MASNET GroupXiuzhen ChengFeb 8, 2006 Terms and Concepts Behind Wireless Communications.

MASNET Group Xiuzhen Cheng Feb 8, 2006

Terms and Concepts Behind Wireless Communications


Basic Wireless Terms

Electromagnetic waves

Frequency

Spectrum

Bandwidth

Capacity


Electromagnetic (EM) Radiation

A natural phenomenon that allows information to be carried from transmitter to a receiver via a medium such as the air or fiber optic cable

Wireless devices, such as cell phones, produce electromagnetic waves of different frequencies that move through space


Frequency

Frequency is the number of times that a wave's peak passes a fixed point in a specific period of time

Point A

10 Cycles / 1 Second = 10 Hertz

1 Second


Frequency (cont.)

Frequency is measured in cycles per second, or Hertz (Hz)

Cellular phones, for example, produce radio waves with frequencies around 800 MHz

“Frequency” and “Spectrum” are used interchangeably by some people, although they are not the same

1,000 Hz = 1 KiloHertz (kHz)1,000,000 Hz = 1 MegaHertz (MHz)

1,000,000,000 Hz = 1 GigaHertz (GHz)


The set of all possible frequencies (an infinite number) is called the "electromagnetic spectrum" The subset of frequencies from 3 kHz to 300 GHz is known as the "radio spectrum"

Electromagnetic Spectrum

Spectrum


Frequency vs. Bandwidth

Frequency is a specific location on the electromagnetic spectrum

Bandwidth is the range between two frequencies Bandwidth is measured in Hertz A cellular operator may transmit signals

between 824-849 MHz, for a total bandwidth of 25 MHz


Bandwidth vs. Capacity

Capacity is usually measured by Mega bits per second (Mbps)

Bandwidth for a particular service is fixed, but the number of calls and the rate of data transmission is not (capacity)


An example: IEEE 802.11b (WiFi)

Operating center frequency: 2.4 GHz. There are 11 channels in 802.11b. Starting

from 2.412 GHz to 2.462 GHz. Spectrum: 2.412 GHz ~ 2.462 GHz Bandwidth: 40 MHz. Capacity: 1, 2, 5.5, and11Mbps. Typical

data rate is about 6.5Mbps.


Parameters that determine the capacity

Capacity is the fundamental concern in wireless networks.

The capacity of a particular bandwidth is determined by following parameters: Signal strength Interference Path loss Lower or higher frequencies Etc


Signal strength

The ability of an electromagnetic wave to persist as it radiates out from its transmitter

Signal strength, or power, is measured in Watts, or more conveniently expressed in decibels (dB)


Power and Interference

Power can increase the strength of a signal, but it can also cause the signal to “bleed” into other frequencies, resulting in interference with other transmissions


Path loss (path attenuation)

Decrease in signal strength over distance due to: Absorption Reflection Diffusion Scattering Free-space loss


Lower frequencies are

Better for mobile services Low powered signals go farther at lower

frequencies, resulting in lower-powered handsets = smaller handsets less interference


Higher Frequencies

Path Loss is greater at higher frequencies Higher frequency signals have difficulty

penetrating buildings and traveling around objects Radio components are more expensive for higher

frequencies Frequency stability (staying in your allocated

bandwidth) more difficult at higher frequencies

As you increase the frequency, the coverage area decreases – but potential data rates increase (why?).


Propagation Characteristics

900MHz Multipath: High Foliage: Pine (Absorb Radiation)

2.4GHz Multipath: Very High (Concrete, Brick, Steel) Foliage: Any Absorbs Radiation (water resonance)

5GHz Multipath: Very High (Concrete, Brick, Steel, Foliage) Foliage: Limited Absorption


More details will be coved in

The section named “Antennas and Propagations”.


Digital Technology

Transfers information in digital format (binary 0’s and 1’s) versus analog (continuous values) Significant improvement in wireless

systems (why?) Reduces many problems associated with

decrease in signal strength We will see more of digital technology in

the section named “Encoding, Spread Spectrum Technology”


Wireless technologies/standards

802.11a 802.11b (Wi-Fi) 802.11g (Wi-Fi) 802.11i (Security) 802.16 2004, e & f

(WiMAX) Bluetooth (802.15) 1G: CDPD (Cellular Digital

Packet Data)

2G: GSM (Global System for Mobile Communications) GPRS (General Packet Radio Service)

3G: CDMA2000, WCDMA EvDO (Evolution Data

Only)


IEEE 802.11a/b/g (Wi-Fi)

802.11a 802.11b 802.11g

5 GHz 2.4 GHz 2.4 GHz

54 Mbps 11 Mbps 54 Mbps

Less interference, more bandwidth

Best over-all coverage range

Faster than 802.11b and better range than 802.11a

Not as widely implemented, shorter range

Not as fast as other technologies

Less range than 802.11b


IEEE 802.16 (WiMAX)

802.16d – A.K.A 802.16-2004 Intended for "last mile" connectivity at high

data rates. Point-to-multipoint only implementation

802.16e – Adds mobility approved in December 2005.


IEEE 802.20 (MBWA)

Mobile Broadband Wireless Access (MBWA) Working Group 1 Mbps Mobile speeds of 100mph Could compete with 3G cellular Licensed band use only


IEEE 802.11i (WPA2)

Provide improvements to WiFi security Address security short comings in WEP Add user authentication


Evolution Data Only (EvDO)

Available in Larger Metro Areas Offered by Sprint, Verizon, Other 700Mbps

Supports Streaming Video


Elements of a wireless network

network infrastructure

wireless hosts

base station

wireless link

Network infrastructure


Elements of a wireless network

Ad hoc mode no base stations nodes can only transmit

to other nodes within link coverage

nodes organize themselves into a network: route among themselves


Why a wireless network is more subjected attacks?

MASNET GroupXiuzhen ChengFeb 8, 2006 Terms and Concepts Behind Wireless Communications.

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