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I'm a bit puzzled about the virtual COM ports used by modern high speeds USB devices that creates several serial interfaces, one of which is used for data transfer. A modern 3G/4G USB modem can reach speed over 20 Mbps, but the virtual COM ports that they use to actually transfer the data, have their baud rate set to 921600 symbols/s.
Do they use some high level bits/symbol modulation ? If yes, which ones?

  • I don't know the actual answer, but a good guess: compression. – Tyson Jul 27 '14 at 0:34
  • @Tyson Yes, is channel compression (not data compression). Instead of using only 2 symbols (0s and 1s), they use more symbols those achieving higher data rates that is multiple of the baud rate. What I would like to know is more details on this matter and how can drivers handle this. There is no special driver used for this virtual COM ports, only a generic one, so it must be a standard. – Chris Jul 27 '14 at 8:05
  • As far as I know, the VCP baudrate,start bits, stop bits, parity etc is only meaningful when the VCP represents a real UART. In that case, the baudrate is used to setup the baudrate of the UART, which affects the rate which bits are transmitted/received on the serial line. On the other hand if the VCP is truly virtual, the baudrate,stop bits,parity, etc can be ignored. The OS/kernel will deliver data to the USB device as fast as it can and vice versa. The USB device can control the rate by ACKing and NAKing IN/OUT transactions as appropriate. – Wayne Uroda Dec 9 '16 at 0:05
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Newer high-speed USB modem devices don't actually transfer data over virtual serial ports. They use them as a control channel to send AT commands to the "modem" but once they establish a network connection they create another virtual device that identifies itself as an ethernet adapter.

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  • My modem (as most of them) doesn't have a QMI interface. The pppd service uses interface ttyUSB0 (which is a serial interface; driver option) for connecting (with AT commands) and transferring data after the modem switched in data mode, after the AT dial command(ATD*99***1#). – Chris Jul 27 '14 at 17:33
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Compression is one factor in speeding up communication, another factor is sending several bits at once by modulation

"baud rate" is sloppishly used for many things.
The actual technical term is "symbol changes per second" and is directly related to modulation (above)
Please note the difference to "bit transfer rate per second"
so technically these are NOT THE SAME and has never been.

If you say "100 mega-bits per second" (the throughput/transfer rate) you're not telling a single 'bit' about the baudrate.

So "bit" and "baud" are mutually exclusive, unless "modulation" is also mentioned.


Nowadays the actual used "baudrate" is almost always LOWER than the throughput.

If you can find an old analogue 1200 bps modem, then you're likely have a device that has the same baudrate and throughput.

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