Wireless LAN Overview

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Wireless LAN Overview

• Abdus Salam ICTP, February 2003
• Presented by
• Ermanno Pietrosemoli
• Latin American Networking School - ULA

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Wireless LAN Overview

• Wireless networks where borne as LANs, but for
  developing countries applications they are more
  useful as MANs or even WANs
• The enormous success of this technology has led
  to a dramatic price reduction of the equipment,
  from 750 in 1992 to 60 in 2002

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Wireless LAN OverviewAgenda

• DSSS Channel Allocation
• Access Point Modes and Types
• Clients Types
• 802.11 Standards
• 802.11 Terminology

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Wireless LAN Overview

• DSSS popularity has eclipsed FHSS, although the
  latter may be more resistant to interference
• We will focus on DSSS

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Channel Overlapping
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Access Point Modes

• Root Mode
• Repeater Mode
• Bridge Mode

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• Repeater and Bridging Functions

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Chipset Manufacturers
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Client Devices
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Client Devices
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Client Devices
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Common options that most wirelessresidential
gateways include are

• Point-to-Point Protocol over Ethernet (PPPoE)
• Network Address Translation (NAT)
• Port Address Translation (PAT)
• Ethernet switching
• Virtual Servers
• Print Serving
• Fail-over routing
• Virtual Private Networks (VPNs)
• Dynamic Host Configuration Protocol (DHCP)
  Server and Client
• Configurable Firewall

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Enterprise Gateway Features

• Enterprise wireless gateways do have features,
  such as Role-Based Access Control (RBAC), that
  are not found in any access points. RBAC allows
  an administrator to assign a certain level of
  wireless network access to a particular job
  position in the company. If the person doing that
  job is replaced, the new person automatically
  gains the same network rights as the replaced
  person. Having the ability to limit a wireless
  user's access to corporate resources, as part of
  the "role", can be a useful security feature.

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Enterprise Gateway Features

• Class of service is typically supported, and
  an administrator can assign levels of service to
  a particular user or role. For example, a guest
  account might be able to use only 500 kbps on the
  wireless network whereas an administrator might
  be allowed 2 Mbps connectivity.

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Configuration and Managementof EG

• Enterprise wireless gateways are installed in the
  main the data path on the wired LAN segment just
  past the access point(s)
• They are configured through console ports using
  telnet, internal HTTP or HTTPS servers, etc.
• Centralized management of only a few devices is
  one big advantage of using enterprise wireless
  gateways. An administrator, from a single
  console, can easily manage a large wireless
  deployment using only a few central devices
  instead of a very large number of access points.

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Configuration and Managementof EWG

• Enterprise wireless gateways are normally
  upgraded through use of TFTP in the same
• fashion as many switches and routers on the
  market today. Configuration backups can often be
  automated so that the administrator won't have to
  spend additional management time backing up or
  recovering from lost configuration files.
  Enterprise wireless gateways are mostly
  manufactured as rack-mountable 1U or 2U devices
  that can fit into your existing data center
  design.

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UNII Bands
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UNII Middle Band

• The middle UNII band is bound by 5.25 GHz and
  5.35 GHz and is specified at 250 mW of output
  power by the FCC. The power output specified by
  IEEE for the middle UNII band is 200 mW. This
  power limit allows operation of devices either
  indoors or outdoors and is commonly used for
  short outdoor hops between closely spaced
  buildings. In the case of a home installation,
  such a configuration might include an RF link
  between the house and the garage, or the house
  and a neighbors house.
• .

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UNII Upper Band

• The upper UNII band is reserved for outdoor
  links and is limited by the FCC to 1 Watt of
  output power. This band occupies the range of
  frequencies between 5.725 GHz and 5.825 GHz, and
  is often confused with the 5.8 GHz ISM band. The
  IEEE specifies the maximum output power for this
  band as 800 mW, which is plenty of power for
  almost any outdoor implementation, except for
  large campuses or long-distance RF links.

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Power Limits

• PtMP links have a central point of
  connection and two or more non-central connection
  points. PtMP links are typically configured in a
  star topology. The central connection point may
  or may not have an omnidirectional antenna It is
  important to note that when an omnidirectional
  antenna is used, the FCC automatically considers
  the link a PtMP link.
• Regarding the setup of a PtMP link, the FCC
  limits the EIRP to 4 Watts in both the 2.4 GHz
  ISM band and upper 5 GHz UNII band. The power
  limit set for the intentional radiator (the
  device transmitting the RF signal) in each of
  these bands is 1 Watt. If the transmitting
  wireless LAN devices are adjustable with respect
  to their output power, then the system can be
  customized to the needs of the user.

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Power Limits

• Suppose a radio transmitting at 1 Watt (30
  dBm) is connected directly to a 12 dBi
  omnidirectional antenna. The total output power
  at the antenna is about 16 Watts, which is well
  above the 4 Watt limit. The FCC stipulates that
  for each 3 dBi above the antenna's initial 6 dBi
  of gain, the power at the intentional radiator
  must be reduced by 3 dB below the initial 30
  dBm. For our example, since the antenna gain is
  12 dBi, the power at the intentional radiator
  must be reduced by 6 dB. This reduction will
  result in an intentional radiator power of 24
  dBm (30 dBm 6 dB), or 250 mW and an EIRP of 36
  dBm (24 dBm 12 dBi), or 4 Watts. Clearly this
  rule can become confusing, but the end result
  must be that the power at the intentional
  radiator must never be more than 1 Watt and the
  EIRP must never be above 4 Watts for a PtMP
  connection.

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Power Limits
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Power Limits
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Power Limits
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Power Limits
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IEEE 802.11g

• 802.11g provides the same maximum speed of
  802.11a,coupled with backwards compatibility for
  802.11b devices. This backwards compatibility
  will make upgrading wireless LANs simple and
  inexpensive.
• IEEE 802.11g specifies operation in the 2.4
  GHz ISM band. To achieve the higher data rates
  found in 802.11a, 802.11g compliant devices
  utilize Orthogonal Frequency Division
  Multiplexing (OFDM) modulation technology. These
  devices can automatically switch to QPSK
  modulation in order to communicate with the
  slower 802.11b- and 802.11- compatible devices.
  With all of the apparent advantages, 802.11gs
  use of the crowded 2.4 GHz band could prove to be
  a disadvantage.

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Wireless Ethernet Compatibility Alliance

• The Wireless Ethernet Compatibility Alliance
  (WECA) promotes and tests for wireless LAN
  interoperability of 802.11b devices and 802.11a
  devices. WECAs mission is to certify
  interoperability of Wi-Fi (IEEE 802.11) products
  and to promote Wi-Fi as the global wireless LAN
  standard across all market segments. As an
  administrator, you must resolve conflicts among
  wireless LAN devices that result from
  interference, incompatibility, or other problems.

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Wireless Ethernet Compatibility Alliance
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European Telecommunications Standards Institute
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European Telecommunications Standards Institute
The website for ETSI is www.etsi.org
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Wireless LAN Association

• The Wireless LAN Association's mission is to
  educate and raise consumer awareness regarding
  the use and availability of wireless LANs and to
  promote the wireless LAN industry in general. The
  Wireless LAN Association (WLANA) is an
  educational resource for those seeking to learn
  more about wireless LANs. WLANA can also help if
  you are looking for a specific wireless LAN
  product or service.
• WLANA has many partners within the industry that
  contribute content to the WLANA directory of
  information. It is this directory, along with the
  many white papers and case studies that WLANA
  provides, that offer you valuable information for
  making your own decisions about wireless LAN
  implementation.
• The website for WLANA is www.wlana.org

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Bluetooth
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Bluetooth
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Bluetooth
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Bluetooth
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The website for IrDA is www.irda.org
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Locating a Wireless LAN
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Service Set Identifier
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Beacons

• Beacons (short for beacon management frame) are
  short frames that are sent from the access point
  to stations (infrastructure mode) or
  station-to-station (ad hoc mode) in order to
  organize and synchronize wireless communication
  on the wireless LAN.
• Beacons serve several functions, including the
  following

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Time Synchronization Beacons synchronize clients
by way of a time-stamp at the exact moment of
transmission. When the client receives the
beacon, it changes its own clock to reflect the
clock of the access point. Once this change is
made, the two clocks are synchronized.
Synchronizing the clocks of communicating units
will ensure that all time-sensitive functions,
such as hopping in FHSS systems, are performed
without error. The beacon also contains
the beacon interval, which informs stations how
often to expect the beacon.
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FH or DS Parameter Sets Beacons contain
information specifically geared to the spread
spectrum technology the system is using. For
example, in a FHSS system, hop and dwell time
parameters and hop sequence are included in the
beacon. In a DSSS system, the beacon contains
channel information.
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SSID Information Stations look in beacons for the
SSID of the network they wish to join. When this
information is found, the station looks at the
MAC address of where the beacon originated and
sends an authentication request in hopes of
associating with that access point. If a station
is set to accept any SSID, then the station will
attempt to join the network through the first
access point that sends a beacon or the one with
the strongest signal strength if there are
multiple access points.
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Traffic Indication Map (TIM) The TIM is used an
as indicator of which sleeping stations have
packets queued at the access point. This
information is passed in each beacon to all
associated stations. While sleeping, synchronized
stations power up their receivers, listen for the
beacon, check the TIM to see if they are listed,
then, if they are not listed, they power down
their receivers and continue sleeping. .
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Supported Rates With wireless networks, there are
many supported speeds depending on the standard
of the hardware in use. For example, an 802.11b
compliant device supports 11, 5.5, 2, 1 Mbps
speeds. This capability information is passed in
the beacons to inform the stations what speeds
are supported on the access point.
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Passive Scanning Passive scanning is the process
of listening for beacons on each channel for a
specific period of time after the station is
initialized. These beacons are sent by access
points (infrastructure mode) or client stations
(ad hoc mode), and the scanning station catalogs
characteristics about the access points or
stations based on these beacons. The station
searching for a network listens for beacons until
it hears a beacon listing the SSID of the network
it wishes to join. The station then attempts to
join the network through the access point that
sent the beacon.
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Active Scanning Active scanning involves the
sending of a probe request frame from a wireless
station. Stations send this probe frame when they
are actively seeking a network to join. The probe
frame will contain either the SSID of the network
they wish to join or a broadcast SSID. If a probe
request is sent specifying an SSID, then only
access points that are servicing that SSID will
respond with a probe response frame. If a probe
request frame is sent with a broadcast SSID, then
all access points within reach will respond with
a probe response frame. The point of probing in
this manner is to locate access points through
which the station can attach to the network. Once
an access point with the proper SSID is found,
the station initiates the authentication and
association steps of joining the network through
that access point. .
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Active Scanning
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