Re: Re the encapsulation I-Ds...

[Dr G Fairhurst <gorry@erg.abdn.ac.uk>]

Richard Gibbons wrote:
> 
> The question is a good one. While IP is at layer 3 there is a HDLC
> router output that provides Sync Serial IP frames that could directly
> interface with a Modem. NASA's Space Network currently uses HDLC as the
> underlying satellite transport. ATM is also possible as the basic
> underlying satellite transport. The comparision between HDLC, DVB and
> ATM for satellite transport is not obvious in spite of the mass produced
> DVB IRDs.


HDLC/PPP, ATM, SONET/SDH, MPEG-2/DVB/ATSC all provide ways of moving
IP packets between interfaces ona router or end-host. That is they
are all at L2.

I'll follow the HDLC thread...

HDLC/PPP uses flag bytes/sequences to delimit the edges of frames.  In MPEG-2
transmission is synchronous, and each MPEG-2 TS frame (confusingly known as
a TS Packet) contains a SYNC byte at the start in the header. The 
synchonisation has similarities to ATM.

HDLC provides link addressing (if you want it, in the first byte). This
is used by SDLC, NRM, LAP-D, etc. It identifies a level 2 flow to a
particular destination (or group of destination addresses).
MPEG-2 has a similar concept in the PID field which identifies a flow,
flows can be (and often are) point-to-multipoint. A receiver
can use a PID to slect an appropriate flow that it wishes to receive.

Unlike ATM and HDLC, a wide cariety of commercial off-the-sheld 
components are available using MPEG-2 that combine both the physical 
radio / "modem" interface  with this link level interface in the same 
board/box. The "DVB", "ATSC", etc standards add the physical layer 
to the MPEG-2 link layer.

draft-fair-ipdvb-req-03.txt gives more details on how this relates to IP.

Gorry

> 
> Richard Gibbons
> 
> Bertrand WENDLING wrote:
> >
> > IP is not a transport protocol, MPEG yes, but you are free to use any other
> > transport format. The other reason is that millions of receiver are able to
> > receive data (video, audio, encapsulated data of various format) in MPEG
> > format. Creating a new format would result in having to start from zero
> > receiver to launch the service....
> >
> > OZGUR.AKSU@startv.com.tr wrote:
> >
> > > Hello ,
> > >
> > > I  have been following you for a long time , I am prepairing a master
> > > thesis , but one point I could not understand that why we need to
> > > encapsulate IP packets with MPEG format, what are the advantages of doing
> > > that ? Is it also possible to transmit data in IP format via satellite , at
> > > this time, what types of problems we would encounter ?
> > >
> > > Thank you.
> > >
> > > Ozgur  AKSU
> >
> > --
> > This message and any attachments (the "message") is intended solely for the
> > addressees and is confidential. If you receive this message in error, please
> > delete it and immediately notify the sender.
> > Any use not in accordance with its purpose, any dissemination or disclosure,
> > either whole or partial, is prohibited except formal approval.
> > The E-Mail transmission can not guarantee the integrity of this message.
> > CANAL+TECHNOLOGIES will not therefore be liable for the message if modified.