If your uplink bandwidth at home is limited and you use Skype or other Internet based voice services you’ve probably experienced more than once that when someone else in your household starts uploading cat videos, your Skype call goes south. That’s because the voice packets are treated like all other packets and start queuing up behind those fat IP packets with parts of the cat video inside. This is not a theoretical scenario, it happens in practice. So how does VoLTE on LTE cope with this?
Unlike at home, VoLTE signaling and voice packets on a smartphone use a different bearer. Think of an LTE bearer as a separate virtual network interface with its own IP address. While normal apps use the virtual network interface for Internet connectivity, the VoLTE IMS stack on the device uses the IMS bearer instead. When the device initially connects to the network the IMS bearer gets a higher priority than the Internet bearer. As a consequence all packets ending up on this virtual network interface and its transmission queue in the radio modem get precedence over whatever is in the queue of the Internet bearer.
When a voice call starts, the network additionally creates a dedicated bearer for the voice packets that runs alongside the IMS bearer. All packets to and from a certain IP address and UDP port get even higher priority than the IMS signaling. In addition, packets transported over this bearer are not repeated if they are not transmitted correctly over the air interface as they would be useless to the receiver. Either the packet arrives in time so it can be converted back into 20 milliseconds of an audible signal or it comes to late and is discarded.
Priorization also works between different users that are transferring data over the same base station. VoLTE packets of one user get precedence over the Internet traffic of other users in the cell.
In practice I recently ran the ultimate test by up- and downloading a huge amount of data from my smartphone while a VoLTE call was ongoing. At a data rate of 50 Mbit/s in the downlink direction and 25 Mbit/s in the uplink direction, which was the maximum the cell could deliver at the time, the call would have been heavily distorted at home over Wifi and my VDSL line, while over LTE, the voice call continued to sound just fine. Also, there was no change in the speech channel’s delay. Very nicely done indeed!
Now we just have to figure out how to reconcile this with network neutrality…
For details on how VoLTE bandwidth negotiation works, have a look here.