Tero makes some good points but perhaps I have a different view on the Layer 1 fragmentation introduced with 802.15.4k.
Firstly, the Layer 1 fragmentation was early on intended to be a general function that could be used with any PHY, that was why it was in the MAC section for 802.15.4k. During the 802.15.4-2015 revision effort we saw that there were some PHYs that would require additional effort on the mechanism (I cannot remember the specifics). Rather than delay the revision further we chose the expedient way to move on which was to put it into the LECIM section, since they were the PHYs (DSSS in particular) that absolutely needed Layer 1 fragmentation. In the next revision I will propose that, once again, we revisit the concept of extending the Layer 1 fragmentation to more PHYs as well as change the moniker LECIM to LPWAN. One of the requirements that we placed on Layer 1 fragmentation was the ability to abort the fragmentation/reassembly process at any point in the process. This was achieved with minimal impact to the overhead by setting the acknowledgment’s fragmentation number field and the acknowledgment content set to zero (ie. no packets were received). While Layer 1 fragmentation has benefits over Layer 2 fragmentation, it does have some disadvantages. Firstly, since the MAC will not accept datagrams that are larger than the size dictated by “aMaxPhyPacketSize” (a PHY constant that dictates the largest frame that the PHY can transmit, currently either 127 or 2047 octets), if this value is set to 127, a Layer 2 fragmentation method is required for datagrams larger than 127 octets. The second that seems important for mesh networks is that a Layer 1 fragmentation is solely link-by-link/hop-by-hop, ie. the reassembly must occur at each receiver. While this is no problem for star topologies, it is a concern for mesh or multi-hop networks. Pat Pat Kinney Kinney Consulting LLC IEEE 802.15 WG vice chair, SC chair ISA100 co-chair, ISA100.20 chair O: +1.847.960.3715 [email protected] <mailto:[email protected]> On 6, Apr2017, at 7:34, Tero Kivinen <[email protected]> wrote: Pascal Thubert (pthubert) writes: > IEEE 802.5.4 understood and started addressing those issues in their > protocols years ago, consider for instance how this is done at PHY > layer in 802.15.4K LECIM - LP-WAN (cc'ing Pat). In the light of that > art, the legacy 6LoWPAN technique is shamefully inadequate. 802.15.4k is really only a phy level fragmentation, and the main issue there is that the max packet size in that PHY is extremely small (less than 30 octets). When designing the IEEE std 802.15.9 we checked 4k that and realized that it is not something that can be used in general case, and thats why the MAC level fragmentation in the 802.15.9 is using different method. Fragmentation was needed in the 802.15.9 as KMP payloads do not fit to the small frames of 802.15.4. This fragmentation method (and the multiplexing) from the 802.15.9 will most likely be adopted in the 802.15.12 later... Btw, the 802.15.9 is now available using the get IEEE 802 program, and you can download it from http://standards.ieee.org/getieee802/download/802.15.9-2016.pdf The fragmentation wire formats are described in the section 7, and state matchines are described in section 9. > My suggestion to the group is to take a holistic view of these > issues, and redesign or fragment support. This is what my draft > does. The 802.15.9 is hop by hop fragmentation, so if you want to be able to forward fragments, you need to specify your own way of doing that... -- [email protected]
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