ASPI comments about draft-clausen-ipdvb-enc-00.txt

[Stephane.Combes@space.alcatel.fr]

Hi,

a few comments on the draft  :

- page header to be corrected : it is not the ID "Requirements..." anymore
- in the introduction, something should be said about bidirectionnal systems
(UDLR, DVB-RCS) and the ones including MPEG switches (satellite OBP for
instance). DVB-RCS (either for transparent or regenerative satellites) is very
interesting since it specifies MPEG-2 transport both for the forward and
(optionally) for the return link.
- when we think about DVB-RCS's MPEG mode for the return link (shared access
between terminals), we may want to have a slightly different encapsulation
scheme. For instance it could include an additional sublayer performing IP
packet segmentation into several SNDU (AAL2 like scheme).
- on the other hand, we could allow IP packets "packing" into a same SNDU
- I tend to agree with P. Cipière comment : why making such a complex use of
PUSI and AFC ? Sticking to the simpler solution that some drivers currently
implement might be better (less complex, less overhead).
- I am not convinced we need to envisage that many SNDU types : aren't IPv4, v6,
Ethernet, MPLS enough ? 2 bytes for this like Ethertype is long...
- the famous "label" needs to be specified. A 16-bit field should be enough for
all kinds of usage, provided it is complemented by an ARP protocol.
- why should the format of bridged payload be specified in this document ?
- is it so important to keep the 16/32 bit alignment ?
- I am not sure to understand well the paragraph about the MPLS header. Why
putting it into the adaptation field ? it is not the case for other higher layer
headers like IP and Ethernet.
- Do you have an adaptation field inserted before each SNDU ? why ?

As far as regenerative satellites are concerned, here are our views :

- currently we only envisage ATM VP or MPEG PID switching at the OBP. Switching
on an extra label would cost a lot... and currently we do not see any concrete
need to do that. Currently forget about Ethernet or IP layer processing on-board
!
- large capacity multi spot-beam "mesh" systems based on regenerative payload
and MPEG switch handle a lot of terminals (hence a lot of "feeds"). Being able
to use the same PID for several feeds may prove to be interesting (PID used as a
"broadcast network" as Horst put it). It is possible to do this, even with a
"simple" MPEG TS level switch a the OBP. The only need is to have an extra label
(e.g. the SNDU label) being used to discriminate the SOURCE of the SNDU and have
only one such SNDU per MPEG cell.
- concerning G. Anissa's mail about "Channel-descriptor" signalling the mapping
Source/Destination @ -> PID, Label : DVB-RCS group is currently working on such
protocol.


Best regards,


Stéphane

ALCATEL SPACE INDUSTRIES
Research Department/Advanced Telecom Satellite Systems
Tel : +33 (0)53435 6938  /  Fax : +33 (0)53435 5560
Porte : F1027  /  E-Mail : stephane.combes@space.alcatel.fr