---
title: BLE 筆記
tags: BLE
---

## BLE 簡介
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### BLE 架構
|layer|用途|
|---|---|
|Physical Layer|SOC 上的無線電的收發|
|Link Layer|定義封包結構、收發及資料加密|
|L2CAP|多工處理上層的通訊協定的接口|
|ATT|將數值轉為屬性中並實作供上層讀取邱改的方法|
|GATT|基於ATT數值進行封裝供上層使用|
|GAP|對LL的功能抽象化，便於開發|
|SMP|安全通信建立|
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### 1. Physical Layer
- Industrial Scientific Medical (ISM)：2.4 GHz
- Radio Frequency (RF) channels：2 MHz 訊號間隔
- BLE RF channel types：廣播通道 (advertising channels)、資料通道 (data channels)
- 通道數：40 channels(advertising：3, data：37)
- 實體層資料傳輸綠：1 Mbps
- 接收端 Bit Error Rate (BER)：$10^{-3}$
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### 1. Link Layer
建立點對點連結 (point-to-point connection) 時，接收端傳送 Request Message (隨機生成 32-bit access code) 給傳送端。
- 極微網 piconet：成對 master 及 slave (星狀拓樸 star topology, 1個 master 對m個 slave)
- 監聽方式：Time Division Multiple Access (TDMA)
- master 提供訊息 slave：跳頻演算法(frequency hopping algorithm)
- connection event：通道分割不重疊的時間單位
- Inter Frame Space (IFS)：150 µs (傳送間隔 transmission space)
- 確認結尾封包：data channel packet More Data (MD)
- 連續封包發生 bit errors 或是 access address field errors 則封包毀損。
- connInterval：7.5 ms－4 s,</br>connSlaveLatency：0－499 timeout,</br>connSupervisionTimeout：100 ms－32 s
- connection method：stop-and-wait,</br>Sequence Number (SN)：identifies the packet number,</br>Next Expected Sequence Number(NESN)： recevied next Sequence number.
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### L2CAP
- 目標：多工處理上面三層的通訊協定
- max L2CAP payload size：23 bytes
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### ATT
- **server** 及 **client** 雙方溝通的規則
- GATT：資料架構的資訊管理儲存
- server 對 client 未定型化溝通模式：
1. notifications, which are unconfirmed.
3. indications, require client to send a confirmation.
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### GATT
GATT 架構使用 ATT 的傳送服務，交換裝置間的腳色問題。
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### Security
確保 BLE 裝置間通信間的安全性
BLE LL 層建立加密 encryption 及認證 authentication
透過 Chaining Message Authentication Code Protocol,CCMP 以及 AES block cipher 做加密及認證

Pairing 有三個步驟
1. 特徵交換：通訊雙方建立輸出輸入的容量及資訊
2. 配對過程產生的Key：
LE legacy pairing:
- Temporary Key(TK)：短暫出現的Key,用來生成STK (128bits)
- Short Term Key(STK)：加密配對時使用的key (128bits)
- Long Term Key(LTK)：加密配對後的key (128bits)
3. 密鑰交換，交換時機：
•STK：LE legacy pairing
• LTK：LE Secure Connections
• shared Link Key：BR/EDR pairing
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### GAP and Application Profiles
- BLE GAP 因應要求定義四種：
- Broadcaster
- Observer
- Peripheral
- Central
- 廣播 advertise：Broadcaster to Observer
- 極微網 piconet(master and slave)：Central to Peripheral
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### Performance Evaluation
1. 耗能 (Energy Consumption)
2. 延遲 (Latency)
3. 最大極微網大小 (Maximum Piconet Size)
4. 吞吐量 (Throughput)