V90 in Practice


In my experience as a modem tester,I have found that the 56kbps is an ideal which in practice will never be achieved.This is a theoritical maximum.You're more likely to get between 45 and 52kbps throughputs. It is worth noting that although you initially connect at , say 50kbps, the connect speed will change, and a few minutes later, the connection has dropped to 42kbps.

Why is this so you may ask?

The actual throughput is highly dependent on line quality. the analogue link from the telco exchange and the modem will suffer from noise and interference,such as intermodulation distortion and echo which cause distortions in the phase frequency, and amplitude of the wave. Some of this noise is introduced by the telco's line repeaters and amplifiers, so the noisier the line, the slower the connect speed you can obtain. The distance from the telco


exhange is another crucial factor affecting line quality: the further away you are, the weaker the signal.

The speed reported by the modem is of little value. For example, K56flex modems are notorious for appearing to connect at a 'low speed' and then gradually retraining to faster speeds. Using ati6 command (in diagnostics, after a connection) on a 3Com/USR modem reveals more more connstuctive information.

For V90 to function, there must be only one analogue to digital conversion between the modem and the server.

Finally, it is VERY IMPORTANT that you are using the latest drivers for your modem, a useful site for K56 (using Rockwell or Lucent chipsets) is Zoltrix, these use generic drivers and seem to work every time.

Other Useful Sites for Modem Information

56k = Unreliable
Modem Help

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