A modulator-demodulator (modem) is a device that connects your computer to the Internet either through a dial-up connection, DSL or cable. The modem converts the digital signals that are exchanged between your computer and the Internet into frequencies that can be streamed over the carrier, which are either phone lines or cable. Where the modem is located in your system depends on what kind of modem you have.
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To locate the modem, find the modem's connection point. The connection will be either an RJ11 jack or a coaxial connector on the back of the computer tower or on a separate box sent to you from your Internet service provider.
RJ11 is the type of connection that is used for phone lines. If you have a typical home phone service, an RJ11 connector looks like the connector on a phone cord and the jack would look like the typical phone wall jack. It is very similar in appearance to an RJ45 connector which is an ethernet cable connector, but the RJ11 connector is slightly smaller.
A coaxial connector is used for cable connections. Typically, a coaxial jack looks like a cylindrical post about 3/4 of an inch long with threads on the outside and a small hole in the center.
A computer tower that has a modem inside it will have an RJ11 jack or a coaxial jack on the back of the tower. The modem might be a separate component inside the computer or it might be integrated onto the computer's motherboard. In either case, the jack will be visible on the back of the tower. If you use broadband Internet access, you might not have an internal modem. Even if a modem is installed in the tower, you are likely not using it. Broadband access generally requires an external modem that can be provided by your Internet service provider.
An external modem is a small box that has an RJ11 jack for DSL or a coaxial jack for cable on it and at least one ethernet jack. It will also have a power source that plugs into an outlet. It is common to find that modern modems are also routers, and so the term "modem" and "router" have almost begun to be interchangeable. It is not accurate to say that modems and routers are the same thing, but the small box supplied by your Internet service provider is likely a combination modem and router.