The Teletransmission Chain

The teletransmission chain model

The teletransmission chain is a model describing the path of information from source to destination. It organizes communication into stages: signal preparation, transmission through a channel, reception, and message recovery. The scheme applies to phone calls, website visits, and video streams.

The five stages of the teletransmission chain

The chain contains five main elements, each with a specific role in preparing, transmitting, and reconstructing the message.

1. Information Source:

   The source generates the information to be sent. Examples include a person speaking into a microphone, a computer with a file to transfer, and a camera capturing video.

2. Transmitter (Processing Unit):

   The transmitter takes information from the source and converts it into a signal suitable for transmission. In an analogue or digital path, this may involve several steps:

   • Conversion: In an analogue path, turning a physical quantity into an electrical signal (e.g., a microphone).
   • Encoding: Representing the information in a standardized format (e.g., ASCII for text, PCM for audio).
   • Modulation: In a passband system, placing information on a carrier wave for transmission through the channel.
3. Transmission Channel:

   The transmission channel is the medium and associated path elements through which the signal travels from transmitter to receiver. Every real channel introduces limitations and impairments that degrade the signal.

   • Noise: Random, unwanted energy that gets added to the signal (e.g., thermal noise, atmospheric interference).
   • Distortion: The alteration of the original signal's waveform caused by the channel's physical characteristics (e.g., limited bandwidth, non-linearities).
4. Receiver (Processing Unit):

   The receiver extracts information from the weakened and corrupted signal received from the channel. It performs operations corresponding to those of the transmitter:

   • Amplification: Raising the level of a weak received signal when required.
   • Demodulation: In a carrier-based system, recovering the information signal from the carrier wave.
   • Decoding: Converting the signal back into a usable message format.
5. Information Destination:

   This is the final stage. The destination is the end-user or device that receives and interprets the message, such as a person listening through a speaker or a computer receiving a file.

The telecommunication channel

The term "telecommunication channel" refers to the set of technical means enabling signal transmission between two points. Channel parameters affect the possible data rate, achievable distance, and communication quality.

To analyze and test channels, engineers may use a reference signal source known as a normal generator. A classical telephone generator has a 600 Ω internal resistance and delivers $P_o = 1 \text{ mW}$ to a matched 600 Ω load, which is the reference level 0 dBm. Its parameters also include electromotive force ($E$) and internal resistance ($R_g$). This allows consistent and repeatable measurements of channel properties such as attenuation.