Modems

Modems work by converting digital signals on the computer to and from the analogue signals needed by telephone lines. Their name is derived by combining the two words, modulator and demodulator. They also have a telephone line socket, which is why they must be approved by the Australian Communications and Media Authority(1) (ACMA) for use in Australia.

Your modem must be compatible with your computer type, so when you buy one make sure you've got the right software and computer cable for your type of computer.

Some modems also come with software that allows your computer to send and receive faxes, or even act as a telephone answering machine.

2) Modem Block Diagram

3) Cross-over cables

Ethernet crossover cables are most often used in home networks when connecting two ethernet computers without a hub. An Ethernet crossover cable has it's send and receive wires crossed. When using a hub or switch, this is automatically done for you.

4) Telephone Circuit Bandwitdth

A POTS (Plain Old Telephone Service) line (in the US and Europe) has a bandwidth of 3kHz. A normal POTS line can transfer the frequencies between 400 Hz and 3.4 Khz. The frequency response is limited by the telephone transmission system (the actual wire from central office to your wall can usually do much more).

Nowadays POTS is sharply bandlimited due to the fact that the line almost always is digitally sampled at 8kHz at some point in the circuit. The absolute, theoretical limit (with perfect filters) is therefore 4kHz - but this isn't reality, 3.4 kHz maximum frequency is.

The bass frequency response is limted because of the limitations in telephone system components: transfromers and capacitors can be smaller if they don't have to deal with lowest frequencies. Other reason to drop out the lowest frequencies is to keep the possibly strong mains frequency (50 or 60 Hz and it's harmonics) hummign away from the audio signal you will hear.

5) Null Modem Cable

The purpose of a null-modem cable is to permit two RS-232 "DTE" devices to communicate with each other without modems or other communication devices (i.e., "DCE"s) between them.

To achieve this, the most obvious connection is that the TD signal of one device must be connected to the RD input of the other device (and vice versa).

Also, however, many DTE devices use other RS-232 pins for out-of-band (i.e., "hardware") flow control. One of the most common schemes is for the DTE (the PC) to assert the RTS signal if it is ready to receive data (yes, it DOES sound backwards, but that's how it works), and for the DCE (the modem) to assert CTS when it is able to accept data. By connecting the RTS pin of one DTE to the CTS pin of the other DTE, we can simulate this handshake.

Also, it is common convention for many DTE devices to assert the DTR signal when they are powered on, and for many DCE devices to assert the DSR signal when they are powered on, and to assert the CD signal when they are connected. By connecting the DTR signal of one DTE to both the CD and DSR inputs of the other DTE (and vice versa), we are able to trick each DTE into thinking that it is connected to a DCE that is powered up and online. As a general rule, the Ring Indicate (RI) signal is not passed through a null-modem connection.

6) Internal and External Modems

They consist either of a box of electronics connected to your computer by a cable (an external modem), or a card which fits inside your computer (an internal modem). Traditionally, internal modems have been dial-up modems that connect to Internet service providers (ISPs) using an analog signal over a telephone line. Since telephones also use analog signals, phones and modems cannot share lines simultaneously. Dial-up is the slowest, but also the least expensive way to connect to the Internet.

External modems can also be dial-up modems, and if so, will be rated at a transfer rate of 56 kbps (kilobytes per second), like their internal twins. External modems are handy when there are no available internal slots, or if the modem needs to be shared between computers that are not networked. In this case an external modem connected to a desktop system, can easily be disconnected and connected to a different desktop, or even a laptop.

External modems for dial-up service are inexpensive and available everywhere computers are sold. They are easy to setup, come with instructions, and most include fax capability at the software level. Recent versions of most operating systems recognize and load drivers for dial-up modems, which are plug-and-play devices.

7) Dial up vs High Speed

High-speed Broadband (External Modems)

Most external modems today are not dial-up modems, but broadband modems. External modems designed for broadband service provide a different kind of Internet connection that is extremely fast, used with high-speed ISPs. Broadband modems typically have transfer rates from 1.5 - 8 mbps (megabytes per second), or higher.

DSL and cable are the two most popular flavors of high-speed Internet access, and more often than not, ISP selling broadband Internet access will provide their customers with external modems pre-configured to use with the service. Depending on the contract terms, the external modems might be leased with a small monthly charge, or provided free to use for the duration of the client's contract. High-speed modems are normally not internal.

External modems used for broadband service will usually have a built-in firewall. A firewall filters traffic for security, disallowing certain connections to be made into your system to keep out malicious traffic. Broadband modems can be wired, or wireless. If wireless, a wireless network adapter must be installed in each computer that is to communicate with the modem. These adapters can be PCMCIA, or external devices that attach to the computer via the USB port.

DSL modems utilize the telephone line for sending digital signals, rather than analog signals. Consequently, you can use the telephone while using DSL service. External modems used for high-speed cable service will not use the telephone line, but rather the cable TV line. Local cable companies provide