Power Save

Overview

IEEE 802.11b defines a Power Save (PS) mechanism that allows stations (STAs) to conserve power by periodically turning off their radios (entering doze mode) while the Access Point (AP) buffers their traffic.

At specific intervals, the AP announces buffered traffic via beacons containing a Traffic Indication Map (TIM). Dozing STAs wake up, check for pending traffic, and either stay awake or return to sleep.

This mechanism provides a tradeoff between power efficiency and latency and operates entirely at the MAC layer (PHY-independent).

Power Save States

State

Description

Active Doze Awake Sleep

STA is fully awake, can transmit and receive. STA radio off, cannot send or receive. Temporary awake period after beacon or data. Long-term power-saving state.

Power Management Bits

Two bits in the Frame Control field control power save behavior:

Bit

Field Name

Meaning (1 = set)

12 13

Power Management More Data

STA will enter PS (doze) mode after this frame AP has additional buffered frames for STA

  • STA sets Power Management = 1 to inform AP it is entering doze mode.

  • AP sets More Data = 1 in downlink frames if more buffered frames remain.

Beacon Operation and TIM

The AP transmits beacons at regular intervals (typically 100 TU = 102.4 ms). Each beacon includes:

  • Timestamp (for TSF synchronization)

  • Capability information

  • TIM element — announces which STAs have buffered unicast frames

  • DTIM element — announces pending broadcast/multicast frames

Traffic Indication Map (TIM)

The TIM element identifies which stations have buffered data at the AP.

Structure:

Field

Description

DTIM Count | Number of beacons until next DTIM beacon DTIM Period | Interval between DTIM beacons Bitmap Control | Offset for partial bitmap Partial Virtual Bitmap| Bits indicating buffered STAs by AID index

Each STA is assigned an AID (Association ID) during association. The AP maps each AID to one bit position in the TIM bitmap.

  • Bit = 1: AP has buffered unicast frame(s) for that STA.

  • Bit = 0: No buffered frames.

When an STA sees its bit set, it stays awake and sends a PS-Poll.

DTIM (Delivery Traffic Indication Message)

The DTIM is a special beacon that informs STAs of pending broadcast/multicast data.

Fields in TIM element:

Field

Description

DTIM Count = 0 DTIM Period

Indicates this beacon is a DTIM beacon. Defines how often DTIMs occur (e.g., 3).

Behavior:

  • STAs must wake for every DTIM beacon if they wish to receive broadcast traffic.

  • The AP transmits all queued broadcast/multicast frames immediately after DTIM beacon.

Unicast Delivery Using PS-Poll

When an STA detects its bit set in the TIM, it requests its buffered frames using a PS-Poll control frame.

PS-Poll Frame

Field

Description

Frame Control AID BSSID TA (Transmitter)

Type: Control / Subtype: PS-Poll STA’s Association ID AP’s MAC address STA’s MAC address

PS-Poll Procedure

  1. STA wakes up at beacon time.

  2. STA reads TIM; sees its AID bit = 1.

  3. STA contends for the medium using DCF (DIFS + backoff).

  4. STA sends PS-Poll to AP.

  5. AP replies with one buffered frame: - More Data = 1: additional frames buffered. - More Data = 0: last buffered frame.

  6. STA acknowledges the frame.

  7. If More Data = 1 → STA sends another PS-Poll.

  8. If More Data = 0 → STA returns to doze mode.

Example Sequence

Beacon(TIM) → PS-Poll → DATA#1 [MoreData=1] → PS-Poll → DATA#2 [MoreData=0] → Sleep

Null Data Frames (State Transitions)

STAs use Null Data frames (no payload) to signal power state transitions.

Bit | Power Management | Meaning |

|------|——————|---------| | 0 | Active mode (stay awake) | | | 1 | Enter PS mode (doze) | |

The STA transmits this frame before going to sleep or after waking up, allowing the AP to update its association table accordingly.

Buffered Frame Delivery (Example)

AP : Beacon + TIM(bit=STA)
STA: Wake → PS-Poll
AP : DATA1 (MoreData=1)
STA: ACK → PS-Poll
AP : DATA2 (MoreData=0)
STA: ACK → Sleep

DTIM and Broadcast/Multicast Delivery

  • AP buffers broadcast/multicast frames until the next DTIM beacon.

  • When DTIM Count = 0, AP transmits all buffered broadcast/multicast frames after beacon transmission.

  • All PS STAs must stay awake until the DTIM transmission completes.

DTIM Beacon → [Broadcast#1] → [Broadcast#2] → End of DTIM → STAs return to sleep

Ad Hoc (IBSS) Power Save (ATIM Window)

In IBSS (Independent BSS) networks, there is no AP to buffer frames. Instead, STAs coordinate via ATIM (Announcement TIM) frames.

Mechanism:

  1. All STAs wake for the Beacon + ATIM window.

  2. During ATIM window, STAs announce buffered data via ATIM frames.

  3. Recipients of ATIM stay awake for the next data period.

  4. STAs without ATIMs return to doze after ATIM window.

Timing:

Beacon → ATIM window (announcements) → Data period → Sleep

Timing Example (Infrastructure Mode)

AP : Beacon (TIM bit=STA)
STA: DIFS + Backoff → PS-Poll
AP : SIFS → DATA (MoreData=1)
STA: ACK → PS-Poll
AP : SIFS → DATA (MoreData=0)
STA: ACK → Sleep

Power Save Timing Relationships

Important MAC Fields

Field

Function

Where Used

Power Mgmt bit More Data bit TIM Bitmap AID DTIM Count / Period

STA sleep/awake signaling AP indicates queued frames Announces buffered STAs Identifies STA bit index Multicast delivery timing

Frame Control (STA→AP) Frame Control (AP→STA) Beacon (TIM element) Association & PS-Poll Beacon (DTIM field)

Edge Cases and Behavior Notes

Case | Behavior |

|-------|———–| | STA misses beacon | STA stays asleep until next beacon, increased latency | | Multiple PS STAs | AP maintains per-STA buffers and bitmap bits | | AP buffer overflow | Queued frames dropped silently | | STA stays awake | Treated as active; AP transmits immediately | | PS-Poll lost | STA retries after timeout | | STA fails to send PS-Poll | Buffered frames eventually expire |

Power Save with Fragmentation and RTS/CTS

  • If buffered frame is fragmented, AP transmits fragments using SIFS spacing.

  • STA stays awake until final fragment (More Data = 0).

  • RTS/CTS may precede PS-Poll response to protect channel access.

  • NAV ensures protection during PS-Poll → DATA exchanges.

Infrastructure vs IBSS Power Save

Feature

Infrastructure (AP-based)

IBSS (Ad Hoc)

Coordination Signaling Mechanism Buffered by Unicast Delivery Broadcast Delivery Wake Period

Centralized (AP) TIM / DTIM beacons AP PS-Poll frames At DTIM Beacon interval

Distributed among peers ATIM window Each STA maintains buffer ATIM + direct data During ATIM window Beacon + ATIM window

Example Beacon TIM Layout

Example TIM element in beacon frame:

Element ID: 5 (TIM)
Length: variable
DTIM Count: 0x01
DTIM Period: 0x03
Bitmap Control: 0x00
Partial Virtual Bitmap: 0x40 0x00  (bit 6 = STA with AID=6 has pending data)

Implementation Guidelines

  • Beacons must always be sent at basic rates.

  • TIM and DTIM fields must accurately reflect buffered traffic.

  • AP must maintain per-STA queues for PS clients.

  • PS STAs must synchronize with AP’s beacon interval.

  • DTIM period determines broadcast delay and power-saving efficiency.

  • AP may drop buffered frames after timeout or queue overflow.

References

  • IEEE Std 802.11-2020, Clause 10.2.1–10.2.3 (Power Management)

  • IEEE Std 802.11b-1999, Clause 18.2.3.6 (TIM, DTIM, PS-Poll)

  • Gast, M. 802.11 Wireless Networks: The Definitive Guide, O’Reilly

  • Tanenbaum & Wetherall, Computer Networks, 5th Edition

  • Heusse et al., Performance Anomaly of 802.11b, IEEE INFOCOM 2003

Figures

TIM bitmap and DTIM structure

Example TIM and DTIM relationship in beacon frame.

Power save sequence timing

PS-Poll and data delivery sequence between STA and AP.