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

Mike Perkins mikep at randomtraveller.org.uk
Sat Aug 20 09:48:52 UTC 2011

On 20/08/11 09:40, sonofzev at iinet.net.au wrote:
> 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.
> 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..
You have another option, and that is to run your HDHR on a different 'network'. 
Put another NIC in your master backend (PCI, PCIe, USB, whatever) and connect 
your HDHR directly to it with a crossover cable - some NICs don't even need a 
crossover any more, they can figure out what to present on what wires so you 
could try a patch cable.

Configure this link with addresses in a different subnet. There is no reason for 
your HDHR to communicate with any other box than your MBE - why clog the tubes 
with that traffic, then?


Mike Perkins

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