Communication Networks

Transcription

1 Communicaion Neworks Chaper 10 Wireless Local Area Neworks According o IEEE Communicaion Neworks: 10. IEEE WLANs According o IEEE Overview Organizaion of a WLAN according o IEEE Curren IEEE sandards IEEE layers IEEE MAC MAC synchronizaion, power saving and roaming WLAN and ad hoc neworks Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 1

2 10.1 Working Modes WLAN in Infrasrucure Mode STA 1 ESS BSS LAN Access Poin BSS 2 Disribuion Sysem Access Poin 802.x LAN Poral STA LAN STA 3 Saion (STA) Device wih access o he wireless medium and conneciviy o he access poin Basic Service Se (BSS) Group of devices working on he same radio frequency Access Poin Device ha allows communicaion beween saions and inegraes hem ino he disribuion sysem Poral Gaeway o some oher nework Disribuion Sysem Connecion of differen WLAN cells o build an Exended Service Se EES Communicaion Neworks: 10. IEEE Working Modes WLAN in Ad-hoc Mode STA 1 BSS LAN STA 2 STA 3 Direc communicaion wih limied range Saion (STA): Device wih access o he wireless medium Basic Service Se (BSS): Group of devices working on he same radio frequency (Independen Basic Service Se IBSS) BSS 2 STA 4 STA LAN Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 2

3 10.1 Working Modes Hidden Node Problem A B C A would like o communicae wih B C is already ransmiing informaion o B using he same channel There are collisions in B Boh A and C canno deec hese collisions Communicaion Neworks: 10. IEEE Working Modes Exposed Node Problem A B C D B is already ransmiing informaion o A C would like o communicae wih D C finds he channel occupied and refrains from sending Free capaciies are unused Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 3

4 10.2 The Sandard IEEE IEEE-Sandard Fixed Terminal Mobile Terminal Server Infrasrucure Nework Applicaion TCP IP MAC PHY MAC PHY Access Poin MAC PHY Applicaion TCP IP MAC PHY Communicaion Neworks: 10. IEEE The Sandard IEEE IEEE Imporan Subsandards Original sandard from 1997 Daa rae 1 or 2 Mbi/s Frequency range o GHz (ISM band) a Exended physical layer, published 1999 Daa rae 54 Mbi/s Frequency range around 5 GHz b Exended physical layer, published 1999 Daa Rae 11 Mbi/s Same frequency range as original g Exended physical layer, published 2003 Daa rae 54 Mbi/s Frequency range o GHz n Muliple-inpu muliple-oupu anennas (MIMO) Publicaion by he IEEE in Ocober 2009 Daa rae up o 600 MBi/s Frequency range 2.4 GHz or 5 GHz ac Published 2013 Theoreical max. daa rae Gbi/s wih 8x8 MIMO Frequency: 5 GHz p Wireless access in vehicular environmens (WAVE) Daa exchange beween high-speed vehicles and beween he vehicles and he roadside infrasrucure in he licensed ITS band of 5.9 GHz ( GHz) Published in November 2010 Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 4

5 Saion Managemen 10.2 The Sandard IEEE Layers LLC Logical Link Conrol MAC Medium Access Conrol MAC Managemen PLCP Physical Layer Convergence Proocol PMD Physical Medium Dependen PHY Managemen Communicaion Neworks: 10. IEEE The Sandard IEEE Funcions MAC Medium Access Segmenaion/Reassembly Ciphering MAC Managemen Synchronizaion Roaming MIB Power Conrol PLCP Clear Channel Assessmen Signal (Carrier Sense) PMD Modulaion Coding PHY Managemen Channel Selecion MIB Saion Managemen Coordinaion of Managemen Funcions Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 5

6 images/80211fh.gif MAC Frame Forma Communicaion Neworks: 10. IEEE Fields in he IEEE MAC Frame Synch Preamble: for FH PHY 80 bis for DSSS PHY 128 bis alernaing '0's and '1's SFD Sar Frame Delimier: 16 bis PLW PLCP-PDU Lengh Word: 12 bis indicaing he number of byes in he packe firs porion of he PLCP header PLCP header is ransmied only a 1 Mbps! PSF PLCP Signaling Field: 4 bis o show he rae of he MAC payload ransmission Bi 0 is reserved and is always '0' Bis 1 o 3 indicae he daa raes Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 6

7 MAC Disribued Foundaion Wireless MAC (DFWMAC) Differen raffic ypes Asynchronous daa ransmission (sandard) Exchange of MAC frames wihou Qualiy of Service ( bes-effor ) Broadcas and mulicas Time-limied ransmission (opional) Poin Coordinaion Funcion (PCF) only in infrasrucure mode Communicaion Neworks: 10. IEEE MAC Procedures Disribued Coordinaion Funcion: Carrier Sense Muliple Access wih Collision Avoidance (DFWMAC-DCF CSMA/CA) (sandard) Collision avoidance based on arbirary backoff algorihm Minimum ime span beween wo MAC frames (so called iner frame spacing) Correc ransmission signaled wih ACK-frame (excep Broadcas or Mulicas) Disribued Coordinaion Funcion wih Reques o Send / Clear To Send Frames (DFWMAC-DCF wih RTS/CTS) (opional) Avoidance of hidden node problem Poin Coordinaion Funcion (DFWMAC-PCF) Lis-based polling done in he Access Poin (opional) Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 7

8 MAC: Iner Frame Spacing Implemenaion of Prioriies No guaranees Shorer iner frame spacing allows earlier sending ime for he frame: (Shor Iner Frame Spacing) Highes prioriy, for ACK, CTS, response o polling PIFS (PCF IFS) Medium prioriy, for ime limied services in PCF (DCF, Disribued Coordinaion Funcion IFS) Lowes prioriy, for asynchronous daa ransmission medium busy PIFS compeiion direc access, if medium is free nex frame Communicaion Neworks: 10. IEEE CSMA/CA I medium busy PIFS Compeiion Window (arbirary backoff ime) nex frame waiing ime ime slo Carrier sense based on clear channel assessmen signal Saion may send, if medium is free for he appropriae IFS If medium is busy, saion ses backoff ime o an arbirary number of ime slos Afer he medium is free again, saion wais he appropriae IFS and he backoff ime If medium ges busy during backoff ime, backoff ime is frozen Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 8

9 CSMA/CA II bo e bo r bo e bo r bo e busy Saion 1 bo e busy Saion 2 Saion 3 busy bo e busy bo e bo r Saion 4 bo e bo r bo e busy bo e bo r Saion 5 busy medium busy (frame, ack ec.) bo e elapsed backoff ime daa arrival a MAC-SAP bo r residual backoff ime Communicaion Neworks: 10. IEEE CSMA/CA III Sending unicas frames Frames can be sen afer (plus backoff ime as described before) Receiver responds immediaely (afer ), if frame has been correcly received (CRC) If an error occurs, he frame is auomaically repeaed Sender Receiver Furher Saions Daa Ack waiing ime compeiion Daa Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 9

10 RTS / CTS Sending unicas frames using RTS/CTS Before ransmiing a daa frame, an RTS frame has o be sen including he duraion of he daa frame (afer ) Receiver acknowledges RTS frame wih a CTS frame (afer ) Sender may hen send he daa frame afer, which is acknowledged as usual Oher saions sore he ime he medium is busy (as conained in he RTS and CTS frames) Sender Receiver RTS CTS daa ACK Furher Saions NAV = Nework Allocaion Vecor NAV (RTS) NAV (CTS) waiing ime compeiion daa Communicaion Neworks: 10. IEEE RTS / CTS: Fragmenaion Sender Receiver Furher Saions RTS CTS frag 1 NAV (RTS) NAV (CTS) ACK 1 frag 2 NAV (frag 1 ) NAV (ACK 1 ) ACK2 compeiion daa Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 10

11 DFWMAC-PCF I 0 1 Super Frame Medium busy Poin Coordinaor Saions PIFS D 1 U 1 D 2 U 2 NAV of he saions NAV Poin Coordinaion Funcion Communicaion Neworks: 10. IEEE DFWMAC-PCF II Poin Coordinaor D 3 PIFS D 4 CF end Saions U 4 NAV of he saions NAV period wihou compeiion compeiion Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 11

12 MAC Address Forma Frame Type o DS from DS Address 1 Address 2 Address 3 Address 4 Ad-hoc Nework 0 0 DA SA BSSID - Infrasrucure Nework, from AP 0 1 DA BSSID SA - Infrasrucure Nework, o AP 1 0 BSSID SA DA - Infrasrucure Nework, in DS 1 1 RA TA DA SA DS AP DA SA BSSID RA TA : Disribuion Sysem : Access Poin : Desinaion Address : Source Address : Basic Service Se Idenifier : Receiver Address : Transmier Address Communicaion Neworks: 10. IEEE MAC Managemen MAC Managemen Synchronizaion Finding and saying in a WLAN Timer ec. Power Managemen Sleep modus wihou loosing a frame Periodically sleeping, buffering of frames, raffic map Associaion / Reassociaion Associaing wih a disribuion sysem Roaming, i.e. changing neworks when changing access poins Scanning, i.e. acively looking for a WLAN MIB - Managemen Informaion Base Adminisering, reading, wriing Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 12

13 MAC Managemen MAC Synchronizaion in Infrasrucure Mode beacon inerval (20ms 1s) access poin medium B B B B busy busy busy busy value of he imesamp B beacon frame Communicaion Neworks: 10. IEEE MAC Managemen MAC Synchronizaion in Ad hoc Mode beacon inerval saion 1 B 1 B 1 saion 2 B 2 B 2 medium busy busy busy busy value of he imesamp B beacon frame random delay Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 13

14 MAC Managemen Power Saving Idea swich he ransceiver off if no needed Saes of a saion sleep awake Timing Synchronizaion Funcion (TSF) saions wake up a he same ime Infrasrucure Traffic Indicaion Map (TIM) lis of unicas receivers ransmied by AP Delivery Traffic Indicaion Map (DTIM) lis of broadcas/mulicas receivers ransmied by AP Ad-hoc Ad-hoc Traffic Indicaion Map (ATIM) announcemen of receivers by saions buffering frames more complicaed - no cenral AP collision of ATIMs possible (scalabiliy?) APSD (Auomaic Power Save Delivery) new mehod in e replacing above scheme Communicaion Neworks: 10. IEEE MAC Managemen Power Saving in Infrasrucure Mode TIM inerval DTIM inerval access poin medium D B busy T T d D busy busy busy B saion p d T TIM D DTIM awake B broadcas/mulicas p PS poll d daa ransmission o/from he saion Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 14

15 MAC Managemen Power Saving in Ad hoc Mode ATIM window beacon inerval B saion 1 A D B 1 1 saion 2 B 2 B 2 a d B beacon frame random delay A ransmi ATIM D ransmi daa awake a acknowledge ATIM d acknowledge daa Communicaion Neworks: 10. IEEE MAC Managemen Roaming No or bad connecion? Then perform: Scanning scan he environmen, i.e., lisen ino he medium for beacon signals or send probes ino he medium and wai for an answer Reassociaion Reques saion sends a reques o one or several AP(s) Reassociaion Response success: AP has answered, saion can now paricipae failure: coninue scanning AP acceps Reassociaion Reques signal he new saion o he disribuion sysem he disribuion sysem updaes is daa base (i.e., locaion informaion) ypically, he disribuion sysem now informs he old AP so i can release resources Fas roaming r e.g. for vehicle-o-roadside neworks Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 15

16 in Ad-hoc Mode IEEE s IEEE amendmen for mesh neworking, since 2012 incorporaed in sandard Broadcas/mulicas and unicas delivery using radio-aware merics over selfconfiguring muli-hop opologies Defaul mandaory rouing proocol Hybrid Wireless Mesh Proocol (HWMP), inspired by a combinaion of AODV (RFC 3561) and ree-based rouing, based on MAC addresses Peer auhenicaion mehods defined for securiy One Lapop per Child projec (lapop.org) uses he s draf sandard for is OLPC XO lapop and OLPC XS school server neworking Communicaion Neworks: 10. IEEE in Ad-hoc Mode Ad-hoc Neworks and QoS (I) Guaranee of QoS even when opology keeps changing all he ime! Normal procedure: Find roue wih enough resources Reserve he required resources Keep on conrolling he achieved QoS For ad-hoc neworks: Limied range and energy Resriced availabiliy of channels / bi rae (shared medium) Unforeseeable radio problems Verical and horizonal handover Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 16

17 in Ad-hoc Mode Ad hoc Neworks and QoS (II) Node canno supply he required QoS Communicaion Neworks: 10. IEEE in Ad-hoc Mode Ad hoc Neworks and QoS (III) Trying o guaranee QoS hrough redundancy If QoS canno be supplied: bes effor ransmission or communicaion breakdown Parallel Roues Simulaneous ransmission Backup roues esablished and reserved Backup roues seleced Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 17

18 in Ad-hoc Mode Ineroperabiliy wih Oher Neworks One of he nodes in he ad-hoc nework allows access o some oher nework Problem: differen nework characerisics Addresses Capaciy / QoS Rouing and Signaling Example: WLAN-based ad hoc nework Ad-hoc Nework LTE Cell Inerne Communicaion Neworks: 10. IEEE in Ad-hoc Mode Mulimedia Transmission in an Ad hoc Nework Idea: Transmiing a video sream in an ad hoc nework based on AODV Problem: Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 18

19 in Ad-hoc Mode Summary on Ad hoc Neworks Currenly many research projecs in his area Sill no very well acceped Securiy? Benefis? Sandards? Well suied o enhance an infrasrucure nework Perfec for communicaion in underdeveloped areas Communicaion Neworks: 10. IEEE References References Gas, Mahew S. (2017): Wireless Neworks. The Definiive Guide. Sebasopol, CA: O'Reilly Media. Olenewa, Jorge L. (2017): Guide o Wireless Communicaions. Fourh ediion. Ausralia: Cengage Learning. Perahia, Eldad; Sacey, Rober (2013): Nex Generaion Wireless LANs n, ac, and Wi-Fi direc. 2nd ediion. Cambridge: Cambridge Universiy Press. Schiller, Jochen H. (2009): Mobile Communicaions. Unied Kingdom: Pearson Educaion Limied. Slingerland, Jane (2018): Wi-Fi. How I Works. Lake Elmo, MN: Focus Readers. Communicaion Neworks: 10. IEEE Prof. Jochen Seiz 19