CWNA Summary Notes: Capacity and Coverage Requirements

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.