文档介绍：: .
Ethernet Switches
-if sense channel busy
then binary backoff
CSMA receiver:
if received OK
return ACK after SIFS
哈工大计算机学院李全龙 Computer Networks 5: Datalink Layer and LAN 6IEEE MAC Protocol
CSMA Protocol: others
NAV: Network Allocation
Vector
frame has
transmission time field
others (hearing data)
defer access for NAV time
units
哈工大计算机学院李全龙 Computer Networks 5: Datalink Layer and LAN 7Hidden Terminal effect
hidden terminals: A, C cannot hear each other
obstacles, signal attenuation
collisions at B
goal: avoid collisions at B
CSMA/CA: CSMA with Collision Avoidance
哈工大计算机学院李全龙 Computer Networks 5: Datalink Layer and LAN 8Collision Avoidance: RTS-CTS exchange
CSMA/CA: explicit
channel reservation
sender: send short
RTS: request to send
receiver: reply with
short CTS: clear to
send
CTS reserves channel for
sender, notifying
(possibly hidden) stations
avoid hidden station
collisions
哈工大计算机学院李全龙 Computer Networks 5: Datalink Layer and LAN 9Collision Avoidance: RTS-CTS exchange
RTS and CTS short:
collisions less likely, of
shorter duration
end result similar to
collision detection
IEEE allows:
CSMA
CSMA/CA: reservations
polling from AP
哈工大计算机学院李全龙 Computer Networks 5: Datalink Layer and LAN 10 : .
Ethernet Switches
ˆ layer 2 (frame) forwarding,
filtering using LAN
addresses