Some wireless equipment does not offer customization for the wireless network in terms of creating SSIDs with different characteristics. All SSIDs created inherit the same radio spectrum configuration profile, i.e. the same channel, output power and channel width.
To work with voice over IP, you first need to carry out a site survey to measure the parameters and make adjustments whenever necessary. Above all, the coverage of the 2.4 GHz spectrum, although more comprehensive compared to 5 GHz radio, only has 3 non-overlapping channels, as shown in the figure below. There is therefore no scope for selecting another channel when there are several SSIDs configured or of external origin.
This implication means that the choice of 5 GHz spectrum provides a greater channel selection range for working with voice over IP.
It is highly recommended to work with 5GHz (IEEE 802.11an standard), 20MHz of bandwidth, and plan for channel reuse whenever possible to operate voice over IP. However, this does not rule out the need to request a site survey beforehand in order to analyze and plan the access point customization parameters. In addition, having a separate voice over IP SSID channel from the others can mitigate interference and performance.
To prevent the wireless device from automatically choosing, or being configured, to operate on 2.4 GHz, it is recommended that the SSID profile only contains the 5GHz setting. This ensures that wireless phones will not register with the SSID operating in the 2.4 GHz frequency range.
This example shows that the use of the 40 MHz bandwidth at 2.4 GHz is almost never desired as for to the lack of available channels for 2.4GHz.
Operating to the 5GHz spectrum offers much more available channels.
Another important item to consider is the adjacent interference coming from overlapping nearby channels.
The design of the wireless network must go through stages such as: spectrum analysis of the site, wireless network coverage, project design, adjustments and testing. In addition, the network requires continuous monitoring due to its nature of being susceptible to interference from other devices, microwaves, very high output power.
In general, the signal level of the receiver (devices such as cell phones, computers, cordless phones) should be between -65dBm and -70dBm, the signal to noise ratio between -25dB and - 30dB and there should be no channel overlap.