Ch' 4 Wireless Topologies

1
Ch. 4 Wireless Topologies
2
Overview

• This chapter is just an overview of many topics
  that we will discuss in much more detail in later
  chapters.

3
Components
4
AP Antennas

• Cisco Aironet AP 2.4 GHz antennas are compatible
  with all Cisco RP-TNC equipped APs.
• The antennas are available with different gain
  and range capabilities, beam widths, and form
  factors.
• Coupling the right antenna with the right AP
  allows for efficient coverage in any facility, as
  well as better reliability at higher data rates.
• A detailed coverage of antennas will be provided
  later in the course.

5
Bridge Antennas

• Cisco Aironet bridge 2.4 GHz antennas provide
  transmission between two or more buildings.
• Antennas operate at Layer 1 of the OSI Model.
• Remember that the physical layer defines the
  electrical, mechanical, procedural, and
  functional specifications for activating,
  maintaining, and deactivating the physical link
  between end systems.
• Characteristics such as voltage levels, timing of
  voltage changes, physical data rates, maximum
  transmission distances, physical connectors, and
  other, similar, attributes are defined by
  physical layer specifications.

6
WLAN Topologies

• Many of these features depend upon the vendor and
  whether the AP is a consumer wireless product or
  business/enterprise wireless product.
• Not all of these features are available on all
  APs or by all vendors.

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Cisco Three-layer Model
Includes APs, wireless bridges

• Be familiar with this model (see online
  curriculum).

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Local area networks (LAN)

• The basic service set (BSS) is the area of RF
  coverage provided by one access point.
• It is also referred to as a microcell.
• BSS can be extended by adding another AP.
• When more than one BSS is connected to a wired
  LAN, it is referred to as an extended service set
  (ESS).

9
Local area networks (LAN)

• Adding an AP is also a way to add wireless
  devices and extend the range of an existing wired
  system.
• If a single cell does not provide enough
  coverage, any number of cells can be added to
  extend the range.
• It is recommended that adjacent BSS cells have a
  10 to 15 percent overlap.

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Minimal Overlap Coverage Option
SSID Student Channel 6
SSID Student Channel 1

• 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 be discussed later.)

11
Wireless repeater
50 overlap
Not covered under 802.11

• A wireless repeater is simply an access point
  that is not connected to the wired backbone.
• This setup requires a 50 overlap of the AP on
  the backbone and the wireless repeater. (So they
  can reach each other.)
• The user can set up a chain of several repeater
  access points.
• However, 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 packet on the same channel.
• 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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System redundancy and load balancing
Load Balancing and Redundancy

• With the Cisco DS systems, the units are set to
  different channels.
• Redundancy If one unit goes down, remote clients
  will hand off to the remaining unit and continue
  working.
• Load balancing This can be configured based on
  the number of users, the bit error rate, or
  signal strength.
• Distributes user connections across available
  access points
• Optimizes aggregate throughput

13
Roaming

• 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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Roaming
IAPP Please send buffered frames for
IAPP Ok!
Packet - Source MAC of client
AP(B) must update MAC address tables on
infrastructure switches to prevent the loss of
data.

• The client initiates the roaming (re-association)
  process.
• As the client is moving out of range of its
  associated AP, the signal strength will start to
  drop off.
• At the same time, the strength of another AP will
  begin to increase.
• The re-association process then occurs, including
  authentication.
• We will look more at Roaming and IAPP later in
  the semester!

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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

• Scans for a better access point if the signal
  strength falls below a threshold value.
• The following options define signal strength and
  wait thresholds that trigger a new scan.

• When Adapter Has Been Associated for at LeastThe
  number of seconds the client adapter waits after
  connecting before searching for a better access
  point. This threshold keeps the client adapter
  from jumping from one access point to another too
  quickly after the initial connection.
• Signal Strength is Less ThanThe signal strength
  threshold below which the client adapter should
  search for a better access point. This threshold
  keeps the client adapter from jumping from one
  access point to another when both have strong
  signals.
• Example When using the default values of 20
  seconds and 50, the client adapter monitors the
  signal level 20 seconds after connecting and
  every second thereafter. If the client detects
  that the signal strength is below 50, it scans
  for a better access point. After the client
  connects to a better access point, this scanning
  process repeats.

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Scalability

• Specifies the channel number and frequency that
  the client adapter uses for communications. The
  channels conform to the IEEE 802.11 Standard for
  your regulatory domain.
• In infrastructure mode, this option is set
  automatically and cannot be changed. The client
  adapter listens to the entire spectrum, selects
  the best access point, and then uses the same
  channel as that access point.
• In ad hoc mode, the channel of the client adapter
  must match the channel used by the other clients
  in the wireless network. If the client adapter
  does not find any other ad hoc client adapters,
  this option specifies the channel on which the
  client adapter broadcasts beacons.

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Channel Setup
19
Multiple Overlapping Networks Coverage Option
SSID Staff Channel 6
SSID Student Channel 1
SSID Public Channel 11

• Multiple networks can operate in the same
  vicinity.
• The architecture provides multiple channels that
  can exist in the same area with virtually no
  interference to each other.
• In this mode, each system is configured with
  different SSIDs and different channels, which may
  (depending on configurations) prevent clients
  from roaming to access points of a different
  wireless network.

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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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Multiple Overlapping Networks Coverage Current
Thinking

• Only three of the 11 channels used by wireless
  hubs in the US can be allocated simultaneously.
• Reason
• In North America, the 802.11b spectrum ranges
  form 2411 MHz to 2473 MHz, and is divided up into
  11 channels. Channels are spaced 5 MHz apart
  from the center.
• However, each channel is 22 MHz wide, so there is
  a great overlap

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Burtons Analysis
An entire 22MHz is not simply swallowed up in a
rectangular pattern with power on the vertical
axis and frequency on the horizontal instead
it's more of a parabola, centered around the
midpoint of the frequency. Thus, as you get
further away from the center, the power drops off
substantially. According to Burton's analysis,
when three channels separate 802.11b frequencies,
there is only about 4 of interference. This is
the case between frequencies 1 and 4, and 8 and
11. Between 4 and 8, the interference is
substantially less than 1.
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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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Access point coverage and comparison

• The Cisco Aironet 2.4 GHz radio delivers 100 mW
  of output (AP and client) and offers a high
  degree of receiver sensitivity. (The power level
  can be decreased to 1 mw)
• The 5 GHz client radio has a 20 mW transmit power
  and the 5 GHz access point has a 40 mW transmit
  power. (The power can be decreased to 5 mw
• It is possible to adjust the power level down, to
  create pico-cells, or smaller coverage cells.
• This would be done, for example, to prevent the
  coverage area of one AP from extending too far
  into the coverage area of another AP.

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• Sets the transmit power level of the radio.
  Select a value for Transmit Power that is no
  greater than the maximum allowed by the
  regulatory body in your country (FCC in the
  United States, ETSI in Europe, and MKK in Japan).
  Reducing the transmit power conserves battery
  power, but it reduces the range of the radio. The
  default power level is the maximum power allowed
  by the regulatory agency in your country.
• Note If World Mode is enabled, the transmit
  power is limited to the maximum level allowed by
  the regulatory agency of the country where the
  adapter is used.

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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

• More on Bridges Later

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Root modes

• Cisco Aironet access points and bridges have two
  different root modes, in which to operate the
  following
• Root ON
• The bridge or AP is a root.
• If it is a bridge, then it is called the master
  bridge.
• Root OFF
• The bridge or AP is not a root, non-root.
• If it is an AP, then it is called a repeater

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Root modes
on
on
off
Associated to the same Root
off
off
off
None root bridges must be configured with the
same root BSSID
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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.

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Point-to-point configuration

• Many corporations would like to have more
  bandwidth between two locations, than the 11 Mbps
  provided by the 802.11b standard.
• Currently, with Cisco IOS, it is possible to use
  Fast Etherchannel or multi-link trunking, to bond
  or aggregate up to three bridges together.
• This gives the customer the potential for 33 Mbps.

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Point-to-multipoint configuration
root
Non-root
Non-root

• 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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VLAN, QoS, and Proxy Mobile IP
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VLAN features

• The Cisco Aironet APs only support the 802.1Q
  Trunking protocol standard.
• Switches will not allow different VLANs to talk
  to one another.
• A router will be needed to allow different VLANs
  to communicate to each other.
• The Cisco Aironet APs can be configured with 16
  different VLANs for system design flexibility.
• WLANs can now fit nicely into the larger network
  because VLANs have been enabled on the APs.
• This allows WLAN users to roam from access point
  to access point maintaining connectivity to the
  proper VLAN.

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Quality of Service (QoS) feature

• Time critical data traffic such as voice and
  video benefit from Quality of Service (QoS),
  which can be configured to give voice and video
  higher priority.
• This allows for smooth voice communication,
  jitter free video, and reliable delivery of
  E-Mail configured with a lower priority.
• Class of Service (CoS) uses the 802.1P standard
  to set the priority field to network traffic.
• 802.11e is supplementary to the MAC layer to
  provide QoS support for LAN applications.

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Proxy mobile IP
Wrong

• Ciscos Proxy Mobile IP is designed for use in
  even the most complex network environments.
• As the wireless station leaves one area and
  enters the next, the new access point queries the
  station for its home agent.
• After it has been located, packet forwarding is
  established automatically between the new and old
  access points to ensure the user can
  transparently exchange data.
• Here is what really happens

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IP Mobility
A mobile node is a device that moves from one
network to another while keeping its original IP
address. Many applications continue
uninterrupted as long as the brief delay involved
in roaming does not prompt a disconnect.
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IP Mobility
39
Proxy Mobile IP How It Works

• The wireless access point acts as a proxy on
  behalf of wireless clients that are not aware of
  the fact that they have roamed onto a different
  Layer 3 network.
• The access point handles the IRDP communications
  to the foreign agent and handles registrations to
  the home agent.
• There are three primary states of operation for
  proxy Mobile IP
• Agent discovery
• Updating the subnet map table
• Device registration
• Tunneling

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Proxy Mobile IP
IRDP