Most computers today use PPP as the data-link protocol when using modems. Modems essentially provide an OSI Layer 1 service, supporting the transmission and reception of a serial bit stream. In fact, a dialed circuit between two modems creates a physical network that has a lot of similarities with a leased point-to-point circuit. Also, analog modems typically transmit traffic asynchronously. PPP supports both synchronous communication, as typically is done over leased point-to-point lines, as well as asynchronous communication, which typically is done over dialed circuits using modems. So, PPP is the logical choice for a data-link protocol when using modems today.
PPP includes some features that are important when using modems to dial into an ISP. PPP includes the capability of dynamically assigning an IP address to a device on the other end of the PPP link. So, when you dial into an ISP, the ISP dynamically assigns an IP address to your computer. Also, PPP supports that Challenge Handshake Authentication Protocol (CHAP), which popularly is used to allow the dial-in user to supply a username and password to gain access to the ISP network. (CHAP is covered in the CCNAICND Exam Certification Guide.)
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