Wireless LAN Topologies

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www.greenITcenter.org

DUE 0903239

Wireless LAN
Topologies


Module 10

www.greenITcenter.org


Wireless LAN Topologies


Computer networks:


Peer
-
to
-
peer


Client/server


Centralized CPU with dumb terminals


Networking
topology



physical

and/or
logical

layout of nodes in a computer network.


Wired topologies:


Bus, Ring, Star, Mesh, Hybrid


Coverage from small area to worldwide


802.11 standard


Three topologies or
services sets

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Originator: Quissannie
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Wireless LAN Topologies


Wireless Networking Topologies


802.11 Topologies


802.11 Configuration Modes

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Wireless Networking Topologies


Wireless Wide Area Network (WWAN
)


Cellular


Wireless Metropolitan Area Network (WMAN)


Wireless Personal Area Network (WPAN
)


Bluetooth,
ZigBee


Wireless Local Area Network (WLAN
)


802.11
-
2007

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Wide
Area Network
(WAN
)


Wide area network (WAN) covers a large
geographic area:


State, region, country or worldwide


The Internet is a WAN


WAN infrastructure


T1, fiber, routers


WAN protocols


Frame relay, ATM, MPLS,
etc

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Wireless Wide Area Network (WWAN)


Also covers a large geographic area:


Wireless transmission medium, rather than wired.


WWANs may
use cellular technologies


GSM


CDMA


TDMA


GPRS


Data rates relatively slow, but improving (4G)


802.11
cannot

be deployed as WWAN

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Wireless Metropolitan Area Network
(WMAN)


Covers a smaller geographical area such as a
metropolitan area (e.g. city + surrounding suburbs)


May use 802.16
standard


Broadband wireless access


Worldwide Interoperability for Microwave Access (
WiMAX
)


competes directly with
DSL and
cable (last
-
mile)


Used as a
backhaul


More info:


ieee802.org/16


www.wimaxforum.org


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Wireless Personal Area Network (WPAN)


Communication with devices within close proximity to
a user.


Devices include:


Laptops, PDAs
,
phones
,
headsets, keyboards, mice,
etc


Most common WPAN is
Bluetooth 802.15


RF technology that uses FHSS


ZigBee

also under 802.15 (wireless sensors; not widely
used)


For WPAN
:


low bandwidth


low throughput

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Originator: Quissannie
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Wireless Personal Area Network (WPAN)


More info:


www.ieee802.org/15


www.bluetooth.com


www.zigbee.org


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Originator: Quissannie
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Wireless Local Area Network (WLAN)


Defined in 802.11
-
2007 standard


802.11 wireless networking ideal for building or
campus environment due to range and speed.


Common
in
SOHO


Common
in
enterprise
n
etworking


May
include several
Aps


Provide end users with access to:


Network resources


Network services


Internet



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Originator: Quissannie
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802.11 Topologies


Main component of an 802.11 wireless network is
the radio card, which is used inside:


Wireless client or station (STA)


Access point (AP)


Three topologies or service sets:


Basic service set (BSS)


Extended service set (ESS)


Independent basic service set (IBSS)


Nonstandard topologies:


Bridging


Repeating


Workgroup bridge


M
esh networking

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Originator: Quissannie
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802.11 Topologies


Review of basic networking terms:


Simplex



One devices transmits to multiple
receivers.


AM and FM radio


Rarely used in computer networks.


Half
-
duplex


Both devices are capable of
transmitting and receiving, but only one device at a
time.


Two
-
way radios, walkie
-
talkies


IEEE 802.11 wireless networks use half
-
duplex


Full
-
duplex


Both devices are capable of
simultaneously transmitting and receiving.


Telephone


802.3 wired networks

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Originator: Quissannie
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802.11 Topologies


Access
points (AP)


Wireless
clients


Integration
service
(IS)


Distribution
system (
DS)


Wireless
distribution
s
ystem
(WDS)


Service s
et
i
dentifier
(SSID)


Basic
service
s
et
(BSS)


Basic s
ervice
s
et identifier
(BSSID)


Extended
service
s
et
(ESS)


Independent b
asic
s
ervice set
(IBSS)


Nonstandard 802.11
topologies


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Access Points


An AP is basically a hub with a radio card.


Remember, a hub communicates in half
-
duplex.


The RF medium allows for only one device
communicating at a time


half
-
duplex.


Contend for the
medium along with clients.


Autonomous APs
have switch
-
like
intelligence versus
lightweight APs
that
require a
WLAN controller
to do their job.


Autonomous APs can address and direct traffic
using layer 2 information (MAC address).


An autonomous AP is a hybrid device with both
switch and hub functionality.



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Originator: Quissannie
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Access Points


802.11 header of wireless frame typically has 3
MAC addresses, but could have as many as four.


AP use layer 2 addresses to forward frame (layer 3
-
7
information) to client or integration device (switch).


Upper layer information is known as
MAC Service Data
Unit (MSDU)
.



AP also functions as:


Media converter (wired


wireless)


Point coordinator (for PCF)



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Client Stations



Any non
-
AP device with a radio card.


Can be any number of devices:


Laptops


PDA

s


Scanners


Printers


Phones


Contend for half
-
duplex wireless medium with each
other and AP.


When connected to AP (layer 2) client is said to be
associated

with an AP

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Integration Service (IS)


Enables
delivery of MSDUs between
the non
-
802.11
LAN and the d
istribution system (DS)


A
frame format transfer/conversion method


MSDU is usually intended for wired network.


Wired (802.3) and wireless (802.11) are difference
physical mediums, we need to transfer from one to
the other.


IS removes 802.11 header and trailer.


MSDU payload is encased inside 802.3 frame.


802.3 frame sent to Ethernet network.

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Originator: Quissannie
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Distribution System (DS)


Defined in 802.11
-
2007, a DS interconnects
a set of
basic service sets
(BSSs)

via
integrated
LANs to
create
an
Extended Service Set (ESS
)
.


Consists of two main
components
:


Distribution System Medium (DSM
)


logical physical
medium used to connect APs. (e.g. 802.3)


Distribution System Services (DSS
)


services built
inside an AP (software) that provide the switch
-
like
intelligence.


Manage client associations, re
-
associations, and disassociations.


Forward MSDU to wireless client or IS







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Modified by: Brierley
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Originator: Quissannie
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www.greenITcenter.org


Wireless Distribution System (WDS)


802.11
-
2007 defines wireless communication
using a frame with four MAC addresses.


This mechanism is a WDS, which is essentially a
DS
that uses the
wireless medium.


Examples of WDS
include:


Bridges


Repeaters


Wireless Mesh Networks

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Originator: Quissannie
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www.greenITcenter.org


Wireless Distribution System (WDS)


Radios in APs not only connect clients, but
communicate with each other as a WDS.


Half
-
duplex medium effects performance.



Improve performance by using dual radios in each AP.


802.3 network is best option for DS.

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Originator: Quissannie
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Wireless Distribution System (WDS)


Repeater extends the cell coverage of original AP.

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Must be on same channel


Must be at least 50% cell overlap.


Transmissions must be sent twice.


Decreased throughput


Increased latency.

Modified by: Brierley
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Feb 2012

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Originator: Quissannie
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www.greenITcenter.org


Service Set Identifier (SSID)


The SSID is the logical name used
to
identify a wireless
network.


The three 802.11 topologies
(BSS, ESS, and
IBSS)

use the SSID so that radio my identify
each other in:


Passive

scanning, or


Active

scanning


SSID


Configurable on clients and Aps


Case sensitive


Up to 32 characters.


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APs can
cloak

or mask the
SSID


considered a
very

weak security measure.

Modified by: Brierley
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Originator: Quissannie
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Basic Service Set (BSS)


The foundational topology of an 802.11 network.


Consists of an access
p
oint
(AP) and any
associated
clients.

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Clients join the
wireless domain of the
AP and begin
communicating
through the AP.


Members of the BSS
have a layer 2
connection to the AP
and are considered
associated.

Modified by: Brierley
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Feb 2012

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Originator: Quissannie
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August 2010

www.greenITcenter.org


Basic Service Set (BSS)


AP typically connected to a DS, but not required
to be a BSS.


If AP is connected to DS, client can communicate with
resources on the DSM.


Clients that wish to communicate with each other do
so through the AP.

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Basic Service
Set Identifier (BSSID)


48
-
bit [6
-
octet] MAC address on AP’s radio card.


e.g.
58:55:ca:f2:a8:
a1


Serves as the layer 2 identifier for each BSS


BSSID versus SSID


Not the same!


SSID is logical and user configurable (WLAN name).


BSSID


A

physical address provided by manufacturer.


Found in header of most 802.11 frames, for identification.


Plays a role in directing traffic within BSS


Required during roaming.

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Basic Service
Set Identifier (BSSID)

Basic Service Area (BSA)


in a BSS, this is the
physical area of coverage provided by the AP.


Client can move throughout BSA and stay connected.

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Within the BSA, data rates can
change as the distance from the
AP changes


dynamic rate
switching
.


Size and shape of BSA depends
on:


AP transmit power


Antenna gain


Physical surroundings

Modified by: Brierley
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Originator: Quissannie
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Extended Service Set (ESS)

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An
ESS

builds upon the BSS, by combining 1
or
more BSS

s connected
by a distribution system
medium (DSM).


Typically a collection of APs and their clients all
connected by a single DSM.


The most common example:


APs with overlapping cells.


Seamless roaming


15

25% overlap

recommended


Seamless roaming

not required.

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Originator: Quissannie
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Extended Service Set (ESS)


Nomadic roaming


no seamless roaming

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Extended Service Set (ESS)


Colocation



multiple APs with totally
overlapping coverage areas
.


Intended to provide increased capacity.

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Extended Service Set (ESS)


Extended Service Set Identifier (ESSID)


the
network name of an ESS.



Access Points in an ESS where roaming is required
must all share the same SSID [logical], but have unique
BSSIDs [physical] for each BSS cell.

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Independent Basic Service
Set
(IBSS
)

Independent Basic Service Set Identifier (IBSS)


a
topology consisting solely of clients


no AP.


Also referred to as an
ad hoc or peer
-
to
-
peer
network.


All clients transmits frames directly to each other.


Only one transmit at a time.


All clients on same

frequency.


Same SSID WLAN name.


BSSID comes from first

station that starts up.

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Nonstandard 802.11 Topologies

Vendors can implement 802.11 radio cards in
nonstandard topologies
with remaining compliant
with 802.11
-
2007.


Wireless bridging


most common. Connects two wired
networks together using wireless link.


Mesh networks


Workgroup bridge


gateway for a
small wired
workgroup (bridge is also a client connected to AP).


Repeater



special AP the forward client data to a root
AP.

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802.11 Configuration Modes


Access Point Modes


Root Mode


Only mode compliant with 802.11 standard.


Default mode.


AP serves as portal to a distribution system (DS).


AP transfers data back and forth between DS
& wireless
clients.


Bridge Mode


AP acts as a wireless bridge.


Workgroup Bridge
Mode


AP acts as a
workgroup
bridge.


Repeater
Mode


AP acts as a repeater AP.


Scanner Mode


AP acts as a senor radio, becoming
part of a
wireless intrusion detection system (WIDS)
.

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802.11 Configuration Modes


Station
Modes


Infrastructure
Mode


Default mode for a client radio card.


Communication via an AP.


Client participates in a BSS or ESS.


Client can access resources on the DS.


Ad Hoc
Mode


Also called peer
-
to
-
peer mode.


Participate in an IBSS.


No AP; all transmissions and exchanges are peer
-
to
-
peer.



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Originator: Quissannie
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Originator: Quissannie
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August 2010

Questions: Feel free to contact
the manager of this material

Mike Qaissaunee

Professor,
Brookdale

Community College,

mqaissaunee@gmail.com







This material is based upon work supported by the National Science Foundation under

Grant No. 0903239. Any opinions, findings, and conclusions or recommendations expressed in this material are those
of the author(s) and do not necessarily reflect the views of the National Science Foundation


Modified by: Brierley
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Feb 2012

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