This is Part 1 of a two-part problem/resolution series
Multicast and broadcast traffic are a normal part of all networks today. Many devices use multicast/broadcast traffic to advertise and discover services on the network. Protocols such as Bonjour and MDNS make streaming to an Apple TV or connecting to a printer simple – but that simplicity comes at a cost. When there are too many devices on a wireless network sending this traffic, the network can become bogged down. To understand why this is, you need to know how multicast/broadcast traffic is handled on WiFi versus a wired network.
On a wired network, each device has a full-duplex connection to its switchport – typically at gigabit speed. It does not share this bandwidth with other devices. As long as the switch backplane can handle the traffic, one user’s traffic isn’t impacted by another’s. WiFi is more analogous to the switch’s predecessor, the hub. On a hub, packets are sent to all devices connected to it. The devices shared the same collision domain and traffic from one port impacted all devices on the other ports. WiFi is very similar to this; all devices operating on a channel (frequency) affect the other devices on the same channel.
Beyond sharing the same bandwidth, multicast/broadcast is handled differently than unicast data on WiFi. In order to ensure that every device associated with the SSID receives the multicast/broadcast traffic, the AP must send it at a so-called “legacy” data rate. The legacy data rates are the original 802.11abg rates: 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48 and 54Mbps. These are much slower rates compared to the higher speeds available in 802.11n (High-throughput rates) and 802.11ac (Very High-throughput rates). In addition to being slower, they also take up more airtime when transmitting. This leaves less bandwidth available for other clients and increases contention on the channel. But the 802.11 standard mandates this behavior in order to allow for backwards compatibility.
Another problem with multicast/broadcast traffic is that the AP needs to buffer this traffic if any associated clients are in powersave mode. The AP transmits the buffered data on a periodic interval known as the DTIM, which leads to bursts of traffic on the network. The AP also prioritizes buffered traffic over all other traffic, so it essentially blocks everything else while it’s being sent.
You may think the solution is to just send all multicast/broadcast data at the highest rate of 54Mbps, but this can cause problems for clients that aren’t close to the AP. With WiFi there’s an inverse relationship between speed and distance: the higher the data rate, the shorter the packet can travel; the lower the data rate, the farther the packet can travel. So it’s up to the system administrator to set a fixed rate at which all multicast/broadcast traffic is sent. This is a classic engineering tradeoff of performance versus reliability and every network is different in its needs.
We know that multicast/broadcast traffic is useful for accomplishing certain tasks on WiFi, but now you have a better understanding of the problems it can cause – especially when there is a lot of it. As systems administrators, we want to ensure that the wireless network is usable by everyone. This means taking steps to keep the amount of multicast/broadcast traffic on a network to a minimum.