IEEE 802.11 Wireless LAN

1. IEEE 802.11 Wireless LAN

2. Why Wireless LAN? • Traditional LANs need wires, which may be difficult to set up in some situations. • Advantages of Wireless LANs • Allow mobility and flexibility • Reduced cost • Applicable scenarios • Offices • Building with open area • Hybrid with wired LANs

3. Architectures Infrastructure mode Infrastructure-less/ distributed/ad-hoc mode

4. Physical Layer • RF: Spread Spectrum, no licensing required. Resistance to interference • Band: 915-Mhz, 2.4 GHz (worldwide ISM), 5.2 Ghz • Direct sequence spread spectrum (DSSS) • broaden the signaling band by artificially increasing the modulation rate using a spreading code. 2M or 10M. • Frequency hopping spread spectrum (FHSS) • hop from narrow band to narrow band within a wide band, using each narrow band for a specific time period.

5. MAC Layer: Hidden Terminal Problem • Node B can communicate with A and C both • A and C cannot hear each other • When A transmits to B, C cannot detect the transmission using the carrier sense mechanism • If C transmits, collision will occur at node B A B C

6. A B C MCAC (Multiple Access with Collision Avoidance) • When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS)to A • On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet • When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer • Transfer duration is included in RTS and CTS both

7. A B C Reliability • Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance. • Mechanisms needed to reduce packet loss rate experienced by upper layers • When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). • If node A fails to receive an Ack, it will retransmit the packet

8. IEEE 802.11 Wireless MAC • Distributed and centralized MAC components • Distributed Coordination Function (DCF) • Point Coordination Function (PCF)

9. A B C IEEE 802.11 DCF • Uses RTS-CTS exchange to avoid hidden terminal problem • Any node overhearing a CTS cannot transmit for the duration of the transfer • Uses ACK to achieve reliability • Any node receiving the RTS cannot transmit for the duration of the transfer • To prevent collision with ACK when it arrives at the sender • When B is sending data to C, node A will keep quite

10. Collision Avoidance • With half-duplex radios, collision detection is not possible • CSMA/CA: Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism • Carrier sense: When a node wishes to transmit a packet, it first waits until the channel is idle • Collision avoidance: Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit

11. Congestion Avoidance • When transmitting a packet, choose a backoff interval in the range [0,cw] • cw is contention window • Count down the backoff interval when medium is idle • Count-down is suspended if medium becomes busy • When backoff interval reaches 0, transmit RTS

12. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 Example B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

13. IEEE 802.11 PCF • Purpose: contention-free data transmission • System components • Access Point (AP): a coordinator controlling the medium access in a poll-and-response manner • Stations: transmit only when being polled • A LAN operates in PCF or DCF mode • The duration in which PCF operates is called contention-free period (CFP) • Before/after a CFP, the network operates in DCF.

14. IEEE 802.11 PCF • Starting • AP seizes the medium by using “priority inter-frame space” (PIFS) • AP sends out a beacon packet to announce the beginning of a CFP (the packet contains the duration of the CFP) • In a CFP • AP may transmit data packets to any station • AP may send a polling packet to a station • The polled station replies with a data packet or a NULL packet (when nothing to send) • Ending • AP sends out an END packert.

15. MAC Management • Synchronization • finding and staying with a WLAN. • Synchronization functions • Power management • sleeping without missing any messages • power management functions, e.g., periodic sleep, frame buffering, traffic indication map • Association and Re-association • joining a network, roaming, moving from one AP to another, scanning

16. Power Management • 802.11 power off station during idle periods • A station can be in one of three states: • transmitter on, • receiver only on, • dozing: both transmitter and receivers off • is transparent to existing protocols • is flexible to support different application

17. Power Management • APs buffer packets for sleeping stations • AP announces which stations have frames buffered • traffic indication map (TIM) sent with every beacon. • All multicasts/broadcasts are buffered • Time Synchronization Function (TSF) assures AP and power save stations are synchronized • stations wake up periodically to hear a beacon • TSF timer keeps running when stations are sleeping • synchronization allows extreme low power operation

18. Summary • Architectures of Wireless LANs • Infrastructure or infrastructure-less • MAC • Hidden terminal problem • collision avoidance • DCF and PCF • MAC management • Power management and others

19. Mobile Ad Hoc Networks

20. What is a MANET (Mobile Ad Hoc Networks)? • Formed by wireless hosts which may be mobile • No pre-existing infrastructure • Routes between nodes may potentially contain multiple hops • Nodes act as routers to forward packets for each other • Node mobility may cause the routes change B A A B C C D D

21. Why MANET? • Advantages: low-cost, flexibility • Ease & Speed of deployment • Decreased dependence on infrastructure • Applications • Military environments • soldiers, tanks, planes • Civilian environments • vehicle networks • conferences / stadiums • outside activities • Emergency operations • search-and-rescue / policing and fire fighting

22. Challenges • Collaboration • Collaborations are necessary to maintain a MANET and its functionality. • How to collaborate effectively and efficiently? • How to motivate/enforce nodes to collaborate? • Dynamic topology • Nodes mobility • Interference in wireless communications

23. Routing Protocols: Overview • Proactive protocols • Determine routes independent of traffic pattern • Traditional link-state and distance-vector routing protocols are proactive • Examples: • DSDV (Dynamic sequenced distance-vector) • OLSR (Optimized Link State Routing) • Reactive protocols • Maintain routes only if needed • Examples: • DSR (Dynamic source routing) • AODV (on-demand distance vector) • Hybrid protocols • Example: Zone Routing Protocol (intra-zone: proactive; inter-zone: on-demand)

24. Routing Protocols: Tradeoff • Latency of route discovery • Proactive protocols may have lower latency since routes are maintained at all times • Reactive protocols may have higher latency because a route from X to Y may be found only when X attempts to send to Y • Overhead of route discovery/maintenance • Reactive protocols may 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 the traffic and mobility patterns

25. Dynamic Source Routing • J. Broch, D. Johnson, and D. Maltz, “The dynamic source routing protocol for mobile ad hoc networks,” Internet-Draft Version 03, IETF, October 1999. • When node S wants to send a packet to node D, but does not know a route to D, node S initiates a routing process • Runs in three phases • Route Discovery  Route Reply  Path Establishment • Route Discovery • Source node S floods Route Request (RREQ) • Each node appends own identifier when forwarding RREQ

26. Route Discovery in DSR Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S

27. Route Discovery in DSR Y Broadcast transmission Z [S] S E F B C M L J A G H D K I N Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ

28. Route Discovery in DSR Y Z S [S,E] E F B C M L J A G [S,C] H D K I N

29. Route Discovery in DSR Y Z S E F [S,E,F,J] B C M L J A G H D K I N [S,C,G,K]

30. Route Reply in DSR • Destination D on receiving the first RREQ, sends a Route Reply (RREP) • RREP is sent on a route obtained by reversing the route appended to received RREQ • RREP includes the route from S to D on which RREQ was received by node D

31. Route Reply in DSR Y Z S RREP [S,E,F,J,D] E F B C M L J A G H D K I N Represents RREP control message

32. Route Reply in DSR • Node S on receiving RREP, caches the route included in the RREP • When node S sends a data packet to D, the entire route is included in the packet header • Hence the name source routing • Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

33. Data Delivery in DSR Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I N Packet header size grows with route length

34. Some Other Routing Protocols • Location information aided protocols • Power-aware protocols • Others … • e.g., considering the stability of topology

35. Location-Aided Routing (LAR) • Y. Ko and N. Vaidya, “Location-aided routing (LAR) in mobile ad hoc networks,” MobiCom'98. • Exploits location information to limit scope of route request flood • Location information may be obtained using GPS • Expected Zone is determined as a region that is expected to hold the current location of the destination • Expected region determined based on potentially old location information, and knowledge of the destination’s speed • Route requests limited to a Request Zonethat contains the Expected Zone and location of the sender node • B. Karp, and H. Kung, “Greedy Perimeter Stateless Routing for Wireless Networks,” MobiCom 2000.

36. Power-Aware Routing • Modification to DSR to make it power aware (for simplicity, assume no route caching): • Route Requests aggregate the weights of all traversed links • Destination responds with a Route Reply to a Route Request if • it is the first RREQ with a given (“current”) sequence number, or • its weight is smaller than all other RREQs received with the current sequence number

37. Geography Adaptive Fidelity • Each node associates itself with a square in a virtual grid • Node in each grid square coordinate to determine who will sleep and how long [Y. Xu, et al. “Geography Adaptive Fidelity in Routing,” Mobicom’2001] Grid head

38. Research in Other Layers • Transport layer • A survey: A. Hanbali, E. Altman, P. Nain, “A Survey of TCP over Mobile Ad Hoc Networks (2004)”. • Application layer • Data management • e.g., B. Xu, A. Ouksel, and O. Wolfson, "Opportunistic Resource Exchange in Inter-vehicle Ad Hoc Networks," MDM, 2004. • Distributed algorithms • clock synchronization • mutual exclusion • leader election • Byzantine agreement

39. Security in Mobile Ad Hoc Networks

40. Problems • Hosts may misbehave or try to compromise security at all layers of the protocol stack • Transport layer: securing end-to-end communication • Need to know keys to be used for secure communication • May want to anonymize the communication • Network layer: misbehaving hosts may create many hazards • May disrupt route discovery and maintenance:Force use of poor routes (e.g., long routes) • Delay, drop, corrupt, misroute packets • May degrade performance by making good routeslook bad • MAC layer: misbehaving nodes may not cooperate • Disobey protocol specifications for selfish gains • Denial-of-service attacks

41. Security in MANET: Agenda • Key management • Securing communications • Dealing with MAC and Network layer misbehaviors

42. Key Management • Challenges • In “pure” ad hoc networks, access to infrastructure cannot be assumed • Network may also become partitioned • Solutions • Distributed public key infrastructure • Self-organized key management • Distributed key certification • TESLA • Others

43. Self-Organized Public Key Management [Capkun03] • Nodes form a “Certificate Graph” • each vertex represents a public key • an edge from Ku to Kw exists if there is a certificate signed by the private key of node u that binds Kw to the identity of some node w. Ku (w,Kw)Pr Ku Kw

44. Self-Organized Public Key Management [Capkun03] • Four steps of the management scheme • Step 1: Each node creates its own private/public keys. Each node acts independently

45. Self-Organized Public Key Management [Capkun03] • Step 2: When a node u believes that key Kw belongs to node w, node u issues a public-key certificate in which Kw is bound to w by the signature of u • u may believe this because u and w may have talked on a dedicated channel previously • Each node also issues a self-signed certificate for its own key • Step 3: Nodes periodically exchange certificates with other nodes they encounter • Mobility allows faster dissemination of certificates through the network

46. Self-Organized Public Key Management [Capkun03] • Step 4: Each node forms a certificate graph using the certificates known to that node Authentication: When a node u wants to verify the authenticity of the public key Kv of node v, u tries to find a directed graph from Ku to Kv in the certificate graph. If such a path is found, the key is authentic.

47. Self-Organized Public Key Management [Capkun03] • Misbehaving hosts may issue incorrect certificates • If there are mismatching certificates, indicates presence of a misbehaving host (unless one of the mismatching certificate has expired) • Mismatching certificates may bind same public key for two different nodes, or same node to two different keys • To resolve the mismatch, a “confidence” level may be calculated for each certificate chain that verifies each of the mismatching certificates • Choose the certificate that can be verified with high confidence – else ignore both certificates

48. Secure Communication • With the previously discussed mechanisms for key distribution, it is possible to authenticate the assignment of a public key to a node • This key can then be used for secure communication • The public key can be used to set up a symmetric key between a given node pair as well • TESLA provides a mechanism for broadcast authentication when a single source must broadcast packets to multiple receivers

49. Secure Communication • Sometimes security requirement may include anonymity • Availability of an authentic key is not enough to prevent traffic analysis • We may want to hide the source or the destination of a packet, or simply the amount of traffic between a given pair of nodes

50. Traffic Analysis • Traditional approaches for anonymous communication, for instance, based on MIX nodes or dummy traffic insertion, can be used in wireless ad hoc networks as well