CIT 384: Network Administration Wireless LANs CIT 384: Network Administration

CIT 384: Network Administration Topics Wireless modes Interference and coverage 802.11 Physical Standards 802.11 MAC Layer WLAN Security CIT 384: Network Administration

CIT 384: Network Administration Wireless vs Ethernet 802.11 wireless similar to Ethernet Uses 48-bit source and dest MAC addresses Wireless networks can experience collisions. 802.11 is different from Ethernet 802.11 frames are acknowledged by recipient. 802.11 uses CSMA/CA instead of CSMA/CD. 802.11 is always half-duplex. CIT 384: Network Administration

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

CIT 384: Network Administration Infrastructure Mode Devices talk with Access Point (AP), not directly with each other. APs bridge wired and wireless networks. CIT 384: Network Administration

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

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

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

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 802.11 standard, Bluetooth. Direct Sequence Spread Spectrum (DSSS) Uses one of several separate channels (freqs.) 11 overlapping channels, 2.402 – 2.483 GHz. Used by 802.11b Orthogonal Frequency Division Multiplexing (OFDM) Used by 802.11a, g, n (n uses multiple antennas) CIT 384: Network Administration

Using nonoverlapping channels CIT 384: Network Administration

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

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

CIT 384: Network Administration 802.11 Standards 802.11 is just another MAC layer. 802.11x are physical layer implementations. Image from 802.11: The Definitive Guide CIT 384: Network Administration

CIT 384: Network Administration 802.11 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

CIT 384: Network Administration 802.11 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

CIT 384: Network Administration 802.11 Frame Format Frame Control Indicates frame type, protocol version (0) 802.11 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 802.11: The Definitive Guide CIT 384: Network Administration

Wired to Wireless Frame Translation Image from 802.11: The Definitive Guide CIT 384: Network Administration

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 802.11: The Definitive Guide CIT 384: Network Administration

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 802.11: The Definitive Guide CIT 384: Network Administration

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

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

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. CIT 384: Network Administration

CIT 384: Network Administration How WEP Works Image from 802.11: The Definitive Guide CIT 384: Network Administration

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

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

Wi-Fi Protected Access (WPA) Marketing standard from Wi-Fi Alliance. WPA based on 3rd draft of 802.11i (TKIP). WPA2 is 802.11i final standard. CIT 384: Network Administration

CIT 384: Network Administration References James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005. Cisco, Cisco Connection Documentation, http://www.cisco.com/univercd/home/home.htm Cisco, Internetworking Basics, http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm Matthew Gast, 802.11 Wireless Networks: The Definitive Guide, O’Reilly, 2005. Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007. CIT 384: Network Administration