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Home Troubleshooting Limiting Connect Speed 

Limiting Connection Speed (Updated 1-Dec-03)

Most modems can be set to connect only at a speed or range of speeds. Lucent (Apollo/Mars/LT Win Modem with V.90 firmware) are limited to autorate, or locking a single connect rate; the upstream rate can also be set by the user. In some situations with Lucent modems, lowering the upstream rate (to say 21.6k) will improve the downstream rate and reliability. This page shows the speed limit commands for:

3Com/USR Modems Broadcom BCM Modem Cirrus/Ambient Modems
ESS [Teledrive] ModemsLucent/Agere based ModemsMotorola SM56 Modems
PCTel-based ModemsRockwell/Conexant based ModemsSmartlink Modems
TOPIC Chipset ModemsVIA Chipset Modem

 Using speed limiting commands may be helpful if you experience variability in your CONNECT rates, and only want to connect on your higher speed connections, or if your modem is connecting at too high a speed so as to choke on errors. For example, I find that when calling my ISP, I'll either get a rate of 32000 or 46666-48000. I've set my dial-up networking to automatically re-dial up to 50 times with a 1-second re-dial delay, and added the &U&N settings to make my connection a minimum of 42666bps. If that rate cannot be achieved, no carrier is obtained, the line is disconnected, and the modem immediately re-dials. Sometimes, it may take 9 or more attempts before a good connection is obtained - and no user intervention is required to reject the low-speed poor connections and re-dial. There are also many people whose modems are "over aggressive" on connect speed who can eliminate disconnect and/or low throughput problems by limiting the maximum speed the modem connects to a slightly lower rate.

There's also a Freeware program - ConnectPro that, among other things, will re-dial if you don't get your specified minimum connect speed. (ConnectPro is available from Modemsite's Download Area.) Another freeware auto/re-dial program - CiDial.

3Com/USR Modems:

The commands: &Un and &Nn (where n is a number between 0 and 39) control the connect speeds your modem will accept. &U control the lowest link speed, and &N controls the highest link speed. If you want to use this, add the appropriate &Un&Nn string in the 'extra settings' box of the Advanced Properties of your Modem from Control Panel. (See the graphic of 56k Troubleshooting page 1.) My extra settings (for 42.6k minimum connect) are: &U28&N39.  IMPORTANT: If you use only &N, the modem will connect only at that speed, and if a connection cannot be achieved at that speed, you will get NO CARRIER. In most cases, you will want to set a speed range for acceptable connects using both &U &N.

I don't limit the highest connect speed - but you can: if you find that you get connects above 50k that are unreliable and error-prone, you can use the &Nn setting to limit the maximum speed.

Note: I have not been able to use this option reliably with x2 servers. It does work quite well for me with V.90. I suspect the reason is that with x2, I will regularly get CONNECTs at 48k that never go above 32k, while with V.90, my actual rate and connect speed are more accurate. 13-Dec-98: The command seems to be partially broken on USR/Sportster with new 5.0.0 firmware - see my 12/13 Update.

&U## - Lowest Link Speed
&N## - Highest Link Speed
Where ## is as follows:

Values for 3Com/USR/Sportster/Courier V.90 modems:
1= 300bps2= 1200bps3= 2400bps4= 4800bps
5= 7200bps6= 9600bps7= 12000bps8= 14400bps
9= 16800bps10= 19200bps11= 21600bps12= 24000bps
13= 26400bps14= 28800bps15= 31200bps16= 33600bps
17= 28000bps18= 29333bps19= 30666bps20= 32000bps
21= 33333bps22= 34666bps23= 36000bps24= 37333bps
25= 38666bps26= 40000bps27= 41333bps28= 42666bps
29= 44000bps30= 45333bps31= 46666bps32= 48000bps
33= 49333bps34= 50666bps35= 52000bps36= 53333bps
37= 54666bps38= 56000bps39= 57333bps0= No Limit

These are the possible values for n with a x2 (non-V.90) 3Com modem:

Values for 3Com/USR/Sportster/Courier x2-only modems:
0 = No Limit1=   300bps2=  1200bps3=  2400bps
4=  4800bps5=  7200bps6=  9600bps7= 12000bps
8= 14400bps9= 16800bps10= 19200bps11= 21600bps
12= 24000bps13= 26400bps14= 28800bps15= 31200bps
16= 33600bps17= 33333bps18= 37333bps19= 41333bps
20= 42666bps21= 44000bps22= 45333bps23= 46666bps
24= 48000bps25= 49333bps26= 50666bps27= 52000bps
28= 53333bps29= 54666bps30= 56000bps31= 57333bps
32= 64000bps   

Also see: Other extra settings.

Rockwell / Conexant Modems:

The command format is:
+MS= <mod> [,[<automode>][,[<min_rate>][,[<max_rate>][,[<x_law>][,[< rb_signaling>]]]]]]<CR>
Newer [RCV-PLL] chipsets have another variation of the above command with an extra field at the end of above which I believe is maximum upstream rate.
except for HCF and Soft56 modems where the command format is:
+MS=<modulation>,<automode>,<min tx rate>,<max tx rate>,<min rx rate>,<max rx rate>

You can determine the format of the +MS command for your modem with the command:
AT +MS=?   which returns all possible values for each field.  The command AT +MS? will return the current settings for each field.

To limit the Rockwell-chipset (Conexant chipset) modem to a minimum of 44,000bps and a maximum of 52k, the following string would be entered in extra settings:

+MS=,,44000,52000  (non HCF)
+MS=,,,,44000,52000  (HCF)

Note: If you place any commands after the +MS= command, you must terminate the +MS portion with a semicolon - ie:  +MS=12,0;s10=7

The preferred modulation can be selected by placing  the following parameter before the first comma:

ModemV.90K56FlexV.34 (no 56k)
Rockwell/Conexant Non-HCF125611
Rockwell/Conexant HCFV90K56V34

+MS=12,,44000,52000  (non HCF) - V.90 is preferred, connect min 44k, max 52k
+MS=V90,,,,44000,52000  (HCF) - +MS=V90,,,,44000,52000  (HCF) -    V.90 is preferred, connect min 44k, max 52k

The default for automode - the second parameter - is 1 which will allow other than the selected modulation; to force V.90 in the prior example:

+MS=12,0,44000,52000  (non HCF) - V.90 is required, connect min 44k, max 52k
+MS=V90,0,,,44000,52000  (HCF) -
   V.90 is required, connect min 44k, max 52k

With some firmware versions, the commands may be broken, or not work correctly. See the auto-redial section.

Lucent/Agere Modems:

New Lucent/Agere AMR and softmodems do not support the -v90= commands in the table below. They will accept the +MS= modulation commands with format similar to Rockwell. The older Lucent product with later firmware will support both the commands below as well as the +MS= format. Agere modems, when set to connect at a 56k speed, will attempt that rate. If it fails, the modem will then attempt a V.34 connection. See the auto-redial section for more on how this behavior can be used to reject and re-dial lower-speed connections.

Lucent modems supporting +MS= ,except AMR & Softmodem, conform to following:
+MS=protocol, auto, 0, upstream limit, 0, downstream limit
 where
protocol = V21,V22,V22B,V23,V32B,V34,V90,56K, or V92
auto = 0 or 1 - where 1 enables auto-protocol negotiation, 0 restricts protocol to specified value
upstream limit = 0 or valid rate between 300 and 33600; 0 = automatic
downstream limit = 0 or valid rate between 300 and 56000; 0 = automatic

AMR & SoftModems: +MS=protocol,auto,min dowstream, max downstream

The Lucent LT Win Modem (Apollo/Mars) flexibility for connect speed options as 3Com/USR and Rockwell/Conexant. It will either do autorate (default), or you can specify a single 56k speed to (try to) connect with. These commands also apply to V.92 connections as V.92 uses V.90 downstream:

     AT-V90=# where # represents the 56k V.90 downstream rate:
0 - V.90 disabled1 - Auto Rate2 - 280003 - 29333
4 - 306665 - 320006 - 333337 - 34666
8 - 360009 - 3733310 - 3866611 - 40000
12 - 4133313 - 4266614 - 4400015 - 45333
16 - 4666617 - 4800018 - 4933319 - 50666
20 - 5200021 - 5333322 - 54666*23 - 56000*

* - -v90=22 & 23 valid only with firmware 5.49 and higher.

The following commands are implemented on both the Lucent Apollo/Mars (LT Win Modem) and Venus (external/PCMCIA/int) modems:
   S38=#  where # disables or selects the KFlex downstream rate:
0 - disable Flex1 - Auto Rate2 - 32k3 - 34k
4 - 36k5 - 38k6 - 40k7 - 42k
8 - 44k9 - 46k10 - 48k11 - 50k
12 - 52k13 - 54k  
   s37=#  where # controls the upstream rate:
  0 - Maximum  3 -   300bps  5 -  1200bps  6 -  2400bps
  7 -  4800bps  8 -  7200bps  9 -  9600bps11 - 14400bps
13 - 19200bps14 - 21600bps15 - 24000bps16 - 26400bps
17 - 28800bps18 - 31200bps19 - 33600bps 

NOTE: With the Venus chipset, s38=# controls both the KFlex and V.90 downstream rate; the KFlex/V.90 preference is set as follows:

S109=#     0=Disable V90 (Flex only)    
           1=auto select V.90/KFlex
           2=Disable KFlex (V90 only)

For the VENUS chipset, s38=# has different meaning for a V.90 connection:

   S38=#  where # disables or selects the V.90 downstream rate for VENUS chipset:
 0 - disable V.90 1 - Auto Rate 2 - 28k 3 - 29.3k
 4 - 30.6k 5 - 32k 6 - 33.3k 7 - 34.6k
 8 - 36k 9 - 37.3k10 - 38.6k11 - 40k
12 - 41.3k13 - 42.6k14 - 44k15 - 45.3k
16 - 46.6k17 - 48k18 - 49.3k19 - 50.6k
20 - 52k21 - 53.3k22 - 54.6k23 - 56k

Also see: Other extra settings.

Motorola SM56 Modems:

SM56 modems are K56Flex and V.90 capable; To limit the speed, put %B# in extra settings where # is as follows:
%B0 - Max Rate%B1 -   300bps%B2 -  1200bps%B3 -  2400bps
%B4 -  4800bps%B6 -  9600bps%B9 - 14400bps%B11- 16800bps
%B12- 19200bps%B13- 21600bps%B14- 24000bps%B15- 26400bps
%B16- 28800bps%B17- 31200bps%B18- 33600bps%B19- 32000bps
%B20- 34000bps F%B21- 36000bps%B22- 38000bps F%B23- 40000bps
%B24- 42000bps F%B25- 44000bps%B26- 46000bps F%B27- 48000bps
%B28- 50000bps F%B29- 52000bps%B30- 54000bps F%B31- 56000bps
%B32- 58000bps F%B33- 60000bps%B34- 28000bps%B35- 29333bps
%B36- 30666bps%B37- 33333bps%B38- 34666bps%B39- 37333bps
%B40- 38666bps%B41- 41333bps%B42- 42666bps%B43- 45333bps
%B44- 46666bps%B45- 49333bps%B46- 50666bps%B47- 53333bps
%B48- 54666bps   
F-These rates are valid only for K56Flex connections
  Rates specified by %B34 through %B48 are valid only for V.90 connects.

Special Thanks to Phillip Julias IV for supplying this Motorola SM56 info.

PC Tel Chipset

To limit the speed on PCTel modems, you must select either V.90 (n0s37=14) or K56Flex (n0s37=13) along with the speed limit s34=# where # is as follows:

     For V.90 - n0s37=14s34=# where # is:
0 = 28000bps1 = 29333bps2 = 30666bps3 = 32000bps
4 = 33333bps5 = 34666bps6 = 36000bps7 = 37333bps
8 = 38666bps9 = 40000bps10 = 41333bps11 = 42666bps
12 = 44000bps13 = 45333bps14 = 46666bps15 = 48000bps
16 = 49333bps17 = 50666bps18 = 52000bps19 = 53333bps
20 = 54666bps21 = 56000bps  
      For K56Flex - n0s37=13s34=#:
0 = 32000bps1 = 34000bps2 = 36000bps3 = 38000bps
4 = 40000bps5 = 42000bps6 = 44000bps7 = 46000bps
8 = 48000bps9 = 50000bps10 = 52000bps11 = 54000bps
12 = 56000bps   
Note: If the PCTel modem is unable to achieve the rate specified, it will connect at the next lower rate which can be achieved.

Special Thanks to Phillip Julias IV for supplying this PCTel info.

Cirrus/Ambient Modems:

Cirrus Logic spun off its modem chipset business to Ambient Technologies. Ambient makes chipsets, not modems, and like Rockwell/Conexant has very poor support and documentation available from its website. While AT command documentation (in .pdf format) is on the web-site, the all-important SPEED LIMIT commands are not included in the Ambient/Cirrus documentation! 

A variation of the +MS= command is used in Cirrus/Ambient modems to select protocol as well as speed limits. Cirrus/Ambient makes a number of different chipsets with different capabilities, and the parameters for +MS= vary among the products. 

The command AT +MS=? will return the possible values for each of the fields in the command; the following comes from a MD5620:

at+ms=?
+MS:
(BELL103,BELL212,V21,V23,V22,V22B,V32,V32B,V34,V34S,X2,V90), (0,1),(0,300-33600),(0,300-33600),(0,2400-33600),(0,2400-57600)

Cirrus/Ambient makes a controller-based x2/V.90 modem chipset (MD565X) for ISA and external configurations which uses this command format:

+MS= <carrier>, <automode>, <min rate>, <max rate>, <min rx rate>, <max rxrate>

<carrier> = Modulation type
<automode> = 0 or 1;  1= enable or 0=disable automatic rate negotiation to obtain highest rate possible
<min rate> <max rate> = 0 for automatic or valid rates from 300-33600 for non 56k-connect, or upstream on 56k connect
<min rx rate> <max rxrate> = 0 for automatic, or valid rates from up to 33.6k for minimum, and 53.3k for maximum.

Cirrus/Ambient also makes:
MD57xx - single-chip integrated DSP and controller
MD566x - to replace MD565X - eliminates need for SRAM in modem circuitry
MD567x - Mac & Windows USB chipset

VIA Chipset Modems:

Via Chipset Modems implement the same MAXIMUM link speed command as Motorola SM56 Modems; The MINIMUM speed uses the same # values with %L##
%B0 - Max Rate%B1 -   300bps%B2 -  1200bps%B3 -  2400bps
%B4 -  4800bps%B6 -  9600bps%B9 - 14400bps%B11- 16800bps
%B12- 19200bps%B13- 21600bps%B14- 24000bps%B15- 26400bps
%B16- 28800bps%B17- 31200bps%B18- 33600bps%B19- 32000bps
%B20- 34000bps F%B21- 36000bps%B22- 38000bps F%B23- 40000bps
%B24- 42000bps F%B25- 44000bps%B26- 46000bps F%B27- 48000bps
%B28- 50000bps F%B29- 52000bps%B30- 54000bps F%B31- 56000bps
%B32- 58000bps F%B33- 60000bps%B34- 28000bps%B35- 29333bps
%B36- 30666bps%B37- 33333bps%B38- 34666bps%B39- 37333bps
%B40- 38666bps%B41- 41333bps%B42- 42666bps%B43- 45333bps
%B44- 46666bps%B45- 49333bps%B46- 50666bps%B47- 53333bps
%B48- 54666bps   
F-These rates are valid only for K56Flex connections
  Rates specified by %B34 through %B48 are valid only for V.90 connects.

VIA Chipset compression commands: %C0 = Compression disabled; %C1 - Enable Compression
\N Error-Correction Mode
\N0 Normal
\N1 Direct
\N4 LAP-M Only
\N6 Reliable
\N7 Auto-Reliable

ESS Teledrive Modems:

Modems with the ESS chipset (Teledrive) use the  +MS= command to control modulation and minimum/maximum connect speeds:

+MS= protocol,automode,minspeed,maxspeed
+MS= 17,0,40000,44000

where minspeed and maxspeed are valid rates between 300 and 57333bps.
In the example above, 17 is the code to specify V.90 modulation, and the zero disables automode which will prevent the modem from connecting if V.90 cannot be achieved.

Valid values for protocol include: 
0 = V.21 (300bps)          1 = V.22 (1200bps)       2 = V.22bis (2400bps)
9 = V.32 (9600bps)      10 = V.32bis (14.4k)      11 = V.34 (33.6k)
17=V.90                      64 = Bell 103 (300bps)

Broadcom BCM Modems:

Modems with the Broadcom BCM chipset use 2 S-registers to control maximum rate. S61 controls the maximum 56k downstream rate (V.90 or x2), and S37 controls the maximum upstream rate. If no 56k connection is obtained, S37 controls both the upstream and downstream maximums:

 To control maximum downstream rate, set S61=# where # is the value indicated below
0 - No Limit (default)3 - 28000bps4 - 29333bps5 - 30666bps
6 - 32000bps7 - 33333bps8 - 34666bps9 - 36000bps
10 - 37333bps11 - 38666bps12 - 40000bps13 - 41333bps
14 - 42666bps15 - 44000bps16 - 45333bps17 - 46666bps
18 - 48000bps19 - 49333bps20 - 51333bps21 - 52000bps
22 - 53333bps   
To control maximum upstream rate, set S37=# where # is the value indicated below
0 - No Limit (default)1 - 300bps5 - 1200bps6 - 2400bps
8 - 4800bps9 - 9600bps10 - 12000bps11 - 14400bps
12 - 7200bps13 - 16800bps14 - 19200bps15 - 21600bps
16 - 24000bps17 - 26400bps18 - 28800bps19 - 31200bps
20 - 33600bps   

Smartlink Modems:

Modems with Smartlink chipsets use the  +MS= command to control modulation and minimum/maximum connect speeds:

+MS= protocol,automode,minspeed,maxspeed
+MS= 90,1,300,56000 (default)

TOPIC Chipset Modems:

The TOPIC Chipset allows user to specify a maximum connection rate (not a range of rates) by using the *i# command; the limit selected depends upon whether the modem is set for PCM (V.90) or not:

To control maximum speed for V.34 & lower connection protocols use: *i# where # is:
0 - 1200 bps1 - 2400bps2 - 4800bps3 - 7200bps
4 - 9600 bps5 - 12000bps6 - 14400bps7 - 16800bps
8 - 19200bps9 - 21600bps10 - 24000bps11 - 26400bps
12 - 28800bps13 - 31200bps14 - 33600bps19 - 31200bps
20 - 33600bps   
To control maximum speed for V.90 (PCM) connection protocol use: *i# where # is:
1 - 28000bps2 - 29333bps3 - 30666bps4 - 32000bps
5 - 33333bps6 - 34666bps7 - 36000bps8 - 37333bps
9 - 38666bps10 - 40000bps11 - 41333bps12 - 42666bps
13 - 44000bps14 - 45333bps15 - 46666bps16 - 48000bps
17 - 49333bps18 - 50666bps19 - 52000bps20 - 53333bps
21 - 54666bps22 - 56000bps  

 

Rockwell & Lucent Modems - Autoredial:

Jim Bell found a solution to allow auto re-dial with Lucent modems (that lack effective speed range commands) and some Rockwell V90 firmware where the +MS command is 'broken':

1.  First, find the highest speed at which your modem connects and gives stable connections.  If your modem speed varies, this should be done only after numerous logons to your ISP.

2.  In Dial Up Networking, Click "Connections" and then "settings".  Check the Redial box.  Under this change "Before giving up retry" to 50 times.  Press "OK".

3.  In Dial-Up Networking, right click your internet connectiod and hit properties.  Under your modem listing, hit "Configure".   Set Maximum Speed to 115200.  Select the Connection tab.  Hit "Advanced".  Change your modem speed by adding the following command in the Extra Settings box:

  Rockwell Modems (except HCF):  +MS=12,1,xxxxx,56000
    where xxxxx is the maximum speed where you get a stable connection, e.g. 49333 or 52000
     Note that 12 is used for V.90 modems.  Change this to 56 for flex connections. (NOTE: If your modem is connecting at too high a speed - ie, high error, unstable connection, limit the maximum speed by replacing the 56000 with a lower V90 speed.)

Rockwell HCF Modems: +MS=V90,1, <min tx rate>, <max tx rate>, <min rx rate>, <max rx rate>
   you can specify minimum and maximum values for both receive & transmit rates; to limit the receive rate to 42-48k with no other limits: +MS=V90,1,,,42000,48000

  Lucent Modems:  -V90=#    or     S38=#  or +MS=
     Find the appropriate selection for # in the Lucent section above.

Hit "OK", "OK", and "OK".

4.  Engage your internet connection.  After hitting "Connect", listen to the modem dial and pay attention to the handshaking sequence.  If your modem does not reach the desired speed, you will hear it begin another handshaking sequence.  Time the seconds it takes between hitting the "Connect" button and the second handshaking sequence.  Add 2 to 3 seconds to this time.
In Dial-Up Networking, again right click your internet connection and select "Properties" and "Configure".  Select the "Connection" tab.  Check the box next to "Cancel the call if not connected within xx seconds" and change the xx to the above number you observed when listening to your modem.  Hit "OK" and "OK" and try reconnecting. If you can't find this setting in your version of Windows, you can also set the time via your init string: s7=## where ## is the time in seconds to allow the modem to complete handshake.
     Your modem should now redial until it gets the rate you specified.  Be sure to listen when the modem cuts off before redialing to make sure that a second handshaking sequence begins.  If it quits too soon, adjust the seconds upwards.
     Also note that adjusting your extra settings box will affect dialing in Hyperterminal.  If you connect to other modems using Hyperterminal which are slower, hit "Cancel" at the dialing prompt, and then Enter.  Issue the command to reset your modem (atz in Rockwell modems) and dial manually (e.g. atdt5551212).


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