Proceedings of the 6th WSEAS International Conference on ENGINEERING EDUCATION

Educational Prototype for Line Coding

DAVID ALEJANDRO LOPEZ-RAMIREZ, MARIO REYES-AYALA, EDGAR ALEJANDRO ANDRADE-GONZALEZ, JOSE ALFREDO TIRADO-MENDEZ Electronics Department Metropolitan Autonomous University San Pablo 180, Col. Reynosa Tamaulipas, Azcapotzalco, ZIP Code 02200, Mexico City MEXICO [email protected]

Abstract: - In many telephone networks the use line coding allows us to adequate the signal to the channel. Some parameters like distance, transmission media, bit rate and bandwidth; defines the performance of a line coding technique. In this paper, the design and implementation of an educational tool is widely analyzed. The tool described here can synthesized a lot of line codes, because the main goal of this work is to emphasize the advantages of each line code.

Key-words: - Line coding, base-band transmission, Bandwidth, PSTN, Hierarchical Networks.

1 Introduction signal can vary the bit rate and the voltage levels of The base-band communication process can face each line code. some problems according to an specific application. The paper is organized in five sections, section Some of these problems are: signal propagation in one is dedicated to an introduction, in section two the transmission media, link distances, clock the line coding techniques are presented, section stabilities, equipment mobility, performance in very three the architecture of the module, in section four noisy channels and the final limited bandwidth [1] the main results of this work are shown and, the [2] [3]. section five are the conclusions. The use of line coding can reduce the consequences of several of the impairments mentioned above. Besides, the use of baseband 2 Line Coding transmission can be used in PCM systems for real- If the bit rate is slow a Non-Return to Zero (NRZ) is time applications like voice and video. In these sort enough. This kind of line code can be divided in of systems the sample time may be divided for three variations NRZ-L, NRZ-M and NRZ-S (L for many users (Time Division Multiplex TDM), see level, M for mark and S for space). In Figure 2 Figure 1. This feature increases the bit rate and the NRZ-L for unipolar systems is plotted. clock instabilities. Public Switched Telephone Network (PSTN) is one of the most important 1 0 1 1 0 0 0 1 0 1 examples of line coding [4] [5] [6] [7]. +V

Amplitude 0V t User 1 User 2 Amplitude Amplitude User N

"1" +V "0" +V ...... 0V t 0V t

Time TS Sample 1 2 3 4 5 6 7 8 Fig. 2 Unipolar NRZ-L.

Fig. 1 TDM. The use of unipolar NRZ-L cause a Direct Current (DC) component, because the signal can be In this paper, the tool design and +V or 0V. The mean of the signal in unipolar NRZ- implementation is analyzed in detail. The output L depends on the probability of logic levels; then this problem provoke a power dissipation in the

ISSN: 1790-2769 232 ISBN: 978-960-474-100-7 Proceedings of the 6th WSEAS International Conference on ENGINEERING EDUCATION transmission media. The level of dissipation is positive and negative alternatively. In Figure 5 the increased if the link distance is also incremented. Alternate Mark Inversion (AMI) code is illustrated. A typical solution to this impairment is to use a AMI is widely used because has some bipolar variation of NRZ-L. In Figure 2 a bipolar important advantages: very low DC component and NRZ-L is illustrated. very short bandwidth. But this technique has a problem, because a large number of consecutive 0V Amplitude levels can produce a synchronization loss. 1 0 1 1 0 0 0 1 0 1 +V Amplitude

t 1 0 1 1 0 0 0 1 0 1 0V +V -V 0V t Amplitude Amplitude -V "1" +V "0" +V Amplitude Amplitude 0V t 0V t +V +V -V -V "1,1" "0" 0V 0V t Fig. 3 Bipolar NRZ-L. -V -V Bipolar NRZ-L produces a small DC component if the probability of each polarity is approximately equal to each other; but the real Fig. 5 NRZ-AMI. applications have random information sources. Then, in general is necessary to employ a different In order to preserve synchronization, there are line code variation. In Figure 4 a Manchester line several kinds of substitution of zeros. In Figure 6 a code is shown. line code variation designed to substitute eight consecutive zeros (B8ZS). The substitution involves Amplitude additional transitions, whom can be identified by 1 0 1 1 0 0 0 1 0 1 destination using a non-alternate rule named +V violation (V).

t 0V 1 0 0 0 0 0 0 0 0 1 -V

Amplitude Amplitude NRZ-AMI

"1" +V "0" +V

V V 0V t 0V t B8ZS -V -V

Fig. 4 Manchester code. Fig. 6 B8ZS. Manchester o bi-phase codes can eliminate the DC component, because each binary symbol is B8ZS is employed in T1 carrier for North- divided in two parts with an identical duration and American telephone hierarchy; but in a higher bit different polarities. The main disadvantage of this rate this technique does not work well. In European line code is a large bandwidth as a result of a lot of standard a four-zero substitution is required for the transitions. Telephone networks have adopted a line first telephone hierarchy (HDB3). This code is code where the polarity of a binary symbol is illustrated in Figure 7.

ISSN: 1790-2769 233 ISBN: 978-960-474-100-7 Proceedings of the 6th WSEAS International Conference on ENGINEERING EDUCATION

0 1 0 0 0 0 1 1 0 0 0 0 defined new line codes and storage them in the NRZ-AMI module memory. In Figure 10 the simplified block diagram of the module is illustrated.

HDB3 OUTPUT V V B V DIGITAL TO ANALOG KEYBOARD DECODER MICROCONTROLLER CONVERTER

2 1

3 4

INPUT 5 Fig. 7 HDB3. KEYBOARD

There are line codes that they do not follow a level rule. This variation is named space or mark Fig. 10 Simplified block diagram. variations. In Figure 8 NRZ-S is plotted, where a zero determines the change of the rule in NRZ line The microcontroller receives the line code code. selected by keyboard and the output changes its rules for the next clock pulse. If necessary the Amplitude Digital to Analog converter can give 256 output levels (128 positive and 128 negative levels for all 1 0 1 1 0 0 0 1 0 1 +V of the line codes). This feature allows us to use a lot of commercial devices and telephone interfaces. 0V t The bit rate of all of line codes can be changed Fig. 8 NRZ-S. in 8 defined values. If a very strange bit rate is necessary it is possible to use the external clock to Spaced and marked line codes offer a very well adjust this parameter. performance in noisy applications, because its rules allows us to identify the transitions of zeros and ones in a binary system. 4 Results Finally, in non binary systems is possible to The module was proved for a lot of combinations of code several bits in a single level. In Figure 9 a the line codes presented here. The module allows to four-level line code is shown. The most important compare many line codes for educational and limitation to multi-level line code are the training purposes. determination of two almost equal levels.

Amplitude

1 0 1 1 0 0 0 1 0 1 11 +3V

10 +2V

01 +1V

00 0V t

Fig. 9 Multi-Level Line Code.

3 Module implementation The module analyzed here can generate all of the Fig. 11 Bipolar NRZ-L. line codes exposed in section two, including a lot of line codes with mixed features. It is possible to From Figure 11 to Figure 14 some of the line codes generated are shown.

ISSN: 1790-2769 234 ISBN: 978-960-474-100-7 Proceedings of the 6th WSEAS International Conference on ENGINEERING EDUCATION

Fig. 14 AMI.

Fig. 12 Unipolar NRZ-L. 5 Conclusions The protoype presented can generate many line

coding techniques. All of the wave forms were obtained with a The main goal of this module is to teach line clock frequency equal to 1 MHz (internal clock of coding in universities and training centers, because the microcontroller); but this frequency can be a lot of systems employ base band digital increased to 32 MHz. This is important because transmission, like telephone, telemetry and maximum frequency of the input is 10% automation industrial systems. approximately of the microcontroller clock This paper describes a cheap and easy to use frequency. prototype, designed to compare many line codes according to: the DC component, synchronization preservation, bandwidth and performance in noisy channels. The commercial options to this work are quite expensive and, they normally are designed to some line codes, few bit rates and fixed voltage levels. Now the results of this paper are being employed to build a new prototype to enhance the features described before.

References: [1] J. C. Bellamy, “Digital Telephony”, Third Edition, Wiley Interscience, 2000. [2] M. J. Miller, “Digital Transmission Systems

and Networks”, Computer Science, USA, 1987. Fig. 13 Manchester line code. [3] www.atmel.com/dyn/resources/prod_documents /doc2502.pdf [4] http://www.atmel.com/dyn/resources/prod_docu ments/doc2502.pdf [5] www.ece.utep.edu/courses/web3376/EE3376/L ab%204.html [6] www.datasheetcatalog.net/es/datasheets_pdf/7/4 /C/9/74C922.shtml [7] http://www.atmel.com

ISSN: 1790-2769 235 ISBN: 978-960-474-100-7