Reliable and Available Wireless Technologies
draft-thubert-raw-technologies-01
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draft-thubert-raw-technologies-01
RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational D. Cavalcanti
Expires: December 8, 2019 Intel
X. Vilajosana
Universitat Oberta de Catalunya
June 6, 2019
Reliable and Available Wireless Technologies
draft-thubert-raw-technologies-01
Abstract
This document presents a series of recent technologies that are
capable of time synchronization and scheduling of transmission,
making them suitable to carry time-sensitive flows with requirements
of both reliable delivery in bounded time, and availability at all
times, regardless of packet transmission or individual equipement
failures.
Status of This Memo
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. On Scheduling . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Benefits of Scheduling on Wires . . . . . . . . . . . . . 4
3.2. Benefits of Scheduling on Wireless . . . . . . . . . . . 4
4. IEEE 802 standards . . . . . . . . . . . . . . . . . . . . . 5
4.1. IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.1. Provenance and Documents . . . . . . . . . . . . . . 5
4.1.2. 802.11ax High Efficiency (HE) . . . . . . . . . . . . 7
4.1.3. 802.11be Extreme High Throughput (EHT) . . . . . . . 10
4.1.4. 802.11ad and 802.11ay (mmWave operation) . . . . . . 11
4.2. IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1. Provenance and Documents . . . . . . . . . . . . . . 12
4.2.2. TimeSlotted Channel Hopping . . . . . . . . . . . . . 14
5. 3GPP standards . . . . . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
When used in math or philosophy, the term "deterministic" generally
refers to a perfection where all aspect are understood and
predictable. A perfectly Deterministic Network would ensure that
every packet reach its destination following a predetermined path
along a predefined schedule to be delivered at the exact due time.
In a real and imperfect world, a Deterministic Network must highly
predictable, which is a combination of reliability and availability.
On the one hand the network must be reliable, meaning that it will
perform as expected for all packets and in particular that it will
always deliver the packet at the destination in due time. On the
other hand, the network must be available, meaning that it is
resilient to any single outage, whether the cause is a software, a
hardware or a transmission issue.
RAW (Reliable and Available Wireless) is an effort to provide
Deterministic Networking on across a path that include a wireless
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physical layer. Making Wireless Reliable and Available is even more
challenging than it is with wires, due to the numerous causes of loss
in transmission that add up to the congestion losses and the delays
caused by overbooked shared resources. In order to maintain a
similar quality of service along a multihop path that is composed of
wired and wireless hops, additional methods that are specific to
wireless must be leveraged to combat the sources of loss that are
also specific to wireless.
Such wireless-specific methods include per-hop retransmissions (HARQ)
and P2MP overhearing whereby multiple receivers are scheduled to
receive the same transmission, which balances the adverse effects of
the transmission losses that are experienced when a radio is used as
pure P2P.
2. Terminology
This specification uses several terms that are uncommon on protocols
that ensure bets effort transmissions for stochastics flows, such as
found in the traditional Internet and other statistically multiplexed
packet networks.
Reliable: That consistently performs as expected, the expectation
for a network being to always deliver a packet in due time.
Available: That is exempt of unscheduled outage, the expectation for
a network being that the flow is maintained in the face of any
single breakage.
PAREO (functions): the wireless extension of DetNet PREOF. PAREO
functions include scheduled ARQ at selected hops, and expect
the use of new operations like overhearing where available.
Track: A DODAG oriented to a destination, and that enables Packet
ARQ, Replication, Elimination, and Ordering Functions.
ARQ: Automatic Repeat Request, enabling an acknowledged
transmission, which is the typical model at Layer-2 on a
wireless medium.
HARQ: Forward error correction, sending redundant coded data to help
the receiver recover transmission errors.
HARQ: Hybrid ARQ, a combination of FEC and ARQ.
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3. On Scheduling
The operations of a Deterministic Network often rely on precisely
applying a tight schedule, in order to avoid collision loss and
guarantee the worst-case time of delivery. To achieve this, there
must be a shared sense of time throughout the network. The sense of
time is usually provided by the lower layer and is not in scope for
RAW.
3.1. Benefits of Scheduling on Wires
A network is reliable when the statistical effects that affect the
packet transmission are eliminated. This involves maintaining at all
time the amount of critical packets within the physical capabilities
of the hardware and that of the radio medium. This is achieved by
controlling the use of time-shared resources such as CPUs and
buffers, by shaping the flows and by scheduling the time of
transmission of the packets that compose the flow at every hop.
Equipment failure, such as an access point rebooting, a broken radio
adapter, or a permanent obstacle to the transmission, is a secondary
source of packet loss. When a breakage occurs, multiple packets are
lost in a row before the flows are rerouted or the system may
recover. This is not acceptable for critical applications such as
related to safety. A typical process control loop will tolerate an
occasional packet loss, but a loss of several packets in a row will
cause an emergency stop (e.g., after 4 packets lost, within a period
of 1 second).
Network Availability is obtained by making the transmission resilient
against hardware failures and radio transmission losses due to
uncontrolled events such as co-channel interferers, multipath fading
or moving obstacles. The best results are typically achieved by
pseudo randomly cumulating all forms of diversity, in the spatial
domain with replication and elimination, in the time domain with ARQ
and diverse scheduled transmissions, and in the frequency domain with
frequency hopping or channel hopping between frames.
3.2. Benefits of Scheduling on Wireless
In addition to the benefits listed in Section 3.1, scheduling
transmissions provides specific value to the wireless medium.
On the one hand, scheduling avoids collisions between scheduled
transmissions and can ensure both time and frequency diversity
between retries in order to defeat co-channel interference from un-
controlled transmitters as well as multipath fading. Transmissions
can be scheduled on multiple channels in parallel, which enables to
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use the full available spectrum while avoiding the hidden terminal
problem, e.g., when the next packet in a same flow interferes on a
same channel with the previous one that progressed a few hops
farther.
On the other hand, scheduling optimizes the bandwidth usage: compared
to classical Collision Avoidance techniques, there is no blank time
related to inter-frame space (IFS) and exponential back-off in
scheduled operations. A minimal Clear Channel Assessment may be
needed to comply with the local regulations such as ETSI 300-328, but
that will not detect a collision when the senders are synchronized.
And because scheduling allows a time-sharing operation, there is no
limit to the ratio of isolated critical traffic.
Finally, scheduling plays a critical role to save energy. In IOT,
energy is the foremost concern, and synchronizing sender and listener
enables to always maintain them in deep sleep when there is no
scheduled transmission. This avoids idle listening and long
preambles and enables long sleep periods between traffic and
resynchronization, allowing battery-operated nodes to operate in a
mesh topology for multiple years.
4. IEEE 802 standards
With an active portfolio of nearly 1,300 standards and projects under
development, IEEE is a leading developer of industry standards in a
broad range of technologies that drive the functionality,
capabilities, and interoperability of products and services,
transforming how people live, work, and communicate.
The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains
networking standards and recommended practices for local,
metropolitan, and other area networks, using an open and accredited
process, and advocates them on a global basis. The most widely used
standards are for Ethernet, Bridging and Virtual Bridged LANs
Wireless LAN, Wireless PAN, Wireless MAN, Wireless Coexistence, Media
Independent Handover Services, and Wireless RAN. An individual
Working Group provides the focus for each area. Standards produced
by the IEEE 802 SC are freely available from the IEEE GET Program
after they have been published in PDF for six months.
4.1. IEEE 802.11
4.1.1. Provenance and Documents
The IEEE 802.11 LAN standards define the underlying MAC and PHY
layers for the Wi-Fi technology. Wi-Fi/802.11 is one of the most
successful wireless technologies, supporting many application
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domains. While previous 802.11 generations, such as 802.11n and
802.11ac, have focused mainly on improving peak throughput, more
recent generations are also considering other performance vectors,
such as efficiency enhancements for dense environments in 802.11ax,
and latency and support for Time-Sensitive Networking (TSN)
capabilities in 802.11be.
IEEE 802.11 already supports some 802.1 TSN standards and it is
undergoing efforts to support for other 802.1 TSN capabilities
required to address the use cases that require time synchronization
and timeliness (bounded latency) guarantees with high reliability and
availability. The IEEE 802.11 working group has been working in
collaboration with the IEEE 802.1 group for several years extending
802.1 features over 802.11. As with any wireless media, 802.11
imposes new constraints and restrictions to TSN-grade QoS, and
tradeoffs between latency and reliability guarantees must be
considered as well as managed deployment requirements. An overview
of 802.1 TSN capabilities and their extensions to 802.11 are
discussed in [Cavalcanti_2019].
Wi-Fi Alliance (WFA) is the worldwide network of companies that
drives global Wi-Fi adoption and evolution through thought
leadership, spectrum advocacy, and industry-wide collaboration. The
WFA work helps ensure that Wi-Fi devices and networks provide users
the interoperability, security, and reliability they have come to
expect.
The following IEEE 802.11 specifications/certifications are relevant
in the context of reliable and available wireless services and
support for time-sensitive networking capabilities:
Time Synchronization: IEEE802.11-2016 with IEEE802.1AS; WFA TimeSync
Certification.
Congestion Control: IEEE802.11-2016 Admission Control; WFA Admission
Control.
Security: WFA Wi-Fi Protected Access, WPA2 and WPA3.
Interoperating with IEEE802.1Q bridges: IEEE802.11ak.
Stream Reservation Protocol (part of IEEE802.1Qat):
AIEEE802.11-2016.
Scheduled channel access: IEEE802.11ad Enhancements for very
high throughput in the 60 GHz band [IEEE80211ad].
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802.11 Real-Time Applications: Topic Interest Group (TIG)
ReportDoc [IEEE_doc_11-18-2009-06].
In addition, major amendments being developed by the IEEE802.11
Working Group include capabilities that can be used as the basis for
providing more reliable and predictable wireless connectivity and
support time-sensitive applications:
IEEE 802.11ax D4.0: Enhancements for High Efficiency (HE). [IEEE802
11ax]
IEEE 802.11be Extreme High Throughput (EHT). [IEEE80211be]
IEE 802.11ay Enhanced throughput for operation in license-exempt
bands above 45 GHz. [IEEE80211ay]
The main 802.11ax and 802.11be capabilities and their relevance to
RAW are discussed in the remainder of this document.
4.1.2. 802.11ax High Efficiency (HE)
4.1.2.1. General Characteristics
The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax
amendment [IEEE80211ax], which includes new capabilities to increase
efficiency, control and reduce latency. Some of the new features
include higher order 1024-QAM modulation, support for uplink multi-
user MIMO, OFDMA, trigger-based access and Target Wake time (TWT) for
enhanced power savings. The OFDMA mode and trigger-based access
enable scheduled operation, which is a key capability required to
support deterministic latency and reliability for time-sensitive
flows. 802.11ax can operate in up to 160 MHz channels and it includes
support for operation in the new 6 GHz band, which is expected to be
open to unlicensed use by the FCC and other regulatory agencies
worldwide.
4.1.2.1.1. Multi-User OFDMA and Trigger-based Scheduled Access
802.11ax introduced a new orthogonal frequency-division multiple
access (OFDMA) mode in which multiple users can be scheduled across
the frequency domain. In this mode, the Access Point (AP) can
initiate multi-user (MU) Uplink (UL) transmissions in the same PHY
Protocol Data Unit (PPDU) by sending a trigger frame. This
centralized scheduling capability gives the AP much more control of
the channel, and it can remove contention between devices for uplink
transmissions, therefore reducing the randomness caused by CSMA-based
access between stations. The AP can also transmit simultaneously to
multiple users in the downlink direction by using a Downlink (DL) MU
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OFDMA PPDU. In order to initiate a contention free Transmission
Opportunity (TXOP) using the OFDMA mode, the AP still follows the
typical listen before talk procedure to acquire the medium, which
ensures interoperability and compliance with unlicensed band access
rules. However, 802.11ax also includes a multi-user Enhanced
Distributed Channel Access (MU-EDCA) capability, which allows the AP
to get higher channel access priority.
4.1.2.1.2. Improved PHY Robustness
The 802.11ax PHY can operate with 0.8, 1.6 or 3.2 microsecond guard
interval (GI). The larger GI options provide better protection
against multipath, which is expected to be a challenge in industrial
environments. The possibility to operate with smaller resource units
(e.g. 2 MHz) enabled by OFDMA also helps reduce noise power and
improve SNR, leading to better packet error rate (PER) performance.
802.11ax supports beamforming as in 802.11ac, but introduces UL MU
MIMO, which helps improve reliability. The UL MU MIMO capability is
also enabled by the trigger based access operation in 802.11ax.
4.1.2.1.3. Support for 6GHz band
The 802.11ax specification [IEEE80211ax] includes support for
operation in the new 6 GHz band. Given the amount of new spectrum
available as well as the fact that no legacy 802.11 device (prior
802.11ax) will be able to operate in this new band, 802.11ax
operation in this new band can be even more efficient.
4.1.2.2. Applicability to deterministic flows
TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide
the underlying mechanism for supporting deterministic flows in a
Local Area Network (LAN). The 802.11 working group has already
incorporated support for several TSN capabilities, so that time-
sensitive flow can experience precise time synchronization and
timeliness when operating over 802.11 links. TSN capabilities
supported over 802.11 (which also extends to 802.11ax), include:
1. 802.1AS based Time Synchronization (other time synchronization
techniques may also be used)
2. Interoperating with IEEE802.1Q bridges
3. Time-sensitive Traffic Stream identification
The exiting 802.11 TSN capabilities listed above, and the 802.11ax
OFDMA and scheduled access provide a new set of tools to better
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server time-sensitive flows. However, it is important to understand
the tradeoffs and constraints associated with such capabilities, as
well as redundancy and diversity mechanisms that can be used to
provide more predictable and reliable performance.
4.1.2.2.1. 802.11 Managed network operation and admission control
Time-sensitive applications and TSN standards are expected to operate
under a managed network (e.g. industrial/enterprise network). Thus,
the Wi-Fi operation must also be carefully managed and integrated
with the overall TSN management framework, as defined in the IEEE
Std. 802.1Qcc specification [IEEE8021Qcc].
Some of the random-access latency and interference from legacy/
unmanaged devices can be minimized under a centralized management
mode as defined in IEEE Std. 802.1Qcc, in which admission control
procedures are enforced.
Existing traffic stream identification, configuration and admission
control procedures defined in IEEE Std. 802.11 QoS mechanism can be
re-used. However, given the high degree of determinism required by
many time-sensitive applications, additional capabilities to manage
interference and legacy devices within tight time-constraints need to
be explored.
4.1.2.2.2. Scheduling for bounded latency and diversity
As discussed earlier, the 802.11ax OFDMA mode introduces the
possibility of assigning different RUs (frequency resources) to users
within a PPDU. Several RU sizes are defined in the specification
(26, 52, 106, 242, 484, 996 subcarriers). In addition, the AP can
also decide on MCS and grouping of users within a given OFMDA PPDU.
Such flexibility can be leveraged to support time-sensitive
applications with bounded latency, especially in a managed network
where stations can be configured to operate under the control of the
AP.
As shown in [Cavalcanti_2019], it is possible to achieve latencies in
the order of 1msec with high reliability in an interference free
environment. Obviously, there are latency, reliability and capacity
tradeoffs to be considered. For instance, smaller Resource Units
(RU)s result in longer transmission durations, which may impact the
minimal latency that can be achieved, but the contention latency and
randomness elimination due to multi-user transmission is a major
benefit of the OFDMA mode.
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The flexibility to dynamically assign RUs to each transmission also
enables the AP to provide frequency diversity, which can help
increase reliability.
4.1.3. 802.11be Extreme High Throughput (EHT)
4.1.3.1. General Characteristics
The 802.11be is the next major 802.11 amendment (after 802.11ax) for
operation in the 2.4, 5 and 6 GHz bands. 802.11be is expected to
include new PHY and MAC features and it is targeting extremely high
throughput (at least 30 Gbps), as well as enhancements to worst case
latency and jitter. It is also expected to improve the integration
with 802.1 TSN to support time-sensitive applications over Ethernet
and Wireless LANs.
The 802.11be Task Group started its operation in May 2019, therefore,
detailed information about specific features is not yet available.
Only high level candidate features have been discussed so far,
including:
1. 320MHz bandwidth and more efficient utilization of non-
contiguous spectrum.
2. Multi-band/multi-channel aggregation and operation.
3. 16 spatial streams and related MIMO enhancements.
4. Multi-Access Point (AP) Coordination.
5. Enhanced link adaptation and retransmission protocol, e.g.
Hybrid Automatic Repeat Request (HARQ).
6. Any required adaptations to regulatory rules for the 6 GHz
spectrum.
4.1.3.2. Applicability to deterministic flows
The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG)
provided detailed information on use cases, issues and potential
solution directions to improve support for time-sensitive
applications in 802.11. The RTA TIG report [IEEE_doc_11-18-2009-06]
was used as input to the 802.11be project scope.
Improvements for worst-case latency, jitter and reliability were the
main topics identified in the RTA report, which were motivated by
applications in gaming, industrial automation, robotics, etc. The
RTA report also highlighted the need to support additional TSN
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capabilities, such as time-aware (802.1Qbv) shaping and packet
replication and elimination as defined in 802.1CB.
802.11be is expected to build on and enhance 802.11ax capabilities to
improve worst case latency and jitter. Some of the enhancement areas
are discussed next.
4.1.3.2.1. Enhanced scheduled operation for bounded latency
In addition to the throughput enhancements, 802.11be will leverage
the trigger-based scheduled operation enabled by 802.11ax to provide
efficient and more predictable medium access. 802.11be is expected to
include enhancements to reduce overhead and enable more efficient
operation in managed network deployments [IEEE_doc_11-19-0373-00].
4.1.3.2.2. Multi-AP coordination
Multi-AP coordination is one of the main new candidate features in
802.11be. It can provide benefits in throughput and capacity and has
the potential to address some of the issues that impact worst case
latency and reliability. Multi-AP coordination is expected to
address the contention due to overlapping Basic Service Sets (OBSS),
which is one of the main sources of random latency variations.
802.11be can define methods to enable better coordination between
APs, for instance, in a managed network scenario, in order to reduce
latency due to unmanaged contention.
Several multi-AP coordination approaches have been discussed with
different levels of complexities and benefits, but specific
coordination methods have not yet been defined.
4.1.3.2.3. Multi-band operation
802.11be will introduce new features to improve operation over
multiple bands and channels. By leveraging multiple bands/channels,
802.11be can isolate time-sensitive traffic from network congestion,
one of the main causes of large latency variations. In a managed
802.11be network, it should be possible to steer traffic to certain
bands/channels to isolate time-sensitive traffic from other traffic
and help achieve bounded latency.
4.1.4. 802.11ad and 802.11ay (mmWave operation)
4.1.4.1. General Characteristics
The IEEE 802.11ad amendment defines PHY and MAC capabilities to
enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave)
band. The standard addresses the adverse mmWave signal propagation
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characteristics and provides directional communication capabilities
that take advantage of beamforming to cope with increased
attenuation. An overview of the 802.11ad standard can be found in
[Nitsche_2015] .
The IEEE 802.11ay is currently developing enhancements to the
802.11ad standard to enable the next generation mmWave operation
targeting 100 Gbps throughput. Some of the main enhancements in
802.11ay include MIMO, channel bonding, improved channel access and
beamforming training. An overview of the 802.11ay capabilities can
be found in [Ghasempour_2017]
4.1.4.2. Applicability to deterministic flows
The high data rates achievable with 802.11ad and 802.11ay can
significantly reduce latency down to microsecond levels. Limited
interference from legacy and other unlicensed devices in 60 GHz is
also a benefit. However, directionality and short range typical in
mmWave operation impose new challenges such as the overhead required
for beam training and blockage issues, which impact both latency and
reliability. Therefore, it is important to understand the use case
and deployment conditions in order to properly apply and configure
802.11ad/ay networks for time sensitive applications.
The 802.11ad standard include a scheduled access mode in which
stations can be allocated contention-free service periods by a
central controller. This scheduling capability is also available in
802.11ay, and it is one of the mechanisms that can be used to provide
bounded latency to time-sensitive data flows. An analysis of the
theoretical latency bounds that can be achieved with 802.11ad service
periods is provided in [Cavalcanti_2019].
4.2. IEEE 802.15.4
4.2.1. Provenance and Documents
The IEEE802.15.4 Task Group has been driving the development of low-
power low-cost radio technology. The Timeslotted Channel Hopping
mode, added to the 2015 revision of the IEEE802.15.4 standard
[IEEE802154], is targeted at the embedded and industrial world, where
reliability, energy consumption and cost drive the application space.
The IEEE802.15.4 physical layer has been designed to support
demanding low-power scenarios targeting the use of unlicensed bands,
both the 2.4 GHz and sub GHz Industrial, Scientific and Medical (ISM)
bands. This has imposed requirements in terms of frame size, data
rate and bandwidth to achieve reduced collision probability, reduced
packet error rate, and acceptable range with limited transmission
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power. The PHY layer supports frames of up to 127 bytes. The Medium
Access Control (MAC) sublayer overhead is in the order of 10-20
bytes, leaving about 100 bytes to the upper layers. IEEE802.15.4
uses spread spectrum modulation such as the Direct Sequence Spread
Spectrum (DSSS).
IPv6 over TSCH is enabled by the work done at the 6TiSCH WG. 6TiSCH
has enabled best effort distributed scheduling to exploit the
deterministic access capabilities provided by TSCH. The group
designed the essential mechanisms to enable the management plane
operation while ensuring IPv6 is supported. Yet the charter did not
focus to providing a solution to establish end to end tracks while
meeting quality of service requirements. 6TiSCH, through the RFC8480
[RFC8480] defines the 6P protocol which provides a pairwise
negotiation mechanism to the control plane operation. The protocol
supports agreement on a schedule between neighbors, enabling
distributed scheduling. 6P goes hand-in-hand with a Scheduling
Function (SF), the policy that decides how to maintain cells and
trigger 6P transactions. The Minimal Scheduling Function (MSF)
[I-D.ietf-6tisch-msf] is the default SF defined by the 6TiSCH WG;
other standardized SFs can be defined in the future. MSF extends the
minimal schedule configuration, and is used to add child-parent links
according to the traffic load.
Time sensitive networking on low power constrained wireless networks
have been addressed by ISA100.11a and WirelessHART. TODO
The 6TiSCH architecture [I-D.ietf-6tisch-architecture] already
identified different models to schedule resources along tracks
exploiting the TSCH schedule structure however these models have not
been standardized. A Track, in the 6TiSCH architecture is considered
a directed path from a source 6TiSCH node to one or more
destination(s) 6TiSCH node(s) through the 6TiSCH network. A Track in
6TiSCH is the implementation of the deterministic path in the Detnet
architecture [I-D.ietf-detnet-architecture] . Along a Track, 6TiSCH
nodes reserve the resources to enable the efficient transmission of
packets while aiming to optimize certain properties such as
reliability and ensure small jitter or bounded latency. The track
structure enables Layer-2 forwarding schemes, reducing the overhead
of taking routing decisions at the Layer-3. Serial Tracks can be
understood as the concatenation of cells or bundles along a routing
path from a node towards a destination. The serial track concept is
analogous to the circuit concept where resources are chained through
the multi-hop topology. For example, A bundle of Tx Cells in a
particular node is paired to a bundle of Rx Cells in the next hop
node following a routing path. More complex approaches are described
in and complemented by extensions to the RPL routing protocol in
[I-D.ietf-roll-nsa-extension]. Reliability measures are for example
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achieved by exploiting concepts such as Replication and Elimination.
In them, packets at origin are replicated and transmitted along
disjoint tracks. This redundancy measure exploiting track forwarding
increases energy consumption of the network nodes but improves
significantly the reliability of the network.
Useful References include:
1. IEEE Std 802.15.4: "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"
[IEEE802154]. The latest version at the time of this writing is
dated year 2015.
2. Morell, A. , Vilajosana, X. , Vicario, J. L. and Watteyne, T.
(2013), Label switching over IEEE802.15.4e networks. Trans.
Emerging Tel. Tech., 24: 458-475. doi:10.1002/ett.2650"
[morell13].
3. De Armas, J., Tuset, P., Chang, T., Adelantado, F., Watteyne,
T., Vilajosana, X. (2016, September). Determinism through path
diversity: Why packet replication makes sense. In 2016
International Conference on Intelligent Networking and
Collaborative Systems (INCoS) (pp. 150-154). IEEE. [dearmas16].
4. X. Vilajosana, T. Watteyne, M. Vucinic, T. Chang and K. S.
J. Pister, "6TiSCH: Industrial Performance for IPv6 Internet-
of-Things Networks," in Proceedings of the IEEE, vol. 107, no.
6, pp. 1153-1165, June 2019. [vilajosana19].
4.2.2. TimeSlotted Channel Hopping
4.2.2.1. General Characteristics
As a core technique in IEEE802.15.4, TSCH splits time in multiple
time slots that repeat over time. The structure is referred as a
Slotframe. For each timeslot, a set of available frequencies can be
used, resulting in a matrix-like schedule (see Fig. Figure 1).
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timeslot offset
| 0 1 2 3 4 | 0 1 2 3 4 | Nodes
+------------------------+------------------------+ +-----+
| | | | | | | | | | | | C |
CH-1 | EB | | |C->B| | EB | | |C->B| | | |
| | | | | | | | | | | +-----+
+-------------------------------------------------+ |
| | | | | | | | | | | |
CH-2 | | |B->C| |B->A| | |B->C| |B->A| +-----+
| | | | | | | | | | | | B |
+-------------------------------------------------+ | |
... +-----+
|
+-------------------------------------------------+ |
| | | | | | | | | | | +-----+
CH-15| |A->B| | | | |A->B| | | | | A |
| | | | | | | | | | | | |
+-------------------------------------------------+ +-----+
ch.
offset
Figure 1: Slotframe example with scheduled cells between nodes A, B
and C
This schedule represents the possible communications of a node with
its neighbors, and is managed by a Scheduling Function such as The
Minimal Scheduling Function (MSF) [I-D.ietf-6tisch-msf]. Each cell
in the schedule is identified by its slotoffset and channeloffset
coordinates. A cell's timeslot offset indicates its position in
time, relative to the beginning of the slotframe. A cell's channel
offset is an index which maps to a frequency at each iteration of the
slotframe. Each packet exchanged between neighbors happens within
one cell. An Absolute Slot Number (ASN) indicates the number of
slots elapsed since the network started. It increments at every
slot. This is a 5 byte counter that can support networks running for
more than 300 years without wrapping (assuming a 10 ms timeslot).
Channel hopping provides increased reliability to multi-path fading
and external interference. It is handled by TSCH through a channel
hopping sequence referred as macHopSeq in the IEEE802.15.4
specification.
The Time-Frequency Division Multiple Access provided by TSCH enables
the orchestration of traffic flows, spreading them in time and
frequency, and hence enabling an efficient management of the
bandwidth utilization. Such efficient bandwidth utilization can be
combined to OFDM modulations also supported by the IEEE802.15.4
standard [IEEE802154] since the 2015 version.
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In the RAW context, low power reliable networks should address non-
critical control scenarios such as Class 2 and monitoring scenarios
such as Class 4 defined by the RFC5673 [RFC5673]. As a low power
technology targeting industrial scenarios radio transducers provide
low data rates (typically between 50kbps to 250kbps) and robust
modulations to trade-off performance to reliability. TSCH networks
are organized in mesh topologies and connected to a backbone.
Latency in the mesh network is mainly influenced by propagation
aspects such as interference. ARQ methods and redundancy techniques
such as replication and elimination should be studied to provide the
needed performance to address deterministic scenarios.
4.2.2.2. Applicability to Deterministic Flows
Nodes in a TSCH network are tightly synchronized. This enables to
build the slotted structure an ensure efficient utilization of
resources thranks to proper scheduling policies. Scheduling is a key
to orchestrate the resources that different nodes in a track or path
are using. Slotframes can be split in resource blocks reserving the
needed capacity to certain needs. Periodic and bursty traffic can be
handled independently in the schedule, using active and reactive
policies and taking advantage of certain cell overprovision. Along a
track, resource blocks can be chained so nodes in previous hops
transmit their data before those that come later. This provides a
tight control to latency along a track. Redundancy is achieved in a
best effort manner by overprovision, giving time to the management
plane of the network to request more resources if needed. -time
synchronization - scheduling capabilities, discuss such things as
Resource Units, time slots or resource blocks. Can we reserve
periodic resources vs. ask each time, what precision can we get in
latency control. - diversity scenarios, what's available, - gap
analysis, e.g. discuss multihop, or what's missing how to do PAREO
features.
5. 3GPP standards
6. IANA Considerations
This specification does not require IANA action.
7. Security Considerations
Most RAW technologies integrate some authentication or encryption
mechanisms that were defined outside the IETF.
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8. Acknowledgments
Many thanks to the participants of the RAW WG where a lot of the work
discussed here happened.
9. References
9.1. Normative References
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
in progress), March 2019.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-13 (work in progress), May 2019.
[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-Power and
Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
2009, <https://www.rfc-editor.org/info/rfc5673>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC8480, November 2018,
<https://www.rfc-editor.org/info/rfc8480>.
9.2. Informative References
[Cavalcanti_2019]
Dave Cavalcanti et al., "Extending Time Distribution and
Timeliness Capabilities over the Air to Enable Future
Wireless Industrial Automation Systems, the Proceedings of
IEEE", June 2019.
[dearmas16]
Jesica de Armas et al., "Determinism through path
diversity: Why packet replication makes sense", September
2016.
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[Ghasempour_2017]
Yasaman Ghasempour et al., "802.11ay: Next-Generation 60
GHz Communications for 100 Gb/s Wi-Fi", December 2017.
[I-D.ietf-6tisch-msf]
Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
draft-ietf-6tisch-msf-03 (work in progress), April 2019.
[I-D.ietf-roll-nsa-extension]
Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P.
Thubert, "RPL DAG Metric Container Node State and
Attribute object type extension", draft-ietf-roll-nsa-
extension-01 (work in progress), March 2019.
[IEEE80211]
"IEEE Standard 802.11 - IEEE Standard for Information
Technology - Telecommunications and information exchange
between systems Local and metropolitan area networks -
Specific requirements - Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY)
Specifications.".
[IEEE80211ad]
"802.11ad: Enhancements for very high throughput in the 60
GHz band".
[IEEE80211ak]
"802.11ak: Enhancements for Transit Links Within Bridged
Networks", 2017.
[IEEE80211ax]
"802.11ax D4.0: Enhancements for High Efficiency WLAN".
[IEEE80211ay]
"802.11ay: Enhanced throughput for operation in license-
exempt bands above 45 GHz".
[IEEE80211be]
"802.11be: Extreme High Throughput".
[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".
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[IEEE8021Qat]
"802.1Qat: Stream Reservation Protocol".
[IEEE8021Qcc]
"802.1Qcc: IEEE Standard for Local and Metropolitan Area
Networks--Bridges and Bridged Networks -- Amendment 31:
Stream Reservation Protocol (SRP) Enhancements and
Performance Improvements".
[IEEE_doc_11-18-2009-06]
"802.11 Real-Time Applications (RTA) Topic Interest Group
(TIG) Report", November 2018.
[IEEE_doc_11-19-0373-00]
Kevin Stanton et Al., "Time-Sensitive Applications Support
in EHT", March 2019.
[morell13]
Antoni Morell et al., "Label switching over IEEE802.15.4e
networks", April 2013.
[Nitsche_2015]
Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz
communication for multi-Gigabit-per-second Wi-Fi",
December 2014.
[vilajosana19]
Xavier Vilajosana et al., "6TiSCH: Industrial Performance
for IPv6 Internet-of-Things Networks", June 2019.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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Dave Cavalcanti
Intel Corporation
2111 NE 25th Ave
Hillsboro, OR 97124
USA
Phone: 503 712 5566
Email: dave.cavalcanti@intel.com
Xavier Vilajosana
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Email: xvilajosana@uoc.edu
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