resources or to browse the Internet. Today, modem speeds can reach 56 K, but as recently as
1994, modem speeds reached only 2.4 K or 9.6 K.
over the PSTN has some issues. Computer data must be converted from digital to analog form
by the modem, transmitted over the PSTN, and then converted back to digital by the modem on
the receiving end. The PSTN was designed for the transmission of analog voice messages, not
digital messages. Many of the telephone lines that exist today were installed in the 1960s, which
can mean poor transmission quality.
very often anymore. However, everything has its place. Analog modems fit into the picture as
backup links very well, as shown in Figure 11-2.
network--leased lines and dialup lines:
period of time, and they generally are available 24 hours a day, seven days a week.
the same pair of wires, or half duplex. With a four-wire link, each pair of wires is dedicated to
one direction of data flow, or full duplex. Most are four-wire. Figure 11-3 shows a four-wire T1
circuit using pins 1 and 2 for transmit and pins 4 and 5 for receive. Pins 3, 6, 7, and 8 are not
used.