[mythtv-users] Interesting Problem - Can't have third PCI Card - USB Suggestions?
linux at thehobsons.co.uk
Sat Aug 20 08:31:55 UTC 2011
Kevin Ross wrote:
> > Well, would I be overtaxed with a 10/100 switch? Maybe I should think
>> of the HDHomeRun... will I be able to record to my PC and then output
>> right out again to another (I have seperate frontend and backends)...
>I think you'd be pushing it. North American broadcast HDTV is 19 Mbps.
>So 19 Mbps from the HDHR to the backend, then another 19 Mbps from the
>backend to the frontend (actually less because of multiple streams on
>the same channel). That's 38 Mbps, of your theoretical 100 Mbps
>maximum. But you don't get the theoretical maximum. If you record two
>shows at once from the HDHR, or watch from two frontends while the HDHR
>is recording, you're going to saturate your bandwidth.
Actually that's not true any longer. Back in the days of half-duplex
and shared bandwidth, there was a rule of thumb (based on testing)
that you'd never get more than around 40% of your available
bandwidth. I've seen graphs where people have done tests, and in the
general case (many-to-many traffic) throughput peaks at about 40% and
then actually drops off. Now you can't buy hubs any more, that's not
With a switched network, each link is normally full-duplex, and you
don't have problems with shared bandwidth.
So taking this case, the data flow from the HD Homerun TO the backend
follows one path. The data flow FROM the backend to the frontend
follows a different path. Because the link between switch and backend
is full duplex, the two traffic flows are largely independent*
Thus you should be able to handle in excess of 50Mbps from tuners to
the backend at the same time as handling in excess of 50Mbps from the
backend to frontends. That's assuming the rest of the system is up to
it. In theory you could run up to 100Mbps each way, but when you get
that full there are some interesting effects that start to creep in.
As an analogy, think of the old ethernet as being like a single lane
road - traffic going one way has to wait if there's traffic going the
other. If you try and push too many vehicles down it, then collisions
occur and throughput drops off dramatically. What's more, you have
junctions all along where traffic is trying to get on and off.
Now replace the road with a dual carriageway - now traffic can flow
freely in both directions without interfering with the other
direction. The junctions are now replaced with arrays of sliproads
etc like you get on motorways (freeways ?) so traffic just has to
buffer in the on slip and drop into the next gap.
Going to Gigabit is like adding some more lanes and increasing the speed limit.
You'd want some decent network cards, since some are known to cause
problems with high interrupt rates when the packet rate is high. Over
on the Xen user list, Intel gigabit cards come highly recommended,
while Realtek don't.
* Whilst the bulk of the flow is packets in one direction, there's be
some small management packets going the other way. So there is some
interaction, but in practice that shouldn't matter as long as you
don't push things to the limit.
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