Data Communication ch04 transmission media

Chapter 4 transmission media

Physical transmission media

Oriented media (twisted pair wires, coaxial cables, optical fibers)

Non-oriented media (ground microwave, Satellite Microwave, broadcast radio, infrared)

Operating frequency

Physical

Transmission features

Wireless Transmission Mode

Ground Wave Propagation, sky wave propagation, line of sight Propagation

Line of sight transmission (DAMAGE)

 

Transmission media is the physical channel between the transmitter and receiver in the data transmission system.

The characteristics and quality of data communication depend on the nature of the transmission media and the characteristics of the transmission signal.

Media orientation depends on the nature of the media.

Non-oriented media mainly refers to the signal bandwidth generated by the sending Antenna

Data transmission system design mainly considers data rate and transmission distance

The larger the data rate, the better the transmission distance.

 

Bandwidth-bandwidth

High bandwidth and high data rate

Transmission damage-Transmission Impairments

Damage limit transmission distance

Twisted Pair wires> coaxial cables> optical fiber cables

Interference-interference

Competing signals in an area with overlapping frequencies

Particularly noteworthy in non-oriented media

Number of receivers-# Of Receivers

Use oriented media to form a point-to-point or link to multiple devices

When multiple devices are connected and shared, there will be attenuation and distortion, and the transmission distance and/or data rate are limited.

 

Twisted Pair wires

Cheapest and most widely used

Use a line as a communication link

Low data rate and short-distance transmission

 

Twisted Pair wires-Application

Telephone Transmission System (analog signal transmission)

Analog Signal: The floor is connected to the local telephone exchange board to form a "User loop"

Digital signal: connecting to a dedicated small switch (PBX) in an office building)

For LAN (transmit digital signals)

Data Rate: 10 Mbps-100 Mbps

 

Transmission Principle:

A pair of wires are used to carry two signals in the same direction. Two signals are used to transmit the same information. One signal is reversed at the receiving end, and the two waveforms are superimposed.

Why use two wires twisted together to form a communication link?

Reduce interference with other lines

The two signal lines generate equal electromagnetic waves in the opposite direction. They are superimposed as standing waves to offset each other and effectively reduce external electromagnetic interference. That is, they do not disturb others as much as possible.

Reduce interference from other lines

Twisted Pair wires carry two reversed-phase signals of the same channel of information. If they are subject to electromagnetic interference during transmission, because they are pairs of twisted pair signal lines, which are very close to each other, they can be considered as similar, the effect of noise on them is equal, that is, the superposition of two signal waveforms to the same extent. One of the signals is reversed at the receiving end. In this way, the signal components in the two waveforms are equal to the same size and direction, while the noise components are in the opposite direction. Then the two waveforms are superimposed, and the signal component is enhanced, while the noise component is eliminated due to offset.

 

Analog transmission mode

Use an amplifier every 5 km to 6 km

Digital Transmission Mode

Use numbers or analog signals

Use a forwarder every 2 km to 3 km

Compared with other oriented transmission media

Transmission distance, bandwidth, and data rate are limited

Susceptible to attenuation, interference, and noise (p81 fig 4.3d)

 

Unshielded twisted pair wires-unshielded twisted pair (UTP)

Common telephone lines

Cheapest

Easy to use and easy to install

Subject to external electromagnetic interference

Shielded twisted pair-shielded twisted pair (STP)

Use a metal mesh cover or skin protection to reduce interference

High data rate, better performance

Expensive

Inconvenient to use

Comparison between the two (P83, table 4.2)

 

On 1991, the American Electronics Industry Association announced EIA-568 standards

EIA-568A standards (1995)

Unshielded twisted pair wires

Cat 3 (voice cable)

Transmission features up to 16 MHz

Exists in most office buildings

The length of each twist is 7.5 to 10 cm.

Cat 4

Transmission features up to 20 MHz

Cat 5 (data-level cable)

Transmission features up to 100 MHz

Usually pre-built in the new office building

The length of each twist is 0.6 to 0.85

Shielded twisted pair wires

100 shielded twisted pair wires

150 shielded twisted pair wires

The reduction of the twist distance increases the performance, but also increases the cost.

 

 

Coaxial Cable

It is composed of a hollow cylindrical outer conductor and an internal conductor inside the cylinder.

 

Advantages:

Less vulnerable to crosstalk and interference than twisted pair wires

Longer transmission distance and higher data rate

Application:

Widely used

Television Broadcasting-television distribution

Cable TV-cable TV

Long-distance telephone transfer-can carry 10,000 voice cballs simultaneously

Being replaced by Optical Fiber gradually

Connections between short-distance devices, such as high-speed I/O channels between computer systems

Lan

 

Transmission features:

Analog Signal

Use an amplifier every several thousands of meters

The higher the frequency, the closer the amplifier interval.

Valid spectrum can be extended to 500 MHz

Digital Signal

One forwarder per kilometer

The higher the data rate, the closer the interval is.

 

Optical Fiber

Light

Light is an electromagnetic energy form

Optical Fiber

Media that is slim, flexible, and capable of transmitting light

Generally, glass and plastic materials are made.

Components

Core

Glass or plastic

Layer

The interface between the core and the core acts as a reflector to prevent the light from escaping from the core.

Shield

 

Advantages:

Larger capacity

Transmit tens of thousands of meters at a data rate of several hundred GB

Smaller, lighter

Lower Attenuation

Isolated Electromagnetic Field

Not afraid of interference such as impulse noise and Crosstalk

More secure and hard to be tapped

The repeater has a longer interval.

Lower cost and less fault points

 

Application

Long-distance lines

Telephone Network, 20 k-60 k voice channel, average length 1500 km

Urban relay circuit

KB voice channel with an average length of 12 km

Agricultural switching trunk Circuit

5 K voice channel, length range: 40-160 km

Optical fiber user Loop

Provides voice, data, images, videos, and other services

LANs

Capacity: 100 Mbps-10 Gb BPS

 

Principle: The beam is reflected and transmitted along the optical fiber forward

Transmission features:

Optical Fiber System Frequency Operation Range: 10 ^ 14 to 10 ^ 15Hz

Refraction and reflection of light

The light from a high-density medium to a low-density medium increases with the angle of incidence, and gradually moves away from the vertical axis to the horizontal axis.

The change of the incident angle increases the angle of refraction to 90 degrees, and the refraction light completely transmits horizontal light along the media interface. The incident angle is called the critical angle or the total reflection angle.

The light will be shifted to a medium with a large refractive index.

 

Multimode

Launch from multiple angles

Multiple propagation paths

Category

Multimode mutation: the refractive index of the external media is fixed, and the refractive index of the internal optical core is also fixed. The internal beam goes forward in line

Disadvantage: the length of each path leads to different transmission times, and the optical pulse is eventually propagated.

Suitable for short-distance transmission

 

Multimode gradient: the refractive index of the external media is fixed, and the refractive index of the internal optical core decreases with the increase of the radius. The internal beam goes forward in a spiral shape

The high center refractive index slows down the speed of light along the axis. As a result, the beams in multiple transmission paths almost reach at the same time.

Commonly Used in LAN

 

Single-Mode

One angle, one transmission path, suitable for long-distance transmission

 

Light Source

Light Emitting Diode-Light Emitting Diode (LED)

Low Price

Wide operating temperature range

Long life

Laser Diode injection-injection laser diode (ILD)

Higher efficiency

High data rate supported

The available bandwidth of light can reach the magnitude of THz.

1thz = 10 ^ 12Hz

 

Antenna

Frequency Range of Wireless Transmission

Radio Wave Zone

30 MHz to 1 GHz, omnidirectional Application

Microwave Frequency

1 GHz to 40 GHz, high direction, point-to-point transmission

Infrared Spectrum

3x10 ^ 11Hz to 2x10 ^ 14Hz, point-to-point and multi-point applications in specific regions

Non-oriented media is sent and received through an antenna.

 

Antenna

Electrical conductor or electrical conductor system that emits or receives electromagnetic energy

Concept

Coaxial Antenna

Equally send energy to all directions

Parabolic reflection Antenna

Antenna Gain

 

Parabolic reflection Antenna

Antenna Gain

The ratio of the output power of the antenna in a specific direction to the power produced by the omnidirectional antenna in any direction

P90, formula 4.1

 

Ground microwave

Ground microwave uses a parabolic disc antenna to merge electromagnetic waves into a fine-beam

Application

Long-distance telecommunication service: such as voice and television

Over-line-of-sight transmission, requiring less amplifier or repeater than coaxial cable

Point-to-point short distance between buildings for closed-circuit television and LAN

Cellular System (chapter 14)

Spectrum range: 1-40 GHz: the higher the frequency, the wider the bandwidth, the higher the data rate, but the greater the attenuation

 

Main loss of Microwave Transmission

Attenuation

Loss formula:

The higher the frequency, the greater the distance, the greater the attenuation

The higher the frequency, the smaller the antenna is, the lower the price.

 

Satellite Microwave

Communication Satellite

Microwave Relay Station

Receives signals from one frequency band, scales up or regresses, and then sends signals from another frequency band.

 

The position of a satellite relative to the Earth is constant.

Height 35,863 km

Application

Television Broadcasting, long-distance telephone transmission, dedicated commercial network (VSAT), Global Positioning

The optimum frequency range is 1 to 10 GHz.

Transmission latency of 0.25 seconds from the earth station to the receiver

Satellite communication is essentially broadcast, so Satellite Microwave is inherently a broadcast Tool

 

Overview of GPS principles

(GPS satellite constellation consists of 27 satellites, 24 of which are working satellites and 3 are backup satellites.

Satellite distribution ensures that at least four satellites can be observed at any point in the world .)

Coordinates of satellite I are (XI, Yi, zi)

The coordinates of the receiver are (x, y, z)

The satellite's transmitting time is Ti, the time when the receiver receives the signal is TR, and the time error is B;

The formula is met:

If you measure with four or more satellites at the same time, you can know the exact location.

 

Broadcast Radio

Omnidirectional

Application

Broadcast FM band

Television broadcasting in the ultra-high frequency and ultra-high frequency

Data Network applications

Transmission features

The bandwidth ranges from 30 MHz to 1 GHz.

The transmission range above 30 MHz is limited to the line of sight range

Major sources of Radio damage

Attenuation (same as microwave)

Multi-path interference

Reflection of natural or man-made objects in the ground, waters

 

Infrared

Transmit/receiver modulated irrelevant infrared light for communication

Direct transmission or reflection within the line of sight range

Unable to penetrate the wall, relatively safe, not interfering

Frequency allocation does not exist.

 

4.3 Wireless Communication

Based on the signal transmission path (P95 table 4.7 ):

Ground Wave Propagation below 2 MHz

2 Mbit/s-30 MHz

Over 30 MHz line of sight Propagation

 

Line of sight transmission

Optical line of sight and radio line of sight p98 fig 4.9

Optical line of sight (distance between antenna and horizon)

Radio line of sight

The maximum distance between two antennas for line-of-sight communication is the sum of the radio line-of-sight between the two antennas.

 

4.4 line of sight transmission

Unique damage type

Free Space Loss

The farther away from the transmitting antenna, the Attenuation Caused by signal diffusion weakens the received signal power.

Atmospheric Absorption

Mainly from water vapor and oxygen

Scattering Caused by rain and fog causes Attenuation

Multi-path

Signals are reflected by obstacles and transmitted along multiple paths. For example, when passing through the water area, the wind causes the reflection surface of the water to move, resulting in multi-path propagation.

Refraction

Atmospheric refraction, altitude change, or other atmospheric state changes

 

Summary of this Chapter

Physical transmission media

Oriented media (twisted pair wires, coaxial cables, optical fibers)

Non-oriented media (ground microwave, Satellite Microwave, broadcast radio, infrared)

Operating frequency

Physical

Transmission features

Wireless Transmission Mode

Ground Wave Propagation, sky wave propagation, line of sight Propagation

Line of sight transmission (DAMAGE)

 

Questions in this Chapter

1. What types of commonly used oriented media are available?

Twisted Pair wires, coaxial cables, optical fiber cables

2. What types of optical fiber transmission modes are available?

Single-mode, multimode. The difference is whether an angle is transmitted and whether a transmission path is transmitted.

3. What are the methods for wireless communication? They are?

Ground Wave Propagation, sky wave propagation, line of sight Propagation