1. Wireless, Mobile Networks6-1 Chapter 6: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!  # wireless Internet-connected devices soon to exceed # 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

2. Wireless, Mobile Networks6-2 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics  CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4 Cellular Internet Access  architecture  standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher- layer protocols 6.9 Summary

3. Wireless, Mobile Networks6-3 Elements of a wireless network network infrastructure wireless hosts  laptop, PDA, IP phone  run applications  may be stationary (non-mobile) or mobile  wireless does not always mean mobility

4. Wireless, Mobile Networks6-4 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, 802.11 access points

5. Wireless, Mobile Networks6-5 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

6. Wireless, Mobile Networks6-6 Characteristics of selected wireless link standards Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km.056.384 1 4 5-11 54 IS-95, CDMA, GSM 2G UMTS/WCDMA, CDMA2000 3G 802.15 802.11b 802.11a,g UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular enhanced 802.16 (WiMAX) 802.11a,g point-to-point 200 802.11n Data rate (Mbps) data

7. Wireless, Mobile Networks6-7 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

8. Wireless, Mobile Networks6-8 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

9. Wireless, Mobile Networks6-9 Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure 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) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET

10. Wireless, Mobile Networks6-10 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 ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

11. Wireless, Mobile Networks6-11 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  given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 10 20 30 40 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4

12. Wireless, Mobile Networks6-12 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C 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 A B C A’s signal strength space C’s signal strength Signal attenuation:  B, A hear each other  B, C hear each other  A, C can not hear each other interfering at B

13. Wireless, Mobile Networks6-13 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”)

14. Wireless, Mobile Networks6-14 CDMA Encode/Decode slot 1 slot 0 d 1 = -1 111 1 1 - 1 - 1 -1 - Z i,m = d i. c m d 0 = 1 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = 1 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

15. Wireless, Mobile Networks6-15 CDMA: two-sender interference

16. Wireless, Mobile Networks6-16 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics  CDMA 6.3 IEEE 802.11 wireless LANs (“Wi-Fi”) 6.4 Cellular Internet Access  architecture  standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher- layer protocols 6.9 Summary

17. Wireless, Mobile Networks6-17 IEEE 802.11 Wireless LAN  802.11b  2.4-5 GHz unlicensed spectrum  up to 11 Mbps  direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code  802.11a  5-6 GHz range  up to 54 Mbps  802.11g  2.4-5 GHz range  up to 54 Mbps  802.11n: multiple antennae  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

18. Wireless, Mobile Networks6-18 802.11 LAN architecture  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 1 BSS 2 Internet hub, switch or router AP

19. Wireless, Mobile Networks6-19 802.11: 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

20. Wireless, Mobile Networks6-20 802.11: passive/active scanning AP 2 AP 1 H1 BBS 2 BBS 1 1 2 2 3 4 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: H1 to selected AP AP 2 AP 1 H1 BBS 2 BBS 1 1 2 3 1 Passive Scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent: H1 to selected AP

21. Wireless, Mobile Networks6-21 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) A B C A B C A’s signal strength space C’s signal strength

22. CSMA/CA  CSMA/CD does not work in wireless channel  difficult to detect collisions in a radio environment Power level varies due to other factors such as fading  difficult to control the wireless channel  hidden and exposed terminal problems  CSMA/CA is used in wireless environments  Sense before transmit  Contend for channel usage  “Collision detection” via the acknowledgment  Time-spacing to set priority  Avoid collisions via carrier sensing and virtual carrier sensing  IEEE 802.11 family Wi-Fi technologies (802.11b)

23. CSMA/CA  Carrier sensing mechanism  Physical layer carrier sensing: physical layer carries out the medium sensing (power sensing)  Virtual carrier sensing: each node monitors the Duration field in all MAC frames and place this information in the station ’ s Network Allocation Vector (NAV) if the value is greater than the current NAV value. The NAV acts like a timer to indicate that the medium is busy with transmissions from other nodes  Two operating modes  PCF mode: Point Coordination Function mode, access point (AP) coordinates the transmission  DCF mode: Distributed Coordination Function mode, also known as peer-to-peer mode, a contention-based operational mode

24. CSMA/CA  Time-spacing  IFS: inter-frame space  SIFS: short IFS – providing highest priority level to access channel, e.g., for ACK, CTS, the 2 nd or subsequent MSDU of a fragment burst  PIFS: PCF IFS, allowing PCF mode to take over  DIFS: DCF IFS, allowing DCF mode to operate

25. Wireless, Mobile Networks6-25 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

26. CSMA/CA  DCF mode operation

27. MAC  Sense before transmit: Choose a random number over the interval [0, CW] Backoff Time = Random() × aSlotTime  If the medium is idle for a backoff slot, the backoff time is decremented by aSlotTime  If the medium is determined to be busy during a backoff slot, the backoff procedure is suspended until the medium is determined to be idle for DIFS period  Whenever the Backoff Timer reaches zero, a packet transmission begins  Whenever the transmission is not successful (due to channel conditions or collisions via acknowledgment or timeout), the contention window size doubles and backoff timer is regenerated

28. Binary Exponential Backoff

29.  Algorithm for delayed transmission: random period of time for all contention- based MAC  Binary exponential backoff algorithm:  Wnenver a transmission failure occurs, set current contention window size CW=2 CW (starting with CW=CWmin), pick random number from [0, CW], say, j, then set backoff time=j* aSlotTime

30. Wireless, Mobile Networks6-30 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!

31. Wireless, Mobile Networks6-31 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

32. CSMA/CA with RTS/CTS  RTS-CTS handshaking protocol  RTS: Request To Send  CTS: Clear To Send  Operations  sender sends RTS to receiver, any node hearing RTS will not transmit  receiver, upon the correct reception of RTS, will send CTS to the sender, any node hearing the CTS will refrain from transmitting  regular data exchange starts after successful RTS-CTS exchange

33. Distributed Foundation Wireless MAC (DFWMAC)  IEEE 802.11 wireless LAN standard  RTS-CTS-DATA-ACK four-way handshake protocol (an option for 802.11 family)  Operations  sender sensing the channel idle will wait for DIFS and then transmit RTS  receiver, upon receiving the RTS correctly, will wait for SIFS and then reply with CTS  sender, upon receiving CTS correctly, will wait for SIFS and transmit data  receiver will send ACK SIFS later after correct data  time spacing (DIFS>SIFS) provides priority to certain message  backoff algorithm will be used if collision

34. IEEE 802.11 MAC Protocol (Distributed)  CSMA/CA  Carrier sensing Physical Carrier Sensing Virtual Carrier Sensing  Interframe Spacing (IFS) Short IFS (SIFS) < DCF IFS (DIFS) DIFS RTS Backoff CTS SIFS DATA SIFS ACK NAV(RTS) NAV(CTS) DIFS RTS … Backoff –Binary Exponential Backoff Randomly chosen from [0, CW] CW doubles in case of collision Transmitter Receiver Others B A ACK … Request to send Clear to send Acknowledgement DATA

35. Distributed Foundation Wireless MAC (DFWMAC)  Collision resolution: backoff algorithm  IEEE 802.11: binary exponential backoff Any node involving in collisions (when the node does not receive the desired CTS for the transmitted RTS) will double the contention window size up to its maximum: i.e., the node picks a random number between 0 and double of the previous window size for the next attempt  Fast collision resolution (FCR) algorithm Upon collision, all nodes with data ready to transmit will expand their contention window size: i.e., even the already deferred nodes will act again to actively avoid future collisions

36. Hidden Terminal Problem  A hidden terminal is the one within the sensing range of the receiver, but not in the sensing range of the transmitter.  The hidden terminal does not know the transmitter is transmitting, hence may transmit to some node, resulting in a collision at the receiver.  Remark: hidden terminal cannot be a receiver either!  Remark: various range definitions are still challenging problem C: the hidden terminal of A Small circle: transmission range Large circle: Sensing/interference range

37. Exposed Terminal Problem  An exposed terminal is the one within the sensing range of the transmitter but not that of the receiver.  The exposed node senses the medium busy and cannot transmit when the transmitter transmits, leading to bandwidth under-utilization C: the exposed terminal Small circle: transmission range Large circle: Sense/interference range

38. Wireless, Mobile Networks6-38 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control 802.11 frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode

39. Wireless, Mobile Networks6-39 Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame R1 MAC addr H1 MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

40. Wireless, Mobile Networks6-40 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version 2 2411111111 802.11 frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for RDT) frame type (RTS, CTS, ACK, data)

41. Wireless, Mobile Networks6-41 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router  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 used to reach H1

42. Wireless, Mobile Networks6-42 802.11: 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) 1020 30 40 SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4 operating point 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

43. Wireless, Mobile Networks6-43 802.11: advanced capabilities Power Management  node-to-AP: “I am going to sleep until next beacon frame”  AP knows not to transmit frames to this node  node wakes up before next beacon frame  beacon frame: contains list of mobiles with AP- to-mobile frames waiting to be sent  node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame