Besides implementation specific aspects of VoLTE, the industry has agreed on an 'IMS Profile for Voice and SMS', which lists the minimum functions and features that a terminal and its network must support. Besides defining mandatory basic IMS capabilities, the document mandates support of several core features for the radio and packet. Amongst other features, it requires support for discontinuous reception, or DRX. This is crucial for optimising VoLTE power consumption. DRX and its two modes Two DRX modes have been defined – idle and connected (cDRX). Idle mode is generally defined as the state in which no RRC connection exists between device and network. Theoretically, there is a paging radio network temporary identifier (P-RNTI) on the physical downlink control channel (PDCCH) every 1ms. Monitoring the channel constantly would impact a device's battery life, so LTE includes a mode for discontinuous monitoring of the PDCCH. The DRX in idle mode allows the device to only monitor certain subframes in a specific radio frame for its paging message. For the rest of the time, the user equipment (UE) enters sleep mode to conserve battery power. The initial configuration for the Idle DRX cycle can come from higher layers and therefore can be device specific. But it can also be broadcast using a Type 2 system information block, where it is common to all devices in a particular cell. Based on the set parameters, the terminal determines the radio frame subframes in which it will look for paging messages. The relevant DRX mechanism for VoLTE and power consumption is cDRX, in which the network sets certain parameters while sending a message. What are these parameters and how do they impact the overall mechanism? Assuming the UE receives a scheduling grant for either the downlink or uplink, the terminal would always reset the drx-inactivityTimer, which has a flexible length of up to 2.56s. If the device does not receive another scheduling grant, it will not reset the timer, which will continue to run. At some point, the timer will expire, which triggers the move into the DRX cycle. There are two DRX cycles – short and long.