03 FWL System ArchitectureandDesign(I)

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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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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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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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Main Teaching Points

WLAN Design Considerations

Wireless Roaming


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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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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



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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



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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






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






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





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





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





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





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





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





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





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





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





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





Follow Up Assignment





Summary of Main Teaching Points





Q & A

Question and Answer Session


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Topic and Structure of next session

Wireless Interference

Interoperability and WLAN Backbones

Next Session

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03 FWL System ArchitectureandDesign(I)

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