Beaconing

Common IEs in IEEE 802.11b Beacons

Element ID	Field	Description
0 1 3 5 42	SSID Supported Rates DS Parameter Set TIM ERP Information	Network name (0–32 bytes) List of basic and extended data rates Current channel number Traffic Indication Map for power save STAs (Optional) for 802.11g coexistence

Beacon Transmission Process

1. AP maintains a 64-bit TSF timer.

2. At each TBTT: - AP assembles beacon with updated TSF timestamp. - Inserts TIM and DTIM information.

3. If channel is idle → transmit immediately.

4. If channel is busy → defer using DIFS + random backoff.

5. Beacon transmitted at lowest basic rate (e.g., 1 Mbps).

6. All STAs receiving beacon update TSF timers.

Power Save Coordination via TIM and DTIM

Traffic Indication Map (TIM)

The TIM IE identifies STAs with buffered unicast data.

Field	Description
DTIM Count | Beacons until next DTIM beacon DTIM Period | Frequency of DTIM beacons Bitmap Control | Offset of partial virtual bitmap Partial Virtual Bitmap | Bits mapping AIDs of PS STAs

• Each associated STA has an AID (Association ID).

• If bit = 1 → AP has buffered data for that STA.

Delivery TIM (DTIM)

DTIMs indicate pending broadcast/multicast traffic.

• When DTIM Count = 0, the beacon is a DTIM beacon.

• After DTIM beacon, AP transmits all queued broadcast/multicast frames.

• PS STAs must wake at every DTIM to receive these frames.

Example:

DTIM Period = 3
Beacons: #1 → #2 → #3 (DTIM)
AP sends broadcast/multicast after #3

Power Save Timing Example

STA: (Doze)........Wake→Receive Beacon(TIM Bit=0)→Sleep
STA: (Doze)........Wake→Beacon(TIM Bit=1)→Send PS-Poll→Receive Data→Sleep

Infrastructure vs IBSS Beaconing

Infrastructure BSS

• AP is the beacon source.

• Beacons define network timing for all associated STAs.

• TSF synchronization is one-way (AP → STA).

IBSS (Ad Hoc) Beaconing

• No centralized AP.

• All STAs maintain their own TSF timers.

• At each TBTT: 1. Each STA sets a random Beacon Delay timer. 2. If another beacon is heard before expiry → cancel own beacon. 3. If timer expires → STA transmits beacon.

• Ensures exactly one beacon per interval.

• All STAs synchronize TSF using the first beacon received.

TSF Synchronization in IBSS

STA1: Random delay = 30 µs
STA2: Random delay = 50 µs
STA3: Random delay = 70 µs
STA1 transmits beacon first → STA2, STA3 sync to STA1 TSF.

Beacon Frame Example (Field Breakdown)

Field | Example Value | Description |

|--------|—————-|-------------| | Timestamp | 0x0000123456789ABC | TSF timer value | | Beacon Interval | 0x0064 | 100 TU (102.4 ms) | | Capability Info | 0x0431 | ESS + Short Preamble + Privacy | | SSID | “MyWiFi” | Network name | | Supported Rates | 1, 2, 5.5, 11 Mbps | Basic data rates | | DS Parameter Set | Channel 6 | Operating channel | | TIM | DTIM Count=1, Period=3, Bitmap=0x40 | PS indication |

NAV and Beacon Duration

• Beacon Duration field = 0, so beacons do not reserve the channel.

• No ACK is transmitted (broadcast frame).

• STAs may begin contention immediately after beacon transmission ends.

Beacon Timing Parameters (802.11b)

Parameter	Typical Value
SlotTime SIFS DIFS Beacon Interval PHY Preamble Rate	20 µs 10 µs 50 µs 100 TU (102.4 ms) 192 µs (long preamble) 1 Mbps (basic rate)

Beacon Filtering and Scanning

• STAs can perform passive scanning (listening for beacons) or active scanning (sending Probe Requests and receiving Probe Responses).

• Filters are applied by SSID, BSSID, and channel.

• Beacons carry all information needed to join a BSS.

Advantages and Tradeoffs

Aspect	Benefit	Tradeoff
Synchronization Power Save Discovery Redundancy	All STAs share common TSF time Enables doze scheduling (TIM/DTIM) Allows passive network detection Regular transmission ensures recovery	Drift if beacons missed Increased latency Beacon overhead on busy channels Collisions possible (IBSS)

Timing Example — Infrastructure Beacon Sequence

|<-- Beacon Interval (100 TU) -->|
AP: [Beacon]..........[Beacon]..........[Beacon]
STA:(Sleep).......Wake→Receive→Sleep→Wake→Receive...

References

• IEEE Std 802.11-2020, Clause 10.2.1.4 (Beacon Generation & Synchronization)

• IEEE Std 802.11b-1999, Clause 18.2.3.5 (Beacon Transmission and TSF)

• 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

Structure of IEEE 802.11b Beacon frame and main fields.

Beacon interval, TSF synchronization, and wake/sleep behavior.