Typically, the data rate available at the radio interface is lower than the data rate available on the network interfaces. Thus, when a certain rate data service is higher than the data rate of the LTE radio interface, this leads to buffering in the UE and the eNodeB. This buffer allows the programmer to the MAC layer some freedom to vary the instantaneous data rate of the physical layer in order to adapt to the current conditions of the radio channel. Thanks to buffer the variations in instantaneous data rate are observed by applying only an unstable transfer delay.
However, when the data rate exceeds the rate achieved by using data provided by the radio interface during a long period, a large number of data buffers may result. This can lead to significant loss of data transmission, without any losses if the transmission is not applied to the carrier or to excessive delay for real time applications.
In the fixed Internet, one of the roles typically performed by routers is to drop packets when the index of a data request exceeds the data rate available on a part of the Internet. An application can then detect packet loss and adapt its data rate to the available rate. A typical example is the handling of TCP transmission window in the TCP transmission window is reduced when a missing packet is detected, thereby adapting the available rate. Other applications such as video or voice calls via IP can also detect lost packets, for example via RTCP (Realtime Transport Control Protocol) feedback, and can adapt the data rate accordingly.
In order for these mechanisms to work, and to prevent excessive delay function discarding included PDCP layer for LTE. This feature is based on the drop timer, wherein for each PDCP SDU, received from higher layers in the transmitter starts a timer, and when the transmission of PDCP SDU has not yet started in the UE upon expiration of the timer is discarded PDCP SDU. If the timer is set to an appropriate value for the required QoS radio, this mechanism can prevent discarding excessive delays and queues at the transmitter.