Also known as: DRX, Discontinuous reception, eDRX, extended DRX
Discontinuous reception (DRX) is a power-saving mechanism in which an idle device’s receiver sleeps between scheduled paging occasions and wakes only briefly to check whether the network is trying to reach it.1 By keeping the receiver off most of the time, DRX trades a little latency — the network may have to wait for the next wake window to deliver a page — for large battery savings. It is present in GSM, LTE, and 5G NR, and its extended form (eDRX) stretches the cycle to minutes for low-power IoT such as NB-IoT and LTE-M.
How it works
When a device registers with the network but has no active data, it enters an idle state and negotiates a DRX cycle — a repeating period whose length both sides know. Within each cycle the device computes, from its own identity, a specific paging occasion: a short window in which it turns its receiver on and listens for a paging message addressed to it. If nothing is there, it powers the receiver back down until the next cycle. Because the network knows the device’s schedule too, it holds any incoming page until that window and transmits it then. The whole scheme rests on both ends agreeing, at registration time, on the cycle length and how the paging occasion is derived.
The trade-off is latency for power. A longer DRX cycle means the receiver is off a greater fraction of the time — dramatically less energy — but an incoming call, message, or data push can wait up to one cycle before the device notices it. Ordinary phones use cycles of a fraction of a second to a couple of seconds, short enough that the added delay is imperceptible while still cutting idle drain sharply. The receiver hardware, not the display, is often the dominant idle consumer, so switching it off between pages is one of the largest levers on standby battery life.
Extended DRX and IoT
For devices that must run years on a small battery, standard DRX does not sleep long enough. Extended DRX (eDRX) stretches the cycle far beyond the normal range — up to minutes, and in some configurations longer — so an NB-IoT or LTE-M sensor can stay asleep for a very long interval and still be reachable, at the cost of the network tolerating that much delay before a downlink reaches it. A related mechanism, Power Saving Mode, goes further still by letting the device become essentially unreachable between its own transmissions. Conceptually none of this is new: it is the same duty-cycling that battery-powered paging receivers have always used — listen briefly on a known schedule, sleep the rest of the time.
Relevance to SDR
DRX is why a monitored idle cellular device is silent on the air most of the time and only briefly active: its receiver, and any uplink it sends, follow the negotiated cycle rather than a continuous stream. An observer sees traffic clustered around paging occasions rather than spread evenly. GopherTrunk decodes land-mobile trunking rather than cellular paging, but the same idea — a receiver that duty-cycles to catch a control-channel page and otherwise stays idle — describes how battery-powered trunking subscribers conserve power while waiting for a call, making DRX useful background for reasoning about intermittent RF activity.
Sources
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Discontinuous reception — Wikipedia, for the DRX cycle, paging occasions, and the extended-DRX variant. ↩