[Sea-sa] SLS comments to slide 9 and 10 of "SEA High Rate comm issue 1Mar21.pptx": Background materials for today's SEA-SA SCCS-ARD discussion
Gian.Paolo.Calzolari at esa.int
Gian.Paolo.Calzolari at esa.int
Tue Mar 9 15:45:33 UTC 2021
[attachment "Spaghetti2021.ppt" deleted by Gian Paolo Calzolari/esoc/ESA]
Dera Peter,
here attached SLS comments to slide 9 and 10 of "SEA High Rate
comm issue 1Mar21.pptx"
We focused on those two slides because of the diagrams that are addressing
books produced in SLS.
Absence on comments on other slides or other parts of your mail does not
imply SLS agreement.
The two original slides are attached here below for easier reference.
I tell you that there was a quite spread opinion about the fact that your
slides 9 and 10 contained obsolete material for which was not worth
commenting in detail before an update.
Nevertheless some focused comments were gathered and they are reported
below.
-----------------------------------------------------------------------------
SLIDE 9 - Current CCSDS Layered Protocol Stack, with CNES & ESA “add-ons”
shown
1) The box Encapsulation service shall be renamed Encapsulation Packet
protocol
2) the box for Encapsulation is linked to 4 out of the 5 SDLP boxes
immediately below (while SPP is connected to all fives boxes). Link to TC
SDLP is missing and shall be added.
3) Diagram can be simplified replacing the two boxes for SPP and EPP with
a single box (connected to the 5 SDLP boxes immediately below) named e.g.
SPP/EPP (even in longer form)
4) The DVB+SCCC separate box is only connected to TM Space Data Link
Protocol. This is wrong. Possibility of transmitting AOS Frames is there
since the beginning while addition of USLP Frames is in progress. However
this is in progress also for 131.0 and the consequence is that the
present diagram shows an unbalanced preference for TM Coding..Of course
one could remove the USLP link to TM Coding book, but it may be better to
simplify the diagram showing that TM/OAS/AOS SDLP's are equally connected
to TM Coding, SCCC, and DVB-S2 books.
5) SLS strongly disagree with the statement: "Issue: These CNES and ESA,
agency unique, “omnibus standards” are not aligned with the rest of CCSDS,
but are glued on the side." Those two standards provides the same type of
service provided by 131.0-B but the WGs agreed to assign them separate
books for their peculiarities without detracting from the other book. At
the extreme, one could state that also the separate book for Proximity-1
is an issue as it is separate from TC Coding. Idem for Proximity-1
physical layer. To be removed.
6) The statement "They combine several layers into one spec, act as
separate, parallel, coding, modulation, and signaling layers to the
“mainline” CCSDS." is at least inaccurate and it is perceived to have the
purpose to give a negative accent to those two standards and to the works
of the WG/Area.To be removed.
7) The statement "Span Data Link, Coding and Sync, Slicing, Modulation,
and Physical Layer signaling" is inaccurate and not precise. It mixes
layers (or sublayers) with procedures within layers (e.g. slicing,
modulation, etc.). Putting together < Data Link, Coding> is highly
ambiguous as it could be understood as either "the Synchronization and
Channel Coding Sublayer of the Data Link layer" or "the complete Data Link
Layer including both the Data Link Protocol Sublayer and the
Synchronization and Channel Coding Sublayer". It can be guessed that the
original intention was the latter and this is clearly wrong because no one
of the three 131.x standards affects the Data Link Protocol Sublayer.
8) The statement/bullets <These “integrated suites” duplicate Coding &
Data Link Layer functions: * variable-to-fixed length conversion * idle
data insertion> is wrong. There is no variable-to-fixed length conversion
performed by the 131.x standards since even the slicing is applied to
fixed length frames in input. There is no idle data insertion performed
by the 131.x standards (note that the word idle is not present in
https://public.ccsds.org/Pubs/131x2b1e1.pdf while in
https://public.ccsds.org/Pubs/131x3b1.pdf it is used only to explain the
unused acronym OID).
9) The layers label on the left are wrong. The label "Data Link Layer"
shall be replaced by " Data Link Protocol Sublayer". The "Coding and
Modulation Sublayer" does not exist and that label shall be replaced by "
Synchronization and Channel Coding Sublayer".
10) The height of the DVB and SCCC boxes is wrong when "linked" to the
labels on the left for the layers. This is another case where the drawing
seems to be done to purposely give a negative accent to those two
standards: while they match the ordering of the boxes on the right they do
not match the SCCC and DVB boxes. First of all SCCC and DVB-S2 shall have
the same height as they covers the same layers. Second they height shall
be such that only "Synchronization and Channel Coding Sublayer" and
"Physical Layer"
11) the labels for books at the centre of the slide are not all necessary.
In the diagram there is no silver book, there are no pink sheets, there is
no white paper.
12) To be picky, the box for 231.0 should extend a little towards the
physical layer because of the PLOP(s). See also Figure 2-1: Relationship
with OSI Layers in https://public.ccsds.org/Pubs/231x0b3.pdf
------------------------------------------------------------------
SLIDE 10 - Detail of SCCC and DVB-S2 “integrated Data Link and Physical
Layers
1) The box labelled TM SDLP shall be labelled "TM/AOS/USLP SDLP". knowing
that USLP is coming soon for all the three 131.x coding books.
2) On the left side of the drawing, the label "Data Link Layer" shall be
replaced by " Data Link Protocol Sublayer"
3) On the left side of the drawing, the label "Coding Sublayer of Data
Link Layer" shall be replaced by "Synchronization and Channel Coding
Sublayer"
4) The SCCC and DVB-S2 boxes shall have the same height.
5) The SCCC box height shall not exceed into the Data Link Protocol
Sublayer
6) The DVB-S2 box height shall not exceed into the Data Link Protocol
Sublayer
7) The Space Data Link Protocols perform no slicing. The box "Slice" in
SDLP box (upper right corner of the drawing)shall be removed.
8) The "Fill frames" box in the SDLP box (upper right corner of the
drawing) shall be renamed "OID Frames"
9) I understand that the SDLP box (upper right corner of the drawing)
tries to give a summary of that processing, but I wonder whether the
actual boxes (after corrections) show something reasonable. I send
separately my proposal to Greg and Matt.
10) in the TM S&CC box, the box names LDPC should be renamed (e.g. "LDPC
+/- slic."?) or there should be two LDPC boxes to show the possibility of
slicing.
11) in the TM S&CC box, it could be good adding something showing the
concatenated codes.
12) In the RF & Mod Box, showing only "Modulation: BPSK, QPSK" looks a
limitation. It could be wise to rename it at least "Modulation: BPSK,
QPSK, etc."
13) In the RF & Mod Box, does the "Filter" box make sense?
14) As for the comment on the previous slide.... The statement "Span Data
Link, Coding and Sync, Slicing, Modulation, and Physical Layer signaling"
is inaccurate and not precise. It mixes layers (or sublayers) with
procedures within layers (e.g. slicing, modulation, etc.). Putting
together < Data Link, Coding> is highly ambiguous as it could be
understood as either "the Synchronization and Channel Coding Sublayer of
the Data Link layer" or "the complete Data Link Layer including both the
Data Link Protocol Sublayer and the Synchronization and Channel Coding
Sublayer".I guess the original intention was the latter and this is
clearly wrong because no one of the three 131.x standards affects the Data
Link Protocol Sublayer.
15) The statement "Parallel to “main-line” CCSDS standards from data link
down" it is not clear.
16) SLS strongly disagree with the stated issue "These agency unique,
down-link only, approaches are not truly interoperable with each other
nor with the rest of CCSDS". What does it mean " interoperable with each
other "? Are e.g. Turbo Codes interoperable with LDPC Codes? Are LDPC
codes interoperable with Concatenated Codes? Is anything preventing a
system implementing either SCCC or DVB-S2 decoding from providing SLE RAF?
Is anything preventing a system implementing either SCCC or DVB-S2
decoding from providing SLE RCF? Actually one company integrating SCCC in
an existing equipment plans to rpovide RAF/RCF independtly from the used
ddecoder.
17) The added comment "And they are now being proposed as suitable for
downlink, uplink, and cross-link for all deployments, including Lunar and
deep space" is irrelevant and forgets that such a proposal has been
accepted by the WG and even by CMC that approved the relevant projects.
18) rightmost column (conventional VCM/RFM), modulation is not only BPSK
and QPSK but all HOMs up to 64-APSK (recommendations 2.4.18 and 2.4.23).
The only difference modulation-wise is that SCCC/DVB-S2 do not have BPSK
since intended for high data rate use.
19) There is only one method for interfacing DVB-S2 specified in CCSDS
DVB-S2 recommendation (131.3-B). This method is called Generic Stream (GS)
mode. It is fully DVB-S2 compatible. So it is not understood what is
method 1 (DVB) and method 2 (non DVB) depicted in this block diagram.
There is only one method : Generic Stream (GS) mode – one of the standard
input mode for DVB-S2. The number of BCH code options is debatable: 2
not 4 : short and normal (FECFRAME). There number of LDPC code options is
debatable (11 and not 28). Moreover, it is not understood why SEA
document/discussion needs a so big level of detail (number of code options
for Turbo, Reed-Solomon, Convolutional, LDPC codes is not mentioned in the
same diagram). In general, details should be in the document defining
them. Spreading them in other books only increases the risk of future
inconsistencies and a domino effect for updates whenever the source
document changes.
------------------------------------------------------------------------
In parallel Ken Andrews, author of the original drawings from which your
two slides were extrapolated, volunteered to produce updated diagrams
implementing most of the technical comments above.
To save time, I am sending you the latest version (*) produced by Ken (and
for which I am very grateful) that I find to be a great improvement with
respect to what originally in your slides.
Please be aware that not all the persons involved in this discussion
commented them in detail (there were a few comments after a first update
by Ken) and for this reason I cannot assure this version is fully
conclusive (i.e there could be additional comments)..
I hope I succeeded in reporting correctly and honestly the interaction
with the other SLS Players.
Best regrads
Gian Paolo
(*) As Ken distributed two versions with the same name, I renamed the
latest one to show the full date. The contents are untouched.
From: "Shames, Peter M\(US 312B\) via SEA-SA" <sea-sa at mailman.ccsds.org>
To: "SEA-SA" <sea-sa at mailman.ccsds.org>
Date: 02-03-21 23:37
Subject: [Sea-sa] Background materials for today's SEA-SA SCCS-ARD
discussion
Sent by: "SEA-SA" <sea-sa-bounces at mailman.ccsds.org>
[attachment "431x1b0_CESG_Approval.pdf" deleted by Gian Paolo
Calzolari/esoc/ESA]
Dear SCCS-ARD sub-team,
During today’s SEA-SA SCCS-ARD discussion we spent quite a period of time
discussing the challenges in create a reasonably compact, and also
accurate, table that reflects the currently documented set of
configurations that are made available by the suite of space data link,
coding, synchronization, modulation, RF (and optical), and physical layer
signaling standards. There are many situations where there is no one,
simple, statement, or even set of statements, that can be made. We have
had to resort to a tabular presentation, Table 6-8 in Sec 6 on protocols,
to address this. A copy of this table is attached, along with the “cheat
sheet” of notes that encode the cells in this table.
Any standards that are expected to come into being within the next 6-12
months, but that are not yet final, are highlighted in yellow. We hope
these are final before we publish this document, but all of those dates
are still rather uncertain.
Note that uplink is separate from downlink, that RF coding and modulation
is separate from optical coding and modulation, and that SCCC and DVB-S2
(which both contain coding, modulation, and physical layer signaling in a
single standard) are separated from the “normal” CCSDS standards that
break these into three separate layers. The new Variable Coding and
Modulation (VCM) spec that is now in progress is also shown as a separate
layer. This VCM spec is related to the “bottom” parts of the DVB and SCCC
specs, but it is different from them in distinct ways.
It became clear during discussion that most of those on the call were
unfamiliar with the details and complexities represented in this table.
Furthermore, most are unfamiliar with the complexities inherent in the
“3-layer sandwich” that SCCC and DVB present, and with how they compare
with the “normal” CCSDS link layer, coding, synch, modulation, physical
layer and RF stack. I have attached a presentation that some of us
constructed in order to make sure that we understood what those
relationships are. It is named “SEA high rate comm issue 1Mar21” and is
attached here. This is a statement of the recent issues and also a set of
diagrams comparing these different protocol sets. It does not address
optical comm.
It should be noted that the “bottom” part of the DVB and SCCC specs
includes a specialized set of physical layer signaling mechanisms. These
are not present in normal CCSDS protocol stacks, where any choices that
are made for different coding, synchronization, and modulation
combinations are made “by management”. That phrase “by management” means
that the mission manages these choices manually, outside of the protocols
themselves, that the protocol layers contain no “signals” as to which
choices were made, and that any changes to the coding and modulation must
be agreed to and managed out of band, by pre-agreement.
In the DVB and SCCC, and in the new draft CCSDS VCM spec (CCSDS 431.1-b-1)
which is attached here as a CESG draft spec, a physical layer signaling
mechanism is introduced. VCM is defined as “variable coded modulation,
VCM: A method to adapt the transmission scheme to channel conditions
following a predetermined schedule. ”. This includes two separate
physical layer structures: 1) the “Pilot Symbols” and 2) the encoded and
modulated data symbols. The CCSDS 431.1 spec describes two different VCM
“types”. Type 1 uses the DVB-S2 VCM pilot symbol and data symbol length
approach, Type 2 uses the SCCC VCM pilot symbol and data symbol length
approach. These pilot symbols are, in both cases, just short blocks of 7
bits, protected by a linear code and BPSK modulation (see attached Table
from Annex E). Five of these bits are used to identify one of the 32
possible sets of code and modulation pairs that are applied to the encoded
and modulated symbols that follow the pilot.
Where these DVB Type 1, SCCC Type 2, and CCSDS Type 1 or 2 schemes differ
is in the length of the symbol strings and the sets of code/modulation
pairs that are allowed.
DVB-S2 has its own set shown in Table 3-4. It allows different code
rates, from 1 / 4 (0.25) up to 9 / 10 (0.9), different input lengths from
2992 up to 58112 bits, different modulations (QPSK, 8-PSK, 16 & 32-APSK)
and its own set of DVB-S2 codes that are patented.
SCCC has its own set shown in Table 3-3. It allows different code rates,
from 0.36 up to 0.9, different input lengths from 5758 up to 43678 bits,
the same set of modulations (QPSK, 8-PSK, 16 & 32-APSK) and its own set of
SCCC codes that are patented.
The CCSDS VCM has its own set shown in Table 3-2. It allows different
(CCSDS standard) code rates, from 1 / 6 (0.16) up to 223/255 (0.875),
different (CCSDS standard) input lengths from 1748 up to 16384 bits, the
same set of modulations plus BPSK (BPSK, QPSK, 8-PSK, 16 & 32-APSK) and
the standard LDPC codes.
You can see that these are similar, and that the modulation set largely
overlaps, but they are different. In all cases specialized equipment will
be needed in the RF front ends to handle the pilot symbols and the
continually changing coding and modulation . The other difference is that
the CCSDS VCM expects to signal a pre-planned set of code & modulation
changes, but the SCCC and DVB-S2 also include adaptive coding and
modulation (ACM), which uses signals sent back from the receiver to the
sender. To quote from SCCC, CCSDS 131x2b1d1, Sec 3.2.7:
NOTE –
Changes of the value of the information block size K are done by a system
to adjust the modulation and coding schemes. This is achieved through,
e.g., one of the following approaches: the ground receiver provides the
signal quality estimation (or prediction) through a feedback channel
(e.g., via telecommand) or the change of modulation and coding schemes is
pre-scheduled for each satellite pass based on geometrical information
(elevation angle).
So the SCCC may use a feedback loop, but no specific protocol appears to
be specified for this. The DVB-S2 standard, as adapted for CCSDS, makes
essentially the same statement. The full ETSI DVB-S2 spec, however,
defines an actual feedback protocol that is, in my opinion, only of use
over a near Earth (or at least a “local”) communications path where the
RTLT is sufficiently short to allow requests for data rate changes to be
responded to. This is not appropriate for use in deep space where the
RTLT may be measures in 10’s of minutes or tens of hours. They also bring
substantial added complexity which, in the general case, may not be worth
the added cost of engineering, testing, etc unless the mission is a) in a
near Earth orbit, and b) can make use of available commercial parts.
As I suggested during the webex, I think we must treat the following
groups of standards separately, because to do otherwise will overly
complicate the core of the CCSDS standard suite, that I estimate meets 95%
of the users.
1. The “CCSDS standard” suite of link layer, coding, synchronization,
modulation, and RF standards
2. A subsection on the Optical coding and modulation standards that
slot in underneath the normal link layer protocols, along with a brief
description
3. A separate subsection on the VCM and the associated SCCC and
DVB-S2 “omnibus” standards that replace the standard CCSDS coding,
synchronization, modulation and add physical layer signaling.
If anyone has issues with this approach please bring them up now. I think
this is the only sensible way to handle this issue of these very different
approaches to the lower layer protocols.
Thanks, Peter
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