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Title: In order of appearance:


1
Introduction to 802.11
83180 - WLAN
  • In order of appearance
  • 195 291 Lukasz Kondrad
  • 195 211 Tomasz Augustynowicz
  • 190 902 Jaroslaw Lacki
  • 195 273 Jakub Jakubiak

2
83180 - WLAN
  • 802.11 Overview Architecture
  • 802.11 MAC
  • 802.11e QoS
  • Power Saving methods

3
IEEE 802.11 Overview and Architecture
83180 - WLAN
802.11 Overview and Architecture
  • 195 291 Lukasz Kondrad

4
Agenda
83180 - WLAN
802.11 Overview and Architecture
  • Overview
  • Scope
  • Purpose
  • Architecture
  • General Architecture of IEEE 802.11
  • Components
  • Services

5
Introduction
83180 - WLAN
802.11 Overview and Architecture
  • Although the IEEE 802.11 standard have been
    around since 1997, work continues to make it more
    adaptable to the demand for higher data rates and
    true wireless flexibility.
  • Wireless LANs satisfy mobility, relocation, and
    ad hoc networking requirements and provide a way
    to cover locations that are difficult to wire
    (ex. manufacturing plants, stock exchange trading
    floors, historical buildings.)

6
83180 - WLAN
802.11 Overview and Architecture
  • The scope of 802.11 standard is to develop
  • Medium Access Control (MAC)
  • Physical Layer (PHY)

7
The purpose of 802.11 standard
83180 - WLAN
802.11 Overview and Architecture
  • Describes the functions and services required by
    an IEEE 802.11 compliant device to operate within
    ad hoc and infrastructure networks as well as the
    aspects of station mobility (transition) within
    those networks.
  • Defines the MAC procedures to support the
    asynchronous MAC service data unit (MSDU)
    delivery services.
  • Defines several PHY signaling techniques and
    interface functions that are controlled by the
    IEEE 802.11 MAC.

8
The purpose of 802.11 standard
83180 - WLAN
802.11 Overview and Architecture
  • Permits the operation of an IEEE 802.11
    conformant device within a wireless local area
    network (LAN) that may coexist with multiple
    overlapping IEEE 802.11 wireless LANs.
  • Describes the requirements and procedures to
    provide privacy of user information being
    transferred over the wireless medium (WM) and
    authentication of IEEE 802.11 conformant devices.

9
General Architecture of IEEE 802.11
83180 - WLAN
802.11 Overview and Architecture
  • Four major parts
  • Physical (PHY)
  • a physical convergence layer protocol (PLCP)
    sublayer
  • a physical medium-dependent (PMD) sublayer
  • the MAC of the data link layer
  • carrier-sense multiple access with collision
    avoidance (CSMA/CA)
  • IEEE 802.1X
  • Upper layer authentication protocol

10
General Architecture of IEEE 802.11
83180 - WLAN
802.11 Overview and Architecture
11
Physical layer (PHY)
83180 - WLAN
802.11 Overview and Architecture
  • Six physical layer are defined
  • 802.11 - FHSS, DSSS
  • 1, 2 Mb/s, 2.4 GHz
  • 802.11b - Complementary Code Keying (CCK)
    modulation
  • 11 Mb/s, 2.4 GHz
  • optional mode - packet binary convolutional
    coding (PBCC)
  • 802.11a - OFDM
  • up to 54 Mb/s, 5 GHz
  • 802.11h adds Dynamic Frequency Selection (DFS)
    and Transmitter Power Control (TPC)
  • 802.11g - OFDM and DSSS
  • up to 54 Mb/s, 2.4 GHZ
  • Infrared

12
802.11 alphabet
83180 - WLAN
802.11 Overview and Architecture
13
Components
83180 - WLAN
802.11 Overview and Architecture
  • STA - Station
  • AP - Access Point
  • BSS - Basic Service Set
  • IBSS - Independent BSS
  • ESS - Extended Service Set
  • A set of infrastructure BSSs.
  • Connection of APs
  • Tracking of mobility
  • DS - Distribution System
  • AP communicates with another

14
IEEE 802 .11 Terminology
83180 - WLAN
802.11 Overview and Architecture
  • Access-Point (AP)
  • Device that contains IEEE 802.11 conformant MAC
    and PHY interface to the wireless medium, and
    provide access to a distribution system for
    associated stations
  • Most often infra-structure products that connect
    to wired backbones
  • Stations select an Access-Point and associate
    with it
  • Access-Points
  • Support roaming
  • Provide time synchronization functions
    (beaconing)
  • Provide Power Management support
  • Traffic typically flows through Access-Point
  • in IBSS direct Station-to-Station communication
    takes place

15
IEEE 802 .11 Terminology
83180 - WLAN
802.11 Overview and Architecture
  • Station (STA)
  • Device that contains IEEE 802.11 conformant MAC
    and PHY interface to the wireless medium, but
    does not provide access to a distribution system
  • Most often end-stations available in terminals
    (work-stations, laptops etc.)

16
IEEE 802 .11 Terminology
83180 - WLAN
802.11 Overview and Architecture
  • Basic Service Set (BSS)
  • A set of stations controlled by a single
    Coordination Function (the logical function
    that determines when a station can transmit or
    receive)
  • A BSS can have an Access-Point (both in
    standalone networks and in building-wide
    configurations), or can run without and
    Access-Point (in standalone networks only)
  • Diameter of the cell is app. twice the
    coverage-distance between two wireless stations

17
Basic Service Set (BSS)
83180 - WLAN
802.11 Overview and Architecture
BSS
18
IEEE 802 .11 Terminology
83180 - WLAN
802.11 Overview and Architecture
  • Independent Basic Service Set (IBSS)
  • A Basic Service Set (BSS) which forms a
    self-contained network in which no access to a
    Distribution System is available
  • A BSS without an Access-Point
  • One of the stations in the IBSS can be configured
    to initiate the network and assume the
    Coordination Function
  • Diameter of the cell determined by coverage
    distance between two wireless stations

19
83180 - WLAN
802.11 Overview and Architecture
Independent Basic Service Set (IBSS)
IBSS
20
IEEE 802 .11 Terminology
83180 - WLAN
802.11 Overview and Architecture
  • Extended Service Set (ESS)
  • A set of one or more Basic Service Sets
    interconnected by a Distribution System (DS)
  • Traffic always flows via Access-Point
  • Diameter of the cell is double the coverage
    distance between two wireless stations
  • Distribution System (DS)
  • A system to interconnect a set of Basic Service
    Sets
  • Integrated A single Access-Point in a standalone
    network
  • Wired Using cable to interconnect the
    Access-Points
  • Wireless Using wireless to interconnect the
    Access-Points

21
Extended Service Set (ESS) single BSS (with
integrated DS)
83180 - WLAN
802.11 Overview and Architecture
BSS
22
Extended Service Set (ESS) BSSs with wired
Distribution System (DS)
83180 - WLAN
802.11 Overview and Architecture
BSS
Distribution System
BSS
23
83180 - WLAN
802.11 Overview and Architecture
Extended Service Set (ESS) BSSs and wireless
Distribution System (DS)
BSS
Distribution System
BSS
24
Services
83180 - WLAN
802.11 Overview and Architecture
  • Station services
  • authentication,
  • de-authentication,
  • key distribution
  • data-authentication
  • replay protection
  • privacy,
  • delivery of data
  • Distribution Services ( A thin layer between MAC
    and LLC sublayer)
  • association
  • disassociation
  • reassociation
  • distribution
  • Integration

25
83180 - WLAN
802.11 Overview and Architecture
26
802.11 Medium Access Control (MAC) layer
83180 - WLAN
802.11 Medium Access Control (MAC)
  • 195211 Tomasz Augustynowicz

27
Agenda
83180 - WLAN
802.11 Medium Access Control (MAC)
  • MAC in general
  • MAC in Ethernet
  • Requirements for MAC in WLAN
  • 802.11 MAC

28
What is MAC?
83180 - WLAN
802.11 Medium Acceess Control (MAC)
  • Medium Access Control protocol is a one of
    sublayers of Data Link layer in OSI model.
  • The MAC is a set of rules to determine how to
    access the medium and data link components. The
    MAC rides on every transmission of user data into
    the air. It provides the core framing operations
    and the interaction with a wired network
    backbone.
  • MAC purpose
  • Coordinates and shares use of bandwidth
  • Timing synchronization
  • User datagram transfer function
  • MAC layer management functions

29
MAC mechanism in Ethernet
83180 - WLAN
802.11 Medium Access Control (MAC)
  • In 802.3 (Ethernet) is used CSMA/CD mechanism
  • (Carrier Sense Multiple Access with Collision
    Detection)
  • Short description
  • If medium is idle, transmit, otherwise next step
  • If medium is busy, continue to listen until the
    channel is idle, then transmit immediately
  • If a collision is detected during transmission,
    transmit a brief jamming signal (32 bits) to
    assure that the stations know that there has been
    a collision and than stop transmission
  • After transmitting the jamming signal, wait a
    random amount of time, than attempt to transmit
    again, maximum number of attempts is 16

30
MAC mechanism in Ethernet (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
31
Requirements of MAC for WLAN
83180 - WLAN
802.11 Medium Access Control (MAC)
  • The MAC protocol must be independent of the
    underlying physical layer
  • The access mechanism must be efficient for both
    bursty and periodic traffic
  • The MAC must handle mobile users
  • Why CSMA/CD cant be used in wireless LAN?
  • Require the implementation of a full duplex
    radio that would increase the price
    significantly.
  • All the stations may not hear each other on a
    wireless environment (which is the basic
    assumption of the CD scheme).

32
MAC mechanism in 802.11
83180 - WLAN
802.11 Medium Access Control (MAC)
  • In 802.11 is used CSMA/CA mechanism
  • (Carrier Sense Multiple Access with Collision
    Avoidance)
  • It is considered to be fair for all users
    because treats them equally.
  • Two models
  • - basic access
  • - RTS/CTS

33
MAC mechanism in 802.11 (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
CSMA/CA basic access
  • used in DCF (Distributed Coordination Function)
  • with DCF, stations contend for access and
    attempt to send frames when there is no other
    station transmitting. If another station is
    sending a frame, stations are polite and wait
    until the channel is free.
  • MAC Layer checks the value of its network
    allocation vector (NAV), it must be zero before a
    station can attempt to send a frame.
  • random back off timer that a station uses if it
    detects a busy medium.
  • receiving station needs to send an
    acknowledgement (ACK) if it detects no errors in
    the received frame.

34
MAC mechanism in 802.11 (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
CSMA/CA basic access
35
MAC mechanism in 802.11 (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
CSMA/CA with RTS/CTS
  • 4-way handshake (RTS, CTS, DATA, ACK), protocol
  • When a sending station wants to transmit data,
    it first sends an RTS and waits for destination
    or AP replying with CTS, If success, then
    transmit data and destination sends an ACK for
    receiving data completely and correctly.
  • All other stations that hear RTS or CTS would
    defer transmission in this duration indicated in
    RTS or CTS. Otherwise the sending station would
    go back to compete media, the CSMA/CA mode.

36
MAC mechanism in 802.11 (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
CSMA/CA with RTS/CTS
37
802.11 MAC frame format
83180 - WLAN
802.11 Medium Access Control (MAC)
  • MAC Header format differs per Type
  • Control Frames (several fields are omitted)
  • Management Frames
  • Data Frames

38
802.11 MAC frame format (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
  • Addr. 1 All stations filter on this address.
  • Addr. 2 Transmitter Address (TA), Identifies
    transmitter to address the ACK frame to.
  • Addr. 3 Dependent on To and From DS bits.
  • Addr. 4 Only needed to identify the original
    source of WDS (Wireless Distribution
    System) frames

39
802.11 MAC frame format (cont.)
83180 - WLAN
802.11 Medium Access Control (MAC)
  • Type and subtype identify the function of the
    frame
  • Type00 Management Frame
  • Beacon (Re)Association
  • Probe (De)Authentication
  • Power Management
  • Type01 Control Frame
  • RTS/CTS ACK
  • Type10 Data Frame

40
MAC management services
83180 - WLAN
802.11 Medium Access Control (MAC)
  • Scanning
  • For station to begin communication it must first
    locate either stations or AP. Passive or active.
    Passive scanning involves only listening for
    802.11 traffic, active requires the scanning
    station to transmit and elicit responses from
    other stations and APs
  • Authentication
  • It consists of an exchange of questions, proofs,
    assertions and results. If the proofs exchanged
    are acceptable, each station would then tell the
    other that its assertion of indentity is
    believed. Two forms of authentication open
    system authentication and shared key.

41
MAC management services
83180 - WLAN
802.11 Medium Access Control (MAC)
  • Association
  • A WLAN requires a station to associate itself
    within BSS because the stations can move from one
    BSS to another. It is a process of mobile station
    connecting to an AP within BSS and through that
    station lets the network know its current
    position in ESS
  • Privacy
  • The need of secure communications is strong when
    wireless medium is used. The IEEE 802.11 Wired
    Equivalent Privacy (WEP) mechanism is designed to
    provide a protection level that is perceived as
    being equivalent to that of a wired LAN

42
802.11e QoS
83180 - WLAN
802.11e QoS
  • 190 902 Jaroslaw Lacki

43
Agenda
83180 - WLAN
802.11e QoS
  • Introduction to QoS
  • MAC modes
  • Enhanced MAC modes
  • - EDCF
  • - EPCF

44
83180 - WLAN
802.11e QoS
Introduction
  • IEEE 802.11e is a standard to define QoS for
    wireless gear
  • Support delay sensitive applications such as
    voice and video
  • Used for IEEE 802.11a and 802.11b DCF and PCF
    MAC modes
  • Real-time communications require QoS support
  • 802.11e is standard from the IEEE that defines
    new QoS capabilities of an access point

45
83180 - WLAN
802.11e QoS
Introduction
  • 802.11a uses OFDM modulation scheme that is
    especially well suited to use in office settings
  • There is less interference with 802.11a than with
    802.11b
  • 802.11a provides more available channels
  • frequency spectrum employed by 802.11b (2.400 GHz
    to 2.4835 GHz) is shared with various household
    appliances and medical devices

46
83180 - WLAN
802.11e QoS
MAC modes
  • The 802.11 MAC protocol designed with two modes
    of communication
  • Distributed Coordination Function (DCF) based on
    Carrier Sense Multiple Access with Collision
    Avoidance (CSMA/CA)
  • - "listen before talk.
  • - station waits for quiet period on network
  • - begins to transmit data
  • - detect collisions

47
83180 - WLAN
802.11e QoS
MAC modes
  • Point Coordination Function (PCF)
  • AP periodically send beacon frames
  • Communicate network identification and management
    parameters specific to the network
  • PCF splits the time into a contention-free period
    and a contention period
  • Station can transmit data during contention-free
    polling periods
  • Transmission times are unpredictable

48
83180 - WLAN
802.11e QoS
QoS needed
  • Special characteristics of wireless link such
  • - high loss rate
  • - bursts of frame loss
  • - packet re-ordering
  • - packet delay
  • - jitter
  • Above characteristics may vary over time and
    location.
  • Mobility of users may cause the end to end path
    to change when users moves but users should
    receive same QoS
  • Multimedia applications require QoS

49
83180 - WLAN
802.11e QoS
DCF enhancement
  • Enhancements for DCF modes to facilitate QoS
  • DCF - Enhanced Distribution Coordination Function
    (EDCF)
  • Concept of traffic categories
  • Each station has eight traffic categories -
    priority levels.
  • Stations detects if the medium is idle
  • Waits a period of time corresponding traffic
    category
  • Traffic category called Arbitration Interframe
    Space (AIFS)
  • Higher-priority traffic category have shorter
    AIFS than lower-priority traffic category

50
83180 - WLAN
802.11e QoS
DCF enhancement
  • To avoid collisions within a traffic category -
    station counts down an additional number of time
    slots - contention window, before attempting to
    transmit data
  • If another station transmits before the countdown
    has ended, station waits for the next idle
    period, after which it continues the countdown
  • Scheduler inside the station avoids collision by
    granting TXOP to the highest priority TC
  • EDCF solves problem of internal collisions but
    external collisions between stations may occur

51
83180 - WLAN
802.11e QoS
DCF enhancement
52
83180 - WLAN
802.11e QoS
PCF enhancement
  • 802.11e extend the polling mechanism PCF
  • Hybrid Coordination Function (HCF)
  • Hybrid controller polls stations during a
    contention-free period
  • The polling grants a station
  • Specific start time
  • Maximum transmit duration
  • HC allocates TXOP after detecting the channel is
    being idle for PIFS
  • PIFS have to be shorter than AIFS

53
83180 - WLAN
802.11e QoS
PCF enhancement
  • TXOP allocation may be scheduled during both CFP
    and CP in order to meet the QoS requirements of
    particular traffic categories
  • During the remainder of the CP, all frames are
    transmitted using the EDCF contention based rules
  • During CFP the starting time and maximum duration
    of transmission is specified by HC using CF-Poll
    frames

54
Power Management
83180 - WLAN
  • 195 273 Jakub Jakubiak

55
83180 - WLAN
802.11 Power management
  • In general radio devices operate in several power
    modes. The power consumption mode normally
    follows the relation
  • PTx gt PRx gt Pidle gtgt Psleep gt Poff
  • Power management has the primary goal of
    minimizing the time when the radio device stays
    in the transmit, receive or idle modes, thus
    maximizing sleep/power off time.

56
83180 - WLAN
802.11 Power management
  • IEEE 802.11 power management protocol is based on
    the fundamental transition strategy
  • sleep on inactivity
  • To support transparent communication at higher
    layers, an application independent on load change
    function is added, messages are buffered at base
    stations, and mobile host wakes up periodically
    to check for awaiting messages.

57
Independent BSS
83180 - WLAN
802.11 Power management
  • Reduction in power consumption plays significant
    role for mobile devices. It determines their
    weight as well as the time interval between
    battery charging
  • Distributed
  • Data frame handshake
  • Wake up every beacon.
  • Awake a period of ATIM (Ad-hoc Traffic Indication
    Message) window after each beacon, on receiving
    ATIM frame send ACK awake until the end of next
    ATIM.
  • Estimate the power saving station and delay until
    the next ATIM.
  • Multicast frame No ACK optional

58
Independent BSS
83180 - WLAN
802.11 Power management
  • Overhead
  • Sender
  • Announcement frame
  • Buffer
  • Power consumption in ATIM
  • Receiver
  • Awake for every Beacon and ATIM

59
Infrastructure BSS
83180 - WLAN
802.11 Power management
  • Centralized in the AP.
  • Greater power saving!!
  • Mobile Station sleeps for a number of beacon
    periods.
  • Awake for multicast traffic indicated in DTIM
    (Delivery Traffic Indication Map) in Beacon.
  • AP buffer, indicate in TIM
  • Mobile requests by PS-Poll
  • Larger DTIM count ? better power efficiency but
    latency for multicast traffic increases

60
Protocol harmonization
83180 - WLAN
802.11 Power management
  • Provides substantial reduction in energy
    consumption
  • Combined tuning of the data link protocol and
    physical layer
  • MAC packets are send with optimal transmit power
  • Exploiting various MAC level mechanisms

61
802.11h DFS and TPC
83180 - WLAN
802.11 Power management
  • DFS (Dynamic Frequency Selection) enables AP to
    change currently used frequency channel if some
    other transmission in the same band is discovered
  • TPC (Transmit Power Control) reduces transmit
    power to minimum to avoid interferences. Driven
    by AP
  • Introduced to enable sharing 5 GHz band with
    other radio devices

62
Bibliography
83180 - WLAN
802.11 Power management
  • J. Heiskala, J. Terry, OFDM Wireless LANs A
    Theoretical and Parctical Guide. Sams Publishing,
    2002
  • T. Cooklev, Wireless Communications Standards, A
    Study of 802.11, 802.15, and 802.16,  IEEE Press,
    2004
  • S. Mangold, S. Choi, P. May, O. Klein, G. Hiertz,
    L. Stibor, IEEE 802.11e Wireless LAN for Quality
    of Service, European Wireless 2002, Florence,
    Italy, February, 2002
  • J. P. Ebertand A. Wolisz Combined Tuning of RF
    Power and Medium Access Control for WLANs, in
    Journal of Mobile Networks Applications
    (Monet), vol. 6, no.5, pp. 417-426, September,
    2000
  • ANSI/IEEE Std 802.11, 1999 Edition, Part 11
    Wireless LAN Medium Access Control (MAC) and
    Physical Layer (PHY) Specifications
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