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'[PIC]: FSK modem for using with cellphones.'
2002\03\02@200857 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi,

   I need to make a audio coupled FSK modem for using with cellphones !! I
know it is weird but the client wants something that can couple to any phone
and receive data at 1200 bps. It does not even have to be bidirectional !

   I have seem the Scenix app-note on a bell 202 modem and I am wondering
if there is a better way to handle the decoding. The encoding is straight
forward but the decoding may be a problem because the audio levels would be
varying a lot, specially because of the audio coupling and in some places
the cellphones would be working at old analog modes. I was thinking of
implementing special filters like the ones that Scott used for DTMF decoding
but I am quite sure I cannot do it fast enough.

   I would really like suggestions for the decoding algorithm and input
conditioning circuit. My basic idea is to use a "frequency meter like" input
circuit that just goes and clips the signal and simple pulse counting to
check the frequencies. I would like to test some time filtering also to see
if I can get better results. Any suggestions ? The project is not going to
be exclusive, so I can publish the results back to the list when it is
working.

Best regards,
Alexandre Guimaraes

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2002\03\02@203807 by mark

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Just an advice :

Before you start pulling your hair off, take an audio signal
generator an see the shit you can pass via a DAMPS phone :-(


On 2 Mar 2002 at 22:07, Alexandre Guimarães wrote:

{Quote hidden}

---
Marcelo Puhl
Mensa Brasil member
http://www.mensa.com.br

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2002\03\02@212621 by mark

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On 2 Mar 2002 at 22:55, Alexandre Guimarães wrote:

> Hi Marcelo
>
> > Just an advice :
> >
> > Before you start pulling your hair off, take an audio signal
> > generator an see the shit you can pass via a DAMPS phone :-(
>
>     Is it really that bad ??!! I just need it to pass 2 frequencies and I
> was thinking to use the "standard" 1300 hz and 2100 hz. That is very well
> inside audio band. I imagine it can't be much worse than our old phone lines
> here in brazil 15 years ago and those signals passed along quite well in the
> old times. It it a frequency response problem ot something else ? I know
> very little about the cellphones structure. What can you tell me more ? I
> already have very few hair to pull off. :-)
>
> Best regards,
> Alexandre Guimaraes
>

If you use an old AMPS phone, no problem, it works.

But the voice CODEC inside the DAMPS phones screws up any audio you
can think to pass on. Even DTMF !!!

Have you ever noted the phone's bad voice quality ?

---
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Mensa Brasil member
http://www.mensa.com.br

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2002\03\02@213038 by Larry Kayser

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>    I need to make a audio coupled FSK modem for using with cellphones !! I
>know it is weird but the client wants something that can couple to any phone
>and receive data at 1200 bps. It does not even have to be bidirectional !

Obviously you like running up hill forever!  This is a very hard problem
especially with the FSK options.  The  envelope delay distortion, codec
delays with more delay distortion, switching holes in the transmission, all
work together to drive this kind of thing nuts.

You really have two options, either go to a multi tone protocol (and get
the bit rate down) or encode the data with lots of overhead and redundancy
so that you can recover programatically from what the transmission medium
does to it.  Remember also that most of the digital cellphone stuff has up
to 70 milliseconds of CODEC delay to start with, at both ends if your into
a cell phone to cell phone connection.

Your best bet is to get a MODEM that has been designed to survive on the
cell phone network, they are much cheaper that a development project would be.

Larry
VA3LK

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2002\03\02@225042 by 859-1?Q?Alexandre_Guimar=E3es?=
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Hi,

>If you use an old AMPS phone, no problem, it works.

   That is an easy option. I can switch the phones to analog mode anywhere
! I though that digital mode would be better.

>But the voice CODEC inside the DAMPS phones screws up any audio you
>can think to pass on. Even DTMF !!!

   I have used DTMF sometimes and did not notice any problems.

>Have you ever noted the phone's bad voice quality ?

   I guess I am a little deaf :-) i have always felt that the cellphones
sounded clearer than a regular phone line, at least the digital ones.

> Obviously you like running up hill forever!  This is a very hard problem
> especially with the FSK options.  The  envelope delay distortion, codec
> delays with more delay distortion, switching holes in the transmission,
all
> work together to drive this kind of thing nuts.

   If the delay is constant that will not be a problem because I plan to
run a simplex link. The switching holes could be compensated if I use a
software PLL and fill the missing transitions !? There are many options to
do the decoding, what do you suggest ?

> You really have two options, either go to a multi tone protocol (and get
> the bit rate down) or encode the data with lots of overhead and redundancy
> so that you can recover programatically from what the transmission medium
> does to it.  Remember also that most of the digital cellphone stuff has up
> to 70 milliseconds of CODEC delay to start with, at both ends if your into
> a cell phone to cell phone connection.

    DTMF would be too slow, even for transfering just 8k of data as I need.
The redundancy is ok, the files are very small. I can transmit the blocks 3
or 4 times and use small blocks to have a better chance of getting trough.
What kind of packet size would seem reasonable ?? The delay would not
matter, as long as it is constant, I am planning to use a simplex link with
no automatic return of information.

> Your best bet is to get a MODEM that has been designed to survive on the
> cell phone network, they are much cheaper that a development project would
be.

   Unfortunately that is not an option ! We were not able to find a single
solution that would work all over the country. We have CDMA, TDMA, AMPS and
GSM phones to deal with ! It has to work with them all.

   I can't believe that it is an impossible task. Maybe it is harder than I
first though but should not be impossible. The trick seems to be in the
decoding. Would amplitude encoding and a single frequency be better ???

best regards,
Alexandre Guimaraes

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2002\03\02@232531 by Dale Botkin

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On Sun, 3 Mar 2002, Alexandre Guimarães wrote:

>     If the delay is constant that will not be a problem because I plan to
> run a simplex link. The switching holes could be compensated if I use a
> software PLL and fill the missing transitions !? There are many options to
> do the decoding, what do you suggest ?

I have to think an XR2211 would be a good place to start.  Hams use these
for 1200BPS data comunication over radio links all the time.  They'll pull
FSK data out of some pretty crappy signals, I'd be a little surprised if
it wouldn't work for you.  Tons of inforamtion all over the 'Net, and the
Exar data sheet shows specfic information for your application -- 1200BPS
data using 1200/2200Hz.

http://www.exar.com/products/XR2211.html

Dale

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2002\03\03@030720 by Russell McMahon

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>      DTMF would be too slow, even for transfering just 8k of data as I
need.
> The redundancy is ok, the files are very small. I can transmit the blocks
3
> or 4 times and use small blocks to have a better chance of getting trough.

In which case 300 baud and forward error correcting (eg Reed Solomon) with
small packet sizes and a simple ack/nack packet protocol may be a better
choice.

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2002\03\03@100258 by Larry Kayser

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> > Your best bet is to get a MODEM that has been designed to survive on the
> > cell phone network, they are much cheaper that a development project would
>be.
>
>     Unfortunately that is not an option ! We were not able to find a single
>solution that would work all over the country. We have CDMA, TDMA, AMPS and
>GSM phones to deal with ! It has to work with them all.

Well that is a nasty bunch of stuff to have to work with.  My suggestion is
multi tone, and chop the bit rate down as far as you possibly can, say as
low as 50 bits per second and try and mimic what a voice looks like.  Some
of the digital CODECS are designed to mess up anything but voice so try and
make the signal look like voice.

If it was me, I would do it with an ack/nak protocol using the standard
touch tone chips, at least you have a chance to make it work reliably.  If
you made the tone length about 150 Ms you should be able to get through
almost anything - and frankly that is about what your trying to do.

>     I can't believe that it is an impossible task. Maybe it is harder than I
>first though but should not be impossible. The trick seems to be in the
>decoding. Would amplitude encoding and a single frequency be better ???

Well nothing is impossible, but making your design/development/life cycle
survive the economic limits will be a big task.

I would avoid the EXAR chips like the plague!  Compensation for the twist
(envelope delay distortion) between the two tones over different system and
mixes of systems would have to be automated and that would an extreme pain
to live with.

Here in this country, Canada, I frequently get calls from the various cell
systems and I am amazed at the terrible quality of the sets and the
transmission medium.  I don't see any evidence of a fixed loss plan, hybrid
echo management in view of massive CODEC delays,  etc.
Good Luck.

Larry
VA3LK

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2002\03\03@142759 by Jim

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   >We were not able to find a single solution that
   >would work all over the country. We have CDMA,
   >TDMA, AMPS and GSM phones to deal with !
   >It has to work with them all.

It sounds like you have a DATA application for
which a DATA solution needs to be found. There
are now several national 'carriers' in the US
that can provide this type of service nationally
(not in isolated islands or isolated service areas,
otherwise I would recommend CDPD, etc) - but it
won't be necessarily using a 'cell phone'.

Is this a system that requires DATA tranfer exclusively?

Several of the national "PCS" carriers even offer
some real DATA capability (again, here in the US).

With the 'data over cell-phone' approach you are fighting
a science which has strived to characterize the human
vocal tract - then convey only those factors/coefficients
from that reduction process necessary to convey the
bare essentials of a human 'speech'.

Jim



{Original Message removed}

2002\03\03@150545 by dbengtson

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On Sat, 2 Mar 2002 22:07:51 -0300, you wrote:

>Hi,
>
>    I need to make a audio coupled FSK modem for using with cellphones !! I
>know it is weird but the client wants something that can couple to any phone
>and receive data at 1200 bps. It does not even have to be bidirectional !
>
.
.
.
.
>Best regards,
>Alexandre Guimaraes


How much data are you looking at moving, and where are you located?
There may be other options than cellphones.

Dave

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2002\03\03@201307 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi,

> Is this a system that requires DATA tranfer exclusively?

   It is a simple problem... Transfering HEX files for programming boards
all around Brazil. The technician is sometimes at quite remote location and
access to a regular phone line is harder than using his own cellphone,
besides that we do not want the data to be available not encrypted. The
problem starts because the different carriers treat the cellphones in
different ways. Sometimes it is digital and have data capabilities,
sometimes digital with not data, sometimes analog. The systems can be CDMA,
TDMA, AMPS, DAMPS and GSM ! The files are very small, 8 Kbytes max.

> Several of the national "PCS" carriers even offer
> some real DATA capability (again, here in the US).

   None of that easily available down here :-(  :-(  :-( We also tried some
old PCMCIA modems for cellular phones and they do not work on digital lines
!

> With the 'data over cell-phone' approach you are fighting
> a science which has strived to characterize the human
> vocal tract - then convey only those factors/coefficients
> from that reduction process necessary to convey the
> bare essentials of a human 'speech'.

   If things get to be too hard I can start digitizing the words "one" and
"zero" and do speech recognition at the target. :-) Just kidding.... Maybe
some other kind of signal that is not a simple senoid can get better results
??? I think I will really connect the digital scope to a cell phone and see
what the signals really look like. Maybe a simples lowpass before the timing
can cope with much of the presented problems. It can not be so difficult to
make a simplex transmission at 1200 bps ! I will experiment with it and let
you know..

Best regards,
Alexandre Guimaraes


> Jim
>
>
>
> {Original Message removed}

2002\03\03@201310 by Bob Ammerman

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> With the 'data over cell-phone' approach you are fighting
> a science which has strived to characterize the human
> vocal tract - then convey only those factors/coefficients
> from that reduction process necessary to convey the
> bare essentials of a human 'speech'.
>
> Jim


Hm.....

Would it be possible to design a modulation/demodulation scheme that takes
advantage of the fact that digital phones model speech? IE: A modulation
scheme that makes the data signal look like speech.

Bob Ammerman
RAm Systems

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2002\03\03@201508 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi

> How much data are you looking at moving, and where are you located?
> There may be other options than cellphones.


   8 kbytes maximum...

   I am in Brazil and we have cell phones everywhere but with different
systems at each place. I have to cover basically all the states and no data
service is available everywhere. The only option is really the cellphones.
At least the only fast option.

Best regards,
Alexandre Guimaraes

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2002\03\03@201512 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi


> >      DTMF would be too slow, even for transfering just 8k of data as I
> need.
> > The redundancy is ok, the files are very small. I can transmit the
blocks
> 3
> > or 4 times and use small blocks to have a better chance of getting
trough.
>
> In which case 300 baud and forward error correcting (eg Reed Solomon) with
> small packet sizes and a simple ack/nack packet protocol may be a better
> choice.

   300 baud maybe a better choice but my usual experience on the old days
of 1200 half duplex modem and 300 baud full duplex modems is that even when
the line is very, very bad the 1200 is much faster. I guess it is because it
is faster when getting a block trough. Less time to get noise into the
signal. We developed a protocol that was adaptive on the block size and went
all the way from 16 bytes per block up to 4096. In very good connections the
protocol went from 256 to 4096 and settled at 1024 as the best troughput
point. At very bad connections usually the best troughput was at 32 bytes !
When we changed the baud rate this changed completely. We could get 1200
baud connections that were usable where 300 baud connections did not tranfer
anything ! It all depends on how long the noise last and what burst lenghts
it has.

   I will have to experiment with that also. The option of using an
external PLL decoder is not exactly what I was hoping to use but maybe the
best option really. It should be less disturbed by noise than a
microprocessor decoder. Does anyone knows about good articles online dealing
with software PLL's ?

Best regards,
Alexandre Guimaraes

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2002\03\03@213428 by Larry Williams

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Look at what alarm companies use.  It works through almost anything.
Uses a strange tone scheme. Works with hayes 1200 baud modems. We built
a one off hook up for a tower company through a nokia phone. Tried to
find data on using it directly but either had to pay megabucks for the
info or else go partners with someone in exchange for the information.
Now, we just use the data connection on the phone and dial away.
the modem is actually in the cell site.  On those systems where we can,
we use motorola bag phones and interface.  There is a device you can
get  from Radio Shack that takes your Nokia phone and gives you a
standard phone line so you can use your cellular/pcs phone for the home
and use your regular telephone to talk with.  The device mearly provides
dialtone as if you had a real phone line.

Alexandre Guimarces wrote:
{Quote hidden}

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2002\03\03@215258 by Jim

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Sounds like you are in the US - where the cellular
(800 MHz) companies still offer *analog* cellular
service along with whatever flavor digital they opted
to use. This means that each sector has *at least*
one radio (actually, two: one for the setup/paging
channel and one for the analog voice traffic). The
balance of the radios (in a mature system) are
digital. This is done to provide some 'universality'
of service from region to region when roaming in another
carrier's area using a cellular phone.

This *isn't* the case everywhere in the world and is
certainly *not* true of the 'PCS' (1800 MHz) carriers
in the US where 1800 MHz PCS operators from the outset
have utilized digital-mode phones.

Jim


{Original Message removed}

2002\03\04@092345 by Jim

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Alexandre, I posed this question "How much of Brazil has
wireless data?" to a friend of mine who has done some
work in Brazil in the wireless telecom area, here is his
reply:

*** Beginning of message. ***

Not a simple question to answer..

First, let me say that the b-band regions mentioned below are only  testing
CDPD & have not deployed it throughout their licensed area.

The country of Brazil was divided into 10 regions for the purpose of issuing
the b-band cellular license.

Our customer operates a network in Region 1 & 10. Their network uses Nortel
infrastructure, where CDPD is easily implemented. The voice protocol in
those regions is strictly TDMA. No AMPS, & no analog control channels.

I know there is some interest in at least 2 other regions, but they have
Ericsson infrastructure, & it is much harder to implement CDPD in their
network.

I think about 5 of the 10 b-band regions chose Nortel infrastructure. So,
there would be 5 where it can be done easily, & the others would be harder.

FYI....here is the areas where CDPD is being tested or there is an interest
in it.

1) Region 1 - Sao Paulo Metropolitan area - Nortel b-band infrastructure -
  TDMA - Testing CDPD
2) Region 10 - The states of Piauí, Ceará, Rio Grande do Norte, Paraíba,
  Pernambuco and Alagoas - Nortel b-band infrastructure - TDMA - Testing
CDPD
3) Region 2 - Sao Paulo state - Ericsson Infrastructure - TDMA - Limited
  Testing of CDPD
4) Region 7 - Federal District (DF), Goiás, Mato Grosso, Mato Grosso do Sul,
  Tocantins, Rondônia and Acre states - Nortel b-band - TDMA - Testing CDPD
5) Region 3 - Rio de Janeiro and Espírito Santo states - Ericsson
  Infrastructure - TDMA - Limited Testing of CDPD

I do not recall the details of other b-band regions. The 5 regions above
covers more than half of the total population of Brazil. The Amazon region
(not listed above) had to be auctioned more than once because nobody wanted
it. The area is very large & mostly unpopulated.

The a-band is a mix of AMPS only, CDMA & AMPS or TDMA & AMPS. Their area of
coverage is also different than the b-band networks.

GSM is the newest networks in Brazil, therefore it has the smallest coverage
areas. Brazil was divided into 3 regions for the purpose of the GSM license.

It's a tough call. I wish the guy luck. If he needs data coverage of all of
Brazil for 8KB of data, may I suggest that he look into a commercial HF
frequency. If he can narrow his service area, perhaps CDPD, TDMA Circuit
Switched Data, or CDMA Circuit Switched Data may be the solution. Also,
Nextel is in Brazil & they have some kind of data service. I do not know
details of their coverage area.

Hope this helps.

XXXXXXXX


*** End of message. ***

Jim



{Original Message removed}

2002\03\04@113236 by Herbert Graf

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Is analog available everywhere? If so then use an analog cellphone,
orinically they are MUCH easier to use for acoustic coupled data. TTYL

> {Original Message removed}

2002\03\04@133708 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi

> The a-band is a mix of AMPS only, CDMA & AMPS or TDMA & AMPS. Their area
of
> coverage is also different than the b-band networks.
>
> GSM is the newest networks in Brazil, therefore it has the smallest
coverage
> areas. Brazil was divided into 3 regions for the purpose of the GSM
license.

> It's a tough call. I wish the guy luck. If he needs data coverage of all
of
> Brazil for 8KB of data, may I suggest that he look into a commercial HF
> frequency. If he can narrow his service area, perhaps CDPD, TDMA Circuit
> Switched Data, or CDMA Circuit Switched Data may be the solution. Also,
> Nextel is in Brazil & they have some kind of data service. I do not know
> details of their coverage area.

   Oh Yeah !!! That is really my country. Now you can imagine why I have to
find something that works on any phone. What a mess people have made with
our cellular network. And it is quite different from the US where the system
got to have different systems because of new technologies. Our network is
quite new and people could have regulated it to have just one system running
but that would not be fun :-)

   It is weird when we have full coverage of cellular telephony and it is
so mixed up with different systems that old HF radio would be the only
option it I were to use it all the time. Managers, marketing and politicians
can really kill any good technology !

   Jim, thanks a lot for the information, it will be very usefull when I
have to transfer bigger volumes of data in one of those areas that are
testing the data systems.

   I will start tomorrow testing how bad the signal passes trough the cell
phones. By the info gathered so far I will probably try a microprocessor at
each end with high degress of redundancy on the data and heavy filtering of
the signal on the receiving end. If the voice passes I will find a way to
pass the small amount of data I have :-) It started as a simple thing to do
and ended up as a challenge. I am quite sure it is doable despite the
difficulties.

   Thanks for everyone for all the information. I will keep you informed of
how it goes..

Best regards,
Alexandre Guimaraes

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2002\03\04@134217 by 859-1?Q?Alexandre_Guimar=E3es?=

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Hi,

> Is analog available everywhere? If so then use an analog cellphone,
> orinically they are MUCH easier to use for acoustic coupled data. TTYL

   As far as I can tell analog is available everywhere.. It will probably
be the option I will use if the digital codecs are so bad as people are
telling me. It is really ironical, the cell phone business grew up so fast
that it is not structred for doing simple things like transfering a few
bytes around.

Best regards,
Alexandre Guimaraes

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2002\03\06@022047 by Jim

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Alexandre, I posed this question "How much of Brazil has
wireless data?" to a friend of mine who has done some
work in Brazil in the wireless telecom area, here is his
reply:

*** Beginning of message. ***

Not a simple question to answer..

First, let me say that the b-band regions mentioned below are only  testing
CDPD & have not deployed it throughout their licensed area.

The country of Brazil was divided into 10 regions for the purpose of issuing
the b-band cellular license.

Our customer operates a network in Region 1 & 10. Their network uses Nortel
infrastructure, where CDPD is easily implemented. The voice protocol in
those regions is strictly TDMA. No AMPS, & no analog control channels.

I know there is some interest in at least 2 other regions, but they have
Ericsson infrastructure, & it is much harder to implement CDPD in their
network.

I think about 5 of the 10 b-band regions chose Nortel infrastructure. So,
there would be 5 where it can be done easily, & the others would be harder.

FYI....here is the areas where CDPD is being tested or there is an interest
in it.

1) Region 1 - Sao Paulo Metropolitan area - Nortel b-band infrastructure -
  TDMA - Testing CDPD
2) Region 10 - The states of Piauí, Ceará, Rio Grande do Norte, Paraíba,
  Pernambuco and Alagoas - Nortel b-band infrastructure - TDMA - Testing
CDPD
3) Region 2 - Sao Paulo state - Ericsson Infrastructure - TDMA - Limited
  Testing of CDPD
4) Region 7 - Federal District (DF), Goiás, Mato Grosso, Mato Grosso do Sul,
  Tocantins, Rondônia and Acre states - Nortel b-band - TDMA - Testing CDPD
5) Region 3 - Rio de Janeiro and Espírito Santo states - Ericsson
  Infrastructure - TDMA - Limited Testing of CDPD

I do not recall the details of other b-band regions. The 5 regions above
covers more than half of the total population of Brazil. The Amazon region
(not listed above) had to be auctioned more than once because nobody wanted
it. The area is very large & mostly unpopulated.

The a-band is a mix of AMPS only, CDMA & AMPS or TDMA & AMPS. Their area of
coverage is also different than the b-band networks.

GSM is the newest networks in Brazil, therefore it has the smallest coverage
areas. Brazil was divided into 3 regions for the purpose of the GSM license.

It's a tough call. I wish the guy luck. If he needs data coverage of all of
Brazil for 8KB of data, may I suggest that he look into a commercial HF
frequency. If he can narrow his service area, perhaps CDPD, TDMA Circuit
Switched Data, or CDMA Circuit Switched Data may be the solution. Also,
Nextel is in Brazil & they have some kind of data service. I do not know
details of their coverage area.

Hope this helps.

XXXXXXXX


*** End of message. ***

Jim



{Original Message removed}

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