Physical coding

The signal coding system has a hierarchy.

Physical Encoding

The lowest level in the coding hierarchy is physical coding, which determines the number of discrete signal levels (voltage amplitudes, current amplitudes, brightness amplitudes).

Physical encoding considers encoding only at the lowest level of the encoding hierarchy - at the physical level and does not consider higher levels in the encoding hierarchy, which include logical encodings of different levels.

From the point of view of physical coding, a digital signal can have two, three, four, five, etc. levels of voltage amplitude, current amplitude, and light amplitude.

None of the versions of Ethernet technology uses direct binary coding of bit 0 with a voltage of 0 volts and bit 1 with a voltage of +5 volts, since this method leads to ambiguity. If one station sends a bit string 00010000, then another station can interpret it either as 10000 or 01000, since it cannot distinguish “no signal” from bit 0. Therefore, the receiving machine needs a way to uniquely determine the beginning, end and middle of each bit without the help of an external timer. The coding of the signal at the physical level allows the receiver to synchronize with the transmitter by changing the voltage in the middle of the bit period.

In some cases, physical encoding solves problems:

• Capacitive resistance - an increase in the wired communication channel of a constant component (spurious capacitance), which interferes with the functionality of electrical equipment ^[5] ;
• Violation of the density of repetition of single pulses - when transmitting a sequence of logical zeros or ones, the transmitter and receiver are out of sync ^[5] .

Logical Coding

The second level in the coding hierarchy is the lowest level of logical coding with different purposes.

Together, physical coding and logical coding form a low-level coding system.

Each bit of the code word is transmitted or recorded using discrete signals, for example, pulses. The way the source code is represented by certain signals is determined by the code format. A large number of formats are known, each of which has its own advantages and disadvantages and is intended for use in certain equipment.

• The BVN format (without returning to zero) ^[8] - a single bit is transmitted within the cycle level does not change. A positive difference means a transition from 0 to 1 in the source code, a negative difference - from 1 to 0. The absence of differences indicates that the values ​​of the previous and subsequent bits are equal. To decode codes in the BVN format, clock pulses are required. The signal corresponding to the code of the BVN format contains low-frequency components (when transmitting long series of zeros or ones, differences do not occur).
• The BVN-1 format (without returning to zero with a difference in transmission 1) is a variant of the BVN format. The signal drops are formed during transmission 1, while transmission 0, the signal level does not change.
• The BVN-0 format (without returning to zero with a difference in transmission 0) is a variation of the BVN format. Signal drops are formed when transmitting 0, when transmitting 1, the signal level does not change. It is used in multi-track digital signal recording systems. A possible option is to record two additional signals corresponding to codes in the BVN-1 and BVN-0 formats.
• VN format (with return to zero) - requires the transmission of a pulse that occupies only part of the clock interval (for example, half), with a single bit. With a zero bit, an impulse is not formed.
• VN-P format (with an active pause) - means the transmission of a pulse of positive polarity with a single bit and negative with a zero bit. A signal of this format has a clock component in the spectrum. It is used in some cases for data transmission over communication lines.
• The DF-0 format (two-phase with a phase jump during transmission 0) - corresponds to the presentation method, in which drops are formed at the beginning of each measure. For single bits, a signal in this format changes with a clock frequency, that is, a level difference occurs in the middle of each clock cycle. When transmitting a zero bit, a differential in the middle of the clock is not formed, that is, a phase jump occurs. Code in this format has the ability to self-synchronize and does not require the transmission of clock signals.

The direction of the differential when transmitting the signal of the unit does not matter. Therefore, changing the polarity of the encoded signal does not affect the decoding result. It can be transmitted along symmetrical lines without a constant component. It also simplifies its magnetic recording. This format is also known as Manchester 1. It is used in the SMPTE address-time code, which is widely used for synchronizing audio and video information carriers.

NRZ (Non Return to Zero)

NRZ (Non Return to Zero, from English - “without returning to zero”) is a two-level code. Logical zero corresponds to the lower level, logical unit corresponds to the upper level. Information transitions occur at the border of significant intervals (significant moment) ^{[3] [7]} .

NRZ code presentation options

There are several options for presenting the code:

• Unipolar code - the logical unit is represented by the upper potential, the logical zero is represented by the zero potential;
• Bipolar code - a logical unit is represented by a positive potential, a logical zero is represented by a negative potential.

Advantages of NRZ code

• Simple implementation;
• High speed data transfer;

NRZ code shortcomings

• The need to transmit a start-stop bit to synchronize the receiver with the transmitter;
• The presence of a constant component (capacitance) ^[5] , because of which it is impossible to provide galvanic isolation using a transformer;
• High requirements for frequency synchronization at the receiving and transmitting end - during the transmission of one word (byte), the receiver should not be lost more than a bit (for example, for a word with a byte length with start and stop bits, that is, only 10 bits of channel information, desync the receiver and transmitter frequencies cannot exceed 10% in both directions, for a word of 16 bits, that is, 18 bits of channel information, the desync should not exceed 5.5%, and even less in physical implementations).

NRZI (Non Return to Zero Invertive)

NRZI (Non Return to Zero Invertive) is a potential code with an inversion at unity, the code is generated by an inverse state when a logical unit is received at the input of the encoder, the state of the potential does not change when a logical zero is received. This method is a modified Non Return to Zero ( NRZ ) method ^[3] .

Since the code is not protected from long sequences of logical zeros or ones, this can lead to synchronization problems. Therefore, before transmitting, it is recommended to pre-encode the given sequence of bits with a code providing for scrambling (the scrambler is designed to give randomness properties to the transmitted data sequence in order to facilitate the selection of the clock frequency by the receiver). Used in Fast Ethernet 100Base-FX and 100Base-T4.

Advantages of NRZI code

• Ease of implementation;
• The method has good error recognition (due to the presence of two sharply different potentials);
• The signal spectrum is located in the low-frequency region with respect to the repetition rate of significant intervals.

NRZI code flaws

• The method does not have the property of self-synchronization. Even in the presence of a high-precision clock, the receiver may make a mistake with the choice of the moment of data acquisition, since the frequencies of two generators are never completely identical. Therefore, at high data exchange rates and long sequences of ones or zeros, a slight mismatch in the clock frequencies can lead to an error in a whole clock cycle and, accordingly, the reading of an incorrect bit value;
• The second serious drawback of the method is the presence of a low-frequency component, which approaches a constant signal when transmitting long sequences of ones and zeros (can be circumvented by compressing the transmitted data). Because of this, many communication lines that do not provide a direct galvanic connection between the receiver and the source do not support this type of encoding. Therefore, in networks, the NRZ code is mainly used in the form of its various modifications, in which both poor code self-synchronization and problems of the constant component are eliminated.

Manchester Coding

With Manchester coding, each measure is divided into two parts. Information is encoded by potential drops in the middle of each measure. There are two options for Manchester coding:

• In accordance with IEEE 802.3, a logical unit is encoded by a difference from a low signal level to a high one, and a logical zero is encoded by a difference from an upper signal level to a lower one in the center of a significant interval.
• Differential Manchester coding (D.E. Thomas) - the logical unit is encoded by the difference from the upper signal level to the low, and the logical zero is encoded by the difference from the lower signal level to the upper one in the center of the significant interval ^[3] .

At the beginning of each measure, a service signal drop can occur if several units or zeros are to be represented in a row. Since the signal changes at least once per transmission cycle of one data bit, the Manchester code has self-synchronizing properties. The obligatory presence of a transition in the center of the bit makes it easy to isolate the clock. The permissible discrepancy in transmission frequencies is up to 25% (this means that the Manchester-2 code is the most resistant to desync, it self-synchronizes in each bit of the transmitted information).

The code density is 1 bit / hertz. In the spectrum of the signal encoded by Manchester-2, there are 2 frequencies - the transmission frequency and half the transmission frequency (it is formed when 0 and 1 or 1 and 0 are near each other. When transmitting a hypothetical sequence of 0 or 1 only the transmission frequency will be present in the spectrum).

Advantages of Manchester Coding

• No DC component (signal change occurs at each data transfer cycle)
• The frequency band in comparison with NRZ coding is the main harmonic in the transmission of a sequence of units or zeros has a frequency of N Hz, and with a constant sequence (when transmitting an alternation of units and zeros) - N / 2 Hz.
• It is self-synchronizing , that is, it does not require special encoding of the clock pulse, which would occupy the data band and therefore is the densest code per unit frequency.
• The ability to provide galvanic isolation using a transformer, since it does not have a constant component
• The second important advantage is the lack of need for synchronizing bits (as in the NRZ code) and, as a result, the data can be transmitted in a row for an arbitrarily long time, due to which the data density in the total code stream approaches 100% (for example, for the NRZ code 1- 8-0 it is equal to 80%).

Miller Code

The Miller code (sometimes called three-frequency) is a bipolar two-level code in which each information bit is encoded by a combination of two bits {00, 01, 10, 11} , and transitions from one state to another are described by a graph ^[9] . When logical zeros or ones are continuously fed to the encoder, polarity switching occurs at intervals of T, and the transition from transmitting units to transmitting zeros at intervals of 1.5T. When the sequence 101 arrives at the encoder, an interval of 2T occurs, for this reason this encoding method is called three-frequency ^[3] .

Benefits

• There is no redundancy in the code (there are no special combinations for synchronization);
• The ability to self-synchronize (the principle is laid down in the code by which it is guaranteed to be synchronized);
• The Miller code bandwidth is half that of Manchester coding.

Weaknesses

• The presence of a constant component, while the low-frequency component is quite large, is overcome in the modified Miller code squared.

RZ (return to zero)

RZ (return to zero) ( English coding from returning to zero) is a bipolar code with returning to zero ^[5] (three-level). According to the RZ code, each bit is transmitted by a difference from one level to zero, in the middle of a significant interval as follows: logical zero corresponds to a transition from the upper level to zero level, a logical unit corresponds to a transition from the lower level to the zero level. It requires 2 times more state switching speed compared to the switching speed according to the NRZ code.

AMI Bipolar Code

AMI (Alternate mark inversion) code - has good synchronizing properties when transferring a series of units and is relatively simple to implement. The disadvantage of the code is the restriction on the density of zeros in the data stream, since long sequences of zeros lead to loss of synchronization. It is used in telephony data transmission layer when multiplexing streams are used ^[3] .

The AMI code ^[5] uses the following bit representations:

• bits 0 are represented by zero voltage (0 V)
• bits 1 are represented alternately by the values ​​-U or + U (V)

HDB3 (third-order high density bipolar code)

The HDB3 code (third-order high-density bipolar code ^[5] ) corrects any 4 consecutive zeros in the original sequence. The rule of code generation is as follows: every 4 zeros are replaced by 4 characters in which there is at least one signal V. To suppress the DC component, the polarity of signal V alternates with successive replacements. Two methods are used for replacement:

1. If the source code contained an odd number of units before the replacement, then the sequence 000V is used
2. If the source code contained an even number of units before the replacement, then the sequence 100V is used

V-signal of the unit prohibited for a given signal polarity

Same as AMI , only the coding of sequences of four zeros is replaced by the code -V / 0, 0, 0, -V or + V / 0, 0, 0, + V - depending on the previous phase of the signal and the number of units in the signal, preceding this sequence of zeros.

MLT-3

MLT-3 (Multi Level Transmission - 3) ( English multi-level transmission) is an encoding method that uses three signal levels. The method is based on the cyclic switching of the levels -U, 0, + U. A unit corresponds to a transition from one signal level to the next. As in the NRZI method, the signal does not change when transmitting a logical zero. The method was developed by Cisco Systems for use in FDDI networks based on copper wires known as CDDI. Also used in Fast Ethernet 100BASE-TX . A unit corresponds to a transition from one signal level to another, and the change in signal level occurs sequentially, taking into account the previous transition. When transmitting zero, the signal does not change.

Benefits of MLT-3 Code

• In the case of the most frequent level switching (long sequence of units), four transitions are necessary to complete the cycle. This allows a fourfold decrease in the carrier frequency relative to the clock frequency, which makes the MLT-3 a convenient method when using copper wires as a transmission medium.
• This code, like NRZI, needs to be pre-encoded. Used in Fast Ethernet 100Base-TX .

Hybrid Ternary Code

4B3T

4B3T (4 Binary 3 Ternary, when 4 binary characters are transmitted using 3 ternary characters) - the signal at the output of the encoder, according to the code 4B3T, is three-level, that is, the signal with three potential levels is generated at the output of the encoder. The code is generated, for example, according to the encoding table MMS43 ^[10] . Coding table:

Decoding Table:

2B1Q (Potential Code 2B1Q)

2B1Q (2 Binary 1 Quaternary) - a potential code 2B1Q (called PAM- 5 in some literature) transmits a pair of bits in one significant interval ^{[1] [2]} . Each possible pair is assigned a level of four potential levels.

Advantage of Method 2B1Q

• The signal speed of this method is two times lower than that of the NRZ and AMI codes, and the signal spectrum is two times narrower. Therefore, using the 2B1Q code, data can be transmitted over the same line twice as fast.

2B1Q Method Disadvantages

• The implementation of this method requires a more powerful transmitter and a more complex receiver, which must distinguish four levels.

• Modulation
• PHY
• Manipulation
• MFM encoding
• Synchronous data transfer method
• Asynchronous Data Transfer Method

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