PPTP allows existing Network Access Server (NAS) functions to be separated using a client-server architecture. Traditionally, the following functions are implemented by a NAS:
1) Physical native interfacing to PSTN or ISDN and control of external modems or terminal adapters.
A NAS may interface directly to a telco analog or digital circuit or attach via an external modem or terminal adapter. Control of a circuit-switched connection is accomplished with either modem control or DSS1 ISDN call control protocols.
The NAS, in conjunction with the modem or terminal adapters, may perform rate adaption, analog to digital conversion, sync to async conversion or a number of other alterations of data streams.
2) Logical termination of a Point-to-Point-Protocol (PPP) Link Control Protocol (LCP) session.
3) Participation in PPP authentication protocols [3].
4) Channel aggregation and bundle management for PPP Multilink Protocol.
5) Logical termination of various PPP network control protocols (NCP).
6) Multiprotocol routing and bridging between NAS interfaces.
PPTP divides these functions between the PAC and PNS. The PAC is responsible for functions 1, 2, and possibly 3. The PNS may be responsible for function 3 and is responsible for functions 4, 5, and 6. The protocol used to carry PPP protocol data units (PDUs) between the PAC and PNS, as well as call control and management is addressed by PPTP.
The decoupling of NAS functions offers these benefits:
Flexible IP address management. Dial-in users may maintain a single IP address as they dial into different PACs as long as they are served from a common PNS. If an enterprise network uses unregistered addresses, a PNS associated with the enterprise assigns addresses meaningful to the private network.
Support of non-IP protocols for dial networks behind IP networks. This allows Appletalk and IPX, for example to be tunneled through an IP-only provider. The PAC need not be capable of processing these protocols.
A solution to the "multilink hunt-group splitting" problem. Multilink PPP, typically used to aggregate ISDN B channels, requires that all of the channels composing a multilink bundle be grouped at a single NAS. Since a multilink PPP bundle can be handled by a single PNS, the channels comprising the bundle may be spread across multiple PACs.