Telephone systems were established to carry voice transmissions using analog signals. An analog signal is composed of continuous waves transmitted over a medium at a certain frequency range, which is the number of complete fluctuations in energy per sound wave. Changes in the wave transmissions reflect changes in voice and sound pitch, or tone. In addition to using telephone lines, some cellular networks, cable television systems, and satellite dishes use analog communications media for carrying voice and sound transmissions.
Computers cannot understand data in analog form. Instead, computers use the binary number system to transform data into digital signals. Newer communications technologies generally employ digital signals.
Whenever digital data is sent from one computing device to another over an analog communications medium, such as a telephone line, both the sending and receiving devices must be equipped with a modem. converts digital signals into analog form so an analog communications medium can send those signals. Modems also convert incoming analog signals back to th digital equivalents so computers can receive those signals. Some newer modems are able to transmit and receive digital data without analog conversion, provide they are connected to the right communications media.
The term modem is derived from the words modulate and demodulate. Modulation refers to the process of changing a digital signal into an a nal, while demodulation refers to the process of changing an analog signal int a digital signal. Originally, personal computers used a type of modem called a dial-up modem, which can dial a telephone number, establish a connection, and close the connection when it is no longer needed. Many personal compute today use a cable modem or a DSL modem that uses a broadband connection to the Internet. analog sig a a
Modems can be internal or external. An internal modem is an electronic board (card) inserted into an expansion slot on a computer's motherboard (Figure 6-3) One end of a standard telephone line is plugged into the modem port and the other end into a telephone outlet. The advantage of an internal modem is that it does not take up space on a user's desktop. An external modem operates in the same fashice as an internal modem, but is a stand-alone device connected by cable to a computer's motherboard (Figure 6-4). The advantage of an external modem is that it c easily be moved from one computer to another.
Notebook and other portable computers use a Personal Computer Memory Card International Association (PCMCIA) modem that inserts into a PCMCIA slot or a USB modem that plugs into a USB port. The modem is connected to a telephone outlet using a standard telephone line. Mobile users without access to standard telephone outlets can use a special cable to connect the modem to a cellular phone.