Two important types of physically transmitted signal in communications media

1. A baseband signal (“digital-over-digital” transmission):

A sequence of electrical pulses or light pulses produced by means of a line coding scheme such as Manchester coding.

This is typically used in serial cables, wired local area networks such as Ethernet, and in optical fiber communication. It results in a pulse amplitude modulated signal, also known as a pulse train.

2. A pass band signal (“digital-over-analog” transmission):

A modulated sine wave signal representing a digital bit-stream. Note that this is in some textbooks considered as analog transmission, but in most books as digital transmission.

The signal is produced by means of a digital modulation method such as PSK, QAM or FSK. The modulation and demodulation is carried out by modem equipment. This is used in wireless communication, and over telephone network local-loop and cable-TV networks.

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According to one definition of digital signal, both baseband and pass band signals are considered as digital, while an alternative definition only con­siders the baseband signal as digital, and the pass band transmission as a form of digital-to-analog conversion.

Data transmission is a subset of the field of data communications, which also includes computer networking or computer communication applica­tions and networking protocols, for example routing, switching and proc­ess-to-process communication.

Although the Transmission control proto­col (TCP) involves the term “transmission”, TCP and other transport layer protocols are typically not discussed in a textbook or course about data transmission.

Digital transmission or data transmission also belongs to telecommunica­tions and electrical engineering.

Courses and textbooks in the field of data transmission as well as digital transmission and digital communications have similar content.

Analog modulation schemes such as AM and FM are used for transferring analog message signals over analog pass band channels without digitization, and are not covered within the field of data transmission.