1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: TSCH CSMA-CA issues
Date Submitted: 9 July, 2018
Source: Tero Kivinen Company -
Re: 6Tisch comments from IETF
Abstract:
There are some issues with the TSCH CSMA-CA processing in the standard.
Purpose: Provide information which kind of changes are needed to the standard.
Notice: This document has been prepared to assist the IEEE P It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P

2. TSCH CSMA-CA issues
Tero Kivinen

3. Issues
CSMA-CA sections and are inconsistent with figures 6-5 and 6- 6.
Section text is misleading and vague.

4. Section 6.2.5.2 The text in section 6.2.5.2:
“If TschCca was set to ON in the MLME- TSCH-MODE.request primitive, the MAC requests the PHY to perform a CCA at the designated time in the timeslot, macTsCcaOffset, without any backoff delays.”
Conflicts with the figure 6-5.
Figure 6-5 does CCA always for shared slots, and does CCA for dedicated slots only if TschCca is on.

5. Section 6.2.5.3 “backoff window”
Section defines backoff window and how it is updated, but will not use it explictly.
There is text using without refering it to with name:
“The MAC sublayer shall delay for a random number in the range 0 to (2BE – 1) shared links (on any slotframe) before attempting a retransmission on a shared link.”

6. Figure 6-6 and Figure 6-5
The figure 6-6 contains right hand side that is copied from the Figure 6-5. This is duplicated can will cause problems.
Also the Figure 6-5 ends with success telling that CCA was done, and device can start transmission. Figure 6-6 is supposed to finish when the frame has been successfully sent out, i.e., node has received ACK from the remote node.

7. Section and Figure 6-6
Figure 6-6 uses things defined in the , i.e., things like NB, BE, etc.
We should propably include text refering to the for those, and also explain that NB gets carried over from the figure 6-5, i.e., every single CCA failure in figure 6-5 does increment NB, and NB might not be 0 when Figure 6-6 starts.

8. Figure 6-6 and BE increntation
In figure there is box:
BE = min(BE – 1, macMinBe)
This is wrong, as it does NOT increment BE. Correct text is:
BE = min(BE + 1, macMaxBe)
This was added as comment i-9 of first Sponson Ballot, and marked as acceped, but not reflected to draft.

9. BE incrementation
The Figure 6-6 will increment BE always, it should only increment it for failures on shared links, as explained in text:
“For each successive failure on a shared link, the device should increase the backoff exponent until the backoff exponent = macMaxBe.”

10. Section 6.2.5.3 text and figure 6-6
The text in section is not exactly matching the figure 6-6:
“A device upon encountering a transmission failure in a shared link shall initialize the BE to macMinBe.”
Figure 6-6 only does this once for initial transmission, not every transmission (including retransmissions).

11. Section 6.2.5.3 and resetting BE
The text in section says that BE is not reset in this case:
“The backoff window does not change when a transmission is successful in a dedicated link and the transmission queue is still not empty afterwards.”
This is not reflected in figure 6-6, which resets BE every time.

12. CSMA-CCA and queue
We might have multiple retranmissions ongoing at the same time, where one of them is using shared links and dedicated link to node A, and packet going to node B, will just use dedicated links to B (as frame to A is in queue before frame to B). This is indicated in the case where it says that we reset backoff window only if transmit queue is empty. This means that the NB needs to be per frame information, not global, but on the other hand BE will be global. We do not have any text about this in the standard.