Chapter 7

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Chapter 7

  1. 1. Chapter 7 Wireless Local Area Networks Some new material added!
  2. 2. Introduction <ul><li>WLANs serve same purpose as LANs </li></ul><ul><ul><li>Connect a set of wireless computers into a wired network </li></ul></ul><ul><ul><li>But can extend a LAN where it is not previously wired therefore making casual connections possible </li></ul></ul><ul><li>Aka WiFi – used by 90% of companies </li></ul><ul><li>This chapter looks at the data link layers and physical layers of several technologies </li></ul>
  3. 3. I. Wireless Ethernet (802.11b/g) <ul><li>WLAN topology looks like wired star with access point at center as hub </li></ul><ul><li>Can apply security settings: encryption </li></ul><ul><li>802.11b – up to 11 Mbps </li></ul><ul><li>802.11g – up to 54 Mbps </li></ul><ul><li>Central access point is a radio transceiver that communicates like hub </li></ul><ul><ul><li>It is a repeater to all clients connected </li></ul></ul><ul><ul><li>Can also be connected to wired network </li></ul></ul>
  4. 4. Access Point <ul><li>Home models are usually wireless routers . </li></ul><ul><ul><li>Act as access point, wired switch , and firewall , NAT </li></ul></ul><ul><ul><li>WAN port; LAN ports; wireless ports </li></ul></ul><ul><li>Business models are access points connected to a central management pt </li></ul><ul><li>ISU uses Cisco access points ~$600 </li></ul>
  5. 5. 802.11b/g technology <ul><li>3 radio frequencies used on 2.4 GHz </li></ul><ul><ul><li>Same band as cordless phones and some microwave ovens </li></ul></ul><ul><ul><li>Can cause problems in apartment-type living </li></ul></ul><ul><li>NIC listens (CSMA) to find strongest channel (may hear several APs) </li></ul><ul><li>As user roams through the network, NIC may reselect a different AP. </li></ul><ul><ul><li>We can stay connected from COB to HMSU! </li></ul></ul>
  6. 6. More Technology <ul><li>Antennas – Fig 7.3 p. 225 </li></ul><ul><ul><li>Directional – narrower, more focused </li></ul></ul><ul><ul><li>Omnidirectional – all directions </li></ul></ul><ul><li>Size of antenna “cloud” affects </li></ul><ul><ul><li>How well users are picked up </li></ul></ul><ul><ul><li>Security – does signal reach outside bldg? </li></ul></ul><ul><li>802.11g can “shift down” to 802.11b but all clients must be b in low-end APs </li></ul>
  7. 7. Wireless Adapters <ul><li>PC Card – Fig 7.2. Laptop slot </li></ul><ul><li>miniPCI card – fits inside laptop with antenna around the screen: better! </li></ul><ul><li>USB adapter – good for desktops or laptops </li></ul>Connector for antenna
  8. 8. Wireless Connection Types <ul><li>Infrastructure (access point) </li></ul><ul><li>Ad Hoc (computer to computer) </li></ul><ul><li>Any available network (AP preferred) </li></ul><ul><li>If you choose the wrong type, it will not work! </li></ul>
  9. 9. Media Access Control <ul><li>Distributed Coordination – each computer listens to see if channel is open </li></ul><ul><ul><li>Not good for wide networks where computers at edge may not be able to hear each other </li></ul></ul><ul><li>Point Coordination – each computer sends a request to send (RTS) to the AP, then it allows one to talk. </li></ul><ul><li>Efficiency – capacity is shared by all active computers on the network (e.g., 11/2 = 5.5) </li></ul>
  10. 10. Speed on 802.11b/g <ul><li>802.11b=11 Mbps, 802.11g=54 Mbps ( shorter range ) </li></ul><ul><li>Actual speed depends on … </li></ul><ul><ul><li>Signal strength effects of range </li></ul></ul><ul><ul><ul><li>up to 200+ feet without obstructions </li></ul></ul></ul><ul><ul><ul><li>Practical is 15-50 feet with obstructions: experiment! </li></ul></ul></ul><ul><ul><ul><li>802.11g is shorter range than 802.11b </li></ul></ul></ul><ul><ul><li>Trans. errors (distance, obstructions, quality of antennas) </li></ul></ul><ul><ul><li>Traffic effects on speed </li></ul></ul><ul><ul><ul><li>802.11b: low (4.8), moderate (1.9), or high (960K) </li></ul></ul></ul><ul><ul><ul><li>802.11g: low (17.2), moderate (6.9), high (3.4) </li></ul></ul></ul><ul><li>Super G = version of 802.11g at 108 Mbps </li></ul><ul><ul><li>Aka Wireless-G Enhanced </li></ul></ul>
  11. 11. Physical Design Concerns <ul><li>Engineering is necessary! </li></ul><ul><ul><li>Cathy’s older sorority house </li></ul></ul><ul><ul><li>ISU wireless project used engineering </li></ul></ul><ul><ul><ul><li>Then did reengineering when the assumptions changed (to cover faculty offices) </li></ul></ul></ul><ul><ul><li>Antenna design makes a big difference </li></ul></ul><ul><ul><li>Hand-off issues for mobile users </li></ul></ul>
  12. 12. Configuration/Security <ul><li>For a client to connect to an access point, must know the … </li></ul><ul><ul><li>SSID of access point (Service Set ID) </li></ul></ul><ul><ul><ul><li>Broadcast SSID (anyone can see it) </li></ul></ul></ul><ul><ul><ul><li>Silent SSID (client must already know it) </li></ul></ul></ul><ul><ul><li>WEP key (wired equivalent privacy Encryption) </li></ul></ul><ul><ul><ul><li>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) </li></ul></ul></ul><ul><ul><ul><li>Store up to 4 WEP keys </li></ul></ul></ul>
  13. 13. Pre-Windows XP Client <ul><li>First install </li></ul><ul><ul><li>Driver for wireless adapter </li></ul></ul><ul><ul><li>Client software for the wireless NIC </li></ul></ul><ul><li>Next attach the wireless adapter </li></ul><ul><li>Configure the client SW for connection </li></ul><ul><ul><li>for each access point set </li></ul></ul><ul><ul><ul><li>SSID (network name) </li></ul></ul></ul><ul><ul><ul><li>WEP (key) if enabled </li></ul></ul></ul><ul><ul><li>Can also configure for “choose any AP” </li></ul></ul>
  14. 14. Windows XP Client <ul><li>Install driver for wireless NIC and install adapter </li></ul><ul><li>Use Windows XP client software – built-in wireless client (it disables legacy client software) </li></ul><ul><ul><li>Properties of the wireless NIC connection </li></ul></ul><ul><ul><li>Use the Wireless Networks tab </li></ul></ul><ul><ul><li>Can set up preferred networks in your order </li></ul></ul><ul><ul><li>You’ll get a message when an AP is in range </li></ul></ul><ul><ul><li>Advanced: enable 802.1x authentication (802.11i) </li></ul></ul><ul><li>Look for connection status in the tray: signal strength color bar (red – yellow – green) </li></ul><ul><li>I have had to disable the wireless bridge (???) </li></ul>
  15. 15. II. Wireless 802.11a (newer) <ul><li>Speedy: 802.11a – up to 54 Mbps! </li></ul><ul><li>This is newer than 802.11b </li></ul><ul><li>Operates in the 5.0 GHz range </li></ul><ul><ul><li>Frequency relatively free from interference (unlike 802.11b) </li></ul></ul><ul><ul><li>http://www.networkcomputing.com/1201/1201ws1.html gives technical details about frequencies of .11a and .11b </li></ul></ul><ul><ul><li>A has more channels (4-12) than B (3) so could have more APs in a given location for more bandwidth </li></ul></ul><ul><ul><li>Each channel has 52 subchannels </li></ul></ul><ul><li>Media access control and packet layout similar to B </li></ul>
  16. 16. III. Bluetooth (802.15) <ul><li>Wireless Personal Area Network (WPAN) </li></ul><ul><li>Strikingly different purpose </li></ul><ul><ul><li>Provide very small area wireless (<30 ft) </li></ul></ul><ul><ul><li>Connects two devices rather that to wired LAN </li></ul></ul><ul><ul><li>Replace short cable between computer and printer, PDA and cell phone, etc. </li></ul></ul><ul><ul><li>Speed is 1 Mbps – slow but OK </li></ul></ul><ul><ul><li>Up to 8 devices connected; mostly 2 </li></ul></ul><ul><ul><li>Not intended to do general networking </li></ul></ul><ul><li>Named after Danish King Bluetooth (really!) </li></ul>
  17. 17. IV. Other Wireless <ul><li>Infrared – requires direct line of sight </li></ul><ul><ul><li>New version can bounce off walls, not direct line of sight, but only in same room </li></ul></ul><ul><ul><li>Infrared used for printers, Palm Pilot PDAs, others </li></ul></ul><ul><li>802.11g – long distance (MAN) </li></ul><ul><li>Joink Fixed wireless – 2-10 mile range at DSL like speeds </li></ul>
  18. 18. V. Best Practice WLAN Design <ul><li>Tradeoff: data rate and cost </li></ul><ul><ul><li>See Fig 7.12 p. 242 on data rate and users </li></ul></ul><ul><ul><li>See Tech Focus 7-1 p. 243 on distance and speed </li></ul></ul><ul><li>Don’t forget the very high cost of installing wiring vs. wireless </li></ul><ul><li>Need for engineering approach </li></ul><ul><ul><li>See Fig 7.13, 7.14 p. 246 for antenna layouts </li></ul></ul><ul><ul><li>We look like Fig 7.14 in COB </li></ul></ul><ul><ul><li>See Fig 7.15 p. 249 for coverage at IU </li></ul></ul><ul><ul><li>Reexamine usage levels for better placement </li></ul></ul>
  19. 19. WLAN Security issues !! <ul><li>Assume these networks are not secure </li></ul><ul><li>Ways to improve security </li></ul><ul><ul><li>Don’t broadcast SSID </li></ul></ul><ul><ul><li>Use WEP </li></ul></ul><ul><ul><ul><li>Change SSID and WEP keys frequently </li></ul></ul></ul><ul><ul><ul><li>Can use EAP – extensible authentication protocol where keys are produced dynamically for each session, then discarded </li></ul></ul></ul>
  20. 20. More WLAN Security Issues <ul><li>Turn off remote management (like mine) so nobody can get in and change things </li></ul><ul><li>Change the admin password in the web server section </li></ul><ul><li>Consider VPN client only for access </li></ul><ul><li>Establish rules on who can connect when </li></ul><ul><ul><li>Can use MAC addresses (but users can spoof an IP) </li></ul></ul><ul><ul><li>Use authentication – 802.11i </li></ul></ul><ul><li>Disable DHCP and preset IP addresses on certain machines – smart and easy. </li></ul><ul><li>Adjust router location to reduce outside footprint </li></ul>
  21. 21. 802.11i – Future Standard <ul><li>This adds client authentication to AP role along with changing keys </li></ul><ul><ul><li>WPA – WiFi Protected Access (scaled down) </li></ul></ul><ul><ul><ul><li>Temporal Key Integrity Protocol </li></ul></ul></ul><ul><ul><ul><li>WPA fixes WEP’s problems by rotating keys </li></ul></ul></ul><ul><ul><li>RSN – Robust Security Network (.11i) </li></ul></ul><ul><ul><ul><li>Dynamic negotiation of authentication and keys </li></ul></ul></ul><ul><ul><ul><li>Improves on WPA </li></ul></ul></ul><ul><ul><ul><li>Radius server does the authentication (AP talks to it) </li></ul></ul></ul>

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