On a PCM (G.711) RTP packet the payload is typically 160 bytes per packet.
But the total size of the frame on the wire is typically ~214 bytes, to carry a 160 byte payload that means 25% of the data being carried is headers.
This is fine for VoIP services operating over fixed lines, but when we’re talking about VoLTE / IMS and the traffic is being transferred over Radio Access Networks with limited bandwidth / resources, it’s important to minimize this as much as possible.
IMS uses the AMR codec, where the RTP payload for each packet is around 90 bytes, meaning up to two thirds of the packet on the wire (Or in this case the air / Uu interface) is headers.
Using ROHC the size of the headers are cut down to only 4-5 bytes, this is because the IPv4 headers, UDP headers and RTP headers are typically the same in each packet – with only the RTP Sequence number, RTP timestamp IPv4 & UDP checksum and changing between frames.
As the traffic is point to point headers vary vary little so is predictable and can be compressed efficiently.
For a VoLTE medea stream a 40 byte IPv6 header, 8 byte UDP header, 12 byte RTP header and 30 bytes of RTP data.
This means that we have 60 bytes of headers and only 30 bytes of data, which is a very inefficient use of resources, so by compressing this data we can shrink this substantially.
Handover Mitigation
When handing over between NodeBs on previous 3GPP RANs packet loss and reordering was common during handovers between NodeBs.
E-UTRAN specs have minimized this as much as possible, the handing off eNB can transfer information using PDCP about data to be transferred to the UE to the eNB the UE is handing over too.
Security
As the radio link is particularly vulnerable to eavesdropping, PDCP offers another independent ciphering and integrity control mechanism to verify data is not modified / intercepted.
Usage of PDCP Functionality
Not all these functionalities are used for all types of traffic, as shown in the table below.
Recap of Radio Interfaces
PDCP is discussed in this post, interfacing the radio interface with the core network.
A summary of the hierarchy is shown here with user data in pink and control data in blue:
The RNTI is shown in a dotted box as the RNTI is not transmitted as a header on the transport block but is logically associated with the transport block thanks to the allocation table.
