[mythtv-users] multiple tuner question

[Bruce Markey]

Tom Trelvik wrote:
> 
>     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)),

Realize that "TCP/IP network" != "TCP" + "IP" over ethernet.
Layer 1 is coax, layer 2 is ATM with nodes identified by MAC
addresses. The cable modems and set top boxes have IP addresses
used for addressing and routing on layer 3. It is packets
encapsulated with IP routing headers that makes it an IP network.

There is a "TCP/IP network" running over the cable network.
I did not mean say that ethernet protocol was used over the
coax and I certainly didn't say (as Gregorio assumed) that
Transmission Control Protocol is the protocol used on layer
4. In fact, TCP would be a horrible choice for a massively
broadcast networks  (imagine the ACK storms!).

> 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.

Right. Lots of wire and wire protocol pairs can be used
for layers 1 and 2 (cat5/ethernet, phone line/ppp, raido
waves/802.11x, fiber optic/FDDI, coax/ATM, etc.) All can
carry packets with IP address headers (layer 3) and devices
can route those packets from one type of physical network
to another. It's this interconnection of different types of
physical and logical networks into the same address space
that leads to the term "internet".

>     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).

I think that's more like layer 4. 3 would be more like an
addressing scheme.  I assume that the FAT channels are
different frequencies so there probably wouldn't be a need
for an addressing scheme if there is only one unidirectional
stream of data being broadcast. Whether or not these packets
are or aren't encapsulated in IP headers is moot. The data
could be sniffed. However, I assume that if it could be
easily decoded, thousands of people would be doing this
already. I know we're not the first people to think it would
be cool to grab the mpeg streams ;-). And, I know it would
be a waste of my time to think I might be the first person
to figure out how to do it.

>  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

Yep. That leaves no doubt that there is an IP network.

--  bjm