6TSCH X. Vilajosana, Ed.
Internet-Draft Universitat Oberta de Catalunya
Intended status: Informational K. Pister
Expires: January 15, 2014 University of California Berkeley
July 14, 2013
Minimal 6TSCH Configuration
draft-vilajosana-6tsch-basic-01
Abstract
This document describes the minimal set of rules to operate a
[IEEE802154e] Timeslotted Channel Hopping (TSCH) network. These
rules can be used during early interoperability testing and
development, when the centralized and distributed solutions developed
by the 6TSCH group are not fully implemented yet, or otherwise not
available.
Status of This Memo
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Basic Schedule . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Slotframe . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Cell Options . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Retransmissions . . . . . . . . . . . . . . . . . . . . . 5
2.4. Time Slot timming . . . . . . . . . . . . . . . . . . . . 5
3. Enhanced Beacons Configuration and Content . . . . . . . . . 6
3.1. Sync IE . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 7
3.2. Frame and Link IE . . . . . . . . . . . . . . . . . . . . 7
3.2.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. IE Content . . . . . . . . . . . . . . . . . . . . . 8
4. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. ACK/NACK Time Correction IE . . . . . . . . . . . . . . . 8
4.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 8
5. Neighbor information . . . . . . . . . . . . . . . . . . . . 9
5.1. Neighbor Table . . . . . . . . . . . . . . . . . . . . . 9
5.2. Time Parent Selection . . . . . . . . . . . . . . . . . . 10
6. Queues and Priorities . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
7.3. External Informative References . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The nodes in a [IEEE802154e] TSCH network follow a communication
schedule. The entity (centralized or decentralized) responsible for
building and maintaining that schedule has very precise control over
the trade-off between the network's latency, bandwidth, reliability
and power consumption. During early interoperability testing and
development, however, simplicity is often more important than
efficiency. The goal of this document is to define the simplest set
of rules for building a [IEEE802154e] TSCH-compliant network, at the
necessary price of lesser efficiency.
2. Basic Schedule
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In order to form a network, a minimum schedule configuration is
required so nodes can advertise the presence of the network, and
allow other nodes to join.
2.1. Slotframe
The Slotframe, as defined in [I-D.palattella-6tsch-terminology], is
an abstraction of the MAC layer that defines a collection of time
slots of equal length and priority, and which repeats over time. In
order to set up a basic TSCH network, nodes need to be synchronized
with the same slotframe configuration so they can exchange Enhanced
Beacons (EBs) and data packets. This document recommends the
following slotframe configuration.
Basic configuration
+------------------------------------+----------------------+
| Property | Value |
+------------------------------------+----------------------+
| Number of time slots per Slotframe | 101 |
+------------------------------------+----------------------+
| Number of available channels | 16 |
+------------------------------------+----------------------+
| Number of EBs cells | 1 (slotOffset 0) |
+------------------------------------+----------------------+
| Number of scheduled cells | 5 (slotOffsets |
| | 1,2,3,4,5) |
+------------------------------------+----------------------+
| Number of unscheduled cells | 95 (from slotOffset |
| | 6 to 100) |
+------------------------------------+----------------------+
| Number of MAC retransmissions (max)| 3 |
+------------------------------------+----------------------+
| Time Slot duration | 15ms |
+------------------------------------+----------------------+
The suggested basic schedule is hard-coded in each node. The
slotframe is composed of 101 time slots. The first slot in the
slotframe is used to send Enhanced Beacons announcing the presence of
the network. These EBs are not acknowledged. Five cells are
scheduled for exchanging data packets, as described in Section 2.2.
These cells are scheduled at slotOffset 1 to 5, and channeOffset 0.
Per the IEEE802.15.4e TSCH standard, data packets sent on these cells
to a unicast MAC address are acknowledged by the receiver. The 95
remaining cells are unscheduled, i.e., the radio of the nodes remains
off.
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Basic schedule overview
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 0 | EB |TxRxS|TxRxS|TxRxS|TxRxS|TxRxS| OFF | ... | OFF |
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 1 | | | | | | | | ... | |
+-----+-----+-----+-----+-----+-----+-----+ +-----+
...
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 15 | | | | | | | | ... | |
+-----+-----+-----+-----+-----+-----+-----+ +-----+
0 1 2 3 4 5 6 100
2.2. Cell Options
Per the [IEEE802154e] TSCH standard, each scheduled cell has a bitmap
of cell options assigned. All scheduled cells in the basic schedule
are configured as Hard cells
[I-D.watteyne-6tsch-tsch-lln-context][I-D.draft-wang-6tsch-6top]
since reallocation is not considered in that simple approach.
The EB cell is assigned the following bitmap of cell options:
b0 = Transmit = 1 (set)
b1 = Receive = 0 (clear)
b2 = Shared = 0 (clear)
b3 = Timekeeping = 0 (clear)
b4 = Hard = 1 (set)
b5-b7 = Reserved (clear)
The data cells are assigned the bitmap of cell options below that
results in "Slotted Aloha" behavior. Because both the "Transmit" and
"Receive" bits are set, a node either transmits, if there is a packet
in its queue, or listens if it has nothing to transmit. Because the
"shared" bit is set,in presence of collisions it uses the backoff
mechanism defined in [IEEE802154e].
b0 = Transmit = 1 (set)
b1 = Receive = 1 (set)
b2 = Shared = 1 (set)
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b3 = Timekeeping = 0 (clear)
b4 = Hard = 1 (set)
b5-b7 = Reserved (clear)
All remaining cells are unscheduled. Thus the nodes can keep their
radio off. In a memory efficient implementation, scheduled cells
could be represented by a circular linked list. Unscheduled cells
should not occupy any memory.
2.3. Retransmissions
The maximum number of MAC-layer retransmissions is set to 3. For
packets which require an acknowledgement, if none is received after a
total of 4 attempts, the transmissions is considered failed at the
MAC layer, and the upper layer needs to be notified. Packets sent to
the broadcast MAC address (including EBs) are not acknowledged and
therefore not retransmitted.
2.4. Time Slot timming
The figure below shows an active timeslot in which a packet is sent
from the transmitter node (TX) to the receiver node (RX), and a MAC
acknowledgement is sent back from the RX to the TX node, indicating
successful reception. The TsTxOffset duration defines the instant in
the timeslot when the first byte of the transmitted packet leaves the
radio of the TX node. The radio of the RX node is turned on TsLongGT
/2 before that instant, and listen for at least TsLongGT. This
allows for a de-synchronization between the two node of at most
TsLongGT. The RX node needs to send the first byte of the MAC
acknowledgement exactly TsTxAckDelay after the end of the last byte
of the received packet. TX's radio has to be turned on TsShortGT/2
before that time, and keep listening for at least TsShortGT.
Time slot internal timing diagram
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/------------------- Time Slot duration --------------------/
| /tsShortGT/ |
| | | | | |
|------------+-----------------+--------------+------+------|
TX | | TX-Packet | |RX Ack| |
|------------+-----------------+--------------+------+------|
|/tsTxOffset/| | | | |
| | | | | |
|------------+-----------------+--------------+------+------|
RX | | | | RX-Packet | |TX Ack| |
|---------+--+--+--------------+--------------+------+------|
| | | | | | | |
| /tsLongGT/ |/TsTxAckDelay/| | |
Start End
of of
Slot Slot
[IEEE802154e] does not define the different durations of a time slot.
It does allow those durations to be sent in the EBs (through a
TimeSlot IE), but for simplicity, this document recommends to hard-
code the different durations to the values listed below.
Timeslot durations
+---------------------------------+------------------+
| IEEE802.15.4e TSCH parmeter | Value |
+---------------------------------+------------------+
| TsTxOffset | 4000us |
+---------------------------------+------------------+
| TsLongGT | 2600us |
+---------------------------------+------------------+
| TsTxAckDelay | 4606us |
+---------------------------------+------------------+
| TsShortGT | 1000us |
+---------------------------------+------------------+
| Time Slot duration | 15000us |
+---------------------------------+------------------+
3. Enhanced Beacons Configuration and Content
[IEEE802154e] does not define how often or which EBs are sent. The
choice of the duration between two EBs needs to take into account
whether EBs are used as the only mechanism to synchronize devices, or
whether a Keep-Alive (KA) mechanism is used in parallel. For a
simplest TSCH configuration, it is recommended to sent EBs at least
once every 10s. For additional reference see
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[I-D.watteyne-6tsch-tsch-lln-context] where different synchronization
approaches are summarized.
EBs must be sent with the Beacon IEEE802.15.4 frame type and this
document recommends that they carry the following Information
Elements (IEs):
3.1. Sync IE
Contains synchronization information such as ASN and join priority.
3.1.1. IE Header
Length (b0-b7) = 0x06
Sub-ID (b8-b14) = 0x1a
Type (b15) = 0x00 (short)
3.1.2. IE Content
ASN Byte 1 (b16-b23)
ASN Byte 2 (b24-b31)
ASN Byte 3 (b32-b39)
ASN Byte 4 (b40-b47)
ASN Byte 5 (b48-b55)
Join Priority (b56-b63)
3.2. Frame and Link IE
Although the schedule is hard-coded in each node, this document
recommends to indicate the schedule in each EB through a Frame and
Link IE. This enables nodes which implement [IEEE802154e] fully to
be able to configure their schedule as they join the network, and
interact with nodes using a hard-coded schedule.
3.2.1. IE Header
Length (b0-b7) = variable
Sub-ID (b8-b14) = 0x1b
Type (b15) = 0x00 (short)
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3.2.2. IE Content
# Slotframes (b16-b23) = 0x01
Slotframe ID (b24-b31) = 0x01
Size Slotframe (b32-b47) = 0x65
# Links (b48-b55) = 0x06
For each link in the basic schedule:
Channel Offset (2B) = 0x00
Slot Number (2B) = from (0x00 to 0x05)
LinkOption (1B) = as described in Section 2.2
4. Acknowledgement
MAC-layer acknowledgement frames are built according to
[IEEE802154e]. Data frames and command frames sent to a unicast MAC
destination address request an acknowledgement. The acknowledgement
frame is of type ACK (0x10). Each acknowledgement contains the
following IE:
4.1. ACK/NACK Time Correction IE
The ACK/NACK time correction IE is used to carry the measured
desynchronization between the sender and the receiver.
4.1.1. IE Header
Length (b0-b7) = 0x02
Sub-ID (b8-b14) = 0x1e
Type (b15) = 0x00 (short)
4.1.2. IE Content
Time Synch Info and ACK status (b16-b31)
The possible values for the Time Synch Info and ACK status are
described in the following table:
ACK status and Time Synch information.
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+-----------------------------------+------------------+
| ACK Status | Value |
+-----------------------------------+------------------+
| ACK with positive time correction | 0x0000 - 0x07ff |
+-----------------------------------+------------------+
| ACK with negative time correction | 0x0800 - 0x0fff |
+-----------------------------------+------------------+
| NACK with positive time correction| 0x8000 - 0x87ff |
+-----------------------------------+------------------+
| NACK with negative time correction| 0x8800 - 0x8fff |
+-----------------------------------+------------------+
5. Neighbor information
[IEEE802154e] does not define how and when each node in the network
keeps information about its neighbors. This document recommends to
keep the following information in the neighbor table:
5.1. Neighbor Table
The exact format of the neighbor table is implementation-specific,
but it should at least contain the following information, for each
neighbor:
Neighbor statistics:
number of transmitted packets to that neighbor
number of transmitted packets that have been acknowledged by
that neighbor
number of received packets from that neighbor
number of received packets that have been acknowledged for that
neighbor
Neighbor address.
ASN when that neighbor was heard for the last time. This can be
used to trigger a keep-alive message.
RPL rank of that neighbor.
A flag which indicates whether this neighbor is a time source
neighbor.
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Connectivity statistics (RSSI, LQI, etc), which can be used to
determine the quality of the link.
In addition of that information, each node has to be able to compute
some RPL objective function (OF) taking into account the neighbor and
connectivity statistics. An example RPL objective function is the
ETX.
5.2. Time Parent Selection
Each node selects a time parent amongst its known neighbors. When a
node joins a network, it has no routing information yet. Its
(possibly temporary) time parent is the node it can hear "best", for
example based on RSSI measurements of the EBs it received. After
having acquired a RPL rank, the RPL routing parents should also be
IEEE802.15.4e time source neighbors.
Optionally, a node can choose to use an counter to avoid frequent
changes in time source neighbor selection. Based on some thresholds
(on RSSI for example), if the quality of the link with time parent
changes over or below the thresholds for a certain number of times
(e.g., 3), the instability counter is incremented and another time
parent is selected.
6. Queues and Priorities
[IEEE802154e] does not define the use of queues to handle upper layer
data (either application or control data from upper layers). This
document recommends to use a single queue with the following rules:
When the node is not synchronized to the network, higher layers
are not able to insert packets into the queue.
Lower-layer packets have a higher priority that packets received
from a higher layer.
IEEE802.15.4 frames of types Beacon and Command have a higher
priority than IEEE802.15.4 frames of types Data and ACK.
One entry in the queue is reserved at all times for a IEEE802.15.4
frames of types Beacon or Command frames.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
7.2. Informative References
[I-D.watteyne-6tsch-tsch-lln-context]
Watteyne, T., Palattella, M., and L. Grieco, "Using
IEEE802.15.4e TSCH in an LLN context: Overview, Problem
Statement and Goals", draft-watteyne-6tsch-tsch-lln-
context-02 (work in progress), May 2013.
[I-D.thubert-6tsch-architecture]
Thubert, P., Assimiti, R., and T. Watteyne, "An
Architecture for IPv6 over Time Slotted Channel Hopping",
draft-thubert-6tsch-architecture-01 (work in progress),
April 2013.
[I-D.palattella-6tsch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terminology in IPv6 over Time Slotted Channel Hopping",
draft-palattella-6tsch-terminology-00 (work in progress),
March 2013.
[I-D.draft-wang-6tsch-6top]
Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TSCH
Operation Sublayer (6top). draft-wang-6tsch-6top-00 (work
in progress) ", July 2013.
[I-D.ohba-6tsch-security]
Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P., and
A. Yegin, "Security Framework and Key Management Protocol
Requirements for 6TSCH", draft-ohba-6tsch-security-01
(work in progress), July 2013.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-12 (work in
progress), March 2013.
[I-D.phinney-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P., and R. Assimiti, "RPL
applicability in industrial networks", draft-phinney-roll-
rpl-industrial-applicability-02 (work in progress),
February 2013.
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7.3. External Informative References
[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendament 1: MAC sublayer", April
2012.
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", June 2011.
[OpenWSN] , "Berkeley's OpenWSN Project Homepage", ,
<http://www.openwsn.org/>.
Authors' Addresses
Xavier Vilajosana (editor)
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Phone: +34 (646) 633 681
Email: xvilajosana@uoc.edu
Kris Pister
University of California Berkeley
490 Cory Hall
Berkeley, California 94720
USA
Email: pister@eecs.berkeley.edu
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