Q: Why is my GPRS, EDGE, or UMTS so slow?

A1: SERIAL PORT BOTTLENECK: Connections between handsets and "tethered"
devices (e.g., notebook computer, PDA) are typically implemented as a serial
port, either real or virtual, and port speed usually defaults to 115 Kbps. (My
own Bluetooth stack is implemented as a virtual serial port that will go as
fast as 921.6 Kbps, but which runs at only 115.2 Kbps by default.) While 115
Kbps is generally fast enough for GPRS and CDMA 1X, it can be a bottleneck for
EDGE, UMTS, and CDMA EV-DO. In general, I recommend port speed* of 230 Kbps
for GPRS and CDMA 1X, 460 Kbps for EDGE, and 920 Kbps for UMTS and CMDA EV-DO.

A2: HANDSET CAPABILITIES: Handsets (and PC Cards) do not all have the same
data performance. In general, all reasonably recent GPRS/EDGE-capable devices
from Ericsson and Sony Ericsson support Class 8 (4+1) and Class 10 (4+2, max
of 5), whereas most Nokia devices are limited to slower Class 2 (2+1), Class 4
(3+1), and Class 6 (3+2, max 4). Motorola has both Class 4 and Class 8
devices. (The first number is the max downlink slots, and the number after the
"+" is the max uplink slots.) This can translate into a substantial difference
in throughput -- if supported by the carrier:
- Class 4 or Class 6 is 50% faster on downlink than Class 2
- Class 8 or Class 10 is 100% faster on downlink than Class 2,
 33% faster on downlink than Class 4 or Class 6.
- Class 10 can be 100% faster on uplink than Class 2, Class 4, or Class 8.

* To set cellular serial port speed in Windows XP:
- Open Network Connections
- Right-click on cellular connection, and choose Properties
- In Properties-General, select checked cellular modem, and click Configure
- In Modem Configuration, select desired Maximum Speed.
- Click OK to close all windows.