IEEE 802.11 Wireless LAN Draft Standard

1
IEEE 802.11 Wireless LAN Draft Standard

• Professor R. A. Carrasco

2
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.

3
802.11 Data Frame
Bytes
2
2
6
6
4
6
2
6
0-2312
Address 1
Frame Control
Check- sum
Seq
Duration
Address 2
Address 3
Address 4
Data
Bits
2
2
4
1
1
1
1
1
1
1
1
To DS
From DS
Re- try
Version
Type
Subtype
Pwr
More
Frame Control
MF
W
O
4
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

5
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.

6
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.

7
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)

8
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_at_ieee.org

9
Overview, 802.11 Architecture
ESS
Existing Wired LAN
AP
AP
STA
STA
STA
STA
BSS
BSS
Infrastructure Network
STA
STA
Ad Hoc Network
Ad Hoc Network
BSS
BSS
STA
STA
10
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

11
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

12
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

13
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

14
802.11 Protocol Stack
Upper Layers
Logical Link Control
Data Link Layer
MAC Sub- layer
802.11 Infrared
802.11 FHSS
802.11 DSSS
802.11a OFDM
802.11b HR-DSSS
802.11g OFDM
Physical Layer
15
Performance of IEEE802.11b
MAC Header 30 Bytes
CRC 4 Bytes
Data
MPDU
DIFS
Backoff
PLCP Preamble
PLCP Header
SIFS
PLCP Preamble
Ack 14 Bytes
Header
MPDU
16
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
17
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)
18
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

19
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
20
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

21
Performance of IEEE802.11b

• So the overall transmission time

Where
is the propagation of collision experienced for
each packet successfully acknowledged at the MAC
22
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
23
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
24
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
25
Performance of IEEE802.11b

• Study
• The UDP traffic
• TCP traffic.
• Flows in IEEE 802.11 WLANs

26
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

27
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

28
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

29
A Fragment Burst
Fragment Burst
Frag1
RTS
Frag2
Frag3
A
ACK
CTS
ACK
ACK
B
NAV
C
NAV
D
Time
30
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

31
Performance, Theoretical Maximum Throughput

• Throughput numbers in Mbits/sec
• Assumes 100ms beacon interval, RTS, CTS used, no
  collision
• Slide courtesy of Matt Fischer, AMD

32
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

33
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

34
What is WiMax?

• Broadband is thus a Broadband Wireless Access
  (BWA) technique
• WiMax offers fast broadband connections over long
  distances
• The interpretability of different vendors
  product is the most important factor when
  comparing to the other techniques.

35
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

36
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

37
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

38
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.

39
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
40
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

41
ALOHA and Packet Broadcasting Channel

• Prof. R. A. Carrasco
• School of Electrical, Electronic and Computer
  engineering2006University of Newcastle-upon-Tyne

42
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

43
IEEE Transactions on Information Theory, March
1985

• Development of the ALOHANET

44
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

45
Problem

• Limited Resource Channel
• Intermittent operation typical of interactive
  computer terminal dont need point-to-point
  channels. (FDMA or TDMA)
• Spread Spectrum is not appropriate to share the
  channel.

46
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

47
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)

48
History

• 1971 start operation in University of Hawaii.
• 1971-72 build additional TCUs.
• 1972 connect to ARPANET using satellite channel.
  (56kbps)
• 1973 Metcalfes 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.

49
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.

50
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.

51
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?

52
Recovery of Lost Packets

• Positive Acknowledgements.
• Transponder Packet Broadcasting.
• Carrier Sense Packet Broadcasting.
• Packet Recovery Codes

53
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 ? packets
• Let Tp denote the time duration of a packet
• The normalised channel traffic G is defined
• G?Tp
• It also called the offered channel traffic

54
ALOHA 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.
55
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

56
Slotted ALOHA Channel Capacity

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

Statistical Analysis It increase the throughput
57
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.
58
Slotted ALOHA Variable Packet Rates

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

59
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.

60
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

61
Question 2

• An alternative derivation for the
• throughput in a pure ALOHA system
• may be obtained from the relation
• GSA, where A is the average
• (normalised) rate of retransmission. Show that
• AG(1-e-2G ) and then solve for S.

62
Question 3

• Consider a pure ALOHA system that is operating
  with a throughput S0.1
• and packets are generated with a
• Poisson arrival rate ?. Determine
• The value of G
• The average number of attempted
• transmissions to send a packet.

63
Question 4

• Consider a CSMA/CD system in which the
• transmission rate on the bus is 10 Mtbits/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.