COMP 3331/9331: Computer Networks and Applications

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1 COMP 3331/9331: Computer Networks and Applications Week 10 Wireless Networks Reading Guide: Chapter 6: Wireless Networks + Security 1 Wireless and Mobile Networks Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer nets: laptops, palmtops, PDAs, Internetenabled phone promise anytime untethered Internet access two important (but different) challenges wireless: communication over wireless link mobility: handling mobile user who changes point of attachment to network Wireless Networks + Security 2 Wireless Networks: Roadmap 6.1 Introduction 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( wi-fi ) REST EXCLUDED Wireless Networks + Security 3 1

2 Elements of a wireless network network infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (nonmobile) or mobile wireless does not always mean mobility Wireless Networks + Security 4 Elements of a wireless network network infrastructure base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers access points Wireless Networks + Security 5 Elements of a wireless network network infrastructure wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance Wireless Networks + Security 6 2

3 Characteristics of selected wireless link standards Data rate (Mbps) n a,g b a,g point-to-point (WiMAX) UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO UMTS/WCDMA, CDMA2000 IS-95, CDMA, GSM 3G 2G data 3G cellular enhanced Indoor 10-30m Outdoor m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km Wireless Networks + Security 7 News: White Spaces TV Spectrum (700Mhz range) is freeing up as we move from analog to digital transmissions White Space: Unused radio spectrum space Idea: Why not use this idle spectrum for wireless communication? This idea can be extended to other frequencies as well Secondary users can transmit over unused spectrum until primary users become active Secondary users should be able to sense the presence of primary users US FCC approved the use of the 700MHz TV spectrum Working prototype developed by Microsoft Wireless Networks + Security 8 Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network Wireless Networks + Security 9 3

4 Elements of a wireless network Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves E.g: wireless sensor networks, vehicular ad-hoc networks Wireless Networks + Security 10 Wireless network taxonomy infrastructure (e.g., APs) no infrastructure single hop host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) multiple hops host may have to relay through several wireless nodes to connect to larger Internet: mesh networks no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET Wireless Networks + Security 11 Wireless Networks: Roadmap 6.1 Introduction 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( wi-fi ) Wireless Networks + Security 12 4

5 Wireless Link Characteristics (1) Differences from wired link. decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving at the destination at slightly different times. make communication across (even a point to point) wireless link much more difficult Wireless Networks + Security 13 Wireless Link Characteristics (2) SNR: signal-to-noise ratio larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs given physical layer: increase power -> increase SNR->decrease BER (bit error rate) given SNR: choose physical layer that meets BER requirement, giving highest throughput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) BER SNR(dB) QAM256 (8 Mbps) SNR (db) = 20 log (amp of signal/amp of noise) QAM16 (4 Mbps) BPSK (1 Mbps) Wireless Networks + Security 14 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal strength C s signal strength Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B space Signal fading: B, A hear each other B, C hear each other A, C can not hear each other interfering at B Wireless Networks + Security 15 5

6 Recall: Multiple Access Protocols Random Access Taking Turns Channel Partitioning TDMA FDMA Now we study CDMA Wireless Networks + Security 16 Code Division Multiple Access (CDMA) used in several wireless broadcast channels (cellular, satellite, etc) standards unique code assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) Wireless Networks + Security 17 CDMA: Encoding and Decoding Assume original data are represented by 1 and -1 Encoded signal = (original data) modulated by (chipping sequence) assume c m = if data is 1, send if data is -1 send Decoding: inner-product (summation of bit-by-bit product) of encoded signal and chipping sequence if inner-product > threshold, the data is 1; else -1 Wireless Networks + Security 18 6

7 CDMA Encode/Decode Code of user m c m : sender data bits code - The number of bits of each chipping sequence is M - The i th data bit is d i ; write it down by duplicating it M times receiver d 1 = d 0 = slot 1 slot 0 received input code slot 1 slot 0 Z i,m = d i. cm channel output Z i,m slot 1 channel output M D i = Σ Z i,m. c m m=1 M slot 0 channel output d 1 = -1 slot 1 channel output d 0 = 1 slot 0 channel output Wireless Networks + Security 19 CDMA: two-sender interference Sender 1 Code 1: Code 2: Sender 2 Wireless Networks + Security 20 CDMA codes CDMA codes are orthogonal. E.g: (1,1,1,-1,1,-1,-1,-1) and (1,-1,1,1,1,-1,1,1) Inner product of the codes should be zero C 1 : C 2 : C 1. C 2 = 1 +(-1) (-1) (-1)+(-1)=0 If there are multiple CDMA codes all of the codes have to be orthogonal to each other. E.g: 3 codes: A, B and C. Then A x B = 0, B x C = 0 and A x C = 0 Wireless Networks + Security 21 7

8 Roadmap 6.1 Introduction 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( wi-fi ) Wireless Networks + Security 22 IEEE Wireless LAN b GHz unlicensed spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps n: multiple antennae GHz range up to 200 Mbps all use CSMA/CA for multiple access all have base-station and ad-hoc network versions Standard ratified in Sept 2009 Wireless Networks + Security LAN architecture BSS 1 AP Internet switch or router AP wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only BSS 2 Wireless Networks + Security 24 8

9 802.11: Channels, association b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! 3 non-overlapping channels in b: 1, 6 and 11 host: must associate with an AP scans channels, listening for beacon frames containing AP s name (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP s subnet Wireless Networks + Security b channels Wireless Networks + Security : passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 AP AP 2 AP AP 2 H1 H1 Passive Scanning: (1) beacon frames sent from APs (host scans for beacons on 11 channels) (2) association Request frame sent: H1 to selected AP (3) association Response frame sent: Selected AP to H1 Active Scanning: (1) Probe Request frame broadcast from H1 (2) Probes response frame sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent: Selected AP to H1 Wireless Networks + Security 27 9

10 IEEE : multiple access avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : no collision detection! Why? difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C A B A s signal strength C s signal strength space Wireless Networks + Security 28 IEEE MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then sender transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) data ACK receiver SIFS Wireless Networks + Security 29 CSMA/CA vs CSMA/CD In if sender senses channel to be busy, the sender backs off for a random time (based on exponential back-off) In Ethernet, if the sender senses channel to be busy, the sender keeps sensing the channel until it becomes free Why this difference? Wireless Networks + Security 30 10

11 Avoiding collisions & dealing with hidden nodes idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA (OPTIONAL used for Longer data frames) RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS after waiting for SIFS duration RTS/CTS heard by all nodes sender transmits data frame after waiting for SIFS duration other stations defer transmissions by setting Network Allocation Vector (NAV) Avoid data frame collisions completely using small reservation packets! Wireless Networks + Security 31 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) reservation collision RTS(B) RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) Wireless Networks + Security 32 RTS/CTS Illustration sender receiver DIFS RTS SIFS CTS SIFS data other stations NAV (RTS) NAV (CTS) defer access DIFS new contention data t RTS A RTS CTS AP CTS B Wireless Networks + Security 33 11

12 frame: addressing frame address address address address duration seq payload CRC control control 4 Address 1: MAC address of wireless host or AP to receive this frame Address 2: MAC address of wireless host or AP transmitting this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode Wireless Networks + Security frame: addressing H1 R1 router Internet AP R1 MAC addr AP MAC addr dest. address source address (Ethernet) frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame Wireless Networks + Security frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) frame address address address address duration seq payload CRC control control 4 Protocol version Type Subtype To AP From AP More frag 1 1 Power Retry mgt More data WEP Rsvd frame type (RTS, CTS, ACK, data) Wireless Networks + Security 36 12

13 802.11: mobility within same subnet H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (link-layer): switch will see frame from H1 and remember which switch port can be used to reach H1 BBS 1 router switch AP 1 AP 2 H1 BBS 2 Wireless Networks + Security : advanced capabilities Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point BER SNR(dB) 1. SNR decreases, BER increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER Wireless Networks + Security : personal area network less than 10 m diameter replacement for cables (mouse, keyboard, headphones) ad hoc: no infrastructure master/slaves: slaves request permission to send (to master) master grants requests : evolved from Bluetooth specification GHz radio band up to 4Mbps Frequency hopping spread spectrum (FHSS) S S P M P S P M Master device S Slave device radius of coverage P Parked device (inactive) P Wireless Networks + Security 39 13

14 802.16: WiMAX (SELF-STUDY) like & cellular: base station model transmissions to/from base station by hosts with omnidirectional antenna base station-to-base station backhaul with point-topoint antenna unlike : range ~ 6 miles ( city rather than coffee shop ) ~14 Mbps Long Term Evolution (LTE) - emerging point-to-point point-to-multipoint Wireless Networks + Security : WiMAX: downlink, uplink scheduling transmission frame down-link sub-frame: base station to node uplink sub-frame: node to base station pream. DL- UL- MAP MAP DL burst 1 DL burst 2 downlink subframe DL burst n Initial request SS #1 SS #2 SS #k maint. conn. uplink subframe base station tells nodes who will get to receive (DL map) and who will get to send (UL map), and when WiMAX standard provide mechanism for scheduling, but not scheduling algorithm Wireless Networks + Security 41 Quiz The protocol does not implement collision detection because Collision detection was found to be useless in wired Ethernet The adapter would not be able to detect all collisions due to hidden terminal Collisions are not possible in In a host can re-associate with a new AP in the subnet without changing its IP address True or False Wireless Networks + Security 42 14

15 Wireless, mobility: impact on higher layer protocols logically, impact should be minimal best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile but performance-wise: packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff TCP interprets loss as congestion, will decrease congestion window un-necessarily delay impairments for real-time traffic limited bandwidth of wireless links Wireless Networks + Security 43 Summary Wireless Networks + Security 44 15

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