Early telegraph communications used Morse code with human operators as transmitter and receiver. The Baudot code introduced a constant 5-bit code length for use with mechanical telegraph transmitters and receivers. A protocol is essential for defining the common message format and procedures for transferring data between all devices on the network. It includes the following important features:. An analog communications channel conveys signals that change continuously in both frequency and amplitude. A typical example is a sine wave as illustrated in Figure 1. On the other hand, digital transmission employs a signal of which the amplitude varies between a few discrete states.
An example is shown in Figure 1. As the signal travels along a communications channel its amplitude decreases as the physical medium resists the flow of the signal energy. This effect is known as signal attenuation. With electrical signaling some materials such as copper are very efficient conductors of electrical energy. However, all conductors contain impurities that resist the movement of the electrons that constitute the electric current. The resistance of the conductors causes some of the electrical energy of the signal to be converted to heat as the signal progresses along the cable resulting in a continuous decrease in the electrical signal.
To allow for attenuation, a limit is set for the maximum length of the communications channel. This is to ensure that the attenuated signal arriving at the receiver is of sufficient amplitude to be reliably detected and correctly interpreted.
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If the channel is longer than this maximum specified length, repeaters must be used at intervals along the channel to restore the signal to acceptable levels. Signal attenuation increases as the frequency increases. This causes distortion to practical signals containing a range of frequencies. This problem can be overcome by the use of amplifiers that amplify the higher frequencies by greater amounts. The quantity of information a channel can convey over a given period is determined by its ability to handle the rate of change of the signal, i.
The bandwidth of an analog channel is the difference between the highest and lowest frequencies that can be reliably transmitted over the channel. These frequencies are often defined as those at which the signal at the receiving end has fallen to half the power relative to the mid-band frequencies referred to as the -3 dB points , in which case the bandwidth is known as the -3 dB bandwidth.
Digital signals are made up of a large number of frequency components, but only those within the bandwidth of the channel will be able to be received. It follows that the larger the bandwidth of the channel, the higher the data transfer rate can be and more high frequency components of the digital signal can be transported, and so a more accurate reproduction of the transmitted signal can be received. The maximum data transfer rate C of the transmission channel can be determined from its bandwidth, by use of the following formula derived by Shannon.
The achievable data transfer rate would then be reduced by half because of the Nyquist rate. It is further reduced in practical situations because of the presence of noise on the channel to approximately bps unless some modulation system is used. As the signals pass through a communications channel the atomic particles and molecules in the transmission medium vibrate and emit random electromagnetic signals as noise.
The strength of the transmitted signal is normally large relative to the noise signal. However, as the signal travels through the channel and is attenuated, its level can approach that of the noise. When the wanted signal is not significantly higher than the background noise, the receiver cannot separate the data from the noise and communication errors occur.
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An important parameter of the channel is the ratio of the power of the received signal S to the power of the noise signal N. A high signal to noise ratio means that the wanted signal power is high compared to the noise level, resulting in good quality signal reception. The theoretical maximum data transfer rate for a practical channel can be calculated using the Shannon-Hartley law, which states that:. Digital transmission channels make use of higher bandwidths and digital repeaters or regenerators to regenerate the signals at regular intervals and maintain acceptable signal to noise ratios.
The degraded signals received at the regenerator are detected, then retimed and retransmitted as nearly perfect replicas of the original digital signals, as shown in Figure 1. Provided the signal to noise ratios is maintained in each link, there is no accumulated noise on the signal, even when transmitted over thousands of kilometers. Simplex A simplex channel is unidirectional and allows data to flow in one direction only, as shown in Figure 1. Public radio broadcasting is an example of a simplex transmission.
The radio station transmits the broadcast program, but does not receive any signals back from the receiver s. This has limited use for data transfer purposes, as we invariably require the flow of data in both directions to control the transfer process, acknowledge data etc. Half-duplex Half-duplex transmission allows simplex communication in both directions over a single channel, as shown in Figure 1.
A line turnaround procedure takes place whenever transmission is required in the opposite direction. The delay in the line turnaround reduces the available data throughput of the channel. Full-duplex A full-duplex channel gives simultaneous communications in both directions, as shown in Figure 1.
Data communications depends on the timing of the signal transmission and reception being kept correct throughout the message transmission. The process of selecting and maintaining these sampling times is called synchronization. In order to synchronize their transmissions, the transmitting and receiving devices need to agree on the length of the code elements to be used, known as the bit time.
The receiver also needs to synchronize its clock with that of the sender in order to determine the right times at which to sample the data bits in the message. A device at the receiving end of a digital channel can synchronize itself using either asynchronous or synchronous means as outlined below. Here the transmitter and receiver operate independently, and the receiver synchronizes its clock with that of the transmitter only at the start of each message frame.
Transmissions are typically around one byte in length, but can also be longer. Often but not necessarily there is no fixed relationship between one message frame and the next, such as a computer keyboard input with potentially long random pauses between keystrokes. At the receiver the channel is monitored for a change in voltage level. The leading edge of the start bit is used to set a bistable multivibrator flip-flop , and half a bit period later the output of the flip-flop is compared ANDed with the incoming signal.
If the start bit is still present, i. The input signal is fed into a serial-in parallel-out shift register. The data bits are then captured by clocking the shift register at the data rate, in the middle of each bit. For an eight-bit serial transmission data plus parity , this sampling is repeated for each of the eight data bits and a final sample is made during the ninth time interval to identify the stop bit and to confirm that the synchronization has been maintained to the end of the frame.
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Figure 1. The receiver here is initially synchronized with the transmitter, and then maintains this synchronization throughout the continuous transmission of multiple bytes. This is achieved by special data coding schemes, such as Manchester, which ensure that the transmitted clock is encoded into the transmitted data stream. This enables the synchronization to be maintained at the receiver right to the last bit of the message, and thus allows larger frames of data up to several thousand bytes to be efficiently transferred at high data rates.
The synchronous system packs many bytes together and sends them as a continuous stream, called a frame. Each frame consists of a header, data field and checksum. Clock synchronization is provided by a predefined sequence of bits called a preamble. Some systems may append a post-amble if the receiver is not otherwise able to detect the end of the message.
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An example of a synchronous frame is shown in Figure 1. Understandably all high-speed data transfer systems utilize synchronous transmission systems to achieve fast, accurate transfers of large amounts of data. The Manchester codes have the advantage that they are self-clocking. The price paid for this is a bandwidth requirement double that which is required by the RZ-type methods. In Manchester encoding, the beginning of a bit interval is used merely to set the stage. The presence or absence of a transition at the beginning of a bit interval indicates the value; the transition in mid-interval just provides the clocking.
There is a transition in the middle of each bit interval. This makes the encoding method self-clocking, and helps avoid signal distortion due to DC signal components. For one of the possible bit values but not the other, there will be a transition at the start of any given bit interval. In differential Manchester encoding, the presence or absence of a transition at the beginning of the bit interval determines the bit value. The transition in the middle of the interval is just for timing. The RZ-type codes consume only half the bandwidth taken up by the Manchester codes. RZ is a bipolar signal-encoding scheme that uses transition coding to return the signal to a zero voltage during part of each bit interval.
It is self-clocking. NRZ is a bipolar encoding scheme. This means that the encoding changes during a transmission. The presence or absence of a transition indicates a bit value, not the voltage level. MLT-3 is a three-level encoding scheme that can also scramble data. This scheme is one proposed for use in FDDI networks. MLT-3 is not self-clocking, so that a synchronization sequence is needed to make sure the sender and receiver are using the same timing.
The Manchester codes, as used for 10 Mbps Ethernet, are self-clocking but consume unnecessary bandwidth at 10 Mbps it introduces a 20 MHz frequency component on the medium. For this reason it is not possible to use it for Mbps Ethernet, even over Cat5 cable. The problem with these, however, is that they are not self-clocking and hence the receiver loses synchronization if several zeros are transmitted sequentially.
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For example, the binary pattern is coded into the five-bit pattern This code table has been designed in such a way that no combination of data can ever be encoded with more than 3 zeros on a row. This allows the carriage of Mbps data by transmitting at MHz, as opposed to the Mbps required by Manchester encoding. All practical data communications channels are subject to noise, particularly where equipment is situated in industrial environments with high electrical noise, such as electromagnetic radiation from adjacent equipment or electromagnetic induction from adjacent cables.
As a consequence the received data may contain errors. To ensure reliable data communication we need to check the accuracy of each message. Asynchronous systems often use a single bit checksum, the parity bit, for each message, calculated from the seven or eight data bits in the message.
Longer messages require more complex checksum calculations to be effective. For example the Longitudinal Redundancy Check LRC calculates an additional byte covering the content of the message up to 15 bytes while a two-byte arithmetic checksum can be used for messages up to 50 bytes in length.
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