Wireless Networks: OSI Model & Technologies

• Wireless communications and networks is available as a book by William Stallings

OSI Model

• The OSI model illustrates the client side and the server side with 7 layers to each
• The 7 layers are Physical, Data-Link, Network, Transport, Session, Presentation, and Application from Layer 1 to Layer 7

Wireless Age

• In 1896, Guglielmo Marconi invented the wireless telegraph
• The wireless telegraph communicated by encoding alphanumeric characters in an analog signal and sent telegraphic signals across the Atlantic Ocean
• Communication satellites were launched in the 1960s
• Wireless technology has advanced through radio, television, mobile telephones, and communication satellites,
• Satellite communications, wireless networking and cellular technology have also advanced recently

Broadband Wireless Technology

• Higher data rates can be obtained with broadband wireless technology, which includes graphics, video and audio
• Broadband wireless shares the advantages of all wireless services which include convenience and reduced cost
• Services can be deployed faster than fixed services because there are no cable plant costs
• These services are also mobile and able to be deployed almost anywhere

Limitations and Difficulties of Wireless Technologies

• Wireless is convenient and less expensive
• Wireless technologies are limited by political and technical difficulties
• There is a lack of an industry-wide standard
• Device limitations include:
  • Small LCDs on mobile telephones which can display a few lines of text
  • Most mobile wireless devices use wireless Hypertext Markup Language (HTML)

Background

• Part One of the wireless book, provides a preview and context
• Part One also covers basic topics such as Data Communications and TCP/IP

Transmission Fundamentals

• Chapter Two provides an overview of transmission topics and data communications concepts
• This includes techniques of analog and digital data transmission
• This chapter covers Channel Capacity, Transmission Media, and Multiplexing

Communication Networks

• Chapter Three compares basic communication network technologies
• The technologies covered are Circuit Switching, Packet Switching, Frame Relay, and ATM

Protocols and the TCP/IP Protocol Suite

• Protocol architecture, an overview of TCP/IP, Open systems interconnection (OSI) reference model, and internetworking are the fourth chapter

Wireless Communication Technology

• Part Two describes the underlying technology of wireless transmission, as well as encoding of analog and digital data for wireless transmission

Antennas and Propagation

• Chapter Five looks at the principles of radio and microwave, antenna performance, wireless transmission modes, and fading

Signal Encoding Techniques

• The sixth chapter covers wireless transmission, analog and digital data, and analog and digital signals

Spread Spectrum

• Frequency hopping, direct sequence spread spectrum, and code division multiple access (CDMA) are discussed

Coding and Error Control

• Forward error correction (FEC), using redundancy for error detection, and automatic repeat request (ARQ) techniques are covered

Wireless Networking

• Part Three examines major types of networks, including, satellite-based networks, cellular networks, cordless systems and fixed wireless access schemes
• The use of mobile IP and Wireless Access Protocol (WAP) to provide Internet and Web access is also examined

Satellite Communications

• Geostationary satellites (GEOS), low-earth orbiting satellites (LEOS), medium-earth orbiting satellites (MEOS), and capacity allocation are discussed

Cellular Wireless Networks

• The tenth chapter studies cellular wireless network design issues
• Topics include first generation analog (traditional mobile telephony service), second generation digital cellular networks, time-division multiple access (TDMA), code-division multiple access (CDMA), and third generation networks

Cordless Systems and Wireless Local Loop

• Cordless systems and Wireless local loop (WLL) and Radio in The Loop are studied

Mobile IP and Wireless Access Protocol

• Modifications to IP protocol to accommodate wireless access to Internet
• Wireless Application Protocol (WAP), which provides mobile users access to telephony and information services including Internet and Web
  • This includes wireless phones, pagers and personal digital assistants (PDAs)

Wireless Local Area Networks

• Part Four examines underlying wireless LAN technology and stardardized approaches to local wireless networking

Wireless LAN Technology

• the overview of LANs and wireless LAN technology and applications are overviewed in this section
• Trasmission techniques of wireless LANs such as spread spectrum, narrowband microwave and infrared technologies are also discussed

IEEE 802.11 Wireless LAN Standard

• The IEEE 802.11 committee defines wireless LAN standards

Bluetooth

• Bluetooth is an open specification for wireless communication and networking
• This includes personal computers, mobile phones, and other wireless devices

Internet and Web Resources

• The book has a webpage which can be found via WilliamStallings.com/Wireless1e.html
• It contains useful websites, errata sheets, figures, tables, slides, internet mailing lists, and wireless courses
• There is also a Computer Science Student Support Site available at WilliamStallings.com/StudentSupport.html
• Relevent newsgroups include comp.std.wireless and comp.dcom.*

Electromagnetic Signal

• This may be expressed as a function of frequency
• Signals consist of components of different frequencies

Time-Domain Concepts

• Analog signal signal intensity varies in a smooth fashion over time and has no breaks or discontinuities
• Digital signal signal intensity maintains a constant level for some period of time and then changes to another constant level
• Periodic signals are analog or digital signal patterns that repeats over time, this is expressed as: s(t +T ) = s(t ) -∞ < t < ∞
  • with T being the period of the signal
• Aperiodic signals are analog or digital signal patterns that do not repeat over time
• Peak amplitude (A) = maximum value or strength of the signal over time, measured in volts
• Frequency (f) is the rate, in cycles per second, or Hertz (Hz) at which the signal repeats
• Period (T) is the amount of time it takes for one repetition of the signal, where T = 1/f
• Phase (Φ) is the measure of the relative position in time within a single period of a signal
• Wavelength (λ) is the distance occupied by a single cycle of the signal or the distance between two points of a corresponding phase of two consecutive cycles

Sine Wave Parameters

• General sine waves follow the formula: s(t ) = A sin(2πft + Φ)
• Figure 2.3 shows the effect of varying each of the three parameters:
• The examples of varying sine waves are A = 1, f = 1 Hz, Φ = 0, thus T = 1s
  • Reduced peak amplitude at A=0.5
  • Increased frequency at f = 2, thus T = ½
  • Phase shift at Φ = π/4 radians (45 degrees)
• 2π radians = 360° = 1 period

Time vs. Distance

• When the horizontal axis is time, graphs display the value of a signal at a given point in space as a function of time
• When the horizontal axis is in space, graphs display the value of a signal at a given point in time as a function of distance
• At a particular instant of time, the intensity of the signal varies as a function of distance from the source

Frequency-Domain Concepts

• Fundamental frequency occurs when all frequency components of a signal are integer multiples of one frequency
• Spectrum is the range of frequencies that a signal contains
• Absolute bandwidth is the width of the spectrum of a signal
• Effective bandwidth (or just bandwidth) is the narrow band of frequencies that most of the signal's energy is contained in
• Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases
• The period of the total signal is equal to the period of the fundamental frequency

Relationship between Data Rate and Bandwidth

• The greater the bandwidth, the higher the information-carrying capacity
• Any digital waveform will have infinite bandwidth, however, the transmission system will limit the bandwidth that can be transmitted
• For any given medium, the greater the bandwidth transmitted also greatly increases the cost
• Limiting the bandwidth creates distortions

Data Communication Terms

• Data refers to entities that convey meaning, or information
• Signals are electric or electromagnetic representations of data
• Transmission is the communication of data by the propagation and processing of signals

Examples of Analog and Digital Data

• Analog data can be video and/or audio
• Digital data can be text and integers

Analog Signals

• Analog signals are continuously varying electromagnetic waves that may be propagated over a variety of media, depending on frequency
• Examples of media are: copper wire, fiber optic cable, and atmosphere or space propagation
• Analog signals can propagate analog and digital data

Digital Signals

• Digital signals are a sequence of voltage pulses that may be transmitted over a copper wire medium
• They are generally cheaper than analog signaling
• Digital signals are less susceptible to noise interference but suffer more from attenuation, although they can propagate both analog and digital data

Analog Transmission

• Transmits analog signals without regard to content
• Attenuation creates limits on the length of the transmission link
• Cascaded amplifiers can boost a signal's energy over longer distances, though these cause distortion
  • Analog data can tolerate this distortion
  • These amplifiers could also introduce errors in digital data

Digital Transmission

• Digital transmission is concerned with the content of the signal
• Attenuation of the signal endangers the integrity of the data
• With a Digital Signal:
  • Repeaters achieve greater distance
  • Repeaters recover the signal and retransmit
• Analog signal carrying digital data:
  • Retransmission device recovers the digital data from the analog signal
  • And generates new, clean analog signal

Channel Capacity

• Impairments, such as noise, limit data rate that can be achieved
• Channel Capacity - The maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

Concepts Related to Channel Capacity

• Data rate - rate at which data can be communicated (bps)
• Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)
• Noise - average level of noise over the communications path
• Error rate - rate at which errors occur
  • Error = transmit 1 and receive 0; transmit 0 and receive 1

Nyquist Bandwidth

• For binary signals, which have two voltage levels: C = 2B
• With multilevel signaling: C = 2B log2 M
  • M = number of discrete signal or voltage levels

Signal-to-Noise Ratio

• Signal-to-noise Ratio is the ratio of the power in a signal to the power contained in the noise that's present at a particular point in the transmission
• It is typically measured at a receiver's input
• The formula for signal-to-noise ratio (SNR, or S/N) is: (SNR)dB = 10log10 signal power / noise power
• High SNR means a high-quality signal, also resulting in a low number of required intermediate repeaters
• SNR sets an upper bound on achievable data rate

Shannon Capacity Formula

• Shannon Capacity's formula can be expressed as: C = Blog2(1+SNR)
• This formula represents the theoretical maximum that can be achieved, in practice, only much lower rates are achievable
• The Shannon formula assumes white noise (thermal noise)
• Impulse noise is not accounted for, nor are attenuation distortion or delay distortion

Example of Nyquist and Shannon Formulations

• Given a Spectru between 3 MHz and 4 MHz with a SNRdB of 24 dB the following can be calculated
• B = 4 MHz – 3 MHz = 1 MHz
• SNRdB = 24 dB = 10log10 (SNR)
• SNR = 251
• C = 106 x log2(1+251)≈ 106 x 8 = 8Mbps

Example of Nyquist and Shannon Formulations

• To calculate the signaling levels that are required use: C = 2Blog2M
• Example: 8x106 = 2 x (106)xlog2 M
• 4 = log2 M
• This means M = 16

Classifications of Transmission Media

• Transmission Medium includes the physical path between transmitter and receiver
• Guided Media uses guides along or through a solid medium
  • examples include copper twisted pair, copper coaxial cable, and optical fiber
• Unguided Media Provides means of transmission but does not guide electromagnetic signals
  • Usually this refers to as wireless transmission. examples include atmosphere or outer space

Unguided Media

• Transmissions and receptions are achieved by means of an antenna
• Wireless transmissions have two configurations of antenna-based transmission: Directional Omnidirectional

General Frequency Ranges

• Radio frequency range is between 30 MHz to 1 GHz and is suitable for omnidirectional applications
• Microwave frequency range is between 1 GHz to 40 GHz and directional beams are possible
  • This range is suitable for point-to-point transmission and is used for satellite communications
• Infrared frequency range is roughly, 3x1011 to 2x1014 Hz
  • This is useful in local point-to-point multipoint applications within confined areas

Satellite Microwave

• Description of communications satellite
  • Microwave relay station
  • Used to link two or more ground-based microwave transmitter/receivers
  • Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink)
• Applications for TV, long-distance telephone and private business

Broadcast Radio

• Broadcast Radio Antennas
  • Omnidirectional antennas
  • Antennas are not required to be dish-shaped
  • Antennas need not be rigidly mounted to a precise alignment
• Broadcast Radio Applications
  • Broadcast radio use a radio antenna -VHF and part of the UHF band which is 30 MHZ to 1GHz
  • Covers FM radio and UHF and VHF television

Multiplexing

• Capacity of transmission medium usually exceeds capacity required for transmission of a single signal
• Multiplexing - carrying multiple signals on a single medium
• More efficient use of transmission medium

Reasons for Widespread Multiplexing Use

• Cost per kbps of transmission facility declines with an increase in the data rate
• Cost of transmission and receiving equipment declines with increased data rate
• Individual data communicating devices usually require relatively modest data rate support

Multiplexing Techniques

• Frequency-division multiplexing (FDM)
  • This takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal
• Time-division multiplexing (TDM)
  • This takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal