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PPP is a layer-2 protocol that only runs over a duplex dedicated or switched serial links. The circuit must operate in either asynchronous or bit-synchronous serial mode, transparent to PPP link layer frames. Originally PPP was designed for simple point-to-point connections over HDLC serial links. However, PPP is now compatible with all major WAN protocols (including ISDN, X.25, FR and SONET). PPP provides facilities for link quality testing, error detection, option negotiation for the network address layer, (optional) authentication, and compression.
Multi-link PPP (MLPPP) has been introduced (which bundles multiple links into one logical pipe). Extra PPP links can be added to an MLPPP connection without interrupting service. Using MLPPP, each PPP link has access to the aggregate bandwidth of all links together. Packets may be transmitted across different physical links or be fragmented across multiple physical links. Like PPP, MLPPP is independent of the underlying physical network (such as ISDN, and synchronous or asynchronous technologies) MLPPP can use many different WAN network technologies (such as FR, ISDN, X.25) simultaneously for the links. MLPPP can open links as needed. Links can be closed if there is little traffic. Inverse multiplexing is used for packet transfer.
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