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 • 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.
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
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)
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)
** 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)
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.
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
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.
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
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)
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.
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
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.
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.
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.
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
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.
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.
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.
How does this RTS/CTS approach solve the hidden node problem? • How does this solve the exposed node problem?
The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.
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
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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.
802.11 is working on several security issues. • To get people to use the security features. • To make them easier to use.
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.
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).
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.
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.
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.”
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.
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.
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.
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: