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