CWNA Summary Notes: Capacity and Coverage Requirements

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Capacity and Coverage Requirements

Factors that should be considered when planning for Capacity:

  • Data applications.
    • For 802.11b/g network, 12 to 15 data users per AP
    • ForVoWiFi
      • SpectraLink recommends max of 12 calls per 11 Mbps cell (with 1 call needing 4.5 % of AP bandwidth), 7 calls per 2 Mbps (with 1 call needing 12% of AP bandwidth). This can be predicted using the Erlang Formula. Erlang – one hour of telephone traffic in one hour of time.
  • User density. A high concentration of human bodies can attenuate the RF signal due to absorbtion.Consider:
    • How many users currently need wireless access
    • How many users many need connectivity in the future
    • Where are the users
  • Peak on/off use
  • Existing transmitters
    • Previously installed 802.11 network
    • Interfering devices
      • microwaves
      • cordless headsets
      • cordless phones
      • wireless machinery
  • Mobile vs Mobility
    • When one is mobile – disconnect may be ok (moving with the laptop from one desk to another). roaming may not be a requirement.
    • For mobility – user must be connected 100% of the time when travelling through the facility eg
      • VoWiFi
      • Warehouse scanning applications
  • 802.11g protection mechanism. This will always affect throughput due to backward compatibility with 802.11b HR-DSSS clients that support a max data raet e of 11 Mbps as compared to 54 Mbps for the 11g clients.

Issues that may cause performance problems in a WLAN network

  • Co-channel interference
  • multipath interference

Issues that may cause roaming problems in a WLAN

  • Interference
  • Lack of adequate coverage
  • Lack of proper cell overlap

Fctors that should be considered and noted before the site survey

  • Current WLAN problems
    • throughput related
    • frequent disconnects
    • roaming issues
  • Known sources of RF interference
    • microwave ovens
    • cordless phones
    • bluetooth for keyboards or mouse
  • Known coverage dead zones. Too little or too much coverage may cause connectivity and roaming issues.
  • Prior site survey data
  • Check what equipment is currently installed.

Roaming may not be required in some areas. The Role-based access control (RBAC) capabilities of a wireless switch or controller can control and segment roaming. Another roaming considertaion is whether or not layer 3 roaming is required. It may not be optimal for delay sensitive applications like VoWiFi.

Security Expectations:

  • AAA requirements
  • Are there plans to implement Wireless Intrusion Detection System (WIDS) against rogue APs and wireless attacks
  • Will layer 3 VPNs be used – they increase WLAN overhead.
  • EAP authentication may affect VoWiFI and this should be considered.

References:

  1. CWNA Certified Wireless Network Administrator Study Guide by David D. Coleman and David A. Wesrcott.

 

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