1.3 Multiplexing, Time-Switching, Point-to-Point versus Buses

Transcription

1 1.3 Multiplexing, Time-Switching, Point-to-Point vesus Buses n R m Aggegation (multiplexing) Distibution (demultiplexing) Simplest Netwoking, like simplest pogamming: Sequential! CS-534, Copyight Univ. of Cete 1 Time-Switching n R m Aggegation (multiplexing) Distibution (demultiplexing) Shaed Medium Communication Demultiplexo detemines whee each piece of infomation is outed to by selecting the time at which each output eceives infomation fom the shaed medium Simplicity: one thing at a time no paallelism Non-scalable! cannot incease R indefinitely Full-capacity (R = n ) o Statistical Multiplexing (R < n ) Implementation issues, point-to-point links vesus bidiectional, shaed physical medium CS-534, Copyight Univ. of Cete 2 CS-534, 1.3: Mux, Time-Switch, Bus 1

2 Statistical Multiplexing C Cn = CM > Cm C1 CM Buffe Cm Cn fixed aggegate capacity multiplexing not fully utilized all the time patitioned capacity (cicuit switching) statistical multiplexing buffe equied to absob tansients when λ(t) > Cm shaed capacity (packet switching) Capacity Cm is bette utilized than capacity CM The aveage aggegate souce ate must not exceed Cm ove time windows detemined by the size of the buffe memoy CS-534, Copyight Univ. of Cete 3 CS-534, Copyight Univ. of Cete 4 CS-534, 1.3: Mux, Time-Switch, Bus 2

3 CS-534, Copyight Univ. of Cete 5 Point-to-Point vesus Shaed Bidiectional Links Point-to-Point: Shaed Medium: xmit1 xmitn xmit cv cv1 cvm (e.g. high-speed coppe o fibe links) Highe Pefomance: no tun-aound delay no abitation ovehead inceased paallelism when used with switches (e.g. wieless links, old ethenet, buses) Lowe Cost: boadcast & select: switching is inheent in the medium natual in some envionments --e.g. wieless without diectional antenas CS-534, Copyight Univ. of Cete 6 CS-534, 1.3: Mux, Time-Switch, Bus 3

4 CS-534, Copyight Univ. of Cete 7 How many bits is the length of the wie? Tun-aound delay expessed as a lost oppotunity to tansmit an amount of infomation equivalent to: How long is a packet on the wie? Duation of each tansmit session (assume one packet), in time o in equivalent packet "length" on the wie: K 10K 100K K 12K d=2km d/c = 10 us d=200m d/c = 1 us d=20m d/c = 100 ns c ~= 200 Mm/s avg IP pck sz. equivalent packet "length" on the wie 20m 200m 2km 20km 200km p a c k e t t i m e 100ns 1us 10us 100us 1ms pck sz = 10 Kb = 1.2 KB pck sz = 1 Kb = 128 B bits Bytes 10M 100M 1G 10G 10M 100M 1G 10G bits/s bits/s link thoughput link thoughput CS-534, Copyight Univ. of Cete 8 CS-534, 1.3: Mux, Time-Switch, Bus 4

5 Link Utilization = f(packet Length, Wie Length, Thoughput) Utilization packetsize Link = = packetsize + tunaoundbitequiv packettime packettime + tunaounddelay = = 99% = = 91% 1 1 = = 50% 128 B, 2 m; o 1.2 KB, 20 m; o 12 KB, 200 m 128 B, 20 m; o 1.2 KB, 200m; o 12 KB, 2 km = = % 128 B, 200 m; o 1.2 KB, 2 km = = Link Utilization 1% 10M 100M 1G 10G bits/s Link Thoughput 128 B, 2 km; o 1.2 KB, 20 km Packet Size, Link Length CS-534, Copyight Univ. of Cete 9 Abitation Ovehead in Shaed Media xmit1 1. eq-1 2. gant Abite 1. eq-n xmitn 3. data cvi Sepaate medium fo equests and gants? inceased media cost, inceased latency. Shaed medium fo all of equest, gant, and data? educed thoughput, inceased latency. Optimistic abitation (CDMA/ethenet style)? limited peak thoughput, vey high latency at high loads. CS-534, Copyight Univ. of Cete 10 CS-534, 1.3: Mux, Time-Switch, Bus 5

6 Sequential vesu Paallel Tansmissions Shaed Medium: Point-to-Point Links + Switches: Single tansmission at a time Multiple tansmissions in paallel CS-534, Copyight Univ. of Cete 11 This Couse: Point-to-Point Links Thoughput in shaed media is athe low time switching (simple), potocols usually in softwae, with little, if any, & simple H/W suppot Shaed media in moden netwoking: almost expelled, except in wieless wieless MAC potocols ae a majo topic in othe couses,and diffe significantly fom the hadwae achitectue of high-speed switches CS-534, Copyight Univ. of Cete 12 CS-534, 1.3: Mux, Time-Switch, Bus 6