5-1 Data Link Layer r Wireless Networks m Wi-Fi (Wireless LAN) Example Problems m RTS/CTS

5-2 Example 1 r Consider an CSMA/CA WLAN with two stations A and C and one Access Point (AP), B. At time, t=0, station A has generated a frame of length 10 slots and station C has generated a frame of length 5 slots both to be transmitted to B. What is the earliest time (T) that both the frames will be successfully received and acknowledged by B? r Assumptions: m DIFS = 2 slots m SIFS = 1 slot m ACK frames = 1 slot r T = ?

5-3 Example 2 r Consider an CSMA/CA WLAN with two stations A and C and one Access Point (AP), B. At time, t=0, station A has generated a frame of length 10 slots and station C has generated a frame of length 5 slots both to be transmitted to B. Assume that after waiting for DIFS, both A and C generated a random backoff number equal to 1. What is the earliest time (T) that both the frames will be successfully received and acknowledged by B? r Assumptions: m DIFS = 2 slots m SIFS = 1 slot m ACK frames = 1 slot r T = ?

5-4 Example 3 r Consider an CSMA/CA WLAN with two stations A and C and one Access Point (AP), B. At time, t=0, station A has generated a frame of length 20 slots and station C has generated a frame of length 10 slots both to be transmitted to B. Assume that after waiting for DIFS, both A and C generated a random backoff number equal to 1. What is the earliest time (T) that both the frames will be successfully received and acknowledged by B? r Assumptions: m DIFS = 2 slots m SIFS = 1 slot m ACK frames = 1 slot r T = ?

6-5 Avoiding collisions – RTS/CTS idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames r sender first transmits small request-to-send (RTS) packets to BS using CSMA m RTSs may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS in response to RTS r CTS heard by all nodes m sender transmits data frame m other stations defer transmissions avoid data frame collisions completely using small reservation packets!

6-6 Collision Avoidance: RTS-CTS exchange AP A C time RTS(A) RTS(C) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

5-7 Example 4 r Consider an CSMA/CA WLAN with two stations A and C and one Access Point (AP), B. At time, t=0, station A has generated a frame of length 20 slots and station C has generated a frame of length 10 slots both to be transmitted to B. Assume that after waiting for DIFS, both A and C generated a random backoff number equal to 1. What is the earliest time (T) that both the frames will be successfully received and acknowledged by B? r Assumptions: m A and C are both RTS/CTS enabled m RTS = CTS = 1 slot m DIFS = 2 slots m SIFS = 1 slot m ACK frames = 1 slot r T = ?

6-8 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C Hidden terminal problem r B, A hear each other r B, C hear each other r A, C can not hear each other means A, C unaware of their interference at B A B C A’s signal strength space C’s signal strength Signal attenuation: r B, A hear each other r B, C hear each other r A, C can not hear each other interfering at B