Topic 2b Wireless MAC Chapter 7 Wireless and Mobile Networks - PowerPoint PPT Presentation

Topic 2b Wireless MAC Chapter 7 Wireless and Mobile Networks Computer Networking: A Top Down Approach 7 th edition Jim Kurose, Keith Ross Pearson/Addison Wesley April 2016 Wireless and Mobile Networks 7-1

Ch. 7: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)!  # wireless Internet-connected devices equals # wireline Internet-connected devices • laptops, Internet-enabled phones promise anytime untethered Internet access  two important (but different) challenges • wireless: communication over wireless link • mobility: handling the mobile user who changes point of attachment to network Wireless and Mobile Networks 7-2

Chapter 7 outline 7.1 Introduction Mobility 7.5 Principles: addressing and Wireless routing to mobile users 7.2 Wireless links, 7.6 Mobile IP characteristics 7.7 Handling mobility in • CDMA cellular networks 6.73 IEEE 802.11 wireless 7.8 Mobility and higher-layer LANs ( “ Wi-Fi ” ) protocols 67.4 Cellular Internet Access • architecture • standards (e.g., 3G, LTE) Wireless and Mobile Networks 7-3

Elements of a wireless network network infrastructure Wireless and Mobile Networks 7-4

Elements of a wireless network wireless hosts  laptop, smartphone  run applications  may be stationary (non- mobile) or mobile network • wireless does not always infrastructure mean mobility Wireless and Mobile Networks 7-5

Elements of a wireless network base station  typically connected to wired network  relay - responsible for sending packets between wired network and network wireless host(s) in its infrastructure “ area ” • e.g., cell towers, 802.11 access points Wireless and Mobile Networks 7-6

Elements of a wireless network wireless link  typically used to connect mobile(s) to base station  also used as backbone link  multiple access protocol coordinates link access network  various data rates, infrastructure transmission distance Wireless and Mobile Networks 7-7

Characteristics of selected wireless links 1300 802.11 ac 450 802.11n 54 802.11a,g 802.11a,g point-to-point Data rate (Mbps) 5-11 802.11b 4G: LTWE WIMAX 4 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 1 802.15 .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor Outdoor Mid-range Long-range 10-30m 50-200m outdoor outdoor 200m – 4 Km 5Km – 20 Km Wireless and Mobile Networks 7-8

Elements of a wireless network infrastructure mode  base station connects mobiles into wired network  handoff: mobile changes base station providing network connection into wired infrastructure network Wireless and Mobile Networks 7-9

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 Wireless and Mobile Networks 7-10

Wireless network taxonomy multiple hops single hop host may have to host connects to relay through several infrastructure base station (WiFi, wireless nodes to WiMAX, cellular) (e.g., APs) connect to larger which connects to Internet: mesh net larger Internet no base station, no connection to larger no no base station, no Internet. May have to connection to larger infrastructure relay to reach other Internet (Bluetooth, a given wireless node ad hoc nets) MANET, VANET Wireless and Mobile Networks 7-11

Chapter 7 outline 7.1 Introduction Mobility 7.5 Principles: addressing and Wireless routing to mobile users 7.2 Wireless links, 7.6 Mobile IP characteristics 7.7 Handling mobility in • CDMA cellular networks 7.3 IEEE 802.11 wireless 7.8 Mobility and higher-layer LANs ( “ Wi-Fi ” ) protocols 7.4 Cellular Internet Access • architecture • standards (e.g., 3G, LTE) Wireless and Mobile Networks 7-12

IEEE 802.11 Wireless LAN 802.11a 802.11b  5-6 GHz range  2.4-5 GHz unlicensed spectrum  up to 54 Mbps  up to 11 Mbps 802.11g  direct sequence spread spectrum  2.4-5 GHz range (DSSS) in physical layer  up to 54 Mbps • all hosts use same chipping 802.11n: multiple antennae code  2.4-5 GHz range  up to 200 Mbps  all use CSMA/CA for multiple access  all have base-station and ad-hoc network versions Wireless and Mobile Networks 7-13

802.11 LAN architecture  wireless host communicates with base station Internet • base station = access point (AP )  Basic Service Set (BSS) (aka hub, switch “ cell ” ) in infrastructure or router mode contains: • wireless hosts • access point (AP): base BSS 1 station • ad hoc mode: hosts only BSS 2 Wireless and Mobile Networks 7-14

802.11: Channels, association  802.11b: 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!  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 [Chapter 8] • will typically run DHCP to get IP address in AP ’ s subnet Wireless and Mobile Networks 7-15

802.11: passive/active scanning BBS 1 BBS 1 BBS 2 BBS 2 1 AP 2 1 1 2 2 AP 2 AP 1 AP 1 3 2 4 3 H1 H1 active scanning : passive scanning: (1) beacon frames sent from APs (1) Probe Request frame broadcast (2) association Request frame sent: H1 to from H1 selected AP (2) Probe Response frames sent (3) association Response frame sent from from APs selected AP to H1 (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent from selected AP to H1 Wireless and Mobile Networks 7-16

IEEE 802.11: multiple access  avoid collisions: 2 + nodes transmitting at same time  802.11: CSMA - sense before transmitting • don ’ t collide with ongoing transmission by other node  802.11: no collision detection! • 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) B A C C C ’ s signal A ’ s signal B strength strength A space Wireless and Mobile Networks 7-17

IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then sender receiver 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 data if no ACK, increase random backoff interval, repeat 2 802.11 receiver SIFS - if frame received OK ACK return ACK after SIFS (ACK needed due to hidden terminal problem) Wireless and Mobile Networks 7-18

Avoiding collisions (more) 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 • RTSs may still collide with each other (but they ’ re short)  BS broadcasts clear-to-send CTS in response to RTS  CTS heard by all nodes • sender transmits data frame • other stations defer transmissions avoid data frame collisions completely using small reservation packets! Wireless and Mobile Networks 7-19

Collision Avoidance: RTS-CTS exchange A B AP reservation collision DATA (A) defer time Wireless and Mobile Networks 7-20

802.11 frame: addressing 6 4 2 2 6 6 6 2 0 - 2312 frame duration address address address address seq payload CRC control 1 2 3 4 control Address 4: used only in ad hoc mode Address 1: MAC address of wireless host or AP Address 3: MAC address to receive this frame of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame Wireless and Mobile Networks 7-21

802.11 frame: addressing Internet router H1 R1 R1 MAC addr H1 MAC addr source address dest. address 802. 3 frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 1 address 2 802. 11 frame Wireless and Mobile Networks 7-22

802.11 frame: more frame seq # duration of reserved (for RDT) transmission time (RTS/CTS) 6 4 2 2 6 6 6 2 0 - 2312 frame duration address address address address seq payload CRC control 1 2 3 4 control 2 2 4 1 1 1 1 1 1 1 1 Protocol To From More Power More Type Subtype Retry WEP Rsvd version AP AP frag mgt data frame type (RTS, CTS, ACK, data) Wireless and Mobile Networks 7-23

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 (Ch. 5): switch will see frame from H1 and “ remember ” which switch port can be H1 BBS 2 used to reach H1 BBS 1 Wireless and Mobile Networks 7-24

802.11: advanced capabilities Rate adaptation 10 -1  base station, mobile 10 -2 dynamically change 10 -3 BER transmission rate 10 -4 (physical layer modulation 10 -5 technique) as mobile 10 -6 moves, SNR varies 10 -7 10 20 30 40 SNR(dB) 1. SNR decreases, BER increase as node moves away from base station QAM256 (8 Mbps) QAM16 (4 Mbps) 2. When BER becomes too BPSK (1 Mbps) operating point high, switch to lower transmission rate but with lower BER Wireless and Mobile Networks 7-25