[opendtv] Re: Femtocells pose threat to voice-over-Wi-Fi, cellular basestation deployment

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Fri, 30 Mar 2007 16:49:00 -0400

John Shutt wrote:

> What possible advantage is there to making cell phone calls VoIP?
> CDMA and GMS are already digital, why bother with IP
> encapsulation?

Craig answered that with a valid question. My answer is simply that if
you route telephone calls using the same IP routers used for data, you
can dispense with the more expensive telecom switches that are currently
required for circuit-based telephony. Makes the whole infrastructure

> So you're saying that the 3G data capacity is separate from the
> voice data capacity? It seems to me that VoIP has much more
> overhead than CDMA or GMS voice, reducing total potential load
> capacity.

Actually, 3G is a rather broad term. 3G in its original versions was
still a circuit-based scheme, where IP data would be sent over just
another circuit. But there were also setup and voice circuits included.
But then came schemes like EV-DO (evolution, data only), which optimized
the 3G interface for packets only. More bit rate in the same channel.

Although all of this wasn't explained in the article, I think what the
telcos are wanting to use the 3G's data path for their voice too, using
SIP to set up the call, and IP routing to route the calls.

> But if there are two separate services and never the twain shall
> meet, then I guess it makes sense to tap into unused 3G capacity
> for additional call traffic in a particular cell.

My guess is that if the cell phone negotiates a narrowband CDMA or TDMA
channel with the base station, the base station will continue to use
circuit based 2G routing. But if the cell phone manages to set up a
wideband CDMA link with the base station, W-CDMA is used for all 3G
systems, then the base station would route calls using VoIP.

For example, my cell phone says it is 3G enabled, which means that it
should be able to use either regular CDMA or W-CDMA, depending what the
base station in range has. I expect, just a guess, that a software
update to the phone would permit the phone to add VoIP to its bag of
tricks. And software updates are very easy to do, just by dialing a
number and then pressing 1. I think that's what they meant when they
said that this new service would work with existing phones.

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