Sharing a ChannelOutlineChannelPacket Delay - continuedSlide 5Queuing DelayQueuing Delay (continued)Slide 8UCBSharing a ChannelJean WalrandU.C. Berkeleywww.eecs.berkeley.edu/~wlrUCBOutlineChannelTDMFDMCDMAStatistical MultiplexingAloha: Slotted, Unslotted, ReservationCSMA/CACollision AvoidanceUCBChannelExamples: Wireless, copper, fiberCan transmit range of frequenciesf1f2Gain(f)fUCBPacket Delay - continuedIllustration Time 0 QD P/R = TRANS L/Speed = PROP Buffer Transmission Line (L km)UCBPacket Delay - continuedExamples: Link 1: Long FiberLink 2: Copper Link 3: WirelessLength L (km) 80 4 10Speed (s/km) 5 4 3.3Transmission Rate1Gbps 1Mbps 10kbps TRANS 1s 1ms 100msPROP 400s 16s 33sUCBQueuing DelayIsolated Packets:Time tX(t) = number of bits at time tX(t)PP/RR bpsQD = 0UCBQueuing Delay (continued)Packet Bursts:Time tX(t) = number of bits at time tX(t)PP/RR bpsAverage value of QD = (0 + 1 + 2)TRANS/3 = TRANSFor an isolated burst of size N: = (0 + 1 + … + N - 1 )TRANS/N = (N – 1)TRANS/2PPP/R P/RQD2QD3QD = (N – 1)TRANS/2UCBQueuing Delay (continued)Rule of thumb …. 80% utilization => QD 4TRANSExample: 0.1Mbps 100Mbps 100Mbps 0.1Mbps R1 R2 Rn 1500km P = 1,000 bits; 80% utilization on each linkQD 4[10ms + (n – 1)0.01ms + 10ms] 80msPROP = 51500s = 7.5msTRANS = 10ms + (n – 1)0.01ms + 10ms 20ms => Delay
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