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  #1  
Old 09-22-2008, 10:59 AM
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bandwidth question...

I tried searching for this, but didn't find anything relevent...

What is the average bandwidth requirement for SD? I record at 3 Mbps. I would *assume* that the requirement for streaming to a SageTV client would be no more than 4 Mbps or maybe even 5...

BUT

The other day, I tried using my WLAN (802.11b) and see if it would support a single SD stream... It didn't I went back to the wired connection, and clocked the network usage at 11 Mbps while viewing a show (and 0 otherwise). Is there some reason why a 3 Mbps video stream needs 11 Mbps to play back??

Thanks!
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Old 09-22-2008, 11:15 AM
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Raw bandwidth isn't the only consideration. Wireless networks have a high rate of lost packets and retransmits, which doesn't matter much for web surfing and file transfers, but tends to play havoc with video streaming. 802.11b is barely adequate under ideal conditions; if there's any RF noise about (cordless phones, microwave ovens, competing WLANs), you're not going to get a satisfactory result. 11g is somewhat better for SD video, but marginal for HD. Stick with wired if at all practical.
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Old 09-22-2008, 03:41 PM
paulbeers paulbeers is offline
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As G stated already, packet loss is a killer. Also if you lose about half of your bandwidth just to the wireless connection (all the encyption / etc.) so even if you have an 11mbps connection, you are lucky if you even have 6mbps of true bandwidth for data. Then throw in packet loss and you are now well below the necessary. Also if you are like me and record at 3.2GB per hour for SD, then that equates to about 7mbps.....11G is the minimum I would even try to do wireless video, but even that can be plagued with packet loss.
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Old 09-22-2008, 04:13 PM
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Quote:
I went back to the wired connection, and clocked the network usage at 11 Mbps while viewing a show (and 0 otherwise).
I think he is wondering why the 3Mbps video is coming across the "Wired" network as 11Mbps?
This should not be due to packet loss on a wired network.

@bbroerman
Can you clarify that you are recording at 3Mbps or 3GB/hr?
Another thing to cosider is if you are recording 3Mbps of video, you also have to take in consideration the audio. Not huge but it does make the 3Mbps a little higher...
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Old 09-24-2008, 01:01 PM
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You also have to take into account communications overhead. The file is not streamed across the network in one big chunk. It gets broken into tiny packets. Each packet gets wrapped with protocol information including the IP Address of the source, IP address of the destination, packet size, packet ID, etc.

Sending large files across a network is kind of like mailing a large book through the mail using regular envelopes - you can only put a few pages into each envelope and each envelope needs an address label and return address label as well as instructions so the person on the other end knows how to reassemble the pages once they are received.

Some protocols are more efficient than others, but all will add some overhead. Most protocols require some sort of acknowledgment from the receiver to the sender so that the sender knows the packets got through. If the acknowledgment isn't received in a timely manner, the packets get re-sent.

Even on a hard-wired network, packets can get lost or corrupted (usually due to electrical interference). If you've ever run a vacuum cleaner or blender while watching TV and the picture got snowy, you've seen electrical interference in action. Still, a hard-wired network tends to be much more reliable and see much less interference than a wireless one.

Another variable is other network traffic. If there are other devices on the Ethernet network, they may try to communicate at the same time. This results in "collisions" and data must be re-sent.
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