One of the main functions of the PDCP header compression is using Header Compression (ROHC) robust protocol defined by IETF (Internet Engineering Task Force) . In LTE, header compression is very important because there is no support for transport of voice circuit-switched (CS) domain. Thus in order to provide voice services (PS ) domain Packet – Switched to a way approaching deficient normally associated with CS services need to be outside IP/UDP/RTP3 compliment which is usually used for Voice over IP ( VoIP) services.

Specified in the IETF “RFC 4995,” a framework that supports a number of “profiles ” different header compression (eg. sets of rules and parameters for performing compression) . Supported header compression profiles for the LTE are listed in the table. This means that the EU can implement one or more of the ROHC Profile. It is important to note that the profiles have been defined in the IETF earlier, “RFC 3095 ” has been redefined in RFC 4995 , in order to increase the strength in some cases. The efficiency of RFC 3095 and RFC 4995 is similar, and UMTS only supports RFC 3095.

Supported header compression protocols in lTE

Support of ROHC is mandatory UE, except UES supporting VoIP. UES supporting VoIP must support at least one RTP compression profile, UDP and IP.The eNodeB RRC signaling controls the ROHC profiles supported by the UE are allowed to be used. UE ROHC compressor and eNodeB then dynamically detect IP flows using a certain configuration IP header compression and choose a suitable profile of profiles allowed and supported.

ROHC header compression works by allowing both the sender and receiver to store the state of the header (e.g. IP addresses of the sender / receiver), and update them only when they change. In addition, the dynamic components (for example, RTP time stamp outside) are compressed from the transmission num different a reference clock maintained.
As part non – thus changing the headers are sent once decompression success depends on proper receipt. Therefore, the feedback is used in order to confirm correct receipt of the initialization information for header decompression. In addition, proper decompression APRIM PDCP PDU is confirmed periodically by packet losses.

As mentioned above, the most important use case for ROHC is VoIP. Typically, for the transport of a VoIP packet, which contains a 32- byte payload, the header will be added to 60 bytes to 40 bytes for IPv6 and IPv4 case – that is, a top head 188 % and 125 %, respectively. By the ROHC, the header compression entities, this overhead can be compressed to four to six bytes, and thus a relative direction of 12.5-18.8 %. This calculation is valid to active periods, but in times of peace size is less useful, such as the relative overhead is large.