Chapter Twenty-Three:Paging and Wireless Data

Networking

Introduction• Paging systems are very common and are now

built into Personal Communication Systems• Wireless modems and LANs are also becoming

more widespread• Personal organizers, like the Palm Pilot™ are now

using wireless communication technologies

Paging and Messaging Systems• Traditional paging systems use widely spaced transmitters, each

covering a large geographic region

• All transmitters in a given system operate on the same frequency– VHF 152 or 158 MHz

– UHF 454.025 or 454.650 MHz

• All pages are transmitted by all the transmitters in the system

• A traditional pager is a fixed-tuned receiver that uses a transmitted code to identify messages intended for it

• Simple paging systems are still quite popular because of their small size, low cost, and long battery life

One-Way Paging Systems• The 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 transmitters

• Frequencies are reused by using the same frequencies for every transmitter

• In a voice system, this would result in low capacity, but as messages are typically very short, the transmission times are also short

• TDMA systems are used for many paging systems• Each pager has a unique address called a capcode

Basic Paging Networks• Because all pages are sent from all transmitters, there

is no need to know the location of any pager• Satellites are often used to transmit pages great

distances to local systems, which are relayed using terrestrial transmitters

• LEO satellites are used for this

Basic Paging Network

Paging Protocols• The most common protocol is POCSAG (Post Office Code

Standardization Advisory Group)– Transmits data at 512, 1200, and 2400 b/s– Modulation scheme is FSK with 4-kHz deviation– POCSAG messages are sent in batches

Two-Way Paging Systems• Motorola’s ReFLEX™ system is the de facto

standard for 2-way alphanumeric paging systems• Two-way paging is much more complex and

expensive than one-way paging in that each pager requires a transmitter

• Message receipt acknowledgement is a possibility with two-way paging systems

ReFLEX™ Paging Systems• ReFLEX™ paging systems operate in the frequency

ranges of 928–932 and 940–941 MHz for the outbound channel and 896–902 MHz for the inbound channel

• The available data rates are:– Outbound: 1600, 3200, and 6400 b/s

– Inbound: 9600 b/s

Voice Paging• Motorola’s InFLEXion™ system is the most popular voice paging protocol• It uses analog compression and SSB AM to transmit voice messages from

the base station to the pagers• Upper and lower sidebands are used, but each sideband constitutes a

separate voice channel• InFLEXion™ pagers normally allow a text reply

Wireless Local-Area Networks• Most offices and even some homes use local-area

networks to connect computers using hard-wired systems

• In facilities or applications where it is necessary to have moveable computers, wireless connections are desirable

• In general, wireless networks are slower and more expensive than traditional wired networks

• Wired Ethernet installations typically have data rates of either 10 or 100 Mb/s compared to 1–11 Mb/s for wireless networks

Radio LANs• There are number of proprietary standards for radio

LANs• Most are using the unlicensed 900-MHz band and

the 2.4-GHz range• There have been some standards developed recently

– IEEE 802.11– Bluetooth

IEEE 802.11• The 802.11 standard, adopted in 1998, envisions

spread-spectrum operation on the unlicensed ISM frequency band from 2.4–2.484-GHz range (the same as microwave ovens)

• The use of spread-spectrum allows these networks to operate in the presence of interference

• The standard also allows for infrared operation

IEEE 802.11 Standards• Standards 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 network

– A 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 coverage

– A 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 Versions• The 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 band

• The 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

Bluetooth• The Bluetooth specification is a joint venture involving

IBM, Intel, Nokia, and Toshiba

• It is designed as an open standard for short-range system— between 10 cm and 10 m—and can be extended to 100 m using RF amplifiers for the transmitters

• Bluetooth 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 Operation• The Bluetooth standard resembles the 802.11 wireless Ethernet standard

• It operates in the 2.4-GHz ISM band

• Bluetooth radios employ spread-spectrum technology using frequency hopping

• Channels are 1 MHz apart, giving room for 79 channels

• The transmitter uses two-level FSK with a frequency deviation between 140 and 175 kHz

• Bluetooth envisions both audio and data transmission

• Full-duplex operation is accomplished by time-division duplex (TDD)

Bluetooth Networks

Wireless Bridges• Wireless bridges connect LAN segments• Typically they are used in different buildings to provide

connections• Some bridges are high-speed microwave devices that are

expensive and require licenses• Lower-cost bridges use the 902-MHz or 2.4-GHz bands• Some 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 Infrared• Infrared connections tend to be short-distance and the signal

will not pass through walls or other opaque objects

• A short-range infrared system called Infrared Data Association (IRDA) has been used for some time to allow devices to communicate with each other

• Some systems are capable of data transfer rates of up to 4 Mb/s

• It is possible to build wireless LANs using infrared technology

Wireless Modems• A typical wireless modem can work

with one modem or several by polling each of them in turn

• Data rates are fairly slow, with a maximum of 19.2 kb/s

• Many wireless modems use the 2.4-GHz ISM band

• Most wireless modems operate at low power levels, but can operate over longer distances because of the lower data transfer rates

Wireless Packet-Data Services• There are two major wireless packet-data networks in

North America– Mobitex networks use a cellular structure in the 900-MHz band,

but are separate from the AMPS system• Narrow channels only 12.5 kHz wide are used• GMSK modulation is used for data rates up to 8 kb/s

– ARDIS (Advanced Radio Data Information Services) was created by IBM along with Motorola for use in working with its outside sales and service personnel

• All ARDIS cells use the same frequency• Data rates are 4.8 kb/s in a 25-kHz channel