 |
| Modems |
|
| The 'modem' is a contraction of the term modulator-demodulator. It is a conversion that facilitates the transmission and reception of the data over the public switched telephone network (PSTN). The data of interest may be digital signals generated by computers or service providers. In such an application, the modulator portion of the modem converts the incoming digital into a standard form suitable for transmission over a telephone channel is the PSTN. |
| |
| The demodulator portion of the modem receives the channel output and reconverts in into the original digital signal format. |
| |
| In yet another application, namely fax modems or more precisely modems with facsimile capability, the data may represent text, graphics, pictures or a combination thereof. In this latter application, the document of interest is coded into a series of compressed picture elements (pixels), which are then transmitted over the telephone channel by modulating their values according to a predefined modulation standard. |
| |
| When the fax modem is in a receiving mode of operation, the demodulator portion of the modem operates on the received analog signal and decompresses the corresponding binary data representation of the demodulated signal into a near or actual duplicate of the original transmitted image. |
| |
| Let us now focus our attention on modems that provide communication between a user and an Internet Service Provider (ISP) over the PSTN. Traditionally, the PSTN has been viewed as an analog network. However, in reality, the PSTN has become an almost entirely digital network. In most cases, the only part of PSTN that has remained analog (and will likely remain so far many years to come) is the local lop which represents the relatively short connection from a home to the central office. |
| |
| Depending on how PSTN is used, there are two distinct classes of modem configurations, symmetric and asymmetric. |
| |
