Page 175 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 11: Wireless WAN

11.1.2 Cellular Digital Packet Data (CFPD) Notes

CDPD allows mobile users to transmit digital signals over the analog cellular telephone network
in the 800 MHz band. It can achieve data rates up to 19.2 Kbps, much faster than a typical analog
cellular phone link being used for data. CDPD transmits very short bursts of data on the cellular
channels when no one is using them. Analog cellular networks are idle about 20 percent of the
time, even when networks that get very heavy use. For example, the time between one call being
disconnected and the next one being connected is idle time for an analog network. A CDPD
modem searches for an idle channel and then transmits the data in 128- byte packets.

11.1.3 Personal Communications Services (PCS)

PCS is a fully digital version of cellular wireless that uses very small cells called microcells.
Because it is fully digital, PCS has enhanced features, better encryption, and uses less power.
Because they use less power, hand held PCS devices can be smaller and lighter. PCS uses a digital
cellular standard called GSM, Global System for Mobilization. GSM is widely used in Europe. The
frequency band between 1.850 and 1.990 GHz was recently allocated to PCS in the United States.
The frequencies were auctioned to companies that planned to provide PCS. These companies
paid the government $7.7 billion for a band of frequencies in 1995 and additional frequencies
were auctioned for $6 billion in 1996.

11.1.4 Antennae

The WNI(Wireless Network Interface) connects to an antenna that transmits and receives the
radio signal. The transmit power of an antenna is measured in watts. Many radio stations use
50,000 watts of power. Wireless networks do not need to send their signals as far as a broadcast
radio station and so they require much less power. Typically, the transmit power for a wireless
network is one watt or less.

An antenna can increase the power of the radio signal so that it can travel farther. A good analogy
is a sprinkler that sprays water in many directions. If all but one of the holes is covered, the
water will spray out of the one open hole with much greater power and will travel much farther.
An antenna increases the power of a radio signal by keeping the radio waves from going up
but allowing them to go out toward the receiver, that is, the antenna compresses the vertical
propagation to increase the horizontal propagation. The increase in power provided by an
antenna is called gain and measured in decibels (dB).

Did u know? As a rule of thumb, a 6dB antenna will increase the power of a signal by 4 and a
9dB antenna will multiply the power by 8. A typical cellular phone antenna has a 3dB gain.

Types of Antenna

Figure 11.2: Omnidirectional Antenna

Source: http://k-12.pisd.edu/currinst/network/11_806A_4-2_SG.pdf

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