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Special Topics on Wireless Ad-hoc Networks. Lecture 3: Wireless LANs. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Covered topic. How to build a small wireless network? considerations Media access issues References Chapter 3 of the book

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Special Topics on Wireless Ad-hoc Networks


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    1. Special Topicson Wireless Ad-hoc Networks Lecture 3: Wireless LANs University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Computer Network

    2. Covered topic • How to build a small wireless network? • considerations • Media access issues • References • Chapter 3 of the book • “Wireless Medium Access control protocols” a survey • “MACAW: A Media Access Protocol for Wireless LAN’s” • “Design alternative for Wireless local area networks”, • “Bluetooth” • SSCH: Slotted Seeded Channel Hopping for Capacity … • ECHOS: Enhanced Capacity 802.11 Hotspots • A backoff Algorithm for Improving Saturation Throughput in IEEE 802.11 DCF • A wireless MAC protocol Using Implicit Pipelining Computer Network

    3. Outlines • Why wireless LANs? Applications? • Wireless LAN’s issues • 802.11 standard • Mac protocols. • Bluetooth • ZigBee Computer Network

    4. 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. Computer Network

    5. 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. Computer Network

    6. Applications ? • 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!. Computer Network

    7. Ideal Wireless LAN? • Wish List • High speed • Low cost • No use/minimal use of the mobile equipment battery • Can work in the presence of other WLAN • Easy to install and use • Etc Computer Network

    8. Wireless LAN Design Alternatives • 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 • Design Choices • Physical Layer: IR or 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? Computer Network

    9. Physical Layer Alternatives • IR • Simple circuitry, cost-effective, no regulatory constraints, no Rayleigh fading (waves are small), also nice for micro-cellular networks... (multiple cells can exist in a room providing more bandwidth) • RF • more complicated circuitry, regulatory constraints (ISM bands) in the U.S. Computer Network

    10. Physical Layer Alternatives Computer Network

    11. Radio Technology • Spread Spectrum Technologies • Frequency Hopping: The sender keeps changing the carrier wave frequency at which its sending its data. Receiver must be in synch with transmitter, and know the ordering of frequencies. • Direct-Sequence: The receiver listens to a set of frequencies at the same time. The subset of frequencies that actually contain data from the sender is determined by spreading code, which both the sender and receiver must know. This subset of frequencies changes during transmission. • Non-Spread Spectrum requires licensing Computer Network

    12. Frequency Hopping versus Direct Sequence • DS advantages • Lower cost • FH advantages • Higher capacity • Interference avoidance capability: If some frequency has interference on it, simply don't hop there. • Multiple networks can co-exist: Just use a different frequency hopping pattern. Computer Network

    13. LAN Industry • WANs are offered as service • Cost of infrastructure • Coverage area • LANs are sold as “end products” • You own, no service charge • Analogy with PSTN/PBX • WLAN vs. WAN Cellular Networks • Data rate (2 Mbps vs. 54 Mbps) • Frequency band regulation (Licensing) • Method of data delivery (Service vs. own) Computer Network

    14. Growth of Home wireless Computer Network

    15. LAN standard • IEEE 802 Standards • 802.3, 802.4, 802.5 are wired LANs • 802.9: ISO Ethernet • 802.6: MAN • 802.11, 802.15, 802.16: Wireless local net • 802.14 Cable modem • 802.10 Security management Computer Network

    16. LAN standard Computer Network

    17. Early Experiences • IBM Switzerland,Late 1970 • Factories and manufacturing floors • Diffused IR technology • Could not get 1 Mbps • HP Labs, Palo Alto, 1980 • 100 Kbps DSSS around 900 Mhz • CSMA as MAC • Experimental licensing from FCC • Frequency administration was problematic, thus abandoned • Motorola, ~1985 • 1.73 GHz • Abandoned after FCC difficulties Computer Network

    18. 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

    19. 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

    20. Access Point Access Point Wireless LAN Architecture Ad Hoc Laptop Laptop Server DS Pager Laptop PDA Laptop Computer Network

    21. Access Point Functions • Access point has three components • Wireless LAN interface to communicate with nodes in its service area • Wireline interface card to connect to the backbone network • MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently Computer Network

    22. Medium Access Control • Wireless channel is a shared medium • Need access control mechanism to avoid interference • MAC protocol design has been an active area of research for many years. See Survey. Computer Network

    23. MAC: A Simple Classification Wireless MAC Centralized Distributed On Demand MACs, Polling Guaranteed or controlled access Random access Our focus SDMA, FDMA, TDMA, Polling Computer Network

    24. Wireless MAC issues • Half duplex operations: difficult to receive data while sending • Time varying channel: Multipath propagation, fading • Burst Channel error: BER is as high as 10-3. We need a better strategy to overcome noises. • Location dependant carrier sensing: signal decays with path length. • Hidden nodes • Exposed nodes • Capture: when a receiver can cleanly receive data from two sources simultaneously, the farther one sounds a noise. Computer Network

    25. Performance Metrics • Delay: ave time on the MAC queue • Throughput: fraction used for data transmission. • Fairness: Not preference any node • Stability: handle instantaneous loads greater than its max. capacity. • Robust against channel fading • Power consumption: or power saving • Support for multimedia Computer Network

    26. Wireless LAN Architecture, Cont… Logical Link Control Layer MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer • Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI Computer Network

    27. Multi-Channel MAC: A simple approach • Divide bandwidth into multiple channels • Choose any one of the idle channels • Use a single-channel protocol on the chosen channel • ALOHA • MACA Computer Network

    28. Multiple Channels • Multiple channels in ad hoc networks: typically defined by a particular code (CDMA) or frequency band (FDMA) • TDMA requires time synchronization among hosts in ad hoc network • Difficult • Many MAC protocols have been proposed Computer Network

    29. MAC & Network Topology • CDMA: Not beneficial under current regulations - difficult to get good spreading codes • FDMA: Inefficient spectrum utilization for bursty traffic • CSMA: Suitable for Peer-to Peer architecture • TDMA: favors Base-Station/Remote-Station architecture Computer Network

    30. CSMA versus TDMA • CSMA Advantages • Can be implemented on an Ethernet chipset • TDMA advantages • simple remote stations • isochronous traffic supported (low-latency, consistent throughput for such things as voice) • high power saving potential (only need to listen at certain times) Computer Network

    31. Integrated CSMA/TDMA MAC Protocol • Supports guaranteed bandwidth traffic and random access traffic • The bandwidth is divided into a random part and a reserved part. • Random part is LBT, reserved part • During high traffic all bandwidth can be used for reserved traffic (like wireless telephony) Reserved-1 H2 LBT H1 Reserved-2 H3 Computer Network

    32. Reservation/Polling MAC Protocol • Works only with AP • Fair and slow. First-in-First-Out • Wireless station send a request. • All requests are queued. • Wireless stations are polled in the same order that the requests have arrive. • All data reception is acknowledged. Computer Network

    33. 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

    34. 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

    35. IEEE 802.11 WLAN, History • 1997 IEEE 802.11 working group developed standard for inter-working wireless LAN products for 1 and 2 Mbps data rates in 2.4 GHz ISM (industrial, scientific, and medical) band (2400-2483 MHz) • Required that mobile station should communicate with any wired or mobile station transparently (802.11 should appear like any other 802 LAN above MAC layer), so 802.11 MAC layer attempts to hide nature of wireless layer (eg, responsible for data retransmission) Computer Network

    36. 802.11 WLAN History, Cont.. • 1999 IEEE 802.11a amendment for 5 GHz band operation and 802.11b amendment to support up to 11 Mbps data rate at 24 GHz • Different standards: a, b, e, etc., differ in physical link properties, services, etc. • MAC sub layer uses CSMA/CA (carrier sense multiple access with collision avoidance) Computer Network

    37. 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

    38. 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

    39. 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

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

    41. Starting an ESS • The infrastructure network is identified by its extended service set ID (ESSID); • All APs will have been set according to this ESSID; • On power up, stations will issue probe requests and will locate the AP that they will associate with. Computer Network

    42. 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

    43. 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

    44. 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

    45. 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

    46. Source SAP Destination SAP Control Data IEEE 802.11 LLC Layer, Cont.. • Each SAP (Service Access Point) address is 7 bits. One bit is added to it to indicate whether it is order or response. • Control has three values • Information, carry user data • Supervisory, for error control and flow control • Unnumbered, other type of control packet Computer Network

    47. IEEE 802.11 LLC <-> MAC Primitives • Four types of primitives are exchanged between LLC and MAC Layer • Request: order to perform a function • Confirm: response to Request • Indication: inform an event • Response: inform completion of process began by Indication Computer Network

    48. Reception of packets • AP Buffer traffic to sleeping nodes • Sleeping nodes wake up to listen to TIM (Traffic Indication Map) in the Beacon • AP send a DTIM (Delivery TIM) followed by the data for that station. • Beacon contains, time stamp, beacon interval, DTIM period, DTIM count, sync info, TIM broadcast indicator Computer Network

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

    50. 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