This section may be skipped by all readers
and used by those interested in wireless network technology.
A D V E R T I S E M E N T
Transmission of waves take place in the electromagnetic (EM) spectrum.
The carrier frequency of the data is expressed in cycles per second
called hertz(Hz). Low frequency signals can travel for long distances
through many obstacles but can not carry a high bandwidth of data. High
frequency signals can travel for shorter distances through few obstacles
and carry a narrow bandwidth. Also the effect of noise on the signal is
inversely proportional to the power of the radio transmitter, which is
normal for all FM transmissions. The three broad categories of wireless
media are:
Radio - 10 Khz to 1 Ghz. It is broken into many bands including
AM, FM, and VHF bands. The Federal communications Commission (FCC)
regulates the assignment of these frequencies. Frequencies for
unregulated use are:
902-928Mhz - Cordless phones, remote controls.
2.4 Ghz
5.72-5.85 Ghz
Microwave
Terrestrial - Used to link networks over long distances but
the two microwave towers must have a line of sight between them.
The frequency is usually 4-6GHz or 21-23GHz. Speed is often
1-10Mbps. The signal is normally encrypted for privacy.
Satellite - A satellite orbits at 22,300 miles above the
earth which is an altitude that will cause it to stay in a fixed
position relative to the rotation of the earth. This is called a
geosynchronous orbit. A station on the ground will send and
receive signals from the satellite. The signal can have
propagation delays between 0.5 and 5 seconds due to the
distances involved. The transmission frequency is normally
11-14GHz with a transmission speed in the range of 1-10Mbps.
Infared - Infared is just below the visible range of light
between 100Ghz and 1000Thz. A light emitting diode (LED) or laser is
used to transmit the signal. The signal cannot travel through
objects. Light may interfere with the signal. The types of infared
are
Point to point - Transmission frequencies are
100GHz-1,000THz . Transmission is between two points and is
limited to line of sight range. It is difficult to eavesdrop on
the transmission.
broadcast - The signal is dispersed so several units may
receive the signal. The unit used to disperse the signal may be
reflective material or a transmitter that amplifies and
retransmits the signal. Normally the speed is limited to 1Mbps.
The transmission frequency is normally 100GHz-1,000THz with
transmission distance in 10's of meters. Installation is easy
and cost is relatively inexpensive for wireless.
Terms:
AMPS - Advanced Mobile Phone Service is analog cellular phone service.
CDMA - Code division multiple access allows transmission of voice and
data over a shared part of radio frequencies. This is also called spread
spectrum.
CDPD - Cellular Digital Packet Data will allow network connections for
mobile users using satellites.
cellular - An 800 Mhz band for mobile phone service.
D-AMPS - Digital AMPS using TDMA to divide the channels into three
channels.
FDMA - Frequency Division Multiple Access divides the cellular network
into 30Khz channels.
GSM - Global System for Mobile Communications.
HDML - Handheld Device Markup Language is a version of HTML only
allowing text to be displayed.
MDBS - Mobile Data Base Station reviews all cellular channels at
cellular sites.
PCS - Personal communications Service is a 1.9 Ghz band.
TDMA - Time Division Multiple Access uses time division multiplexing to
divide each cellular channel into three sub channels to service three users
at a time.
wireless bridge - Microwave or infared is used between two line of site
points where it is difficult to run wire.
WML - Wireless markup language is another name for HDML.
Categories of LAN Radio Communications
Low power, single frequency - Distance in 10s of meters. Speed in
1-10Mbps. Susceptible to interference and eavesdropping.
High power, single frequency - Require FCC licensing and high power
transmitter. Speed in 1-10Mbps. Susceptible to interference and
eavesdropping.
Spread spectrum - It uses several frequencies at the same time. The
frequency is normally 902-928MHz with some networks at 2.4GHz. The speed of
902MHz systems is between 2 and 6Mbps. If frequency-hopping is used, the
speed is normally lower than 2Mbps. Two types are:
Direct sequence modulation - The data is broken into parts and
transmitted simultaneously on multiple frequencies. Decoy data may be
transmitted for better security. The speed is normally 2 to 6 Mbps.
Frequency hopping - The transmitter and receiver change
predetermined frequencies at the same time (in a synchronized manner).
The speed is normally 1Gbps.
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