Copyright © Syed Ali Khayam 2007

CC-885: Wireless Networks

Syed Ali KhayamNUST Institute of Information Technology (NIIT)National University of Sciences & Technology (NUST)Pakistan

Copyright © Syed Ali Khayam 2007

Credits and Acknowledgements In this course, I will be teaching concepts from a

number of recent research papers, which will be referenced appropriately

I am thankful to Prof. Nitin Vaidya of UIUC, Prof. Hari Balakrishnan of MIT, and Prof. Andrew S. Tanenbaum of Vrije Universiteit for putting up some excellent slides on the web, from which I will be borrowing material intermittently

I will be borrowing material and examples from one of my favorite books: Computer Networks by Prof. Andrew S. Tanenbaum It is not possible to a add another text/reference book for

this course, otherwise I would have added this book

Copyright © Syed Ali Khayam 2007

Wireless MAC Challenges

CSMA/CD in a switched configuration works like a charm in wired networks

But it doesn’t work quite as well on a wireless network because of: Varying Link Quality Hidden and Exposed Terminal Problems Tradeoff between Packet Length and

Errors

Copyright © Syed Ali Khayam 2007

C is transmitting to B But A doesn’t know that because it is out of

C’s range A starts transmitting to B, resulting in a

collision

Wireless MAC Challenges: The Hidden Terminal Problem

A B C

Copyright © Syed Ali Khayam 2007

B is transmitting to C A wants to transmit to D, which is out of the

range of B But A thinks that the channel is busy, and

defers its transmission

Wireless MAC Challenges: The Exposed Terminal Problem

A B CD

Copyright © Syed Ali Khayam 2007

Wireless MAC Challenges: Solution to the Hidden Terminal Problem

Potential solution to the hidden terminal problem: Both the sender and the receiver

announce their intention to communicate This mechanism is called RTS/CTS:

The sender broadcasts a Request to Send (RTS) packet

In response, the receiver broadcasts a Clear to Send (CTS) packet

Mul

tiple

Acc

ess

with

Col

lisio

n

Avo

idan

ce (M

AC

A)

Copyright © Syed Ali Khayam 2007

RTSCTS

Wireless MAC Challenges: Solution to the Hidden Terminal Problem

B CA

Copyright © Syed Ali Khayam 2007

Wireless MAC Challenges: Hidden and Exposed Terminal Problems

When will MACA fail? Both A and C send RTS to B simultaneously Due to channel interference, A does not receive

the CTS Due to channel interference, C does not receive

the CTS Assuming that ample bandwidth is available,

can there be another solution that will solve both the hidden and the exposed terminal problems? Time or Frequency Division Multiplexing

Can be resolved by retrans