Modems Tutorial

I dont know who will find this useful, but I had to write some of this for my Cisco class, I figured that I would post it for the betterment of the Antionline community. :)



Dial-Up Modems:

Definition and Description:

Modem-


(Modulator/demodulator) An electronic device for converting between serial data (typically EIA-232) from a computer and an audio signal suitable for transmission over telephone lines. In one scheme the audio signal is composed of silence (no data) or one of two frequencies representing 0 and 1.

Modems are noticable by the maximum baud rate they support. Baud rates can range from 75 baud up to 56000 and beyond. Data to the computer is sometimes at a lower rate than data from the computer on the assumption that the user cannot type more than a few characters per second.

Different data compression and error correction programs are needed to support the highest speeds. Other optional features are auto-dial (auto-call) and auto-answer which allow the computer to start and accept calls without human connecting. Most modems support a number of different protocols, and two modems, when first connected, will automatically negotiate to find a common protocol (this process may be audible through the modem or computer's loudspeakers). Some modem protocols allow the two modems to renegotiate ("retrain") if the initial choice of data rate is too high and gives too many transmission errors.

A modem may either be internal or external. The actual speed of transmission in characters per second depends on not just the modem-to-modem data rate, but also on the speed with which the processor can transfer data to and from the modem, the kind of compression used and whether the data is compressed by the processor or the modem, the amount of noise on the telephone line and the serial character format. When a user connects with a a Dial-Up Modem the user will recieve a different IP address evertime they connect.

In 1998, 56K modems went mainstream, after the V.90 modem standard was declared by the ITU (the international standards organization) in February. Modem makers quickly began shipping V.90 compliant products. The V90 standard is now complete by most major ISPs (and AOL), and consumers have adopted widespread use of V.90 modems. Currently the Dial-Up Modems are used more than any type of broadband modem for home usage.