Question

how do nodes exchange wireless signals and specifically how this relates to the wireless spectrum

Answer 1

All Wireless Signals are carried through the air by electromagnetic waves. The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. Identify potential obstacles successful wireless transmission and their repercussions such as interference and reflection. Continuum of electromagnetic waves characteristics as follows :

·         Data, voice communication

·         Arranged by frequencies Lowest to highest

·         Spans 9 KHz and 300 GHz

Wireless services associated with one area

FCC oversees United States frequencies,

ITU oversees international frequencies

·         Air signals propagate across borders.

Antennas : Each type of wireless service required an antenna specifically design for that service. The service’s specification determines the antenna’s power output, frequency and radiation pattern.

Signal Propagation: LOS uses the least amount of energy and results in the reception of the clearest possible signal. When there is an obstacle in the way the signal may pass through the object or be absorbed by the object or may be subject to reflection diffraction or scattering.

Signal Degradation : Fading occurs as a signal hits various objects. Because of fading the strength of the signal that reaches the receiver is lower than the transmitted signal strength. The further a signal moves from its source the weaker it gets signal are also affected by noise. (The electromagnetic interference).

Frequency Ranges : older wireless devices used the 2.4 GHZ band to send and receive signal. This had 11 communication channels that are unlicensed. Newer wireless devices can also use the 5 GHZ band which has 24 unlicensed bands.

Narrowband, Broadband and Spread Spectrum Signals:

   Narrowband: a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies.

Broadband – uses a relatively wide band of the wireless spectrum and offers higher thoughts than narrowband technologies.

Spread-spectrum : Multiple frequencies used to transmit signals offers secutiry.

Fixed Vs Mobile: Each type of wireless communication falls into one of two categories

1. Fixed – the location of the transmitted and receiver do not move (results in energy saved because weaker signal strength is possible with directional antennas). No wasted energy issuing signals. More enrgy used for signal itself.
2. Mobile – the location can change. Receiver located anywhere within transmitter’s range. Receiver can roam.

WLAN Architecture :

There are two main types of arrangements:

1. ADHOC : Data us sent directly between devices – good for small local devices, poor performance
2. Infrastructure Mode – a wireless access point is placed centrally that all devices connect with
3. Access point: Accepts wireless signals from multiple nodes retransmit signals to network. Base stations wireless routers, wireless gateway.

The most popular wireless stands used on contemporary LANs those developed by IEEE’s 802.11 committee.

Four of the best known standards are also referred to as Wi-Fi. They are

1. 802.11b
2. 802.11a
3. 802.11g
4. 802.11n

These four standards share many characteristics i.e.

All 4 use half duplex signalling

Follow the same access method.

Access Method

802.11 standards specify use of CSMA/CA to access a shared medium. Using CSMA/Ca before a station begins to send data on an 802.11 network. It checks for exiting wireless transmissions.

Association:

Two types of scanning…

1.    Active – station transmits a special frame, known as a prove, on all available channels within its frequency range. When an access point finds the probe frame, it issues a probe response.

2.    Passive – wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point – the beacon frame contains information necessary to connect to the point.

Re-association occurs when a mobile user moves out of one access point’s range and into the range of another.

Frames

Read page 378 – 381 about frames and specific 802.11 protocols