Bluetooth Mesh Networking: The Packet

There are many reasons to believe that Bluetooth mesh networking will be the most successful  low-power IoT standard. Its V1.0 delivers much more than anybody anticipated. It has all essential ingredients, including an extremely deep and flexible application layer (addressed in the Mesh Model and Mesh Device Properties specifications). Before it changes the way things around us communicate, let’s find out what makes this new global technology so revolutionary.

There are many novel and unique concepts in mesh, but the key asset and differentiator is the packet. It is extremely compact. This compactness contributes to the spectral efficiency and throughput of Bluetooth mesh networks.

Inside the Packet

Radio is a shared medium and collisions are one of the major problems to address. The math is simple: a shorter packet means less collisions. But how short can it be? The answer is: up to 29 bytes, as described in the section 3.4.4 of the Mesh Profile Specification

Bluetooth mesh…is the first wireless standard capable of meeting the enormous expectations of the IoT.

Of course, such design begins with the basics: compressed binary payload instead of a text representation. Covering a broad set of use cases (including connected lighting, building automation, and sensors), 11 bytes for the application payload seems appropriate. 1-2 bytes for an opcode and up to 10 bytes for parameters, such as a value measured by a sensor, or a multidimensional light (lightness, hue, saturation) with a transition time.

Security and Control

On top of that there are two items, which may be considered an overhead (but it is a necessary overhead): addressing/propagation control (SRC, DST, CTL+TTL: 5 bytes in total) and security (IVI+NID, SEQ, AppMIC and NetMIC). The IVI+NID is 1 byte. This byte helps identifying a network (is this a network I know and have keys to interact with?). SEQ is 3 bytes and, together with the unique concept of a slowly propagated IV Index, forms a 7-byte long sequence number. Each packet sent on a mesh network has a unique sequence number, per given SRC address. The smart part here is we’re including only 3 bytes in the air interface packet. The remaining 4 bytes are slow changing and are known to the network. Sequence is essential in two areas: detecting replayed packets (very trivial security attack) and also being the key ingredient of both network and application nonces – see Section 3.8.5 of the aforementioned spec.

Message Integrity Checks

MICs, or Message Integrity Checks, define the level of security of the system. Bluetooth Mesh has a dual layer security — the network layer and the application layer. Messages may be secured with two independent keys. This is useful for relay nodes to authenticate a message on a network layer and still not be able to tamper with the application payload. A light bulb that relays a message to a door lock cannot change the payload from openclose, but checks if the packet belongs to its own network. The network layer MIC can be either 8 or 4 bytes long. In its shorter form it is combined with the application layer MIC that can again be 8 or 4 bytes long. The end result is an application payload that is sufficient for almost any building automation, lighting control, and sensor applications with strong security and flexible addressing. And this all comes in an extremely compact form factor. Combined with the modulation scheme offered by Bluetooth Low Energy (LE), it is also feather light and includes all necessary radio interface fields, such as a preamble, an access address, and a CRC, it totals 47 octets. As a result, a single transmission on a single frequency lasts less than 400μs. This is 10x less than it takes to transmit a comparable message using other existing wireless technologies. When using the new 2M PHY introduced by Bluetooth 5, this advantage can potentially be doubled.

 Spectrally Efficient

The success of any wireless system fundamentally relies on the spectral efficiency. It is like the success of an airliner fundamentally relies on its fuel efficiency. In the low power, ultra-short message category, Bluetooth mesh delivers an order of magnitude more than other wireless solutions. As far as data transmission is concerned, it is the first wireless standard capable of meeting the enormous expectations of the IoT era.

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