PSTN
An excerpt from a recent manual we've been reading entitled, INTERNET WORKING - A Guide to Network Communications LAN to LAN; LAN to WAN by Mark A. Miller, P.E.
Chapter 3.1 Dial-Up Telephone Network Facilities - starting with paragraph 2
A telephone channel is defined as a "frequency band, or its equivalent in the time domain, established in order to provide a communications path between a message source and its destination". The telephone channel is therefore a specific frequency band that has been optimized for voice communications. For a number of reasons, the required channel -- called a passband -- must pass frequencies that range from approximately 300 to 3300 Hertz (Hz).
The bandwidth is the width, measured in Hz, of the passband, and is found by subtracting the upper and lower frequency limits (thus equaling 3000 Hz). The passband is determined by the electrical inductance and capacitance of the cable itself, and by filters connected to every analog telephone line in the telephone company's Central Offices (C.O.s).
If all we wanted to transmit was an audio signal, it's easy to see that the telephone channel characteristics would define the frequencies of interest. Frequencies below 300 Hz or above 3300 Hz would be attenuated, and not be transmitted with as great an amplitude as those frequencies within the passband.
Data communication signals, however, are measured in terms of bits per second (bps), not Hz, and therefore some mechanism must be employed to associate the transmission channel characteristic (in Hz) to the data rate (in bps).
Three points are worth emphasizing. First, the terms "baud" and "bits per second" are often confused. Modem designers, and others who must deal with the actual signal that is placed on the telephone channel, will speak of those signals in terms of "baud". Those of us who wish to use those same channels to transfer a file, for example, will speak in terms of "bits per second." Second, upper limits to the data transmission characteristics of the telephone channel exist. These limits are 6000 baud and 30,000 bps. Thus, while it is possible to transmit data at rates greater than 30,000 bps over twisted pair transmission media, it is not possible to do so using the Public Switched Telephone Network (PSTN).
Current modem technology, such as the V.34 standard which transmits at 28.8 Kbps (28,800 bps), is pushing the upper limit of [Nyquist's] theorem. If faster data rates are required, two options are available. The first option is to compensate for the signal degradation caused by the transmission channel through a process known as conditioning. The second is to use a digital, instead of analog, transmission channel.
...dial up transmission facilities are the most ubiquitous transmission option. As we have all experienced, dial-up connections have a varying degree of quality -- sometimes you get a circuit that is noisy, and sometimes you get one that sounds like the other party is next door. In other words, switched facilities have a randomness associated with them, and this is due to the switched nature of the connection itself. Every time a dial-up circuit is established, a different path is chosen. Some of those paths are noisy, some are not, and as [Shannon's Capacity Theorem] (C = B log2 ((1+S/N)) bps) illustrates, the amount of noise has a direct relationship to the maximum data rate.