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Lecture 11:

Lecture 11:. 802.11 WLAN’s and Other Recent Technological Developments. This lecture provides discussion of the latest technological advances in the following areas. Wireless Local Area Networks based on the IEEE 802.11 family of standards IEEE 802.16 and 802.20

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Lecture 11:

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  1. Lecture 11: 802.11 WLAN’s and Other Recent Technological Developments

  2. This lecture provides discussion of the latest technological advances in the following areas. • Wireless Local Area Networks based on the IEEE 802.11 family of standards • IEEE 802.16 and 802.20 • Fourth generation cellular systems • Emerging technologies considered to be most promising in the further development of wireless technologies • OFDM • Ultra Wideband • Space-time processing • The goal is to give insight into areas of potential research and economic development.

  3. I. The IEEE 802 Family of Standards • The Institute of Electrical and Electronics Engineers • A technical, professional, and student society. • Publishes many journals and magazines. • Also has developed a few technical standards. • Most notably Local Area Network standards. • Ethernet (802.3) and others. • 802.11 is the working group for Wireless LAN’s

  4. Created by the IEEE LAN /MAN Standards Committee (LMSC) • Started in 1980 • Working Groups • 802.1 High Level Interface (HILI) Working Group (active) • 802.2 Logical Link Control (LLC) Working Group (hibernating) • 802.3 CSMA/CD Working Group (active) – Ethernet, standard for wired LAN’s • 802.4 Token Bus Working Group (hibernating)

  5. 802.5 Token Ring Working Group (hibernating) • 802.6 Metropolitan Area Network (MAN) Working Group (hibernating) • 802.7 Broadband Technical Adv. Group (BBTAG) (hibernating) • 802.9 Integrated Services LAN (ISLAN) Working Group (hibernating) • 802.10 Standard for Interoperable LAN Security (SILS) Working Group (hibernating)

  6. ** 802.11 Wireless LAN (WLAN) Working Group (active) • 802.12 Demand Priority Working Group (hibernating) • 802.14 Cable-TV Based Broadband Communication Network Working Group (disbanded, no publications) • 802.15 Wireless Personal Area Network (WPAN) Working Group (active) • ** 802.16 Broadband Wireless Access (BBWA) Working Group (active) • 802.17 Resilient Packet Ring (RPR) (active) • 802.18 Radio Regulatory Technical Advisory Group (active) • 802.19 Coexistence Technical Advisory Group (active) • ** 802.20 Mobile Wireless Access Working Group (active)

  7. IEEE 802.11 Wireless LAN’s • Source: Andrew S. Tanenbaum, Computer Networks, Fourth Edition, Prentice Hall, 2003. Pages 68-71, 267-270, and 292-302.

  8. II. Background • As stated before, 802.11 WLAN’s are prime competitors for providing high speed data access within buildings, including public places like airports and restaurants. • 802.11 was first standardized in 1997. • 802.11 – 1 Megabit per second (Mbps) and 2 Mbps capabilities. • In unlicensed 2.4 GHz band (ISM). • 802.11b (1999) – 11 Mbps at 2.4 GHz • 802.11a (1999) – 54 Mbps at 5.8 GHz • 802.11g (2001) – 54 Mbps at 2.4 GHz

  9. III. 802.11 Operation • Two operating modes 1. With a base station • Base station is called an access point. 2. Without a base station • Computers talk to each other directly • Ad hoc networking approach. • Defines how devices cooperate without a central controller. • Especially concerned with how to cope with packet collisions.

  10. Compatibility with Ethernet • Since Ethernet was a very popular LAN standard (IEEE 802.3) for wired environments, 802.11 was made compatible with it. • 802.11 Physical Layers • 802.11 – 3 modes – 1 to 2 Mbps in 2.4 GHz band • Infrared • FHSS • DSSS

  11. Infrared • 0.85 to 0.95 microns, 1 Mbps or 2 Mbps • FHSS • Frequency Hopped Spread Spectrum • 79 channels • Each 1 MHz wide • Dwell time less than 400 msec • DSSS • Direct Sequence Spread Spectrum • 1 Mega symbols per second (Megabaud) • 1 Mbps is one bit per symbol using Differential BPSK • 2 Mbps is two bits per symbol using Differential QPSK • 11 chips per symbol (11 Mega chips per second)

  12. Uses a bandwidth of 22 MHz per channel • No security from DSSS, since all stations use the same chip sequence • Allows 11 frequency channels to be used in the 2.4 GHz ISM band. • Channels are spaced 5 MHz apart and overlap • Overlapping coverage areas should use different channels.

  13. 802.11a – 54 Mbps in the 5.8 GHz band • Uses OFDM (Orthogonal Frequency Division Multiplexing) • More on OFDM later in the lecture • 48 frequencies each at 250,000 symbols per second

  14. 802.11b – up to 11 Mbps in the 2.4 GHz band • Not a follow-up to 802.11a • The 802.11b standard was approved first and got to market first. • 1, 2, 5.5, and 11 Mbps • Rate may be adapted to achieve best performance under current noise and load. • In practice, 11 Mbps is nearly always used.

  15. Uses HR-DSSS (High rate DSSS) • 1 and 2 Mbps rates use the same technique as 802.11 • 5.5 and 11 Mbps run at 1.375 Mbaud with 4 or 8 bits per symbol • Better range than 802.11a • About 7 times larger range • Named “Wi-Fi” by the Wireless Ethernet Compatibility Alliance (www.wi-fi.com) • Goal is to promote interoperability between vendors’ products.

  16. 802.11g – 54 Mbps in the 2.4 GHz band • Two upgrade options from 802.11b. • First upgrade: Add a 256-state convolutional code to 802.11b CDMA. • Creates rates of 22 and 33 Mbps. • Higher rates are possible because of the coding gain • Second upgrade. Uses OFDM like 802.11a. • Uses direct sequence SSM for the header, then OFDM for the payload. • Payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.

  17. Problems Unique to Wireless LAN’s • Traditional Ethernet LAN’s • Listen until the channel is not busy • Send a message • If it collides with another message, wait a random time then retry. • Called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • Assumes all stations can hear all the other transmissions • Assumes that a collision can be detected. • But not all collisions can be detected when using wireless. Additional challenges in Wireless LAN’s

  18. Problem: All users may not be able to hear each other.

  19. Problem (a): Hidden node problem • C is sending to B. • A cannot hear C and thinks it could also transmit to B. • A’s and C’s packets will collide at B.

  20. Problem (b): Exposed node problem. • A is transmitting to station X. • If B listens, it will think the radio channel is busy. • So it will falsely conclude it cannot send to C. • But C would hear no interference if B sent a packet to it. • C would not also hear the one from A, since C is out of range from A. • So, B could have transmitted but will not. • Note: B can send to C, but cannot receive from C.

  21. Solution: RTS and CTS • Potential senders send a Request to Send (RTS). • Tells how long of a message it wishes to send. • Potential receiver sends a Clear to Send (CTS) in response. • Also tells how long of a message will be sent. • Assumption: If I hear something from Y, I am in Y’s range and Y is in mine.

  22. How does this RTS/CTS approach solve the hidden node problem? • How does this solve the exposed node problem?

  23. The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.

  24. C is within range of A but not within range of B. • It hears the RTS from A but not the CTS from B. • As long as it does not interfere with the CTS, it is free to transmit while the data frame is being sent. • solve the exposed node problem

  25. D is within range of B but not A. • It does not hear the RTS but does hear the CTS. • Hearing the CTS tips it off that it is close to a station that is about to receive a frame, so it defers sending anything until that frame is expected to be finished. • solve the hidden node problem • E hears both control messages and, like D, must be silent until the data frame is complete.

  26. Notes: • This assumes all stations have the same range. • Collisions still might occur between RTS messages. • For example, B and C could both send RTS frames to A at the same time, These will collide and be lost. • The RTS/CTS procedure slows down communications somewhat. • Summary: • Hear a CTS, don’t send. • Only hear an RTS, assume okay

  27. This approach is called CSMA/CA • Carrier Sense Multiple Access (CSMA) • Stations listen to the channel • Collision Avoidance (CA) • RTS/CTS are used to prevent collisions of data packets

  28. 802.11 Distributed Coordination Function (DCF) • Used when there are no access points (ad hoc mode) • Uses CSMA/CA • The figure below shows the timing for all stations. • When A is sending a packet to B.

  29. A sends RTS, waits for CTS, sends data, then waits for Acknowledgement (ACK). • B sends CTS and then ACK when done. • C hears the RTS • Learns the intended length of the transmission • Creates an internal Network Allocation Vector (NAV) from this information. • NAV tells C how long to wait until it should try to send its own RTS. • Note: If C does not also hear the CTS, it can abandon its NAV. • D hears the CTS (not the RTS) • D also uses a NAV from info in the CTS.

  30. A fragment burst

  31. 802.11 Point Coordination Function (PCF) • The access point polls other stations to give those stations a chance to send something. • No collisions occur, so CSMA/CA is not needed here. • DCF and PCF are used simultaneously. • This is done by coordinating the amount of time between successive messages. • Different amounts of dead time are required between messages. • Messages in 802.11 are called frames.

  32. Interframe spacing in 802.11

  33. Message waiting times • Short InterFrame Spacing (SIFS) • To next control message (ACK, CTS, etc.) • Or next fragment in a block of fragments all being sent in succession. • Only one station is expected to send one of these messages. • PCF InterFrame Spacing (PIFS) • Now the base station (access point) is allowed to try to send a polling message.

  34. DCF InterFrame Spacing (DIFS) • Now any station can send an RTS to attempt to grab the channel. • If a collision of RTS occurs, stations wait a random amount of time and try again. • Extended InterFrame Spacing (EIFS) • Used for a station to tell that it has received a bad or unknown frame.

  35. PCF will not always transmit when it has a chance. • This would starve DCF. • A time interval is defined. • In the first part of the superframe, the AP polls in a round-robin fashion all stations configured for polling. • The AP idles for the remainder of the superframe. • Which allows DCF.

  36. The 802.11 data frame.

  37. 802.11 Services • Several services are provided by 802.11 to perform necessary functions. • Distribution Services – Related to stations connecting with base stations. • Association - to connect to base stations. • Disassociation - to disassociate with base stations • Reassociation - change a preferred base station, without losing data in the handover. • Distribution - determine how to route frames sent to the base station. • Integration - handles translation from the 802.11 format into another format required by a destination network.

  38. Station Services - For activity during communications • Authentication - stations identify themselves as valid before being permitted to send data. • Deauthentication - to make sure a user that leaves can no longer use the network. • Privacy - encryption capabilities to keep information sent over a wireless LAN confidential. • Data delivery - ways to transmit and receive data as has been discussed already.

  39. 802.11 is working on several security issues. • To get people to use the security features. • To make them easier to use.

  40. V. IEEE 802.16 • IEEE 802.16 - Broadband Wireless Access (BBWA) Working Group • Called the IEEE 802.16 WirelessMAN Standard • Published April 2002. • Designed as an alternative to fiber, cable modems, or DSL. • Much quicker to deploy and potentially less costly. • Consists of point-to-multipoint connections between end locations and base stations located on buildings or poles.

  41. Operates in various frequencies in the range of 10 to 66 GHz. • Uses line-of-sight connections. What are the benefits and drawbacks of using line-of-sight? • Antennas would need to be installed on the outside of a building. • The higher the frequency, the more difficult to penetrate through walls, vegetation, etc. • Some non line-of-sight is being considered in an amendment for 2-11 GHz (802.16a).

  42. Range and Data Rate • Range: Up to 31 miles. • Data Rate: 70 Mbps. • Quality of Service • The standard defines different handling of packets, depending on whether they are voice/video or data. • Modulation is Adaptive • Adjusted almost instantaneously for optimal data transfer. • Uses Reed-Solomon block coded FEC. • In combination with QPSK, 16-QAM, or 64-QAM. • Also uses a convolutional code to protect critical data, such as frame control and initial accesses.

  43. VI. IEEE 802.20 • IEEE 802.20 - Mobile Broadband Wireless Access (MBWA) Working Group • Goals • Packet based air interface • Optimized for the transport of Internet Protocol based services. • Affordable, ubiquitous, always-on and interoperable multi-vendor mobile broadband wireless access networks.

  44. Scope • Licensed bands below 3.5 GHz. • Greater than 1 Mbps. • Vehicle mobility up to 250 km/hr. • Seeks “spectral efficiencies, sustained user data rates and numbers of active users that are all significantly higher than achieved by existing mobile systems.”

  45. Flash OFDM • A very interesting technology for possible use in MBWA. • By Flarion (http://www.flarion.com/products/flash_ofdm.asp). • Claims: • 3 times greater physical layer capacity by using OFDM instead of CDMA. • Dedicated bandwidth to each flow for QoS • Adaptive error control coding.

  46. Main ideas with MBWA • Design networks for data first. • Support voice as a data service. • Protect voice quality using special packet prioritization mechanisms. • Can achieve substantial increases in spectral efficiency.

  47. VII. OFDM • Orthogonal Frequency Division Multiplexing • Enabled by new capabilities for hardware-based Digital Signal Processing. • Instead of transmitting one signal in a frequency band, transmit many signals at different carriers.

  48. Each one is narrower bandwidth • With lower bit rate. • Small frequency spacing between carriers. • And overlap is allowed because the carriers are chosen carefully (to be orthogonal). • Conceptual picture:

  49. Specification from the 802.11a standard:

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