IEEE 802.11 Wireless LAN Draft

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IEEE 802.11 Wireless LAN Draft
Standard






Professor R. A. Carrasco

Introduction


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 Frame

Address 1

Frame

Control

Duration

Address 2

Address 3

Seq

Address 4

Data

Check
-

sum

Bytes

2

2

6

6

6

2

6

0
-
2312

4

Version

Type

Subtype

To

DS

From

DS

MF

Re
-

try

Pwr

More

W

O

Bits

2

2

4

1

1

1

1

1

1

1

1

Frame Control

Contents


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 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.


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
Group


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 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, v.hayes@ieee.org


Overview, 802.11 Architecture

STA

STA

STA

STA

STA

STA

STA

STA

AP

AP

ESS

BSS

BSS

BSS

BSS

Existing
Wired LAN

Infrastructure
Network

Ad Hoc
Network

Ad Hoc
Network

Overview, 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 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.


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, 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 Stack

Logical Link Control

802.11

Infrared

802.11

FHSS

802.11

DSSS

802.11a

OFDM

802.11b

HR
-
DSSS

802.11g

OFDM

MAC

Sub
-

layer

Upper

Layers

Data

Link

Layer

Physical

Layer

Performance of IEEE802.11b

MAC Header

30 Bytes

CRC

4 Bytes

MPDU

DIFS

Backoff

PLCP

Preamble

PLCP

Header

MPDU

SIFS

PLCP

Preamble

Header

Ack

14 Bytes

Data

Performance of IEEE802.11b


Successful transmission of a signal frame


PLCP = physical layer convergence protocol
preamble




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

Performance of IEEE802.11b


DIFS =

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

=

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

=

(Short PLCP header)

Performance of IEEE802.11b


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:




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



Performance of IEEE802.11b


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

Where the constant overhead

Performance of IEEE802.11b


The overall frame transmission time experienced
by a single host when competing with N


1 other
hosts has to be increased by time interval
t
cont

that
accounts for the time spent in contention
procedures

Performance of IEEE802.11b

So the overall transmission time


Where

is the propagation of collision experienced for each
packet successfully acknowledged at the MAC

Performance of IEEE802.11b


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

Where

is the data frame length in bits

Performance of IEEE802.11b


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

and

the associated overhead time

Let

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

Performance of IEEE802.11b


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

We can express the channel utilization of the slow host as

where

Performance of IEEE802.11b


Study:


The UDP traffic &


TCP traffic.




Flows in IEEE 802.11 WLANs

Frequency Hopping and Direct Sequence
Spread 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 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.


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 Burst

Frag1

ACK

RTS

Frag2

Frag3

CTS

ACK

ACK

NAV

NAV

A

B

C

D

Time

Fragment Burst

Security


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
Throughput


Throughput numbers in Mbits/sec:


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


Slide courtesy of Matt Fischer, AMD

Background for broadband wireless
technologies


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?


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?


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 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 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 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
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 802.16a

Parameters

802.16a
(WiMax)

802.11
(WLAN)

802.15
(Bluetooth)

Frequency Band

2
-
11GHz

2.4GHz

Varies

Range

~31miles

~100meters

~10meters


Data transfer rate

70 Mbps

11 Mbps


55
Mbps

20Kbps


55
Mbps


Number of Users

Thousands

Dozens

Dozens




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:
Standard (WiMAX)


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

ALOHA and Packet Broadcasting
Channel





Prof. R. A. Carrasco


School of Electrical, Electronic and Computer engineering

2006

University of Newcastle
-
upon
-
Tyne

Packet Broadcasting Related Works by
Metcalfe 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
Theory, March 1985





Development of the ALOHANET

ALOHA 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

Problem


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.


Approach


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 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)


History


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 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
Broadcasting


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 broadcasting


Elimination of routing and switches.


System simplicity


Some channels are basically broadcast channel. (satellite, ..)



Needs unified presentation of packet broadcasting theory.


Packet 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 Packets


Positive Acknowledgements.



Transponder Packet Broadcasting.



Carrier Sense Packet Broadcasting.



Packet Recovery Codes


ALOHA 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 T
p

denote the time duration of a packet



The normalised channel traffic G is defined


G=
λ
T
p


It also called the offered channel traffic


ALOHA Capacity


Errors reduce the ALOHA Capacity


Random noise errors


Errors caused by packet overlap.



Statistical Analysis:



S: Channel Throughput

G: Channel Traffic

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


ALOHA 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 Capacity


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



Statistical Analysis


It increase the throughput

Mixed 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 Rates


Assume ALOHA used by
n

users with different channel traffic.

ALOHA


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 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

Question 2


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.

Question 3


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.

Question 4


Consider a CSMA/CD system in which the


transmission rate on the bus is 10 M
τ
bits/s. The


bus is 2 Km and the propagation delay is 5
μ
s/Km.


Packets are 1000 bits long.


Determine:


i) The end
-
to
-
end delay

d
.

ii) The packet duration T
p

iii) The ratio

d
/T
p

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


rate.