What is a Data Transmission Signal?

A data transmission signal is the physical representation of digital information. While analog signals are continuous, a digital signal is discrete, meaning it represents information as a sequence of distinct states. In binary systems, these states correspond to logical '0' and '1'. The process of converting this binary sequence into a physical waveform, like a varying voltage, is called line coding.

Data transmission in most telecommunication systems is done serially, meaning the bits are sent one after another over a single communication channel.

Visual Representation and Key Parameters

A simple data signal can be visualized as a waveform where different voltage levels correspond to binary values. This waveform is defined by a few key parameters.

Signal Parameters

• Voltage Levels: The signal uses distinct voltage levels to represent bits. For example, a high voltage ($u_H$) can represent a '1', and a low voltage ($u_L$) can represent a '0'. This is a form of bipolar signaling.
• Bit Duration ($T$ or $\tau$): Also known as the bit period, this is the amount of time the signal spends representing a single bit. It is measured in seconds [s].
• Bit Rate ($R_b$): The bit rate is the number of bits transmitted per second. It is the inverse of the bit duration.
  Formula: $R_b = \frac{1}{T}$. Its unit is bits per second [bit/s or bps].

The Need for Channel Coding

Sending a "raw" stream of bits as simple voltage levels (as shown above) presents several practical challenges. A long sequence of identical bits (e.g., 00000000 or 11111111) can cause problems:

• Loss of Synchronization: If the signal does not change for a long time, the receiver's clock can drift out of sync with the sender's clock, leading to errors in reading the bits.
• DC Component Build-up: A long string of '1's (or '0's in unipolar schemes) creates an average DC voltage on the line, which cannot pass through certain network components like transformers and can interfere with signal detection.
• Bandwidth Issues: The sharp edges of an ideal square-wave signal contain infinite high-frequency components, which no real-world channel can transmit perfectly.

To overcome these issues, raw binary data is transformed using various line coding schemes (such as Manchester, AMI, or HDB-3). These codes are designed to ensure frequent signal transitions for clock recovery, eliminate the DC component, and shape the signal's spectrum to fit the channel's characteristics.