Beaconing
============

Common IEs in IEEE 802.11b Beacons
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

.. code-block:: none

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

Power Save Timing Example
~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: none

   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
~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: none

   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         | 20 µs                   |
| SIFS             | 10 µs                   |
| DIFS             | 50 µs                   |
| Beacon Interval  | 100 TU (102.4 ms)       |
| PHY Preamble     | 192 µs (long preamble)  |
| Rate             | 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  | All STAs share common TSF time         | Drift if beacons missed              |
| Power Save       | Enables doze scheduling (TIM/DTIM)     | Increased latency                    |
| Discovery        | Allows passive network detection       | Beacon overhead on busy channels     |
| Redundancy       | Regular transmission ensures recovery  | Collisions possible (IBSS)           |
+------------------+----------------------------------------+--------------------------------------+

Timing Example — Infrastructure Beacon Sequence
------------------------------------------------

.. code-block:: none

   |<-- 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
--------

.. figure:: _static/beacon_frame_structure.svg
   :align: center
   :alt: Beacon frame format

   Structure of IEEE 802.11b Beacon frame and main fields.

.. figure:: _static/beacon_timing_sequence.svg
   :align: center
   :alt: Beacon interval and TSF synchronization

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