Is 56kbps really transmitting 56kbits in 1 sec?

[Desmond Kwan]

I've written a routine to read in from the COM1 port. However, the bitrate cannot be met. If baud rate is set to 56kbps, does that mean that at the receiving end, I should see 7000 bytes (or characters) every second? I'm seeing only about 50% of the data.

[Manish Malik]

Are you sure you are talking about "baud rate" and not "bit rate" of 56 kbps? It should be bit-rate.

Anyway, theoretically, the maximum transfer speed of a 56 Kbps modem is approx. 53 Kbps, and usually it wanders in between 33 Kbps and 53 Kbps on most telephone lines.

Read this for more detail: http://www.skypoint.com/services/56k.html

[Radius]

Ok, modem theory in a nutshell. :)

The transfer rates on modems are in k-bits/sec. This means in theory a 56K modem can transfer 56, 000 bits a second. However, this doesn't mean you can transfer 7000 characters a second.

Remember, there are start and stop bits, so if your modem is configured as 8, N, 1 the modem automatically generates one start bit and one stop bit, giving you a total of 10 bits per character. This number can increase depending on other factors, like number of stop bits and such. So, for a standard 56K connection, optimal performance would give you 5600 characters a second.

Unfortunately, these speeds are rarely realized due to the quality of phone lines, and in some cases quality goes down depending on if you are using modem compression protocols to send an already compressed file. This makes the data somewhat larger at times, so it takes even longer. This last point is rarely a factor on modern modems however.

Hope this helps make things a little more clear. :)

[Chris Richardson]

There's also the fact that the FCC won't allow transfer of data at 56 kilo-bits. Most modems of 56K can only transmit (in a perfect situation) 53K per second.

Chris Richardson

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