Aoucemets Readig Chapter 4 (4.1-4.2) Project #4 is o the web ote policy about project #3 missig compoets Homework #1 Due 11/6/01 Chapter 6: 4, 12, 24, 37 Midterm #2 11/8/01 i class 1
Project #4 otes IPv6Iit, IPv6socket, routig IPv6Iit, IPv6socket, routig Recv thread, Forwards packets IPv6Iit, IPv6socket, routig 2
Trasmissio: Satellites Differet Orbits Possible orbit affects may commuicatio properties Geosychroous always over the same spot o the earth 36,000 Km orbit is required oly 180 slots possible uses oe uplik ad oe dow lik frequecy large roud-trip latecies LEO (Low Earth Orbit) each satellite keeps movig ito ad out of rage solutio: use a large umber of satellites sort of like cells, but the cells are the oes movig lower roud-trip latecy 3
Sedig More Tha oe Sigal At Oce Called multiplexig origial goal of Bell was to MUX multiple telegraph sigals Time Divisio Multiplexig everyoe gets whole badwidth but oly whe its their tur 193 Bit frame (125 msec) Chael 1 Chael 2 Chael 3 Chael 4 Chael 24 1 0 Bit 1 is a framig code 7 Data bits per chael per sample Bit 8 is for sigalig figure copyright, 1996, Adrew S. Taebaum 4
Frequecy Divisio Multiplexig Frequecy Divisio everyoe gets to talk at oce but oly i their ow frequecy Chael 1 1 Atteuatio factor Chael 2 1 Chael 3 1 Chael 2 Chael 1 Chael 3 60 64 Frequecy (khz) (c) 68 72 300 3100 Frequecy (Hz) (a) 60 64 68 72 Frequecy (khz) (b) figure copyright, 1996, Adrew S. Taebaum 5
ATM Switchig Requiremets be able to switch 360,000 cells/sec per iput lik switch cells with as low a discard rate as possible ever reorder the cells o a virtual circuit Issues multiple cells destied for the same output at oce eed to buffer oe of them must esure fairess is maitaied head-of-lie blockig possible that a blocked output is holdig up cells that could be delivered 6
Switchig Fabric (space divisio) Cross bars are great, but require O( 2 ) wires Ca use a collectio of smaller cross bar switches pealty: a request to coect may block Crossbars k k k k Iputs = 16, = 4, k = 2 = 16, = 4, k = 3 k Crossbars Crossbars k k k k outputs Crossbars k k k k Iputs k Crossbars Crossbar s k k k k outputs (a) From: Computer etworks, 3 rd Ed. by Adrew S. Taebaum, (c)1996 Pretice Hall. (b) 7
Batcher-baya Switchig Baya ca do a good or poor job of switchig due to collisios if the iputs are sorted, we get performace Batcher sorts traffic base o full address of destiatio compares two collidig packets ad uses fial destiatio to select output port requires O(log 2 ) odes (2x2 switchig elemets) Title: (Adobe Illustrator (R) Versio 5.0 Level 2 Emulatio) Creator: Adobe Illustrator(TM) 5.0 CreatioDate: (04/10/93) () From: Computer etworks, 3 rd Ed. by Adrew S. Taebaum, (c)1996 Pretice Hall. 8
Medium Access Layer Broadcast etworks share a commo resource for commuicatio bus, wire, air, etc. eed to coordiatio access to this resource Limits of Static Chael Allocatio suitable for costat rate traffic of similar speeds however, bursty traffic results i poor chael utilizatio cosider oe queue vs. separate queues for each perso queues with bursty arrival have mea delay times 1 queue Dyamic Allocatio oly use chael whe have somethig to sed eed to cotrol access to the chael 9
Shared Chael Model Statio model idepedet statios each wats to sed λ frames per secod a statio may ot sed aother frame util the first is set Sigle Chael Assumptio all statios commuicate over a sigle shared chael Collisios: two statios attempt to sed at oce Time either trasmissio succeeds cotiuous time: frame trasmissios ca start aytime discrete time: clock esures all seds iitiate at the start of a slot Carrier Sese statios ca tell if chael is i use before sedig statios must wait to kow if chael was i use 10
Aloha Statios groud based radio statios o islads Pure Aloha If sed data at will, collisios will happe o collisio, wait a radom amout of time & try agai use stadard, fixed size packets what is chael efficiecy (assumig Poisso distributio)? S ew frames per frame time assume G total frames tryig to be set per frame time P 0 = probability of exactly oe seder sedig S = G P 0 probability of k frames geerated durig a frame time Pr[k] = G k e -G / k! P o = e -G e -G = e -2G, so S= Ge -2G 11
Performace of Aloha Collides with the start of the shaded frame t Collides with the ed of the shaded frame to to+ t to+ 2t to+ 3t Time Vulerable From: Computer etworks, 3 rd Ed. by Adrew S. Taebaum, (c)1996 Pretice Hall. S (throughput per frame time) 0.40 0.30 Slotted ALOHA: S = Ge -DG 0.20 0.10 Pure ALOHA: S = Ge -D2G 0 0.5 1.0 1.5 2.0 3.0 G (attempts per packet time) From: Computer etworks, 3 rd Ed. by Adrew S. Taebaum, (c)1996 Pretice Hall. 12
Aloha (cot.) Slotted Aloha Use a cetral clock Each statio oly seds at the start of frame Reduces collisio widow by 1/2 S = G e -G 13