In packet transmission systems, the data is organized into packets and transmitted along with control information such as the size (or start and end) of the data, and the identity of the receiver (at least when communicating over shared media). In many systems the time is divided into time frames, where each frame carries zero or more packets, along with the mentioned control information.
A receiver in such a system monitors the transmitted control information for the presence of packets that should be received by that receiver. In situations where the bitrate required by the application is much lower than the transmission bitrate, the receiver is most of the time essentially idle, just monitoring the control information.
Examples of the above include all existing cellular packet transmission techniques, i.e. GPRS/EDGE, WCDMA (R99, HSDPA, and E-UL), and CDMA2000 (1x, Ev-DO, and Ev-DV).
With many transmission technologies, actively monitoring the control information can be very power consuming, even if the control information itself contains little information and there is no data that needs to be received. For example, a wideband radio transmission technique, designed for high data bit rates, may require that the receiver demodulates and processes the entire frequency band even though it is only interested in the control information. This is the case, e.g. for WCDMA HSDPA and may well turn out to be the case for OFDM-based systems such as the long-term evolution (LTE) of UMTS.
This means that for a low-rate service (such as voice) the power consumption can be significantly larger when using a wideband system compared to a narrowband system.
If the mobile terminal occasionally could enter a “sleep mode”, meaning that it does not monitor the control information during a period of time, power consumption could be reduced. This type of sleep mode is currently used when the mobile terminal has entered a paging state. In paging state the mobile terminal only occasionally listens for a “wake up” or “paging” signal, from the network. However this method requires additional signaling between the network and the mobile terminal, in order to “wake it up”, and wouldn't be appropriate to use for short time intervals e.g. between successive packets in an ongoing real-time data session.