BLE_star

IP-over-BLE Star Network

Introduction

IP-over-BLE was tested as a Proof-of-Concept (PoC), and has several limitations compared to Wi-Fi mesh, including Inflexibility of topology, Shorter range, and Lower bandwidth/data rate, with the only noteworthy advantage being that BLE is significantly more energy/power efficient compared to Wi-Fi. Nevertheless, it could be handy for facilitating short-range, peer-to-peer transactions if a strong use case were identified.

There are considerable ongoing efforts to define IPv6-over-BLE network standards, however for our workshop we required IPv4 connectivity, which has no officially recognized network standard for integration with a BLE link layer. This feature can however be easily implemented by leveraging BlueZ, which is the bluetooth stack implemented in Linux. Linux kernels 3.4 and later include Bluez 5.0 or later, and in particular version 5.55-3.1+rpt1 was used for our testing. To simplify BlueZ implementation of IPv4-over-BLE, we additionally used the bluez-tools package (tested with version 2.0-20170911.0.7cb788c-4) which has some functionality to readily implement bluetooth network access.

Required packages

• Bluez: $ sudo apt-get install bluez
• Bluez-tools: $ sudo apt-get install bluez-tools

Description

Bluetooth uses Time-Division Multiple Access (TDMA) for channel access, meaning that in a bluetooth network, one node is designated as the master, and this master polls the other (slave) nodes for communications to occur. This is the reason for the inflexible topology, which is essentially a star with the master node in the center, as slaves may only communicate directly with the master when polled. To communicate with other nodes, the master must relay the information.

Additionally, for IP-over-BLE, the master node acts as a DHCP server to issue IP addresses to connecting bluetooth peers.

Topology

Instructions

It is strongly recommended to set the bluetooth hostname on each device before attempting to configure anything. Otherwise you will end up with several bluetooth devices all named raspberrypi and no idea which is which when trying to discover & peer. To set the bluetooth hostname, create the /etc/machine-info file with the following content, replacing <device-name> with your desired bluetooth name:

PRETTY_HOSTNAME=<device-name>

To configure the Master node:

1. Create the bridge interface file (ex. pan0) at /etc/systemd/network/pan0.netdev with the following content:

[NetDev]
Name=pan0
Kind=bridge

2. Create the network settings file at /etc/systemd/network/pan0.network with the following content:

[Match]
Name=pan0

[Network]
Address=172.25.1.1/24
DHCPServer=yes

N.B. - If connected devices should not use the BLE channel to accessing external network, add the EmitRouter=no line in the Network section of the above file.

3. Register the bluetooth agent service by creating the following file at /etc/systemd/system/bt-agent.service:

[Unit]
Description=Bluetooth Auth Agent

[Service]
ExecStart=/usr/bin/bt-agent -c NoInputNoOutput
Type=simple

[Install]
WantedBy=multi-user.target

4. Register the bluetooth network access service by creating the following file at /etc/systemd/system/bt-network.service:

[Unit]
Description=Bluetooth NAP PAN
After=pan0.network

[Service]
ExecStart=/usr/bin/bt-network -s nap pan0
Type=simple

[Install]
WantedBy=multi-user.target

5. Enable & start the required services:

$ sudo systemctl enable --now systemd-networkd
$ sudo systemctl enable --now bt-agent
$ sudo systemctl enable --now bt-network

To configure Slave nodes:

No configuration is required; simply enable bluetooth, discover & peer with the master, and run the following command:

bt-network -c <bluetooth name or address of Master> nap

Note - The above command uses the bluetooth name or 48-bit bluetooth address or address of the master, not the IP hostname or IP address.

Known Issues & Workarounds:

1. The built-in bluetooth manager for Raspberry Pi OS often has issues detecting, pairing, and/or connecting with other devices (especially other Pi's). We found that the behavior was more consistent using Blueman bluetooth manager: $ sudo apt-get install blueman. It is recommended to reboot after installing Blueman.

2. Over repeated implementations of this topology we found that occasionally, after configuring the master and attempting to connect a slave, the slave would give an error saying Network service is not supported by this device. After removing/deleting the configuration changes described in the above section "To configure the Master node", rebooting the master, and performing those configuration changes again (doing nothing differently), we found that on subsequent attempts the slaves would connect without issue.

3. Occasionally we would observe page/segmentation faults crashing the bt-network connection from the slave nodes. It was unclear what triggered these errors, but they generally were not a significant issue. Reconnecting (by running the script in the above section "To configure Slave nodes") or rebooting generally resolved these problems.

Final Note - This setup is very much a hack and should not be considered an enterprise solution; stability and consistency may vary.

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