[opendtv] Re: News: TV Braces for the Apple Tablet

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Fri, 29 Jan 2010 18:54:07 -0600

Craig Birkmaier wrote:

> Sorry Bert, but with IP Multicast, telco networks are far more
> spectrally efficient than ATSC MHP for video delivery.

By the way, even this is wrong, Craig, assuming you are talking about cellco 
wireless nets here.

If you want to transmit IP multicast to individual cell phones, and NOT just 
use IP multicast within the wired backhaul nets between cell towers, and then 
broadcast the signal, you need to establish two-way links to each cell phone. 
Because that's how IP multicast works, Craig. What makes IP multicast 
"efficient" is that it reduces the traffic between IP routers. IP routers are 
in the backhaul network.

Now, for IP multicast to the cell phone, each individual cell phone needs to 
communicate using unicast transmissions with its local cell tower. Each 
individual cell phone needs to tell the cell tower which multicast group it 
wants to participate in, and it needs to keep doing this every time it gets a 
query from the cell tower. These are frequent, like once a minute at the least.

Not only that, but the two-way link between each cell phone and the cell tower 
is unique to that cell phone. Its power is adjusted according to the distance 
from cell tower to cell phone, and it goes through a handover procedure as the 
cell phone reaches the edge of a given tower's coverage area.

So as you can see, the spectrum required in the wireless parts of the cell 
network is not used any more efficiently here than it would be unicasting that 
video. The advantage of IP multicast is only in the backhaul network. On the 
other hand, if the cell phone uses a broadcast-only chunk of spectrum, one 
which does not eat into cell tower's two-way link capacity, that's when you'll 
see the advantage.

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