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3D Spotlight : Tweaking : Tweaking your modem (Windows Millennium edition)


Tweaking your modem
Last Updated on September 18, 2000 by Thomas McGuire - Page 2/10

Init strings

In the Extra settings field I suggest entering the following Init string into the space provided (NOTE - The strings below is intended for 3com/US Robotics modems, do NOT use it on non-3Com/USR modems, refer to your modem manual for appropriate strings). You can find modem Init strings for other brand modems at 56k.com. You should try to reference it with the appropriate version of the ones recommended below.


US Robotics Courier owners with a 25Mhz clock (not on earlier models) should use this AT%G1&K3S15=2S11=38S27=16S0=0

Explanation of settings

Here is an explanation of each string setting, as taken from the modem manual.

AT%G1. Enables a higher DTE rate of 230400. This may not work with all modems, unless they have a 25Hz clock speed, or fast serial port connection. Refer to your manual if unsure.

&K3. Disables MNP5 compression (which often adds latency to connections).

S15=2. Disables retrains.

S11=38. This sets the duration & spacing, in ms, for tone dialling (You can increase/decrease this value as you wish by changing the 38).

S27=16. Disables MNP 2-4 compression.

S0=0. Disables Auto-answer.

Users of Diamond Supra modems might want to try the following Init string. This string however needs some testing. It is similar to the 3COM/USR string above. Email me with your experiences using this string, or if you have a better one.


MTU, TTL & DefaultRcvWindow

First a few definitions;

MTU. Maximum Transmission Unit. This specifies the maximum transmission unit size of a packet. A packet larger than the MTU size will be fragmented into smaller packets. Using the (Maximum) MTU your ISP allows can result in less fragmentation of packets.

TTL. Time To Live. This sets the number of hops a packet is allowed to pass between "you" & it's destination.

DefaultRcvWindow. This sets the size of the TCP send & receives windows, which is the amount of data that can be accepted in a single transaction.

Before setting your MTU you must find it. To start with you should set your MTU to a high value. This way the following procedure will be accurate, i.e. If you MTU is currently 576 then the procedure won't find an MTU an larger than 576. Basically skip to the Setting MTU/TTL/RWIN section & set your MTU to 1500 or greater.

Click on Start, Programs, Accessories, MS-DOS prompt once connected to the Internet. Type in ping f l xxxx www.yourisp.com. Replace www.yourisp.com with whatever address your ISPs is (Or the IP address). Replace xxxx with numeric values until you get the message Packet needs to be fragmented by DF set. Use whatever the largest value is before that error message & add 28 to it to find your MTU.

f. Sets a Dont Fragment flag in packet.

l xxxx. This sends a buffer sized xxxx.

E.g. ping f l 1472

1472 + 28 = 1500. As a result Id set my MTU to 1500 as higher values return the fragment message.

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