Chapter 14:Wireless WANs Business Data Communications, 6e
Reasons for Wireless Networks • Mobile communication is needed. • Communication must take place in a terrain that makes wired communication difficult or impossible. • A communication system must be deployed quickly. • Communication facilities must be installed at low initial cost. • The same information must be broadcast to many locations.
Problems with Wireless Networks • Operates in a less controlled environment, so is more susceptible to interference, signal loss, noise, and eavesdropping. • Generally, wireless facilities have lower data rates than guided facilities. • Frequencies can be more easily reused with guided media than with wireless media.
Cellular Wireless Networks • One of the most revolutionary developments in telecommunications • Supports users in locations that are not easily served by wired networks • Used for mobile telephones, personal communications systems, wireless Internet and wireless Web applications, and more
Cellular Network Organization • Uses multiple low-power transmitters (≤100W) • Areas divided into cells, each one served by its own antenna. • Each cell allocated a band of frequencies, and is served by a base station • Adjacent cells are assigned different frequencies to avoid interference or crosstalk • Cells sufficiently distant from each other can use the same frequency band
Frequency Reuse Patterns • Each cell has a base transceiver • Generally 10 to 50 frequencies assigned to each cell • Each cell can have K/N frequencies – where K = total number of frequencies and N = number of cell within the pattern
Increasing Capacity • Adding new channels • Frequency borrowing: Frequencies are taken from adjacent cells by congested cells • Cell splitting: Cells in areas of high usage can be split into smaller cells. • Cell sectoring: Cell divided into wedge-shaped sectors. Each sector is assigned a separate subset of the cell's channels, and directional antennas at the base station are used to focus on each sector. • Microcells: Useful in city streets in congested areas, along highways, and inside large public buildings
Mobile to Base Channels • Control channels are used to exchange information having to do with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest BS • Traffic channels carry a voice or data connection between users
Steps in a Mobile Call • Mobile unit initialization • Mobile-originated call • Paging • Call accepted • Ongoing call • Handoff
Other Mobile Functions • Call blocking • Call termination • Call drop • Calls to/from fixed and remote mobile subscriber
Mobile Telephony • First Generation • analog voice communication using frequency modulation. • Second Generation • digital techniques and time-division multiple access (TDMA) or code-division multiple access (CDMA) • Third Generation • evolving from second-generation wireless systems • will integrate services into one set of standards.
Multiple Access • Four ways to divide the spectrum among active users • frequency-division multiple access (FDMA) • time-division multiple access (TDMA) • code-division multiple access (CDMA) • space-division multiple access (SDMA)
CDMA • Based on direct sequence spread spectrum (DSSS) • Provides immunity from various kinds of noise and multipath distortion. (The earliest applications of spread spectrum were military, where it was used for its immunity to jamming.) • Can be used for hiding and encrypting signals. • Several users can independently use the same (higher) bandwidth with very little interference
Third Generation Systems • Intended to provide provide high speed wireless communications for multimedia, data, and video • Reflects trend toward universal personal telecommunications and communications access • Personal communications services (PCSs) and personal communication networks (PCNs) are objectives for 3G wireless. • Planned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity
IMT-2000 3rd Generation Capabilities • Voice quality comparable to PSTN • 144 kbps data rate for motor vehicles • 384 kbps for pedestrians • Support for 2.048 Mbps for office use • Support for packet and circuit switched data services • Adaptive Internet interface • More efficient spectrum use • Support for a wide variety of mobile equipment • Flexibility
Wireless Application Protocol (WAP) • Designed to work with all wireless technologies • Programming model based on the WWW Programming Model • Wireless Markup Language, adhering to XML • Specification of a small browser suitable for a mobile, wireless terminal • A lightweight communications protocol stack • A framework for wireless telephony applications (WTAs)
Wireless Markup Language • Does not assume a standard keyboard or a mouse; designed to work with telephone keypads, styluses, and other input devices common to mobile, wireless communication • Documents are subdivided into small, well-defined units of user interaction called cards; users navigate by moving back and forth between cards. • Uses a small set of markup tags appropriate to telephony-based systems
Microbrowser • Based on a user interface model appropriate for mobile, wireless devices. • Traditional 12-key phone keypad is used to enter alphanumeric characters • Users navigate among the WML cards using up and down scroll keys rather than a mouse. • Navigation features familiar from the Web (e.g., Back, Home, and Bookmark) are provided as well.
Satellite Communications • Two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in space • The antenna systems on or near the earth are referred to as earth stations • Transmission from an earth station to the satellite is an uplink, from the satellite to the earth station is downlink • The transponder in the satellite takes an uplink signal and converts it to a downlink signal
Geostationary Satellites • Circular orbit 35,838 km above the earth’s surface • Rotates in the equatorial plane of the earth at exactly the same angular speed as the earth • Remains above the same spot on the equator as the earth rotates
Advantages of Geostationary Orbits • Satellite is stationary relative to the earth, so no 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 simplified. • One satellite can communicate with roughly a fourth of the earth; three satellites separated by 120° 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 weaken after traveling that distance • Polar regions and the far northern and southern hemispheres are poorly served • Even at speed of light, the delay in sending a signal 35,838 km each way to the satellite and back is substantial
LEO Characteristics • Circular or slightly elliptical orbit < 2000 km • Orbit period is in the range of 1.5 to 2 hours • Diameter of coverage is about 8000 km • Round-trip signal propagation delay is < 20 ms • Maximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is up to 20 minutes • System must be able to cope with large Doppler shifts, which change the frequency of the signal • Significant atmospheric drag on a LEO satellite results in gradual orbital deterioration.
LEO Advantages • Reduced propagation delay • Received LEO signal is much stronger than that of GEO signals for the same transmission power • LEO coverage can be better localized so that spectrum can be better conserved. • On the other hand, to provide broad coverage over 24 hours, many satellites are needed.
Types of LEOs • Little LEOs: Intended to work at communication frequencies below1 GHz using no more than 5 MHz of bandwidth and supporting data rates up to 10 kbps • Big LEOs: Work at frequencies above 1 GHz and supporting data rates up to a few megabits per second
MEO Characteristics • Circular orbit at an altitude of 5000 to 12,000 km • Orbit period is about 6 hours • Diameter of coverage is 10,000 to 15,000 km • Round trip signal propagation delay < 50 ms • Maximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is a few hours • Require fewer hand-offs than LEOSs
Satellite Network Applications • Television distribution • Long-distance telephone transmission • Private business networks