Dial-Up Modems
Dial-Up Modem Network Architecture
Before the internet, computers were connected to other computers via the Public Switched Telephone Network (PSTN). As shown in figure a below, the modem gets the digital information from the computer, converts it to analog representation and sends it to the telephone company Central Office (CO). At the CO the data is converted from analog to digital and then transferred to the CO located near the destination computer.
Today, users connects other computer via the internet, as described in figure b below.
The computer connects to the telephone company central office as in figure a (with D/A and A/D converters), but then the data is transferred to the ISP, in contrast to figure a, where it connects to other CO.
Note that the ISP is connected to the CO of the PSTN via digital lines. This type of connection makes the 56 Kbps technology possible, as described later in this chapter.
As mentioned earlier in this chapter, dial-up modems technology enables maximum data rate of 56 Kbps. This restriction exist because of "historical reasons". Long ago, the telephone companies decided that voice frequency range that will be transmitted over telephone lines will be 300-3300 Hz. The telephone switching system filters higher frequencies in order to reduce noise. Unfortunately, this filtering equipment prevents from using the higher frequencies to get higher throughput, and limits the maximal data rate of dial-up modems to 56 Kbps.
In the ADSL chapter we will see how ADSL modems uses these higher frequencies and reaches data speeds of 8 Mbps.
Dial-Up Modems
Dial-Up Modem Network Architecture