UCB Sharing a Channel Jean Walrand U C Berkeley www eecs berkeley edu wlr UCB Outline Channel TDM FDM CDMA Statistical Multiplexing Aloha Slotted Unslotted Reservation CSMA CA Collision Avoidance Channel UCB Examples Wireless copper fiber Can transmit range of frequencies Gain f f f1 f2 UCB Packet Delay continued Illustration Buffer Transmission Line L km 0 QD P R TRANS Time L Speed PROP UCB Packet Delay continued Examples Link 1 Long Fiber Link 2 Copper Link 3 Wireless Length L km 80 4 10 Speed s km 5 4 3 3 1Gbps 1Mbps 10kbps 1 s 1ms 100ms 400 s 16 s 33 s Transmission Rate TRANS PROP UCB Queuing Delay Isolated Packets X t number of bits at time t R bps X t P P R Time t QD 0 UCB Queuing Delay continued X t number of bits at time t Packet Bursts R bps P X t QD3 P Average value of QD 0 1 2 TRANS 3 TRANS For an isolated burst of size N 0 1 N 1 TRANS N N 1 TRANS 2 QD2 QD N 1 TRANS 2 P P R P R P R Time t UCB Queuing Delay continued Rule of thumb 80 utilization QD 4TRANS Example 0 1Mbps 100Mbps R1 100Mbps R2 0 1Mbps Rn 1500km P 1 000 bits 80 utilization on each link QD 4 10ms n 1 0 01ms 10ms 80ms PROP 5 1500 s 7 5ms TRANS 10ms n 1 0 01ms 10ms 20ms Delay 108ms
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