A number of types of physical channel are defined. All Bluetooth physical channels are characterized by a set of PHY frequencies combined with temporal parameters and restricted by spatial considerations. For the basic and adapted piconet physical channels frequency hopping is used to change frequency periodically to reduce the effects of interference and for regulatory reasons.
In the BR/EDR core system, peer devices use a shared physical channel for communication. To achieve this their transceivers need to be tuned to the same PHY frequency at the same time, and they need to be within a nominal range of each other. Given that the number of RF carriers is limited and that many Bluetooth devices may be operating independently within the same spatial and temporal area there is a strong likelihood of two independent Bluetooth devices having their transceivers tuned to the same RF carrier, resulting in a physical channel collision. To mitigate the unwanted effects of this collision each transmission on a physical channel starts with an access code that is used as a correlation code by devices tuned to the physical channel. This channel access code is a property of the physical channel. The access code is present at the start of every transmitted packet. Access code is used for timing synchronization, offset compensation, paging and inquiry.
Several BR/EDR physical channels are defined. Each is optimized and used for a different purpose.
- Two of these physical channels (the basic piconet channel and adapted piconet channel) are used for communication between connected devices and are associated with a specific piconet.
- Other BR/EDR physical channels are used for discovering (the inquiry scan channel) and connecting (the page scan channel) Bluetooth devices.
- The synchronization scan physical channel is used by devices to obtain timing and frequency information about the Connectionless Slave Broadcast physical link or to recover the current piconet clock.
A Bluetooth device can only use one BR/EDR physical channel at any given time. In order to support multiple concurrent operations the device uses timedivision multiplexing between the channels.
Basic piconet channel
The basic piconet channel is used for communication between connected devices during normal operation.
- The basic piconet channel is characterized by a pseudo-random sequence hopping through the PHY channels. The hopping sequence is unique for the piconet and is determined by the Bluetooth device address of the master. The phase in the hopping sequence is determined by the Bluetooth clock of the master. All Bluetooth devices participating in the piconet are time- and hopsynchronized to the channel.
- The channel is divided into time slots where each slot corresponds to an PHY hop frequency. Consecutive hops correspond to different PHY hop frequencies. The time slots are numbered according to the Bluetooth clock of the piconet master. Packets are transmitted by Bluetooth devices participating in the piconet aligned to start at a slot boundary. Each packet starts with the channel access code, which is derived from the Bluetooth device address of the piconet master.
- On the basic piconet channel the master controls access to the channel. The master starts its transmission in even-numbered time slots only. Packets transmitted by the master are aligned with the slot start and define the piconet timing. Packets transmitted by the master may occupy up to five time slots depending on the packet type.
A basic piconet channel may be shared by any number of Bluetooth devices, limited only by the resources available on the piconet master device. Only one device is the piconet master, all others being piconet slaves. All communication is between the master and slave devices. There is no direct communication between slave devices on the piconet channel.
There is, however, a limitation on the number of logical transports that can be supported within a piconet. This means that although there is no theoretical limit to the number of Bluetooth devices that share a channel there is a limit to the number of these devices that can be actively involved in exchanging data with the master.
Adapted piconet channel
The basic piconet channel is used for communication between connected devices during normal operation. It differs from the basic piconet channel in two ways:
- the frequency on which a slave transmits is the same as the frequency used by its master in the preceding transmission. In other words the frequency is not recomputed between master and subsequent slave packets.
- the adapted piconet channel may be based on fewer than the full 79 frequencies. A number of frequencies may be excluded from the hopping pattern by being marked as “unused”. The remainder of the 79 frequencies are included. The two sequences are the same except that whenever the basic pseudo-random hopping sequence selects an unused frequency, it is replaced with an alternative chosen from the used set. The set of frequencies used may vary between different physical links on the same adapted piconet channel.
Inquiry scan channel
- In order for a device to be discovered, an inquiry scan channel is used. A discoverable device listens for inquiry requests on its inquiry scan channel and then sends a response to that request.
- In order for a device to discover other devices, it iterates (hops) through all possible inquiry scan channel frequencies in a pseudo-random fashion, sending an inquiry request on each frequency and listening for any response.
- Inquiry scan channels follow a slower hopping pattern and use an access code to distinguish between occasional occupancy of the same radio frequency by two co-located devices using different physical channels.
- The access code used on the inquiry scan channel is taken from a reserved set of inquiry access codes that are shared by all Bluetooth devices. One access code is used for general inquiries, and a number of additional access codes are reserved for limited inquiries. Each device has access to a number of different inquiry scan channels. As all of these channels share an identical hopping pattern, a device may concurrently occupy more than one inquiry scan channel if it is capable of concurrently correlating more than one access code.
- A device using one of its inquiry scan channels remains passive on that channel until it receives an inquiry message on this channel from another Bluetooth device. This is identified by the appropriate inquiry access code. The inquiry scanning device will then follow the inquiry response procedure to return a response to the inquiring device.
- In order for a device to discover other Bluetooth devices it uses the inquiry scan channel to send inquiry requests. As it has no prior knowledge of the devices to discover, it cannot know the exact characteristics of the inquiry scan channel.
- The device takes advantage of the fact that inquiry scan channels have a reduced number of hop frequencies and a slower rate of hopping. The inquiring device transmits inquiry requests on each of the inquiry scan hop frequencies and listens for an inquiry response. Transmissions are done at a faster rate, allowing the inquiring device to cover all inquiry scan frequencies in a reasonably short time period.
Inquiring and discoverable devices use a simple exchange of packets to fulfill the inquiring function. The topology formed during this transaction is a simple and transient point-to-point connection.
Page scan channel
- A connectable device (one that is prepared to accept connections) listens for a page request on its page scan channel and, once received, enters into a sequence of exchanges with this device.
- In order for a device to connect to another device, it iterates (hops) through all page scan channel frequencies in a pseudorandom fashion, sending a page request on each frequency and listening for a response.
- The page scan channel uses an access code derived from the scanning device’s Bluetooth device address to identify communications on the channel. The page scan channel uses a slower hopping rate than the hop rate of the basic and adapted piconet channels. The hop selection algorithm uses the Bluetooth device clock of the scanning device as an input.
- A device using its page scan channel remains passive until it receives a page request from another Bluetooth device. This is identified by the page scan channel access code. The two devices will then follow the page procedure to form a connection. Following a successful conclusion of the page procedure both devices switch to the basic piconet channel that is characterized by having the paging device as master.
- In order for a device to connect to another Bluetooth device it uses the page scan channel of the target device in order to send page requests. If the paging device does not know the phase of the target device’s page scan channel it therefore does not know the current hop frequency of the target device. The paging device transmits page requests on each of the page scan hop frequencies and listens for a page response. This is done at a faster hop rate, allowing the paging device to cover all page scan frequencies in a reasonably short time period.
- The paging device may have some knowledge of the target device’s Bluetooth clock (indicated during a previous inquiry transaction between the two devices, or as a result of a previous involvement in a piconet with the device), in this case it is able to predict the phase of the target device’s page scan channel. It may use this information to optimize the synchronization of the paging and page scanning process and speed up the formation of the connection.
Paging and connectable devices use a simple exchange of packets to fulfill the paging function. The topology formed during this transaction is a simple and transient point-to-point connection.
Synchronization scan channel
In order to receive packets sent on the CSB logical transport, a device must first obtain information about the timing and frequency channels of those packets. If a device misses a Coarse Clock Adjustment notification, it needs to recover the current piconet clock. The synchronization scan channel is provided for these purposes. A scanning device listens for synchronization train packets on the synchronization scan channel. Once a synchronization train packet is received, the device may stop listening for synchronization train packets because it has the timing and frequency information necessary to start receiving packets sent on the CSB logical transport or to recover the piconet clock.
The synchronization scan channel uses an access code derived from the Bluetooth device address of the synchronization train transmitter to identify synchronization train packets on the channel. Once a synchronization train packet is received, the scanning BR/EDR Controller may start receiving packets sent on the CSB logical transport, depending on the needs of the Host and any applicable profile(s).
The topology formed during this scan is transient and point-to-multipoint. There can be an unlimited number of scanning devices simultaneously receiving synchronization train packets from the same synchronization train transmitter.