Bluetooth phones for local communication
Bluetooth was announced in 1994 by the Swedish communications company Ericsson as a technology which could be used to connect computers and computing devices wirelessly to other computing devices or mobile phones. The technology had great promise because it eliminated cables and provided great convenience for connecting data or computing devices such as the PDA without the hassles of wired connectivity.
Other manufacturers of computer equipment also expressed an interest in the possibilities and the Bluetooth special interest group consisting of Ericsson, Nokia, IBM, Intel and Toshiba was formed to further develop the technology. As a result of the efforts of the partners in the Bluetooth special interest group, Bluetooth has now evolved as a global technology standard and many devices with Bluetooth connectivity are now available. The number of technology partners has also increased to include firms like Microsoft, Agere and Motorola amongst others.
The 2.4 GHz industry, scientific and medical or ISM band is used by Bluetooth. The exact frequency range is 2400 - 2483.5 MHz in Europe as well as in many other countries. There are 79 channels available in this frequency band and the relation f = 2402 + k [MHz], where’s k = 0, …, 78 holds for carrier frequencies with a lower guard bands of 2 MHz and an upper guard band of 3.5 MHz. An alternative set of frequencies that are utilised in France and Spain is the 2446.5 - 2483.5 MHz band and for this frequency ranges the relation f = 2454 + k [MHz], where’s k = 0… 22 hold. For the frequencies that are available in France and Spain, there is a lower guard band of 7.5 MHz and an upper guard band of 3.5 MHz. This band does not require any licensing and it is shared by many devices ranging from garage doors, cordless phones, baby monitors to microwave ovens.
The effective range of Bluetooth communications is ideally 10m, but may be extendable to 100m. Bluetooth does not require line of site communications and its frequency hopping connectivity makes it secure, but not as secure as infrared communications. Hackers will have to be within close physical range to listen to transmissions, but it is possible for other Bluetooth devices to attack a system. Although it is designed for low power battery operated devices, the power consumption can be relatively high because of a requirement to maintain a connection. Bluetooth provides a shared bandwidth of 1 Mbps and the component costs are expected to drop to $5 when economies of production come into play. Many Bluetooth enabled devices such as mobile phones, printers, PDA’s etc are now available. However, every Bluetooth device has to have a type approval and qualification with a unique Bluetooth address being defined for identification.
Bluetooth devices can support one asynchronous voice and data channel, or three simultaneous voice channels. A 100 mW Bluetooth transmitter takes about 0.55 mA when in the quite listening mode and this consumption rises to about 35 mA when a connection with a network of devices has been established. When transmitting, the peak power is 75 mA and in the low power mode, only 60 micro Ampere is consumed. However, the power consumption is considered to be relatively high when connected to other devises.
The Bluetooth Protocol Stack and the Piconet
Just like the internet, the Bluetooth protocol may be viewed as a number of protocols that are operating in synchronism in order to enable communications between various Bluetooth devices. The four most important protocols in the Bluetooth suite of protocols consist of the baseband protocol, the link manager protocol or LMP, the logical link manager and adaptation protocol or the L2CAP and the service discovery protocol or the SDP. The LMP, L2CAP and the SDP are used when needed while the baseband is used for all communications between devices. A diagrammatic representation of various Bluetooth protocols is shown below.
The Bluetooth Protocol Stack
A group of two to eight Bluetooth devices can form a piconet when they are in range, with there being a master device for a piconet and all the other devices being slaves. It is the master which controls the communications in a piconet and the maximum number of devices in a piconet is deliberately kept at eight in order to support the relatively high capacity connectivity between all devices as well as minimizing the addressing requirements. The baseband protocol and the LMP handle the communications between the master and the slave in a piconet. Any device that establishes the connections to form a piconet is the master and once the piconet is cancelled, the other devices can become masters by establishing a new piconet. Because Bluetooth devices are constantly on the move, piconets are constantly formed and cancelled as the device moves into and exits the vicinity of other Bluetooth devices. A scatternet can be formed as a result of the coming together of two piconets when a member of one piconet is also a member of another piconet. Only one device can only be a master in one piconet and a master in one piconet can be a slave in another piconet. The pseudo-random frequency hopping of Bluetooth devices is based on the clock of the piconet master. The master device in a piconet manages the frequency hopping sequence in a piconet, dictating when hops occur, determines the frequency used in the piconet and polls the slaves which respond on polling. If there are more then a total of eight Bluetooth devices in a piconet, then devices other then the eight connected devices will be in a low power mode.
In a piconet, Bluetooth devices may be in the active mode, hold mode, sniff mode or the park mode with progressively lower power consumption associated with each mode. Active mode for two devices occurs when a master and a slave are actively exchanging data or communicating over a channel. In the sniff mode, the master and the slave arrange a sniff interval and the slave goes into the active mode at the beginning of the interval, awaiting the arrival of a packet. If the slave receives a packet in the active mode, it stays active, otherwise it goes back low-power sleep mode until the next sniff time. In the hold mode, there is an agreed hold time which is determined by the master in which there is no requirement to exchange data and the slave can either switch off its radio and conserve energy during the hold time or start a process of discovery to determine what other Bluetooth devices are available. The park mode refers to the situation in which a slave stays synchronised with the master by periodically listening to master transmissions, but it is not part of the piconet and maintains itself in the minimal energy consumption mode, permitting other devices to take an active part in the piconet.
The exchange of data between the master and the slave takes place in base band packets of one, three or five timeslots. In the hold mode, the slave actively listens to the established channel and is able to scan, page or enquire about other devices in the area. In the sniff mode, the master can start transmission to a slave within specified regularly spaced time slots as the channel is divided into 1600 time slots per second as a result of the 1600 hops per second and this makes it possible to form scatternets. The connection between a master and a slave may be a Synchronous Connection Oriented Link SCO or an Asynchronous Connection Less link ACL. In the SCO link situation, the master polls the slave at regular intervals while when in the ACL link situation, the polling is possible in more then one ways, depending on the master. A slave that has been polled in a SCO situation responds with a SCO packet. When a master has no data for a slave, it sends a POLL packet to the slave which is an empty packet. A polled slave will respond with a single packet or a NULL packet when it has nothing to send to the master. Obviously, SCO polling is possible after a piconet has been formed and the maximum data transfer rate is 64 kb/sec.
The ACL link supports packet switching and a point-to-point or a point-to-multipoint link between the master and slaves in a piconet. The ACL link is established primarily to exchange data asynchronously or isochronously. The ACL data can be interrupted when a set bits of data has arrived in a buffer or in the isochronous mode, data flows at a set rate to enable the application to handle the data flow. Data integrity in the ACL mode is ensured by using retransmission schemes. It is also possible to broadcast in the ACL mode when a packet not addressed to a particular slave is picked up by all slaves. The maximum data transmission rate using ACL connectivity is 732.2 kb/sec. ACL links can be maintained only one at a time and can be established while a SCO link is under way. ACL links are activated on a predetermined transmission slot.
Every Bluetooth device is provided with a 48 bit address that is a unique global address by an assigning authority. The Bluetooth address whole is made up of the lower address part or the LAP, the upper address part or the UAP and the non-significant address part or the NAP. An organization is assigned the UAP and the NAP by the assigning authority while the LAP is assigned by the organization which is using the Bluetooth device. In a piconet, the device address of the master is used as an input to the pseudo-random sequence generator which determines the frequency hopping on the channel. The Bluetooth packet that is sent between devices consists of an access code, a header and a payload. The access code bit length can range from 68 to 72 bits. It is 68 bits long if there is an empty payload and 72 bits long with a payload. A baseband packet header of 54 bits is used and the size of the packet payload can vary from 0 to 2745 bits. The access code part of the Bluetooth packet is used to identify all the packets that are being exchanged on the Bluetooth channel of the piconet. Channel Access Code CAC Device Access Code DAC or the Inquiry Access Code IAC may be sent on the Bluetooth channel.
The CAC is included in all packets exchanged in a piconet, while the DAC is used for paging purposes. IAC interrogations may consist of the General Inquiry Access Code GIAC or the Dedicated Inquiry Access Code DIAC are used to determine the Bluetooth devices which are in range of a device. The DAC and the IAC will not always contain a header and the size of the DAC or IAC without a header is 68 bits. The access code consists of a preamble, a sync word and a trailer as shown in the diagrams below. The preamble is 0101if the sync word is zero and 1010 for a non-zero case. The sync word is derived from the LAP of the master. The trailer is a 1010 for a zero last bit of sync word and 0101 otherwise. The packet is encoded using the 1/3 FEC code which results in a 54 bit header. The header packet prior to FEC coding is shown in the diagrams below and contains a 3 bit active member address AM_ADDR for each active slave on the piconet and this address is included in all communications between the master and the slave. The TYPE defines the type of packet which is being exchanged while the FLOW bit is used for flow control on ACL links. The ATQN bit indicates the correct receipt of a payload after CRC check and the SEQN bit is used for retransmissions. The 8 bit HEC is used to check the integrity of the packets.
Tags: bluetooth, communication, effective range, Ericsson, mobile phones, protocol
