2.1 Modem Functions
2.1.4 Modem operation
Modems perform their basic operations as follows (in one direction) and as seen in the Figure:
  1. Outgoing data from an originating DTE comes into the sending modem via the TxD pin.
  2. If the sending modem’s buffer is nearly full, the modem can control flow (via hardware) by lowering the CTS signal, thus instructing the DTE to not use TxD.
  3. The data is compressed by using a proper algorithm (MNP 5 or V.42bis) that is mutually agreed upon between the two communicating modems.
  4. The data is then packetized, where windowing, checksum, error control (using MNP 4 or LAP-M), and retransmission are performed.        

Note: Here, the term packetized is not referring to an IP packet or Layer 3 protocol data unit (PDU). Rather, it refers to the preparation of the data by the modem.

  1. The digital data is modulated into analog signals and sent out through the telephone network.
  2. When the data reaches the receiving modem, it goes through the same steps in reverse order. The signal will be demodulated, and the data will be depacketized, decompressed, and delivered to the destination DTE. The DTE can use RTS to indicate that it is unable to receive data on RxD.