802'11 Introduction

1
802.11 Introduction

• Over recent years, the market for wireless
  communications has enjoyed tremendous growth.
  Wireless technology now reaches or is capable of
  reaching virtually every location on the face of
  the earth. Hundreds of millions of people
  exchange information every day using pagers,
  cellular telephones, and other wireless
  communication products. Seeing the tremendous
  success of wireless telephony and messaging
  services, wireless communication has been applied
  to the realm of personal and business computing.
  The protocol that is used to communicate over a
  wireless medium is 802.11.

2
Introduction (Continued)

• This protocol was adopted by IEEE standards
  committee in 1997 and is the first wireless LAN
  (WLAN) standard. This standard defines the media
  access control (MAC) and physical (PHY) layers
  for a LAN with wireless connectivity. It
  addresses local area networking where the
  connected devices communicate over the air to
  other devices that are within close proximity to
  each other

3
802.11

• The IEEE 802.11 protocol is an extension of the
  IEEE 802.3 protocol for wired networks.
• The IEEE 802.3 protocol defines the Media Access
  Control (MAC) and physical layers for a wired
  network.

4
802.3

• The IEEE 802.3 standard is most widely used
  standard for wired network which was developed
  out of the original work done on Ethernet.
  Ethernet was developed by Xerox corporation's
  Palo Alto research center (PARC) in the 1970s and
  was the technological basis for the IEEE 802.3
  specification, which was initially released in
  1980. Today, the term Ethernet is often used to
  refer to all carrier sense multiple
  access/collision detection (CSMA/CD) LANs that
  generally conform to Ethernet specifications,
  including IEEE 802.3.

5
802.3 (Continued)

• The 802.3 protocol has many implementations that
  are available and to distinguish between them the
  committee has developed a concise notation.
• ltdata rate in Mbpsgtltsignaling methodgtltmaximum
  length in hundreds of metersgt
• The defined alternatives for a 10Mbps date rate
  are
• 10BASE5
• 10BASE2
• 10BASE-T (T Twisted Pair)
• 10BASE-F (F Optical Fiber)

6
802.3 (Continued)

• The defined alternatives for a 100Mbps data rate
  are
• 100BASE-TX (two pairs of data grade twisted-pair
  wire)
• 100BASE-FX (a two-strand optical fiber cable)
• 100BASE-T4 (four pairs of telephone twisted pair
  wire)

7
802.11 Versus 802.3

• The 802.11 standard is similar in most
  respects to the IEEE 802.3 Ethernet standard.
  Specifically, the 802.11 standard addresses
• Functions required for an 802.11 compliant device
  to operate either in a peer-to-peer fashion or
  integrated with an existing wired LAN
• Operation of the 802.11 device within possibly
  overlapping 802.11 wireless LANs and the mobility
  of this device between multiple wireless LANs
• MAC level access control and data delivery
  services to allow upper layers of the 802.11
  network
• Several physical layer signaling techniques and
  interfaces
• Privacy and security of user data being
  transferred over the wireless media

8
802.11 versus 802.3

• There are a number of characteristics that are
  unique to the wireless environment (as compared
  to a wired LAN) that the 802.11 standard must
  take into consideration. The physical
  characteristics of a wireless LAN introduce range
  limitations, unreliable media and dynamic
  topologies where stations move about,
  interference from outside sources, and lack of
  the ability for every device to hear every
  other device within the WLAN.

9
802.11 versus 802.3

• These limitations force the WLAN standard to
  create fundamental definitions for short-range
  LANs made up of components that are within close
  proximity of each other. Larger geographic
  coverage is handled by building larger LANs from
  the smaller fundamental building blocks or by
  integrating the smaller WLANs with an existing
  wired network.

10
Mobility

• Mobility of wireless stations is the most
  important feature of a wireless LAN. A WLAN would
  not serve much purpose if stations were not able
  to move about freely from location to location
  either within a specific WLAN or between
  different WLAN segments.

11
Mobility (Continued)

• For compatibility purposes, the 802.11 MAC must
  appear to the upper layers of the network as a
  standard 802 LAN. The 802.11 MAC layer is
  forced to handle station mobility in a fashion
  that is transparent to the upper layers of the
  802 LAN stack. This forces functionality into the
  802.11 MAC layer that is typically handled by
  upper layers.

12
802.11 WLAN Architecture

• The 802.11 architecture is comprised of several
  components and services that interact to provide
  station mobility transparent to the higher layers
  of the network stack.Wireless LAN StationThe
  station (STA) is the most basic component of the
  wireless network. A station is any device that
  contains the functionality of the 802.11
  protocol, that being MAC, PHY, and a connection
  to the wireless media. Typically the 802.11
  functions are implemented in the hardware and
  software of a network interface card (NIC).A
  station could be a laptop PC, handheld device, or
  an Access Point. Stations may be mobile,
  portable, or stationary and all stations support
  the 802.11 station services of authentication,
  de-authentication, privacy, and data delivery.

13
802.11 WLAN Architecture

• Basic Service Set (BSS)802.11 defines the Basic
  Service Set (BSS) as the basic building block of
  an 802.11 wireless LAN. The BSS consists of a
  group of any number of stations.

14
802.11 Topologies

• Independent Basic Service Set (IBSS)The most
  basic wireless LAN topology is a set of stations,
  which have recognized each other and are
  connected via the wireless media in a
  peer-to-peer fashion. This form of network
  topology is referred to as an Independent Basic
  Service Set (IBSS) or an Ad-hoc network.

15
IBSS

• In an IBSS, the mobile stations communicate
  directly with each other. Every mobile station
  may not be able to communicate with every other
  station due to the range limitations. There are
  no relay functions in an IBSS therefore all
  stations need to be within range of each other
  and communicate directly.

Independent Basic Service Set (IBSS)
16
Infrastructure Basic Service Set

• An Infrastructure Basic Service Set is a BSS with
  a component called an Access Point (AP). The
  access point provides a local relay function for
  the BSS. All stations in the BSS communicate with
  the access point and no longer communicate
  directly. All frames are relayed between stations
  by the access point. This local relay function
  effectively doubles the range of the IBSS.The
  access point may also provide connection to a
  distribution system.

Distribution System
17
Distribution System

• The distribution system (DS) is the means by
  which an access point communicates with another
  access point to exchange frames for stations in
  their respective BSSs, forward frames, to follow
  mobile stations as they move from one BSS to
  another, and exchange frames with a wired
  network.
• As IEEE 802.11 describes it, the distribution
  system is not necessarily a network nor does the
  standard place any restrictions on how the
  distribution system is implemented, only on the
  services it must provide. Thus the distribution
  system may be a wired network like 803.2 or a
  special purpose box that interconnects the access
  points and provides the required distribution
  services.

18
Extended Service Set

• Extending coverage via an Extended Service Set
  (ESS) 802.11 extends the range of mobility to an
  arbitrary range through the Extended Service Set
  (ESS). An extended service set is a set of
  infrastructure BSSs, where the access points
  communicate amongst themselves to forward traffic
  from one BSS to another, to facilitate movement
  of stations between BSSs.The access point
  performs this communication through the
  distribution system. The distribution system is
  the backbone of the wireless LAN and may be
  constructed of either a wired LAN or wireless
  network

19
ESS

• Typically the distribution system is a thin layer
  in each access point that determines the
  destination for traffic received from a BSS. The
  distribution system determines if traffic should
  be relayed back to a destination in the same BSS,
  forwarded on the distribution system to another
  access point, or sent into the wired network to a
  destination not in the extended service set.
  Communications received by an access point from
  the distribution system are transmitted to the
  BSS to be received by the destination mobile
  station.

20
ESS

• Network equipment outside of the extended service
  set views the ESS and all of its mobile stations
  as a single MAC-layer network where all stations
  are physically stationary. Thus, the ESS hides
  the mobility of the mobile stations from
  everything outside the ESS. This level of
  indirection provided by the 802.11 architecture
  allows existing network protocols that have no
  concept of mobility to operate correctly with a
  wireless LAN where there is mobility.

21
ESS
22
Distribution Services

• Distribution services provide functionality
  across a distribution system. Typically, access
  points provide distribution services. The five
  distribution services and functions detailed
  below include association, disassociation,
  re-association, distribution, and integration.

23
Association

• The association service is used to make a logical
  connection between a mobile station and an access
  point. Each station must become associated with
  an access point before it is allowed to send data
  through the access point onto the distribution
  system. The connection is necessary in order for
  the distribution system to know where and how to
  deliver data to the mobile station.The mobile
  station invokes the association service once and
  only once, typically when the station enters the
  BSS. Each station can associate with one access
  point though an access point can associate with
  multiple stations.

24
Disassociation

• DisassociationThe disassociation service is used
  either to force a mobile station to eliminate an
  association with an access point or for a mobile
  station to inform an access point that it no
  longer requires the services of the distribution
  system. When a station becomes disassociated, it
  must begin a new association to communicate with
  an access point again.An access point may force
  a station or stations to disassociate because of
  resource restraints, the access point is shutting
  down or being removed from the network for a
  variety of reasons. When a mobile station is
  aware that it will no longer require the services
  of an access point, it may invoke the
  disassociation service to notify the access point
  that the logical connection to the services of
  the access point from this mobile station is no
  longer required.

25
Disassociation

• Stations should disassociate when they leave a
  network, though there is nothing in the
  architecture to assure this happens.
  Disassociation is a notification and can be
  invoked by either associated party. Neither party
  can refuse termination of the association.

26
Re-association

• Re-Association enables a station to change its
  current association with an access point. The
  re-association service is similar to the
  association service, with the exception that it
  includes information about the access point with
  which a mobile station has been previously
  associated. A mobile station will use the
  re-association service repeatedly as it moves
  through out the ESS, loses contact with the
  access point with which it is associated, and
  needs to become associated with a new access
  point.

27
Re-association

• By using the re-association service, a mobile
  station provides information to the access point
  to which it will be associated and information
  pertaining to the access point which it will be
  disassociated. This allows the newly associated
  access point to contact the previously associated
  access point to obtain frames that may be waiting
  there for delivery to the mobile station as well
  as other information that may be relevant to the
  new association.The mobile station always
  initiates re-association.

28
Distribution

• Distribution is the primary service used by an
  802.11 station. A station uses the distribution
  service every time it sends MAC frames across the
  distribution system. The distribution service
  provides the distribution with only enough
  information to determine the proper destination
  BSS for the MAC frame.The three association
  services (association, re-association, and
  disassociation) provide the necessary information
  for the distribution service to operate.
  Distribution within the distribution system does
  not necessarily involve any additional features
  outside of the association services, though a
  station must be associated with an access point
  for the distribution service to forward frames
  properly.

29
Integration

• The integration service connects the 802.11 WLAN
  to other LANs, including one or more wired LANs
  or 802.11 WLANs. A portal performs the
  integration service. The portal is an abstract
  architectural concept that typically resides in
  an access point though it could be part of a
  separate network component entirely.The
  integration service translates 802.11 frames to
  frames that may traverse another network, and
  vice versa.

30
802.11 Media Access Control

• The 802.11 MAC layer provides functionality to
  allow reliable data delivery for the upper layers
  over the wireless PHY media. The data delivery
  itself is based on an asynchronous, best-effort,
  connectionless delivery of MAC layer data. There
  is no guarantee that the frames will be delivered
  successfully.
• The 802.11 MAC provides a controlled access
  method to the shared wireless media called
  Carrier-Sense Multiple Access with Collision
  Avoidance (CSMA/CA). CSMA/CA is similar to the
  collision detection access method deployed by
  802.3 Ethernet LANs.

31
802.11 Media Access Control

• Another function of the 802.11 MAC is to protect
  the data being delivered by providing security
  and privacy services. Security is provided by the
  authentication services and by Wireless
  Equivalent Privacy (WEP), which is an encryption
  service for data delivered on the WLAN.

32
CSMA/CA

• The fundamental access method of 802.11 is
  Carrier Sense Multiple Access with Collision
  Avoidance or CSMA/CA. CSMA/CA works by a "listen
  before talk scheme". This means that a station
  wishing to transmit must first sense the radio
  channel to determine if another station is
  transmitting. If the medium is not busy, the
  transmission may proceed.

33
CSMA/CA

• The CSMA/CA protocol avoids collisions among
  stations sharing the medium by utilizing a random
  backoff time if the stations physical or logical
  sensing mechanism indicates a busy medium. The
  period of time immediately following a busy
  medium is the highest probability of collisions
  occurring, especially under high utilization.

34
CSMA/CA

• The CSMA/CA scheme implements a minimum time gap
  between frames from a given user. Once a frame
  has been sent from a given transmitting station,
  that station must wait until the time gap is up
  to try to transmit again. Once the time has
  passed, the station selects a random amount of
  time (the backoff interval) to wait before
  "listening" again to verify a clear channel on
  which to transmit. If the channel is still busy,
  another backoff interval is selected that is less
  than the first. This process is repeated until
  the waiting time approaches zero and the station
  is allowed to transmit. This type of multiple
  access ensures judicious channel sharing while
  avoiding collisions.

35
802.11 Physical Layer (PHY)

• The 802.11 physical layer (PHY) is the interface
  between the MAC and the wireless media where
  frames are transmitted and received. The PHY
  provides three functions. First, the PHY provides
  an interface to exchange frames with the upper
  MAC layer for transmission and reception of data.
  Secondly, the PHY uses signal carrier and spread
  spectrum modulation to transmit data frames over
  the media. Thirdly, the PHY provides a carrier
  sense indication back to the MAC to verify
  activity on the media.

36
802.11 Physical Layer (PHY)

• 802.11 provides three different PHY definitions
  Both Frequency Hopping Spread Spectrum (FHSS) and
  Direct Sequence Spread Spectrum (DSSS) support 1
  and 2 Mbps data rates.

37
802.11b

• Operating in the 2.4GHz frequency range, 802.11b
  (aka Wi-Fi) has a nominal maximum data rate of
  11Mbps, with the potential of three simultaneous
  channels. 802.11b has a great advantage in that
  it is accepted worldwide. One of the more
  significant disadvantages of 802.11b is that the
  frequency band is crowded, and subject to
  interference from other networking technologies,
  microwave ovens, 2.4GHz cordless phones (a huge
  market), and Bluetooth. There are drawbacks to
  802.11b, including lack of interoperability with
  voice devices, and no QoS provisions for
  multimedia content. Interference and other
  limitations aside, 802.11b is the clear leader in
  business and institutional wireless networking
  and is gaining share for home applications as
  well.

38
802.11a

• 802.11a, is much faster than 802.11b, with a
  54Mbps maximum data rate (actually increased to
  72Mbps or 108Mbps in a non-standard double-speed
  mode depending on the chipset vendor and
  component manufacturer). 802.11a (Wi-Fi5)
  operates in the 5GHz frequency range and allows
  eight simultaneous channels. One big advantage to
  802.11a is that it isn't subject to interference
  from Bluetooth or any of the other 2.4GHz
  frequency denizens.

39
802.11a

• One big disadvantage is that it is not directly
  compatible with 802.11b, and requires new
  bridging products that can support both types of
  networks, although if you don't mind spending the
  money for access points for both 11a and 11b, you
  can plug them into hubs or better yet, switches
  on your network and they'll work just fine. Other
  clear disadvantages are that 802.11a is only
  available in half the bandwidth in Japan (for a
  maximum of four channels), and it isn't approved
  for use in Europe, where HiperLAN2 is the
  standard.
• Like 802.11b, 802.11a has no provisions to
  optimize voice or multimedia content.

40
802.11g

• 802.11g, operates in the 2.4GHz frequency with
  mandatory compatibility with 802.11b but with a
  maximum data rate of 54Mbps
• The standard operates entirely in the 2.4GHz
  frequency, but uses a minimum of two modes (both
  mandatory) with two optional modes. The mandatory
  modulation/access modes are the same CCK
  (Complementary Code Keying) mode used by 802.11b
  (hence the compatibility with Wi-Fi) and the OFDM
  (Orthogonal Frequency Division Multiplexing) mode
  used by 802.11a (but in this case in the 2.4GHz
  frequency band). The mandatory CCK mode supports
  11Mbps and the OFDM mode has a maximum of 54Mbps.
  There are also two modes that use different
  methods to attain a 22Mbps data rate--TI's
  PBCC-22 (Packet Binary Convolutional Coding,
  rated for 6 to 54Mbps) and Intersil's CCK-OFDM
  mode (with a rated max of 33Mbps).

41
802.11g

• The obvious advantage of 802.11g is that it
  maintains compatibility with 802.11b (and
  802.11b's worldwide acceptance) and also offers
  faster data rates comparable with 802.11a. The
  number of channels available, however, is not
  increased, since channels are a function of
  bandwidth, not radio signal modulation and on
  that score, 802.11a wins with its eight channels,
  compared to the three channels available with
  either 802.11b or 802.11g. Another disadvantage
  of 802.11g is that the 2.4GHz frequency will get
  even more crowded

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Next Presentation

• WEP
• Failure Of WEP
• WEP 2 with Temporal Key Integrity Protocol
• Enhanced Security Network (ESN).