BLUETOOTH (IEEE 802.15.1)

Protocol for PAN communication, based on 2.4 frequencies band,it aims to the following goals:

low cost device
low distance communication
voice and data management

This comes with the cost of lower bandwidth so lower data transfer rate

PROTOCOL STACK

Bluetooth is a complex stack of protocols at different layer and devices can implement only a subset of functionality and not all

ARCHITECTURE

Bluetooth architecture is called PICO-NET where a node plays the role of master and manages the communication with the other nodes and between nodes

flowchart TD
A[MASTER]
B[node]
C[node]
D[node]
A --> B & D & C

Where:

there is always a unique master node
maximum of 7 active nodes
communication happens on a single channel using frequency hopping

PREPARATION PHASE

In order to communicate 2 nodes need to complete a preparation phase in order to construct the topology, this phase is spit in 2 sub-phases:

INQUIRY PHASE a node start the discovery of nearby nodes and become the master
PAGING PHASE master establish a bidirectional communication channel in frequency hopping with the slave nodes

flowchart TD
subgraph inquiry mode
A((master))
end
subgraph inquiry scan mode
B((slave1))
C((slave2))
D((slave3))
end
subgraph nodes out of range
E((node1))
F((node2))
end
A --> B
A --> C
A --> D

One of the main constraint of Bluetooth communication is that nodes need to have their clock synchronized, this is done by imposing the master clock to the slaves that adapts their clocks

COMMUNICATION WITH MASTER

Time is divided by time slots in which only 2 nodes can communicate This is done in order to avoid collisions

The master decide which node can communicate, the master can communicate in all the odd slots

SCO CHANNELS

SCO packets are sent in pre-allocated time slots, in order to guarantee a fixed bandwidth (must have for audio streaming applications)

In order to avoid the consumption of all time slots the master can establish 3 SCO channels at the same time

In this mode re transmissions are not allowed

ACL CHANNELS

ACL packets are best effort communication with no guaranteed bandwidth this allow for higher bandwidth, it also support asymmetric bandwidth allocation for the 2 directions

In this connection types the slave can transmit only if in the previous time slot has received a packet from the master

In this mode retransmissions are allowed

MULTI HOP COMMUNICATION (SCATTER NET)

Bluetooth supports communication between PICO-NETs if the range allows it

flowchart TD
subgraph piconet1
A[1]
B[2]
C[3]
end
subgraph piconet2
D[4]
E[5]
F[6]
end
A <--> B & C
D <--> E & F
A <--> D

In this configuration a node for each net is selected as the gateway that can forward traffic to the other net. This is a possibility allowed by the protocol but it’s not implemented for performance reasons

SERVICE DISCOVERY PROTOCOL

Bluetooth has a discovery protocol in order to identify nodes in the range, this allows also to discover what service the device can offer

BLUETOOTH LOW ENERGY BLE

Improvements in energy consumption with the same performances different set of channels (40 2-MHz channels). Within a channel, data is transmitted using Gaussian frequency shift modulation. Bitrate is 1 Mbit/s, and the maximum transmit power is 10 mW

The main difference is in the protocol of discovery and advertisement on the availability for piconets which is based on discovery packet broadcasting

All BLE devices implements the generic attribute (GATT) profile

BLUETOOTH VS WIFI

WIFI	BLUETOOTH
multi node communication	point to point communication only
no discovery needed	need of discovery phase in order to communicate with a node
communication can be done in broadcast mode	no efficient broadcast support

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