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

sonofzev at iinet.net.au sonofzev at iinet.net.au
Sat Aug 20 08:40:06 UTC 2011

On Sat Aug 20 18:31 , Simon Hobson  sent:

>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

In my experience with Gigabit (which is really a cheap solution these days
anway).. I have all Realtek chipsets on all cards and no problems from them at
all.. .. throughput is generally very good and I regularly am streaming 2 HD
streams from the backend to the frontend  while other operations  are going (file
copying and backup operations) I don't have an HD Home Run on the network though.. 

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