For VoIP packets, the size of headers (IP/UDP/RTP) is usually larger than the data itself. Compressing these protocol headers over end to end (User to user) connection is not feasible since the headers play important role, but there is possibility to compress them over the air interface i.e. between UE and eNB/NB/BTS etc. Over the air interface, protocol headers do not play any role, so therefore there is considerable reduction in the data over head on radio interface. For IPv4, UDP and RTP, the amount of overhead due to headers is 40 bytes while RoHC can compress this to 2 or 3 bytes. RoHC is described in RFC 3095
In LTE, the role of Packet data convergence protocol layer (PDCP) is to implement RoHC as shown below (Only uplink traffic shown).
RoHC is applied to user plane traffic only while this feature is transparent to control plane traffic.
Use of RoHC with conversational bearer (QCI 1) in LTE will result in efficient use of radio resources since it can reduce the user plane traffic by several bytes over air interface.