Data Transmission Signal

What is a data transmission signal?

A data transmission signal is the physical representation of digital information. Analog signals are continuous, whereas 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, such as a varying voltage, is called line coding. Channel coding is a separate operation that adds redundant information for error detection or correction.

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 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 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'. With opposite-polarity levels, 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].

Digital transmission challenges and line coding

Sending a raw stream of bits as simple voltage levels, as shown above, presents several practical challenges. A long sequence of identical bits, such as 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: The DC component is the time average of the signal. A long sequence of identical symbols, including the bit values 0 or 1, can produce a non-zero average value when its assigned level has non-zero voltage. In a unipolar scheme, this mainly concerns a long sequence of 1s, whose DC component cannot pass through some network components such as 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 limit these issues, binary data is transformed using line codingschemes such as Manchester, AMI, or HDB-3. Manchester coding provides a transition in the middle of every bit, AMI limits the DC component through alternating pulse polarity, and HDB-3 additionally limits long runs without transitions. All of these codes shape the signal spectrum to fit the channel's characteristics.