Want to fill in somewhat to William's
detailed answer.
1) There is a proposal in DVB-RCS
to utilise the adaptation field to perform inband signalling for the DVB-RCS
network. This signalling has to be independent of the higher layers, as
it has to be available before these are up (same applies for Ethernet,
which may of course not even be present). This was the reason for bringing
this up in the first place. The problem is more general though. The adaptation
field can be used by the MPG2-TS layer (ref. William's detailed explanation).
Explicitly prohibiting this in the ULE specification seems to be in violation
with the MPG2 specification (i.e. ISO/IEC-13818-1).
2) All receivers should as a minimum
be able to rip off the field. As William S. says this is part of MPG processing
and should not concern ULE at all (unless you have a specific design in
mind....). In any case the performance hit of checking one bit and throwing
away some data every now and then (most likely never in most systems) should
not affect performance much.
--Tor Brekke
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Sent by: owner-ip-dvb@erg.abdn.ac.uk
13.04.2004 17:59
Please respond to
ip-dvb@erg.abdn.ac.uk
To
ip-dvb@erg.abdn.ac.uk
cc
Subject
Re: Adaptation field use
in ULE / MPG2-TS specification
So... as I recall, the rationale behind the current
rev. was:
1) There was no stated requirement for the Adaptation Field
when used with ULE, if this requirement has emerged, then
I think we need to know what function is being propossed
and why this is performed at the MPEG-2 layer, rather than
at the IP/Ethernet layers. Please send text!
2) There is a cost at adding Adaptation Field processing, in
that it impacts implementation/performance of the ULE receiver.
This has a performance hit, if it is mandatory to support it.
If it is optional, then there is a compatability hit.
Thoughts?
Gorry
P.S. Note: ULE does *NOT* use the PES syntax for the data
sent on these streams.
William StanisLaus wrote:
> Here ULE is not preventing use of lower level MPEG2 packet header
> fields, rather we should figure out why we drop the packet based on
> the adaptation field control, is there any reason behind to specify
in
> the draft to drop packets other than AFC is "01"
>
> In the case of adaptation field, which is coupled with MPEG2-TS
> header, itself will be decoded and action could have taken well before
> the control reaches ULE. Hence the goal of adaptation field is
> accomplished, if it is going to be a sequential/linear processing.
But
> the question is, when the AFC is "11" in such case MPEG2-TS
payload
> contains both adaptation field private section data and actual payload
> (SNDU), SNDU payload processing will be dropped as per the ULE
draft.
>
> Best Regards,
> William.
>
> -----Original Message-----
> From: owner-ip-dvb@erg.abdn.ac.uk
> [mailto:owner-ip-dvb@erg.abdn.ac.uk]On Behalf Of Allison,
Art
> Sent: Tuesday, April 13, 2004 6:51 PM
> To: 'ip-dvb@erg.abdn.ac.uk'
> Subject: RE: Adaptation field use in ULE / MPG2-TS specification
>
> I agree with Tor (and am a bit chagrined that I didn't
notice
> this). The ULE draft should not prevent use of the 'lower'
level
> MPEG-2 packet header fields, and that included the adaptation
> field. A key concept ( Tor's point #2) in MPEG-2
systems is the
> layering with each layer having a separable function.
Building
> vertically integrated designs that constrain this reduce
long term
> flexibility and can have other impacts like the one
Tor identified.
>
> However, now that I glance at the header structure...to
send this
> data in TS packets, 'payload start indicator' must
be set to a
> value. It is one bit, where 0= no PES start in this
packet, and 1=
> PES or PSI start in this packet. Suggest define that
it be set to 0.
>
> Art
> ::{)>
> Art Allison
> Director Advanced Engineering
> NAB
> 1771 N St NW
> Washington DC 20036
> 202 429 5418
>
> -----Original Message-----
> From: William StanisLaus [mailto:williams@calsoft.co.in]
> Sent: Tuesday, April 13, 2004 8:18 AM
> To: ip-dvb@erg.abdn.ac.uk
> Subject: RE: Adaptation field use in ULE
/ MPG2-TS specification
>
> Hi Marie-Jose,
> I understand your point
here about extension headers. But
> actually Adaptation field specified here
is part of
> MPEG2-TS header and not ULE, in that case
even ULE is a
> payload for MPEG2-TS. If adaption field
control is 10 or 11 in
> bits, MPEG2-TS contains private adaption
field, and incase of
> "10" we need not care about
anything, but still ULE draft says
> anything other than " 01" in
AFC, receiver MUST discard.
> Thanks for your comment
Tor Brekke, but i wonder why we
> have such limitation in ULE, even though
we are not worried in
> ULE about the MPEG2-TS private section
of Adaption field, if
> we are not going to take any action based
on the adaption
> field and control field we can just ignore.
I see this as an
> implementation issue and should not be
limited in
> specification/draft.
>
> transport_packet(){
>
> sync_byte
> 8 bslbf
>
> transport_error_indicator
> 1 bslbf
>
> payload_unit_start_indicator
> 1 bslbf
>
> transport_priority
> 1 bslbf
>
> PID
> 13 uimsbf
>
> transport_scrambling_control
> 2 bslbf
>
> adaptation_field_control
> 2 bslbf
>
> continuity_counter
> 4 uimsbf
> if(adaptation_field_control=='10'
||
> adaptation_field_control=='11'){
>
adaptation_field()
> }
> if(adaptation_field_control=='01'
||
> adaptation_field_control=='11') {
>
for (i=0;i<N;i++){
>
> data_byte
>
8 bslbf
>
}
> }
> }
>
> here N data_byte's is SNDU as specified
by ULE.
>
> Also ULE cannot limit anything on exisiting
underlying
> protocol as such MPEG2-TS. But definitly
it can limit or
> provide more services to above layers.
>
>
> Best Regards,
> William StanisLaus
> CalSoft, Nortel Networks Division
>
>
> -----Original Message-----
> From: owner-ip-dvb@erg.abdn.ac.uk
> [mailto:owner-ip-dvb@erg.abdn.ac.uk]On
Behalf Of
> Marie-Jose Montpetit
> Sent: Tuesday, April 13,
2004 4:00 PM
> To: ip-dvb@erg.abdn.ac.uk
> Subject: Re: Adaptation
field use in ULE / MPG2-TS
> specification
>
> Thanks for you input. If
you follow recent discussions
> there is a proposal to use
extension headers in ULE to
> support system specific
signalling (and other). It would
> be interesting to know more
about how you process the
> information but I think
our philosophy would be to use the
> headers in the same way
as you describe: transparent to
> system who do not know what
to do with them, processed by
> the ones who do. This is
consistent with IPv6 extension
> headers and also with other
implementations of extension
> headers (cable for example).
>
> So I do believe we could
use ULE in RCS. Actually in a
> recent study sponsored by
ESA we showed that for traffic
> with a lot of ACK bursts
(http: for example) ULE was very
> efficient because of it's
packing capability.
>
> Again thanks for your input.
>
> Marie-Jose
>
>
>
>
> ----- Original
Message -----
> From: Tor
Brekke <mailto:tor.brekke@nbs.nera.no>
> To: ip-dvb@erg.abdn.ac.uk
<mailto:ip-dvb@erg.abdn.ac.uk>
> Sent: Tuesday,
April 13, 2004 3:02 AM
> Subject: Adaptation
field use in ULE / MPG2-TS
> specification
>
>
> Hello,
>
> I am jumping
into this discussion at a fairly late
> stage. Having
just read the ULE draft I have to make a
> comment about
the use of the adaptation field though.
> In the draft
it says that "TS Packets from a ULE
> Encapsulator
MUST be sent with an AFC value of '01'",
> i.e. without
adaptation field. Now the adaptation
> field contains
a private field which according to the
> MPG2-TS specification
(ISO/IEC 13818-1) can be used to
> transmit system
dependent data.
> Now to the
question: Why is this limitation imposed on
> ULE?
>
> I have two
main reasons to ask this.
> 1) Firstly
there is ongoing work to use the private
> adaptation
field for network internal signalling in
> the DVB-RCS
standard. This means that ULE will not
> work over
DVB-RCS systems employing the adaptation
> field signalling.
> 2) As far
as I have been able to determine all other
> encapsulation
forms over MPG2-TS are transparent to
> the adaptation
field, i.e. the adaptation field and
> payload are
handled independently (even though there
> may be an
implicit connection between them). It seems
> strange to
break this logicinf the MPG2- TS mechanism
> for a new
encapsulation type.
>
> Best Regards
> Tor Brekke
> Nera Broadband
Satellite AS.
>