Bluetooth Baseband

This is the most comprehensive part of the Bluetooth protocol and one which is most important. This tutorial page touches the different aspects of baseband only very briefly. For more information on the subject in context, please click   More .. on the right side wherever the link is provided.

Baseband

Baseband is the physical layer of the Bluetooth which manages physical channels and links apart from other services like error correction, data whitening, hop selection and Bluetooth security. Baseband lies on top of Bluetooth radio in Bluetooth stack and essentially acts as a link controller and works with link manager for carrying out link level routines like link connection and power control. Baseband also manages asynchronous and synchronous links, handles packets and does paging and inquiry to access and inquire the Bluetooth devices. Baseband transceiver applies a time-division duplex (TDD) scheme. (alternate transmit and receive). Therefore apart from different hopping frequency (frequency division), the time is also slotted. In the normal connection mode, the master shall always start at even numbered slots and slave transmission shall always start at odd numbered slots (though they may continue to transmit regardless of the number of the slot).

ACL and SCO links

Baseband handles two types of links : SCO (Synchronous Connection-Oriented) and ACL (Asynchronous Connection-Less) link. The SCO link is a symmetric point-to-point link between a master and a single slave in the piconet. The master maintains the SCO link by using reserved slots at regular intervals (circuit switched type). The SCO link mainly carries voice information. The master can support up to three simultaneous SCO links while slaves can support two or three SCO links. SCO packets are never retransmitted. SCO packets are used for 64 kB/s speech transmission.

The ACL link is a point-to-multipoint link between the master and all the slaves participating on the piconet. In the slots not reserved for the SCO links, the master can establish an ACL link on a per-slot basis to any slave, including the slave already engaged in an SCO link (packet switched type). Only a single ACL link can exist. For most ACL packets, packet retransmission is applied.

Logical Channels

Bluetooth has five logical channels which can be used to transfer different types of information. LC (Control Channel) and LM (Link Manager) channels are used in the link level while UA, UI and US channels are used to carry asynchronous, isosynchronous and synchronous user information.

Bluetooth Addressing

There are basically four types of device addresses in Bluetooth:

Bluetooth packets

The data on the piconet channel is conveyed in packets. The general packet is shown below:

Access code are used for timing synchronization, offset compensation, paging and inquiry. There are three different types of Access code: Channel Access Code (CAC), Device Access Code (DAC) and Inquiry Access Code (IAC). The channel access code identifies a piconet (unique for a piconet) while DAC is used for paging and its responses. IAC is used for inquiry purposes. The header contains information for packet acknowledgement, packet numbering for out-of-order packet reordering, flow control, slave address and error check for header. The packet payload can contain either voice field, data field or both. The packet can occupy more than one slot (multi-slot packets) and can continue transmission in the next slot. The payload also carries a 16-bit CRC for error detection and correction in the payload. SCO packets do not include CRC.

There are five common types of packet, four SCO packets and seven ACL packets. There brief description is given below. For more information click   More ..

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Error correction

There are three kinds of error correction schemes: 1/3 rate FEC, 2/3 rate FEC and ARQ scheme. In 1/3 rate FEC every bit is repeated three times for redundancy, in 2/3 a generator polynomial is used to encode 10 bit code to a 15 bit code, and in ARQ scheme a packet is retransmitted till an acknowledgement is received (or timeout is exceeded). Bluetooth uses fast, unnumbered acknowledgement in which it uses positive and negative acknowledgements by setting appropriate ARQN values. If timeout is exceeded, Bluetooth flushes the packet and proceeds with the next.

Bluetooth Controller

Flow control and synchronization

Bluetooth recommends using FIFO queues in ACL and SCO links for transmission and receive. Link Manager fills these queues and link controller empties the queues automatically.

If these RX FIFO queues are full, flow control is used to avoid dropped packets and congestion. If data cannot be received, a STOP indication is transmitted inserted by the Link Controller of the receiver into the header of the return packet. When the transmitter receives the STOP indication, it freezes its FIFO queues. If receiver is ready it sends a GO packet which resumes the flow again.

We already know that Bluetooth transceiver uses a time-division duplex (TDD) scheme. This means that it alternately transmits and receives in a synchronous manner. The average timing of master packet transmission must not drift faster than 20 ppm relative to the ideal slot timing of 625 microseconds. Jitter from average timing should be less than 1 microsecond. The piconet is synchronized by the system clock of the master. The Bluetooth Device Address (BD_ADDR) of the master determines the frequency hopping sequence and the channel access code; the system clock of the master determines the phase in the hopping sequence. Master controls the traffic on the channel by a polling scheme. The master never adjusts its system clock during the existence of the piconet. The slaves adapt their native clocks with a timing offset in order to match the master clock. The Bluetooth clocks should have a resolution of 312.5 microseconds.

A 20 microsecond uncertainty window is allowed around the exact receive time in order for the access correlator for the receiver to search for the correct channel access code and get synchronized with the transmitter. When a slave returns from the hold mode, it can correlate over a bigger uncertainty window till they dont overlap slots. A parked slave periodically wakes up to listen to beacons from the master and re-synchronizes its clock offset.

Controller States

Bluetooth controller operates in two major states: Standby and Connection . There are seven substates which are used to add slaves or make connections in the piconet. These are page, page scan, inquiry, inquiry scan, master response, slave response and inquiry response .

The Standby state is the default low power state in the Bluetooth unit. Only the native clock is runnning and there is no interaction with any device whatsoever. In the Connection state, the master and slave can exchange packets. using the channel (master) access code and the master Bluetooth clock. The hopping scheme used is the channel hopping scheme.

Normally, a connection between two devices occur in the following fashion. First master uses the GIAC and DIAC to inquire about the Bluetooth devices in the range (Inquire substate). If any nearby Bluetooth device is listening for these inquiries (Inquiry scan substate), it responds to the master by sending its address and clock information (FHS packet) to the master (Inquiry response substate). After sending the information, the slave may start listening for page messges from the master (Page scan). The master after discovering the in range Bluetooth devices may page these devices (Page substate) for connection setup. The slave in page scan mode if paged by this master will respond (Slave response substate) with its device access code (DAC). The master after receiving the response from the slave, may respond by transmitting the master's real time clock, master's BD_ADDR, the BCH parity bits and the class of the device (FHS packet). After slave has received this FHS packet, both enter into Connection state.

Below we describe each of these states briefly:

Connection States

The Connection state starts with a POLL packet sent by the master to verify that slave has switched to the master's timing and channel frequency hopping. Th slave can respond with any type of packet.

A Bluetooth device in Connection state can be in any of the four following states: Active, Hold, Sniff and Park mode. One of the challenges in Bluetooth is to move between these states especially from Park to Active and vice versa. These modes are described briefly below:

Bluetooth Security

Bluetooth security is an important if we have to allow keyless doors and automatic billing in super-stores. At the link layer, security is maintained by authentication of the peers and encryption of the information. For this basic security we need a public address which is unique fro each device (BD_ADDR), two secret keys (authentication keys and encryption key) and a random number generator. First a device does the authentication by issuing a challenge and the other device has to then send a response to that challenge which is based on the challenge, it's BD_ADDR and a link key shared between them. After authentication, encryption may be used to communicate.

There are four types of link keys: combination key, unit key, temporary key and initialization key .

Move to Link Manager

After looking at the Baseband and its various important features, lets move to understanding Link Manager Protocol (LMP) which is a link layer above Baseband.