Mr. Liebowitz's response points out many of the advantages of
IP/MPEG/DVB. My company chose MPEG/DVB because of the addressability of
channels it offered; i.e. the ability to make "virtual channels" over the
satellite, which you can't do with HDLC. With HDLC all your receivers "see"
everything that's in the carrier, since the carrier typically operates in
single-channel-per-carrier SCPC mode. We use MPEG to let us operate in a
multi-channel-per-carrier MCPC mode, and then we command each receiver to
listen to only the channel of choice for that receive site.
As to bandwidth, MPEG is actually more efficient in many ways than
HDLC. In typical HDLC fashion, you have the output of a router spewing out
WAN (HDLC) traffic at say 10 Mbps. This gets encapsulated by a satellite
modulator and sent over the air. What happens if there is no traffic to
send out that one router? I've only seen synchronous HDLC with satellite
equipment, so it is continually sending out something even when there is no
Internet traffic coming out the router. With MPEG you send out packets only
whenever there is actually Internet traffic there. For the idle times, then
idle packets are sent. These idle packets can be used by other services.
For instance, we have customers sending video and audio streams as well
as Internet traffic, with all streams completely independent of one another.
We simply multiplex these other streams into our multiplexer (at full
transponder speeds) and so you get to use up those idle packets with other
services. Or our stream gets multiplexed in other multiplexers (we've done
that as well).
Again, the advantage is MPEG gives you an opportunity to reclaim the
unused part of the outgoing stream, and it also allows you to create a
bundle of pipes in the sky (8059 of them) and then command each receiver to
de-multiplex the pipe(s) you want it to. The DVB definitely is less
expensive, and the Reed-Solomon gives you near fiber-optic BERs on a typical
day. We have customers augmenting ISPs with wider forward-channel
bandwidth, and others providing news service feeds, and they really utilize
these advantages which MPEG/DVB gives them that HDLC does not.
- - - - - - - - - - - -
Dave Bohlmann, Principle Systems Engineer
dbohlmann@adaptivebroadband.com <mailto:dbohlmann@adaptivebroadband.com>
Adaptive Broadband, Corp.
SpectraCast Products, Elkhart IN
-----Original Message-----
From: owner-tcpsat@lerc.nasa.gov
<mailto:owner-tcpsat@lerc.nasa.gov> [mailto:owner-tcpsat@lerc.nasa.gov]
<mailto:[mailto:owner-tcpsat@lerc.nasa.gov]>
Sent: Tuesday, April 13, 1999 10:38 AM
To: tcpsat@grc.nasa.gov; <mailto:tcpsat@grc.nasa.gov;>
Jon Mansey
Subject: RE: transports: MPEG vs HDLC
Several organizations are planning or already using MPEG/DVB
for
transmission of IP packets. The advantages over HDCL
include:
* availability of low cost receivers, which
are low cost
because of mass production for the consumer market. There
are set top boxes
available in the $1000 range. DVB cards for PCs cost $200 to
$300.
* security and program control provided by the
conditional
access mechanism
* ability to filter local information from a
large carrier.
Use of large carriers improves statistical multiplexing
therby increasing
bandwidth efficiency.
* ability to use large carriers that saturate
a transponder.
This picks up some power, allowing the use of smaller
antennas or more less
impactive FEC.
* use of Reed-Solomon coding for improved
signal processing.
To counter balance these benefits there is a disadvantage
due to MPEG packet
overhead.
DVB uses fixed 188 byte cells, four of these bytes are
overhead. There is
also a 16 byte overhead for section packing, if you use
multiprotocol
encapsulation. Also, incomplete cells are padded, causing
furthor loss. This
loss depends on packet size and could be quite substantial.
Some vendors
will pack packets from the same service in an MPEG packet
which minimizes
the impact of short packets. Packing, however, implies
delay, because the
processor must wait some time to see if there is another
packet ready to
transmit. So in the end it is a tradeoff of efficiency vs.
delay. If you
want the packing feature you must make sure that both the
encapsulator at
the uplink and the receiver support this feature.
Overall, the advantages of MPEG/DVB are quite significant,
especially if you
can saturate a transponder using a single carrier.
Burt Liebowitz
> ----------
> From: Jon Mansey[SMTP:jon@interpacket.net]
<mailto:[SMTP:jon@interpacket.net]>
> Sent: Saturday, April 10, 1999 9:58 PM
> To: tcpsat@GRC.NASA.GOV <mailto:tcpsat@GRC.NASA.GOV>
> Subject: transports: MPEG vs HDLC
>
> Hi List,
>
> After the interesting discussion recently on BER, I want
to throw another
> one out there.
>
> What are the lists opinions of MPEG/DVB as an IP transport
over satellite,
> vs HDLC?
>
> Is there any published research comparing them?
>
> Im particularly interested in comparing 3 aspects, link
performance,
> encapsulation and decapsulation overhead, and equipment
requirements at
> each end.
>
> Jon.
> jon@interpacket.net <mailto:jon@interpacket.net>
Chief Science Officer
>
------------------------------------------------------------------
> "Low-cost & high-speed access to the US Internet via
Satellite"
> InterPacket Group, Inc.
http://www.interpacket.net <http://www.interpacket.net>
> 1901 Main St. 2nd Floor tel (310)
382 3300
> Santa Monica, California 90405 fax (310)
382 3310
>
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>
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>
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