CS422 Compuer Neworks Lecure 2 Physical Layer Dr. Xiaobo Zhou Deparmen of Compuer Science CS422 PhysicalLayer.1 Quesions of Ineress How long will i ake o ransmi a message? How many bis are in he message (ex, image)? How fas does he nework/sysem ransfer informaion? Can a nework/sysem handle a voice (video) call? How many bis/second does voice/video require? A wha qualiy? How long will i ake o ransmi a message wihou errors? How are errors inroduced? How are errors deeced and correced? Wha ransmission speed is possible over radio, copper cables, fiber,? CS422 PhysicalLayer.2
A Transmission Sysem Transmier Receiver Communicaion channel Transmier Convers informaion ino signal suiable for ransmission Injecs energy ino communicaions medium or channel Telephone convers voice ino elecric curren Modem convers bis ino ones Receiver Receives energy from medium Convers received signal ino form suiable for delivery o user Telephone convers curren ino voice Modem convers ones ino bis CS422 PhysicalLayer.3 Transmission Impairmens Transmier Transmied Signal Received Signal Receiver Communicaion channel Communicaion Channel Pair of copper wires Coaxial cable Radio Ligh in opical fiber Ligh in air Transmission Impairmens Signal aenuaion Signal disorion Spurious noise Inerference from oher signals Infrared CS422 PhysicalLayer.4
Bi Raes of Digial Transmission Sysems Sysem Bi Rae Observaions Telephone 33.6-56 kbps 4 khz elephone channel wised pair Eherne wised pair 10 Mbps, 100 Mbps 100 meers of unshielded wised copper wire pair Cable modem 500 kbps-4 Mbps Shared CATV reurn channel ADSL wised pair 64-640 kbps in, 1.536-6.144 Mbps ou Coexiss wih analog elephone signal 2.4 GHz radio 2-11 Mbps IEEE 802.11 wireless LAN 28 GHz radio 1.5-45 Mbps 5 km mulipoin radio Opical fiber 2.5-10 Gbps 1 wavelengh Opical fiber >1600 Gbps Many wavelenghs CS422 PhysicalLayer.5 Fundamenal Issues in Transmission Media d meers Communicaion channel = 0 = d/v Propagaion speed of signal c = 3 x 10 8 meers/second in vacuum = c/ speed of ligh in medium where is he dielecric consan of he medium = 2.3 x 10 8 m/sec in copper wire; = 2.0 x 10 8 m/sec in opical fiber CS422 PhysicalLayer.6
The Theoreical Basis for Daa Communicaion Daa informaion can be ransmied on wires by varying some physical propery as volage. Fourier Analysis Bandwidh-Limied Signals Maximum Daa Rae of a Channel CS422 PhysicalLayer.7 Guided Transmission Daa Wha is he purpose of he physical layer? Magneic Media Twised Pair Coaxial Cable Fiber Opics CS422 PhysicalLayer.8
Magneic Media Given a 200GB ape, a box 60 X 60 X 60cm can hold abou 1000 of hese apes. A box can be delivered anywhere domesically in 24 hours. Wha is he effecive bandwidh? Wha are he major problems wih magneic media? CS422 PhysicalLayer.9 Twised Pair A wised pair consiss of wo insulaed copper wires, ypically abou 1mm hick. More wiss per cm leads o less crossalk and beer qualiy over longer disance Why wised pairs are widely used? (a) Caegory 3 UTP (16 MHz). (b) Caegory 5 UTP (100 MHz). CS422 PhysicalLayer.10
Coaxial Cable A good combinaion of high bandwidh and excellen noise immuniy. Used o be in elecom for long-disance lines (replaced by fiber opics), bu sill widely used in cable TV and MANs. A coaxial cable. CS422 PhysicalLayer.11 Fiber Opics An opical ransmission sysem has hree key componens: he ligh source, ransmission medium, and he deecor Main issue: ligh leaking (a) Three examples of a ligh ray from inside a silica fiber impinging g on he air/silica boundary a differen angles. (b) Ligh rapped by oal inernal reflecion (mulimode fiber). The achievable bandwidh is more han 50 Tbps. Why he curren pracical singling limi is abou 10Gbps (100 Gbps in Labs)? Who wins he rae beween compuing and communicaion? CS422 PhysicalLayer.12
Fiber Cables (a) Side view of a single fiber. (b) End view of a sheah wih hree fibers. A coaxial cable. CS422 PhysicalLayer.13 Wireless Transmission The Elecromagneic Specrum Radio Transmission Microwave Transmission Infrared and Millimeer Waves Lighwave Transmission CS422 PhysicalLayer.14
Communicaion Saellies Geosaionary Saellies Medium-Earh Orbi Saellies Low-Earh Orbi Saellies Saellies versus Fiber CS422 PhysicalLayer.15 Communicaion Saellies Magneic fields Communicaion saellies and some of heir properies, including aliude above he earh, round-rip delay ime and number of saellies needed for global coverage. Why hree regions for saellie placemens? Where are he 24 GPS saellies? CS422 PhysicalLayer.16
Low-Earh Orbi Saellies Iridium Iridium: o provide worldwide elecommunicaion service using hand-held devices ha communicae direcly wih he (66) Iridium saellies. Wha are heir behaviors a low-earh orbi? (a) (a) The Iridium saellies from six necklaces around he earh. (b) 1628 moving cells cover he earh. Why Iridium los business? (b) CS422 PhysicalLayer.17 Public Swiched Telephone Sysem Srucure of he Telephone Sysem The Poliics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Muliplexing Swiching CS422 PhysicalLayer.18
Srucure of he Telephone Sysem PSTN (Public Swiched Telephone Nework): voice or daa (56 kbps vs. 1 Gbps) (a) Fully-inerconneced nework. (b) Cenralized swich. (c) Two-level hierarchy. CS422 PhysicalLayer.19 Srucure of he Telephone Sysem (2) Three major componens in he elephone sysem Local loops ( las mile, analog - > digial) Trunks (Muliplexing) Swiching offices A ypical circui roue for a medium-disance call. CS422 PhysicalLayer.20
Major Componens of he Telephone Sysem Local loops Analog wised pairs going o houses and businesses Trunks Digial fiber opics connecing he swiching offices Swiching offices Where calls are moved from one runk o anoher CS422 PhysicalLayer.21 The Local Loop: Modems, ADSL, and Wireless The use of boh analog and digial ransmissions for a compuer o compuer call. Conversion is done by he modems and codecs. How many acive connecions can be handled by ISP1? CS422 PhysicalLayer.22
Modems Today s modem is called V.90 The baud rae: 8000 samples (symbols) /sec The sample (symbol) size: 8 bis (7 daa + 1 conrol in US) TCM: (a) V.32 for 9600 bps. (b) V.32 bis for 14,400 bps. CS422 PhysicalLayer.23 Simplex, Half Duplex, and Full Duplex Simplex connecion Half-duplex connecion Full duplex connecion Give an example per each ype? Wha does a modem belong o? CS422 PhysicalLayer.24
Topology Impac on Transmission Pahs A packe-swiching nework has a sar opology wih a cenral swich. I has n nodes. Wha are he bes-, average-, and wors-case ransmission pahs in hops? How abou a bidirecional ring, and a fully inerconneced packe-swiching neworks? CS422 PhysicalLayer.25 Trunks and Muliplexing Muliplexing many conversaions ino a single physical runk FDM (Frequency Division Muliplexing) The overall frequency specrum is divided ino frequency bands, wih each user having exclusive possession of some band. TDM (Time Division Muliplexing) The users ake urns (Round-Robin), each one periodically geing he enire bandwidh for a lile burs of ime. CS422 PhysicalLayer.26
Swiching Circui Swiching Message Swiching Packe Swiching CS422 PhysicalLayer.27 Circui Swiching vs. Packe Swiching (a) Circui swiching. (b) Packe swiching. CS422 PhysicalLayer.28
Message Swiching Message Message Message Message swiching invened for elegraphy Enire messages muliplexed ono shared lines, sored & forwarded Headers for source & desinaion addresses Rouing a message swiches Connecionless Source Message Swiches Desinaion CS422 PhysicalLayer.29 Message Swiching Delay Source T Swich 1 Swich 2 Desinaion Delay Minimum delay = 3 + 3T Addiional queueing delays possible a each link Wha are he major problems? CS422 PhysicalLayer.30
Long Messages vs. Packes 1 Mbi message source BER=p=10-6 BER=10-6 des How many bis need o be ransmied o deliver message? Approach 1: send 1 Mbi message Probabiliy message arrives correcly Approach 2: send 10 100- kbi packes Probabiliy packe arrives correcly 6 6 6 5 6 10 10 10 1 P (1 10 ) e e 1/ 3 c (1 10 ) 10 5 6 6 10 10 0.1 e e 0. 9 On average i akes abou 3 ransmissions/hop Toal # bis ransmied 6 Mbis CS422 PhysicalLayer.31 P c On average i akes abou 1.1 ransmissions/hop Toal # bis ransmied 2.2 Mbis Packe Swiching - Daagram Messages broken ino smaller unis (packes) A packe has maximum size Source & desinaion addresses in packe header Connecionless, packes roued independenly (daagram) Packe 2 Packe 1 Packe 1 Packe may arrive ou of order Pipelining of packes across nework can reduce delay, increase hroughpu Packe 2 Packe 2 Lower delay ha message swiching, suiable for ineracive raffic CS422 PhysicalLayer.32
Packe Swiching Delay Assume hree packes corresponding o one message raverse same pah 1 2 3 1 2 Delay 3 1 2 Packe pipelining Minimum Delay = 3τ + 5(T/3) (single pah assumed) Addiional queueing delays possible a each link Packe pipelining enables message o arrive sooner 3 CS422 PhysicalLayer.33 Delay for k-packe Messge over L Hops Source 1 2 3 Swich 1 1 2 3 Swich 2 1 2 3 Desinaion 3 hops L hops 3 + 2(T/3) firs bi received L + (L-1)P firs bi received 3 + 3(T/3) firs bi released L + LP firs bi released 3 + 5 (T/3) las bi released L + LP + (k-1)p las bi released or L + (L-1)P + kp where T = k P (wha is P indeed?) CS422 PhysicalLayer.34
Circui, Message, Packe Swichings CS422 PhysicalLayer.35 (a) Circui swiching. (b) Message swiching. (c) Packe swiching Swiching Performance Wha is he delay of sending an X-bi message over a L-hop pah in a circui-swiched nework? The circui seup ime is s sec, he propagaion delay is sec per hop, and he daa rae is b bps. Wha is he delay of sending an X-bi message over a L-hop pah in a lighly-loaded packe-swiched nework? The propagaion delay per packe is sec per hop, he packe size is p bis, and he daa rae is b bps. CS422 PhysicalLayer.36
Packe Swiching A comparison of circui swiched and packe-swiched neworks. CS422 PhysicalLayer.37 The Mobile Telephone Sysem Firs-Generaion Mobile Phones: Analog Voice Second-Generaion Mobile Phones: Digial Voice D-AMPS, GSM, CDMA Third-Generaion Mobile Phones: Digial Voice and Daa CS422 PhysicalLayer.38
Inerne over Cable Wha are wo major challenges facing he cable operaors if hey wan o ge ino he Inerne/elephone business? How he cable indusry ackle he sharing problem? Is here bandwidh compeiion beween you and your neighbors if you use DSL? Cable or DSL, which faser? CS422 PhysicalLayer.39 Reading and Homework Chaper 2 of he exbook. Reading follows he lecuring some conceps no covered in he lecures and heir readings are opional herefore. Homework Posed on he course web sie. CS422 PhysicalLayer.40