EE4L Computer & Communication Networks

1. EE4LComputer & Communication Networks Part IV – Wireless networking Dr Costas Constantinou

2. Wireless Networking • Overview • Wireless MAC layer protocols • Wireless LANs • Mobile ad hoc networks Acknowledgements: Slides adapted from numerous sources. Thanks go (alphabetically) to, Dr Romit Roy Choudhury, Duke University; Prof Jim Kurose, Univ of Massachusetts; Prof Nitin Vaidia, Univ of Illinois at Urbana-Champaign

3. 1. Overview • Wireless networks are becoming ubiquitous • The edge of the Internet is fast becoming wireless • Single hop networks: • Wireless LANs • Cellular • Multi-hop networks: • Personal area networks • Military

4. Microsoft, Intel, Cisco … Internet Mesh Networks and Wireless Backbones Personal Area Networks Motorola, Intel, Samsung … Citywatchers, Wal-Mart Intel, Philips, Bosch … RFID and Sensor Networks 1. Overview Future network vision:

5. network infrastructure 1. Overview Elements of a wireless network wireless hosts • laptop, PDA, IP phone • run applications • may be stationary (non-mobile) or mobile • wireless does not always mean mobility

6. network infrastructure 1. Overview Elements of a wireless network 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

7. network infrastructure 1. Overview Elements of a wireless network wireless link • connects mobiles to base station • sometimes used as a backbone link • multiple access protocol coordinates link access • data transmission rate is a function of distance (SNIR really)

8. network infrastructure 1. Overview Elements of a wireless network infrastructure mode • base station connects mobiles into wired network • handover/handoff: mobile changes base station

9. 1. Overview Ad hoc mode • no base stations • nodes can only transmit to other nodes within coverage • nodes organize themselves into a network: route among themselves

10. 1. Overview Characteristics of selected wireless link standards 200 802.11n 54 802.11a,g 802.11a,g point-to-point data 5-11 802.11b 802.16 (WiMAX) 3G cellular enhanced 4 UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 3G UMTS/WCDMA, CDMA2000 2G .056 IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km

11. 1. Overview • Wireless networks are unlike other networks • Fundamental difference is that the concept of link as a mathematical graph edge joining a pair of nodes is not applicable • A node broadcasts its messages, it cannot send them each to a chosen neighbour • Broadcast domains do not have well-defined boundaries, are time-varying and are subject to interference, multipath, noise, etc. • Broadcast domains are (almost always) partially overlapping • Interference happens at the receiver; interference (and carrier sense) range is longer than communication range • Nodes may move

12. 1. Overview • Wired vs. wireless media access • Both are on shared media • Then, what’s really the problem? • Wired network: • Collision Detection • Nodes can transmit and receive at the same time • If (Transmitted_Signal ≠ Sensed_Signal)  Collision • Channel Condition is identical at Tx and Rx • Wireless network: • No collision detection possible • Nodes cannot transmit and receive simultaneously on same channel • Channel condition varies from node to node and is never identical at Tx and Rx

13. 1. Overview B A C C C’s signal strength A’s signal strength B A space Hidden terminal problem • A, C can not hear each other • means that A, C unaware of their interference at B Spatial signal variation: • B, A hear each other • B, C hear each other • A, C can not hear each other interfering at B

14. X Y Z 1. Overview Hidden terminal problem • X is transmitting to Y • Z cannot sense X • Z transmits to Y • Collision at Y; high collision rate; wastes bandwidth • Absence of carrier does not always mean it is safe to transmit Exposed terminal problem • W is transmitting to X • Y wants to transmit to Z but senses transmission of W and defers • W does not exploit possible simultaneous transmission to Z; high idle rate; wastes bandwidth • Presence of carrier does not always mean it is not safe to transmit X W Y Z

15. 2. Wireless MAC • Assume that you have some basic knowledge of wired MAC such as Aloha, CSMA, CSMA/CD (a.k.a. IEEE802.3) • Wireless MAC proved to be non-trivial • research by [Karn90] (MACA) • research by [Bhargavan94] (MACAW) • Led to IEEE 802.11 committee • The standard was ratified in 1999 • The predominant wireless MAC protocol is IEEE802.11 and its variants

16. RTS = Request To Send CTS = Clear To Send 2. Wireless MAC • IEEE802.11 basic operation/handshake M Y S RTS D CTS X K

17. 2. Wireless MAC • IEEE802.11 basic operation/handshake silenced M Y silenced S Data D ACK silenced X K silenced

18. 2. Wireless MAC

19. 2. Wireless MAC • Two modes • CSMA/CA – A contention based protocol. In 802.11 this mode is known as Distributed Coordination Function (DCF) • Priority-based access – A contention free access protocol usable on the infrastructure mode. Known as Point Coordination Function (PCF)

20. 2. Wireless MAC • 802.11 Steps • All backlogged nodes choose a random number • R = rand (0, CW_min) • Each node counts down R • Continue carrier sensing while counting down • Once carrier busy, freeze countdown • Whoever reaches ZERO transmits RTS • Neighbours freeze countdown, decode RTS • RTS contains (CTS + DATA + ACK) duration = T_comm • Neighbours set NAV = T_comm • Remains silent for NAV time

21. 2. Wireless MAC • 802.11 Steps (cont.) • Receiver replies with CTS • Also contains (DATA + ACK) duration. • Neighbours update NAV again • Tx sends DATA, Rx acknowledges with ACK • After ACK, everyone initiates remaining countdown • Tx chooses new R = rand (0, CW_min) • If RTS or DATA collides (i.e., no CTS/ACK returns) • Indicates collision • RTS chooses new random no. R1 = rand (0, 2*CW_min) • Note Exponential Backoff Ri = rand (0, 2^i * CW_min) • Once successful transmission, reset to rand(0, CW_min)

22. 2. Wireless MAC • 802.11 basic flow control • Sender sends RTS with NAV (Network Allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) after waiting for DIFS • Receiver acknowledges via CTS after SIFS (if ready to receive) • CTS reserves channel for sender, notifying possibly hidden stations; • any station hearing CTS should be silent for NAV • Sender can now send data at once DIFS RTS data sender SIFS SIFS CTS receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

23. 2. Wireless MAC • 802.11: RTS/CTS + ACK, the Final Version • 802.11 adds ACK in the signaling to improve reliability • implication: to avoid conflict with ACK, any station hearing RTS should not send for NAV • thus a station should not send for NAV if it hears either RTS and CTS • Note: RTS/CTS is optional in 802.11, and thus may not be always turned on---some network interface cards turn it on only when the length of a frame exceeds a given threshold DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

24. 2. Wireless MAC • 802.11: PCF for Polling SIFS PIFS D D point coordinator SIFS U polled wireless stations NAV NAV contention free period t medium busy contention period D: downstream poll, or data from point coordinator U: data from polled wireless station

25. DIFS 2. Wireless MAC • How to integrate PCF and DCF? • Basic Solution: Using Inter Frame Spacing to Prioritize Access • Different inter frame spacing (IFS): if the required IFS of a type of message is short, the type of message has higher priority • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (Point Coordination Function Spacing) • medium priority, for time-bounded service using PCF • DIFS (Distributed Coordination Function Spacing) • lowest priority, for asynchronous data service DIFS PIFS SIFS medium busy contention next frame t Access point access if medium is free  DIFS random direct access if medium is free  DIFS

26. RTS CTS 2. Wireless MAC • RTS/CTS: Does it solve hidden terminal problem? • Assuming carrier sensing zone = communication zone E F A B C D E does not receive CTS successfully  Can later initiate transmission to D. Hidden terminal problem remains.

27. 2. Wireless MAC • HT: How about increasing carrier sense range? • E will defer on sensing carrier  no collision !!! RTS E F CTS A B C D Data

28. 2. Wireless MAC • HT: But what if barriers/obstructions exist? • E doesn’t hear C  Carrier sensing does not help RTS E F CTS A B C D Data

29. 2. Wireless MAC • ET: B should be able to transmit to A • RTS prevents this E RTS CTS A B C D

30. 2. Wireless MAC • ET: B should be able to transmit to A • Carrier sensing makes the situation worse E RTS CTS A B C D

31. 2. Wireless MAC • 802.11 does not solve HT/ET completely • Only alleviates the problem through RTS/CTS and recommends larger CS zone • Large CS zone aggravates exposed terminals • Spatial reuse reduces  A tradeoff • RTS/CTS packets also consume bandwidth • Moreover, backing off mechanism is also wasteful • 802.11 is still being optimized • Thus, wireless MAC research still alive

32. Internet 3. Wireless LANs • Wireless 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 hub, switch or router AP AP BSS 1 BSS 2

33. 3. Wireless LANs • 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 neighbouring AP! • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (service set identifier – SSID) and MAC address • selects AP to associate with • may perform authentication • will typically run DHCP to get IP address in AP’s subnet

34. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 3. Wireless LANs • 802.11 frame: addressing Address 4: used only in ad hoc mode 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 2: MAC address of wireless host or AP transmitting this frame

35. router AP Internet R1 MAC addr AP MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame 3. Wireless LANs • 802.11 frame: addressing H1 R1

36. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd 3. Wireless LANs • 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame type (RTS, CTS, ACK, data)

37. 3. Wireless LANs • Mobility within the same subnet • H1 remains in same IP subnet: IP address can remain same • switch: which AP is associated with H1? • self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1 router hub or switch BBS 1 AP 1 AP 2 H1 BBS 2

38. 4. MANETs • Mobile ad hoc Network (MANET) • Formed by wireless hosts which may be mobile • Without (necessarily) using a pre-existing infrastructure • Routes between nodes may potentially contain multiple hops

39. 4. MANETs • May need to traverse multiple links to reach a destination

40. 4. MANETs • Mobility causes route changes

41. 4. MANETs • Why ad hoc Networks ? • Ease of deployment • Speed of deployment • Decreased dependence on infrastructure • Applications • Personal area networking • cell phone, laptop, ear phone, wrist watch • Military environments • soldiers, tanks, planes • Civilian environments • taxi cab network • meeting rooms • sports stadiums • boats, small aircraft • Emergency operations • search-and-rescue • policing and fire fighting

42. 4. MANETs • Many variants • Fully Symmetric Environment • all nodes have identical capabilities and responsibilities • Asymmetric Capabilities • transmission ranges and radios may differ • battery life at different nodes may differ • processing capacity may be different at different nodes • speed of movement • Asymmetric Responsibilities • only some nodes may route packets • some nodes may act as leaders of nearby nodes (e.g., cluster head) • Traffic characteristics may differ in different ad hoc networks • bit rate • timeliness constraints • reliability requirements • unicast / multicast / geocast • host-based addressing / content-based addressing / capability-based addressing

43. 4. MANETs • Many variants (cont.) • May co-exist (and co-operate) with an infrastructure-based network • Mobility patterns may be different • people sitting at an airport lounge • taxis • kids playing • military movements • personal area network • Mobility characteristics • speed • predictability • direction of movement • pattern of movement • uniformity (or lack thereof) of mobility characteristics among different nodes

44. 4. MANETs • MANET challenges • Must address all of these issues • Limited wireless transmission range • Broadcast nature of the wireless medium • Packet losses due to transmission errors • Mobility-induced route changes • Mobility-induced packet losses • Battery constraints • Potentially frequent network partitions • Ease of snooping on wireless transmissions (security hazard) • No protocol solution fits all MANET scenarios • Protocol performance metrics do not scale well with increasing mobility and/or number of nodes

45. 4. MANETs • Unicast routing in MANETs • What’s special about MANET routing? • Host mobility • link failure/repair due to mobility may have different characteristics than those due to other causes • Rate of link failure/repair may be high when nodes move fast • New performance criteria may be used • route stability despite mobility • energy consumption • Unicast MANET routing protocol classes • Proactive protocols • Determine routes independent of traffic pattern • Traditional link-state and distance-vector routing protocols are proactive • Reactive protocols • Maintain routes only if needed • Hybrid protocols

46. 4. MANETs • Routing protocol trade-offs • Delay in route discovery • Proactive protocols may have lower delay since routes are maintained at all times • Reactive protocols may have higher delay because a route from X to Y will be found only when X attempts to transmit to Y • Overhead of route discovery/maintenance • Reactive protocols have lower overhead since routes are determined only if needed • Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating • Which approach achieves a better trade-off depends on traffic & mobility patterns

47. 4. MANETs • We shall examine briefly • Flooding • Dynamic Source Routing (DSR) protocol • Ad hoc On-Demand Distance Vector (AODV) routing protocol 4.1 Flooding for Data Delivery • Sender S broadcasts data packet P to all its neighbours • Each node receiving P forwards P to its neighbours • Sequence numbers used to avoid the possibility of forwarding the same packet more than once • Packet P reaches destination D provided that D is reachable from sender S • Node D does not forward the packet

48. 4.1 Flooding Y Z S E F B C M L J A G H D K I N Represents a node that has received packet P Represents connected nodes that are within each other’s transmission range

49. 4.1 Flooding Y Broadcast transmission Z S E F B C M L J A G H D K I N Represents a node that receives packet P for the first time Represents transmission of packet P

50. 4.1 Flooding Y Z S E F B C M L J A G H D K I N Node H receives packet P from two neighbours: Potential for collision