Wireless networks - Lecture 7: CSMA and spread spectrum

Wireless Networks

Lecture 7
CSMA and Spread Spectrum
Dr. Ghalib A. Shah

1


 Review of previous lecture #6
 CSMA
► Versions of CSMA
► CSMA/CA
► Example

 Spread Spectrum
► Frequency Hoping
► Direct Sequence

 Summary of today’s lecture
2


Last Lecture Review





FDMA
TDMA

CDMA
Random Access
► ALOHA
► Slotted ALOHA
► Reservation-based ALOHA

3


Carrie r S e ns e  Multiple  Ac c e s s  (CS MA)
 Dis advantag e s  o f ALOHA 
► us e rs  do  no t lis te n to  the  c hanne l be fo re  (and
while) trans mitting
► suitable for networks with long propagation delays

 Carrie r S e ns e  Multiple  Ac c e s s
► po lite  ve rs io n o f 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

4


Versions of CSMA
 Emplo ys  diffe re nt no de  be havio ur whe n 
c hanne l fo und bus y
► no n­pe rs is te nt CS MA

• after sensing busy channel, node waits entire back-off
period before sensing again

► pe rs is te nt CS MA

• after sensing busy channel, node continues sensing until
the channel becomes free; then …

► 1­pe rs is te nt CS MA

• node transmits immediately with probability 1

► p­pe rs is te nt CS MA

• node transmits with probability p; or, it defers transmission
with probability (1-p)
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CS MA / Co llis io n Avo idanc e
 Us e d whe re  CS MA/CD c anno t be  us e d 
► 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 inte rvals  –contention window –intervals used for contention
and transmission of packet frames
Bac ko ff c o unte r–used only if two or more stations compete for
transmission
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CSMA/CA Algorithm
Frame to
transmit
Medium
Idle?

No

Yes

Wait IFS

Wait IFS

Still
Idle?

Wait until
Trans ends

No

Yes
Transmit frame

Still
Idle?

No

Yes
Exp b/o while
Medium idle

If medium becomes busy during the backoff
time, the backoff timer is halted and
resumes when the medium becomes idle.

Transmit frame

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Example

8

Spread Spectrum
 Pro ble m o f Radio  Trans mis s io n
► fre que nc y de pe nde nt fading  c an wipe  o ut 
narro wband s ig nals  fo r duratio n o f inte rfe re nc e

 solution:
► spread narrow band signal into a broad band signal
using a special code
► initially developed for military in order to combat
jamming  and inte rc e ptio n
► power of spread signal is the same as of narrow
band signal, resulting in a lower power spectral
density due to larger bandwidth
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10


Spread Spectrum

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 Types of spreading:
► dire c t s e que nc e  s pre ad s pe c trum (DS S S )
► fre que nc y ho pping  s pre ad s pe c trum (FHS S )

12


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

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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

14

Frequency Hoping Spread Spectrum

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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 ­ slow­frequency­hop spread spectrum
 Tc < Ts ­ fast­frequency­hop spread spectrum

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Slow-frequency Hop Spread Spectrum using
MFSK
PN Sequence 00
MFSK

0 1

11
1 1

0 0

01
1 1

1 1

10
0 1

1 0

00
0 0

ycneuqer F

Wd

Ws

Wd

Wd

Wd
T

M =4, k=2

Ts

Tc

17

0 0

1 1


Fast-frequency Hop Spread Spectrum using MFSK
PN Sequence
MFSK

00
0

11

01

1

1

10

00
1

0

10

00

11

0

1

1

ycneuqer F

Wd

Ws

Wd

Wd

Wd
T

M =4, k=2

Tc
Ts

18

10
1

00
1


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

19


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 exclusive­OR

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Direct Sequence Spread Spectrum (DSSS)

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Summary
 CSMA
► Versions of CSMA
► CSMA/CA
► Example

 Spread Spectrum
► Frequency Hoping
► Direct Sequence

 Next Lecture
► Evolution of wireless networks
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