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28 terms

Wireless Networks

Wireless Medium
Wireless connections are desirable when a physical connectivity is prohibited due to distance, obstacles, or costs.
-Wireless may include any wavelength of the electromagnetic spectrum.
-Different wavelengths have different propagation properties that need to be considered when designing a wireless digital communication device
802.11 refers to a family of specifications developed by the IEEE for wireless LAN technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients.
802.11 Goals
Access Method
Frame Format
MAC Management (think mobile)
-Physical Layer
Spectrum Selection
Frequency Hopping
Direct Sequence
802.11 Problems
-Mobile network devices. A device may move from one network to the next.
-Reuse of radio spectrum
-Interference from overlapping devices
Interference from other devices (cordless phones)
-RF propagation, delay and multipath
applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.
(also referred to as 802.11 High Rate or Wi-Fi) -- an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
applies to wireless LANs and provides 54 Mbps in the 2.4 GHz band (backwards compatible with 802.11b).
(MIMO)- Multiple In Multiple Out , multiple transmitters, higher speeds, spectrum still under review. 600 mbps (4 radios)
Spread Spectrum Radio
Spread Spectrum technology is a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems.
-Uses a narrowband carrier that changes frequency in a pattern that is known to both transmitter and receiver.
-Properly synchronized, the net effect is to maintain a single logical channel.
-To an unintended receiver, FHSS appears to be short-duration impulse
-Generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code).
-The longer the chip, the greater the probability that the original data can be recovered.
-Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission.
Orthogonal frequency-division multiplexing
-FDM modulation technique for transmitting large amounts of digital data over a radio wave. OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions. 802.11a WLAN, 802.16 and WiMAX technologies use OFDM.
Using RF as a Medium
To use the electromagnetic spectrum as a digital network medium devices must:
-Create and encode a signal
-Transmit a signal via an antenna
-Receive a signal via an antenna
-Decode the received signal
Gain is the amount of signal increase. In reality you can't increase the signal strength without an amplifier, you can only focus the signal.
Antennas are designed to shape the power of the transmitter into a pattern that concentrates the signal in the direction you wish to send.
OR it gathers signals from the direction you expect to receive.
Omni Antenna
An omni antenna transmits its signal in a doughnut shaped pattern
As gain increases doughnut gets flatter
Yagi Antenna
Narrow Cone Shaped Pattern.
Pattern expressed as beam width in degrees
Horizontal and vertical beam width are same.
Panel/Sector Antenna
Many designs for desired pattern.
Horizontal and vertical beam width determined by design
Adhoc Network Connection Mode
Set of 802.11b wireless stations that communicate directly with one another without using an access point or any connection to a wired network.
Infrastructure Network Connection Mode
An 802.11 network in which devices communicate with each other by first going through an Access Point (AP).
Access Point (AP)
An access point is a base station in a wireless LAN. Access points are typically stand-alone devices that plug into an Ethernet hub, switch, or router.
Two APs that are configured to act as bridges to two (typically wired) Ethernet networks.
Wireless Station
An 802.11 Network Interface Card, typically in a client PC
Wired Equivalent Privacy (WEP)
Shared private key encryption
Same key used for all nodes (typically)
MAC Filtering
AP is configured to only pass traffic from specific MAC addresses
Wireless USB
Used to connect peripherals such as keyboards, printers, mice, camera, etc.
3.1-10.6 Ghz
480/110 Mbps
3-10 Meters Distance
Used to connect peripherals such as headsets, keyboards, printers, mice, camera
Used for data exchange
2.4 ghz
V1 1 Mbps
V2 3 Mbps
V3 24 Mbps?
WiMedia Alliance 53-480 Mbps (proposed)