Chapter 13:Wireless Networks

Business Data Communications, 4e

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.

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.

AMPS - Advanced Mobile Phone Service

Mobile Switching Center (MSC)

Mobile Unit

Base Transceiver Station

AMPS Components

Mobile Units contains a modem that can switch

between many frequencies 3 identification numbers: electronic

serial number, system ID number, mobile ID number

Base Transceiver full-duplex communication with the

mobile

Mobile Switching Center (MSC)

AMPS Spectral Allocation

Two 25-MHz bands 864-894 MHz: Base Station Mobile Unit 824-849 MHz: Mobile Unit Base Station

Each band is split in two (i.e. 12.5 MHz)416 channels (30kHz apart) 21: control 395: call

Spatial Allocation Frequency Reuse Patterns (p.343)

GSM - Global System for Mobile Communication

Developed to provide common 2nd-generation technology for Europe200 million customers worldwide, almost 5 million in the North AmericaGSM transmission is encryptedSpectral allocation: 25 MHz for base transmission (935–960 MHz), 25 MHz for mobile transmission (890–915 MHz)

GSM Layout

Mobile Services Switching

Center

MSSC

Subscriber

Base Transceiver

Base Station Controller (BSC)

GSM Network Architecture

HLR: Home Location RegisterVLR: Visitor Location Register

AuC: Authentication CenterEIR: Equipment Identity Register

GPRSGeneral Packet Radio Services

GSN: GPRS Support NodeSGSN: Serving-GSNGGSN: Gateway-GSN

To support packet data To support packet data serviceservice

Multiple Access

Four ways to divide the spectrum among active users frequency-division multiplexing (FDM) time-division multiplexing (TDM) code-division multiplexing (CDM) space-division multiplexing (SDM)

Choice of Access Methods

FDM, used in 1st generation systems, wastes spectrumDebate over TDMA vs CDMA for 2nd generation TDMA advocates argue there is more successful

experience with TDMA. CDMA proponents argue that CDMA offers

additional features as well, such as increased range.

TDMA systems have achieved an early lead in actual implementations

CDMA seems to be the access method of choice for third-generation systems

Code Division Multiplexing

Based on direct sequence spread Spectrum (DSSS)Break each bit into k chips. k: spreading factor

Ex. k = 61:

0:Code : <1,-1,-1,1,-1,1>

CDMA Example

k =6

1,-1,-1,1,-1,1

1, 1,-1,-1,1,1

1, 1,-1, 1, 1,-1

CDMA Example (cont.)

Receiver receives a chip patternd =<d1,d2,d3,d4,d5,d6>

Code of user uu =<u1,u2,u3,u4,u5,u6>

Decoding functionSu(d)=

d1×u1+d2×u2+d3×u3+d4×u4+d5×u5+d6×u6

Orthogonal SA(cB)=0 SA(-cB)=0

SA(cA)=6 SA(-cB)=-6

1 0

Third Generation Systems

Intended to provide provide high speed wireless communications for multimedia, data, and videoPersonal communications services (PCSs) and personal communication networks (PCNs) are objectives for third-generation wireless.Planned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity

Wireless Application Protocol (WAP)

Programming model based on the WWW Programming ModelWireless Markup Language, adhering to XMLSpecification of a small browser suitable for a mobile, wireless terminalA lightweight communications protocol stackA framework for wireless telephony applications (WTAs)

WAP Programming Model

WAP’s Optional Proxy Model

WAP 1.0 Protocol Stack

WAP 1.0 Gateway

WAP 2.0 Proxy

What’s New in WAP 2.0

WAP Push User Agent Profile Wireless Telephony Application (WTA) External Functionality Interface (EFI) Persistent Storage Interface Data Synchronization (SyncML) Multimedia Message Service (MMS) Provisioning Pictogram

WTA Logical Architecture

Geostationary Satellites

Circular orbit 35,838 km above the earth’s surfacerotates in the equatorial plane of the earth at exactly the same angular speed as the earth will remain 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 > 35,000 kmPolar regions and the far northern and southern hemispheres are poorly served Even at speed of light, about 300,000 km/sec, the delay in sending a signal from a point on the equator beneath the satellite 35,838 km to the satellite and 35,838 km back is substantial.

LEO and MEO Orbits

Alternatives to geostationary orbitsLEO: Low earth orbitingMEO: Medium earth orbiting

Satellite Orbits

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 kbpsBig LEOs: Work at frequencies above 1 GHz and supporting data rates up to a few megabits per second