CC BY
15Investigation of the computational complexity of the formation of checksums for the Cyclic Redundancy Code algorithm depending on the width of the generating polynomial
Odilzhan A. Turdiev a, Vladimir A. Smagin b and Vladimir N. Kustov a
a Petersburg State Transport University, 9 Moskovsky pr, Saint Petersburg, 190031, Russian Federation
b International Academy of Informatization St. Petersburg, Saint Petersburg, 190031, Russian Federation
Abstract
Formulation of the problem: The need to ensure the integrity of data transmitted in communication networks makes the issue of ensuring the formation of checksums actual. At the same time, it is advisable to reduce the complexity of algorithms for generating checksums to improve data integrity. The well-known CRC (Cyclic Redundancy Code) checksum generation algorithm has high computational complexity. The aim of the work is to perform search studies to substantiate the fundamental possibility of reducing the computational complexity of the algorithm for generating CRC checksums and searching for possible ways of practical implementation. The novelty of the work lies in the fact that in order to achieve the goal of the work, the computational complexity of the CRC algorithm is investigated depending on various generating polynomials and their bit width. Result: on the basis of the conducted search studies, the possibility of reducing the computational complexity of the algorithm for generating CRC checksums for data transmitted in communication networks was confirmed. Additionally, a set of programs is presented for assessing the complexity of the CRC checksum generation algorithm depending on the length of the polynomial, as well as the analysis of the computational complexity of the CRC-based data integrity check algorithm. Practical relevance: The research results can be applied to reduce the computational complexity of the algorithm for generating CRC checksums in protocols of data transmission networks, as well as to substantiate promising ways for further research in this direction.
Keywords
computational complexity, generator polynomial, cyclic redundancy code, CRC, burst errors, bit errors, data integrity.
1. Introduction
One of the important tasks in modern communication networks is to ensure data integrity. The most common algorithm for determining the integrity of transmitted data is the CRC (Cyclic Redundancy Check) algorithm.1.
Models and Methods for Researching Information Systems
in Transport, Dec. 11-12, St. Petersburg, Russia
EMAIL: odiljan.turdiev@mail.ru (O.A. Turdiev);
va_smagin@mail.ru (V.A. Smagin).
ORCID: 0000-0002-1651-5493 (O.A. Turdiev).
© □ 2020 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
r=T-=~l CEUR Workshop Proceedings (CEUR-
WS.org)
The CRC algorithm is based on the theory of cyclic error correction codes. This algorithm was first proposed by V.V. Peterson and D.T. Brown, in [1]. The CRC algorithm computes a short binary sequence that has a certain constant length, known as a check value or CRC algorithm code. The CRC is calculated for each individual block of data to be transmitted over the network and added to the block to form a codeword. When a codeword is received on the receiving side, one of two things is done. Either the received CRC check value is compared with the value of the CRC that is generated anew for the transmitted data on the receiving side, or the CRC is generated again on the receiving side for the entire received
codeword and the resulting CRC check value is compared with the expected residual constant. If the control values do not match, then the transmitted data contains an error and the receiving device can take actions to correct them, such as re-reading the block or sending it again.
The theoretical provisions of the functioning of the CRC algorithm are given in [2-5]. It is assumed that when a data block and its check CRC are received correctly, integrity is ensured for that data block. The process of generating and checking the CRC can be quite laborious when using network devices with low speed or high data transfer rates. In this regard, reducing the computational complexity of the CRC algorithm is an urgent scientific and practical task.
In addition, in real transmission conditions, the communication channel can be affected by various kinds of interference, which appear in the process under study in the form of erroneous bits, which lead to a violation of data integrity [6]. In the work of V.V. Yakovlev [7] proposed the choice of the generating polynomial to increase the probability of error recognition when generating checksums in the transmitted data.
Summarizing the above, it can be noted that the purpose of the work is to perform search studies to substantiate the fundamental possibility and possible ways to reduce the computational complexity of the CRC checksum generation algorithm used to control the integrity of the transmitted data.
To find ways to reduce the computational complexity in the work, a study of the computational complexity of the formation of CRC checksums was carried out depending on various generating polynomials and their bit width. To assess the computational complexity of the CRC, we simulated the process of transmitting binary data over a symmetric channel.
2. Basic concepts and
characteristics of CRC codes
Cyclic redundancy codes CRC are a subclass of block codes and are used in HDLC, Token Ring, Token Bus protocols, Ethernet protocol families and other link layer protocols [8]. Computing resources mean memory, processor power, and the number of shift registers. One of the ways to represent a cyclic code is to represent it in the form of a generating polynomial - the set of all polynomials of degree (r - code-1), containing some fixed polynomial G (x) as a common factor. The polynomial G (x) is called the generating polynomial of the code. For example, x4 + x +1, here r-code = 5, since the binary sequence looks like 10011. The standardized and recommended generator polynomials for the CRC algorithm are given in Table 1, where the name of the standard and the generator polynomial are shown: for example, the notation x4 + x + 1 is equivalent to 1- x4 + 0- x3 + 0- x2 + 1-x + 1- x0 = 10011 (in binary).
Table 1.
Popular standardized generator polynomials [9].
nomination Generating polynomials
CRC-4-TU 4 + x + 1 (ITU G.704)
CRC-5-EPC 5 + x3 + 1 (Gen 2 RFID)
CRC-5-ITU 5 + x4 + x2 + 1 (ITU G.704)
CRC-5-USB 5 + x2 + 1 (USB token packets)
CRC-6-ITU 6 + x + 1 (ITU G.704)
CRC-7 7 + x3 + 1 (ITU-T G.707, ITU-T G.832, MMC, SD)
CRC-8-CCITT 8 + x2 + x + 1 (ATM HEC), ISDN Header Error Control and Cell
»elineation ITU-T I.432.1 (02/99)
CRC-8-Dallas/Maxim 8 + x5 + x4 + 1 (1-Wire bus)
CRC-8 8 +x7 + x6 + x4 + x2 + 1 (ETSIEN302 307, 5.1.4)
CRC-8-SAE J1850 x8 + x4 + x3 + x2 + 1
CRC-10 x10 + x9 + x5 + x4 + x + 1
CRC-11 11 + x9 + x8 + x7 + x2 + 1 (FlexRay)
CRC-12 12 + x11 + x3 + x2 + x + 1 (системы телекоммуникации)
CRC-15-CAN CRC-16-IBM CRC-16-CCITT CRC-16-T10-DIF CRC-16-DNP
CRC-24
CRC-24-Radix-64
CRC-30
CRC-32-IEEE 802.3
CRC-32C (Castagnoli)
CRC-32K (Koopman)
CRC-32Q CRC-64-ISO CRC-64-ECMA
x
15
+ x14 + x10 + x8 + x7 + x4 + x3 + 1
16 + x15 + x2 + 1 (Bisync, Modbus, USB, ANSI X3.28,) 16 + x12 + x5 + 1 (X.25, HDLC, XMODEM, Bluetooth, SD h gp.) 16 + x15 + x11 + x9 + x8 + x7 + x5 + x4 + x2 + 1 (SCSI DIF) 16 + x13 + x12 + x11 + x10 + x8 + x6 + x5 + x2 + 1 (DNP, IEC 870,
1-Bus)
24 + x22 + x20 + x19 + x18 + x16 + x14 + x13 + x11 + x10 + x8 +
+ x3 + x + 1 (FlexRay)
24 + x23 + x18 + x17 + x14 + x11 + x10 + x7 + x6 + x5 + x4 +
+ x + 1 (OpenPGP)
30 + x29 + x21 + x15 + x13 + x12 + x11 + x8 + x7 + x6 + x2 + x +
(CDMA)
32 + x26 + x23 + x22 + x16 + x12 + x10 + x8 + x7 + x5 + x4 +
+ x + 1 (V.42, MPEG-2, PNG, POSIX cksum)
32 + x28 + x27 + x26 + x25 + x23 + x22 + x20 + x19 + x18 + x14 +
13
+ x11 + x10 + x9 + x8 + x6 + 1
32 + x28 + x27 + x26 + x25 + x23 + x22 + x20 + x19 + x18 + x14 + 13 + x11 + x10 + x9 + x8 + x6 + 1
32 + x31 + x24 + x22 + x16 + x14 + x8 + x7 + x5 + x3 + x + 1
64 4 3
+ x4 + x3 + x + 1 (HDLC — ISO 3309)
64 + x62 + x57 + x55 + x54 + x53 + x52 + x47 + x46 + x45 + x40 + 39 + x38 + x37 + x35 + x33 + x32 + x31 + x29 + x27 + x24 + x23 +
22
+ x21 + x19 + x17 + x13 + x12 + x10 + x9 + x7 + x4 + x + 1
19
17
13
12
10
9
7
4
6
3
2
The value of the result of the implementation of the CRC algorithm is the remainder of dividing the binary sequence M (x) corresponding to the input data by the generating binary sequence G (x) of degree r.
Each final sequence of bits is one-to-one associated with a binary polynomial, the sequence of which is the original sequence. For example, bit sequence 1011010 corresponds to
the binary polynomial M(x) = 1- X6 + 0- X5 + 1-
A ^ O 1 HA
X + 1- X + 0- X + 1-X + 0- x=x +
4 3 X + X + x.
Let's consider examples in which addition and subtraction are performed without carrying the digits in accordance with the "exclusive OR" operation, which corresponds to the addition modulo 2 of binary arithmetic.
10011011 00110011 11110000 01010101 +11001010 + 11001101 +10100110 + 10101111
01010001 11111110 01010110 11111010
Polynomial division is performed in the binary system, with the difference that the subtraction is performed modulo 2.
The use of polynomial codes during transmission is as follows. The sender and the receiver pre-select the same generator polynomial G (x), whose coefficients at the higher term and at the lower term must be equal to 1. When calculating the checksums of a block of m bits in size, the following condition m > r must be met. Further, implementing the CRC calculation algorithm, the sender adds the checksum to the transmitted block, considered as a polynomial M (x), so that the transmitted block with the checksum is a multiple of G (x). When the recipient receives the checksum block, he divides it by G (x). If a nonzero remainder is formed, then this indicates the occurrence of an error during transmission [10,11]. Checksum calculation algorithm:
1. Add r zeros to the end of the block so that it contains m + r digits, the result is a polynomial xrM (x).
2. Divide the polynomial xrM (x) by G (x) modulo 2, the quotient is ignored.
3. Subtract modulo 2 the remainder (its length does not exceed r bits) from the string corresponding to xrM (x). The result is a block with a checksum.
Currently, most network technologies most often use the following three types of generating polynomials G (x) [12]: CRC-12 = x12+X11+x3+x2+x + 1 CRC-16-IBM = x16 + x15 + x2 + 1 CRC-16-CCITT = x16 + x12 + x5 + 1
CRC-12 is used to transmit 6-bit characters. The other two are for 8-bit. CRC-16 and CRC-CCITT detect all single and all double errors, an odd number of isolated errors, single bursts of errors less than 16 in length, and many burst errors greater than 16 with a 99.997% probability.
After getting acquainted with the basic concepts and characteristics of CRC codes, let us consider an example of calculating the remainder of cyclic redundancy codes and estimate the computational complexity of this process.
3. An example of calculating the remainder for constructing a CRC code word and estimating the computational complexity
Calculation of CRC. Information frame - 1101011011 Generator polynomial - 10011 Frame with additional zeros - 11010110110000
частное
10110 loot 1 T OlOIO
00000 4
IOIOO IQOlI » Oil 10 00000 1110 remainder (CRC)
CRC codeword (transmitted frame) -11010110111110 [13-15].
Thus, the example shows that with the generator polynomial CRC-4-TU (10011) and the bit length of the information frame equal to 10 bits, 10 divisions and 50 modulo 2 additions are required. In the general case, the relationship between the computational complexity (the number of additions and divisions), the size of the generating polynomial and the size of the information polynomial.
For the best understanding of the number of divisions and additions in the CRC algorithm with various generator polynomials and information frames, software has been created to assess the computational complexity of the CRC algorithm.
4. A program for evaluating the computational complexity of the CRC algorithm
The program is designed to estimate the complexity of the CRC algorithm by the width of the polynomial using the CRC checksum method, which is shown in the example of cyclic redundancy codes. By estimating the complexity of the implementation of the CRC algorithm by performing additions according to the width of the polynomial, we obtain an approximate number of additions (shift into cells). This study opens up possible ways to reduce the computational complexity of the CRC checksum generation algorithm used to control the integrity of transmitted data [16,17].
For the considered CRC generation method, the "CRC calculation" application was written in Java, presented in Appendix 1, which allows for a user-specified frame size and generating polynomial to estimate the required number of additions and divisions to obtain the final CRC checksum. The javaFX library was used to write the application. The CRC and TableData classes are used from this library. There are three operations in the CRC class:
1. Operation (calculation) - it calculates the CRC code.
2. Operation (initialize).
3. Operation (xor).
iiooooioio 10011 I 11010110110000
10011 I LOO 11 10011 OOOOl 00000 i 00010 00000 i OOlOl
00000 4 01011 00000 I
The initialize operation processes the bit sequence array and passes it to the table, the xor operation emulates the operations of logical addition of the bits of the input code and the CRC code to the input and gives two parameters X and Y - a binary code, 1 and 1, or 1 and 0, or 0 and 1, or 0 and 0.
5. Analysis of the results of evaluating the computational complexity of the CRC algorithm
The calculation of the number of additions, which is one of the important components of the CRC algorithm, is carried out in accordance with formula 1. Note that the value obtained as a result of the calculations should not be small, as this will lead to the need to retransmit the data. In this case, the transmission algorithm is repeated, which leads to secondary calculations. Polynomials are chosen so that there is no downtime in the data transmission channels.
The calculation of the number of additions is carried out according to the formula:
R = (P-W) mod 2, (1)
here R is the number of addition operations modulo 2; P - packet size;
N - is the length of the polynomial.
Let us consider an example in which the digit capacity of 8 polynomials has 48 divisions, therefore the number of additions according to the formula (1) R = 48 • 8 = 384. The results of evaluating the computational complexity of the CRC generation algorithm, expressed in the number of operations, are shown using the graph in Fig. 1 and are confirmed a simulation model.
Figure 1: The result of evaluating the computational complexity of the CRC generation algorithm, expressed in the number of operations.
Conclusions
The article investigates the computational CRC algorithm. It is shown that with an increase in the number of bits of generating polynomials, the number of operations increases, while the number of division operations remains unchanged. With an increase in the number of digits and division operations, the generating polynomials remain unchanged, but the number of additions increases.
It is shown that a possible way to reduce the computational complexity of the CRC checksum generation algorithm used to control the integrity of the transmitted data is to reduce the number of addition operations while maintaining the number of division operations in the algorithm under consideration.
As a result of this work, on the basis of research on generating polynomials and the number of division operations, an expression for calculating the number of addition operations was obtained. The research results pave the way for further research to reduce the computational complexity of the operation of cyclic redundancy codes.
Appendix 1.
By the logic of the method, the logical operator "or" is emulated, that is, xor, as shown in Listing 1.
public static int xorfint x.irit y){ if(x == y) returnO);
else
return(l);
Listing 1: Emulation of the logical operator "or".
The CRC class contains fields with the original message, that is, a variable that stores a
bit sequence of a certain length, as shown in Listing 2.
Listing 2: Variable storing a bit sequence of a certain length.
Next, a polynomial of a certain bit depth is generated and the CRC code is calculated in the calculation method.
The program simulates the process of data transfer between the source and the receiver (Fig. 2), and also provides the following functions:
1. Specifying the initial data in the specified table ("polynomial" and "information frame", Fig. 2) creates a number generator using a unique seed.
2. Calculation of the initial data is displayed in the program window (button "Calculation", Fig. 2).
3. As a result of the initial data, the number of additions and the formation of CRC checksums of cyclic redundancy codes ("parameters and data" field, Fig. 2) is calculated.
4. Additionally, at the discretion of the user, you can reset and start the calculation over again (the "Reset" button, Fig. 2).
Figure 2: CRC calculation.
After the simulation of the data calculation process, the number of addition of the CRC checksum algorithm of cyclic redundancy codes is calculated again, which are then compared with those previously calculated to reveal the fact of calculating the addition of the checksum algorithm of the transmitted data (Fig. 2); field "CRC calculation".
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