1-7 Media and Topologies
Posted February 22nd, 2007 by admin
1.7 Specify the general characteristics (For example: carrier speed, frequency, transmission type and topology) of the following wireless technologies:
> Infrared
Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength.
> Bluetooth
Is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices like personal digital assistants (PDAs), mobile phones, laptops, PCs, printers and digital cameras via a secure, low-cost, globally available short range radio frequency.
| 802.11 | 802.11x | Infrared | Bluetooth | |
| Speed | 500 Kbps | 802.11a > 54 Mbps 802.11b > 11 Mbps 802.11g > 54 Mbps |
115.2 Kbps |
1.2 > 720 Kbps 2.0 > 2.1 Mbps |
| Frequency | Radio Wave |
Radio Wave 802.11a > 5 GHz |
Light Wave |
Radio Wave 2.45 GHz. In order to avoid interfering with other protocols which use the 2.45 GHz band, the Bluetooth protocol divides the band into 79 channels (each 1 MHz wide) and changes channels up to 1600 times per second. |
| Transmission | FHSS | DSSS and OFDM | Light (modulated, switched on and off, to encode the data.) | FHSS |
| Topology | Various | Various | Various | Various |
FHSS Frequency-hopping spread spectrum is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
Spread-spectrum transmission offers these advantages over a fixed-frequency transmission:
- Highly resistant to noise and interference.
- Signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.
- Transmissions can share a frequency band with many types of conventional transmissions with minimal interference. As a result, bandwidth can be utilized more efficiently.
DSSS direct-sequence spread spectrum is a modulation technique where the transmitted signal takes up more bandwidth than the information signal that is being modulated, which is the reason that it is called spread spectrum.
Comparison of DSSS and Frequency Hopped SS
DSSS
- Flexible support of variable data rates
- High capacity is possible with enhancements (interference cancellation, adaptive antenna, etc.)
- Suffers from near-far effect
FHSS
- Suitable for ad hoc networks (no near-far problem)
- Robust to interference
- Limited data rate
OFDM Orthogonal frequency-division multiplexing, also called discrete multitone modulation (DMT), is a transmission technique based upon the idea of frequency-division multiplexing (FDM).
- Used in some wireless LAN applications, including WiMAX and IEEE 802.11a/g
- Used in many communications systems such as: ADSL, Wireless LAN, Digital audio broadcasting.