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

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

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

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

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

The MACA Protocol TANENBAUM Computer Networks 1 6

Required Reading Tanenbaum Sections 4.1 4.2 TANENBAUM Computer Networks 1 7