BLE (Bluetooth Low Energy)
- 2 types of Bluetooth Communication are there
- BR/EDR – Bluetooth Basic data Rate / Enhanced data Rate
- BLE – Bluetooth Low Energy
- LE Designed for lower power consumption, lower complexity and lower cost than Bluetooth BR/EDR
- BLE Protocol stack
- Controller has the PHY and LL (Link Layer). This is made separate from Host because multiple host can use the same controller
-
Controller’s job is to take a packet and manage the medium access and physical transmission
- Uses the 2.402 GHz to 2.480 GHz band. Has 40 channels of 2 MHz each:
- 37 channels (with index 0 till 36) are connection oriented channels
- 3 channels (index 37, 38 and 39) are broadcast oriented channels
- Uses Frequency Hopping Spread Spectrum carrier
- PHY uses 2 data rates
- 1M PHY has 3 types
- Uncoded, so 1 Mbps is the data rate
- S=2 Coded PHY, so 500 Kbps is data rate
- S=8 Coded PHY, so 125 Kbps is data rate
- 2M PHY
- Only has uncoded, so 2 Mbps is data rate
- 1M PHY has 3 types
- Both FDMA and TDMA is used for Multiple access
- 37 data channels participate in FDMA
- Each channel is subdivided using TDMA into events
- Each event, one device only is allowed to speak
- 4 types of Events
- Advertising event
- Extended advertising event
- Periodic advertising event
- Connection event (where actual application payload is transmitted)
Advertising
- Advertising happens on the broadcast channels
- channels 37, 38 and 39 are for primary advertisement
- connection oriented channels can be used for secondary advertisement
BLE Mesh
- BLE Mesh stack:
- Uses the BLE layer up to GAP as the “PHY” Layer for Mesh
- 2 types of bearers for bearer layer:
- Advertising bearer which uses only the primary advertisement channels of BLE
- GATT bearer which uses GATT connections for proxy support and legacy support
- Network layer manages:
- decryption on network PDU
- decision whether to relay/transport
- decision whether to pass this packet up to lower transport layer based on address
- Lower transport layer manages:
- Segmentation and Reassembly
- Upper transport layer manages:
- app layer encryption decryption
- message integrity
- Access layer manages:
- Binding of keys and models
- controling of encryption/decryption at upper transport layer
- forwarding the correct packet to correct element
- Model layer manages:
- definitions of messages, states and models to build and maintain the mesh network
- BLE mesh uses a managed flooding approach instead of routing
- Routing requires more code and memory to maintain and manage routes, neighbour tables and routing tables
- In flooding approach, each device rebroadcasts a packet that is not meant for itself
- To control flooding, 2 things are required
- Network Message Cache – Table to remember and discard recently received messages to avoid infinite loops
- TTL value in network PDU is used as a hop counter. With each hop TTL is decreased. If TTL is 0, message won’t be rebroadcasted. Max TTL value is 126
- There are 2 security keys, network key and app key. Since BLE Mesh is a prodcast network, the definition of a network is not through physical channels but through encryption.
- A network is defined by its netkey. All nodes in the same network share the same netkey. This is the network layer key required to decrypt the network PDU.
- The provisioner creates the netkey and appkey.
Key terminologies and definitions
- Network – Network in a BLE Mesh means collection of all devices which share the following four resources:
- Network Key – Used to secure the network payload and define the network
- App Key – Used to secure the access payload and provide extra security
- Network Address - For addressing at network layer
- IV Index - Used for encryption as well as to extend network life
- Provisioner – Special device with Network Address 0 that provisions a new node into the network and provides it a unique unicast address
- Configuration Client model – Model responsible for assigning and maining the remaining network resources to any device.
- It has the capability to modify the configuration of a mesh node.
- It binds network key and app key to models on another device that implements configuration server
- It also knows the device key used to distribute changed network and app keys later.
- Device – Devices can be of two types
- Unprovisioned device – Doesn’t have the network resources. Advertises its presence to a provisioner who can decide whether to accept this device.
- Node (Provisioned Device) – Is part of at least one network. Any node can be part of more than 1 networks. Nodes might implement 3 additional features
- Relay node
- Friend node
- Low power node
Network topology
- Message – Example of Mesh message payload (unsegmented access message total = 29 bytes):
| Network header | Lower Transport header | Access Header | User data | Upper Transport Layer Footer |
|---|---|---|---|---|
| 13 bytes | 1 byte | 1 or 3 bytes | 10 or 8 bytes | 4 bytes |
-
Application layer can transmit at most 10 bytes using SIG models and 8 bytes using custom vendor models per packet. This is mentioned clearly in section 2.3.3 of Mesh Profile
-
Network PDU
| IVI | NID | CTL | TTL | SEQ | SRC | DST | Transport PDU | NetMIC |
|---|---|---|---|---|---|---|---|---|
| 1 bit | 7 bit | 1 bit | 7 bit | 24 bit | 16 bit | 16 bit | upto 128 bit | 32 or 64 bit |
- Network layer continued
- SRC and DST are the network address fields. Each element has exactly one Unicast address assigned to it during provisioning. The valid range of unicast addresses is 1 to 32767. This means total number of nodes in a mesh network can be 32767
- SEQ number increments with each packet sent out of this node.
- SRC is the unicast address of the node that sent this packet
- DST is the destination address, it can be either a unicast or group or virtual address
- Addresses are of 5 types
- 0 is the unassigned address
- 0x0001 – 0x7FFF are unicast addresses (total = 32767)
- 0x8000 – 0xBFFF are virtual addresses (total = 16383)
- 0xC000 – 0xFEFF are group addresses (total = 16127)
- 0xFF00 – 0xFFFF are reserved group addresses (total = 256)
- LPN (Low Power Node) and Friend Node
- Battery powered nodes
- Sleep, therefore need to be managed. Require a Friend node
- Friend Node is AC powered. It buffers messages for the LPN it is friend of
- LPN should wake up every PollTimeout seconds and poll the friend for any packets that are buffered for it. Min = 1 sec, Max = 345,599 sec (Ref Table 3.27 of Mesh Profile)
In depth (To be done)
- Provisioning
- Segmentation and Reassembly
- Security
- Beacons
- Proxy Feature
- LPN and Friendship details
References
[2] Mathworks BLE waveform generation
[4] BLE Mesh Profile