Information is conveyed by varying the electromagnetic field of a particular medium over time. The rate at which this field (e.g., voltage) changes can be represented by a certain bandwidth that characterizes the information. Transducers such as those that facilitate RS-232/RS-422 serial links place the information that is presented to them essentially unmodified onto the transmission medium. In other words, the bandwidth of the information entering the transducer is equivalent to that leaving the transducer. Such a system operates at baseband:
the bandwidth inherent to the raw information. Baseband operation is relatively simple and works well for a transmission medium that can carry raw binary signals with minimal degradation (e.g., various types of wire, or fiber optic cable, strung directly from transmitter to receiver). However, there are many desirable communications media that are not well suited to directly carrying bits from one point to another. Two prime examples are freespace and acoustic media such as a telephone. To launch raw information into the air or over a telephone, the bits must be superimposed upon a carrier that is suited to the particular medium. A carrier is a frequency that can be efficiently radiated from a transmitter and detected by a remote receiver. The process of superimposing the bits on the carrier is called modulation . The reverse process of detecting the bits already modulated onto the carrier is demodulation . For the purposes of this discussion, one of the simplest forms of modulation, binary amplitude modulation (AM), is presented as an example. More precisely, this type of AM is called amplitude shift keying (ASK). With two states, it is called 2-ASK and is illustrated in Fig. 5.9. Each time a 1 is to be transmitted, the carrier (shown as a sine wave of arbitrary frequency) is turned on with an arbitrary amplitude. Each time a 0 is to be transmitted, the carrier is turned off with an amplitude of zero. If transmitting over free space, the carrier frequency might be anywhere from hundreds of kilohertz to gigahertz. If communicating over a fiber optic cable, the carrier is light. If an acoustic medium such as a telephone is used to send the data, the carrier is audible in the range of several kilohertz.
the bandwidth inherent to the raw information. Baseband operation is relatively simple and works well for a transmission medium that can carry raw binary signals with minimal degradation (e.g., various types of wire, or fiber optic cable, strung directly from transmitter to receiver). However, there are many desirable communications media that are not well suited to directly carrying bits from one point to another. Two prime examples are freespace and acoustic media such as a telephone. To launch raw information into the air or over a telephone, the bits must be superimposed upon a carrier that is suited to the particular medium. A carrier is a frequency that can be efficiently radiated from a transmitter and detected by a remote receiver. The process of superimposing the bits on the carrier is called modulation . The reverse process of detecting the bits already modulated onto the carrier is demodulation . For the purposes of this discussion, one of the simplest forms of modulation, binary amplitude modulation (AM), is presented as an example. More precisely, this type of AM is called amplitude shift keying (ASK). With two states, it is called 2-ASK and is illustrated in Fig. 5.9. Each time a 1 is to be transmitted, the carrier (shown as a sine wave of arbitrary frequency) is turned on with an arbitrary amplitude. Each time a 0 is to be transmitted, the carrier is turned off with an amplitude of zero. If transmitting over free space, the carrier frequency might be anywhere from hundreds of kilohertz to gigahertz. If communicating over a fiber optic cable, the carrier is light. If an acoustic medium such as a telephone is used to send the data, the carrier is audible in the range of several kilohertz.