Chapter 17Wireless WANs

Reasons for Wireless Networks

Wireless communications is likely to be viewed as an essential part of an enterprise network infrastructure when:

Mobile communication is neededCommunication must take place in a hostile or difficult terrain that makes wired communication difficult or impossibleA communication system must be deployed quicklyCommunication facilities must be installed at low initial costThe same information must be broadcast to many locations

Disadvantages of Wireless Networks

Operates in a less controlled environment, so is more susceptible to interference, signal loss, noise, and eavesdroppingGenerally, wireless facilities have lower data rates than guided facilitiesFrequencies can be more easily reused with guided media than with wireless media

Cellular Wireless Networks

One of the most revolutionary developments in telecommunicationsSupports users in locations that are not easily served by wired networksUsed for mobile telephones, personal communications systems, wireless Internet and wireless Web applications, and more

Cellular Network Organization

Cellular Geometries

Frequency Reuse

Each cell has a base transceiverTransmission power is carefully controlled to allow communication within the cell using a given frequency while limiting the power at that frequency that escapes the cell into adjacent onesGenerally 10 to 50 frequencies are assigned to each cell

Frequency Reuse

Patterns

Increasing Capacity

Table 17.1Typical Parameters for Macrocells

and Microcells

Cellular System Overview

Example of Mobile

Cellular Call

Mobile Telephony

Multiple Access

Four ways to divide the spectrum among active users:

Code Division Multiple Access (CDMA)

Based on direct sequence spread spectrum (DSSS)Provides immunity from various kinds of noise and multipath distortionThe earliest applications of spread spectrum were military, where it was used for its immunity to jammingCan be used for hiding and encrypting signalsSeveral users can independently use the same (higher) bandwidth with very little interference

CDMA Example

Cellular Multiple Access

Schemes

Third Generation Systems

Intended to provide high speed wireless communications for multimedia, data, and video in addition to voiceReflects trend toward universal personal telecommunications and communications accessPersonal communications services (PCSs) and personal communication networks (PCNs) are objectives for 3G wirelessPlanned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity

The ITU’s International Mobile Telecommunications for the year 2000 (IMT-2000) initiative defined the ITU’s view of third generation capabilities as follows:

Voice quality comparable to the PSTN144 kbps data rate available to users in high-speed motor vehicles over large areas384 kbps data rate available to pedestrians standing or moving slowly over small areasSupport for 2.048 Mbps for office useSupport for both packet and circuit switched data services

Adaptive Internet interface to reflect efficiently the common asymmetry between inbound and outbound trafficMore efficient use of the spectrum in generalSupport for a wide variety of mobile equipmentFlexibility to allow the introduction of new services and technologies

Wireless Application Protocol (WAP)

WAP Specifications:

A programming model based on the WWW Programming ModelA markup language, the Wireless Markup Language (WML), adhering to XMLA specification of a small browser suitable for a mobile, wireless deviceA lightweight communications protocol stackA framework for wireless telephony applications (WTAs)

WAP Programming Model

Wireless Markup Language

Does not assume a standard keyboard or a mouse as an input deviceDesigned to work with telephone keypads, styluses, and other input devices common to mobile, wireless communicationDocuments are subdivided into small, well-defined units of user interaction called cardsUsers navigate by moving back and forth between cardsUses a small set of markup tags appropriate to telephony-based systems

Microbrowser

Wireless Telephony Applications (WTAs)

Provides an interface to the local and wide area telephone systemsUsing WTA application developers can use the microbrowser to originate telephone calls and to respond to events from the telephone network

WAP Network Schematic

Fourth Generation (4G) Network Requirements

According to the ITU, an IMT-Advanced (or 4G) cellular system must fulfill a number of minimum requirements:

Be based on an all IP packet-switched networkSupport peak data rates of up to 100 Mbps for high mobility mobile access and up to 1Gbps for low mobility accessDynamically share and use the network resources to support more simultaneous users per cellSupport smooth handovers across heterogeneous networksSupport high quality of service for next generation multimedia applications

Table 17.2 Third Generation (3G) versus

Fourth Generation (4G) Networks

ThirdGeneration

VersusFourth

GenerationCellular

Networks

Orthogonal Frequency Division Multiple Access (OFDMA)

4G Network Evolution

Because they use new multiple access schemes such as

OFDMA and support significantly higher uplink and downlink

speeds, the rollout of 4G networks and services will also be

tempered by the evolution of 4G capable smartphones

3G CDMA based solutions are likely to remain as the core

cellular service offering for mobile operators while 4G

technologies evolve and mature

With an ever increasing number of mobile device uses the

migration period to 4G technologies is likely to be lengthy

Satellite Communications

The heart of a satellite communications system is a satellite based antenna in a stable orbit above the earthTwo or more stations on or near the earth communicate via one or more satellites that serve as relay stations in spaceThe antenna systems on or near the earth are referred to as earth stationsTransmission from an earth station to the satellite is an uplink Transmissions from the satellite to the earth station are downlinksThe transponder in the satellite takes an uplink signal and converts it to a downlink signal

Geostationary Earth Orbit

Advantages of Geostationary Orbits

Satellite is stationary relative to the earth so there is no problem with frequency changes due to the relative motion of the satellite and antennas on earth (Doppler effect)Tracking of the satellite by its earth stations is simplifiedAt 35,863 km above the earth, the satellite can communicate with roughly a fourth of the earth’s surface

Three satellites in GEO separated by 120o can cover most of the inhabited portions of the entire earth excluding only the areas near the north and south poles

Problems withGeostationary Orbits

Signal can get quite weak after traveling over 35,000 kmPolar regions and the far northern and southern hemispheres are poorly served by GEOSsEven at the speed of light, the delay in sending a signal from a point on the equator beneath the satellite to the satellite and back is substantial

Low-Earth and Medium-Earth

Orbits

LEO CharacteristicsCircular or slightly elliptical orbit < 2000 kmOrbit period is in the range of 1.5 to 2 hoursDiameter of coverage is about 8000 kmRound-trip signal propagation delay is < 20 msMaximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is up to 20 minutesSystem must be able to cope with large Doppler shifts, which change the frequency of the signalSignificant atmospheric drag on a LEO satellite results in gradual orbital deterioration

LEO AdvantagesReduced propagation delay Received LEO signal is much stronger than that of GEO signals for the same transmission powerLEO coverage can be better localized so that spectrum can be better conservedOn the other hand, to provide broad coverage over 24 hours, many satellites are needed

Types of LEOs

MEO Characteristics

Circular orbit at an altitude in the range of 5000 to 12,000 kmOrbit period is about 6 hoursDiameter of coverage is from 10,000 to 15,000 kmRound trip signal propagation delay is less than 50 msMaximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is a few hoursRequire fewer hand-offs than LEOSs

Satellite Communication Configurations

Satellite Network ApplicationsAmong the most important applications for satellites are:

Typical VSAT Configuration

Summary Cellular wireless network

Cellular network organization Operations of cellular systems

Multiple access CDMA Which access method to use

Satellite communications Satellite orbits Satellite network

configurations Applications

Chapter 17: Wireless WANs

Third generation wireless communication WAP WML WTAs Microbrowser

Fourth generation wireless communication 4G network

requirements OFDMA 4G network evolution