6TiSCH J. Munoz, Ed.
Internet-Draft Inria
Intended status: Informational X. Vilajosana
Expires: January 3, 2019 Universitat Oberta de Catalunya
T. Chang
Inria
July 2, 2018
Problem Statement for Generalizing 6TiSCH to Multiple PHYs
draft-munoz-6tisch-multi-phy-nodes-00
Abstract
The present document describes the needs that arise when considering
to use more than one PHY in a IPv6 over the TSCH mode of
IEEE802.15.4e (6TiSCH) network. These considerations are present in:
the choice of the PHY, the MAC layer -TSCH- configuration, the 6top
protocol, 6LoWPAN and RPL.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Neighbor Considerations . . . . . . . . . . . . . . . . . . . 3
3. MAC Sub-Layer Considerations . . . . . . . . . . . . . . . . 4
3.1. Network Formation . . . . . . . . . . . . . . . . . . . . 4
3.2. Discovering Node PHY Capabilities . . . . . . . . . . . . 4
3.3. TSCH Configuration . . . . . . . . . . . . . . . . . . . 5
3.3.1. Timeslot Duration . . . . . . . . . . . . . . . . . . 5
3.3.2. Channel Hopping Sequence . . . . . . . . . . . . . . 5
4. 6top Sub-Layer Considerations . . . . . . . . . . . . . . . . 6
4.1. Resource Allocation . . . . . . . . . . . . . . . . . . . 6
4.2. Duty Cycle Regulations . . . . . . . . . . . . . . . . . 6
5. 6LoWPAN Considerations . . . . . . . . . . . . . . . . . . . 6
6. RPL Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
9.3. Other Informative References . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The protocol stack in a IPv6 over the TSCH mode of IEEE802.15.4e
(6TiSCH) network is defined by multiple protocols covering multiple
layers starting from the link layer, up to the application layer
[I-D.ietf-6tisch-architecture]. This protocol stack sits on top of
the IEEE802.15.4 O-QPSK PHY, at 2.4 GHz, that allows the exchange of
frames of 127 B over 16 frequencies at 250 kbps. Since 2012, more
PHYs are available within the IEEE802.15.4 specification, e.g. the
IEEE802.15.4g amendment [IEEE802154g] of the IEEE802.15.4 standard,
designed for Smart Utility Networks (SUN) application, introducing
the SUN-OFDM, SUN-FSK and SUN-O-QPSK PHYs. The main differences with
the previous IEEE802.15.4 O-QPSK PHY is support of link-layer frames
up to 2047 B long, the possibility of being used either at the same
2.4 GHz band or in sub-GHz, regionally defined bands, and variable
data rate that goes from 6.25 kbps up to 800 kbps.
Radio chips supporting all these new PHY configurations are now
available, giving the opportunity to implementers to exploit the
benefits of this diversity in terms of throughput, range and
reliability that each PHY brings with it.
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However, the adoption of a PHY different from IEEE802.15.4 O-QPSK
poses new design considerations across the 6TiSCH protocol stack.
Even though layer separation exists between protocols, from the link
layer upwards, 6TiSCH protocols have been designed considering one
PHY only. This approach of having links over multiple PHYs in the
same network is new, and poses up-to-now unknown considerations for
network designers.
This document describes how the behavior of each item of the 6TiSCH
protocol stack may be impacted by the inclusion of multiple PHYs with
such different properties.
This document makes the assumption that the reader is familiar with
the [I-D.ietf-6tisch-terminology] and [I-D.ietf-6tisch-architecture],
as well as the protocols mentioned there.
Solutions for the considerations here exposed are out of the scope of
this document. This document is to be considered only for
informative purposes.
2. Neighbor Considerations
In a low-power wireless network with a single PHY, a neighbor node to
a particular node A is any node within its interference domain. If
nodes are able to use multiple PHYs, a pair of nodes using a specific
PHY may be within the same interference domain and when using another
PHY, they may not. In addition, nodes also can communicate over
different frequency bands. So now the definition of a neighbor node
changes to any device within the same interference domain for a given
PHY configuration and frequency band. This modifies how nodes can
manage their neighbors' information.
Neighbor information is accessed by both the MAC and routing layers.
Letting which layer to handle the multiple PHY information changes
the network protocol stack significantly. In case of handling by the
MAC layer, an entity between MAC and Routing to choose which PHY
layer to use is required. This entity could be part of Scheduling
Function (Section 4.3). In case of handling by Routing layer, each
PHY layer could be considered as a neighbor. For RPL, if only one
DODAG exist through the network, a dedicated Objective Function for
multiPHY features is required. If each PHY layer has a DODAG
corresponding with, the OF for 6TiSCH could be used with little
modification. However, this increases the complexity of the network.
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3. MAC Sub-Layer Considerations
The considerations that arise according to the used PHY include
network formation, node discovery, and TSCH configuration.
3.1. Network Formation
Getting nodes to join the network as fast as possible is a major
interest to minimize energy consumption. Radio activity is the most
power consuming task for nodes, therefore the more time nodes spend
listening to get an Enhanced Beacon (EB) the more energy they
consume, reducing their lifespan. Considering a current 6TiSCH
network, with just one PHY and one frequency band (16 frequencies),
nodes have to tune their radios in one frequency wait for an EB.
Nodes which are already part of the network transmit EBs in a round-
robin fashion on these 16 frequencies. If the node did not receive
any EB after some time, it may tune its radio on a different
frequency and listen again for an EB. This means a node listen for a
long time before hearing an EB.
In the case of multiple PHYs, nodes attempting to join the network
need to over even more PHYs, until hearing an EB. A mechanism might
be needed to reduce join time, for example use a particular PHY for
joining.
3.2. Discovering Node PHY Capabilities
For 6TiSCH networks using one PHY configuration, discovering the PHY
neighbor node's capabilities is not necessary. But in a new multiPHY
network context, knowing the capabilities of neighbor nodes is
important. Once a node is part of the network, it may have not have
joined using the most convenient PHY configuration for this pair of
nodes. Any of these nodes might then (a) unicast a request its
neighbors to get the information about their PHY capabilities, or (b)
discover the PHY capabilities of the neighbors by listening for EBs
at specific times over other PHYs.
If using (a), further choices need to be taken to decide whether
nodes would use shared slots or negotiate dedicated timeslots to test
the connectivity over other PHYs. Agreeing on which PHY to test and
when has to be done under the already tested PHY configuration, and
the energy consumption footprint of this process may be too heavy.
If using (b), it may take long time until the most efficient PHY
configuration is discovered between two nodes.
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3.3. TSCH Configuration
A multi-PHY approach has an impact on timeslot duration and channel
hopping sequence.
3.3.1. Timeslot Duration
The diversity of data rates of the PHYs in the IEEE802.15.4-2015
standard makes it challenging to find a timeslot duration that is
both efficient and fits all PHYs options. In current 6TiSCH
networks, a common practice is to have a timeslot of 10 ms. This is
time enough for transmitting a 127 B frame using IEEE802.15.4 O-QPSK,
taking roughly 4 ms, to wait for the acknowledgment, leaving a
handful of milliseconds for data processing with proper guard times.
But for multiple PHYs with data rates going from 6.25 kbps up to 800
kbps and with maximum frame size of 2047 B, the time of transmission
for a full size packet varies from 0.020 s (800 kbps) to 2.62 s (6.25
kbps). With such disparity, considering a timeslot long enough (>
2.62 s) to allow the transmission (and its acknowledge frame) of the
maximum frame size with the slowest data rate results in a waste of
time (network resources) if faster PHY can be used, by leaving the
most part of the timeslot unused. Such a long timeslot would cause
slotframes to have a duration in the order of minutes (considering
for example a slotframe of 101 timeslots), and as tight
synchronization is mandatory, multiple KA frames would have to be
sent within the same slotframe, considerably reducing the network
resources and efficiency.
On the other hand, choosing a shorter timeslot poses a rigid
limitation in the size of the frames when slow data rates PHYs are
used. By having timeslots in the order of 10's of ms, the frame size
for slow data rate is heavily reduced: with a 100 ms timeslot, only
78 B can be transmitted using 6.25 kbps, without considering time for
acknowledgment and guard times.
Multi-PHY designs should therefore tune these parameters to find the
right trade-off between shorter or longer timeslots (limiting sizes
of frames with some PHYs), as well as the size of the slotframe.
3.3.2. Channel Hopping Sequence
Current 6TiSCH implementations use the 2.4 GHz band, with 16
frequencies separated by 5 MHz and 2 MHz wide. Channel hopping
sequences use only the frequency number identification. By
introducing multiple PHYs, these do not have the same characteristics
of channel spacing, bandwidth nor channel numbering. Moreover,
channels from different PHYs may overlap.
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As a result, by only referring to a channel by some index doesn't
carry over to multiple PHYs. Multi-PHY designs need to solve how to
identify channels.
4. 6top Sub-Layer Considerations
The 6top sub-layer [I-D.ietf-6tisch-6top-protocol] is responsible for
allocating cells between pairs of neighbor nodes. In a multi-PHY
environment, cells have different capabilities depending on the PHY
used. Moreover, in some frequency bands, duty cycle regulation must
be met.
4.1. Resource Allocation
Current 6TiSCH networks account the network resources allocation in
the amount of cells per slotframe a pair of nodes needs. In a multi-
PHY design, allocating cells does not provide enough information,
since depending on the PHY used, more or less data can fit in a
timeslot. Multi-PHY designs have to define how to network resource
needs are measured.
4.2. Duty Cycle Regulations
Duty cycle regulations apply to most frequency bands. These
regulations vary from country to country, so multi-PHY designs need
to comply with local regulations.
5. 6LoWPAN Considerations
6LoWPAN has been initially designed with IEEE802.15.4 O-QPSK in mind.
Header compression, fragmentation and reassembly are the main tasks
of this adaptive layer. However, in this new context, other PHYs
allow to send more than 127 bytes in one frame. 6LoWPAN
functionalities should be adapted to efficiently fit in the layer
below. This includes the sizes of the fragments, that should be
calculated depending on the PHY to be used and the maximum amount of
data that can transport, given the length of the timeslot.
6. RPL Considerations
In multi-PHY design, RPL is impacted in several ways: Objective
Functions must now consider more than one PHY, and each node's rank
must be calculated accordingly.
A multi-PHY design may consider new Objective Functions that take
into account the difference in throughput, resource occupancy and
energy consumption of each PHY. For example, in OF0 [RFC6552] , the
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'rank_factor' can have a different value for each PHY, depending on
its characteristics.
7. Security Considerations
This document discusses a number of elements to consider when
designing a multi-PHY solution based on 6TiSCH. It does not define a
new protocol.
8. IANA Considerations
This document does not require any IANA actions.
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-14 (work
in progress), April 2018.
[I-D.ietf-6tisch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terms Used in IPv6 over the TSCH mode of IEEE 802.15.4e",
draft-ietf-6tisch-terminology-10 (work in progress), March
2018.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, DOI 10.17487/RFC6552, March 2012,
<https://www.rfc-editor.org/info/rfc6552>.
9.2. Informative References
[I-D.ietf-6tisch-6top-protocol]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer Protocol (6P)", draft-ietf-6tisch-6top-
protocol-12 (work in progress), June 2018.
9.3. Other Informative References
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[IEEE802154g]
IEEE standard for Information Technology, "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 as amended by IEEE Std.
802.15.4g, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendment 3: Physical Layer (PHY)
Specifications for Low Data-Rate, Wireless, Smart Metering
Utility Networks", April 2012.
Authors' Addresses
Jonathan Munoz (editor)
Inria
2 rue Simone Iff
Paris 12 75012
France
Email: jonathan.munoz@inria.fr
Xavier Vilajosana
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Email: xvilajosana@uoc.edu
Tengfei Chang
Inria
2 rue Simone Iff
Paris 12 75012
France
Email: tengfei.chang@inria.fr
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