Simple Telecom : Transmission Systems

Transmission systems interconnect communication devices (end nodes) by guiding signal energy in a particular direction or directions through a transmission medium such as copper, air, or glass. A transmission system will have at least one transmitting device, a transmission medium, and a receiving device. The transmitting communication device is capable of converting an information signal into a form of electrical, electromagnetic wave (radio), or optical signal that allow the information to be transferred through the transmission medium. The receiving communication device converts the transmitted signal into another form that can be used by the device or other devices that are connected to it. Transmission systems can be unidirectional (one direction) or they can be bi-directional (two directions).

The basic types of transmission mediums include copper wire, coaxial cable, free space/air, fiber optic cable, and mechanical transmission line. Copper and coaxial wire is primarily used for low to moderate frequency transmission over a few miles. Free space/air systems can transmit hundreds of miles but have limited bandwidth and are susceptible to noise interference. Fiber optic cable is capable of carrying high-speed data signals (as light pulses) over thousands of miles. Mechanical (acoustic wave) transmission lines transmit over very short distances (only a few millimeters) and are used for signal filtering components.

Different types of transmission lines have varying performance characteristics and may be susceptible to interference during signal transmission. These characteristics include the available frequency bandwidth (frequency response), how much signal leakage may occur (cross talk), and the susceptibility of absorbing other signals (signal ingress). The construction of the transmission line itself may cause distortions in the transmitted signal. This includes unterminated line splices (bridge tap reflections), poor line splices, and line resistance (signal attenuation). Other characteristics such as varying delays to different frequency ranges may cause group dispersion (smearing) of the desired signal.

To allow devices to communicate with each other over a transmission line, carrier systems specify the signal types and levels along with specific protocol controls (communication rules). These carrier systems are often specific to the transmission medium such as copper or fiber. Some of the more popular carrier systems include plain old telephone service (POTS), digital signaling carrier (DSx), digital subscriber line (DSL), and optical carrier (OCx).

To coordinate the transmission line, signaling messages are sent between communication devices. Some of these control messages are sent along with the data on the transmission line (called in-band signaling) and others are sent through another path or network (called out-of-band signaling).

In some cases, a transmission path may only be a portion of a path (a logical path) through a transmission line. The length of a transmission may be extended through the use of amplifiers or repeaters.

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