NET 205: Data Transmission and Digital Communication

NET 205: Data Transmission and Digital Communication
Transmission medias 2nd semester NET 205: Data Transmission and Digital Communication

205NET LOC 1-Introduction to Communication Systems and Networks architecture OSI Reference Model. 2- Data Transmission Principles 3- Transmission medias

Outline Transmission Media Guided Media Wireless Transmission
Antennas and Antenna Gain Wireless Propagation Electromagnetic Spectrum Channel Impairments Channel Capacity

Transmission Medias A transmission media is the channel that provides the connection between the transmitter and the receiver. It can be a physical or non-physical link The transmission media moves electromagnetic energy from one or more source to one or more receiver. Which medium should be used? Maximize data rate Maximize distance Minimize transmission impairments Minimize cost

Types of Transmission Medias
The Transmission media or channels can be classified as : Analog Channels: These channels can carry analog signals. Examples: telephone system, commercial radio system Digital Channels: These channels can carry digital signals. Example: computer communications

Types of Transmission Medias
Medias also can be classified as: Bounded (guided) medias : signals are confined to the medium and do not leave it. Examples: electrical cables: twisted pair and coaxial cable and optical fiber Unbounded (unguided) medias : the signals originated by the source travel freely into the medium and spread throughout the medium. Unguided media employ an antenna for transmitting through air or water

Electrical Cables Transmit electrical signals on a conductor, e.g. copper Cable carrying electrical current radiates energy, and can pick- up energy from other sources Can cause interference on other cables Other sources can cause interference on the cable Interference results in poor quality signals being received. To minimize interference: Keep the cables away from other sources Design the cables to minimize radiation and pick-up To minimize interference: Keep the cable lengths short Keep the cables away from other sources Design the cables to minimize radiation and pick-up Use materials to shield from interference Organize multiple wires so they do not interfere with each other

Twisted Pair Cable A twisted pair consists of two insulated copper wires twisted together in a helical form

Twisted Pair Cable Most commonly used and least expensive medium
Used in telephone networks and in-building communications Telephone networks designed for analog signaling (but supporting digital data) Also used for digital signaling Two varieties of twisted pair: shielded (STP) and unshielded (UTP); also multiple categories (CAT5)

Coaxial Cable Coaxial cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

Coaxial Cable Provide much more shielding from interference than twisted pair: Higher data rates; more devices on a shared line; Longer distances. Widely used for cable TV, as well as other audio/video cabling. Used in long-distance telecommunications, although optical fiber is more relevant now

Fiber Optic Cables These cables carry the transmitted information in the form of a fluctuating beam of light in a glass fiber.

Fiber Optic Cables Used in long-distance telecommunications, as well as telephone systems, LANs, and city-wide networks Advantages of optical fiber over electrical cables: 1. Lower loss: can transfer larger distances 2. Higher bandwidth: a single fiber is equivalent to 10's or 100's of electrical cables 3. Small size, light weight: lowers cost of installation 4. Electromagnetic isolation

Comparison of Guided Media
Electrical Cables Moderate data rates: 1Gb/s Maximum distance: 2km (twisted pair); 10km (coaxial) Cheapest for low data rates UTP: easy to install, susceptible to interference STP, Coaxial Cable: rigid, protection against interference Optical Cables Very high data rates: 100Gb/s+ Maximum distance: 40km Expensive equipment, but cost effective for high data rates Difficult to install

Wireless Transmission Model
Common wireless systems for communications include: Terrestrial microwave, e.g. television transmission Satellite microwave, e.g. IP star Broadcast radio, e.g. IEEE WiFi (wireless LAN) Infrared, e.g. in-home communications

Wireless Transmission Model
Transmit electrical signal with power Pt Tx antenna converts to electromagnetic wave; introduces a gain Gt Signal loses strength as it propagates; loss L Rx antenna converts back to electrical signal, gain Gr Receive signal with power Pr

Wireless Transmission Issues
What is the role of an antenna? What is antenna gain? How does the signal propagate in different environments? How much power is lost when it propagates?

Antenna An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, electrical energy  electromagnetic energy For reception of a signal, electromagnetic energy  electrical energy For transmission of a signal, electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water) For reception of a signal, electromagnetic energy impinging on the antenna is converted into electrical energy and fed into the receiver.

Antenna Waves are within the Radio and Microwave bands of 3kHz to 300 GHz Antenna characteristics are same for sending or receiving Direction and propagation of a wave depends on antenna type: Isotropic, Omni-directional , and Directional.

Antenna Types Isotropic antenna radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center. (ideal) Omni-directional antenna radiates power in all directions on one plane (circle , donut). Directional antenna: radiates power in particular direction. Dish and Yagi are two common types.

Antenna Patterns

Antenna Gain In a transmitting antenna, the gain describes how well the antenna converts electrical power into electromagnetic waves headed in a specified direction. In a receiving antenna, the gain describes how well the antenna converts electromagnetic waves arriving from a specified direction into electrical power. The gain of an antenna (in any given direction) is defined as the ratio of the antenna power in a given direction to the power of a isotropic antenna in the same direction. In a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction. In a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power. Gain. The gain of an antenna (in any given direction) is defined as the ratio of the power gain in a given direction to the power gain of a reference antenna in the same direction. It is standard practice to use an isotropic radiator as the reference antenna in this definition. Note that an isotropic radiator would be lossless and that it would radiate its energy equally in all directions. That means that the gain of an isotropic radiator is G = 1 (or 0 dB). It is customary to use the unit dBi (decibels relative to an isotropic radiator) for gain with respect to an isotropic radiator. Gain expressed in dBi is computed using the following formula: GdBi = 10*Log (GNumeric/GIsotropic) = 10*Log (GNumeric) Occasionally, a theoretical dipole is used as the reference, so the unit dBd (decibels relative to a dipole) will be used to describe the gain with respect to a dipole. This unit tends to be used when referring to the gain of omnidirectional antennas of higher gain. In the case of these higher gain omnidirectional antennas, their gain in dBd would be an expression of their gain above 2.2 dBi. So if an antenna has a gain of 3 dBd it also has a gain of 5.2 dBi.

Example: Isotropic Antenna (2D)
Transmit with power Pt Measure received power 1m away to be Pr Received power is same at any point equidistant from transmitter (black circle)

Example: Directional Antenna (2D)
Transmit with same power Pt Blue shape: at each point, received power is Pr Measure received power 1m away to be Px Gain of antenna (compared to isotropic) is Px/Pr

Wireless Propagation A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight (LOS). Ground waves: The signal follows the curvature of the earth’s surface, e.g. AM radio. Sky waves: The signal bounces back and forth between the earth’s surface and the earth’s ionosphere (for the higher HF frequencies), e.g. amateur radio, international radio stations. Because it depends on the Earth's ionosphere, it changes with the weather and time of day. I Frequency of signals affect how signal propagates I Different frequencies impacted by water, atmospheric noise, cosmic noise, temperature

Wireless Propagation Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon). To facilitate beyond-the-horizon propagation, satellite or terrestrial repeaters are used I Increased frequency, increased attenuation I Obstacles affect signals differently I Signals may reflect o obstacles, multiple copies of same source signal received at dierent times (multipath)

Multipath Propagation
In unguided channels, signals are not only transmitted directly from source to destination but also a lot of paths from source to destination by reflection, diffraction , …etc. So the receiver receive multiple copies (components) of transmitted signal. Line of sight (LOS) is the fastest component reaching to destination.

Electromagnetic Spectrum
Electromagnetic spectrum is used by many applications International and national authorities regulate usage of spectrum Aim: minimize interference between applications/users, while allowing many applications/users The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. We will talk about two ranges of frequencies: microwave range and radio range

Microwave and Radio Signals
Microwave signals are higher frequency signals. Higher frequency  carry large quantities of information. It is highly directional so it follow LOS propagation. The required antenna is smaller due to shorter wavelength (due to higher frequencies) ( the size of the antenna required to transmit a signal is proportional to the wavelength (λ) of the signal).

Microwave and Radio Signals
Microwave is quite suitable for point-to-point transmission and it is also used for satellite communications. Radio frequency is lower frequency signals suitable for omnidirectional applications. It follow ground or sky wave propagation

Channel Impairments Imperfect characteristics of the channel  a signal is subjected to different types of impairments. As a consequence, the received and the transmitted signals are not the same. These impairments introduce random modifications in analog signals leading to distortion. error in the bit values in digital signals. When a signal is transmitted over a communication channel, it is subjected to different types of impairments because of imperfect characteristics of the channel. As a consequence, the received and the transmitted signals are not the same. These impairments introduce random modifications in analog signals leading to distortion. On the other hand, in case of digital signals, the impairments lead to error in the bit values.

Channel Impairments

Attenuation Irrespective of whether a medium is guided or unguided, the strength of a signal falls off with distance. When a signal travels through a medium it loses energy overcoming the resistance of the medium Attenuation means loss of energy  weaker signal. The attenuation leads to several problems: 1. To be able to detect correctly the signal, the signal strength should be sufficiently high . If the strength of the signal is very low, the signal cannot be detected and interpreted properly at the receiving end. An amplifier can be used to compensate the attenuation of the transmission line. So, attenuation decides how far a signal can be sent without amplification through a particular medium.

Attenuation 1- Attenuated signal cannot be detected and interpreted properly  Amplifier 2. Attenuation Distortion 2. Attenuation Distortion - Attenuation of all frequency components is not same. - Some frequencies are passed without attenuation, some are weakened and some are blocked. As an example, after sending a square wave through a medium, the output is no longer a square wave because of more attenuation of the high-frequency components in the medium.

Delay Distortion Means that the signal changes its form or shape
How this happen A composite signal made of different frequencies components Each signal component has its own propagation speed through a medium and, therefore, 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. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same.

Noise As signal is transmitted through a channel, noise gets mixed up with the signal, along with the distortion introduced by the transmission media. Noise is any unwanted energy tending to interfere with the signal to be transmitted. The noise either be: External Noise. This is noise originating from outside the communication system Internal Noise: This is noise originating from within the communication system. As signal is transmitted through a channel, undesired signal in the form of noise gets mixed up with the signal, along with the distortion introduced by the transmission media.

Some Examples of Noise Thermal Noise: This noise is due to the random and rapid movement of electrons in any resistive component. Electrons “bump” with each other. Impulse noise is irregular pulses or noise spikes of short duration Cross talk is a result of bunching several conductors together in a single cable. Signal carrying wires generate electromagnetic radiation, which is induced on other conductors because of close proximity of the conductors

Signal-to-Noise Ratio
In the study of noise, it is not important to know the absolute value of noise. Even if the power of the noise is very small, it may have a significant effect if the power of the signal is also small. What is important is a comparison between noise and the signal. The signal-to-noise ratio (SNR) is the ratio of signal power to noise power.

SNR = Ps / Pn

Ideally, SNR = ∞ (when Pn = 0). In practice, SNR should be high as possible. A high SNR ratio means a good-quality signal. A low SNR ratio means a low-quality signal. The SNR is normally expressed in decibels, that is: SNR = 10 log10 (Ps / Pn) dB

Figure 3.30 Two cases of SNR: a high SNR and a low SNR

Example The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB SNR = 10 × / = 10,000 SNRdB = 10 log10 (10 × 10-3 / 10-6) = 10 log10 (10,000) = 40 dB

Channel Capacity It is the maximum rate at which data can be correctly communicated over a channel in presence of noise and distortion. The capacity of an analog channel is its bandwidth is the difference between the lowest and highest frequency an analog channel can convey to a receiver The capacity of a digital channel is the number of digital values the channel can convey in one second (bps). The capacity of a digital channel is the number of digital values the channel can convey in one second. It is usually measured in bits per second (bps)

Relationship Between Bandwidth and Datarate
Digital signals consist of a large number of frequency components. If digital signals are transmitted over a channel with a limited bandwidth, only those components that are within the bandwidth of the transmission medium are received. The faster the data rate of a digital signal, the higher the bandwidth will be required since the frequency components will be spaced farther apart. Therefore, a limited bandwidth will also limit the data rate that can be used for transmission

Noiseless Channel: Nyquist Bit Rate
For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate of a transmission medium as a function of its bandwidth C = 2 x BW x log2 m bits/sec, C is known as the channel capacity, BW is the bandwidth of the channel and m is the number of signal levels used.

Examples Consider a noiseless channel with a bandwidth of 3kHz transmitting a signal with two signal levels. What is the Nyquist bit rate? C=2 x 3000 x log2 2 =6000 bps Consider the same noiseless channel transmitting a signal with four signal levels. What is the Nyquist bit rate? C=2 x 3000 x log2 4 = 12,000 bps

Noisy Channel: Shannon Capacity
In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a noisy channel C = BW x log2 (1 +SNR)

C=B log2 (1 + SNR) =B log2 (1 + 0) =B log2 1 =B x 0 = 0
Example Consider an extremely noisy channel in which the value of the SNR is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as C=B log2 (1 + SNR) =B log2 (1 + 0) =B log2 1 =B x 0 = 0 This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot receive any data through this channel.

Example A telephone line normally has a bandwidth of 3000 Hz (300 to Hz) assigned for data communications. The SNR is usually For this channel the capacity is calculated as C =B log2 (1 + SNR) =3000 log2( ) = 3000 log = x = 34,860 bps This means that the highest bit rate for a telephone line is kbps. If we want to send data faster than this, we can either increase the bandwidth of the line or improve the SNR

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NET 205: Data Transmission and Digital Communication

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