Appunti Internet of Things

Notes - IoT

1. Introduction 3 2. IoT Connectivity 3 3. Hardware & Hardware Abstraction 3 3.1 Wireless Devices 4 4. Communication Technologies for the IoT 5 4.1 Networking fundamentals 5 4.2 Application Layer Protocols for IoT 5 4.3 HTTP 5 4.4 COAP - Constrained Application Protocol 5 4.5. MQTT- Message Queuing Telemetry 6 5. IoT Platforms 7 6. Wired Connectivity 7 6.1 Introduction to wireless Connection 8 7. Long Range Communication 8 7.1 Class A device & Message Format 9 7.2 LoraWAN uplink - ALOHA 9 7.3 Performance 9 8. ZigBee Errore. Il segnalibro non è definito. 8.1 Network Layer 12 9. 6LowPAN 14 9.1 Types of Routing protocols: 14 10. RFID - Radio frequency identification 15 10.1 RFID HF 15–10.2 RFID UHF/SHF Electromagnetic coupling 15 10.3 Slotted ALOHA protocol Errore. Il segnalibro non è definito. 10.4 Framed ALOHA 16 11. Used Distribution 17 11.1 Poisson Distribution 17 11.2 Geometric Distribution 17.

1. Introduction

In nowadays internet mobile clients not only download data from the internet, but they can also push information. The real value of the IoT system is the data that they collect. They are spreading among the world because of: cost reduction, miniaturization of CMOS, energy efficiency, resiliency.

2. IoT connectivity

IoT connectivities are based on these features: coverage, bandwidth, reliability, network architecture.

High frequencies: high bandwidth, less crowded but propagation is difficult due to low penetration.

Low frequencies: low bandwidth and many interference.

The same antenna can be used to receive and transmit information.

• Antenna gain: ratio between the power density in the direction of maximum radiation and the power density of an isotropic radiator at reference distance.
• Antenna directivity: ratio between the power density in the direction of maximum radiation and the average power density at reference distance.

Quite often empirical model are adopted to calculate attenuation due to distance with approximate formulas, one of the most famous is the Okumura-Hata model that take in consideration several scenarios: big city to rural areas with distance >1k.

3. Hardware & hardware abstraction

A sensor node (or mote) is a device with the capability of: sensing external phenomena, storing and processing information and communicating with other motes. The actuator instead receives input signals, processes and stores information on industrial processes.

We have different kind of hardware:

• Microcontroller: a single integrated circuit used for a specific application. High flexibility with low cost, but not so powerful.
• Digital Signal Processors: a DSP is a specialized microprocessor designed to process discrete signals through digital filters. High speed and fine for data intensive operation, but low flexibility.
• Application Specific IC: IC that can be customized applications. High speed and customization, but high cost and scarce flexibility.
• FieldProgrammableGateArrays: similar high-level architecture of ASICs, high speed and reconfigurability but high complexity and cost.

Sampling: reading the time-continuous signal at given points in time. Bandwidth is the inverse of the = 1/sampling interval, = 2 Nyquist Theorem: where B is the signal bandwidth.

Quantization means assigning a discrete value to a continuous interval. Resolution is the lowest variation in the input causing a codeword change.

We have a different approach to IoT devices: use applications to directly control hardware to improve performance and efficiency or use an OS layer to achieve better compatibility and fluency control, to focus only on application (connectivity, sensors…). Outsourcing generic tasks.

3.1 Wireless devices

This device has a limited power source and lifetime depends on battery. Communication subsystems are usually the most battery drain component of the system, and drain energy mostly when it is active.

In WakeUp time no information can be managed. Sleep time is the time to shut down the system, it can be transported. The main energy consumption is to keep the connection alive and send information.

Note that WakeUp energy is the same of TX/RX enable one.

What is the consumed energy per bit for transmitting a packet of L bits?

The emitted power is a parameter that is chosen to the lowest value which allow a “good reception”.

• Formula: (1-BEP)= P to success in bit sending.
• Length: l = length of the package.
• Formula: (1-BEP)^l = P to success into pack sending (assumption).

BER and PER depend on the level of noise in the TX\RX channel.

BER can be computed once given the specifics of the channel.

Receiver Sensibility: the minimum input signal power needed at receiver input to provide adequate SNR at receiver output to do data demodulation.

Last 2 are dissipation terms dependent on the architecture.

Local data processing is crucial in minimizing power consumption because it limits the data transfer. Also memory access is has to be count in power consumption and we have to choose the compatible memory (flash writing\erasing is expensive).

4. Communication technologies for the IoT

We can divide the IoT traffic in: cyclic\acyclic and multimedia\backhand traffic. Some definitions:

Throughput: the number of bits per second that can be sent across a link.

Delivery Time: time necessary to send one service data unit from a source to a destination. It depends on transmission time, propagation time and protocol execution time.

Minimum Cycle Time (MCT): minimum time required to execute a cycle in a control loop. = | − |/

Jitter: precision\reliability in performing periodic operations.

4.1 Networking fundamentals

To define a network we have to define:

• Infrastructure: host, link and network devices.
• Architecture: the topology of the network, that could be ring, mesh, tree and star.
• Set of communication protocols: set of rules that define the information exchange between 2 entities.

4.2 Application layer in communication protocols for IoT

An application is a piece of software executed by the host able to communicate with another program through a network. Due to fragmentation of the communication technology, typically the application layer is used also to uniform the different languages used to communicate.

Different approach:

• Web browsing approach: HTTP, COAP.
• Client\Server.
• Publish\Subscribe: MQTT.

4.3 HTTP

Each resource in the web is: managed by a server, identified by a Uniform Resource Location (URL), accessed synchronously by a client.

The client (active) issues HTTP requests for a specific resource and the server (passive) answer backs. The transfer is stateless.

4.4 COAP - Constrained Application Protocol

Web solutions are hardly applicable on IoT (too costly), so COAP redesign a web-based service for IoT. It is an embedded asynchronous web transfer protocol that uses UDP and a subset of the HTTP methods (gets, post, put, delete).

COAP is characterized by:

• Messages with 4 bytes header.
• Reliable exchange though ACK (COAP take).