Some modem basics
Bit - binary digit, byte - 8 bits, this gives 256 combinations. ASCII has 126 characters therfore 7 bits is sufficient to handle them. This means the eigth bit is always 0 when handled by the computer.
The modem can send either 7 bit or 8 bit ASCII though 8 bit is more common.
Serial data - 1 bit at a time
Parallel data- one byte at a time
Internal data bus typically exists of eight wires, each carrying a bit which is read simultaneously, i.e. in parallel
Because a phone line is only 2 wires, data has to be transmitted one bit at a time, i.e serially
Aync- use stop and start parity bits enclosing each byte.
Synchronous- more uncommon, like a pipeline sending strams of bytes with no parity bits. Normally transmission is managed by special protocols