1. Lecture 7 CSMA and Spread Spectrum Dr. Ghalib A. Shah
Wireless Networks
Lecture 7
CSMA and Spread Spectrum
Dr. Ghalib A. Shah

2. Review of previous lecture #6 CSMA
Versions of CSMA
CSMA/CA
Example
Spread Spectrum
Frequency Hoping
Direct Sequence
Summary of today’s lecture

3. Last Lecture Review FDMA TDMA CDMA Random Access ALOHA Slotted ALOHA
Reservation-based ALOHA

4. Carrier Sense Multiple Access (CSMA)
Disadvantages of ALOHA
users do not listen to the channel before (and while) transmitting
suitable for networks with long propagation delays
Carrier Sense Multiple Access
polite version of ALOHA
Listen to the channel before transmitting
if sensed channel busy, back-off (defer transmission), and sense channel again after a random amount of time
if channel idle, transmit entire frame

5. Versions of CSMA
Employs different node behaviour when channel found busy
non-persistent CSMA
after sensing busy channel, node waits entire back-off period before sensing again
persistent CSMA
after sensing busy channel, node continues sensing until the channel becomes free; then …
1-persistent CSMA
node transmits immediately with probability 1
p-persistent CSMA
node transmits with probability p; or, it defers transmission with probability (1-p)

6. CSMA / Collision Avoidance
Used where CSMA/CD cannot be used
e.g. in wireless medium collision cannot be easily detected as power of transmitting overwhelms receiving antenna
CSMA/CA is designed to reduce collision probability at points where collisions would most likely occur
when medium has become idle after a busy state, as several users could have been waiting for medium to become available
key elements of CSMA/CA:
IFS –interframe spacing –priority mechanism–the shorter the IFS the higher the priority for transmission
CW intervals –contention window –intervals used for contention and transmission of packet frames
Backoff counter–used only if two or more stations compete for transmission

7. CSMA/CA Algorithm Frame to transmit Medium Idle? Wait IFS Still
Transmit frame
Exp b/o while
Medium idle
Wait until
Trans ends No
Yes
If medium becomes busy during the backoff
time, the backoff timer is halted and
resumes when the medium becomes idle.

8. Example

9. Spread Spectrum Problem of Radio Transmission solution:
frequency dependent fading can wipe out narrowband signals for duration of interference
solution:
spread narrow band signal into a broad band signal using a special code
initially developed for military in order to combat jamming and interception
power of spread signal is the same as of narrow band signal, resulting in a lower power spectral density due to larger bandwidth

11. Spread Spectrum

12. Types of spreading: direct sequence spread spectrum (DSSS)
frequency hopping spread spectrum (FHSS)

13. Frequency Hoping Spread Spectrum (FHSS)
Signal is broadcast over seemingly random series of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals
Transmitter operates in one channel at a time
Bits are transmitted using some encoding scheme
At each successive interval, a new carrier frequency is selected
Channel sequence dictated by spreading code

14. Frequency Hoping Spread Spectrum
Receiver, hopping between frequencies in synchronization with transmitter, picks up message
Advantages
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at knocking out a few bits

15. Frequency Hoping Spread Spectrum

16. FHSS Using MFSK
MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal
For data rate of R:
duration of a bit: T = 1/R seconds
duration of signal element: Ts = LT seconds
Tc  Ts - slow-frequency-hop spread spectrum
Tc < Ts - fast-frequency-hop spread spectrum

17. Slow-frequency Hop Spread Spectrum using MFSK
PN Sequence
MFSK Wd
Frequency Wd Ws Wd Wd T
M = 4, k=2 Ts Tc

18. Fast-frequency Hop Spread Spectrum using MFSK
PN Sequence
MFSK Wd
Frequency Wd Ws Wd Wd T
M = 4, k=2 Tc Ts

19. FHSS Performance Considerations
Large number of frequencies used
Results in a system that is quite resistant to jamming
Jammer must jam all frequencies
With fixed power, this reduces the jamming power in any one frequency band

20. Direct Sequence Spread Spectrum (DSSS)
Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band
Spread is in direct proportion to number of bits used
One technique combines digital information stream with the spreading code bit stream using exclusive-OR

21. Direct Sequence Spread Spectrum (DSSS)

22. Summary CSMA Spread Spectrum Next Lecture Versions of CSMA CSMA/CA
Example
Spread Spectrum
Frequency Hoping
Direct Sequence
Next Lecture
Evolution of wireless networks