[linux-dvb] Re: IP over DVB receiver cards for Linux

[Wolfgang Wershofen <wolfgang@wershofen.com>]

Dienstag, 17. September 2002 at 12:08 Felix Domke wrote:

>> Do you really think, this is a problem? Normally, SAT-providers state
>> their bandwidth in kBit/s and not MByte/s. So I believe you won't hardly
>> be able to exploit the bandwidth within your card with IP-Networking.
>> The bottelneck of SAT-IP-Networking is the shared bandwidth on the
>> transponder and not the hardware on the DVB-card.
> But these 1.5MB/s are the *theoretical* bandwidth. I think the card doesn't
> offer any busmastering (correct me if i'm wrong), so when you read from the
> card, say, 200kb, you need
> 200k/1.5M = ~130ms. In these 130ms you cannot do anything else, so you have
> a VERY HIGH cpu load with higher bandwiths. It get's critical if you want to
> do timeshifting and stuff (ok, that's not your concern). It's like an IDE
> harddisk which does about 2MB/s without DMA. Are you willing to use this to
> record movies? Movies are ~500kb/s. But it won't work very well.

Interesting view. I didn't take this into account. But, let's see it
from a different point. To receive the 200kB you mentioned with - let's
say "normal" DSL-speed of 768kBit/s, you need (200kByte * 8) / 768 kBit
~= 2.1 seconds. So this is much more time than the card needs to
transfer this data, means: Most of the time, the card will be waiting
for data and not do anything. Am I right?
So as long as we are talking about IP-Networking, there should be no
problem. For video, I fully aggree with you.

> Some multicasts on EON (or other) have bandwidths of up to 8MBit/s, and then
> it really gets hot. I know that some PCI-cards (not the Siemens ones) had a
> problem here and thus couldn't be used for this.

Ok, but even with 8MBit/s you only get 1 MByte/s from the transponder
and this is still below the mentioned 1,5 MB/s.

>> I have no personal experience with the full featured cards, but I know
>> people using T-DSL via Satellite from Deutsche Telekom with such cards
>> and they are getting equal transmission rates like I have with my budget
>> card.
> Off course. It *WILL* work. But for myself, i'd like to experiment with all
> streams i find on the transponders, especially the multicast stuff. And, as
> said, for this, the bandwidth provided by some cards isn't just enough. In
> contrast, the bandwidth provided by the Nova is always enough. So i strongly
> recommend these Cards for everyone who doesn't want to watch TV.
> (Additionally, the delivery of the full, unmodified, unfiltered TS is an
> unique feature of this cards.) And they are cheaper.

FULL ACK. If you only want to do SAT-IP-networking, the budget-cards are
the right choice and even if you want to do DTV and IP over Satellite,
I'd prefer using a full featured DVB for DTV and an additional
budget-card for IP (should be affordable due to it's small price).

So we're not so far from each other ;-)

-- 
Best regards
Wolfgang Wershofen                            mailto:wolfgang@wershofen.com



-- 
Info:
To unsubscribe send a mail to listar@linuxtv.org with "unsubscribe linux-dvb" as subject.