Explain functioning of Bluetooth baseband layer.

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written 8.4 years ago by

teamques10 ★ 68k

Bluetooth is a universal radio interface operating in the ISM band.
It provides ad-hoc wireless connectivity between portable electronic devices located at a short range from each other.
The Bluetooth protocol stack contains various layers like radio layer, baseband layer, L2CAP layer etc. Let’s discuss the baseband layer in depth:
The following figure describes the components of a Bluetooth packet at baseband layer.
The baseband is the digital engine of a Bluetooth system. It is responsible for
- constructing and decoding packets,
- encoding and managing error correction,
- encrypting and decrypting for secure communications,
- calculating radio transmission frequency patterns,
- maintaining synchronization,
- controlling the radio,
- All of the other low level details necessary to realize Bluetooth communications.

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Access code:

This field is needed for timing synchronization and piconet identification
The access code is unique to a frequency hopping (FH) channel and also to a piconet.
It may also contain special codes during paging activity or during inquiry.
A station listens tp the access code of a packet it has just received and checks if the packet has been sent by a station belonging to the same piconet.
If not the, the rest of the packet is ignored.
An access code consist of
- A 4-bit preamble.
- 64- bit synchronization field.
- 4-bit trailer (optional-used if a packet header is attached)

Packet header:

The header is a packet field which follows the access code field. Some of the fields it contains are :

AM address	3-bit	Also called as Active Member address, it represents the active address of the slaves which are assigned temporarily in a piconet. It’s a 3-bit value
Type	4-bit	It determines the type of packet. It is based on the type of connection (seen in Payload section) the applied coding rate, symmetry/asymmetry of connection.
Flow	1-bit	Its meant to implement a flow control mechanism. If flow=0 ; Asynchronous data → Stop flow of data If flow=1 ; Synchronous data → Resume the flow
ARQN SEQN	1-bit 1-bit	ARQN stands for acknowledgement number and SEQN stands for sequence number. They are used for packet acknowledgement purposes.
HEC	8-bit	The Header Error Check (HEC) is used to protect the packet header

Payload:

Upto 343 bytes of payload data can be transferred via a single packet.
A basic requirement for Bluetooth was to provide both voice as well as data transmission facility.
Depending on the requirement (i.e. voice or data ) we classify the links into two which are:
Synchronous Connection Oriented (SCO):
- Used for supporting symmetrical, point-to-point, and circuit-switched connection which are meant for voice transmissions.
- They are realized by reserving two slots at fixed slots intervals for forward and reverse transmission.
Asynchronous Connection-less (ACL):
- These links are used for bursty data transmissions
- It supports packet switched, symmetrical and asymmetrical conditions.

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