Introduction Analog and Digital Signal Advantages & Disadvantages Digital Signal Transmission Impairment Data Rate Limit Bandwidth of the human voice Telephone System Bandwidth Digital Channel Capacity Digital Signal Regeneration

Introduction Data communications are the exchange of data between two devices via some form of transmission such as a wire cable. For data communications to occur, the communicating devices must be part of communication system made up of a combination of hardware (physical equipment) and software (programs). See Figure 1.0 A data communications system has five components such as: 1. Message The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2. Sender The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

Introduction (continue …) 3. Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. 4. Transmission medium The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable and radio waves. 5. Protocol A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person speak only Japanese.

Introduction (continue …) Figure 1.0 : Five components of data communications

Analog & Digital Signals. Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete value. Like the data they represent, signals can be either analog or digital. An analog signal has infinitely many levels of intensity over a period of time. As the wave moves from value A to value B, it passes through and include an infinite number of values along its path.

Figure 1.1 (a): Analog signals. Example of Analog signal: A typical Commercial Radio system

Analog & Digital Signals (continue ...) A digital signal can have only a limited number of defined values. Although each value can be any number, it is often as simple as 1 and 0. See Figure 1.1 (b). The constraint for both analog and digital communication is the physical capabilities of the communications media.

Figure 1.1 (b): Digital electrical signals. Example of Digital signal: Signal on a typical printer cable.

Advantages of Digital Signal Digital circuits are subject to less distortion and interference Error correction is possible. Encryption and privacy is possible Digital circuit is simple and cheap The receiver can request a retransmission of bad information

Disadvantages of Digital Signal Interface to analogue is needed. A digital system requires a greater bandwidth than analogue to carry the same information. Generally digital communication system require synchronization but analogue do not require.

Transmission Impairment. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. Three causes of impairment are attenuation, distortion and noise.

Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. In simple term means a decrease in the electrical signal. To composite for this loss, amplifiers are used to amplify the signal. See Figure 1.2 (a).

Attenuation Figure 1.2 (a): Attenuation

Means that the signal changes its form or shape. Distortion Means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed through a medium, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. See Figure 1.2(b)

Distortion Figure 1.2 (b): Distortion

Induced noise comes from sources such as motors and appliances. Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise, crosstalk and impulse noise, may corrupt the signal. Thermal Noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. Crosstalk is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise is a spike that comes from power lines, lightning and so on. Figure 1.2(c) shows the effect of a noise on a signal.

Noise Figure 1.2 (c): Noise

Data Rate Limit A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors: The bandwidth available The level of the signals we use The quality of the channel (the level of noise) Two theorectical formulas were developed to calculate the data rate: Nyquist bit rate for a noiseless channel BitRate = 2 * bandwidth * log2 L Shannon Capacity for a noisy channel Capacity = bandwidth * log2 (1 + SNR)

Data Rate Limit Channel Capacity Bandwidth Bandwidth in Hertz The maximum possible rate information rate that data can be transmitted over a given communication path or channel under given condition. Bandwidth One characteristics that measures network performance is bandwidth. However, the term can be used in two different measuring value: bandwidth in hertz and bandwidth in bits per second. Bandwidth in Hertz Bandwidth in Hertz is the range of frequencies contained in a composite signal or the range of frequencies a can pass. For example: We can say the bandwidth of a subscriber telephone line is 4kHz. Bandwidth in Bits per Seconds Bandwidth in Bits per seconds refer to the speed of bit transmission in a channel, a link or even a network can transmit. For example: One can say the bandwidth of a Fast Ethernet network is a maximum of 100 Mbps. This means that the network can send 100 Mbps.

Data Rate Limit Relationship between bandwidth in Hertz and bandwidth in bps There is an explicit relationship between the bandwidth in hertz and bandwidth in bits per seconds. Basically, an increase in bandwidth in hertz means an increase in bandwidth in bits per second. The relationship depends whether we have baseband transmission or broadband transmission. Baseband Transmission: Transmission of digital or analog signal without modulation using a low-pass channel. Low pass channel is a channel with bandwith starts from zero. See Figure 1.3.1

Data Rate Limit Baseband Transmission: Figure: 1.3.1 : Baseband Transmission

Data Rate Limit Baseband Transmission: Figure: 1.3.2 (a) and (b) : Bandwidth of two low-pass channel

Data Rate Limit Broadband Transmission: Transmission of signals using modulation of a higher frequency signal. It means changing the digital signal to analog for transmission and modulation allows us to use a bandpass channel – a channel with a bandwith that does not start with zero. See Figure 1.3.3 (a) Figure: 1.3.3 (a): Bandwidth of a bandpass channel

Data Rate Limit Broadband Transmission: Figure: 1.3.3 (b): Modulation of a digital signal for a transmission on a bandpass channel

Bandwidth of the human voice Figure 1.4.1(a) shows the sound power a human vocal system can produce at various frequencies. The power of human sounds at lower frequencies, or the base pitches, is much higher than the power at higher frequencies. The human frequency range is from near zero to over 12,000 Hz i.e. bandwidth of over 12,000Hz. Modern stereo equipment can reproduce most of this range

Figure 1.4.1(a) Bandwidth of the human voice.

Telephone System Bandwidth Due to technology limitations and cost tradeoffs the public telephone system can handle only a small part of the total bandwidth of the human voice. The system provides coverage for the portion of the voice bandwidth that can produce the greatest power. The range is only from 300 to 3,300Hz which is sufficient to convey messages to distant listener.

Figure 1.4.1(b) shows frequency range vendors use to convey data communications through the telephone system Figure 1.4.1 (b) Telephone signal amplitude versus frequency.

Digital Channel Capacity The number of digital values the channel can convey in one second. A digital communications channel has limitations that determine how often the signal can change states over a period. These limitations establish the maximum rate at which data can flow through the channel.

A variety of baseband signaling techniques are used to convey information or data. Digital systems may have more than two discrete changes as shown in Figure 1.5.1. A binary system has only two discrete energy levels A digital system can have many discrete energy levels.

Figure 1.5.1 Digital system electrical signals.

Figure 1.5.2 Digital signal distortion and attenuation with distance

Bandwidth versus length characteristics as shown in Figure 1. 5 Bandwidth versus length characteristics as shown in Figure 1.5.2 can be used to determine the length of channel they want to use for specific applications. high-volume application requires a high bandwidth such as a direct connection between two mainframe computers, a vendor can limit the length of the communications channel to a short distance. A low-volume application such as the connection between a personal computer and a low speed printer, the vendor can specify a longer channel.

Figure 1.5.3 Medium bandwidth versus length

Digital Signal Regeneration Provide devices that regenerate a digital signal. Repeaters receive the signal and rebuild it to its original strength and shape. The repeater catches the signal before it degrades to the point that it is unusable. Digital signal cannot be amplified to increase their distance range in a channel.

If you amplify a digital signal, you also amplify the noise that contaminated the signal. The amplified noise can become a substantial part of the signal. A repeater removes the noise from a signal while it is regenerating the signal.

Figure 1.5.4 Digital signal regeneration.