Data Transmission and Encoding Techniques
Data Elements and Signals
A data element is an entity that can represent one bit. A signal element is the short, in time, digital signal of 1A. Data elements need signals to send data; the signal is what sends these elements. Data rate is the number of data elements (bits) sent in one second. Modulation rate, or baud rate, is the number of signal elements sent per second.
Signal Characteristics and Coding
Bandwidth: The theoretical bandwidth of a digital signal is infinite; the effective bandwidth is finite.
Baseline variations: This is the long string of 0s or 1s that may hinder the receiver’s correct decoding.
DC component: The DC component of the signal has a very low level, almost zero at times, and results in problems with systems that cannot handle lower frequencies or systems that use electrical coupling through a transformer.
Internal synchronization: The bit intervals of the receiver must correspond exactly with the intervals of the issuer.
Built-in error detection: Some codes can detect errors that occur during transmission.
Noise and interference immunity: A code is desired that is immune to noise and other interferences.
Complexity: A complex scheme is more costly than a simple one to implement.
NRZ Encoding
NRZ: This is a bipolar signal with two levels. A positive voltage defines a 1 bit, and a 0 voltage defines a 0 bit.
NRZ-L: This is a polar voltage. In NRZ-L, the voltage level determines the bit value.
NRZ-I: This is also a polar voltage. In NRZ-I, the absence or presence of inversion determines the bit value.
RZ Encoding
RZ uses three values: positive, negative, and 0. The signal changes during the bit. The terms are known, so when a 1 bit begins, it rises and then subsides.
Manchester Encoding
Manchester: The bit length is divided in half. The voltage remains at one level during the first half and moves to another level in the second half. The transition in the middle of the bit provides synchronization.
Differential Manchester: There is always a transition in the first half of the bit. The bit value is determined by the earlier bit. If the next bit is 0, there is a transition; if the next bit is 1, there is not.
Bipolar AMI Encoding
AMI (Alternate Mark Inversion): The voltage alternates. A neutral value is a binary 0. The 1s are represented by alternating positive and negative voltages.
Pseudoternary: A 1 bit is coded as a 0 voltage, and a 0 bit is encoded with alternating positive and negative voltages.
4D-PAM5
4D-PAM5 stands for 4-Dimensional Pulse Amplitude Modulation with 5 levels. The term “4D” means that data is sent using four cables simultaneously. It uses five voltage levels: -2, -1, 0, 1, and 2. However, the 0 level is used only for error detection. If we assume the first dimension is the code, the four levels create something like 8b4q. In other words, the first word of 8 bits is translated into four different signal levels. The rate is virtually a 4/8 or 1/2 signal.
Parallel vs. Serial Transmission
Parallel Transmission: Binary data bits are organized into groups. Computers produce and consume data in groups of bits. These data groupings can be sent n bits at a time rather than one at a time. The advantage of parallel transmission is speed.
Serial Transmission: One bit follows another, so only one communication channel is needed. The advantage of serial transmission is the reduction of transmission cost.
Asynchronous Transmission
In asynchronous transmission, signal timing is not important. Information is received using patterns, which allows the receiver to recover information without knowing the arrival rate. To alert the receiver of the arrival of a group of bits, an extra bit (usually a 0, called the start bit) is added to the beginning of each group. To let the receiver know the end of the group, one or more bits (usually 1s, called stop bits) are added to the end of each group.