Embedded Communication

SPI vs I2C vs UART comparison

Wireless Protocol Landscape

Choosing a wireless protocol involves tradeoffs between range, power consumption, data rate, and network topology.

| Protocol | Range | Data Rate | Power | Topology | Best For | |---|---|---|---|---|---| | BLE 5.x | 100-400m | 2 Mbps | Very low | Star, mesh | Wearables, beacons, sensors | | Zigbee | 10-100m | 250 kbps | Low | Mesh | Home automation, lighting | | Thread | 10-100m | 250 kbps | Low | Mesh | Smart home (with Matter) | | LoRa | 2-15 km | 0.3-50 kbps | Very low | Star-of-stars | Agriculture, utilities, asset tracking | | NB-IoT | 10+ km | 250 kbps DL | Medium | Cellular star | Metering, city infrastructure | | Wi-Fi | 50-100m | 1+ Gbps | High | Star | Streaming, high-bandwidth IoT | | Matter | Depends on transport | Varies | Varies | Mesh/star | Unified smart home standard |

Bluetooth Low Energy (BLE)

BLE is optimized for intermittent data transfer with minimal power consumption. A BLE device can run for years on a coin cell battery.

Key Concepts

GAP (Generic Access Profile): controls advertising and connections
GATT (Generic Attribute Profile): defines data exchange via services and characteristics
Advertising: device broadcasts small packets (31 bytes) periodically
Connections: bidirectional, reliable data exchange between central and peripheral

GATT Hierarchy

Profile
  └── Service (UUID: 0x180F = Battery Service)
        └── Characteristic (UUID: 0x2A19 = Battery Level)
              ├── Value: 85 (percent)
              ├── Properties: Read, Notify
              └── Descriptor (Client Characteristic Configuration)

BLE on Embedded Rust (nRF52 with SoftDevice)

// BLE GATT service definition (UUID = "180f" Battery Service)
STRUCTURE BatteryService
    battery_level: byte    // characteristic UUID "2a19", read + notify

ASYNC TASK BLE_TASK(softdevice, server)
    LOOP
        conn ← AWAIT ADVERTISE_CONNECTABLE(softdevice, adv, config)

        // Update battery level and notify connected client
        server.battery.SET_BATTERY_LEVEL(conn, 85)

Zigbee

IEEE 802.15.4-based mesh protocol for low-power, low-data-rate networks. Supports up to 65,000 nodes per network.

Node Roles

Coordinator: forms the network, assigns addresses, one per network
Router: relays messages, extends network range, must be mains-powered
End Device: sleeps most of the time, communicates through parent router

Zigbee Mesh Routing

Messages hop through routers to reach distant nodes. If one path fails, the mesh self-heals by finding alternative routes. AODV (Ad-hoc On-demand Distance Vector) is the typical routing protocol.

LoRa and LoRaWAN

LoRa (Physical Layer)

Chirp Spread Spectrum (CSS) modulation that trades data rate for range and noise immunity. Operates in unlicensed ISM bands (868 MHz EU, 915 MHz US, 433 MHz Asia).

LoRaWAN (Network Layer)

Defines the network architecture, device classes, and MAC protocol.

  End Devices ──── Gateway ──── Network Server ──── Application Server
  (LoRa radio)    (LoRa+IP)     (deduplication,     (business logic)
                                 routing, MAC)

Device Classes

| Class | Receive Windows | Latency | Power | Use Case | |---|---|---|---|---| | A | After each uplink only | High | Lowest | Sensors, meters | | B | Scheduled (beacon-synced) | Medium | Medium | Actuators | | C | Continuous listening | Low | Highest | Powered actuators, gateways |

LoRa Parameters

| Spreading Factor | Range | Data Rate | Airtime | |---|---|---|---| | SF7 | Shortest | ~5.5 kbps | ~50 ms | | SF12 | Longest | ~0.3 kbps | ~1.5 s |

Higher spreading factor = longer range but lower throughput and more airtime (regulatory limits apply).

NB-IoT (Narrowband IoT)

Cellular LPWAN technology that operates in licensed spectrum alongside LTE:

Deployed by mobile operators -- no gateway infrastructure needed
Deep indoor penetration (20 dB improvement over LTE)
Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) for battery life
Ideal for devices sending small amounts of data infrequently

Thread

IPv6-based mesh networking protocol for smart home devices:

Built on IEEE 802.15.4 radio (same as Zigbee)
Native IP connectivity -- no application-layer gateway needed
Border Router connects Thread mesh to Wi-Fi/Ethernet networks
Self-healing mesh with router-eligible devices
Low power with Sleepy End Devices

Matter

A unifying smart home standard that runs over Thread, Wi-Fi, and Ethernet:

Common application layer across transport protocols
Device types defined by data models (lights, locks, thermostats)
Secure commissioning via QR codes
Multi-admin: devices can be controlled by multiple ecosystems simultaneously
Backed by Apple, Google, Amazon, Samsung, and others

Application-Layer Protocols

MQTT (Message Queuing Telemetry Transport)

Publish-subscribe protocol designed for constrained devices and unreliable networks.

  Publisher ──── Broker ──── Subscriber
  (sensor)      (server)     (dashboard)

  Topic: "home/livingroom/temperature"
  Payload: "23.5"
  QoS: 0 (at most once), 1 (at least once), 2 (exactly once)

MQTT Features

Retained messages: broker stores last message per topic for new subscribers
Last Will and Testament (LWT): broker publishes a message if client disconnects unexpectedly
Keep-alive: periodic PINGREQ/PINGRESP to detect dead connections
Topic wildcards: + (single level), # (multi level)

MQTT on Embedded

// MQTT client for embedded (no dynamic allocation, buffer size 256)
mqtt ← MQTT_NEW(socket, clock, broker_config, buffer_size ← 256)

// Publish sensor data
MQTT_PUBLISH(mqtt,
    payload ← "23.5",
    topic ← "sensors/temp",
    qos ← AtLeastOnce)

// Poll for incoming messages
MQTT_POLL(mqtt, CALLBACK(client, topic, payload, properties):
    IF topic = "actuators/led" THEN
        SET_LED(payload[0] = '1')
)

CoAP (Constrained Application Protocol)

RESTful protocol for constrained devices, built on UDP instead of TCP:

| Feature | MQTT | CoAP | |---|---|---| | Transport | TCP | UDP | | Pattern | Pub/Sub | Request/Response (REST) | | Header size | 2+ bytes | 4 bytes | | Discovery | No | Yes (/.well-known/core) | | Observe | Via subscribe | Built-in observe option | | Security | TLS | DTLS |

CoAP maps naturally to HTTP methods (GET, POST, PUT, DELETE) and supports resource observation for push updates.

Mesh Networking

Mesh Topology Benefits

Self-healing: automatically reroutes around failed nodes
Range extension: each node can relay for others
Scalability: add nodes without infrastructure changes

Mesh Challenges

Increased latency (multi-hop)
Higher power for routing nodes
Routing table overhead
Potential for routing loops

BLE Mesh

Managed flooding (no routing tables needed)
Publish/subscribe model for group communication
Up to 32,767 nodes
Used in commercial lighting and building automation

Gateway Design

Gateways bridge constrained device networks to IP infrastructure.

 Sensor ─── (BLE/Zigbee/LoRa) ──── Gateway ──── (Wi-Fi/Ethernet) ──── Cloud
  MCU                               Linux SBC                          Server
  Battery                           Mains power
  Constrained                       Protocol translation

Gateway Responsibilities

Protocol translation: convert sensor protocol to MQTT/HTTP
Data aggregation: batch messages to reduce cloud traffic
Local processing: edge filtering, anomaly detection
Security: TLS termination, device authentication
Store and forward: buffer data during network outages
Device management: firmware updates, configuration

Gateway Architecture

┌─────────────────────────────────────────┐
│  Gateway                                │
│                                         │
│  ┌──────────┐  ┌──────────┐  ┌───────┐ │
│  │ Radio    │  │ Protocol │  │ Cloud │ │
│  │ Driver   │──│ Handler  │──│ Client│ │
│  │(BLE/LoRa)│  │(parse,   │  │(MQTT/ │ │
│  │          │  │ validate)│  │ HTTP) │ │
│  └──────────┘  └──────────┘  └───────┘ │
│       │            │             │      │
│  ┌──────────────────────────────────┐   │
│  │  Local Storage / Message Queue   │   │
│  └──────────────────────────────────┘   │
└─────────────────────────────────────────┘

Key Takeaways

Protocol selection depends on range, power budget, data rate, and infrastructure availability.
BLE dominates short-range personal devices; LoRa excels at long-range, low-power sensing.
Thread and Matter are converging the smart home on IP-based, interoperable mesh networking.
MQTT's pub/sub model suits IoT's many-to-many data flows; CoAP fits resource-constrained RESTful interactions.
Gateways are critical infrastructure that handle protocol translation, security, and reliability.