Update from Wireless Standards
Pascal ThubertTechnical Leader
Cisco Systems
Presented at the ODVA2014 Industry Conference & 16th Annual Meeting
March 11-13, 2014
Phoenix, Arizona, USA

Abstract:
The next percentile point of process optimization yields hundreds of billions of savings, and will be gained byleveraging the collection of currently unused measurements by widely distributed sensing devices and analytic
capabilities. Gathering and processing this unused data will require the combination of the best of IT and OT
technologies together, forming the IT/OT convergence, aka the Industrial Internet.
Factory networks primarily rely on wired communication networks to support Industrial Automation and Control
applications. On the other hand, Wireless Sensor Networks have the power to extend the reach of Monitoring andControl to gather unused measurements beyond what is physically and economically possible with wires; to this day,the deployment of wireless communications in industrial environments has been hampered by a perceived lack ofdeterminism.
The IEEE and the IETF are creating a new generation of Low Power wireless protocols suitable for Time Sensitive
Industrial applications. The Deterministic IPv6 over IEEE802.15.4e Time slotted Channel Hopping (6TiSCH) is a
new IETF Work Group that formed to enable IPv6 over the TSCH mode of the IEEE802.15.4e standard. The scopeof the WG includes one or more Low-power Lossy Networks (LLNs), federated by a backbone through one or more
BackBone Routers (BBRs). The Work Group will deliver a networking architecture and put together a collection ofstandards for centralized and distributed routing and scheduling over a TSCH MAC.
This paper will introduces the work being conducted by this new working group to the ODVA constituency.
Keywords:
Deterministic Networking, Time-Sensitive Networking, IPv6, IEEE802.15.4e, Time slotted Channel Hopping,
TSCH, 6TiSCH, RPL, 6LoWPAN
Definition of terms:
Please refer to the 6TiSCH terminology document [10]2014 ODVA Industry Conference

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Introduction
For the last 40 years or more, Operational Technology (OT) in industrial networks developed in parallel to the
Public Switched Telephone Network (PSTN), and to early data networks such as Bitnet, which was based on IBM’s
Systems Network Architecture (SNA).
For the last 25 years, the Internet Protocol (IP) has become the de-facto standard for the networking layer of theInformation Technology (IT) and for the Internet at large. For the last 15 years, Voice over IP (VoIP) enabled a
progressive convergence of voice and data networking technologies, to be soon followed by video streaming andconferencing, leading to the convergence of voice, data, and video over the IP infrastructure.
The integration of IT and OT technologies now appears to be the natural step. Due to its different goals, OT has
evolved focusing on highly secure, reliable and Time-Sensitive networks, with limited scalability over a bounded
area, in a sharp contrast to IT, which leveraged the end-to-end principle to scale any-to-any connectivity to the
proportions of today’s Internet, with much less scrutiny on delivery ratio or latency.
So, though appealing for its promised saving on both OpEx and CapEx, enabling OT traffic to be transported over ashared IT infrastructure yields a number of new challenges, which are different as we converge the plant network totransport supervisory flows, the control network to converge automation and monitoring flows, or the field network
to access the devices themselves, be they wired or wireless.
•

Achieving the convergence the Plant Network is probably now a matter of validation and acceptance. Highend fabrics propose flow isolation with
Virtual LANs (VLANs) and Virtual
Routing and Forwarding (VRF), guaranteedbandwidth (QoS) and High Availability(HA) features, only getting easier and more
economical to manage with Software
Defined Networking (SDN) and Network
Function Virtualization (NFV).
• The Control Network requires
more determinism for control operations,improved Packet Reliability, and moreautonomic, zero-touch commissioning tosimplify new deployments and allow
scalable monitoring applications. Thesefeatures are well on the way ofstandardization and productization.
• The convergence of the Field
Network is more in its early stage, though
ISA100.11a is showing the way with the
integration of IPv6. Both ISA100.11a andWirelessHART feature Time Sensitive
Networking properties through a centrally
controlled routing optimization; but newwork is needed to design the distributed
routing and scheduling protocol suite thatwill enable large and affordable monitoring applications in combination with the centralized approach thatwas proven by those protocols. The remainder of this paper discusses the convergence of bleeding edge
wireless IT and OT technologies, and the key capabilities they bring to the Industrial Internet.

IEEE 802.15.4e TSCH
Wireless communications enable a new level of cost effectiveness compared to wire, with a very low incremental
cost per device once the access point (AP) is installed. Though cables are often more trusted for control operations,they can be inappropriate for multiple use cases such as rotating devices, moving cranes, robots arms or mobile
handheld devices, and yield high installation and maintenance cost, long lead time and extreme difficulties to repair.
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On the other hand, radios can reach from near field to orbital trajectory and are appropriate for large scale Machineto-Machine (M2M) deployments where they can enable new usages such as widespread monitoring.
A Wireless Field Network is typically a Low power and Lossy Network (LLN) that interconnects memoryconstrained battery-operated field devices to form a wireless mesh network. In some monitoring and metering usecases, the capability for the wireless mesh to scale to the thousands of nodes is required. An additional level ofscalability, beyond the actual capabilities of a given mesh technology, can be achieved by deploying border routers(BRs) as gateways between LLNs and a federating backbone, either operating at the network layer (L3) or at theapplication layer (L5) of the Internet Protocol (IP) reference model.
At the IETF, the 6LoWPAN, ROLL and CoRE Working Groups (WGs) have defined new protocols to address theconstrained LLN devices, including an IPv6 adaptation layer, 6LoWPAN [1], a routing protocol, RPL [2] and a web
transfer protocol, CoAP [3]. This protocol suite was initially defined for use on IEEE802.15.4 low-power radios [4],but limitations linked to random access and related co-channel interferences soon became apparent in the context ofmesh-networking applications.
To overtake those limitations, the IEEE802.15.4e standard [5] was published in 2012 as an amendment to theIEEE802.15.4-2011 Medium Access Control (MAC) protocol [4], adding in particular the Time slotted Channel
Hopping (TSCH) mode, which is still at this time the state-of-the-art technology for ultra-lower power Time
Sensitive Networking in LLNs. At the core of the TSCH mode is a medium access technique that uses timesynchronization and scheduled transmissions to achieve ultra-low-power operation, coupled with a channel-hopping
method to add diversity and cope with multi-path fading and external interferences.
With TSCH, time is sliced in a Time Division Multiplexing (TDM) fashion into time slots of equal duration; a timeslot is just long enough for a MAC frame of maximum size to be sent and acknowledged. A shared schedule
indicates, for each active (transmit or receive) time slot, a channelOffset and the address of the communicatingneighbors. The channelOffset is translated into a frequency using a specific translation function which causes pairsof neighbors to “hop” between the different available frequencies when communicating. In essence, this technology
is similar to that used in industrial networking technologies such as WirelessHART and ISA100.11a, resulting incomparable performances
IEEE802.15.4e only defines the link-layer mechanisms [6] and does not amend the 802.15.4 PHY layer. Neither
does it define how the network communication schedule is built and matched to the traffic requirement of the
network. The definition of an adaptation layer and architectural recommendations are needed for those highly
efficient LLN networks to transition to end-to-end IPv6-based solutions.
IETF 6TiSCH
It must be noted that TSCH does not amend the physical layer, i.e., it can operate on any IEEE802.15.4-compliant
hardware. The clean layer model of IEEE802.15.4e allows for TSCH to fit under IPv6 in a protocol stack for LLNs,
as exemplified in Figure 1 with the 6TiSCH client stack. But the addition of a Logical Link Control (LLC) layer isrequired between those two layers to provide IP with the link abstraction over which it can operate.
Additionally, IPv6 comes with a suite of protocols that can be difficult to apprehend from the standpoint of aStandard Defining Organization (SDO) that is external to the IETF, such as ODVA, HCF, ISA or IEC. There is aclear need to position the required Request For Comments (RFC) documents that will come into play in an open
standard LLN solution, which role they cover in the broad picture, and how they relate with one another.
To fill this gap, a new Working Group (WG) called 6iTSCH has formed at the IETF. 6TiSCH aims to link
IEEE802.15.4e TSCH capabilities with prior IETF 6LoWPAN and ROLL standardization efforts andrecommendations [7]. In detail, it will (i) define an open standard-based architecture, re-using existing protocols
when possible, and (ii) specify the required LLC operation to provide a link abstraction for IPv6.
Doing so, 6TiSCH will inherit from architecture work that was done for ZigbeeIP, ISA100.11a and WirelessHART
and experience gathered during their deployments. As it goes, most of the 6TiSCH architecture blocks are alreadydeployed, either as standard-based or proprietary implementations, and the new work mostly consists in selecting
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existing and proven IETF components and adapting them to the constraints of the LLN devices. 6TiSCH will
document an open standards-based architecture, highlight best practices, and standardize the missing components toachieve industrial-grade performance in terms of jitter, latency, scalability, reliability and low-power operation forIPv6 over IEEE802.15.4e TSCH [5]. Although not addressed directly by 6TiSCH, it is envisioned that the resulting
techniques will be applicable to technologies other than 2.4GHz IEEE802.15.4 [4].
Figure 1 summarizes the 6TiSCH client stack. A major ingredient of this stack, compared to ISA100.11a andWirelessHART, is the addition of distributed routing with the RPL routing protocol. The RPL protocol is already in
use and deployed with ZigbeeIP and WiSUN for application in energy distribution with the SmartGrid.

Figure 1

With 6TiSCH, a RPL parent is delegated a number of responsibilities centralized such as time slots allocation thatare classically; this allows the central controller to maintain a more abstract view of the network and thus allows thesystem to scale to large numbers.
Figure 2

As illustrated in Figure 2, the scope of thearchitecture is an IPv6 multi-link subnet thatis spread over a high speed powered
backbone and a number of IEEE802.15.4e
TSCH wireless mesh networks connected tothe backbone by synchronized backbone
routers (BBRs).
The 6TiSCH architecture (in progress) [9]and its associated terminology [10] specify
how packets that belong to a deterministic
IPv6 flow are marked and routed or
forwarded over the mesh within jitter andlatency budgets. The document covers link
management for the IPv6 network layer,neighbor discovery and routing. The
security will probably be described in a separate document initially but should be merged ultimately for publication.
The state of the current work on the LLC layer is found in the 6top draft [11]. Cells– time slots- are grouped in abundle of equal quality of service (QoS), and 6top is responsible for matching outgoing IP packets to available cells,
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based on their QoS requirements. Ultimately, the 6top sublayer will be able to negotiate with its peers a variable
number of cells associated with the IP link abstraction, so as to provide dynamic bandwidth allocation that follows
closely the needs of the best effort forwarding plane.
RPL and distributed Routing.
Existing industrial Wireless Field Network protocols rely on a central route computation to establish an end-to-end
sequence of time slots along a path between nodes. This enables a degree of determinism for time-sensitive flows,
whereby the path and the time of arrival are determined, though the actual delivery ratio can be optimized but notguaranteed.
The modus operandi is really designed with control loops inmind; in practice though, wireless deployments are mostly
aimed at monitoring, with oftentimes less strict
requirements of time sensitivity but higher demands for
scalability and lower cost. This is where a distributed
routing protocol becomes fashionable; and RPL [2], which
was designed at the IETF for routing inside an LLN, is the
protocol of choice.
RPL provides a generic distance-vector (DV) operation thatis designed to be complemented by an Objective Function(OF) to address any particular use case. An OF ties together
metrics and logic that will optimize the routing topology forthe particular needs, and produce as an outcome a scalar
metric for use by RPL to form a logical topology, as adirected acyclic graph (DAG).
Figure 3 A RPL DAG

The DAG is directed towards a root, and indicates the default route for traffic. Roots can be interconnected by a
backbone to form a larger IPv6 subnet, in which case they need to advertise their devices in some fashion, forinstance by extending RPL to the backbone with a virtual root,
or by proxying IPv6 ND on behalf of their attached LLN
devices. To serve multiple needs for different optimizations,
RPL introduces the concept of an instance, each instancedefining its own topology based on its own Objective Function.
Compared to a link state (LS) protocol, the DV operation limits
the amount of topological information that has to be exchanged
and maintained at each node to the extreme. In addition, RPL
allows for lazy routing updates, and routing inconsistencies canbe detected through and fixed reactively based on informationthat is placed in the packets that would travel through a broken
route, in a process called the data-path validation.
To address ultra-constrained nodes, RPL proposes a nonstoring mode that relies on source routing to reach individual
devices from a root of the DAG. For more capable devices, orwhen the use case such as long lines that are not appropriate forrouting headers, RPL also proposes a more classical storing
mode, whereby packets are routed up the DAG till the common
parent and then down. The routing stretch is the classicalconsequence of an economy of routing state, and the result isthat only routes towards and from the root are really optimized
whereby any-to-any will incur a stretch.

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6TiSCH routing and forwarding
With 6TiSCH, routing states are computed either in a centralized fashion by a Path Computation Entity (PCE),
which is located either on the backbone or farther in the IPv6 network over a backhaul, or in a distributed fashionusing RPL. Moreover, the allocation of a sequence of time slots along a path (aka a track) can be globally optimized
and then pushed on the network from the PCE that computes the routes, and/or managed by a distributed schedulingprotocol along routes that are computed by RPL.
The architecture will ultimately specify a framework for scheduling frames over time slots that supports threemodels:

1. A centralized route computation that builds and maintains the communication schedule, and distributes it to
the nodes. This schedule includes forwarding information associated to time slots; RPL operations only
apply to emergency repair actions when the reference topology becomes unusable. A number of existing
protocols could be extended to push the schedule from the PCE to the device, including the PCE
communication Protocol (PCEP) [12], Forwarding and Control Element Separation (ForCES) [13],Software-Defined Networking (SDN) OpenFlow or even through network management over theConstrained Application Protocol (CoAP) [3].

Figure 4 track (G-MPLS like [14], [15]) forwarding for reserved bandwidth2.

A distributed resource reservation and signaling protocol that establishes tracks between source anddestination nodes along multi-hop routes identified by RPL. The track may be setup by extensions to thelegacy Resource ReSerVation Protocol (RSVP) [16] or the more recent but rather heavy Next Steps in
Signaling (NSIS) protocol [17].

3. A best effort resource allocation that is used to transport data frames on a per hop basis in the absence of areservation protocol.

Figure 5 best effort (L3) forwarding

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6TiSCH scalability and backbone operation
As detailed in Figure 1, when possible, the 6TiSCH group will reuse existing protocols such as IP6 Neighbor
Discovery (ND) [8], IPv6 Low power Wireless Personal Area Networks (6LoWPAN) [1], and the Routing Protocol
for Low Power and Lossy Networks (RPL) [2], with the minimum adaptation required to meet criteria for reliability
and determinism within the mesh,and scalability over the backbone.
As illustrated in Figure 2, the
6TiSCH architecture also addresses
how multiple BBRs are supported
for a higher degree of scalability
and reliability, and how nodes
maintain synchronization in thepresence of multiple BBR.
This work implies new IPv6 ND
operations called Wireless ND,
extending RFC 6775 6LoWPAN
ND registration [18] for use in a
generic low power of wirelessnetwork as described in [19].
Figure 6 RFC 6775 6LoWPAN ND registration

The Backbone Router Internet draft[20] specifies how host routes learnt from RPL can be turned into ND proxy states, and defended over the backbone.
This way, the backbone router attracts backbone packets that are destined to LLN nodes, and then forwards themover the LLN along RPL routes, as illustrated in Figure 6 Backbone Router operation.

As a result, the 6TiSCH architecture
will enable a new range of use cases
for LLNs, including both:(a)Control loops in a wirelessprocess control network, in whichhigh reliability and a fully
deterministic behavior are required,based on PCE computed routes; and(b)
Scalable monitoring
networks covering potentially large
area or long area, based on
distributed route computation with
RPL.

Figure 7 Backbone Router operation2014 ODVA Industry Conference

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Conclusion
Existing industrial Wireless Sensor Network technologies have demonstrated that the IEEE802.15.4e Time slotted
Channel Hopping (TSCH) effectively enables industrial grade deterministic properties for slow speed control loops
with low latency, ultra-low jitter and a high reliability. It makes sense to extend this support, which is essentiallybased on centralized routing, to a distributed mode that can be cheaper and simpler to deploy, at the expense of the
optimization that only a centralized approach can obtain.
The IETF is now starting an effort, called 6TiSCH, which will provide both centralized and distributed operation,
based on open standards, and which will enable a new range of applications in scalable monitoring and automation(home, city, building) as well as man-to-machine interfaces (cars, planes), thus optimizing processes and savingenergy and optimizing resources for a greener planet.
Deeper dive-in into TSCH and 6TiSCH is available in a paper presented at ESIOT in July [21] and a book chapter
soon to be published [22].
References (optional):[1] N. Kushalnagar, G. Montenegro, and C. Schumacher, IPv6 over Low-Power Wireless Personal Area Networks(6LoWPANs): Overview, Assumptions, Problem Statement, and Goals, RFC 4919, Internet Engineering Task Force
RFC 4919, August 2007.
[2] T. Winter, P. Thubert, A. Brandt, J. Hui, R. Kelsey, P. Levis, K. Pister, R. Struik, J. P. Vasseur, and R.
Alexander, RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks, RFC 6550, Internet Engineering Task
Force RFC 6550, March 2012.
[3] Z. Shelby, K. Hartke, C. Bormann, and B. Frank, Constrained Application Protocol (CoAP), IETF CoRE
Working Group, February 2011.
[4] IEEE802.15.4-2011: IEEE Standard for Local and metropolitan area networks – Part. 15.4: Wireless Medium
Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area
Networks(LR-WPANs), Standard for Information Technology Std., September 2011.
[5] IEEE802.15.4e-2012: IEEE Standard for Local and Metropolitan Area Networks. Part 15.4: Low-Rate Wireless
Personal Area Networks (LRWPANs) Amendment 1: MAC Sublayer, Institute of Electrical and Electronics
Engineers Std., 16 April 2012.
[6] T. Watteyne, Ed., M.R. Palattella, L.A. Grieco, Using IEEE802.15.4e TSCH in an LLN context: Overview,
Problem Statement and Goals. draft-ietf-6tisch-tsch-00 (work in progress), November 2013.
[7] T. Phinney, P. Thubert, RA. Assimiti, RPL applicability in industrial networks. draft-ietf-roll-rpl-industrialapplicability-02, October 2013[8] T. Narten, E. Nordmark, W. Simpson, and H. Soliman, Neighbor Discovery for IP version 6 (IPv6), RFC 4861,
September 2007.
[9] Thubert, P., Ed., Assimiti, RA., Watteyne, T., An Architecture for IPv6 over Time Synchronized Channel
Hopping. Draft-ietf-6tisch-architecture-00 (work in progress), November 2013.
[10] Palattella, MR., Ed., Thubert, P., Watteyne, T., and Q. Wang, Terminology in IPv6 over Time Slotted Channel
Hopping. draft-ietf-6tisch-terminology-00 (work in progress), November 2013.
[11] Wang, Q., Ed., Vilajosana, X., and Watteyne, T., 6top Adaptation Layer Specification. draft-wang-6tisch-6top00 (work in progress), October 2013.

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[12] JP. Vasseur, JL. Le Roux, Path Computation Element (PCE) Communication Protocol (PCEP) RFC 5440,
Internet Engineering Task Force RFC 5440, September 2009.
[13] A. Doria, J. Hadi Salim, R. Haas, H. Khosravi, W. Wang, L. Dong, R. Gopal, J. Halpern, Forwarding andControl Element Separation (ForCES) Protocol Specification RFC 5810, Internet Engineering Task Force RFC
5810, September 2010.
[14] D. Fedyk, L. Berger and L. Andersson, ‘Generalized Multiprotocol Label Switching (GMPLS) Ethernet Label
Switching Architecture and Framework’. Internet Engineering Task Force RFC 5828, March2010.
[15] A. Morell , X. Vilajosana , J.L. Vicario , T. Watteyne, Label Switching over IEEE802.15.4e Networks.
Transactions on Emerging Telecommunications Technologies. April 2013[16] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin, Resource ReSerVation Protocol (RSVP) : Version 1
Functional Specification. Internet Engineering Task Force RFC 2205, September 1997.
[17] R. Hancock, G. Karagiannis, J. Loughney, S. Van den Bosch, Next Steps in Signaling (NSIS): Framework.
RFC 4080, Internet Engineering Task Force RFC 4080, June 2005.
[18] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann, Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs), RFC 6775, November 2012.
[19] Chakrabarti, S., Nordmark, E., Thubert, P and M. Wasserman, Wired and Wireless IPv6 Neighbor Discovery
Optimizations, draft-chakrabarti-nordmark-6man-efficient-nd-04 (work in progress), October 2013.
[20] P. Thubert, 6LoWPAN Backbone Router. draft-thubert-6lowpan-6lowpan-backbone-router-03 (work inprogress), February 2013.
[21] P. Thubert, T. Watteyne, M.R. Palattella, X. Vilajosana, Q. Wang, 'IETF 6TiSCH: Combining IPv6
Connectivity with Industrial Performance", Proc. of IEEE esIoT, Workshop on Extending Seamlessly to the Internetof Things 2013, July 2013, Taichung, Taiwan.
[22] M.R. Palattella, P. Thubert, X. Vilajosana, T. Watteyne, Q. Wang, T. Engel, " 6TiSCH Wireless Industrial
Networks: Determinism Meets IPv6”, in S. C. Mukhopadhyay (Edt), " Internet of Things: Challenges andOpportunities”, Springer-Verlag, Lecture series of Smart Sensors, Instrumentation and Measurements, to be
published in 2014.

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