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IEEE802.11

IEEE802.11. What is IEEE 802.11?. Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE Intended for home or office use (primarily indoor) 802.11 standard describes the MAC layer, while other substandards (802.11a, 802.11b) describe the physical layer

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IEEE802.11

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  1. IEEE802.11

  2. What is IEEE 802.11? • Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE • Intended for home or office use (primarily indoor) • 802.11 standard describes the MAC layer, while other substandards (802.11a, 802.11b) describe the physical layer • Wireless version of the Ethernet (802.3) standard

  3. Wi-Fi Standard (802.11x) • Mission: promote 802.11 interoperability as the global wireless LAN standard • Wi-Fi Board members include AMD, Apple, Cisco, Compaq, Dell, Epson, Ericsson, Fujistu, Gateway, HP, IBM, Intel, Microsoft, NEC, Nokia, Nortel, Philips, Samsung, Sharp, Sony, TDK, Toshiba,

  4. 802.11b LAN Configuration Access Point Device

  5. Example 802.11b Wireless LAN Equipment

  6. Intended Use • Wireless Internet access inside hotel lobbies, conference rooms, etc. • Wireless with your Latte? • Wireless home networking • Wireless at the Airport

  7. Wi-Fi Market in the News • Wireless LAN equipment market • $969 Million in 2000 to estimated $4.5 Billion in 2006 • In 2001: • Microsoft adds 802.11 in Windows XP • Major hotel chains install Wi-Fi Internet access • 462 Starbucks stores offer wireless Internet • Microsoft joins WECA board (the 802.11 alliance) • Intel Joins WECA board • Most PC/Laptop manufacturers offer Wi-Fi

  8. 802.11b Interoperability • The Wireless Ethernet Compatibility Alliance (WECA) certification provides a standard for wireless technology, ensuring interoperability of products from different vendors. WECA Certification Mark

  9. Competing Short-Range Wireless Technologies • Short-range wireless solutions: • Bluetooth • 802.11 (Wi-Fi) • HomeRF (not as popular) • Who will prevail? • 802.11 more suitable for wireless LANs (office, hotel, airport,…) • Bluetooth is designed for personal area networks – smart appliances, printers, scanners, etc.

  10. Commercial WirelessData Communications 802.11a Technology Near Future Digital Cellular Bluetooth Technology Home RF Proprietary Systems 802.11b Wireless LANs Cellular Present 11M 128K 721K 54M 19K 384K Data Rate Bits per second

  11. IEEE 802.11b Technology • Standard adopted by Institute of Electrical and Electronics Engineers (IEEE) in September 1999. • The standard is believed to be very good, will be widely accepted and will allow hardware prices to decrease. • Developed by consortium of major companies with focus on interoperability. • Optimized for wireless LANs. • Uses radio frequency signals in unlicensed 2.4GHz band to send and receive data. • Uses Direct Sequence Spread Spectrum (DSSS) RF method. • Equipment dynamically selects lower data rates as RF signal quality decreases: 11, 5.5, 2, 1 Mbits. • Allows roaming among radio access points.

  12. IEEE 802.11b TechnologyMore Specifically • 802.11b defines how the RF channel is used, allowing multiple devices to communicate on the channel as if it were a wire. • This in itself can form a standalone network as if wired devices were connected to a hub. • The Access Point sits on both the wireless network segment (space) and the wired segment, acting as a bridge from the wireless to the wired segments. • A bridge forwards data packets from one side to the other at the MAC layer.

  13. Flavors of 802.11x • 802.11 (1 Mbps) • Older standard • 802.11b (11 Mbps) • Current technology • 802.11g (22+ Mbps) • Expected future standard

  14. 802.11 Under the Hood(Wireless Ethernet) • Why can’t we use regular Ethernet for wireless? • Ethernet: A sees B, B sees C,  A sees C • Wireless: Hidden node problem A sees B, B sees C, yet A does not see C C A B

  15. 802.11 Under the Hood(Wireless Ethernet) • Why can’t we use regular Ethernet for wireless? • Ethernet: B sees C, C sees D  B & C can’t send together • Wireless: B can send to A while C sends to D B C D A

  16. 802.11 Under the HoodThe Protocol • Sender A sends Request-to-Send (RTS) • Receiver B sends Clear-to-Send (CTS) • Nodes who hear CTS cannot transmit concurrently with A (red region) • Nodes who hear RTS but not CTS can transmit (green region) • Sender A sends data frame • Receiver B sends ACK • Nodes who hear the ACK can now transmit CTS RTS B A

  17. 802.11 Collision Resolution • Two senders might send RTS at the same time • Collision will occur corrupting the data • No CTS will follow • Senders will time-out waiting for CTS and retry with exponential backoff

  18. More Wireless Challenges • Problem: mobility • Nodes can move and get “out of touch” • How to deal with intermittent connectivity? • Solution: hierarchical structure • A set of access points hooks into a wired distribution system • Access points cover the target area • Roaming nodes connect to nearest access point at any given time

  19. Hierarchical Structure: • Communication between roaming nodes is via the distribution system Distribution System A E F AP1 AP3 AP2 C D B

  20. Choosing the Access PointActive Scanning • Roaming node periodically sends probe frame • All APs within range reply with a probe response frame • Roaming node chooses best AP and replies to it with association request • AP acknowledges request with association response. AP notifies previous AP (if any) of the established association (handover).

  21. Choosing the Access PointPassive Scanning • APs advertise themselves periodically by sending beacon frames • Roaming nodes can associate themselves with an AP by sending the association request • AP acknowledges request as before with an association response

  22. Frame Format Immediate Sender (AP3) Intermediate Destination (AP1) Source (A) Ultimate Destination (E) Control Duration Addr1 Addr2 Addr3 Control Addr4 Data CRC Distribution System Frame Type (RTS,CTS,…) ToDS FromDS A E F AP1 AP3 AP2 C D B

  23. Physical Properties • 802.11 • Frequency hopping • Direct sequence • Diffused infrared

  24. Range from Access Point to Computer 100Feet Access Point 11Mb 5.5Mb Computers 11Mbits 5.5Mb 2Mb 1Mb 0Mb Range = 0 to 700 feet

  25. Electrical Engineering Building 3rd 2nd 1st Basement

  26. Network Setup • Basic Network Setup is Cellular • Mobile Terminals (MT) connect with Access Points (AP) • Standard also supports ad-hoc networking where MT’s talk directly to MT’s

  27. IEEE 802.11 Physical Layers

  28. Media Access Control- Ethernet • CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • If media is sensed idle, transmit • If media is sensed busy, wait until idle and then transmit immediately • Collisions can occur if more than one user transmits at the same time • If a collision is detected, stop transmitting. • Reschedule transmission according to exponential backoff

  29. Media Access Control (802.11) • Would like to use CSMA • Nice for bursty traffic • Make for seamless replacement of wired LANs with wireless LANS • Use CSMA, but can’t use CD • PT/PR ratio is too high • Don’t want to waste energy on mobiles • Use Collision Avoidance instead • Don’t always start transmitting immediately after someone else

  30. CSMA/CA Details • SIFS (Short Interframe Space) • DIFS (Distributed Interframe Space) • Scenario: • B and C want to transmit, but A currently has control of medium • B randomly selects 7 slots of backoff, C selects 4 slots • C transmits first, then B

  31. What is HIPERLAN/2? • European standard developed by ETSI/BRAN (European Telecommunications Standards Institute/Broadband Radio Access Networks) • Physical Layer is very similar to 802.11a (OFDM operating in the 5 GHz spectrum) • Standard based on wireless ATM (Asynchronous Transfer Mode)

  32. HIPERLAN/2 MAC BCH – Miscellaneous header FCH – Details how the DL and UL phases will be allocated ACH – Feedback on which resource requests were received RCH – Random access resource request

  33. Performance Comparison

  34. Conclusions • IEEE 802.11 is a widely accepted standard in the United States for wireless LANs • Primarily a “cellular” random access scheme with provisions for ad hoc networking and contention free access • 802.11b products are available now, but better to wait for 802.11a products later this year • HIPERLAN/2 is being pushed in Europe • Wireless ATM solution for real-time traffic • Standard reflects the network topology • There is an effort to agree on one world-wide standard, keep your fingers crossed

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