Wireless Data Communications

Chapter Twenty-Three:Paging and Wireless Data Networking

Chapter 23: Paging and Wireless Data Networking

IntroductionPaging systems are very common and are now built into Personal Communication SystemsWireless modems and LANs are also becoming more widespreadPersonal organizers, like the Palm Pilot are now using wireless communication technologies


Paging and Messaging SystemsTraditional paging systems use widely spaced transmitters, each covering a large geographic regionAll transmitters in a given system operate on the same frequencyVHF 152 or 158 MHzUHF 454.025 or 454.650 MHzAll pages are transmitted by all the transmitters in the systemA traditional pager is a fixed-tuned receiver that uses a transmitted code to identify messages intended for itSimple paging systems are still quite popular because of their small size, low cost, and long battery life


One-Way Paging SystemsThe traditional way to handle paging is to have a network of powerful transmitters, all of which transmit all of the pages on all of the transmittersFrequencies are reused by using the same frequencies for every transmitterIn a voice system, this would result in low capacity, but as messages are typically very short, the transmission times are also shortTDMA systems are used for many paging systemsEach pager has a unique address called a capcode


Basic Paging NetworksBecause all pages are sent from all transmitters, there is no need to know the location of any pagerSatellites are often used to transmit pages great distances to local systems, which are relayed using terrestrial transmittersLEO satellites are used for this


Basic Paging Network


Paging ProtocolsThe most common protocol is POCSAG (Post Office Code Standardization Advisory Group)Transmits data at 512, 1200, and 2400 b/sModulation scheme is FSK with 4-kHz deviationPOCSAG messages are sent in batches


Two-Way Paging SystemsMotorolas ReFLEX system is the de facto standard for 2-way alphanumeric paging systemsTwo-way paging is much more complex and expensive than one-way paging in that each pager requires a transmitterMessage receipt acknowledgement is a possibility with two-way paging systems


ReFLEX Paging SystemsReFLEX paging systems operate in the frequency ranges of 928932 and 940941 MHz for the outbound channel and 896902 MHz for the inbound channelThe available data rates are:Outbound: 1600, 3200, and 6400 b/sInbound: 9600 b/s


Voice PagingMotorolas InFLEXion system is the most popular voice paging protocolIt uses analog compression and SSB AM to transmit voice messages from the base station to the pagersUpper and lower sidebands are used, but each sideband constitutes a separate voice channelInFLEXion pagers normally allow a text reply


Wireless Local-Area NetworksMost offices and even some homes use local-area networks to connect computers using hard-wired systemsIn facilities or applications where it is necessary to have moveable computers, wireless connections are desirableIn general, wireless networks are slower and more expensive than traditional wired networksWired Ethernet installations typically have data rates of either 10 or 100 Mb/s compared to 111 Mb/s for wireless networks


Radio LANsThere are number of proprietary standards for radio LANsMost are using the unlicensed 900-MHz band and the 2.4-GHz rangeThere have been some standards developed recentlyIEEE 802.11Bluetooth


IEEE 802.11The 802.11 standard, adopted in 1998, envisions spread-spectrum operation on the unlicensed ISM frequency band from 2.42.484-GHz range (the same as microwave ovens)The use of spread-spectrum allows these networks to operate in the presence of interferenceThe standard also allows for infrared operation


IEEE 802.11 StandardsStandards include:A set of wireless nodes called Basic Service Set (BSS), and a network consisting of only a BSS with no access points called an ad-hoc networkA network can consist of only wireless nodes communicating with each other - an Independent Basic Service Set (IBSS)There can be multiple access points for extended coverageA network with multiple access points is called an Extended Service Set (ESS)Wireless units can roam within the ESS


IEEE 802.11 Networks


IEEE 802.11 - A & B VersionsThe IEEE 802.11b standard adopted in September 1999 allows for a maximum bit rate of 11 Mb/s using direct-sequence spread-spectrum operation in the 2.4-GHz bandThe IEEE 802.11a is designed as a future standard for data rates up to 54 Mb/s in the 5-GHz band, and is still being developed


BluetoothThe Bluetooth specification is a joint venture involving IBM, Intel, Nokia, and ToshibaIt is designed as an open standard for short-range system between 10 cm and 10 mand can be extended to 100 m using RF amplifiers for the transmittersBluetooth devices are designed to be small and inexpensive enough to be built into cellular and PCS phones, notebook computers, personal digital assistants, and peripheral computer devices


Bluetooth OperationThe Bluetooth standard resembles the 802.11 wireless Ethernet standardIt operates in the 2.4-GHz ISM bandBluetooth radios employ spread-spectrum technology using frequency hoppingChannels are 1 MHz apart, giving room for 79 channelsThe transmitter uses two-level FSK with a frequency deviation between 140 and 175 kHzBluetooth envisions both audio and data transmissionFull-duplex operation is accomplished by time-division duplex (TDD)


Bluetooth Networks


Wireless BridgesWireless bridges connect LAN segmentsTypically they are used in different buildings to provide connectionsSome bridges are high-speed microwave devices that are expensive and require licensesLower-cost bridges use the 902-MHz or 2.4-GHz bandsSome wireless bridges can operate at up to 10 km under good line-of-sight conditions, at a data rate of about 2 Mb/s


Connections Using InfraredInfrared connections tend to be short-distance and the signal will not pass through walls or other opaque objectsA short-range infrared system called Infrared Data Association (IRDA) has been used for some time to allow devices to communicate with each otherSome systems are capable of data transfer rates of up to 4 Mb/sIt is possible to build wireless LANs using infrared technology


Wireless ModemsA typical wireless modem can work with one modem or several by polling each of them in turnData rates are fairly slow, with a maximum of 19.2 kb/sMany wireless modems use the 2.4-GHz ISM bandMost wireless modems operate at low power levels, but can operate over longer distances because of the lower data transfer rates


Wireless Packet-Data ServicesThere are two major wireless packet-data networks in North AmericaMobitex networks use a cellular structure in the 900-MHz band, but are separate from the AMPS systemNarrow channels only 12.5 kHz wide are usedGMSK modulation is used for data rates up to 8 kb/sARDIS (Advanced Radio Data Information Services) was created by IBM along with Motorola for use in working with its outside sales and service personnelAll ARDIS cells use the same frequencyData rates are 4.8 kb/s in a 25-kHz channel


Date post:	09-Mar-2016
Category:	Documents
Upload:	hmnaing077690
View:	218 times
Download:	0 times

Wireless Data Communications

Documents