Analog Channel
A circuit-switched communication path which is intended to carry 3.1 Khz audio in each direction.
Digital Channel
A circuit-switched communication path which is intended to carry digital information in each direction.
Call
A connection or attempted connection between two terminal endpoints on a PSTN or ISDN--for example, a telephone call between two modems.
Control Connection
A control connection is created for each PAC, PNS pair and operates over TCP [4]. The control connection governs aspects of the tunnel and of sessions assigned to the tunnel.
Dial User
An end-system or router attached to an on-demand PSTN or ISDN which is either the initiator or recipient of a call.
Network Access Server (NAS)
A device providing temporary, on-demand network access to users. This access is point-to-point using PSTN or ISDN lines.
PPTP Access Concentrator (PAC)
A device attached to one or more PSTN or ISDN lines capable of PPP operation and of handling the PPTP protocol. The PAC need only implement TCP/IP to pass traffic to one or more PNSs. It may also tunnel non-IP protocols.
PPTP Network Server (PNS)
A PNS is envisioned to operate on general-purpose computing/server platforms. The PNS handles the server side of the PPTP protocol. Since PPTP relies completely on TCP/IP and is independent of the interface hardware, the PNS may use any combination of IP interface hardware including LAN and WAN devices.
Session
PPTP is connection-oriented. The PNS and PAC maintain state for each user that is attached to a PAC. A session is created when end-to-end PPP connection is attempted between a dial user and the PNS. The datagrams related to a session are sent over the tunnel between the PAC and PNS.
Tunnel
A tunnel is defined by a PNS-PAC pair. The tunnel protocol is defined by a modified version of GRE [1,2]. The tunnel carries PPP datagrams between the PAC and the PNS. Many sessions are multiplexed on a single tunnel. A control connection operating over TCP controls the establishment, release, and maintenance of sessions and of the tunnel itself.