communication. This is done by building the schedule with
dedicated links in such a way that at most one link occupies each
slotOffset/channelOffset cell. Multiple pairs of neighbor motes can
exchange data at the same time, but on different frequencies. If a
deployment is done over a large area, slotOffset/channelOffset cells
can be reused by pairs of neigbors sufficiently far appart not to
interfere.
B.2. Multi-Channel vs. Channel Hopping
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 empty data frame, often referred to as a "Keep-alive"
message, to re-synchronize. The frequency at which such message 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. When acknowledgment-based synchronization is
used, re-synchronizing consists in sending any valid frame to the
time source neighbor and using the timing information in the ACK to
realign the clocks. 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 can be considered
almost negligible in many applications.
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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 slot, 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;
2. Lower slotframe identifiers take precedence over higher slotframe
identifiers.
In the example above, the mote would transmit to mote B on slotFrame
2.
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Author's Address
Thomas Watteyne (editor)
Linear Technology
30695 Huntwood Avenue
Hayward, CA 94544
USA
Phone: +1 (510) 400-2978
Email: twatteyne@linear.com
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