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ccna 200 301 book free download.Of course, there is a negative side to dial-up access. You can only choose between surfing the Internet and talking in a language, not both. To use the Internet, you have to make a phone call first, and then the Internet is not always "always on". But the biggest problem was the speed, the fast modem transmitted the entire line at a bit rate of 56kbit/s, which is quite low by today's standards.As time went on, telephone companies around the world began to work on ways to improve the use of analog modems. One of the early improvements was the use of a completely new technology called Integrated Services Digital Network (ISDN). ISDN took the sum of several of the same overhead advantage factors and used them on an analog modem to obtain faster speeds. For example.ISDN uses the same local loopback (local phone line) that most people already have... Just like a regular modem. the ISDN requires the equivalent of a 1SP phone. In each local calling area, the ISP has established a PoP to support the analog modem, so these ISDN calls no longer require any long distance charges.Another of the larger advantages of ISDN is speed. ISDN uses digital signals throughout the local loopback, rather than analog signals. In addition.319 Chapter 15 Identifying Other WAN Types It supports two calls at the same time and at 64kbit/s each on a local loopback call line. both calls (channels) can dial into the ISP for I28kbit/s Internet service. The overhead of ISDN is slightly higher-you pay the phone company to upgrade your ISDN service-but you can browse the Internet and make voice calls at the same time, plus faster speeds than with an analog modem.Figure 15-8 shows some of the details of ISDN. The ISDN client uses a line called the Basic Rate Interface (BRI), and it has two 64kbit/s user transport channels. For physical connectivity, ISDN uses some kind of ISDN-aware (ISDN-aware) device instead of an analog modem. This device is often referred to as an ISDN modem.The ISP side of the connection is also capable of using a number of different technologies, including the ISDN technology known as Primary Rate Interface (PRI). This technology converts a Tl physical line into 23 ISDN channels ready to accept those ISDN calls, as shown on the right in Figure 15-8.Both analog modems and ISDN met the needs of early Internet access. Using existing telephone lines that people have already paid for is the ideal business model. However, their relatively low speeds drove the emergence of faster Internet access-from telephone companies around the world and their then-emerging competitor, the cable companies. Table 15-5 summarizes some of the key parameters of comparison to date.Table 15-5 Comparison among Internet Access Technologies