Wireless Computing

Introduction to Wireless

Wireless Technologies


Network Security



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Wireless Technologies

Internet Architecture - The 802.11 Standard - Bluetooth

Wireless LANs

Wireless LANs do some of the same functions as wired LANs:

  1. provide a bit pipe for data to flow
  2. share common medium - radio wave or infrared light
  3. synchronization and error control mechanisms in order to keep the data intact
  4. routing mechanisms in order to move data
  5. connectivity software on a server

Architecture for a radio-based wireless LAN

wireless architecture

Logical Link Control (LLC)

The LLC exchanges data between users on either end of a LAN. This is used by IEEE 802.2.

Medium Access Control (MAC)

MAC, a data link layer for radio-based wireless LANs, allows many wireless computers, or any wireless appliances, to share the same frequency. The data needs to be transmitted at different times, so there are not collisions. The MAC layer performs the addressing to support the LLC. This is to help prevent collisions during a transmission.

Physical Layer

The Physical later transmits the bits of data through the channel by defining electrical, mechanical, and procedural specifications. Modulation, a physical layer function, prepares the signal for air waves. Spread Spectrum, a modulation technique, puts the data over a wider band of frequencies, which results in less noise. There are two common ways for spread spectrum to transmit data over the frequency.

Frequency Hopping "hops" the data from frequency to frequency. If there is traffic on one frequency, the signal will hop to a different frequency. FCC regulations state that it must use 75 or more frequencies for each channel. The maximum dwell time, which is the time the data spends on each frequency for a given interval, is 400 ms.

Direct Sequence combines data signal with higher data rate bit sequence with a chipping code that consists of 0s and 1s. FCC has that the minimum linear processing gain is 10. Most commercial products use under 20. A different sequence is used for the 0s and 1s. For example, if the data stream were 101, the first 1 would be added to the sequence being used for 1s, and then get sent as that number. The 0 would be added to the sequence being used for 0s, and then get sent as that number, and so on.

Direct sequence spread spectrum chip codes


Infrared is an alternative to radio waves for transmitting data. Infrared has a range of 1 meter and needs an unobstructed path. It has a data rate of 4 Mbps. However, it has several disadvantages, the most serious being that it requires a nearly unobstructed line-of-sight, something impractical in many wireless implementations.

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