Networks are a shared resource and as such suffer from the tragedy of the commons. In networking terms this means that applications and protocols are designed to do their job (self-interest) with no regard to the impact on the network (commons).
This is especially true for WiFi, where the RF medium is shared by users connected to the AP’s radio and by users on neighboring APs on the same channel. An over-subscribed AP or channel will cause application slowness or failure and general user dissatisfaction.
Blocking, pruning, and optimization tactics can be used to get the most out of this limited resource. For example, features like guest access, band steering, and load balancing block users from an AP or its radio band and persuade the client to use a different resource. Application visibility and application firewall block and optimize applications on the network. The focus of this post is how these tactics can be used on broadcast and multicast packets to get the most out of your RF.
There are two network rules that make broadcast and multicast (BCMC) packets troublesome.
- All BCMC packets must be forwarded across a Layer 2 (L2) device: an access point or a switch with ports on the same VLAN.
- All BCMC packets are transmitted in the air at a basic (mandatory) data rate (usually the lowest data rate). This means that an 802.11ac AP will throttle back to send these packets at a very low 802.11n data rate, typically 6, 9, or 12 Mbps.
These two rules can use up a lot of valuable airtime on your wireless network. Here are three ways to deal with them.
The best way to clean up your RF is to prevent unnecessary packets from being transmitted in the first place. Many applications and protocols use broadcast and multicast packets as part of their communication. For example, clients send a broadcast DHCP request to the DHCP server so it can get an IP address. This packet must be allowed upstream from the client to the AP, but should be blocked from being forwarded downstream because your DHCP server should never be connected to your network via a wireless link. Blocking DHCP requests prevents packets from every client from being forwarded out of every AP and transmitted at a low data rate.
Not all traffic is as easy to identify and block as a DHCP request. Some packets, such as multicast streams (streaming audio or video), are necessary in some cases but not in others. In this situation we need to prune the packets to allow them to be forwarded where they are wanted, but prevent them from going if no one is interested. The Internet Group Management Protocol (IGMP) was developed to facilitate control of multicast. The IGMP snooping feature is used to determine if there is someone on an AP who is interested in a particular multicast stream. If no client informs the AP that it wants to receive the stream, it will be pruned.
It is possible to transmit BCMC packets in the air more efficiently. BCMC packets are sent at the default basic rates, the lowest data rates for reliability: 1-11 Mbps for 802.11b/g/n, and 6 Mbps for 802.11a/n/ac. Setting a higher data rate like 24 Mbps will significantly reduce the amount of time it takes to transmit BCMC packets.
Take care when changing the basic data rate, and test it! This is also the rate at which AP beacon frames are sent, and increasing the beacon data rate decreases the AP’s coverage area.
Mojo Wireless Manager has many new features that can help clean up your RF environment. Click the button below to learn more about it. Customers can read the Broadcast/Multicast Optimizations Application Note at our Support Portal.