If transmission power is too strong：
　→ Radio waves spread further than necessary
　→ Interfere with adjacent access point
　→ Increase power consumption in vain
　→ Radio wave end up reaching to outside of building (Security Risk)

If transmission power is too weak：
　→ Radio wave cannot reach to an expected range
　→ Connection is unstable

Automatic adjust depends on the situation is needed
　→Deliver radio waves only where needed 
　→ Decrease interference
　→ Save power consumption

Volume control for microphone in meeting

When you talk to person close to you
　→Small volume is enough（ Decrease electric power）

When you talk to person far from you
　→Big volume is required（ Increase electric power）

TPC automatically adjusts the volume for you

1. Devices connect to an access point
　↓
2. Devices measure the strength of the radio waves they receive
　↓
3. Devices report the strength to the access point
　↓
4. The access point adjust transmission power depends on the information
　↓
5. The access point transmits at only power level required

1. Reduce interference
　→ Do not spread radio wave more than it is needed
　→ Decrease interference with an adjacent access point

2. Save power consumption
　→ Use only electric power is needed
　→ Extend battery life

3. Enhance security
　→ Ensure range radio wave reach 
　→ Prevent radio wave leaking outside a building

4. Improve overall network efficiency
　→ Multiple access points work with adequate electric power
　→ Improve performance

 IEEE 802.11h  is a Wi-Fi standard that formally defines TPC and DFS as standard specifications 

Background：
5GHz band has been originally used in military, weather radar, satellite communication

Problem：
Once Wi-Fi began using the same frequency band, interference with existing radar systems became a concern.

Solution：
Standardize TPC and DFS with 802.11h
　→ Don’t output radio wave more than needed ( TPC )
　→ Avoid frequency interfering with radar ( DFS )

TPC（ Transmit Power Control）
　→ Adjust transmission power
　→ Decrease interference and save power consuming

DFS（ Dynamic Frequency Selection）
　→ Automatically switch to other frequency when it detects radar
　→ Avoid interference with radar systems

TPC    = Mechanism (concept)  for automatically adjusting transmission power
802.11h = Defining TPC and DFS as standard

Although TPC existed as a concept before 802.11h, it was formally defined as an official standard by 802.11h

It is a duty in mainly Europe

EU regulation：
Wi-Fi devices that use the 5GHz band are required to comply with 802.11h（TPC・DFS）

In fact, it is also common in Japan and North America

Layer 7  Application  │
Layer 6  Presentation │
Layer 5  Session      │  The layers TPC is not applicable
Layer 4  Transport    │
Layer 3  Network      │
Layer 2  Data Link    │
─────────────────────────────
Layer 1  Physical     ← The  layer TPC works
─────────────────────────────

TPC ：The method to automatically adjust transmission power of Wi-Fi

How it works： Devices measure the strength of received raido waves and report back to the access points
　→ Access Point automatically adjust transmission power

Pros：
・Reduce interference
・Save power consumption
・Improve security
・Improve efficiency in whole network

Relationship with 802.11h：
TPC = Concept for automatic adjusting transmission power
802.11h = Define TPC and DFS as standard
　→ It is created for prevent interference from radar in 5GHz band

OSI Layer： Physical Layer（ Layer 1）

TPC = The method to automatically adjust the strength of radio wave depending on the situation 802.11h = The definition of TPC as standard