[NCLUG] Data Through Cellular Technology?

[John L. Bass]

	AT&T's data offerings don't seem that good to me.  I think the three you 
	need to look at are Verizon, SprintPCS, and Voicestream->Tmobile.

	AT&T uses a CDPD analog system, last I knew.  I think they are 
	transitioning to a GSM interface, though.  I think this is the system 
	that Sean et al at Tummy have been using.  It is kinda slow, but it 
	works great for them.

	Verizon and Sprint both use CDMA systems.  They are currently limited to 
	14.4k.  Both are supported by Sierra Wireless pcmcia cards, though only 
	the older models support Linux.  The rest are akin to winmodems. :( 

Technically, all wireless communications are analog at some point
when modulated on to the RF ... but CDPD service is really IP packet
oriented digital service at it's descriptive hardware interface.

The CDPD technology that both AT&T and Verizon support today is
the same - and the CDPD modem cards are usable interchangably on
both services. The fact that AT&T uses TDMA for digital voice,
and Verizon/Sprint use CDMA is a choice of how much range and
buildout you need to service a coverage area. There are small
advantages to the CDMA approach in high density metropolitian
areas, and huge advantages to TDMA in rural and suburban areas.
As a result, AT&T offers a much broader coverage area outside metro
areas.

Even after G3 services are deployed, CDPD will be around for a
long time ... simply because the lower bandwidth gives the technology
2-4x the range/building penitration.  As you double the data rate,
you halve the power per bit, which takes the effective signal to
noise margins down by 3db. The same is true with 802.11 ... the 1mbps
data links are 10db better reciever sensitivity (about -93dm) as
compared to 11mbps data rates (about -83dm) which gives the lower
data rate 4x the range simply because of power per bit physics.
Likewise, the newer higher data rate 802.11 technologies continue
to bump modulation bandwidth which cutting range even further as a
tradeoff.

Another advantage is that CDPD is implemented in the 800mhz band,
while G3 is implemented in the 1,900mhz bands (and higher). Similar
to the power per bit problem we have a problem that as you double the
carrier frequency you lose half (3db) of your usable signal to noise
resulting in reduced range and building penitration,

Additionally, 800mhz CPDP has much less problems with phase noise
caused by multipath - the higher data rate 1.9Ghz systems are much
more sensitive to multipath (as are 802.11 systems).

See http://www.tapr.org/tapr/html/ve3jf.dcc97/ve3jf.dcc97.html

Have Fun,
John