Recommended WLAN Access Point Configuration
Recommended WLAN Access Point Configuration

Recommended WLAN Access Point Configuration

PTT Express communicates half-duplex voice traffic at a QoS of VOICE with sample rates of 200 ms for Group and 100 ms for Private calls. It is recommended that the network support a DTIM of 1 or 2 based on a Beacon Interval of 100 ms. To ensure timely delivery against lower priority traffic a QoS of VOICE should be honored across the network by supporting WMM.
Multicast rates on the AP configuration should be optimized. There is some variation between manufacturers (Cisco, Zebra etc). Customers should use the following guidelines to set the multicast rates on their APs to optimize performance.

Multicast Rate Selection

  • Choosing a Multicast Rate - PTT Express uses multicast distribution to implement Group Call. In contrast to unicast mode, each AP in the ESS transmits the multicast packet once (no retries), at a fixed rate. Choice of MC rate is similar to picking a Beacon rate, and should strike a balance between:
    • Lower rates propagate further. Multicast Tx rate should be low enough for a reception rate of at least 90% anywhere within a cell, by all of the target mobile devices.
    • However, an MC rate lower than necessary wastes airtime, and contributes to co-channel interference.
  • Setting the Multicast Rate - The MC rate policy varies with AP vendor, but usually cues on the Basic Rate settings. Two examples:
    • Setting the Multicast Rate The MC rate policy varies with AP vendor, but usually cues on the Basic Rate settings. Two examples:
    • Cisco: MC rate is different for the Wireless Controllers and standalone APs:
      • Access points running recent Cisco IOS versions are transmitting multicast and management frames at the highest configured basic rate, and is a situation that could cause reliability problems.
      • Access points running LWAPP or autonomous IOS should transmit multicast and management frames at the lowest configured basic rate …. If reliable reception is a goal, then multicasts should be transmitted at a low data rate.
  • Summary - Determine how the multicast rate is derived, typically from the Basic Rate settings, for the specific vendor, model, and firmware revision in play. Then, lower multicast rate as described above.
    • Quick Start - Use just one basic rate. Start with 24 Mb/sec basic, and adjust downwards for good performance. As a safety margin, drop the rate one more step.

Other Settings

IGMP Snooping - Must be disabled.
Session Timer - The infrastructure session timer should be disabled or set to longer than the default value (24 hours is the suggested value).