| 6TSCH | X. Vilajosana, Ed. |
| Internet-Draft | Universitat Oberta de Catalunya |
| Intended status: Informational | June 20, 2013 |
| Expires: December 22, 2013 |
Minimal 6TSCH Configuration
draft-vilajosana-6tsch-basic-00
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.
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The nodes in a [IEEE802154e] TSCH network follow a communication schedule. The entity 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 defines the simplest set of rules for building a [IEEE802154e] TSCH-compliant network, at the necessary price of lesser efficiency.
In order to form a network, a minimum schedule configuration is required so nodes can advertise the presence of the network, and exchange data.
The slotframe, as defined by [I-D.draft-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 minimal TSCH network, nodes need to use the same slotframe configuration so they can exchange Enhanced Beacons (EBs) and data packets. This document recommends the following slotframe configuration.
Basic configuration configuration
+---------------------------------+----------------------+ | Property | Value | +---------------------------------+----------------------+ | Number of time slots | 101 | +---------------------------------+----------------------+ | Number of available channels | 16 | +---------------------------------+----------------------+ | Number of EBs slots | 1 (slotOffset 0) | +---------------------------------+----------------------+ | Number of active cells | 5 (slotOffsets | | | 1,2,3,4,5) | +---------------------------------+----------------------+ | Number of inactive slots | 95 (from slotOffset | | | 6 to 100) | +---------------------------------+----------------------+ | Number of MAC retransmissions | 3 | +---------------------------------+----------------------+ | Time Slot duration | 15ms | +---------------------------------+----------------------+
This 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 to announce the presence of the network. These EBs are not acknowledged. Five cells are scheduled for exchangeing 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 inactive, i.e. the radio of the nodes remains off.
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
Per the [IEEE802154e] TSCH standard, each active cell is assigned a bitmap of cell options.
The EB cell is assigned the following bitmap of cell options:
The data cells are assigned the bitmap of cell options below. This results in "Slotted Aloha" behavior. Because both the "Transmit" and "Receive" bits are set, the either transmits, or listens when it has nothing to transmit. Because the "shared" bit it set, it uses the backoff mechanism defined in [IEEE802154e] TSCH to resolve collisions.
All remaining cells are inactive, and so the nodes' radios remain off. In a memory efficient implementation, active cells could be represented by a circular linked list. Inactive cells should not occupy any memory.
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.
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
/------------------- 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 duration | Value | +---------------------------------+------------------+ | TsTxOffset | 4000us | +---------------------------------+------------------+ | TsLongGT | 1300us | +---------------------------------+------------------+ | TsTxAckDelay | 4606us | +---------------------------------+------------------+ | TsShortGT | 500us | +---------------------------------+------------------+ | Time Slot duration | 15000us | +---------------------------------+------------------+
[IEEE802154e] does not define when 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.
EBs must be sent with the Beacon IEEE802.15.4 frame type and this document recommends that they carry the following Information Elements (IEs):
Contains synchronization information such as ASN and join priority.
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.
For each link in the basic schedule:
MAC-layer acknowledgement frames are built according to [IEEE802154e]. Data frames and command frames sent to a unicast MAC destination address request an acknowledged. The acknowledgement frame is of type ACK (0x10). Each acknowledgement contains the following IE:
The ACK/NACK time correction IE is used to carry the measured desynchronization between the sender and the receiver.
The possible values for the Time Synch Info and ACK status are described in the following table:
ACK status and Time Synch information.
+-----------------------------------+------------------+ | 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 | +-----------------------------------+------------------+
[IEEE802154e] does not define when information is be kept at each node about each neighbor. This document recommends to keep the following information in the neighbor table:
The exact format of the neighbor table is implementation-specific, but it should at least contain the following information, for each neighbor:
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.
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.
[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:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |