Electronic Design

A Low-Power, Frequency-Hopping System

Bluetooth implementation requires both hardware and software components. On the hardware side, a typical system includes the RF, baseband, and host controller interface (HCI). An RF or Bluetooth radio operates in the license-free ISM band of 2.4- to 2.4835-GHz, frequency hopping at 1600 hops/s within 79 1-MHz channels. The nominal link range, which is between 10 cm and 10 m, can be extended to more than 100 m by increasing the transmit power.

A wireless Bluetooth personal area network (PAN) is called a Piconet. Several Piconets can be established and linked together in an ad hoc manner to form scatternets, which allow communication and data exchange in flexible configurations. Baseband communication in a Piconet is based on a master-slave relationship.

Access is synchronized via a master identity whose Bluetooth address determines the frequency-hopping sequence. The system clock determines the phase. Each slave follows the hop sequence and adds an offset to its clock to follow the master. Every Bluetooth packet has a fixed format that starts with a unique 72-bit access code based on the master's identity. A 54-bit header follows with error correction, retransmission, and control information. Finally, a payload ranging from 0 to 2475 bits terminates the packet.

Bluetooth provides full-duplex operation using time division duplexing (TDD) to divide the channel into a number of 625-µs timeslots with a 220-µs guard time. The Bluetooth protocol is a combination of circuit and packet switching, implemented for audio and data applications, respectively. Bluetooth supports three synchronous 64-kbit/s voice channels as well as 721/57.6-kbit/s upstream/downstream and symmetrical 432.6-kbit/s data rates.

It also supports authentication and encryption. When combined with frequency hopping, this gives the technology a robust security capability. Link-level security for each pair of Bluetooth devices in a connection is based on a secret 128-bit random number link key employed for authentication and encryption. Moreover, Bluetooth uses forward error correction, including one- and two-third-rate, as well as an ARQ scheme for data.

The HCI allows uniform access to Bluetooth hardware. It contains a command interface to the baseband controller and link manager, as well as access to hardware status. Typical connections with standard interfaces include USB and PCMCIA.

On the software side, a full or partial Bluetooth SIG-approved protocol stack is supported (see the figure). The link manager (LM) is responsible for link setup between Bluetooth devices. It manages such tasks as the master/slave switch, the low-power mode, clock offset, and packet-size negotiation. The LM also handles generation, exchange, control of the link, and the en-cryption key.

The Logical Link Control and Adaptation Protocol (L2CAP) supports multiplexing of protocols like SDP, RFCOMM, and Telephony Control Specification (TCS). And, it performs the segmentation and reassembly of packets. The Service Discovery Protocol (SDP) is employed to discover the services of available Bluetooth servers and their characteristics.

RFCOMM, a serial port emulation protocol, is based on the ETSI 07.10 specification, which emulates RS-232 control and data over the Bluetooth baseband. It's used to facilitate upper-layer services that use serial lines for transport. One example is PPP.

The TCS binary is a bit-oriented protocol that defines call-control signaling for the establishment of speech and data calls between Bluetooth devices. Additionally, it defines mobility management procedures for handling groups of Bluetooth TCS devices. TCS is specified in ITU-T recommendation Q.931. Finally, the Bluetooth protocol stack supports adopted protocols, including PPP, TCP/UDP/IP, OBEX, and WAP.

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