56K Modem Throttle Adjustments:

Fine-tuning your modem's performance

Most 56K modems are designed to find the fastest stable speed and connect there, automatically. Some models have trouble with this, however, and may try to use a higher speed than your line can support. If your modem does this, you may see lots of pauses and stalled downloads.

Fortunately, there is a workaround for this. If your dialer allows custom init strings (Microsoft Dial-Up Networking does, as does FreePPP for Mac), you can fine tune your modem for a particular range of connect speeds.

The bad news: there's no standard "throttle setting" for all modems. These change from brand to brand. We've included settings for the brands we know here. If you have throttle settings for a model we don't list, please let us know.

• Common commands
• Modem-specific settings

Most V.90 modems will also support one of the two proprietary 56K protocols (KFlex or X2). Within these two categories, some init string settings are consistent:

Setting accurate connect-speed reporting:

Most modems come from the factory set to display the "DTE" speed when connected. This is the speed between your computer and your modem, and is pretty much meaningless.

(This is also why your modem might seem to be going at outrageous speeds like 115K.)

If you want to see the actual "DCE" speed (the actuall speed of the modem-to-modem link) in the "Connected" display, you need to add a custom command to the init string.

For most brands, the W2 command will do this. There are a few exceptions:

• Motorola ModemSURFR models use the \V0 command instead.
• US Robotics, Megahertz, and 3Com modems don't seem to contain a way to change this. Sorry.

Setting V.90/X2 modems for X2-compatibility mode:

All modems with a US Robotics-affiliated chipset (all X2 or V.90/X2 modems) use the S32 register to control the connection type. Here are the settings:

S32=2 enables all available connect protocols.
S32=34 tells it to use V.90 instead of X2.
S32=66 tells it to use X2 instead of V.90.
S32=98 disables both X2 and V.90 (drops modem to 33.6K).
S32=114 forces modem to connect at 28.8K or below.

This can be very useful for troubleshooting.

If you have trouble connecting at V.90, try S32=66 to force X2. We've had a bunch of reports that X2 seems more stable than USR's V.90 with our equipment.

USR-affiliated modems all use the &N and &U AT commands to set the allowed speed range for X2 and V.90. If your modem isn't listed in the modem-specific section of this page, just look up those commands in your modem's user manual.

This section contains speed-control tables for models we've dealt with here at Panix, as a courtesy. If you have your modem's user manual, we recommend that you use its speed table, or at least check it against our copy before you start. (Modem makers love to change these things.)

X2 and X2/V.90 modems:

Connection speed is controlled with the &N and &U AT commands. &N sets the maximum speed, &U is the minimum.

If you declare a maximum speed with &N, be sure to declare a value to &U as well. (If you leave &U at 0, it'll take the &N value as the only allowed speed.

(In other words, if you customize &N, be sure to set &U to something above 0 too.)

Details for specific brands' &N and &U settings:

Cardinal X2-only: 14 = 28,800 bps V.34 15 = 31,200 bps V.34 16 = 33,600 bps V.34 17 = 28,800 bps X2 18 = 31,200 bps X2 19 = 33,600 bps X2 20 = 33,333 bps X2 21 = 37,333 bps X2 22 = 41,333 bps X2 23 = 42,666 bps X2 24 = 44,000 bps X2 25 = 45,333 bps X2 26 = 46,666 bps X2 27 = 48,000 bps X2 28 = 49,333 bps X2 29 = 50,666 bps X2 30 = 52,000 bps X2 31 = 53,333 bps X2 32 = 54,666 bps X2 33 = 56,000 bps X2 34 = 57,333 bps X2

US Robotics V.90/X2: 14 = 28,800 bps V.34 15 = 31,200 bps V.34 16 = 33,600 bps V.34 17 = 28,800 bps V.90 18 = 29,333 bps V.90 19 = 30,666 bps V.90 20 = 32,000 bps V.90 21 = 33,333 bps V.90 22 = 34,666 bps V.90 23 = 36,000 bps V.90 24 = 37,333 bps V.90 25 = 38,666 bps V.90 26 = 40,000 bps V.90 27 = 41,333 bps V.90 28 = 42,666 bps V.90 29 = 44,000 bps V.90 30 = 45,333 bps V.90 31 = 46,666 bps V.90 32 = 48,000 bps V.90 33 = 49,333 bps V.90 34 = 50,666 bps V.90 35 = 52,000 bps V.90 36 = 53,333 bps V.90 37 = 54,666 bps V.90 38 = 56,000 bps V.90 39 = 57,333 bps V.90

US Robotics X2-only: 14 = 28,800 bps V.34 15 = 31,200 bps V.34 16 = 33,600 bps V.34 17 = 28,800 bps X2 18 = 37,333 bps X2 19 = 41,333 bps X2 20 = 42,666 bps X2 21 = 44,000 bps X2 22 = 45,333 bps X2 23 = 46,666 bps X2 24 = 48,000 bps X2 25 = 49,333 bps X2 26 = 50,666 bps X2 27 = 52,000 bps X2 28 = 53,333 bps X2 29 = 54,666 bps X2 30 = 56,000 bps X2 31 = 57,333 bps X2

For example, if you want to tell your modem to connect somewhere between 28,800 and 52,000, just use the following init string:

AT&F1&N30&U14

If you still have trouble connecting, just reduce the &N value until you get solid performance.

Modems with Lucent Chipsets:

Lucent makes the chipset for the following modem manufacturers:

ActionTec
Multitech
Xircom
Zoom

These all support KFlex, and the newer models do V.90 too.

Some of these modems have trouble connecting, when they try to negotiate a speed higher than the line can handle.

The syntax for manual speed adjustments is different between KFlex and V.90 mode, however. We'll start with the V.90; if your modem supports only KFlex, just jump down to the KFlex section.

To check your Lucent V.90 modem, start by using this init string:

AT&FS38=0-V90=21

This sets your modem for a V.90 connection at a maximum of 53333 bps. ("AT&F" loads the factory settings. "S38=0" turns off KFlex. "-V90=21" sets the V.90 rate to 53333.)

If your connection is still choppy, start lowering the "-V90=" number one at a time, until you get a reliable connection. Here's the chart of "-V90" settings and corresponding speeds:

    0 ..... disables V.90 (defaults to V.34)
    1 ..... sets V.90 for "Auto Rate" (default)
    2 ..... sets V.90 for 28000 bps
    3 ..... sets V.90 for 29333 bps
    4 ..... sets V.90 for 30666 bps
    5 ..... sets V.90 for 32000 bps
    6 ..... sets V.90 for 33333 bps
    7 ..... sets V.90 for 34666 bps
    8 ..... sets V.90 for 36000 bps
    9 ..... sets V.90 for 37333 bps
    10 .... sets V.90 for 38666 bps
    11 .... sets V.90 for 40000 bps
    12 .... sets V.90 for 41333 bps
    13 .... sets V.90 for 42666 bps
    14 .... sets V.90 for 44000 bps
    15 .... sets V.90 for 45333 bps
    16 .... sets V.90 for 46666 bps
    17 .... sets V.90 for 48000 bps
    18 .... sets V.90 for 49333 bps
    19 .... sets V.90 for 50666 bps
    20 .... sets V.90 for 52000 bps
    21 .... sets V.90 for 53333 bps
  

Lucent modems with KFlex-only chipsets don't have a "-V90" setting, of course. For these modems, the S38 register controls the top speed.

If you have trouble maintaining a stable connection, start with this init string:

AT&FS38=12

This sets your modem to a maximum speed of 52 Kbps. If you still have trouble, just decrease that final number by 1 until the trouble disappears. (S38=11, 10, 9, etc.)

Here's the list of allowable settings for S38:

    0 ..... disables KFlex (defaults to V.34)
    1 ..... sets KFlex for "Auto Rate" (default)
    2 ..... sets KFlex for 32 Kbps
    3 ..... sets KFlex for 34 Kbps
    4 ..... sets KFlex for 36 Kbps
    5 ..... sets KFlex for 38 Kbps
    6 ..... sets KFlex for 40 Kbps
    7 ..... sets KFlex for 42 Kbps
    8 ..... sets KFlex for 44 Kbps
    9 ..... sets KFlex for 46 Kbps
    10 .... sets KFlex for 48 Kbps
    11 .... sets KFlex for 50 Kbps
    12 .... sets KFlex for 52 Kbps
    13 .... sets KFlex for 54 Kbps
    14 .... sets KFlex for 56 Kbps
  

Modems with Rockwell Chipsets:

Rockwell makes chipsets for these brands:

Best Data
Boca
Creative Labs (DigiCom)
Diamond/Supra
Global Village
Hayes (Hayes, Practical Peripherals, Cardinal)
Maxtech
TDK
Viking
Xircom
Zoom

This chipset uses a different command set from the Lucent one. Both KFlex-only and V.90 speeds are controlled by the "+MS" command.

A sample init string for a Rockwell modem would be:

AT&F+MS=12,1,28000,56000

To break this down:

• The AT&F sets the modem for factory defaults.
• The +MS= activates the command for 56K protocol choosing and speed control.
• The first number (a 12 in this case) tells the modem which protocol to use:
  12 sets the modem to V.90 mode.
  56 sets the modem to KFlex mode.
• The second number should always be 1.
• The third and fourth numbers dictate the lowest and highest allowed speeds. I recommend keeping the low speed at 28000.

  If your connection is choppy, you can decrease the highest allowed speed until you get a reliable link every time.

  Allowable speeds for a V.90 (type "12") setting:
  28000, 29333, 30667, 31200, 32000, 33333, 33600, 34000, 34667, 36000, 37333, 38000, 38667, 40000, 41333, 42000, 42667, 44000, 45333, 46000, 46667, 48000, 49333, 50000, 50667, 52000, 53333, 54000, 54667, 56000.

  Allowable speeds for a KFlex (type "56") setting:
  28000, 30000, 32000, 34000, 36000, 38000, 40000, 42000, 44000, 46000, 48000, 50000, 52000, 54000, 56000.

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