WiFi Multicast Configuration

Configuring and selecting the right access point is a crucial part of setting up a functional multicast solution. In this section we will provide advice on how we deal with various WiFi issues and how we configure our WiFi access points to get the best possible performance.

If you have additional advice on how to improve WiFi multicast please get in touch - we would love to talk with you: Contact Us.

WiFi Multicast Issue

Performance of WiFi multicast suffers from several different issues. These issues make it hard for multicast services to get decent performance over WiFi networks.

Most of the issues have been described in the following two IETF RFCs:

1. Multicast Wifi Problem Statement

2. Multicast Considerations over IEEE 802 Wireless Media

Access Point Configuration

Given the outlined WiFi multicast issues here are some configuration options that can be adjusted on the WiFi access point to get better multicast performance.

Reducing the Beacon Interval & DTIM Period

WiFi access points announce their presence by sending out beacons, however beacons are also used to announce whether it has traffic queued up for power saving clients. This information is delivered inside a bitmap in the beacon called the Traffic Indication Map (TIM).

The AP uses a special type of Traffic Indication Map (TIM) to to announce that the it’s about to transmit all buffered broadcast and multicast frames called the Delivery Traffic Indication Map (DTIM). Since all clients are expected to receive broadcast/multicast traffic - all clients are expected to wake up and listen for the traffic. How often a beacon includes a DTIM is controlled by the DTIM period.

For these reasons you should consider the following configurations:

  • Setting the DTIM period to 1 will make every beacon a DTIM beacon. Multicast/broadcast traffic will be sent after each beacon.
  • Set a low beacon interval e.g. 15ms (most access points use per default 100ms)

Reducing Max Listen Interval (OpenWrt)

The OpenWrt option max_listen_int states the maximum number of beacons a station (e.g. mobile device) may sleep before checking if data is pending. Reducing this may keep devices awake to receive the broadcast packets more often. If the option is set too low, the devices can not associate with the access point. For some Huawei tablets the threshold seems to be 5, while the Samsung galaxy s6 does not connect to the network if the value is below 10. Either way, in practice this option does not make devices stay awake for more broadcast packets than usual.

Force Physical Layer Data Rate - PHY Rate (OpenWrt)

Default multicast PHY rate varies from access point to access point. But, the WiFi standard mandates that it be at least 1 Mbps for 2.4 Ghz and 6 Mbps for 5 Ghz. This is typically not enough for e.g. HD video streaming. We therefore typically have to increase it. With OpenWrt we can do this using the following approach:

When logged onto the access point e.g. via SSH the physical layer data rate can be configured with the following command:

iw dev wlan0 set bitrates legacy-5 24 36 48

This will set the minimum data rate for the 5 GHz network to 24 Mbps.

Which Access Point to Buy

We have tested a few access points and had reasonable results with following consumer grade equipment.

Attention

We are in no way affiliated with any of these brands. We we take no responsibility for the performance you may experience.