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Chapter 7
Wireless Local Area Networks
Some new material added!
Chapter 7 - Wireless Networks
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Introduction
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WLANs serve same purpose as LANs
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Connect a set of wireless computers into a wired
network
But can extend a LAN where it is not previously
wired therefore making casual connections
possible
Aka WiFi – used by 90% of companies
This chapter looks at the data link layers and
physical layers of several technologies
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I. Wireless Ethernet (802.11b/g)
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WLAN topology looks like wired star
with access point at center as hub
Can apply security settings: encryption
802.11b – up to 11 Mbps
802.11g – up to 54 Mbps
Central access point is a radio
transceiver that communicates like hub
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It is a repeater to all clients connected
Can also be connected to wired network
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Access Point
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Home models are usually wireless
routers.
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Act as access point, wired switch, and
firewall, NAT
WAN port; LAN ports; wireless ports
Business models are access points
connected to a central management pt
ISU uses Cisco access points ~$600
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802.11b/g technology
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3 radio frequencies used on 2.4 GHz
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Same band as cordless phones and some
microwave ovens
Can cause problems in apartment-type living
NIC listens (CSMA) to find strongest channel
(may hear several APs)
As user roams through the network, NIC may
reselect a different AP.
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We can stay connected from COB to HMSU!
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More Technology
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Antennas – Fig 7.3 p. 225
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Size of antenna “cloud” affects
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Directional – narrower, more focused
Omnidirectional – all directions
How well users are picked up
Security – does signal reach outside bldg?
802.11g can “shift down” to 802.11b
but all clients must be b in low-end APs
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Wireless Adapters
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Connector for
antenna
PC Card – Fig 7.2.
Laptop slot
miniPCI card – fits
inside laptop with
antenna around the
screen: better!
USB adapter – good
for desktops or
laptops
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Wireless Connection Types
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Infrastructure (access point)
Ad Hoc (computer to computer)
Any available network (AP preferred)
If you choose the wrong type, it will not
work!
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Media Access Control
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Distributed Coordination – each computer
listens to see if channel is open
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Not good for wide networks where computers at
edge may not be able to hear each other
Point Coordination – each computer sends a
request to send (RTS) to the AP, then it
allows one to talk.
Efficiency – capacity is shared by all active
computers on the network (e.g., 11/2 = 5.5)
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Speed on 802.11b/g
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802.11b=11 Mbps, 802.11g=54 Mbps (shorter range)
Actual speed depends on …
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Signal strength effects of range
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Trans. errors (distance, obstructions, quality of antennas)
Traffic effects on speed
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up to 200+ feet without obstructions
Practical is 15-50 feet with obstructions: experiment!
802.11g is shorter range than 802.11b
802.11b: low (4.8), moderate (1.9), or high (960K)
802.11g: low (17.2), moderate (6.9), high (3.4)
Super G = version of 802.11g at 108 Mbps
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Aka Wireless-G Enhanced
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Physical Design Concerns
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Engineering is necessary!
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Cathy’s older sorority house
ISU wireless project used engineering
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Then did reengineering when the assumptions
changed (to cover faculty offices)
Antenna design makes a big difference
Hand-off issues for mobile users
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Configuration/Security
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For a client to connect to an access point,
must know the …
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SSID of access point (Service Set ID)
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Broadcast SSID (anyone can see it)
Silent SSID (client must already know it)
WEP key (wired equivalent privacy Encryption)
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This seems like a good idea but it can be quickly broken
ala Enigma Machine (periodic status reports allow
working backward to get the WEP key)
Store up to 4 WEP keys
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Pre-Windows XP Client
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First install
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Driver for wireless adapter
Client software for the wireless NIC
Next attach the wireless adapter
Configure the client SW for connection
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for each access point set
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SSID (network name)
WEP (key) if enabled
Can also configure for “choose any AP”
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Windows XP Client
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Install driver for wireless NIC and install adapter
Use Windows XP client software – built-in wireless
client (it disables legacy client software)
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Properties of the wireless NIC connection
Use the Wireless Networks tab
Can set up preferred networks in your order
You’ll get a message when an AP is in range
Advanced: enable 802.1x authentication (802.11i)
Look for connection status in the tray: signal strength
color bar (red – yellow – green)
I have had to disable the wireless bridge (???)
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II. Wireless 802.11a (newer)
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Speedy: 802.11a – up to 54 Mbps!
This is newer than 802.11b
Operates in the 5.0 GHz range
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Frequency relatively free from interference (unlike 802.11b)
http://www.networkcomputing.com/1201/1201ws1.html
gives technical details about frequencies of .11a and .11b
A has more channels (4-12) than B (3) so could have more
APs in a given location for more bandwidth
Each channel has 52 subchannels
Media access control and packet layout similar to B
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III. Bluetooth (802.15)
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Wireless Personal Area Network (WPAN)
Strikingly different purpose
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Provide very small area wireless (<30 ft)
Connects two devices rather that to wired LAN
Replace short cable between computer and printer, PDA and
cell phone, etc.
Speed is 1 Mbps – slow but OK
Up to 8 devices connected; mostly 2
Not intended to do general networking
Named after Danish King Bluetooth (really!)
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IV. Other Wireless
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Infrared – requires direct line of sight
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New version can bounce off walls, not
direct line of sight, but only in same room
Infrared used for printers, Palm Pilot PDAs,
others
802.11g – long distance (MAN)
Joink Fixed wireless – 2-10 mile range
at DSL like speeds
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V. Best Practice WLAN Design
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Tradeoff: data rate and cost
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See Fig 7.12 p. 242 on data rate and users
See Tech Focus 7-1 p. 243 on distance and speed
Don’t forget the very high cost of installing
wiring vs. wireless
Need for engineering approach
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See Fig 7.13, 7.14 p. 246 for antenna layouts
We look like Fig 7.14 in COB
See Fig 7.15 p. 249 for coverage at IU
Reexamine usage levels for better placement
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WLAN Security issues !!
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Assume these networks are not secure
Ways to improve security
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Don’t broadcast SSID
Use WEP
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Change SSID and WEP keys frequently
Can use EAP – extensible authentication
protocol where keys are produced dynamically
for each session, then discarded
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More WLAN Security Issues
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Turn off remote management (like mine) so nobody
can get in and change things
Change the admin password in the web server
section
Consider VPN client only for access
Establish rules on who can connect when
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Can use MAC addresses (but users can spoof an IP)
Use authentication – 802.11i
Disable DHCP and preset IP addresses on certain
machines – smart and easy.
Adjust router location to reduce outside footprint
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802.11i – Future Standard
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This adds client authentication to AP role
along with changing keys
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WPA – WiFi Protected Access (scaled down)
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Temporal Key Integrity Protocol
WPA fixes WEP’s problems by rotating keys
RSN – Robust Security Network (.11i)
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Dynamic negotiation of authentication and keys
Improves on WPA
Radius server does the authentication (AP talks to it)
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