ieee 802.11 wireless lan draft standard

IEEE 802.11 Wireless LAN Draft IEEE 802.11 Wireless LAN Draft StandardStandard

Professor R. A. Carrasco

IntroductionIntroduction

• IEEE 802.11 Draft 5.0 is a draft standard for Wireless Local Area Network (WLAN) communication.

• This tutorial is intended to describe the relationship between 802.11 and other LANs, and to describe some of the details of its operation.

• It is assumed that the audience is familiar with serial data communications, the use of LANs and has some knowledge of radios.

802.11 Data Frame802.11 Data Frame

Address 1FrameControl Duration Address 2 Address 3 Seq Address 4 Data

Check-sum

Bytes 2 2 6 6 6 2 6 0-2312 4

Version Type SubtypeToDS

FromDS MF

Re-try

Pwr More W O

Bits 2 2 4 1 1 1 1 1 1 1 1

Frame Control

ContentsContents

• Glossary of 802.11 Wireless Terms• Overview• 802.11 Media Access Control (MAC)• Frequency Hopping and Direct Sequence Spread

Spectrum Techniques• 802.11 Physical Layer (PHY)• Security• Performance• Inter Access Point Protocol• Implementation Support• Raytheon Implementation

Glossary of 802.11 Wireless TermsGlossary of 802.11 Wireless Terms

• Station (STA): A computer or device with a wireless network interface.

• Access Point (AP): Device used to bridge the wireless-wired boundary, or to increase distance as a wireless packet repeater.

• Ad Hoc Network: A temporary one made up of stations in mutual range.

• Infrastructure Network: One with one or more Access Points.• Channel: A radio frequency band, or Infrared, used for shared

communication.• Basic Service Set (BSS): A set of stations communicating

wirelessly on the same channel in the same area, Ad Hoc or Infrastructure.

• Extended Service Set (ESS): A set BSSs and wired LANs with Access Points that appear as a single logical BSS.

Glossary of 802.11 Wireless Terms, cont.Glossary of 802.11 Wireless Terms, cont.

• BSSID & ESSID: Data fields identifying a stations BSS & ESS.

• Clear Channel Assessment (CCA): A station function used to determine when it is OK to transmit.

• Association: A function that maps a station to an Access Point.

• MAC Service Data Unit (MSDU): Data Frame passed between user & MAC.

• MAC Protocol Data Unit (MPDU): Data Frame passed between MAC & PHY.

• PLCP Packet (PLCP_PDU): Data Packet passed from PHY to PHY over the Wireless Medium.

Overview, IEEE 802, and 802.11 Working Overview, IEEE 802, and 802.11 Working GroupGroup

• IEEE Project 802 charter:– Local & Metropolitan Area Networks– 1Mb/s to 100Mb/s and higher– 2 lower layers of 7 Layer OSI Reference Model

• IEEE 802.11 Working Group scope:– Wireless connectivity for fixed, portable and moving stations

within a limited area– Appear to higher layers (LLC) the same as existing 802

standards• Transparent support of mobility (mobility across router ports is being

address by a higher layer committee)

Overview, IEEE 802.11 CommitteeOverview, IEEE 802.11 Committee

• Committee formed in 1990– Wide attendance

• Multiple Physical Layers– Frequency Hopping Spread Spectrum– Direct Sequence Spread Spectrum– Infrared

• 2.4GHz Industrial, Scientific & Medical shared unlicensed band– 2.4 to 2.4835GHz with FCC transmitted power limits

• 2Mb/s & 1Mb/s data transfer• 50 to 200 feet radius wireless coverage• Draft 5.0 Letter Ballot passed and forwarded to Sponsor Ballot

– Published Standard anticipated 1997• Next 802.11 - November 11-14, Vancouver, BC

– Chairman - Victor Hayes, [email protected]

Overview, 802.11 ArchitectureOverview, 802.11 Architecture

STASTA

STA STA

STASTASTA STA

APAP

ESS

BSS

BSSBSS

BSS

Existing Wired LAN

Infrastructure Network

Ad Hoc Network

Ad Hoc Network

Overview, Wired vs. Wireless LANsOverview, Wired vs. Wireless LANs

• 802.3 (Ethernet) uses CSMA/CD, Carrier Sense Multiple Access with 100% Collision Detect for reliable data transfer

• 802.11 has CSMA/CA (Collision Avoidance)– Large differences in signal strengths– Collisions can only be inferred afterward

• Transmitters fail to get a response• Receivers see corrupted data through a CRC error

802.11 Media Access Control802.11 Media Access Control

• Carrier Sense: Listen before talking• Handshaking to infer collisions

– DATA-ACK packets

• Collision Avoidance– RTS-CTS-DATA-ACK to request the medium– Duration information in each packet– Random Backoff after collision is determined– Net Allocation Vector (NAV) to reserve bandwidth– Hidden Nodes use CTS duration information

802.11 Media Access Control, cont.802.11 Media Access Control, cont.

• Fragmentation– Bit Error Rate (BER) goes up with distance and

decreases the probability of successfully transmitting long frames

– MSDUs given to MAC can be broken up into smaller MPDUs given to PHY, each with a sequence number for reassembly

• Can increase range by allowing operation at higher BER• Lessens the impact of collisions

• Trade overhead for overhead of RTS-CTS• Less impact from Hidden Nodes

802.11 Media Access Control, cont802.11 Media Access Control, cont

• Beacons used convey network parameters such as hop sequence

• Probe Requests and Responses used to join a network

• Power Savings Mode– Frames stored at Access Point or Stations for

sleeping Stations– Traffic Indication Map (TIM) in Frames alerts awaking

Stations

802.11 Protocol Stack802.11 Protocol Stack

Logical Link Control

802.11Infrared

802.11FHSS

802.11DSSS

802.11aOFDM

802.11bHR-DSSS

802.11gOFDM

MACSub-layer

UpperLayers

DataLinkLayer

PhysicalLayer

Performance of IEEE802.11bPerformance of IEEE802.11b

MAC Header30 Bytes

CRC4 Bytes

trt

MPDUsec5 contt

prt

DIFS BackoffPLCP

PreamblePLCP

HeaderMPDU SIFS

PLCP Preamble

Head

er Ack14 Bytes

sec10 prt ackt

Data


• Successful transmission of a signal frame • PLCP = physical layer convergence protocol

preamble

prt Header transmission time (varies according to the bit rate used by the host

SIFS = 10 sec (Short Inter Frame Space) is the MAC acknowledgement transmission time (10 sec if the selected rate is 11Mb/sec, as the ACK length is 112 bits


• DIFS = sec5

trt = is the frame transmission time, when it transmits at 1Mb/s, the long PLCP header is used and

prt = sec192

If it uses 2, 5.5 or 11 Mb/s, then

prt = sec96 (Short PLCP header)


• For bit rates greater than 1Mb/s and the frame size of 1500 Bytes of data (MPDU of total 1534 Bytes), proportion p of the useful throughput measured above the MAC layer will be:

70.01534

1500

T

TP tr

• So, a signal host sending long frames over a 11Mb/s radio channel will have a maximum useful throughput of 7.74Mb/s


• If we neglect propagation time, the overall transmission time is composed of the transmission time and a constant overhead

ovtr ttT

Where the constant overhead

ackprprov ttSIFStDIFSt


• The overall frame transmission time experienced by a single host when competing with N – 1 other hosts has to be increased by time interval tcont that accounts for the time spent in contention procedures


So the overall transmission time

22

)(1)(

)()(

minCW

N

NPSLOTNt

NtttNT

ccont

contovtr

Where )(NPc is the propagation of collision experienced for each packet successfully acknowledged at the MAC


• Consider how the situation in which N hosts of different bit rate compete for the radio channel. N-1 hosts use the high transmission rate R = 11Mb/s and one host transmits at a degraded rate R = 5.5, 2, or 1Mb/s

T

ST

R

ST d

trd

tr or

dSWhere is the data frame length in bits


• The MAC layer ACK frame is also sent at the rate that depends on the host speed, thus we denote by

Rovt

Tovt

fT

contdR

ovf tR

StT

and the associated overhead time

Let be the overall transmission time for a “fast” host transmitting at rate R


• Similarly, let Ts be the corresponding time for a “slow” host transmitting at rate T:

contdT

ovs tT

StT

fsjam TN

TN

t )2

1(2

NtNPTTN

TU

jamcsf

ss

)()1(

We can express the channel utilization of the slow host as

where


• Study:

The UDP traffic &

TCP traffic.

Flows in IEEE 802.11 WLANs

Frequency Hopping and Direct Sequence Frequency Hopping and Direct Sequence Spread Spectrum TechniquesSpread Spectrum Techniques

• Spread Spectrum used to avoid interference from licensed and other non-licensed users, and from noise, e.g., microwave ovens

• Frequency Hopping (FHSS)– Using one of 78 hop sequences, hop to a new 1MHz channel (out of the

total of 79 channels) at least every 400milliseconds• Requires hop acquisition and synchronization• Hops away from interference

• Direct Sequence (DSSS)– Using one of 11 overlapping channels, multiply the data by an 11-bit

number to spread the 1M-symbol/sec data over 11MHz• Requires RF linearity over 11MHz• Spreading yields processing gain at receiver• Less immune to interference

802.11 Physical Layer802.11 Physical Layer

• Preamble Sync, 16-bit Start Frame Delimiter, PLCP Header including 16-bit Header CRC, MPDU, 32-bit CRC

• FHSS– 2 & 4GFSK– Data Whitening for Bias Suppression

• 32/33 bit stuffing and block inversion• 7-bit LFSR scrambler

– 80-bit Preamble Sync pattern– 32-bit Header

• DSSS– DBPSK & DQPSK– Data Scrambling using 8-bit LFSR– 128-bit Preamble Sync pattern– 48-bit Header

802.11 Physical Layer, cont.802.11 Physical Layer, cont.

• Antenna Diversity– Multipath fading a signal can inhibit reception– Multiple antennas can significantly minimize– Spacial Separation of Orthoganality– Choose Antenna during Preamble Sync pattern

• Presence of Preamble Sync pattern• Presence of energy

• RSSI - Received Signal Strength Indication

• Combination of both

• Clear Channel Assessment– Require reliable indication that channel is in use to defer transmission– Use same mechanisms as for Antenna Diversity– Use NAV information

A Fragment BurstA Fragment Burst

Frag1

ACK

RTS Frag2 Frag3

CTS ACK ACK

NAV

NAV

A

B

C

D

Time

Fragment Burst

SecuritySecurity

• Authentication: A function that determines whether a Station is allowed to participate in network communication– Open System (null authentication) & Shared

Key• WEP - Wired Equivalent Privacy

• Encryption of data

• ESSID offers casual separation of traffic

Performance, Theoretical Maximum Performance, Theoretical Maximum ThroughputThroughput

• Throughput numbers in Mbits/sec:

– Assumes 100ms beacon interval, RTS, CTS used, no collision

– Slide courtesy of Matt Fischer, AMD

1 Mbit/sec 2 Mbit/sec

MSDU size(bytes)

DS FH (400mshop time)

DS FH (400mshop time)

128 0.364 0.364 0.517 0.474

512 0.694 0.679 1.163 1.088

512(frag size = 128)

0.503 0.512 0.781 0.759

2304 0.906 0.860 1.720 1.624

Background for broadband wireless Background for broadband wireless technologiestechnologies

• UWB – Ultra Wide Band – High speed wireless personal area network

• Wi-Fi – Wireless fidelity – Wireless technology for indoor environment (WLANS)– broader range that WPANs

• WiMAX – Worldwide Interoperability for Microwave Access – Wireless Metropolitan Area Networks (WMANs)– For outdoor coverage in LOS and NLOS environment – Fixed and Mobile standards

• 3G – Third generation– Wireless Wide Area Networks (WMANs) are the broadest range

wireless networks – High speed data transmission and greater voice capacity for mobile

users

What is WiMax?What is WiMax?

• WiMAX is an IEEE802.16/ETSI HiperMAN based certificate for equipments fulfilling the interoperability requirements set by WiMAX Forum.

• WiMAX Forum comprises of industry leaders who are committed to the open interoperability of all products used for broadband wireless access.

• The technique or technology behind the standards is often referred as WiMAX

What is WiMax?What is WiMax?

• Broadband is thus a Broadband Wireless Access (BWA) technique

• WiMax offers fast broadband connections over long distances

• The interpretability of different vendor’s product is the most important factor when comparing to the other techniques.

The IEEE 802.16 StandardsThe IEEE 802.16 Standards

• The IEEE 802.16 standards family - broadband wireless wideband internet connection- wider coverage than any wired or wireless connection before

• Wireless system have the capacity to address broad geographic areas without the expensive wired infrastructure

• For example, a study made in University of Oulu state that WiMax is clearly more cost effective solution for providing broadband internet connection in Kainuu than xDSL

The IEEE 802.16 StandardsThe IEEE 802.16 Standards

• 802.16, published in April 2002- A set od air interfaces on a common MAC protocol - Addresses frequencies 10 to 66 GHz- Single carrier (SC) and only LOS

• 802.16a, published in January 2003- A completed amendment that extends the physical layer to the 2 to 11 GHz both licensed and lincensed-exempt frequencies - SC, 256 point FFT OFDM and 2048 point FFT OFDMA- LOS and NLOS

• 802.16-2004, published in July 2004- Revises and replaces 802.16, 802.16a and 802.16 REVd.- This announcements marks a significant milestone in the development of future WiMax technology - P802.16-2004/Corl published on 8.11.2005

IEEE 802.16: Broadband Wireless IEEE 802.16: Broadband Wireless MAN Standard (WiMAX)MAN Standard (WiMAX)

• An 802.16 wireless service provides a communications path between a subscriber site and a core network such as the public telephone network and the Internet. This wireless broadband access standard provides the missing link for the "last mile" connection in metropolitan area networks where DSL, Cable and other broadband access methods are not available or too expensive.

Comparison Overview of IEEE 802.16aComparison Overview of IEEE 802.16a

Parameters 802.16a (WiMax)

802.11 (WLAN)

802.15 (Bluetooth)

Frequency BandFrequency Band 2-11GHz2-11GHz 2.4GHz2.4GHz VariesVaries

RangeRange ~31miles~31miles ~100meters~100meters ~10meters~10meters

Data transfer rateData transfer rate 70 Mbps70 Mbps 11 Mbps – 55 11 Mbps – 55 MbpsMbps

20Kbps – 55 20Kbps – 55 MbpsMbps

Number of UsersNumber of Users ThousandsThousands DozensDozens DozensDozens

• IEEE 802.16 and WiMAX are designed as a complimentary technology to Wi-Fi and Bluetooth. The following table provides a quick comparison of 802.16a with to 802.11b

Protocol Structure -IEEE 802.16: Protocol Structure -IEEE 802.16: Standard (WiMAX)Standard (WiMAX)

• IEEE 802.16 Protocol Architecture has 4 layers: Convergence, IEEE 802.16 Protocol Architecture has 4 layers: Convergence, MAC, Transmission and physical, which can be map to two OSI MAC, Transmission and physical, which can be map to two OSI lowest layers: physical and data linklowest layers: physical and data link

ALOHA and Packet Broadcasting ALOHA and Packet Broadcasting ChannelChannel

Prof. R. A. Carrasco

School of Electrical, Electronic and Computer engineering2006

University of Newcastle-upon-Tyne

Packet Broadcasting Related Works by Packet Broadcasting Related Works by Metcalfe and AbransomMetcalfe and Abransom

1) 1970: N. Abramson, “The ALOHA System – Another alternative for computer communications.”, in Proc. AFIPS Press, vol 37, 1970

2) 1973: R. M. Metcalfe, “Packet communication,” MIT, Cambridge, MA, Rep. MAC TR-114, July 1973.

3) 1977: N. Abramson, “The Throughput of Packet Broadcasting Channels,” IEEE Trans. Commun., vol. COM-25, no. 10, Jan 1977

4) 1985: N. Abramson, “Development of the ALOAHANET,” IEEE Trans. Info. Theory., March 1985

IEEE Transactions on Information IEEE Transactions on Information Theory, March 1985Theory, March 1985

• Development of the ALOHANET

ALOHA ProjectALOHA Project

• Started In September 1968• Goal

– To build computer network in University of Hawaii.

– To investigate the use of radio communications as an alternative to the telephone system for computer communication.

– To determine those situations where radio communications are preferable to conventional wire communications

ProblemProblem

• Limited Resource: Channel• Intermittent operation typical of

interactive computer terminal don’t need point-to-point channels. (FDMA or TDMA)

• Spread Spectrum is not appropriate to share the channel.

ApproachApproach

• Packet Broadcasting Channels– Each user transmits its packets over the

common broadcast channel.– Key innovation of ALOHANET.

There are basically two types of ALOHA systems

--Synchronized or slotted and--Unsynchronized or unslotted

System DesignSystem Design

• 1968, they decided main approach (Packet Broadcasting) for design simplicity.

• Frequency Band: two 100KHz bandwidth channels at 407.350MHz and 413.475MHz.

• TCU (Terminal Control Unit):– Formatting of the ALOHA packets.– Retransmission protocol.– A Terminal attached TCU by means of RS232.– Half duplex mode. (too expensive memory)

HistoryHistory

• 1971: start operation in University of Hawaii.

• 1971-72: build additional TCUs.

• 1972: connect to ARPANET using satellite channel. (56kbps)

• 1973: Metcalfe’s doctorial dissertation about packet broadcasting.

• 1973: PACNET, international satellite networks. (9600 bits/s)

• 1973 ~ : Many researches about “packet broadcasting”. • 1976: slotted ALOHA.

• 1984: unslotted ALOHA in the UHF band by Motorola.

Strategic Theoretical RealitiesStrategic Theoretical Realities

• An appreciation of the basic capacity of the channels and the matching of that capacity to the information rate of the signals.

– In data network, distinguish between the average data rate and the burst data rate

– Network design: to handle different kinds of signals from different source.

• Deals with the problem of scaling for large system.

– Packet broadcasting channel is more scalable than point-to-point channel or switching.

• Theoretical analysis give good guide to design network, but the converse also is true. The operation of a real network can be a valuable guide to the

selection of theoretical problems.

Packet Switching and Packet Packet Switching and Packet BroadcastingBroadcasting

• Packet switching can provide a powerful means of sharing communication resources.

• But it employ point-to-point channels and large switches for routing.

• By use of packet broadcastingElimination of routing and switches.System simplicitySome channels are basically broadcast channel.

(satellite, ..)

• Needs unified presentation of packet broadcasting theory.

Packet Broadcasting ChannelPacket Broadcasting Channel

• Each user transmits packets over the common broadcast channel completely unsynchronized.

• Loss due to the overlap. • How many users can share a channel?

Recovery of Lost PacketsRecovery of Lost Packets

• Positive Acknowledgements.

• Transponder Packet Broadcasting.

• Carrier Sense Packet Broadcasting.

• Packet Recovery Codes

ALOHA Systems and ProtocolsALOHA Systems and Protocols

• We assume that the start time of packets/s that are transmitted is a Poisson point process

• An average rate of λ packets

• Let Tp denote the time duration of a packet

• The normalised channel traffic G is defined G=λTp

It also called the offered channel traffic

ALOHA CapacityALOHA Capacity

• Errors reduce the ALOHA Capacity– Random noise errors– Errors caused by packet overlap.

Statistical Analysis:

S: Channel ThroughputG: Channel Traffic

Throughput is maximum 1/2e when channel traffic equals 0.5.

ALOHA CapacityALOHA Capacity

• Meaning of the result– ALOHA: 9600 bits/s– Terminal: 5bits/s

– 9600 X 1/2e = about 1600 bits/s– The channel can handle the traffic of over

300 active terminals and each terminal will operate at a peak data rate 9600 bits/s

Slotted ALOHA Channel CapacitySlotted ALOHA Channel Capacity

• Each user can start his packet only at certain fixed instants.

Statistical Analysis

It increase the throughput

Mixed Data RatesMixed Data Rates

• Unslotted ALOHA: Variable Packet Lengths = Long Packet Length/ Short Packet Length

• G1 = Short Packet Traffic• G2 = Long Packet Traffic

Total channel throughput can undergo a significant decrease.

Slotted ALOHA: Variable Packet RatesSlotted ALOHA: Variable Packet Rates

• Assume ALOHA used by n users with different channel traffic.

ALOHAALOHA

• Meaning of the result– In a lightly loaded slotted ALOHA

channel, a single user can transmit data at rates above the limit 1/e. : Excess Capacity.

– Important for the network consisting of many interactive terminal users and small number of users who send large but infrequent files.

Question 1Question 1

• In a pure ALOHA system, the channel bit rate is 2400bits/s. Suppose that each terminal transmits a 100-bit message every minute on average.

i) Determine the maximum number of terminals that can use the channel

ii) Repeat (i) if slotted ALOHA is used


• An alternative derivation for the

throughput in a pure ALOHA system

may be obtained from the relation

G=S+A, where A is the average

(normalised) rate of retransmission. Show that

A=G(1-e-2G ) and then solve for S.


• Consider a pure ALOHA system that is operating with a throughput S=0.1

and packets are generated with a

Poisson arrival rate λ. Determine:

i) The value of G

ii) The average number of attempted

transmissions to send a packet.


• Consider a CSMA/CD system in which the transmission rate on the bus is 10 Mτbits/s. The bus is 2 Km and the propagation delay is 5 μs/Km.

Packets are 1000 bits long. Determine:

i) The end-to-end delay d.ii) The packet duration Tp

iii) The ratio d/Tp

iv) The maximum utilization of the bus and the maximum bit rate.

ieee 802.11 wireless lan draft standard

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