LTE Bearers Are Like Virtual Network Interfaces

In LTE, data is transferred over the air interface over ‘Default Bearers’ and ‘Dedicated Bearers’. The terms are a bit confusing but actually the concepts behind them can easily be explained: Virtual Network Interfaces and Traffic Shaping Rules.

In practice a smartphone that is not VoLTE capable usually has one default bearer which is established when the device is switched on. So far things are simple, you get an IPv4 address, or an IPv4 address and an IPv6 prefix from advanced networks and you are good to go.

Things get a bit more complicated when a device is VoLTE capable. In addition to the first default bearer a second default bearer is established that is used for VoLTE signaling traffic. Think of a default bearer as a virtual network interface and it’s implementation immediately becomes obvious: VoLTE devices have at least two virtual network interfaces, one for Internet traffic and one for IMS signaling traffic. The TCP/IP stack automatically chooses the correct virtual network interface (Default Bearer) based on entries in the IP routing table which are made when a network interface is started.

In addition, VoLTE uses a Dedicated Bearer for speech packets. A dedicated bearer is not a virtual network interface, it is just a set of rules that describe which packets take precedence over an existing virtual network interface (the bearer for IMS traffic in this case).

And a final thought: LTE Bearers are not only ‘like’ virtual network interfaces, but mobile operating systems such as Android actually implement them as such. If you know your way around ‘adb’ have a look with ‘ifconfig’ and ‘netcfg’.