Modems, Cable Modems, USB Modems

  A modem is a device that converts between analog and digital signals. Digital signals, which are used by computers, are made up of separate binary units, represented by a series of 1's and 0's. Analog signals vary continuously like sine wave or sound wave. Modems are often used to enable computers to communicate with each other across telephone lines.
 

  A modem converts the digital signals of the sending computer to analog signals that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer. If both modems can transmit data to each other simultaneously, the modems are operating in full duplex mode; if only one modem can transmit at a time, the modems are operating in half duplex mode.

Cable Modems         

   A cable modem is a device that enables you to hook up your PC to a local cable TV line. The data rate for cable modems can far exceed that of the archaic 28.8 and 56 Kbps telephone modems and the up to 128 Kbps of Integrated Services Digital Network (ISDN) and is about the data rate available to subscribers of Digital Subscriber Line (DSL) telephone service. A cable modem can be added to or integrated with a set-top box that provides your TV set with channels for Internet access. In most cases, cable modems are furnished as part of the cable access service and are not purchased directly and installed by the subscriber.
A cable modem has two connections: one to the cable wall outlet and the other to a PC or to a set-top box for a TV set. Although a cable modem does modulation between analog and digital signals, it is a much more complex device than a telephone modem. It can be an external device or it can be integrated within a computer or set-top box. Typically, the cable modem attaches to a standard 10BASE-T Ethernet card in the computer.

USB Modem

   The term USB modem refers to any type of data/fax/voice modem device which can be connected to a computer using USB port. The term more specifically describes a specific portable USB device that looks similar to a USB flash drive. These small portable USB fax modems do not require a power source and can be plugged into any USB port on your PC, notebook, or Macintosh computer and can also be disconnected from the computer without turning off the system. One end of the portable USB modem will have a USB interface, while the other end will have an RJ-11 port for connecting your phone line.