Download IEEE 802.11 Wireless LAN Draft Standard

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
Bytes
2
Frame
Control
Bits
2
6
6
6
6
2
Duration Address 1 Address 2 Address 3 Seq Address 4
2
2
4
Version
Type
Subtype
1
1
1
To From
MF
DS DS
1
1
RePwr More W
try
O
1
1
1
0-2312
4
Data
Checksum
Frame Control
Contents
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•
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•
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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, [email protected]
Overview, 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
STA
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
STA
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
Upper
Layers
Logical Link Control
Data
Link
Layer
MAC
Sublayer
802.11
Infrared
802.11
FHSS
802.11
DSSS
802.11a
OFDM
802.11b
HR-DSSS
802.11g
OFDM
Physical
Layer
Performance of IEEE802.11b
ttr
MAC Header
30 Bytes
CRC
4 Bytes
Data
MPDU
t cont
DIFS
Backoff
10  sec
t pr
PLCP
Preamble
PLCP
Header
MPDU
SIFS
t pr
PLCP
Preamble
t ack
Header
5 sec
Ack
14 Bytes
Performance of IEEE802.11b
• Successful transmission of a signal frame
• PLCP = physical layer convergence protocol
preamble
t pr  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 = 5 sec
ttr
= is the frame transmission time, when it transmits at 1Mb/s, the
long PLCP header is used and
t pr
=
192  sec
If it uses 2, 5.5 or 11 Mb/s, then
t pr
=
96 sec
(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:
P 
Ttr
1500

 0.70
T
1534
• 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
T  ttr  tov
Where the constant overhead
tov  DIFS  t pr  SIFS  t pr  t ack
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 tcont that
accounts for the time spent in contention
procedures
Performance of IEEE802.11b
So the overall transmission time
T ( N )  ttr  tov  tcont ( N )
1  Pc ( N ) CWmin
tcont ( N )  SLOT 

2N
2
Where
Pc (N ) 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
Sd
Sd
Ttr 
or Ttr 
R
T
Where
Sd
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
t ovR
and
T
t ov
the associated overhead time
Let T f be the overall transmission time for a “fast” host transmitting at
rate R
Sd
Tf  t 
 tcont
R
R
ov
Performance of IEEE802.11b
• Similarly, let Ts be the corresponding time for a
“slow” host transmitting at rate T:
Sd
Ts  t 
 tcont
T
T
ov
We can express the channel utilization of the slow host as
Ts
Us 
( N  1)T f  Ts  Pc ( N )  t jam  N
where
t jam 
2
2
Ts  (1  )T f
N
N
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
Fragment Burst
A
B
C
D
Frag1
RTS
CTS
Frag2
ACK
NAV
NAV
Time
Frag3
ACK
ACK
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
1 Mbit/sec
DS
FH (400ms
2 Mbit/sec
MSDU size
(bytes)
128
DS
0.364
0.364
0.517
0.474
512
0.694
0.679
1.163
1.088
512
0.503
0.512
0.781
0.759
0.906
0.860
1.720
1.624
hop time)
FH (400ms
hop time)
(frag size = 128)
2304
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
• 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
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
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 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 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 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.