1. Copyright © Syed Ali Khayam 2007 CC-885: Wireless Networks Syed Ali Khayam NUST Institute of Information Technology (NIIT) National University of Sciences & Technology (NUST) Pakistan

2. 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

3. 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

4. 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 AB C

5. 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 ABCD

6. 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 Multiple Access with Collision Avoidance (MACA)

7. Copyright © Syed Ali Khayam 2007 RTSCTS Wireless MAC Challenges: Solution to the Hidden Terminal Problem BCA

8. 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