Wireless Topologies

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Wireless Topologies

• By
• Dr. R.K.Rao

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WLAN Components

• Laptops Workstations
• Mobile computers, PDAs, and Barcode Readers
• Clients Adapters
• Access Points Bridges
• Antennas
• Wireless Routers

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Laptops Workstations

• Increasingly popular
• Businesses are providing Laptops to employees
• User mobility is enhancing productivity
• Most Laptops are equipped with wireless NICs
• All desktops Laptops operate at all seven
  layers of OSI reference model

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Mobile computers, PDAs etc.

• Mobile computers come in different sizes and
  shapes
• Use different operating systems
• Some use integrated wireless NIC, whereas other
  use compactFlash based cards
• Designs OS
• Key-based computers pen/touch computers
  stationary and vechile mounted terminals
• OS Palm OS, Windows CE Windows XP embedded,
  Linux

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Clients Adapters

• As with Ethernet, a driver is needed to
  communicate with the OS on the computer.
• NDIS Driver Version - The version of the NDIS 3
  device driver that is installed on the computer.
  (Network Driver Interface Specification)

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Access Points and Bridges

• The AP (and bridges) operates at Layers 1 and 2
  of the OSI reference model
• An AP is a WLAN transceiver that can act as the
  center point of a standalone wireless network
• AP can also be used as the connection point
  between wireless and wired networks
• In large installations, the roaming functionality
  provided by multiple APs allows wireless users to
  move freely throughout the facility

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Wireless Bridges

• Are designed to connect two or more networks that
  are typically located in different buildings
• It delivers high data rates and superior
  throughput for data-intensive line-of- sight
  applications

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Wireless Bridges (contd.)

• Can be configured for point-to-point or
  point-to-multipoint applications
• Bridges connect hard-to-wire sites, satellite
  offices, school or corporate campus settings,
  temporary networks warehouses etc.

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Antennas

• Antennas operate at Layer 1 of the OSI model
• APs are available with either dipole antennas or
  Connectors, which let a customer attach different
  type of antennas for their applications
• Antennas are available with different gain, range
  capabilities, and beam widths
• Coupling the right antenna with right AP allows
  for efficient coverage in any facility

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Antennas (contd.)

• Antennas work at Layer 1 (Physical Layer).
  Physical layer defines the electrical,
  mechanical, procedural and functional
  specifications for activating, maintaining, and
  deactivating the physical link
• DSSS, OFDM, FHSS, IR wireless technologies each
  have their own specific physical layer details

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Access Point Antennas
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Bridge Antennas
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Wireless Routers

• Wireless Routers are 3 devices in one box
• Wireless AP that lets you connect to the network
• There is built-in 4 port full-duplex 10/100
  switch to connect to wired Ethernet devices
• The router function ties it all together and lets
  the whole network share a high-speed cable or DSL
  Internet connection

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WLAN Topologies

• Understanding topologies is very important when
  designing networks
• Two important classes of Wireless topologies are
• Wireless in-building LANs
• Wireless building-to-building bridging

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Three-Layer Hierarchical Model
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Wireless in-building LANs

• WLANs are an extension to the wired LAN network
• With WLANs mobile users can do the following
• Move freely around the facility
• Enjoy real-time access to the wired LAN at
  near-wired-Ethernet speeds
• Access all resources of the wired LAN

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Basic Service Set (BSS)

• Basic area of RF coverage provided by on Access
  Point (AP)
• It is also called the microcell
• BSS can be extended by adding another AP
• When more than one BSS is connected to a wired
  LAN, it is called Extended Service Set (ESS)

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Basic Service Set
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Extended Service Set
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Peer-to-Peer ad hoc topology
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Minimal Overlap Coverage

• By arranging the access points so that the
  overlap in a coverage area is minimized, a large
  area can be covered with minimal cost.
• The total bandwidth available to each wireless
  client device depends on the amount of data each
  mobile station needs to transfer and the number
  of stations located in each cell.
• Seamless roaming is supported as a client device
  moves in and out of range of each access point,
  thereby maintaining a constant connection to the
  wired LAN.
• Each device in the radio network must be
  configured with the same SSID to provide roaming
  capability. (Roaming will discussed later.)

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Minimal Overlap Coverage
SSID Student Channel 1
SSID Student Channel 6
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Wireless Repeaters

• In an environment where extended coverage is
  needed, but access to the backbone is impractical
  or unavailable, a wireless repeater can be used.
• A wireless repeater is simply an AP that is not
  connected to the backbone.
• The user can set up a chain of several repeaters
  APs

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Wireless Repeater
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Wireless Repeater

• The throughput for client devices at the end of
  the repeater chain will be quite low.
• This is because each repeater must receive and
  then re-transmit each frame on the same channel,
  similar to a wired repeater.
• For each repeater added to the chain, throughput
  is cut in half.
• It is recommended that not more than two hops be
  used.

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Wireless Repeaters

• Guidelines for configuring APs as repeaters
• Use repeaters to serve clients that do not
  require high throughput
• Omni directional antennas are best suited for
  repeater APs

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System Redundancy

• In a LAN where it is essential to have
  communications, some customers will require
  redundancy.
• With the direct sequence spread spectrum (DSSS)
  products of a different vendor, both AP units
  would be set to the same frequency and data rate.
• Since these units timeshare the frequency, only
  one unit can be talking at a time.
• If that one unit goes down for some reason, the
  remote clients will hand off to the other active
  unit.
• While this does provide redundancy, it does not
  provide any more throughput than a single AP
  provides.

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System Redundancy
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Roaming

• A WLAN designer must determine whether clients
  will require seamless roaming from access point
  to access point.

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Roaming

• Several factors need to be considered when
  designing a WLAN with seamless roaming
  capabilities
• Coverage must be sufficient for the entire path.
• A consistent IP address should be available
  throughout the entire path.
• Until standardized by IEEE 802.11, access points
  will most likely need to be from the same vendor.

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Roaming
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• Initial Association
• Probing (Probe Request, Probe Response)
• Note 802.11 does not specify how the client
  determines which AP to associate with , so it
  depends on vendor implementation.
• Authentication (Authentication Request,
  Authentication Response)
• Association (Association Request, Association
  Response)
• 802.11 does not allow associating with more than
  one AP.

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Channel Setup

• Channel Setup
• There are two critical steps for a good WLAN
  deployment
• 1. Determine placement of access points or
  bridges
• This includes determining where they should be
  placed and deciding how many are required for the
  desired coverage.
• Very few gaps in the coverage should be left.
• These gaps are essentially dead air and the
  client will lack connectivity in these locations.
  
• As discussed before, bandwidth requirements have
  an impact on the coverage areas.
• 2. Map out the channel assignments
• There should be as little overlap as possible
  between channels that use the same frequency.

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Channel setup
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DSSS Channels
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Access Point Coverage

• Access point coverage and comparison
• As a client roams away from the access point, the
  transmission signals between the two attenuate
  (weaken).
• Rather than decreasing reliability, the AP shifts
  to a slower data rate, which gives more accurate
  data transfer.
• This is called data rate or multi-rate shifting.
• As a client moves away from an 802.11b access
  point, the data rate will go from 11 Mbps, to
  5.5Mbps, to 2 Mbps, and, finally, to 1 Mbps.
• This happens without losing the connection, and
  without any interaction from the user.

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IEEE 802.11b AP Coverage
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Multirate implementation

• Provides for seamless roaming, but not at a
  constant speed.
• This example takes advantage of multi-rate
  technology, to step down in bandwidth and gain
  greater coverage distances, with a single access
  point.
• If 11 Mbps is required everywhere, the access
  points would need to be relocated, so that only
  the 11-Mbps circles are touching each other, with
  some overlap.
• This would require a greater number of APs, but
  consistent bandwidth would be achieved.

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Multirate implementation
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Channel usage and interference

• Remember that the 802.11 standard uses the
  unlicensed spectrum and, therefore, anyone can
  use these frequencies.

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Bridge Topologies

• Point-to-point configuration
• When using point-to-point wireless bridges, two
  LANs can be located up to 40 km (25 miles) apart.
• The antennas must have line-of-site with each
  other.
• Obstacles such as buildings, trees, and hills
  will cause communication problems.
• In this configuration, the Ethernet segments in
  both buildings act as if they are a single
  segment.
• The bridge does not add to the Ethernet repeater
  count because this segment is viewed by the
  network as a cable.

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Point-to-point Configuration
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Point-to-point configuration
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Point-to-multipoint configuration

• For multipoint bridging, an omni directional
  antenna is typically used at the main site.
• Directional antennas are used at the remote
  sites.
• In this configuration, again, all the LANs appear
  as a single segment.
• Traffic from one remote site to another will be
  sent to the main site and then forwarded to the
  other remote site.
• Remote sites cannot communicate directly with one
  another.
• Line of sight must be maintained between each
  remote site and the main site.

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Point-to-multipoint configuration