Wireless Communication: Amplitude Driven vs. Radio Waves, Slides of Computer Networks

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Wireless Communications

1 - Wireless Spectrum

http://et.nmsu.edu/~etti/spring97/techtips/spectrum.html

Infrared Light - Positive Options

• Inexpensive
• Compatible with fiber-optic links
• Not bandwidth limited
• No licensing required (FCC)
• Transmissions may be aimed (1 to 2 kilometers)
• Transmissions may be omni-directional (30-60 feet)
• Amplitude driven – little interference
• Range of 1-2 kilometers (approximately ½ to 1 ½ miles)
• Highest bandwidth and throughput
• InfraLAN – product with infrared transmission

Infrared Light – Negative Aspects

• Spectrum is shared with the sun and other lighting sources
• LAN may become “useless” with enough interference
• Signals will not permeate opaque objects (walls, dividers,etc)

Microwave - Positive Options

• Higher throughput without spread spectrum
• 5.8ghz band – using a narrow-band transmission
• RadioLAN - product using microwave transmission

Microwave – Negative Aspects

• Expensive to build infrastructure
• Must operate at less than 500 milliwatts (strict FCC regulations)
• Not commonly used – less available knowledge pool

Radio Waves - Positive Options

• Commonly understood technology
• Not subject to interference of light waves like infrared
• Less expensive than microwave
• Long-range medium – several architectures exist
• Commonly used – larger knowledge pool
• WaveLAN, BreezeNet pro, Proxim Rangelan2, & RadioLAN

Radio Waves – Negative Aspects

• Spread spectrum technology – high overhead
• Lower rates of data transmissions – due to overhead required
• Subject to some interference – causing delays in transmission

Radio Waves – Direct Sequence Spread Spectrum (DSSS)

• Spreads signal over a band (Example, 50 MHz)
• Random Binary String Modulates the Transmission Signal
• String is known as a “Spreading Code”
• Bits are mapped out as “chips” and mapped back as “bits”
• Spreading Ratio – The number of “chips” per bit

DSSS Spreading Ratios

• Higher Ratios Resist Interference Better
• Lower ratios allow for use of more bandwidth
• FCC dictates spreading ratios must be more than ten
• IEEE 802.1 standard requires a spreading ratio of eleven
• Sender & Receiver must synchronize to the spreading code
• Orthogonal spreading codes allow sharing of the band between LANs
• DSSS systems use wide subchannels, limiting LANs possible
• Recovery is faster with DSSS due to ability to spread the signal over a wider band
• Example Product = WaveLAN

Frequency Hopping Spread Spectrum (FHSS) (Continued)

• Used by military and law enforcement
• Jamming is very difficult as the whole band must be jammed
• Orthogonal hopping sequence allows for co-location of

multiple LANs

• Allows for more co-located LANs than DSSS
• Common new product method for wireless
• Product Example = BreezeNet

Multipath Interference

• Interference caused by signals bouncing off of physical objects and arriving at a receiver at differing times
• Multipath is a problem for all wireless modes
• DHSS resists the issue by hopping to other frequencies
• Anti-Multipath algorithms exist to resist the problem
• Rayleigh fading is a subset of Multipath and can completely cancel out the signal
• Infrared resists Rayleigh fading due to small wavelengths

End Of Module

2 - Examples of Wireless protocols and technologies

The two main protocols and technologies discussed in this sections

are:

• Wireless ATM
• Wireless Application protocol