Lecture 14: Point-to-Point Data Links

PSTN For Internet Access

Modems are nowadays most commonly used for dial-in Internet access, connecting to a dial-in router, a technology used by retail Internet Service Providers (see later) to deliver Internet access to isolated computer systems.

The usual configuration of a dial-in router is that hosts which connect via modem appear^[2] to be directly connected to the ISP's own LAN. Note also that the diagram is basically unchanged if an ISDN link is used: the modem at the customer premises is replaced with a TA, and the ISP has ISDN dial-in phone lines.

It's important to note in this diagram that the telecommunications service -- the phone line, sometimes called (in Australia) the "Basic Carriage Service" -- is provided by a "Telco": a telcommunications provider. This is conceptually separate from the service provided by the ISP: that of routing IP packets to and from the home user.

^[2] That is, the network (and subnet) parts of their IP address are the same as those of the computers on the ISP's internal LAN. The point-to-point link is "invisible" in terms of Internet routing.

previous | start | next