Pure (unslotted)ALOHA unslotted Aloha:simpler,no synchronization when frame first arrives o transmit immediately collision probability increases: o frame sent at to collides with other frames sent in [to-1,to+1] will overlap will overlap with start of with end of ←-i's frame+←-i's frame node i frame tol 5:DataLink Layer 5-26
5: DataLink Layer 5-26 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t0 collides with other frames sent in [t0 -1,t0+1]
Pure Aloha efficiency P(success by given node)=P(node transmits). P(no other node transmits in [po-1,Po] P(no other node transmits in [Po-1,Po] =p·(1-p)N-1.(1-p)N-1 =p·(1-p)2N-1) .choosing optimum p and then letting n->infty. Even worse!=1/(2e)-.18 5:DataLink Layer 5-27
5: DataLink Layer 5-27 Pure Aloha efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [p0 -1,p0 ] . P(no other node transmits in [p0 -1,p0 ] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) . choosing optimum p and then letting n -> infty . = 1/(2e) = .18 Even worse !
CSMA (Carrier Sense Multiple Access) CSMA:listen before transmit: If channel sensed idle:transmit entire frame If channel sensed busy,defer transmission OHuman analogy:don't interrupt others! 5:DataLink Layer 5-28
5: DataLink Layer 5-28 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others!
CSMA collisions spatial layout of nodes -space- collisions can still occur: propagation delay means two nodes may not hear 0 each other's transmission collision: entire packet transmission time wasted note: role of distance propagation delay in determining collision probability 5:DataLink Layer 5-29
5: DataLink Layer 5-29 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability
CSMA/CD (Collision Detection) CSMA/CD:carrier sensing,deferral as in CSMA o collisions detected within short time o colliding transmissions aborted,reducing channel wastage ▣collision detectio: o easy in wired LANs:measure signal strengths, compare transmitted,received signals o difficult in wireless LANs:receiver shut off while transmitting human analogy:the polite conversationalist 5:DataLink Layer 5-30
5: DataLink Layer 5-30 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist