VoLTE Packet Size
As it turns out, that question doesn’t have a simple answer. It depends on a lot of variables, including the voice coder choices, the RF conditions in the cell, the eNB’s scheduler algorithm, the protocol options, and so on. To keep this discussion at manageable levels, let’s concentrate on one particular aspect of VoLTE capacity: how many Physical Resource Blocks (PRBs) are needed to deliver the traffic for one VoLTE call over a typical LTE air interface?
Let’s assume for the moment that the operator has deployed 10 MHz LTE radio channels. This is fairly typical (at least in the US), and provides 50 PRBs per millisecond on the downlink (somewhat less on the uplink, depending on the PUCCH configuration). Let’s further presume that VoLTE is configured to use the Adaptive Multi-Rate Wideband (AMR-WB) 12.65 coder, and that Robust Header Compression (RoHC) is enabled over the air interface.
The AMR-WB 12.65 coder generates 253 bits of coded speech every 20 ms (a net data rate of 12.65 kbps, hence the name). In order to deliver each voice sample to the UE, additional protocol headers are needed: an RTP header (typically 12 bytes), a UDP header (8 bytes), and an IPv6 header (40 bytes). This brings the total packet length up to some 733 bits every 20 ms.
RoHC, however, will replace with RTP, UDP and IP headers with a much smaller RoHC header before the packet is actually transmitted over the air. The length of the RoHC header will vary depending on the particular circumstances, but it will average around 3 bytes, or 24 bits. The RLC and MAC layers will add their own overhead, so the end result is that the air interface will have to transport roughly 300 bits of data for every VoLTE packet.
