Understanding Thermal Noise and Wireless Medium

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)