Chapter Summary

Modems allow data to be sent over the PSTN. To do so, one modem calls the phone number of another modem. After the phone company has set up the call, it can send data by putting a particular analog signal on the circuit to mean binary 0, and another to mean binary 1.

Often, modems are used to send and receive data to and from a router that an ISP owns. By doing so, a PC at a house can connect to the Internet, thereby enabling it to send and receive IP packets from almost every IP host in the world. To keep the service inexpensive, ISPs try to locate a router in a POP in each local calling area.

DSL allows data to be sent using digital signals, over the same telephone line to the home. This allows you to use the Internet while also talking on the phone. To accommodate this service, the telco connects the local loop line to a DSLAM at the CO. The DSLAM splits the analog voice out to the telephone switch and the data over to a router that the ISP owns.

Cable modems use the same basic model as DSL, with a device at the CATV's office splitting out the data from the television traffic. Cable modems use the CATV cable for transmission of the data, using frequencies that would otherwise be used for more TV channels. Cable modems allow you to make phone calls, watch TV, and surf the Internet simultaneously.