[Sea-sa] It is a smaller sandwich: Background materials for today's SEA-SA SCCS-ARD discussion
Gian.Paolo.Calzolari at esa.int
Gian.Paolo.Calzolari at esa.int
Wed Mar 3 18:18:35 UTC 2021
Dear Peter,
I must say that I am quite puzzled by this mail.
I still need to digest all the aspects of your message, however I can
ensure that SLS (with proper coordination) will try to provide some
punctual technical comments on the individual items.
Here, after a check with SLS colleagues, I want to comment on your
statement about the “3-layer sandwich” .
Indeed that sandwich is not so big as you claim.
Actually all the following documents
https://public.ccsds.org/Pubs/131x2b1e1.pdf
https://public.ccsds.org/Pubs/131x3b1.pdf
https://public.ccsds.org/Pubs/431x0b1.pdf
clearly state that "This Recommended Standard covers the functions of both
the Synchronization and Channel Coding Sublayer and the Physical Layer";
i.e. the layers combined are two (or let's say 1 and half as one layer and
one sublayer are combined).
They all also show this (even with some minor differences) in their
"Figure 2-1: Relationship with OSI Layers" showing together the
1) Synchronization and Channel Coding Sublayer that provides methods of
synchronization and channel coding for transferring Transfer Frames over a
space link and the
2) Physical Layer that provides the RF and modulation methods for
transferring a stream of bits over a space link in a single direction
For reference the three figures are attached as snapshots.
Best regards
Gian Paolo
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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