|  | 4.2 | PPP Session Establishment
| 4.2.1 | The four phases through which PPP establishes a point-to-point connection
PPP provides a method of establishing, configuring, maintaining, and terminating a point-to-point connection. In order to establish communications over a point-to-point link, PPP goes through four distinct phases: Link establishment and configuration negotiation-An originating PPP node sends LCP frames to configure and establish the data link. Link-quality determination-The link is tested to determine whether the link quality is sufficient to bring up network-layer protocols. Note that this is an optional phase. Network-layer protocol configuration negotiation-The originating PPP node sends NCP frames to choose and configure network-layer protocols. The chosen network-layer protocols-such as IP, Novell IPX, and AppleTalk-are configured, and packets from each network-layer protocol can be sent. Link termination-The link remains configured for communications until LCP or NCP frames close the link or until some external event occurs (for example, an inactivity timer expires or a user intervenes). There are three classes of LCP frames: Link establishment frames-Used to establish and configure a link. Link termination frames-Used to terminate a link. Link maintenance frames-Used to manage and debug a link. LCP frames are used to accomplish the work of each of the LCP phases: (1) Link establishment; (2) Link quality; (3) Network-layer protocol; (4) Link termination. These phases will be described in the following sections.