Publisher Summary Within instrumentation there is often a need for telemetry in order to transmit data or information between two geographical locations. The transmission is required to enable centralized supervisory data logging, signal processing, or control to be exercised in large-scale systems that employ distributed data logging or control subsystems. Telemetry has traditionally been provided by either pneumatic or electrical transmission. Pneumatic transmission is used extensively in process instrumentation and control. Pneumatic instruments are intrinsically safe, and can therefore be used in hazardous areas. They provide protection against electrical power failure, since systems employing air storage or turbine-driven compressors can continue to provide measurement and control during power failure. In addition, communication channels are discussed with transmission lines that are used to guide electromagnetic waves, and in instrumentation these commonly take the form of a twisted pair, a coaxial cable, or a telephone line. The primary constants of such lines in terms of their resistance, leakage conductance, inductance, and capacitance are distributed. Along with this, digital signals are transmitted over transmission lines using either serial or parallel communication. For long-distance communication serial communication is the preferred method. The serial communication may be either synchronous or asynchronous. In synchronous communication the data are sent in a continuous stream without stop or start information. Asynchronous communication refers to a mode of communication in which data are transmitted as individual blocks framed by start and stop bits.