Do you have a zyxel series p600, 660R-D1 dsl modem user manual? I have one and have a range of kbps of 3050 to 240 kblps. I thought I would read and see what to expect, Earthlink and the phone company blame the other for the failure to maintain connection and dsl speeds. thanks

The user manual or user guide of the 660R-D1 is available on the zyxel.com site, if you go into the following path:
Support and downloads --> download library ---> up at the right hand side top you see a drop down menu with list of products, choose 660R Compact series. This brings you to the download page and you can choose out
Model : P-660R-D1
Version : 3.40
Language : English
OS : N/A
Release Date : 07-14-2006
Size : 7 MB

Citating the part from the manual that I think is important (see down at the bottom of this post the speeds, and the note that it may be influenced by your ISP standard. Ask your ISP what they use.):

1.1

The ZyXEL Device is an ADSL2+ gateway that allows super-fast internet access over analog (POTS) or digital (ISDN) telephone lines (depending on your model).
In the ZyXEL Device product name, "R" denotes an integrated router and "D" denotes a chip set standard.
Your ZyXEL Device product name ends with a number. Models ending in "1", for example P660R-D1, a denote a device that works over the analog telephone system, POTS (plain old telephone service). Models ending in "3" denote a device that works over ISDN (Integrated Services Digital Network). Models ending in "7" denote a device that works over T-ISDN (UR-2)

1.2

The DSL RJ-11 (ADSL over POTS models) or RJ-45 (ADSL over ISDN models) connects to your ADSL-enabled telephone line. The ZyXEL Device is compatible with the ADSL/ADSL2/ADSL2+ standards. Maximum data rates attainable for each standard are shown in the next table:

ADSL:
Up: 832 kbps Down: 8Mbps
ADSL2:
Up: 3,5Mpbs Down: 12Mpbs
ADSL2+:
Up: 3,5Mbps Down: 24Mbps

Note: if your ZyXEL Device does not support Annex M, the max ADSL2/2+ upstream data rate is 1,2Mbps.
ZyXEL Devices which work over ISDN do not support Annex M. The standard your ISP supports determines the maximum upstream and downstream speeds attainable. Actual speeds attained also depend on the distance from your ISP, line quality etc.


Please let me know if the above answered your question:)