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Special Topics on Wireless Ad-hoc Networks

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  1. Special Topicson Wireless Ad-hoc Networks Lecture 12: Wireless 802.11 University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Computer Network

  2. Covered topic • How wireless LAN, 802.11 works • References • Chapter 3 of the book • “Wireless Medium Access control protocols” a survey • “MACAW: A Media Access Protocol for Wireless LAN’s” • SSCH: Slotted Seeded Channel Hopping for Capacity … • ECHOS: Enhanced Capacity 802.11 Hotspots • Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANS • A wireless MAC protocol Using Implicit Pipelining Computer Network

  3. Outlines • Why wireless LAN • 802.11 • 802.11 MAC • Some improvement • Performance Analysis. Computer Network

  4. Why wireless networks? • Mobility: to support mobile applications • Costs: reductions in infrastructure and operating costs: no cabling or cable replacement • Special situations: No cabling is possible or it is very expensive. • Reduce downtime: Moisture or hazards may cut connections. Wireless Ad hoc/Sensor Networks

  5. Why wireless networks? (cont) • Rapidly growing market attests to public need for mobility and uninterrupted access • Consumers are used to the flexibility and will demand instantaneous, uninterrupted, fast access regardless of the application. • Consumers and businesses are willing to pay for it Wireless Ad hoc/Sensor Networks

  6. The Two Hottest Trends inTelecommunications Networks Millions Mobile Telephone Users Internet Users Year Wireless Ad hoc/Sensor Networks Source: Ericsson Radio Systems, Inc.

  7. Growth of Home wireless Wireless Ad hoc/Sensor Networks

  8. Why is it so popular? • Flexible • Low cost • Easy to deploy • Support mobility Wireless Ad hoc/Sensor Networks

  9. Applications ? • Ubiquitous, Pervasive computing or nomadic access. • Ad hoc networking: Where it is difficult or impossible to set infrastructure. • LAN extensions: Robots or industrial equipment communicate each others. Sensor network where elements are two many and they can not be wired!. • Sensor Networks: for monitoring, controlling, e Wireless Ad hoc/Sensor Networks

  10. What is special on wireless? • Channel characteristics • Half-Duplex • Location dependency • Very noisy channel, fading effects, etc., • Resource limitation • Bandwidth • Frequency • Battery, power. • Wireless problems are usually optimization problems. Wireless Ad hoc/Sensor Networks

  11. What is special on wireless? • Mobility in the network elements • Very diverse applications/devices. • Connectivity and coverage (internetworking) is a problem. • Maintaining quality of service over very unreliable links • Security (privacy, authentication,...) is very serious here. Broadcast media. • Cost efficiency Wireless Ad hoc/Sensor Networks

  12. Big issues! • Integration with existing data networks sounds very difficult. • It is not always possible to apply wired networks design methods/principles here. Wireless Ad hoc/Sensor Networks

  13. Problems • Host mobility is not considered in the initial Internet design. • There is a hierarchal design in Internet. How Ad hoc wireless networks can be handled • A layered design. Layer should be independent of each other. It is not work at all in wireless • TCP • Battery shortages; • Etc,. Wireless Ad hoc/Sensor Networks

  14. High availbility requirements • No QoS assumed from below • Reasonable but non-zero loss rates • What’s minimum recovery time? • 1 RTT • But conservative assumptions end-to-end • TCP RTO - min(1s)! • Interconnect independent networks • Federation makes things hard: • My network is good. Is yours? Is the one in the middle? • Scale • Routing convergence times, etc. Wireless Ad hoc/Sensor Networks

  15. Growing Application Diversity Collision Avoidance:Car Networks Mesh Networks Wired Internet Access Point Sensor Relay Node Ad-Hoc/Sensor Networks Wireless Home Multimedia Wireless Ad hoc/Sensor Networks

  16. Challenge: Diversity • New architectures must accommodate rapidly evolving technology • Must accommodate different optimization goals • Power, coverage, capacity, price Wireless Edge Network INTERNET INTERNET Wireless Edge Network 2005 2010 Wireless Ad hoc/Sensor Networks

  17. Spectrum Scarcity • Interference and unpredictable behavior • Need better management/diagnosis tools • Lack of isolation between deployments • Cross-domain and cross-technology Why is my 802.11 not working? Wireless Ad hoc/Sensor Networks

  18. Other Challenges • Performance: Nothing is really work well • Security: It is a broadcast media • Cross layer interception • TCP performance Wireless Ad hoc/Sensor Networks

  19. Ideal Wireless Area network? • Wish List • High speed (Efficiency) • Low cost • No use/minimal use of the mobile equipment battery • Can work in the presence of other WLANs (Heterogeneity) • Easy to install and use • Etc Computer Network

  20. Wireless LAN Design Goals • Wireless LAN Design Goals • Portable product: Different countries have different regulations concerning RF band usage. • Low power consumption • License free operation • Multiple networks should co-exist Computer Network

  21. Wireless LAN Design Alternatives Design Choices Physical Layer: diffused Infrared (IR) or Radio Frequency (RF)? Radio Technology: Direct-Sequence or Frequency-Hopping? Which frequency range to use? Which MAC protocol to use. Peer-Peer architecture or Base-Station approach? Univ. of Tehran Computer Network 21

  22. Wireless Standards Computer Network

  23. Distance vs. Data Rate Computer Network

  24. WiFi • Almost all wireless LANs now are IEEE 802.11 based • Competing technologies, e.g., HiperLAN can’t compete on volume and cost • 802.11 is also known as WiFi = “Wireless Fidelity” • Fidelity = Compatibility between wireless equipment from different manufacturers • WiFi Alliance is a non-profit organization that does • the compatibility testing (WiFi.org) Computer Network

  25. Architectures • Distributed wireless Networks: also called Ad-hoc networks • Centralized wireless Networks: also called last hop networks. They are extension to wired networks. Computer Network

  26. Access Point Access Point Centralized Wlan Ad Hoc Laptop Laptop Server DS Pager Laptop PDA Laptop Computer Network

  27. IEEE 802.11 Topology • Independent basic service set (IBSS) networks (Ad-hoc) • Basic service set (BSS), associated node with an AP • Extended service set (ESS) BSS networks • Distribution system (DS) as an element that interconnects BSSs within the ESS via APs. Computer Network

  28. Starting an IBSS • One station is configured to be “initiating station,” and is given a service set ID (SSID); • Starter sends beacons; • Other stations in the IBSS will search the medium for a service set with SSID that matches their desired SSID and act on the beacons and obtain the information needed to communicate; • There can be more stations configured as “starter.” Computer Network

  29. ESS topology • connectivity between multiple BSSs, They use a common DS Computer Network

  30. Base-Station Approach Advantages over Peer-Peer • No hidden terminal: base station hears all mobile terminals, are relays their information to ever mobile terminal in cell. • Higher transmission range • Easy expansion • Better approach to security • Problem? • Point of failure, • Feasibility? Computer Network

  31. 802.11 Logical Architecture • PLCP: Physical Layer Convergence Procedure • PMD: Physical Medium Dependent • MAC provides asynchronous, connectionless service • Single MAC and one of multiple PHYs like DSSS, OFDM, IR • and FHSS. Computer Network

  32. MAC Header Frame Control Duration Addr 1 Addr 3 Sequence Control Address 4 User Data CRC Addr 2 Protocol Version Type Sub type To DS From DS Last Fragment Retry Power Mgt EP RSVD 802.11 MAC Frame Format Bytes 34~2346 32 6 6 2 6 4 6 2 2 6 Bytes Encrypted to WEP Bits 2 1 4 2 1 1 Computer Network

  33. 802.11 MAC Frame Format • Address Fields contains • Source address • Destination address • AP address • Transmitting station address • DS = Distribution System • User Data, up to 2304 bytes long Computer Network

  34. Special Frames: ACK, RTS, CTS bytes 2 2 6 4 • Acknowledgement • Request To Send • Clear To Send Frame Control Duration Receiver Address CRC ACK bytes 2 2 6 6 4 Frame Control Duration Receiver Address Transmitter Address CRC RTS bytes 2 2 6 4 Frame Control Duration Receiver Address CRC CTS

  35. 802.11 Features • Power management: NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. Put to sleep, etc. • Bandwidth: To compress data • Security: • Addressing: destination address does not always correspond to location. Computer Network

  36. Power Management • Battery life of mobile computers/PDAs are very short. Need to save • The additional usage for wireless should be minimal • Wireless stations have three states • Sleep • Awake • Transmit Computer Network

  37. Power Management, Cont… • AP knows the power management of each node • AP buffers packets to the sleeping nodes • AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when? • The node is awake only when it is sending data, receiving data or listening to TDIM. Computer Network

  38. IEEE 802.11 LLC Layer • Provides three type of service for exchanging data between (mobile) devices connected to the same LAN • Acknowledged connectionless • Un-acknowledged connectionless, useful for broadcasting or multicasting. • Connection oriented • Higher layers expect error free transmission Computer Network

  39. Frame type and subtypes • Three type of frames • Management • Control • Asynchronous data • Each type has subtypes • Control • RTS • CTS • ACK Computer Network

  40. Frame type and subtypes, Cont.. • Management • Association request/ response • Re-association request/ response: transfer from AP to another. • Probe request/ response • privacy request/ response: encrypting content • Authentication: to establish identity • Beacon (Time stamp, beacon interval, channels sync info, etc.) Computer Network

  41. Frame type and subtypes, Cont.. • Management… • TIM (Traffic Indication Map) indicates traffic to a dozing node • dissociation Computer Network

  42. 802.11 Management Operations • Scanning • Association/Reassociation • Time synchronization • Power management Computer Network

  43. Scanning in 802.11 • Goal: find networks in the area • Passive scanning • Not require transmission • Move to each channel, and listen for Beacon frames • Active scanning • Require transmission • Move to each channel, and send Probe Request frames to solicit Probe Responses from a network Computer Network

  44. Time Synchronization in 802.11 • Timing synchronization function (TSF) • AP controls timing in infrastructure networks • All stations maintain a local timer • TSF keeps timer from all stations in sync • Periodic Beacons convey timing • Beacons are sent at well known intervals • Timestamp from Beacons used to calibrate local clocks • Local TSF timer mitigates loss of Beacons Computer Network

  45. Authentication • Three levels of authentication • Open: AP does not challenge the identity of the node. • Password: upon association, the AP demands a password from the node. • Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key. Computer Network

  46. Inter Frame Spacing • SIFS = Short inter frame space = dependent on PHY • PIFS = point coordination function (PCF) inter frame space = SIFS + slot time • DIFS = distributed coordination function (DCF) inter frame space = PIFS + slot time • The back-off timer is expressed in terms of number of time slots. Computer Network

  47. 802.11 Frame Priorities • Short interframe space (SIFS) • For highest priority frames (e.g., RTS/CTS, ACK) • PCF interframe space (PIFS) • Used by PCF during contention free operation • DCF interframe space (DIFS) • Minimum medium idle time for contention-based services DIFS PIFS contentwindow Frame transmission Busy SIFS Time Computer Network

  48. SIFS/DIFS SIFS makes RTS/CTS/Data/ACK atomic Example:Slot Time = 1, CW = 5, DIFS=3, PIFS=2, SIFS=1, Computer Network

  49. Priorities in 802.11 • CTS and ACK have priority over RTS After channel becomes idle • If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle • If a node wants to send RTS, it waits for DIFS > SIFS Computer Network

  50. SIFS and DIFS DATA1 ACK1 backoff RTS DIFS SIFS SIFS Computer Network