Wireless LAN Planning and Configuration

1. Wireless LAN Planning and Configuration MIS 4700 Dr. Garrett

2. Wireless Design Considerations

3. Planning & Design Issues • RF design issues • WLAN site qualification • Capacity and coverage • WLAN mode and topology • Existing network issues

4. RF Design Issues • IEEE 802.11 WLANs primarily operate on the 2.4 GHz Industrial, Scientific, and Medical (ISM) frequency band

5. ISM Band Issues • Competing devices • ISM is an unlicensed band and may contain many transmitting devices such as Bluetooth, cordless phones, emergency radio services • Distance • Signal strength and bandwidth decrease as the distance increases from the transmitter • 802.11b transmits at 11 Mbps at 30 meters but looses about 50% of its bandwidth every 30 meters beyond that • 802.11a transmits at looses about one third of its bandwidth every 25 meters

6. 802.11b Bandwidth The bandwidth of an 802.11b signal fades as its distance increases.

7. ISM Band Issues • Metal boxes • A metal box is any metallic or magnetic object that blocks or absorbs a transmitted RF signal • Building, air conditioning unit, metal partitions • Stationary objects • Walls, fireplaces, trees and large bushes • Shared bandwidth

8. RF Signal Attenuation by Common Materials Used with permission from Intel Corporation

9. RF Signal Attenuation by Common Materials Used with permission from Intel Corporation

10. RF Signal Attenuation by Common Materials Used with permission from Intel Corporation

11. WLAN Site Qualification • Scope and size of the network • Ability for the site to support the bandwidth requirements and number of nodes • 20 concurrent (active) users per wireless access point based on number of channels available • Type of traffic – small, bursty packets such as e-mail and Web traffic

12. Capacity and Coverage • Balance capacity with coverage needed • Capacity is the number of nodes and amount of shared bandwidth available to each node • Ensuring capacity ensures coverage

13. WLAN Coverage • Access points should generally be placed in the center of the area to provide as much coverage as possible • Multiple access points may be needed in larger areas • Overlapping coverage areas may provide redundancy

14. WLAN Capacity • 802.11b and 802.11g provide for three non-overlapping channels • 802.11b provides for eight non-overlapping channels • Bandwidth is shared by the users connected to the AP • Capacity changes based on the type of data and the load

15. WLAN Capacity • Increase capacity through more APs and using different WLAN standards • Devices may combine 802.11b and 802.11g, or 802.11a and 802.11g standards • Sometimes a device may combine all three

16. Ad-hoc Infrastructure Independent Basic Service Set (IBSS) Used by ad-hoc Basic Service Set (BSS) Used by Infrastructure Extended Service Set (ESS) Used by Infrastructure WLAN Mode & Topology

17. Ad-hoc Mode • Devices directly connect with each other • Similar to peer-to-peer networking • Does not require and access point An ad-hoc WLAN is created from two or more IBSS devices.

18. Infrastructure Mode • Requires at least one access point, or base station • Using one AP creates a BSS • Using two or more APs creates a ESS

19. BSS and ESS A BSS requires the inclusion of an access point (base station). Multiple access points in a single WLAN create an ESS topology.

20. Expanding a Wired Network • Expanding an existing wired network with a WLAN requires considering: • Dynamic Host Configuration Protocol (DHCP) usage • Roaming • APs must all be on the same subnet serviced by the same router • Throughput may be affected by adding wireless devices

21. Expanding a Wired Network • Coverage area of the network changes when an AP and wireless clients are added • Clients will expect the same capacity or bandwidth as with the wired network • Compatibility with existing nodes • Topology may change from BSS to ESS • Expandability in the future

22. Site Surveys

23. Site Survey • A site survey is a preliminary test of a proposed WLAN site • Tests an area to determine how RF signals will perform in its environment • Determines the amount of RF interference (RFI) or electromagnetic interference (EMI) present

24. Conducting Site Tests • Use a variety of antenna types • Use different combinations of APs

25. Site Survey Steps • Document the site through blueprints, existing network infrastructure, power outlets, and potential sources of interference • Perform a walk-through and visual inspection of the area • Determine radio coverage and interference patterns using site survey tools

26. Site Survey Steps • Identify the best locations for and the positioning of the WLAN access points and antennas • Identify the areas of the WLAN where additional antennas, access points, or repeaters may be needed to overcome interference, signal loss, or range problems

27. Site Survey Steps • Locate available power sources (outlets) and determine the need for UPSs • Install an AP in each area and retest • Note data transfer rate, bandwidth, signal strength • Document every test and finding

28. Site Survey Tools • Site surveys may be conducted using a portable PC with a wireless NIC and the proper software • Access Point • Antenna • Network adapter • Paper for documentation • Portable PC

29. Site Survey Tools • Battery pack and AC/DC converter if power is not available • Network or protocol analyzer (sniffer) to receive and analyze packets • Site survey software to perform RF environmental testing and data analysis • Site survey devices designed for that specific purpose • Spectrum analyzer to locate interference

30. AirMagnet Surveyor A screenshot of the AirMagnet Surveyor site survey software. Image courtesy of AirMagnet, Inc.

31. Site Survey Tools A handheld WLAN spectrum analyzer. Photo courtesy of Rohde & Schwarz GmbH & Co. KG A handheld special-purpose site survey device. Photo courtesy of Berkeley Varitronics Systems, Inc.

32. WLAN Modeling & Testing • The planned location for each AP and antenna should be modeled using a portable PC and wireless NIC Overlapping the RF ranges of wireless access points in a WLAN ensures coverage throughout an area.

33. Range & Coverage Modeling • Place your portable PC & wireless NIC in every location where a wireless station will be located – check signal strength Most wireless NICs are shipped with monitoring and configuration software.

34. Bandwidth Modeling • Bandwidth is shared in a WLAN • More than one AP may be needed to support the bandwidth required • Depends on the 802.11 standard being used, whether the network is indoor or outdoor, and how bursty network access is

35. Pilot Network Modeling • A pilot network is a small network that is a subset of the planned network • Provides live testing

36. Site Survey Software • Indicates which access point provides coverage in an area and the amount of overlap • Receive signal strength indicator (RSSI) shows the strength of the signal • Signal strength ranking

37. AirMagnet Surveyor The signal strength display of the AirMagnet Surveyor site survey software. Image courtesy of AirMagnet, Inc.

38. Seamless roaming Disconnects from one cell to establish a connection to the next cell in an overlapping manner Nomadic roaming Devices adjust their transceiver to the frequency and channel of the strongest signal Wireless Roaming

39. Roaming WLAN devices roam between wireless access points seeking an open channel on which to associate.

40. Channels • Access points use three frequency channels (channels 1, 6, and 11) because the frequencies used on these channels don’t overlap The 802.11b frequency channels defined for the U.S. and Canada.

41. WLAN Roaming • Uses a “break before make” sequence • Device breaks its connection with one AP before connecting to another • If a WLAN device roam from one ESS (roaming domain) to another, its IP configuration must be updated • Inter-Network Roaming (INR) helps LAN-to-WAN roaming • Roaming duration is the time it takes to disassociate with one device and re-associate with another