chapter 7 - wireless networks1 chapter 7 wireless local area networks some new material added!

Chapter 7 - Wireless Networks 1

Chapter 7

Wireless Local Area Networks Some new material added!


Introduction WLANs serve same purpose as LANs

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


I. Wireless Ethernet (802.11b/g) 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 It is a repeater to all clients connected Can also be connected to wired network


Access Point Home models are usually wireless

routers. 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


802.11b/g technology 3 radio frequencies used on 2.4 GHz

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. We can stay connected from COB to HMSU!


More Technology Antennas – Fig 7.3 p. 225

Directional – narrower, more focused Omnidirectional – all directions

Size of antenna “cloud” affects 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


Wireless Adapters 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

Connector for antenna


Wireless Connection Types

Infrastructure (access point) Ad Hoc (computer to computer) Any available network (AP

preferred)

If you choose the wrong type, it will not work!


Media Access Control Distributed Coordination – each computer

listens to see if channel is open 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)


Speed on 802.11b/g 802.11b=11 Mbps, 802.11g=54 Mbps (shorter

range) Actual speed depends on …

Signal strength effects of range up to 200+ feet without obstructions Practical is 15-50 feet with obstructions: experiment! 802.11g is shorter range than 802.11b

Trans. errors (distance, obstructions, quality of antennas) Traffic effects on speed

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 Aka Wireless-G Enhanced


Physical Design Concerns

Engineering is necessary! Cathy’s older sorority house ISU wireless project used engineering

Then did reengineering when the assumptions changed (to cover faculty offices)

Antenna design makes a big difference

Hand-off issues for mobile users


Configuration/Security For a client to connect to an access point,

must know the … SSID of access point (Service Set ID)

Broadcast SSID (anyone can see it) Silent SSID (client must already know it)

WEP key (wired equivalent privacy Encryption) 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


Pre-Windows XP Client First install

Driver for wireless adapter Client software for the wireless NIC

Next attach the wireless adapter Configure the client SW for connection

for each access point set SSID (network name) WEP (key) if enabled

Can also configure for “choose any AP”


Windows XP Client Install driver for wireless NIC and install adapter Use Windows XP client software – built-in

wireless client (it disables legacy client software)

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 (???)


II. Wireless 802.11a (newer) Speedy: 802.11a – up to 54 Mbps! This is newer than 802.11b Operates in the 5.0 GHz range

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


III. Bluetooth (802.15) Wireless Personal Area Network (WPAN) Strikingly different purpose

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!)


IV. Other Wireless Infrared – requires direct line of sight

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


V. Best Practice WLAN Design Tradeoff: data rate and cost

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

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


WLAN Security issues !!

Assume these networks are not secure

Ways to improve security Don’t broadcast SSID Use WEP

Change SSID and WEP keys frequently Can use EAP – extensible authentication

protocol where keys are produced dynamically for each session, then discarded


More WLAN Security Issues 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

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


802.11i – Future Standard This adds client authentication to AP

role along with changing keys WPA – WiFi Protected Access (scaled down)

Temporal Key Integrity Protocol WPA fixes WEP’s problems by rotating keys

RSN – Robust Security Network (.11i) Dynamic negotiation of authentication and keys Improves on WPA Radius server does the authentication (AP talks to

it)

chapter 7 - wireless networks1 chapter 7 wireless local area networks some new material added!

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