Once hardware connectivity is in place, the PPP provides the bridge between the Linux networking stack and the modem. The PPP daemon (pppd) is responsible for establishing and managing a Point-to-Point Protocol session over the serial port of the modem. This negotiates the PPP session, performs authentication if required (such as CHAP or PAP), and sets up the ppp0 network interface on the host. This includes assigning an IP address, configuring routing, and setting DNS servers so that the system can send and receive IP traffic over the connection.

The modem is controlled through AT commands to register with the cellular network and start a data session. AT commands are an industry standard, providing a text-based interface to the functionality of the modem. For example, AT+CGDCONT=1,"IP","<APN>" configures the PDP context with the desired APN, and ATD*99# initiates the actual connection. Once the modem returns CONNECT, the PPP session becomes active and the modem functions as a transparent link-layer pipe, carrying encapsulated IP packets over the cellular network.

Data from the operating system flows through the ppp0 network interface. The IP packets are encapsulated by PPP and transmitted over the serial link to the modem. The modem converts these packets into cellular data and communicates with the GSM or LTE network, sending and receiving data to the wider internet. In this way, the PPP session bridges the IP layers of the host with the cellular network through the serial interface to the modem.