How to go as fast as you can with a modem

Choose the right modem

Modems that support the same protocols vary in performance quite a bit. The differences come in a couple of different areas: Port speed supported. V.34 and V.32bis modems all support the V.42bis compression protocol. This protocol can achieve 4 to 1 compression of data when sending text. This means that a 14400bps modem that is sending text can actually get close to 57600bps. Similarly, a 28800bps modem can get close to 115200bps.

Many modems are limited by the speed with which they can talk to the computer: not all 14.4 modems can talk at 57.6 and not all 28.8 modems can talk at 115.

Ability to handle compression. Compressing data is hard work and not all modems have CPUs that can do it quickly. With most modems, transmissions are delayed in order to compress them. The faster the CPU on the modem, the less delay.

Fall forward, fall back. When lines get noisy, modems fall back to slower modulations. When the lines clear up, only some modems resume operating at a higher speed.

Idiom supports 56kbps modems using Lucent's K56Flex technology and V.90.

Flow Control

Flow Control It is very important to use hardware flow control between your computer and your modem. Flow control is required because sometimes your computer will not be able to keep up with the data that Idiom sends it and sometimes your modem will not be able to keep up with the data that your computer sends it.

Software flow control can be used, but it is less efficient and can cause many problem because the characters used for software flow control are also used to send data. If you are using PPP, it is possible to tell PPP not to use certain characters. The characters to avoid are control-S and control-Q. Port

Port Speed

Speed As mentioned above, a 14.4kpbs modem can send data at 57,600bps and a 28.8kpbs modem can send data at 115200bps. Not all computers can keep up with 38400, let alone 115200. Keeping up requires both the right hardware and the right software. On PC's, the right hardware is 16550 serial chips instead of 16450 serial chips.

For software, something better than Chameleon Sampler is required. With slower computers, Windows cannot always keep up. There is a patch for the windows serial driver that can sometimes speed things up. The easiest thing to do is to keep raising your port speed until your modem/computer stops working. Start with 38400, try 57600. If that works, try 115200. If 115200 doesn't work, try 76800.

In general: V.42bis compression should be on; hardware flow control should be on.

About K56Flex & V.90 Connections

Not all telephone lines can support higher speed connections. Connections with K56Flex & V.90 modems typically are at 40kbps -> 44kbps but not higher.