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ccna 200 301 book download.Using Modems and ISDN Dial-up AccessThe following two Internet access technologies require us to review the early development of the Internet from some perspective. The Internet went through several vigorous phases of development, with the early part of the twentieth century being considered its leapfrog phase, when commercial activity began to drive the Internet's growth significantly.In the early days of the Internet, for customers, most people used dial-up to access the Internet; that is, they used analog phone lines and analog modems and, in essence, dialed into an ISP.As a brief background, when using a home phone line, the phone uses an analog signal to create a circuit. Computers use digital signals; to use an analog circuit, you must convert from digital to analog. The solution: an analog modem.Analog modems are located at both ends of a phone call -------------- at the customer site and one at the ISP. To send digital data from the customer PC or router, the modem modulates or converts the digital signal to an analog signal. The modem at the sending end then transmits the analog signal to the modem at the receiving end, immediately after which the modem at the receiving end demodulates the analog signal back to the original digital. (The term modem comes from an abbreviated form of these two terms: modulation and demodulation.)Figure 15-7 illustrates this basic concept explicitly with two examples. One is a PC with an external modem, which means that the PC is connected to a modem with a cable. The other shows an internal modem. The ISP will then have a matching set of modems, called a modem bank. Calls to the ISP's phone number will ring any available modem, allowing the user to connect to any of the ISP's modems and then to the Internet.Most ISPs now refer to this method as dialaccess or simply dial. Although ISPs have been doing this for decades, most ISPs still offer dial-up services today. Dial-up is cheaper in some markets and is a viable service for remote areas where faster Internet access options are not available.Figure 15-7 Internet Access Using External and Internal ModemsTip: The telephone company refers to the telephone cable that encounters the local loopback of the customer's home or business.Dial-up access happens to have several overhead advantages over other user Internet access options. ISPs deliberately place access points (PoPs) in most local calling areas, so calls to the Internet are free and there are no long distance charges. As the cost of equipment decreases rapidly over time, then the price to start is relatively low. In many markets, almost every household has a home phone, so there is no need to spend more money on a physical access link. So, the only additional overhead is the cost incurred by the ISP for accessing the Internet.