[mythtv-users] multiple tuner question

[Tom Trelvik]

	Thanks for the pdf link, Bruce, it was a really interesting read!  It 
did, however, disagree with your statement of digital cable running over 
TCP/IP.  As Gregorio pointed out, IP is briefly mentioned as going over 
an out-of-band transmission, but that's all.

	It actually runs over ATM AAL5 (network layer 2 stuff (IP is layer 3, 
and TCP is layer 4, for reference)), which makes a lot of sense for a 
cable providers to use, as ATM is a very efficient standard designed to 
allow voice video and data to coexist side by side.  It's also capable 
of getting much higher utilization of bandwidth than we're used to 
seeing on ethernet.  Ethernet performance gets really bad at about 
30-40% utilization, whereas ATM can easily handle >90-95% utilization 
without blinking.  So when you plug your ethernet card into your 
cablemodem, it's just touching network layers 1 (the actual cabling) and 
  2 (translating the ethernet 802.3 frames into the ATM AAL5 cells), 
without needing to touch any of the higher layer protocols like IP & TCP.

	Gregorio also mentioned the A/V side of things using ISO/IEC 13818-1, 
which is the "MPEG-2 Transport Stream" protocol (equivalent to layer 3 
IP, if I understand correctly).  The doc also referred to ISO/IEC 
13818-2 occasionally, which is just MPEG-2 video.

	I found this item from that PDF's glossary informative as well:

FDC Forward Data Channel - A data channel carried from the headend to the
terminal device in a modulated channel at a rate of 1.544 to 3.088 Mbps.
The FDC carries IP traffic only for:
• Conditional access for analog signals
• Entitlement management messages for digital signals
• General messaging
• Application download
• PC data services
• Variable bit rate (VBR) download
• Broadcast data
• Network management

Bruce Markey wrote:
>>     Digital cable runs over TCP/IP?! 
> 
> Uh-huh, that's what makes it digital =). It is essentially

	Not really, there's plenty of digital stuff that doesn't have anything 
to do with TCP/IP.

	Anyway, that was an interesting side track (for me, anyway), but I'll 
let the networking thread drop now.  =)

Thanks,

Tom