1. CIT 384: Network Administration
Wireless LANs
CIT 384: Network Administration

2. CIT 384: Network Administration
Topics
Wireless modes
Interference and coverage
Physical Standards
MAC Layer
WLAN Security
CIT 384: Network Administration

3. CIT 384: Network Administration
Wireless vs Ethernet
wireless similar to Ethernet
Uses 48-bit source and dest MAC addresses
Wireless networks can experience collisions.
is different from Ethernet
frames are acknowledged by recipient.
uses CSMA/CA instead of CSMA/CD.
is always half-duplex.
CIT 384: Network Administration

4. CIT 384: Network Administration
Ad-hoc WLANs
Devices send frames directly to each other.
Similar to shared bus Ethernet.
No specialized network device is necessary.
CIT 384: Network Administration

5. CIT 384: Network Administration
Infrastructure Mode
Devices talk with Access Point (AP), not directly with each other.
APs bridge wired and wireless networks.
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6. CIT 384: Network Administration
Wireless Modes
Mode
Service Set Name
Description
Ad Hoc
Independent Basic Service Set (iBSS)
Allows two devices to communicate directly. No AP needed.
Infrastructure (1 AP)
Basic Service Set
(BSS)
A single WLAN created with an AP and devices.
Infrastructure
(multiple APs)
Extended Service Set (ESS)
Multiple APs create a single WLAN, allowing roaming.
CIT 384: Network Administration

7. Service Set Identifier (SSID)
Each WLAN is identified by a SSID.
Set by APs in infrastructure WLANs.
Set by first client in ad hoc WLANs.
SSID is 1-32 octets.
Often human readable ASCII text.
Broadcast via beacon frames to clients.
SSID Cloaking
Turns off beacon frames from AP.
Client gets SSID by sending probe frames.
Doesn’t really provide any security.
CIT 384: Network Administration

8. Wireless Transmission
Wireless uses set of frequencies like radio.
Interferes w/ other devices in frequency range.
Frequency Range
Name
Sample devices
900 MHz
Industrial, Scientific, Mechanical (ISM)
Cordless telephones.
2.4 GHz
ISM
Cordless phones; WLANs; bluetooth, wireless game controllers.
5 GHz
Unlicensed NII
Newer ordless phones; WLANs.
CIT 384: Network Administration

9. CIT 384: Network Administration
Wireless Encoding
Frequency Hopping Spread Spectrum (FHSS)
Uses all frequencies in band, hopping btw them.
Avoids interference by using slightly difference frequencies for consecutive transmissions.
Original standard, Bluetooth.
Direct Sequence Spread Spectrum (DSSS)
Uses one of several separate channels (freqs.)
11 overlapping channels, – GHz.
Used by b
Orthogonal Frequency Division Multiplexing (OFDM)
Used by a, g, n (n uses multiple antennas)
CIT 384: Network Administration

10. Using nonoverlapping channels
CIT 384: Network Administration

11. Electromagnetic Interference (EMI)
Matter, especially metal, absorbs EM waves.
Can also cause reflection and scattering.
Other wireless devices can interfere
Bluetooth
Wireless controllers
Microwave ovens can interfere
Uses 2.45 GHz frequency, which produces heat by exciting water molecule dipoles.
Metal mesh reduces but does not eliminate EMI.
CIT 384: Network Administration

12. CIT 384: Network Administration
Coverage
The coverage area is a space in which two WLAN devices can communicate.
Speeds are highest near AP.
Coverage is not even—objects can interfere.
Improving coverage
Specialized antennas.
Higher power (FCC rules).
Remove interference.
CIT 384: Network Administration

13. CIT 384: Network Administration
Standards
is just another MAC layer.
802.11x are physical layer implementations.
Image from : The Definitive Guide
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14. CIT 384: Network Administration
Physical Standards
Protocol
Date
Max Speed
Throughput
Frequency
Indoor
Range
802.11b
1999
11 Mbps
4.3 Mbps
2.4 Ghz
35 meters
802.11a
54 Mbps
23 Mbps
5 Ghz
38 meters
802.11g
2003
19 Mbps
802.11n
2009
248 Mbps
74 Mbps
2.4,5 Ghz
50 meters
CIT 384: Network Administration

15. CIT 384: Network Administration
MAC Addresses
48-bit addresses
Taken from same address pool as Ethernet.
Use same vendor prefixes (OUIs)
Stored in ARP tables along w/ Ethernet MACs.
Up to 4 addresses per frame
Source and destination addresses.
Receiver address: indicates which wireless receiver should process the frame. If destination is wireless, receiver=destination. If destination is on Ethernet LAN, then the addresses differ.
Transmitter address: indicates which wireless interface transmitted the frame onto the wireless medium.
CIT 384: Network Administration

16. CIT 384: Network Administration
Frame Format
Frame Control
Indicates frame type, protocol version (0)
has specialized management frame types.
Duration/ID
Indicates time medium expected to be busy with current transmission (helps avoid collisions.)
Sequence control
4-bit fragment number for dealing with fragments
12-bit sequence number to identify frames
Image from : The Definitive Guide
CIT 384: Network Administration

17. Wired to Wireless Frame Translation
Image from : The Definitive Guide
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18. CIT 384: Network Administration
Association
Association allows distribution system to forward frames to the correct AP.
Required by infrastructure networks, not ad hoc.
Initiated by the wireless client.
AP assigns association ID (AID) to client.
Image from : The Definitive Guide
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19. CIT 384: Network Administration
Reassociation
Reassociation is the process of moving an association from an old AP to a new AP.
Necessary when a client moves from one AP’s coverage area to another.
Some clients monitor signal strength and will reassociate to AP with best signal.
Image from : The Definitive Guide
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20. CIT 384: Network Administration
Mobility
Mobility means that applications work no matter where the client is located.
Potential problems
Will moving the client change its IP address?
Will moving the client break long-running connections to network filesystems, databases...?
Will reestablishing network connectivity after being moved take so long that existing connections timeout?
CIT 384: Network Administration

21. CIT 384: Network Administration
Wireless Security
Wireless security problems
No definite physical boundary to network.
Unauth clients can easily obtain network access.
Clients can easily sniff network data.
Clients can alter data during transmission.
Security solutions
Wired Equivalent Privacy (WEP)
802.11i
CIT 384: Network Administration

22. Wired Equivalent Privacy
WEP is insecure
Don’t use it.
However, older devices only support WEP.
WEP encryption
Encrypts payload using RC4 stream cipher.
Uses 40-bit key + 24-bit initialization vector.
Uses Integrity Check Value (ICV) to check for tampering.
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23. CIT 384: Network Administration
How WEP Works
Image from : The Definitive Guide
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24. CIT 384: Network Administration
WEP Security Issues
WEP is easy to break with free tools
Attackers can decrypt any traffic.
Longer keys don’t slow attack significantly.
Solutions
Tunnel connections over a higher level secure encrypted protocol like ssh or SSL.
Dynamic WEP is a vendor extension that lets you change WEP key automatically every few minutes to limit exposure to broken keys.
CIT 384: Network Administration

25. CIT 384: Network Administration
Temporal Key Integrity Protocol (TKIP)
Backward-compatible with WEP hardware.
Uses less secure RC4 encryption hardware.
TKIP Improvements over WEP
Key distribution.
Derives a unique RC4 key for each frame.
Sequence counter prevents replay attacks.
Michael Integrity Check replaces insecure ICV.
Counter Mode with CBC-MAC Protocol (CCMP)
Built on better known secure AES cipher.
Developed from ground up to be secure, unlike TKIP.
CIT 384: Network Administration

26. Wi-Fi Protected Access (WPA)
Marketing standard from Wi-Fi Alliance.
WPA based on 3rd draft of i (TKIP).
WPA2 is i final standard.
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27. CIT 384: Network Administration
References
James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.
Cisco, Cisco Connection Documentation,
Cisco, Internetworking Basics,
Matthew Gast, Wireless Networks: The Definitive Guide, O’Reilly, 2005.
Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.
CIT 384: Network Administration