SIU 2022 · 30th Signal Processing and Communications Applications Conference · IEEE
Improving Synchronization Time in 6TiSCH Networks with Smart Antennas
Summary
At a Glance
Steerable antennas reach farther and suffer less interference, but their narrow beam makes it hard for a new node to hear the network. A new scan packet lets nodes learn their neighbours' directions before they synchronize; beacons are then sent and heard in those directions.
Minute values are read from the vector plots of Figs. 3 and 4 (Cooja, 10 runs each).
I–II · Introduction and Background
Smart Antennas Help, Until a Node Has to Join
Steerable (ESPAR) antennas
- stronger transmission in the chosen direction, longer range
- ignore interference from unwanted directions
- more parallel transfers on the same frequency
- electronically steered, cheap enough for IoT nodes
The price
- a node must know which direction to use for each neighbour
- a narrow beam covers little: work an omni antenna does once must be repeated around the node
- joining the network and finding neighbours takes longer
- existing fixes for slow network formation assume omni antennas
II.B · 6TiSCH
How a 6TiSCH Network Forms
Until it has its own schedule, every node uses one or more shared cells for EBs, join traffic and RPL messages.
Nodes far from the root wait for nearer neighbours to join first, with the radio always on.
Collisions in the shared cell slow formation even with omni antennas; a narrow beam slows it further.
III.A · Fig. 1 and Eq. (1) · Interactive
Antenna Pattern and Link Budget
RSS = PTX + PPL − 10K·log10(d/d0) + G(θ12) + G(θ21), with PTX = 0 dBm, PPL = −40 dB, K = 2.5, d0 = 1 m. Pattern: smooth curve through the paper's 5, −5, −15 dBi; threshold: an omni link reaches 50 m.
III.B · Fig. 2, Algorithms 1–2
The Scan Packet
- Synchronized nodes send Enhanced Beacons; unsynchronized nodes send the new scan packet.
- Unsynchronized nodes keep their radio on, so they hear scan packets with high probability.
- The receiver stores the sender's MAC ID with its own current antenna direction and RSSI: a virtual neighbour list built before synchronization.
- After it receives an EB and synchronizes, a node stops sending scan packets and starts sending EBs.
- In the shared cell, EBs are sent, and nodes listen, towards listed neighbours instead of random directions.
- The radio layer must report direction and RSSI to the MAC layer, so both layers were redesigned.
id ← extract_id(packet) n ← get_neighbor(id, list) if n is null: # new neighbour insert(create(id, current_dir, current_rssi)) else if current_rssi > n.rssi: n.dir ← current_dir # better direction n.rssi ← current_rssiAlgorithm 2: direction in the shared cell
i ← random(list_size)
if i is 0:
return random(2π) # P = 1 / (neighbours + 1)
else:
return list[i].dir # towards a known neighbourIII · Why Directions Matter · Interactive
Beam Alignment: Will One Beacon Be Heard?
Illustrative model, not a result of the paper: one transmission, one listener, no collisions, link budget of the previous slide. Neighbour-aware: each side points exactly at the other with probability 1/(k+1) (Algorithm 2), otherwise at a random angle.
IV · Table I
Test Setup
| Parameter | Value |
|---|---|
| Slotframe length | 101 slots |
| Shared cells | 1 |
| EB interval | 16 s to 50 s |
| Range, omni antenna | 50 m |
| Range, smart antenna | 15 m to 80 m |
| Area | 200 m × 200 m, random placement |
| Nodes | 25, 35, 45, 55, 65, 75 |
| Test length | until the network is synchronized |
Nodes run Contiki in emulation; the radio medium was extended to include relative antenna angles, not only distance.
Each configuration is repeated ten times and averaged.
Omni, steerable with random directions, steerable with scan packets, and both with autonomous cells (control messages copied per neighbour, as in Orchestra).
IV · Fig. 3 · Interactive
Synchronization Time with One Channel
Click a legend entry to hide a curve; hover for values. Omni is fastest in small networks but explodes with density (collisions). Steerable antennas first gain from more neighbours in reach, then lose to interference. Scan packets keep the time flat in dense networks; autonomous cells are short and scalable for both antenna types.
IV · Fig. 4 · Interactive
Scan Packets with One and Two Channels
| Nodes | 1 ch. | 2 ch. |
|---|
Table: change in time with scan packets (green = faster). A second channel makes joining slower, because a new node must listen on each channel in turn. The gain grows with density; in small one-channel networks scan packets cost time.
IV · Discussion
What Drives Synchronization Time?
More nodes mean more neighbours in reach but more collisions in the shared cell. Omni antennas suffer most; narrow beams shield steerable nodes.
Random beams rarely meet. Scan packets turn guesses into aimed transmissions and receptions.
More frequency resources lengthen joining: an unsynchronized node must find the right channel as well as the right direction.
A node that misses EBs for too long leaves the network and must join again, lengthening total formation time.
V · Conclusion
Conclusion
- 6TiSCH was designed for omni antennas; steerable antennas slow down joining and neighbour discovery.
- Scan packets let nodes discover neighbours before synchronization; shared-cell traffic then follows known directions, improving synchronization time by about 25% depending on density.
- Copying control messages to autonomously allocated cells in each neighbour's direction gives a solution that does not depend on network size.
- Future work: machine learning for more efficient use of shared cells.
Supported by TÜBİTAK under project 118E289.
V · Conclusion
Check Questions
- Which two phases must complete before a 6TiSCH network is fully functional?
- Why does a steerable antenna make joining harder than an omni antenna?
- What does a node store when it hears a scan packet, and why can it hear one before it is synchronized?
- Why does Algorithm 2 still pick a random direction with probability 1/(neighbours + 1)?
- Why is the omni antenna fastest at 25 nodes but slowest at 75?
Show answers
- Synchronization through Enhanced Beacons, then building the RPL tree with DIO messages.
- The beam covers only part of the surroundings; the node must also find the right direction, and random directions rarely line up.
- The sender's MAC ID, its own current antenna direction and the RSSI; unsynchronized nodes keep the radio on while they search.
- So that new nodes, not yet in the list, can still be discovered.
- In small networks it reaches everyone at once; in dense ones its wide coverage causes more collisions and lost packets.
References
Selected References
- Kulcu, S., Gormus, S., Jin, Y. (2021). Integration of steerable smart antennas to IETF 6TiSCH protocol for high reliability wireless IoT networks. IEEE Access, 9, 147780–147790.
- Vilajosana, X. et al. (2019). IETF 6TiSCH: A tutorial. IEEE Commun. Surveys Tuts., 22(1), 595–615.
- Vilajosana, X., Pister, K., Watteyne, T. (2017). Minimal IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) configuration. RFC 8180.
- Vallati, C. et al. (2018). Improving network formation in 6TiSCH networks. IEEE Trans. Mobile Comput., 18(1), 98–110.
- Duquennoy, S. et al. (2015). Orchestra: Robust mesh networks through autonomously scheduled TSCH. ACM SenSys, 337–350.
See the full paper for the complete reference list. Result curves are redrawn from the paper's figures.