Wireless Transmission and Services

Chapter 9

Objectives

Associate electromagnetic waves at different points on the wireless spectrum with their wireless services

Identify characteristics that distinguish wireless transmission from wire-bound transmission

Explain the architecture and access methods used in cellular net-works and services

Understand the differences between wireless and wireline local loops

Describe the most popular WLAN standards, including their advantages, disadvantages, and uses

Identify the major satellite positioning schemes and list several telecommunications services that rely on satellite transmission

The Wireless Spectrum

The Wireless Spectrum

The Wireless Spectrum

Characteristics of Wireless Transmission

Antennas

Radiation pattern - the relative strength over a three dimensional area of all the electromagnetic energy the antenna sends or receives.

Directional antenna - issues wireless signals along a single direction

Antennas

Omni-directional antenna - issues and receives wireless signals with equal strength and clarity in all directions.

Signal Propagation

Reflection - the wave encounters an obstacle and bounces back towards its source.

Diffraction - a wireless signal splits into secondary waves when it encounters an obstruction.

Scattering - the diffusion, or the reflection in multiple different directions of a signal.

Signal Propagation

Signal Propagation

Fading and Delay

Fading: a change in signal strength as result of some of the electromagnetic energy being scattered, reflected, or diffracted after being issued by the transmitter.

Diversity - the use of multiple antennas or multiple signal transmissions to compensate for fading and delay.

Signal Propagation

Attenuation - after a signal has been transmitted, the farther it moves away from the transmission antenna, the more it weakens.

Interference - because wireless signals are a form of electromagnetic activity, they can be hampered by other electromagnetic energy, resulting in interference.

Narrowband, Broadband, and Spread Spectrum Signals

Narrowband - a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies.

Broadband - a type of signaling that uses a relatively wide band of the wireless spectrum.

Spread spectrum - the use of multiple frequencies to transmit a signal.

Fixed vs. Mobile

Fixed vs. Mobile

Cellular Communications

Mobile telephone service - a system for providing telephone services to multiple, mobile receivers using two-way radio communication over a limited number of frequencies.

Mobile wireless evolution: First generation Second generation Third generation

Principles of Cellular Technology

Cells

Cellular Call Completion

Components of a signal: Mobile Identification Number (MIN) - an

enclosed representation of the mobile telephone’s 10-digit telephone number.

Electronic Serial Number (ESN) - a fixed number assigned to the telephone by the manufacturer.

System Identification Number (SID) - a number assigned to the particular wireless carrier to which the telephone’s user has subscribed.

Cellular Call Completion

Call Completion

Advanced Mobile Pone Service (AMPS)

A first generation cellular technology that encodes and transmits speech as analog signals.

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Each voice signal is digitized and assigned a unique code, and then small components of the signal are issued over multiple frequencies using the spread spectrum technique.

Global System for Mobile Communications (GSM)

A version of time division multiple access (TDMA) technology, because it divides frequency bands into channels and assigns signals time slots within each channel.

Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common in the United States.

Makes use of silences in a phone call to increase its signal compression, leaving more open time slots in the channel.

Emerging Third Generation (3G) Technologies

The promise of these technologies is that a user can access all her telecommunication services from one mobile phone.

CDMA2000 - a packet switched version of CDMA.

Wideband CDMA (W-CDMA) - based on technology developed by Ericson, is also packet-based and its maximum throughput is also 2.4 Mbps.

Wireless Local Loop (WLL)

A generic term that describes a wireless link used in the PSTN to connect LEC central offices with subscribers.

Acts the same as a copper local loop.

Used to transmit both voice and data signals.

Local Multipoint Distribution Service (LMDS)

A point-to-multipoint, fixed wireless technology that was conceived to supply wireless local loop service in densely populated urban areas and later on a trial basis to issue television signals.

A disadvantage is that its use of very high frequencies limits its signal’s transmission distance to no more than 4km between antennas.

Multipoint Multichannel Distribution System (MMDS)

Uses microwaves with frequencies in the 2.1 to 2.7 GHz range of the wireless spectrum.

One advantage is that because of its lower frequency range, MMDS is less susceptible to interference.

MMDS does not require a line-of-sight path between the transmitter and receiver.

WLAN Architecture

WLAN Architecture

WLAN Architecture

Wireless Networking Standards

802.11 - IEEE’s Radio Frequency Wireless networking standard committee.

802.11b - uses direct sequence spread spectrum (DSSS) signaling. Also used the 2.4 - 2.4835 GHz frequency range and separates it into 14 overlapping 22-MHz channels.

802.11g - designed to be just as affordable as 802.11b while increasing its maximum capacity from 11 Mbps through different encoding techniques.

802.11a - uses multiple frequency bands in the 5 GHZ range. Like 802.11g, 802.11a provides a maximum throughput of 54 Mbps.

Bluetooth

A mobile wireless networking standard that uses direct sequence spread spectrum (DSS) signaling in the 2.4 GHz band to achieve a maximum throughput of less than 1 Mbps.

Designed to be used on small networks composed of personal communications devices, also known as personal area networks.

Personal Area Networks

Satellite Positioning

The original method for positioning satellites above the earth was in geosynchronous orbit.

Geosynchronous satellites are positioned approximately 35,800 km (22,300 miles) above the earth’s equator.

An alternative to GEO satellites are low earth orbiting (LEO) satellites.

Satellite Positioning

Satellite Services

Digital broadcasting - To deliver content to subscribers, networks (or other multimedia providers) uplink their audio and video signals to a satellite, which then downlinks the signals, in a broadcast fashion, to earth.

Analog broadcasting - Traditional analog television and radio signals can be issued from a terrestrial transmitter to a satellite and then downlinked to another terrestrial location within seconds.

Mobile Wireless - Services such as cellular telephone, paging, and other PCS applications are well suited to LEO or MEO satellite transmission.

Satellite Services

Tracking and monitoring - Two-way satellite communications can be used to monitor the whereabouts and condition of wildlife, mobile weather sensors, marine vessels, and so on anywhere in the world.

Global positioning service (GPS) - A service that expands on remote monitoring functions, GPS allows a mobile station on earth to exchange signals with a satellite to determine its precise location.

Wide area networks - Private companies use satellite transmission to connect multiple locations on their WANs.

Summary

The wireless spectrum, the range of frequencies within the electromagnetic spectrum that are used for telecommunications services, starts at 9 KHz and ends at 300 GHz.

Cellular telephone service is distinguished from other mobile two-way radio services by its use of cells to reuse limited frequencies within a certain geographical area.

Wireless LANs (WLANs) use the same protocols and a similar architecture as wire-bound LANs.