1. Wireless LAN Protocols
Lalyer-1 & Layer-2
Wireless LAN Protocols

2. Wireless LAN Protocols
Difference between mobile and portable
Portable: Requires wired connection to a fixed network
E.g. Notebook Computer
Mobile: Requires wireless communication
E.g Portable computers communicating by radio
Basic assumption:
Whenever a receiver is within range of two active transmitters, the resulting signal is garbled
TANENBAUM
Computer Networks 1 2

3. CSMA for Wireless LAN
This is not a good choice because what matters is the interference at the receiver, not at the sender
Hidden Station Problem
A station may not able to detect a communication with its neighbor
Exposed Station Problem
A station may falsely conclude that it can not communicate with its neighbor due to communication going on the other end
TANENBAUM
Computer Networks 1 3

4. Hidden and Exposed Station Problems
A is transmitting to B,
If C senses the medium, it won’t hear A. Because A is out of C’s range
C will falsly conclude that it can transmit to B
If C start transmitting to B, it will interfere at B
B is transmitting to A,
If C senses the medium, it will hear an ongoing tranasmission and falsly conclude that it may not send to D.
TANENBAUM
Computer Networks 1 4

5. Multiple Access with collision Avoidance Protocol
A sends Request To Send (RTS) to B that indicates the length of the data frame that will eventually follow
B replies with a Clear To Send (CTS) that contains the length of the data expected (copied from RTS)
As it receives CTS, A begins transmission
Note that hidden and exposed station problems are solved in this protocol
Hidden station hears RTS
Exposed station hears CTS
TANENBAUM
Computer Networks 1 5

6. The MACA Protocol
TANENBAUM
Computer Networks 1 6

7. Required Reading Tanenbaum Sections 4.1 4.2 TANENBAUM
Computer Networks 1 7