Wireless networks - Lecture 7: CSMA and spread spectrum

Wireless networks - Lecture 7: CSMA and spread spectrum. The main topics covered in this chapter include: carrier sense multiple access (CSMA); employs different node behaviour when channel found busy; problem of radio transmission; direct sequence spread spectrum (DSSS);...

Wireless Networks

Lecture 7 CSMA and Spread Spectrum

Dr Ghalib A Shah

 Review of previous lecture #6

Last Lecture Review

Carrier Sens e Multiple Acces s  (CSMA)

 Dis advantages  of ALOHA 

► us ers  do not lis ten to the channel before (and

while) trans mitting

► suitable for networks with long propagation delays

 Carrier Sens e Multiple Acces s

► polite vers ion 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

CSMA / Collis ion Avoidance

 Us ed where CSMA/CD cannot be us ed 

► 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

Wait IFS

Transmit frame

Still Idle?

No

Yes

No No

Yes Yes If medium becomes busy during the backoff time, the backoff timer is halted and

resumes when the medium becomes idle.

Example

► 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

Spread Spectrum

 Types of spreading:

► direct s equence s pread s pectrum (DSSS)

► frequency hopping s pread s pectrum (FHSS)

Frequency Hoping Spread Spectrum

 MFSK signal is translated to a new frequency every Tc 

seconds by modulating the MFSK signal with the FHSS  carrier signal

Slow-frequency Hop Spread Spectrum using

MFSK

M = 4, k=2

T Ts

Fast-frequency Hop Spread Spectrum using MFSK

M = 4, k=2

T Tc

Direct Sequence Spread Spectrum (DSSS)