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: received signal strength overwhelmed by local transmission strength human analogy the polite conversationalist
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: received signal strength overwhelmed by local transmission strength • human analogy: the polite conversationalist
CSMA/CD Collision Detection ace collision detect/abort time
CSMA/CD Collision Detection
Taking turns"MAC protocols channel partitioning mac protocols share channel efficiently and fairly at high load inefficient at low load: delay in channel access, L/n bandwidth allocated even if only 1 active nodel Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead taking turns" protocols look for best of both worlds
“Taking Turns” MAC protocols channel partitioning MAC protocols: – share channel efficiently and fairly at high load – inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols – efficient at low load: single node can fully utilize channel – high load: collision overhead “taking turns” protocols look for best of both worlds!
Taking Turns" MAC protocols Polling o master node invites" slave nodes to transmit in turn master · typically used with data dumb"slave devices concerns slaves polling overhead latency single point of failure(master)
“Taking Turns” MAC protocols Polling: • master node “invites” slave nodes to transmit in turn • typically used with “dumb” slave devices • concerns: – polling overhead – latency – single point of failure (master) master slaves poll data data
Taking turns"MAC protocols Token passing: control token passed from one node to next sequentially (nothin token message to send) o concerns token overhead latency single point of failure (token) data
“Taking Turns” MAC protocols Token passing: • control token passed from one node to next sequentially. • token message • concerns: – token overhead – latency – single point of failure (token) T data (nothing to send) T