Wireless Protocols

Connected Mode DRX

Q: What is the difference between connected mode DRX and idle mode DRX?

Idle mode DRX applies in RRC_IDLE, where the UE has no dedicated radio bearer and only wakes at paging occasions tied to the network paging cycle (typically 320 ms to 2560 ms) to check for incoming calls or data. Connected mode DRX (C-DRX) applies in RRC_CONNECTED, where a context exists and the UE could be scheduled at any moment. C-DRX cycles are usually much shorter, with long cycles of 40 to 320 ms and short cycles of 2 to 64 ms, because the network may need to schedule data quickly. The key difference is responsiveness versus savings: idle DRX maximizes sleep because traffic is unexpected, while C-DRX must balance sleep against the active session that is already running.

Q: How do the on-duration timer and inactivity timer interact in C-DRX?

At the start of each DRX cycle the UE wakes and runs the drx-onDurationTimer, during which it monitors PDCCH for scheduling. If no grant arrives before the on-duration expires, the UE returns to sleep until the next cycle. If a grant does arrive, the UE starts the drx-InactivityTimer (commonly 10 to 200 ms) and keeps monitoring PDCCH for that whole window after each new transmission. Each fresh grant restarts the inactivity timer, so a busy session effectively keeps the UE awake. Only when the inactivity timer expires without new activity does the UE drop back to DRX sleep, optionally entering short DRX first before transitioning to the long DRX cycle.

Q: How much battery does connected mode DRX actually save?

Savings depend on the duty cycle, which is on-duration divided by DRX cycle length. With a 320 ms long cycle and a 10 ms on-duration, the receiver is active about 3 percent of idle time, so the modem RF front end and baseband can be clock-gated or rail-collapsed for roughly 97 percent of the period. In practice C-DRX commonly reduces modem power during background or bursty traffic by 40 to 60 percent, and aggressive configurations on smartphones can extend standby and talk-time battery life by hours. The trade-off is added downlink latency of up to one DRX cycle, which is why latency-sensitive services such as VoLTE use short cycles or disable DRX during active speech frames.

/kə-NEK-təd mohd D-R-X/ (also "C-DRX")

Defined in the 3GPP MAC specifications, connected mode DRX (C-DRX) is a discontinuous-reception mechanism that lets a UE in the RRC_CONNECTED state switch off its receiver between brief, scheduled wake windows instead of monitoring the PDCCH continuously. The UE wakes for a configured on-duration at the start of each DRX cycle; if no grant arrives, it sleeps until the next cycle. Long cycles run 40 to 320 ms and short cycles 2 to 64 ms in LTE and 5G NR, cutting modem power 40 to 60% during bursty traffic. C-DRX differs from power saving mode because the radio context stays alive, so the device remains schedulable within one cycle of latency.

Category: Wireless Protocols

Long DRX Cycle: 40 to 320 ms

RRC State: RRC_CONNECTED

How C-DRX Schedules Sleep While the Radio Bearer Stays Up

When a UE is in RRC_CONNECTED it has live radio bearers and could be scheduled by the base station at any subframe, which normally forces it to decode the PDCCH every transmission time interval. That constant monitoring keeps the receiver chain, the local oscillator, and the baseband demodulator powered, draining the battery even when no user data is flowing. Connected mode DRX breaks that requirement: the network configures a DRX cycle, and the UE only listens during a short on-duration at the front of each cycle. Outside that window the device is permitted to gate its clocks and collapse RF power rails, then re-synchronize before the next on-duration.

The behavior is governed by a small set of MAC timers signaled in the RRC reconfiguration. The drx-onDurationTimer sets how long the UE listens at the start of each cycle. If a grant is received, the drx-InactivityTimer starts and the UE keeps monitoring; every new grant restarts it, so an active burst naturally extends the awake period. When the inactivity timer expires the UE optionally moves through a short DRX cycle for a configured number of cycles before settling into the longer cycle, giving the scheduler a grace period in case traffic resumes.

Because the radio context is retained, the cost of C-DRX is bounded latency rather than the multi-second re-attach penalty of leaving connected mode entirely. A downlink packet that arrives just after the UE sleeps must wait at most one DRX cycle before the next on-duration, which is why latency-sensitive bearers such as VoLTE pair C-DRX with very short cycles or suspend it during active talk spurts.

C-DRX Timers and Duty Cycle

Receiver Duty Cycle:
D ≈ T_onDuration / T_DRXcycle

Average Receiver Power:
P_avg ≈ D × P_active + (1 − D) × P_sleep

Worst-Case Added Downlink Latency:
L_max ≈ T_DRXcycle − T_onDuration

Where T_onDuration = on-duration window, T_DRXcycle = long DRX cycle period, P_active = receiver-on power, P_sleep = gated sleep power. Example: T_on = 10 ms, T_cycle = 320 ms → D ≈ 3.1%, so the receiver is idle for ≈ 97% of the cycle.

DRX Modes and Parameter Ranges

Mode / Timer	Typical Range	RRC State	Purpose	Effect on Latency
Idle mode DRX	320 to 2560 ms paging cycle	RRC_IDLE	Wake at paging occasions only	High (up to paging cycle)
C-DRX long cycle	40 to 320 ms	RRC_CONNECTED	Deep sleep during quiet periods	Moderate (≤ 1 cycle)
C-DRX short cycle	2 to 64 ms	RRC_CONNECTED	Bridge after inactivity timer	Low
drx-onDurationTimer	1 to 200 ms	RRC_CONNECTED	Mandatory listen window per cycle	None (always awake)
drx-InactivityTimer	10 to 200 ms	RRC_CONNECTED	Stay awake after each grant	None (extends wake)

Common Questions

Frequently Asked Questions

What is the difference between connected mode DRX and idle mode DRX?

Idle mode DRX runs in RRC_IDLE, where the UE only wakes at paging occasions on a 320 ms to 2560 ms paging cycle. C-DRX runs in RRC_CONNECTED, where a radio context exists and data may be scheduled at any time, so cycles are shorter: 40 to 320 ms long and 2 to 64 ms short. Idle DRX maximizes sleep because traffic is unexpected; C-DRX trades some sleep for the responsiveness an active session needs.

How do the on-duration timer and inactivity timer interact in C-DRX?

Each cycle the UE wakes and runs drx-onDurationTimer to monitor the PDCCH. With no grant it sleeps again until the next cycle. If a grant arrives it starts drx-InactivityTimer (10 to 200 ms typical) and keeps listening; every new grant restarts that timer, so busy sessions stay awake. Only when the inactivity timer expires does the UE return to DRX sleep, optionally through short DRX before the long cycle.

How much battery does connected mode DRX actually save?

It scales with duty cycle, on-duration divided by cycle length. A 10 ms on-duration on a 320 ms long cycle keeps the receiver active about 3% of idle time, gating RF and baseband for roughly 97% of the period. Real deployments cut modem power 40 to 60% during bursty traffic. The cost is up to one DRX cycle of added downlink latency, which is why VoLTE uses short cycles or suspends DRX during speech.

Related Terms

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Wireless Infrastructure

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