[mythtv-users] Interesting Problem - Can't have third PCI Card - USB Suggestions?

Simon Hobson 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 
an issue.

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.
-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.


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