Bluetooth Technology and Implications- Page 2/3
December 14, 1999
By
Heidi Monson
Bluetooth technology
Bluetooth definitions
Piconet: Devices connected in an ad hoc fashion, that is, not requiring predefinition and planning, as with a standard network. Two to eight devices can be networked into a piconet. It is a peer network, that is, once connected, each device has equal access to the others. However, one device is defined as master, and the others as slaves.
Scatternet: Several piconets may form a larger scatternet, with each piconet maintaining independence.
Master unit: The master in a piconet whose clock and hopping sequence synchronizes the other devices.
Slave unit: Devices in a piconet that are not the master.
MAC address: Three bit address that distinguishes each unit in a piconet.
Parked units: Piconet devices that are synchronized but don't have MAC addresses.
Sniff and hold mode: Power-saving mode of a piconet device.
Network arrangements
Bluetooth network arrangements (topology) can be either point-to-point or point-to-multipoint. Any unit in a piconet can establish a connection to another piconet to form a scatternet. See the figure to the right, which diagrams a scatternet in which piconet A, which consists of four units, is connected to piconet B, consisting of two units. Note that the master unit of A is not the link between the two piconets.
Transmission types and rates
The baseband (single channel per line) protocol combines circuit and packet switching. To assure that packets do not arrive out of order, slots (up to five) can be reserved for synchronous packets. As noted earlier, a different hop signal is used for each packet. Circuit switching can be either asynchronous or synchronous. Up to three synchronous (voice) data channels, or one synchronous and one asynchronous data channel, can be supported on one channel. Each synchronous channel can support a 64 Kb/s transfer rate, which is fully adequate for voice transmissions. An asynchronous channel can transmit as much as 721 Kb/s in one direction and 57.6 Kb/s in the opposite direction. It is also possible for an asynchronous connection to support 432.6 Kb/s in both directions if the link is symmetric.
Radio frequency and spectrum hopping
What if there's a lot of radio noise? Won't that interfere with Bluetooth connections? As a rule, the answer is no. It is designed to use fast acknowledgement and frequency hopping, which will make connections robust. It is packet-based, and will jump to a new frequency after each packet is received, which not only helps limit interference problems, but also adds to security. Data rates are one megabyte/second, including headers. Full duplex transmissions (both directions at once) are accomplished via time division multiplexing.
The Bluetooth radio chip functions at 2.4 gigahertz, which is in the unlicensed ISM (Industrial Scientific Medical) band. It separates the 2.4 gigahertz frequency band into 79 hops one megahertz apart, starting with 2.402 and ending with 2.480 (though this bandwidth is narrower in Japan, France, and Spain). This spread spectrum is used to hop from one channel to another, pseudo-randomly, which adds a strong layer of security. Up to 1600 hops per second can be made. The standard frequency range is 10 centimeters to 10 meters, and can be extended to at least 100 meters by increasing transmission power.
Connection protocol
Bluetooth connections are established via the following techniques:
Standby: Devices not connected in a piconet are in standby mode. In this mode, they listen for messages every 1.28 seconds over 32 hop frequencies (fewer in Japan, Spain, and France).
Page/Inquiry: If a device wishes to make a connection with another device, it sends out a page message, if the address is known, or an inquiry followed by a page message, if it is unknown. The master unit sends out 16 identical page messages on 16 hop frequencies to the slave unit. If there is no response, the master retransmits on the other 16 hop frequencies. The inquiry method requires an extra response from the slave unit, since the MAC address is unknown to the master unit.
Active: Data transmission occurs.
Hold: When either the master or slave wishes, a hold mode can be established, during which no data is transmitted. The purpose of this is to conserve power. Otherwise, there is a constant data exchange. A typical reason for going into hold mode is the connection of several piconets.
Sniff: The sniff mode, applicable only to slave units, is for power conservation, though not at as reduced a level as hold. During this mode, the slave does not take an active role in the piconet, but listens at a reduced level. This is usually a programmable setting.
Park: Park mode is a more reduced level of activity than the hold mode. During it, the slave is synchronized to the piconet, thus not requiring full reactivation, but is not part of the traffic. In this state, they do not have MAC addresses, but only listen enough to keep their synchronization with the master and check for broadcast messages.
Data transmission
As noted earlier, data can be transmitted both synchronously and asynchronously. The Synchronous Connection Oriented (SCO) method is used primarily for voice, and Asynchronous Connectionless (ACL) is primarily for data. Within a piconet, each master-slave pair can use a different transmission mode, and modes can be changed at any time. Time Division Duplex (TDD) is used by both SCO and ACL, and both support 16 types of packets, four of which are control packets that are the same in each type. Because of the need for smoothness in data transmission, SCO packets are generally delivered via reserved intervals, that is, the packets are sent in groups without allowing other transmissions to interrupt. SCO packets can be transmitted without polling by the sending unit. ACL links support both symmetric and assymetric transmissions.
Bandwidth is controlled by the master unit, which determines how much of the total each slave unit can use. Slaves cannot transmit data until they have been polled by the master, and the master can broadcast messages to the slave units via ACL link.
Error correction and security
Three error correction techniques have been defined: 1/3 rate forward error correction code (FEC), 2/3 rate forward error correction code FEC, and automatic repeat request (ARQ). The FEC methods are designed to reduce the number of retransmissions. However, the overhead significantly slows transmissions, so is generally not used in relatively error-free environments, with the exception of packet headers. The ARQ scheme requires that the header error and cyclic redundancy checks are okay. When they are, an acknowledge is sent. When they aren't, the data is resent.
Security is provided in three ways: through pseudo-random frequency band hops, authentication, and encryption. Frequency band hops make it difficult for anyone to eavesdrop. Authentication allows a user to control connectivity to only devices specified. Encryption uses secret key lengths of 1, 40, and 64 bits. The quality of security is excellent for most applications. However, it is not the highest level available, and for those users who require it, the suggestion is to investigate separate network transfer protocols and security software.
Control of link connections
The basic part of the Bluetooth system consists of the radio chip and controller, as shown in the figure above. The Link Manager (LM) is software that controls link setup, authentication, link configuration, and other protocols. The hardware underlying the LM is the link controller (LC). These two perform the following tasks:
Sending and receiving data.
Paging and inquiries.
Setting up connections.
Authentication.
Negotiating and setting up link types, i.e., SCO or ACL.
Events
Bluetooth network arrangements (topology) can be either point-to-point or point-to-multipoint. Any unit in a piconet can establish a connection to another piconet to form a scatternet. See the figure to the right, which diagrams a scatternet in which piconet A, which consists of four units, is connected to piconet B, consisting of two units. Note that the master unit of A is not the link between the two piconets.
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