Lecture 14: Point-to-Point Data Links

Dial-In "Access" Technologies

A modem link is the most basic Internet Access Technology -- enabling isolated computers (and/or networks) to link to an Internet Service Provider (ISP). Terminology: we call this dial-in Internet Access, and the modem connection is made to a dial-in router at the ISP premises.

The usual configuration of a dial-in router is that hosts which connect via modem appear to be directly connected to the ISP's own LAN -- in effect, only one end of the dial-in link has an associated IP address^[2].

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] The network (and subnet) parts of the dial-in machine's 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.

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