03 fwl system architectureanddesign(i)
TRANSCRIPT
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WLAN System Architecture
and Design (I)
Fundamentals of Wireless LANs
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Topic & Structure of the lesson
WLAN Design Considerations
Wireless Roaming
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Learning Outcomes
At the end of this module,YOU should be able to:
Explain the important factors and requirements for designing
WLANs.
Explain the issues and requirements to implement wireless roaming
for WLANs.
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Key Terms you must be able to use
If you have mastered this topic, you should be able to use the
following terms correctly in your assignments and exams:
Access Points (AP)
Stations Distribution System
Wireless Medium
BSS transition
ESS transition
SSID
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Main Teaching Points
WLAN Design Considerations
Wireless Roaming
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Standards and Design
802.11 networks consist of four major physical
components, which are summarized in the following figure:
WLAN Design Consideration
The components are:
Stations
Networks are built to transfer data between stations
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Stations are computing devices with wireless network
interfaces
There is no reason why stations must be portable
computing devices, though
In some environments, wireless networking is used to
avoid pulling new cable, and desktops are connected by
wireless LANs
Large open areas may also benefit from wireless
networking, such as a manufacturing floor using awireless LAN to connect components
Access points
Frames on an 802.11 network must be converted to
another type of frame for delivery to the rest of the world
WLAN Design Consideration
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Computer Network Infrastructure
WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Devices called access points perform the wireless-to-
wired bridging function
Access points perform a number of other
functions, but bridging is by far the most
important
Initially, access point functions were put into
standalone devices, though several newer products are
dividing the 802.11 protocol between "thin" access
points and AP controllers Wireless medium
To move frames from station to station, the
standard uses a wireless medium
WLAN Design Consideration
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Several different physical layers are defined; the
architecture allows multiple physical layers to be
developed to support the 802.11 MAC
Distribution system
When several access points are connected to form a
large coverage area, they must communicate with each
other to track the movements of mobile stations
The distribution system is the logical component of
802.11 used to forward frames to their destination
802.11 does not specify any particular technology for
the distribution system
WLAN Design Consideration
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
In most commercial products, the distribution system is
implemented as a combination of a bridging engine and
a distribution system medium
which is the backbone network used to relay frames
between access points; it is often called simply the
backbone network
In nearly all commercially successful products,
Ethernet is used as the backbone network technology
WLAN Design Consideration
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Introduction
Mobility is usually the primary motivation for deploying an
802.11 network
Transmitting data frames while the station is moving will dofor data communications what mobile telephony did for voice
802.11 provides mobility between basic service areas at
the link layer
However, it is not aware of anything that happens abovethe link layer
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
When designing and deploying 802.11, network engineers
must take care so that the seamless transition at the radio
layer is also supported at the network protocol layer that the
station IP address can be preserved
As far as 802.11 is concerned, there are three types of
transitions between access points:
No transition
When stations do not move out of their current
access point's service area, no transition isnecessary
This state occurs because the station is not moving
or it is moving within the basic service area of its
current access point
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
BSS transition
Stations continuously monitor the signal strength
and quality from all access points administratively
assigned to cover an extended service area
Within an extended service area, 802.11 providesMAC layer mobility
Stations attached to the distribution system can
send out frames addressed to the MAC address of
a mobile station and let the access points handlethe final hop to the mobile station
Distribution system stations do not need to be
aware of a mobile station's location as long as it is
within the same extended service area
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
The following figure illustrates a BSS transition:
The three access points in the picture are all
assigned to the same ESS
At the outset, denoted by t=1, the laptop with an
802.11 network card is sitting within AP1's basic
service area and is associated with AP1
BSS transition
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
When the laptop moves out of AP1's basic service
area and into AP2's at t=2, a BSS transition occurs
The mobile station uses the re-association service
to associate with AP2, which then starts sending
frames to the mobile station BSS transitions require the cooperation of access
points. In this scenario, AP2 needs to inform AP1
that the mobile station is now associated with AP2
802.11 does not specify the details of the
communications between access points during
BSS transitions
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Note that even though two access points are
members of the same extended set, they may
nonetheless be connected by a router, which is a
layer 3 boundary
In such a scenario, there is no way to guarantee
seamless connectivity using 802.11 protocols only
ESS transition
An ESS transition refers to the movement from
one ESS to a second distinct ESS 802.11 does not support this type of transition,
except to allow the station to associate with an
access point in the second ESS once it leaves
the first
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Higher-layer connections are almost
guaranteed to be interrupted.
It would be fair to say that 802.11 supports
ESS transitions only to the extent that it is
relatively easy to attempt associating with an
access point in the new extended service area
Maintaining higher-level connections requires
support from the protocol suites in question
The following figure shows an ESS transition:
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Four basic service areas are organized into two
extended service areas Seamless transitions from the lefthand ESS to
the righthand ESS are not supported
ESS transitions are supported only because
the mobile station will quickly associate with an
access point in the second ESS
ESS transition
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Designing networks for mobility
Most networks are designed so that a group of
access points provides access to a group of
resources All the access points under control of the
networking organization are assigned to the same
SSID, and clients are configured to use that SSID
when connecting to the wireless network
As client systems move around, they continuously
monitor network connectivity, and shift between
access points in the same SSID
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
802.11 ensures that clients will be able to move
associations between the access points in the
same SSID, but network architects must build the
network to support mobile clients
Small networks are often built on a single VLANwith a single subnet, in which case there is no need
to worry about mobility
Larger networks that span subnet boundaries must
apply some additional technology to provide
mobility support
Wireless Roaming
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Follow Up Assignment
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Summary of Main Teaching Points
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WLAN System Architecture and Design (I)
CT031-3-3-Fundamentals of Wireless LANs
Q & A
Question and Answer Session
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WLAN System Architecture and Design (I)
Topic and Structure of next session
Wireless Interference
Interoperability and WLAN Backbones
Next Session