1 / 13

Wireless Medium

Wireless Medium. David Holmer dholmer@jhu.edu. Thermal Noise. Ever-present thermal noise in wireless medium Sums with any wireless transmission Potentially causes errors in reception (digital) or degradation of quality (analog)

arama
Download Presentation

Wireless Medium

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Wireless Medium David Holmer dholmer@jhu.edu

  2. Thermal Noise • Ever-present thermal noise in wireless medium • Sums with any wireless transmission • Potentially causes errors in reception (digital) or degradation of quality (analog) • Effectively limits transmission range when transmitting signal strength falls below noise floor • -174 dBm/Hz

  3. Thermal Noise Calculation • Depends on channel bandwidth • About 25 MHz for 802.11b or 802.11a channel • = -174dBm/Hz + 10log(bandwidth in Hz) • So for 802.11 • Noise Floor is about -100 dBm • -100 dBm = 10log( .0000000000001 Watts )

  4. Noise Limits Transmitting Distance Signal to Noise Ratio (SNR) Short range transmission (low path loss) + = High Long range transmission (high path loss) = + Low

  5. Physical Channel Properties Review • Wireless signal strength • Transmit power • Loss over distance (falls off by d2) • Shadowing (e.g. absorption by walls) • Multi-path (e.g. bouncing off of metal objects) • Noise • Thermal noise floor • Environmental noise (e.g. microwave ovens) • Channel quality • Related to signal to noise ratio

  6. Multi-transmitter Interference Problem • Similar to multi-path or noise • Two transmitting stations will constructively/destructively interfere with each other at the receiver • Receiver will “hear” the sum of the two signals (which usually means garbage)

  7. Medium Access Control • Protocol required to coordinate access • I.E. transmitters must take turns • Similar to talking in a crowded room • Also similar to hub based Ethernet

  8. Carrier Sense Multiple Access (CSMA) • Procedure • Listen to medium and wait until it is free (no one else is talking) • Wait a random back off time then start talking • Advantages • Fairly simple to implement • Functional scheme that works • Disadvantages • Can not recover from a collision (inefficient waste of medium time)

  9. Carrier Sense Multiple Accesswith Collision Detection (CSMA-CD) • Procedure • Listen to medium and wait until it is free • Then start talking, but listen to see if someone else starts talking too • If a collision occurs, stop and then start talking after a random back off time • This scheme is used for hub based Ethernet • Advantages • More efficient than basic CSMA • Disadvantages • Requires ability to detect collisions

  10. Collision Detection Problem • Transmit signal is MUCH stronger than received signal • Due to high path loss in the wireless environment (up to 100dB) • Impossible to “listen” while transmitting because you will drown out anything you hear • Also transmitter may not even have much of a signal to detect due to geometry

  11. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Procedure • Similar to CSMA but instead of sending packets control frames are exchanged • RTS = request to send • CTS = clear to send • DATA = actual packet • ACK = acknowledgement

  12. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Advantages • Small control frames lessen the cost of collisions (when data is large) • RTS + CTS provide “virtual” carrier sense which protects against hidden terminal collisions (where A can’t hear B) A B

  13. Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA) • Disadvantages • Not as efficient as CSMA-CD • Doesn’t solve all the problems of MAC in wireless networks (more to come)

More Related