Explanation/Reference:
Section: Considerations for Expanding an Existing Network Explanation
Explanation:
In a Voice over wireless LAN (VoWLAN), high data rate signals require higher signal-to-noise ratios (SNRs) than low data rate signals. In addition, signals from adjacent cells on nonoverlapping channels should have an overlap between 15 and 20 percent to ensure smooth roaming. The sensitivity of an
802.11 radio decreases as the data rate goes up. Thus the separation of valid 802.11 signals from background noise must be greater at higher data rates than at lower data rates. Otherwise, the 802.11 radio will be unable to distinguish the valid signals from the surrounding noise. For example, an 802.11 radio might register a 1Mbps signal at -45 decibel milliwatts (dBm) with -96 dBm of noise. These values produce an SNR of 51 decibels (dB). However, if the data rate is increased to 11 Mbps, the radio might register a signal of -63 dBm with -82 dBm of noise, thereby bringing the SNR to 19 dB. Because the sensitivity of the radio is diminished at the higher data rate, the radio might not be able to distinguish parts of the signal from the surrounding noise, which might result in packet loss. Therefore, the optimal cell size is determined by the configured data rate and the transmitter power of the access point (AP).
Packet loss can also be mitigated by maintaining an overlap between 15 and 20 percent on nonoverlapping channels for all adjacent cells in a VoWLAN. By providing at least 15 percent overlap between adjacent cells, a wireless client has a greater chance of completing the roaming process without incurring too much delay or packet loss. If the overlap is less than 15 percent, the client might drop its connection with one AP before it has completed associating with the next AP. This can result in degraded voice quality and disconnected calls.
VoWLANs require higher signal strengths than data-only wireless LANs (WLANs). Data traffic can tolerate delayed or dropped packets because its associated applications typically do not operate in real time. If a wireless client breaks its connection with an AP and packets are delayed or lost, the client can retransmit the missing packets when it reconnects. By contrast, real-time data, such as voice traffic, is particularly sensitive to delay, variations in delay, and packet loss. If packets are delayed too long or lost because a client breaks its connection with an AP, the quality of the client's voice stream is degraded. If there is enough delay or packet loss, the call will be disconnected by the client device.
Likewise, VoWLANs require higher SNRs than data-only WLANs. A high SNR indicates that a device can easily distinguish valid wireless signals from the surrounding noise. The greater the separation between signal and noise, the higher the likelihood that wireless clients will not experience packet loss due to signal interference. Cisco recommends maintaining a minimum signal strength of -67 dBm and a minimum SNR of 25 dB throughout the coverage area of a VoWLAN to help mitigate packet loss.
Increasing the strength of a signal can increase its SNR. By increasing the strength of a transmitted signal, the difference between the signal and any associated noise can be increased at the receiving station. A wireless LAN controller (WLC) can be configured to adjust the signal strength of a lightweight AP (LAP) if it registers a low SNR value from one of the LAP's associated devices.
Reference:
Cisco: Site Survey Guide: Deploying Cisco 7920 IP Phones: Getting started