Standard GSM is limited to 35 km by timing advance (TA).
Permissible TA values vary from zero to 63 in increments of
one, with each step representing about 554 metres.

ER (Extended Range) GSM (range limited by BTS sensitivity)
sacrifices half its traffic capacity by allocating its
timeslots in pairs.  So each user has two BTS timeslots to use,
instead of the single timeslot allocated in standard GSM.
Range is typically about 70 km, with the negotiated TA value
being 63 for all distance beyond 35 km.

EER (Enhanced Extended Range) GSM is an ER variant which adds
more BTS (base station) sensitivity to push usable
line-of-sight range out to about 120 km.

Very few countries use ER, and those which do use it on very few
cells.  The EER version would be even rarer.

When you mention packets, do you mean you're using GPRS?  As far
as I know, Ericsson is the only BTS manufacturer to sell ER
BTSs, and Ericsson have not made any GPRS-compatable ER
equipment.

So unfortunately you can forget trying to use GPRS at over 35
km.  A circuit-switched data call is compatible with ER cells,
but even EER is doubtful at 75 miles.  But you might achieve
comms with line-of-sight to the BTS and a high-gain external
antenna.

UMTS (3G GSM, or 3GSM) uses neither TA, timeslots nor GPRS, and
is not distance-limited in this way.  Sensitivity and noise are
what distance-limits UMTS.

John

Using a handheld phone, over water, with a clear view of the cell site,
which was on a mountain, I ran out of link margin before I ran out of timing
advance. In order to get the maximum range (35 km) I had to use an external
Yagi antenna. At that point it was like running into a brick wall. The phone
would look like it had a signal and would try to connect but it was
hopeless. To really make use of ER you need a very good antenna and maybe a
full-power (Is that Class I or II?) unit. I know of no US carriers who use
ER GSM.

The timing advance limits range to 70km roundtrip. The only way to get
beyond that is to sacrafice the adjacent timing slot, so a channel
only supports 4 calls instead of 8.

It actually is done in a few places, like the Australian outback, but
certainly is not the general case..  

You can often 'see' the BTS are ranges far beyond 35km, but because of
the timing advance issues, you cannot get your packet into the timing
slot at the BTS, so the BTS just sees you as interference/noise.