Bit rate and frequency in data communications

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T his chapter is about the communications theory, transmission modes, modulation and data compression techniques related to communications system. It describes how data flow is represented in terms of bits per time unit, and figures out maximum data transfer rate between two end points referenced to channel binary signals are transmitted at a constant rate of 1000 per second, signal to noise ratio and the conversion of digital signals into analogue format over a voice-graded network.

Upon completion of this chapter, you should be able: Understand the basic transmission theory, binary signals are transmitted at a constant rate of 1000 per second figure out the maximum data rate.

Identify the three transmission modes: Simplex, Half-duplex and Full-duplex. Classify the differences between serial and parallel transmission in terms of cost, data rate and suitability. Describe various analogue and digital modulation techniques. Understand various data compression techniques. Introduction to Information Transmission Theory. Information as discussed in chapter one can be transmitted in a transmission medium as a representation of passing information to the receiver.

The transmission medium can be one of the following: The signal relies on the variation of physical property such as the voltage level and current value. These varying physical properties can mathematically be represented as a function of time. Using Fourier transformation, any reasonably behaved periodic function can be represented as a summation of Sines and Cosines. The information such as digital data between your PC and modem is a periodic signal where the period depends on the modem speed.

Can you figure out the transmission period for bps? The advantages offered by using Fourier series include: Any Sin or Cos waveforms as given in figure can be measured by three physical quantities namely Amplitude, Phase, and Frequency: Quantity Description Amplitude Absolute measure of the height of the wave in voltage or Peak-to-Peak value.

Wavelength It is a measure of a distance for a periodic cycle. Phase Relative measure of the difference in time between waves. The unit is in either degree or radian. Frequency Absolute measure of the number of times a wave repeats per unit time. Can you list the peak-to-peak value in above picture? The velocity V of a wave travelling is determined by frequency and wavelength as given below: The speed is close to light speed in the air and is roughly half the speed for electronics travelling in copper wire.

The propagation delay for an electronic from Tsim Sha Tsui to Shum Chung is therefore around x 10 -9 second, assume the distance is 40 km and the speed is half the light speed. Can you figure out the propagation delay between Hong Kong and Peking? Any analogue signal is not binary signals are transmitted at a constant rate of 1000 per second by a single frequency if it is expanded in terms of Fourier series.

In fact, the waveform such as voice produced by human being consists of waveforms of many different frequencies. The bandwidth as shown in figure is defined by the difference between the points: Bandwidth is characterized by: Based on the conversion to frequency spectrum, any periodic time varying signal can be viewed as a series of frequency signals with limited bandwidth.

The bandwidth for a copper signal is around 10KHz, MHz for coaxial cable, and MHz for single mode optical fiber. Also note that coaxial cable can carry video signal while telephone wire can support voice and low speed data.

Signals are usually grouped into broadband or baseband depending on the signal characteristics. Baseband transmission refers to sending the digital data along the transmission channel by means of voltage fluctuation such as IEEE Note that to form a digital waveforms, more harmonic signals are required as shown in Figure. The square waveform will be distorted after passing through a low pass filter as shown in the same Figure.

Channel capacity refers to the maximum data rate for a finite bandwidth transmission medium in the presence of random noise. It is concerned about the quality of a specific communications channel and was identified by Shannon. The relation is governed by: Practically, this limit is seldom reached.

To increase the transmission rate, the designer should either increase the signal power or use alternative medium with higher bandwidth.

The bandwidth for a telephone network is usually restricted between Hz to Hz by telephone exchange. As a result, signal that is out of this range cannot be transmitted over the PSTN. That is to say, if you inject a signal of 10K Hz over the speaker, the remote cannot hear it.

A symbol can be n multiple digital levels instead of 0 or 1. As the signal to power ratio is usually quite significant, a better representation in communications is used to express the ratio of two values in logarithmic format. The values can be power, voltage or current. It is not an absolute unit, just a relative Figure and is expressed in: It is often used to measure the ratio of signal to noise in a communications channel due to large quantity of signal power.

For example, if the signal power is 1K Watt and the noise power is 1m Watt, there is no point to have a ratio written inWhat is the symbol level? You should find out the theoretical data rate first using the formula in section 1. How is bandwidth measured in what unit? As discussed above, the transmission rate is related to the bandwidth of transmission medium and signal to noise ratio. To increase the transmission rate, one can extend the signal to multiple level. This approach to increase the information transmission rate is suitable for computer to process the faster information as shown in Figure.

Binary signals are transmitted at a constant rate of 1000 per second on Coding level. It seems to be if you can extend the coding level, we can achieve higher speed. However, this is not the case as the coding level for an information is restricted by: Noise is always presented in the transmission medium. There is no method to get rid of them.

It is technically feasible to reduce the binary signals are transmitted at a constant rate of 1000 per second level. As a result, the coding signal cannot be extended to an unlimited level. Any more reasons to explain why it cannot support more levels?

In section 2, if the signalling rate is changed to 40ms, what will be the new data rate? For the same question, if the level is extended to 64, what will binary signals are transmitted at a constant rate of 1000 per second the new data rate?

Irrespective of direction of data transfer, there are THREE types of transmission channels being used to exchange information as shown in Figure. Type Description Simplex One party in the communication can send data to the other, but cannot receive data from the other end such as the radio pager. It is usually not restricted by the transmission medium, but the nature of communication devices. Half duplex Both parties can send and receive information from the other end, but not at the same time such as walkie talkie.

Each time the sender has to press the transmission button before transmitting data. Full duplex Both parties can send and receive information at the same time such as computer to computer communication or telephone to telephone. A full duplex can be logically regarded as two half duplex operating in reverse direction. The digital information regardless of channel type channel can be classified, in terms of transmssion format, into serial or parallel transmission.

Serial transmission means to transmit the data bit by bit, whereas parallel transmission means to transmit data byte by byte, word by word or even more. The bits are transmitted one after another on the same channel such as terminal to computer communication. Figure is a series of data stream transmitting from the right handed side binary signals are transmitted at a constant rate of 1000 per second to the left. It is interesting to note that the first bit to be transmitted is B1 which is the least significant bitnot the most significant one.

The bits are transmitted all binary signals are transmitted at a constant rate of 1000 per second once byte by byte such as computer to printer communication as shown in Figure. In serial transmission, the transmission format can be further classified into Asynchronous and Synchronous. It is guarded by start and stop bits and the character to character space is random as shown in Figure.

The initial and final states are idle which corresponds to volts in terms of voltage level. An odd parity bit is appended to the data bits for the detection of transmission error as shown in Figure. The characters are packed together and there is no gap between two characters. Sync is a special character used to synchronize the data reception as shown in Figure. Usually for a bisynchronous format, there are four synchronous characters preceding the data.

In case there is no character to be delivered by the transmitter, a special character such as 7E in hexadecimal is used to pad the data. Start, Stop and Parity bits are not required in synchronous transmission. As a result, the transmission throughput is roughly twice the asynchronous transmission for the same operating speed. Synchronous data is usually driven by a clock. The clock signals can be either external provided by the modem or internal provided by the computer port Pin number 24 in RSD.

The clocking signal provided by internal clock is usually lower than external clock as extra CPU time is required to produce the clock by the computer as shown in Figure.

If the clock is generated by modem, it is called external clock. The modem speed can be dynamically changed by the user even a file is transferring. This is one of the acceptance tests to verify whether the communication software is robust or not.

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