Introduc)on to Wireless Networks. Dino Lopez

Transcription

1 Introduc)on to Wireless Networks Dino Lopez

2 2

3 The ISM frequency band hdp:// 3

4 Wireless standards coverage range classifica)on TAGGING Ø RFID WPAN Ø IEEE IEEE Bluetooth IEEE UWB (Ultra Wide Band) IEEE ZigBee WLAN Ø IEEE (Wi-Fi) Ø IEEE b, a, g Ø IEEE n Ø IEEE s WMAN Ø IEEE Ø IEEE Ø IEEE e/IEEE (Wi-Mobile) WRAN Ø IEEE U)lisa)on des bandes TV (Wi-RAN) 4

5 Wireless Spectrum Sharing Bandwidth (Mbps) 100 4G Bluetooth 2.0 Bluetooth BLE 3G 2.5G (GPRS) 2G (GSM) Mobility (Km) 5

6 Wireless Local Area Network The Physical Layers 6

7 About the IEEE model IEEE is the world s largest professional associa)on dedicated to advancing technological innova)on in electricity and electronic 802 Project Ø Project began in February 1980 Ø Standards for LAN and MAN (network with variable-size packets) Ø Define the physical and Link Layer of the OSI model developed by the ISO (Interna)onal Organiza)on Standardiza)on) Ø Ethernet Ø Wi-Fi 7

8 Wi-Fi IEEE Completely compa)ble with IP networks Ø Only the two lowest layers are modified. i.e. the Data Link and the Physical Layers 2 working modes Ø Ad hoc mode. Mobile nodes (MN) communicates with neighbors (MNs also) in the coverage area Ø Infrastructure mode. Nodes communicates with a non mobile node (the Access Point AP) 3 physical layers Ø Direct Sequence Spread Spectrum - DSSS Ø Frequency Hopping Spread Spectrum FHDD Ø Infrared 8

9 Wi-Fi The Infrastructure Mode 9

10 Wi-Fi The Ad hoc mode 10

11 IEEE The DSSS Physical Layer DSSS Direct Sequence Spread Spectrum Ø The 2.4GHz band is divided in 14 channels (13 channels in France and 11 in US), each one with a 22MHz bandwidth Ø There are 3 non overlapping channels 11

12 IEEE The FHSS Physical Layer FHSS Frequency Hopping Spread Spectrum 75 channels of 1MHz each Switching channel every 400ms Rate = 2Mbps 12

13 Medium Access Control 13

14 Objec)ves of the MAC protocols Shared medium Ø Ideally, number of channels > number of users One channel per user Ø Usually, number of channels << number of users Only one channel Ø Large bandwidth, high conten)on Several sub channels Ø Lower bandwidth, less conten)on Control the access to the channel Ø Who can talk? Ø When? Ø Which channel? Ø How to detect collisions? Ø What should be done in case of collisions? 14

15 The case of Ethernet Ethernet (802.3) uses the CSMA/CD protocol Ø Carrier Sense Mul)ple Access with Collision Detec)on How to avoid collisions? Ø Listen the medium before sending a frame Channel is busy? Wait Ø Listen the medium while sending a frame React in case of collision What to do in case of collision? Ø When a host detects a collision, all others computers on the LAN should detect it Length of a frame must be equal or higher than a TimeSlot (TS) Add a padding if needed A TimeSlot is defined to be twice of the )me needed by a signal to travel between the furthest two nodes of the network In Ethernet, the TS = 51.2microsecs Ø Execute the binary exponen)al backoff (BEB) Algorithm Resend the frame amer k*ts, with k = rand(0,2 c 1) and c = # of retry 15

16 Why should frame length > TS? Frame length < TS Frame length > TS 16

17 Some numbers If TS = 51.2 microsecs, what is the minimum frame length at 10Mbps? Ø 51.2 * 10^7 / 10^6 = 512 bits If the speed of electricity in a copper cable is 200x10^6 m/s, what is the maximum theore)cal length of a 10 Mbps network? Ø Time to send 512 bits = 512 / 10x10^6 Ø Maximum distance amer 512 transmided bits = speed of signal * )me to send 512 bits = 200x10^6 * 512 / 10x10^6 Ø Maximum network length = maximum distance amer 512 bits / 2 = 5120 m 17

18 The MAC Layer In wired networks, one sta)on can send and listen the medium at the same )me. This is not possible in Wireless networks CSMA/CA: Carrier Sense Mul)ple Access / Collision Avoidance Ø CSMA/CA is a conten)on-based protocol making certain that all sta)ons first sense the medium before transmiqng Ø CSMA/CA is the equivalent of CSMA/CD for wired networks U)liza)on of ACKs 18

19 CSMA/CA Conten)on Window at try i sensing period before sending a frame Ø CW i = min(cwmax, CWmin*2 i-1 ) Ø CWmin = 15 Ø CWmax = 1023 Back-off Ø T = rand(0,cw)*ts TS depends on the Physical Layer FHSS = 50microsecs DSSS = 20microsecs OFDM = 9microsecs 19

20 Distributed Coordina)on Func)on From Balador et al. An Adap)ve Conten)on Window Control for Improving DCF Throughput and Fairness. In European Journal of Scien)fic Research 45(2): September 2010 SIFS = Short Inter-Frame Spacing DIFS = Distributed IFS = SIFS + 2TS RTS = Ready-To-Send CTS = Clear-To-Send 20

21 DCF (cont) From hdp:// Mobility/emob41dg/emob41dg-wrapper/ch5_QoS.html 21

22 Maximum Throughput and Minimum Delay T CYCLE = T DIFS + T BO + T D_DATA + 2τ + T SIFS + T D_ACK T D_DATA = T P + T PHY + T H_DATA + T DATA T D_ACK = T P + T PHY + T ACK Maximum Throughput, MT = L DATA / T CYCLE Ø T BYTE = 8 / Rate Throughput Upper Limit Ø TUL = L DATA / (2T P + 2T PHY + 2τ + T DIFS + T SIFS + (CWmin * Tslot / 2)) Delay Lower Limit Ø DLL = T P + T PHY + τ + T DIFS + (CWmin * Tslot/2) 22

23 Throughput Upper Limit of a From Xiao, Yang and Rosdahl, Jon. "Throughput and delay limits of IEEE " IEEE Communica,ons Le0ers 6, no. 8 (2002):

24 Performance Anomaly in Sta)ons might nego)ate different rates with the AP Performance Anomaly: whatever the nego)ated physical rate is, nodes will obtain the same throughput of the slowest node if they use equal frame sizes Throughput seen by the ith node Th i = DATA L i N CYCLE T i=1 i Adding the conten)on )me Th i = N i=1 L i DATA T CYCLE i + P c (N) t jam N 24

25 The Hidden Node and the Exposed Node Problems 25