Data Transmission: Signals and Media

• Successful data transmission depends on the signal quality and transmission medium characteristics.
• Data transmission occurs between a transmitter and a receiver via a transmission medium.
• Guided media involves a physical path like twisted pair, coaxial cable, or optical fiber.
• Unguided media is wireless, using air, water, or a vacuum.

Transmission Terminology

• A direct link is a transmission path from transmitter to receiver wih no intermediate devices other than amplifiers
• Point to point is a direct link between two devices sharing a medium, applying to unguided media.
• Multipoint involves more than two devices sharing the same medium.
• Simplex: Signals transmit in one direction, like cable television.
• Half-duplex: Both stations transmit, but one at a time, for example, police radio.
• Full-duplex: Both stations transmit simultaneously, such as telephone.

Analog vs. Digital Signal

• Analog signal :Signal intensity varies smoothly, continuously over time without breaks.
• Digital signal: Signal intensity maintains a constant level for some time, then abruptly changes to another constant level, also known as discrete signals.

Sine Wave

• Peak amplitude (A): Maximum strength of signal, usually in volts.
• Frequency (f): Rate at which a signal repeats, measured in Hertz (Hz) or cycles per second.
• Period (T): Time to repeat, where T = 1 / f.
• Phase: Relative position in time within a single period.

Wavelength

• Wavelength is the distance occupied by a single cycle or the distance between two points of corresponding phase of two consecutive cycles
• Given a signal with velocity v, then wavelength (λ) = vT or λf = v.
• If considering a signal at the speed of light (v = c), then v = c = 3 x 10^8 m/s.

Frequency Domain Concepts

• Signals are composed of multiple frequencies.
• Components are sine waves.
• Fourier analysis determines the components of a signal at different frequencies.
• Each component is a sinusoid.
• You can plot frequency domain functions.

Spectrum and Bandwidth

• Spectrum: Range of frequencies contained in a signal (e.g., f and 3f).
• Absolute bandwidth: Width of the spectrum (e.g., 2f).
• Effective bandwidth: Narrow band of frequencies containing most of the energy in the signal.

Data Rate and Bandwidth

• Transmission systems have a limited frequency band, limiting carried data rate.
• Square waves possess infinite components and bandwidth.
• Most energy exists in initial components.
• Limiting bandwidth leads to to distortions.

Data, Signals, and Transmission

• Data: Entities conveying information.
• Signals: Electric or electromagnetic data representations.
• Signaling: Physical signal propagation along a medium.
• Transmission: Data communication via propagation and signal processing.

Digital Data & Signals

• Text is character strings coded into bit sequences according to standards like IRA (ASCII), typically using a 7-bit code with a parity bit.
• Images are coded into pixels with a number of bits per pixel and may be compressed.
• Digital signals are cheaper and less prone to noise, but suffer more from attenuation than analog signals.

Transmission Impairments

• Signals received can differ from those transmitted, possibly degrading the signal quality or causing bit errors.
• Attenuation, delay distortion, and noise are the most significant impairments.

Attenuation

• Attenuation is the signal strength loss over distance.
• Attenuation varies depending upon frequency, which is higher at at higher frequencies.
• Received signal's strength must be high enough to be detected and higher than noise to be received without error.
• Strength increases with repeaters or amplifiers.
• Adjust for attenuation by amplifying more at higher frequencies.

Delay Distortion

• It occurs as propagation velocity varies with frequency.
• Various frequency components arrive at different times and result in phase shifts between frequencies.
• Delay Distortion is particularly critical for digital data as it causes intersymbol interference.

Noise

• Noise is unwanted signals inserted between transmission and reception.
• It is a major limiting factor in system performance.

Categories of Noise

• Thermal Noise: Thermal agitation of electrons, uniformly distributed across bandwidths, referred to as "white noise".
• Intermodulation Noise: Produces unwanted signals at a frequency that is the sum or difference of two original frequencies. For example, signals at 4 KHz and 8 KHz may cause noise at 12 KHz and interfere with a 12 KHz signal.
• Crosstalk consists of a signal from one line being picked up by another due to electrical coupling between nearby twisted pairs or microwave antennas.
• Impulse Noise is caused by external electromagnetic interferences, consisting of non-continuous, irregular pulses or spikes of short duration and high amplitude; it is a minor annoyance for analog signals, but a major source of error in digital data. A sharp spike of energy of 0.01 s duration would not destroy any voice data but would wash out 560 bits of digital data transmitted at 56 kbps.

Channel Capacity

• Channel Capacity is the maximum data rate transmitted over a communications channel under given conditions.
• Four Concepts relating to channel capacity
  • Data rate: bps
  • Bandwidth: Cycles per second (Hz)
  • Noise: average noise level over the signal path
  • Error rate: Rate of corrupted bits
• Limitations are due to physical properties.
• Noise is the main constraint on achieving efficiency.

Nyquist Bandwidth

• Nyquist bandwidth concerns the capacity of a noise-free channel.
• The rate of signal transmission is 2B, the signal frequencies are no greater than B
• Given bandwidth B, highest signal rate is 2B.
• For binary signals, 2B bps needs bandwidth B Hz.
• Increase the rate by using L signal levels.
• Data rate can be increased by increasing signals and L values
• Nyquist Formula: C = 2B log₂(L).

Shannon Capacity Formula

• It considers the relationship of data rate, noise and error rate.
• Faster data rates shorten each bit so more noise corrupts more bits.
• Given noise level, there are higher errors at higher rates
• Shannon developed a formula relating signal to noise ratio (in decibels)
• The Capacity Formula is C = B log₂(1+SNR), where SNR is signal-to-noise ratio.

Transmission Media

• Physical path between transmitter and receiver
• Conducted or guided media : Uses conductors like wires or fiber optic cables.
• Wireless or unguided media: Uses radio waves of different frequencies do not need a wire or cable.

Guided Transmission Media

• Transmission capacity depends on distance and if the medium is point to point or multipoint
• Examples include twisted pair wires, coaxial cables, and optical fiber

Twisted Pair Wires

• Consist of two insulated copper wires arranged in a regular spiral pattern and minimizes electro magnetic interference between adjacent pairs
• Twisted Pairs are used at customer facilities to carry voice and data communications.
• It's a low frequency transmission medium. -STP : The pair is wrapped with metallic foil or braid to insulate the pair
• UTP : Each wire is wrapped in an insulated plastic wrap, but the pair is encased in an outer covering

Category of Twisted Pair Wires

• Category 3 UTP : A data rate of up to 16 Mbps are achievable
• Category 5 UTP: A data rate of up to 100 Mbps are achievable and are more tightly twisted than cat 3
• Category 7 STP : Data rates in excess of 10 Gbps; more expensive, harder to work with

Twisted Pair Advantages & Disadvantages

• Advantages
  • It's Inexpensive and readily available
  • Flexible, lightweight, easy to install
• Disadvantages
  • Is susceptible to interference and noise
  • Has an attenuation problem -For analog, repeaters are needed every 5-6 km
    • For digital, repeaters are needed every 2-3 km
  • And has a low bandwidth (100 Mhz)

Coaxial Cables

• Coaxial Cables have a bandwidth of up to 500 MHz
• Consists of an inner conductor surrounded by a braided mesh
• Share a common axial center, hence the name "co-axial"

Coaxial Cable Adv & Disadvantages

• Advantages
  • Higher Bandwidth of 400-600 Mhz
  • Handles over 10,000 simultaneous voice conversations
  • Relatively less susceptible to interference vs twisted pair
• Disadvantages
  • Can be tapped easily (Pros and Cons)
  • Requires repeaters every 2 to 3 kilometers as high attenuation makes it expensive over long distance -Is bulky

Fiber Optic Cables

• Fiber optic cables are used by telephone companies instead of long distance trunk lines.
• Private companies also implement fiber optic cables within local data networks.
• It consist's of 3 concentric sections.

Types of Fiber Optic Cables

• Multimode step-index fiber: It the walls of the fiber move the light pulses to the receiver
• Multimode graded-index fiber: It acts to refract the light toward the center of the fiber by variations in density
• Single mode fiber: The light is guided down the center of an extremely narrow core

Fiber Optic Advantages & Disadvantages

• Advantages
  • Greater capacity (Hundreds of Gbps)
  • Smaller Size and lighter weight
  • Lower Attenuation
  • Immunity to environmental interference
  • Greater Repeater spacing - 10s of Km
  • Is highly secure due to tap difficulty and lack of signal radiation
• Disadvantages
  • Expensive over short distance
  • Requires highly skilled installers
  • Adding additional nodes is difficult

Wireless Transmission

• Transmission and reception are achieved by means of an antenna
• Directional antenna, at higher frequencies, transmits antenna, the transmitter and receiver must be aligned
• Omnidirectional antenna, at lower frequencies, signal spreads out in all directions, can be received by many antennas.
• Examples: Terrrestrial microwave transmission , satellite transmission, broadcast radio, infrared

Terrestrial Microwave

• Uses the radio frequency spectrum between 2 to 40 GHz.
• Requires a parabolic dish transmitter, mounted as high as possible.
• It is used by Carriers and private networks.
• Line of sight must be unbostructed between source and receiver.
• Due to curvature, earth requires repeaters spaced ~50km apart.

Microwave Applications

• Long-haul telecommunications service carried via voice and TV.
• Short Point-to-Point links for closed circuit TV or between LANs.
• Bypass Local telephone company to reach long distance routes.

Microwave Data Rates

• Band (GHz) vs Bandwidth (MHz) vs Data Rate (Mbps)
  • 2 GHz is 7 MHz at 12 Mbps
  • 6 GHs is 30 MHz at 90 Mbps
  • 11 GHs is 40 MHz at 135 Mbps
  • 18 GHz is 220 MHz at 274 Mbps

Microwave Advantages & Disadvantages

• Advantages
  • The device needs no cabling between sites. -Has wide bandwidth. -Can Multi Channel Transmission
• Disadvantages -The device is subject to interference from airplanes and rain -The device requires a line of sight and uses expensive towers/repeaters.

Satellite Transmission

• Functions as a microwave station in space to relay long distance signals
• Geostationary Satellites remain ~35,863km above the equator and move around the Earth in time with the Earth's rotation

Satellite Transmission Links:

• Earth stations communicate with satellites on an uplink (sending signals)
• The satellite will then communicate (repeat) on a downlink(receiving signals)
• The downlinks broadcast make satellites attractive for TV or programming

Satellite Applications

• Is used for tv distribution. A network provides programming from a central location via direct broadcast satellite (DBS)
• Provides long-distance telephone transmission via high-usage international trunks.
• Private businesses have private networks that use satellites.
• Global positioning services using GPS.

Principle Satellite Band Frequencies:

• C Band: 4(downlink) - 6 (uplink) GHz; the first to be designated.
• Ku Band: 12(downlink) - 14(uplink) GHz smaller, cheaper earth stations, Rain interference is problematic.
• Ka Band: 20(downlink) - 30(uplink) GHz,even smaller, cheaper receivers, even greater attenuation.

Satellite Advantages & Disadvantages

• Advantages
  • Satellites can reach large geographically.
  • High Bandwidth
  • It is cheaper over long distances.
• Disadvantages
  • Has a high initial cost
  • Susceptible to noise and interference
  • Causes propagation delay(1/4 Second)

Radio

• Omnidirectional, easily received in range of 30 MHz to 1 GHz

Forms of Radio

• Broadcast radio at 30 MHz to 1 GHz in the bands FM, UHF, VHF.
• Mobile telephony bands below 1GHZ.
• Wireless LAN with a 2.4 GHz range for 11 MB up to 525 ft.

Infrared

• Modulates incoherent infrared light, that does not penetrate walls.
• The device functions at a wave length of 900 nm, at 2 Mbps -Requires no licensing