Modem Speed

A modem will never connect higher than what it is rated for, although it can connect to a higher speed modem. That is, a 28.8k modem can connect to a 56k modem but it will never connect at higher than 28.8k speeds. Modems generally come in the following speeds, 2400, 9600, 14.4k, 28.8k, 33.6k and 56k.

The Telephone Companies Switch

Whenever you call someone your call goes through to the phone companies Central Office, where it is converted to a digital signal. It then goes through to the Central Office of whomever you are calling and is translated back into an analog signal on the other end. The switching in the phone companies office usually goes about 56k. This is why 56k is the theoretical maximum limit a modem can go. In the past, modems could only communicate at up to 33.6k. This is because if a modem calls another modem that is on a normal telephone line, there is a mathematical law which says that you can only get about half the speed of your switch, which with all the maths ended up being 33.6k in most cases. In order to get to the higher speeds one of the ends needs to be digital. A normal 56k modem calling another normal 56k modem cannot get above 33.6k speeds.

FCC Regulation

In order to get 56k speeds, modems need to produce higher voltages on the phone line. The FCC has a regulation that limits the voltages that can be placed on phone lines in order to prevent crosstalk between lines. For this reason, 56k modems can only effectively go up to 53k.

Phone Line Conditions (static)

If you can hear audible static on a phone line, then more than likely the modem will not only connect very slowly, but they may not connect at all, or disconnect very quickly. Even if you can't hear static on a phone line, there may be faint static that is beyond the human hearing spectrum that is causing the modems to not get their full speeds.

Distance from the Phone Company's Central Office

This mostly affects 56k modems, but may also have some play with V.34 modems. If a user is further than 3 miles from their phone company's central office, they will not generally get above 33.6k speeds. The reason distance makes a difference has to do with the voltage level that modems need to produce, and the further you are from the phone company's office, the more impedance the phone lines have. Also the more further away, the more chance for noise to happen on the lines.

How the phone lines are wired to a location

In order to reduce costs many developers of new housing complexes are putting in systems that will run more phone lines over less pairs of wires. Sometimes this is called pair gain, although this may not be the proper technical term. If one of these systems is in place, it will stop a person from getting over 28.8k speeds. Also the phone company may put filters, or other devices between the central office and the end user that will slow down the speed the modem may go.

Internal Wiring

How the phone lines are wired inside a house can make a large difference on modem speed and reliability. If a person has 2 phone lines, sometimes the noises from one line will bleed into the other. This is called "crosstalk". It is very rare that this will cause major problems but it will slow down a modem. Some people also get remote phone line boxes that plug into a power outlet and allow them to run phone lines into rooms they don't have a jack in. Although these are advertised to be used with modems, in general they will cause a huge performance decrease and will often cause the modems to disconnect.

Brands and type of Modem

Different brands and types of modem vary widely in quality and have a large effect on speed and reliability. See the section on Modem Brands for more detailed information.