Bluetooth is a low-cost cable replacement technology that wirelessly connects devices using a WPAN or Wireless Personal Area Network. Bluetooth can also be described more technically as a short range frequency hopping radio link. The average connection distance is approximately 10 meters. The technology is described as frequency hopping because, it changes channels or frequencies to allow multiple devices to create connections within the same range of each other. Bluetooth maintains the ability to change frequncies without reestablishing a connection.
As soon as the Bluetooth devices experience any type of interference, the devices shift to another frequency that is clear of interference. This ensures a stable connection. Compared to similar technologies, Bluetooth sends smaller packets, or chunks of information, and is capable of changing frequency much faster. Each connection is and exclusive bond between two devices. Bluetooth chips operate in unlicensed frequencies in the 2.4 Ghz range. This range referred to at the ISM band, is very similar to the band used by other short range communications equipment such as cordless phones and a WiFi hotspot.
There are two versions of Bluetooth. Bluetooth 1.1 is more limited in terms of distance of the connection, and power consumptions to the devices used. Bluetooth 1.1 has a range of about 30 feet. Bluetooth 1.2 however, has a range of near 100 feet, and uses far less battery life to operate. Bluetooth devices all have 2 things in common, Specifications, and protocols.
Bluetooth specifications set up all the functions that are capable of working with your device. Each specification has its very own job, and allow the protocols to function. The radio layer, baseband, Link Manager Protocol (LMP), Host Controller Interface (HCI), Logical Link Control and Adaption Protocol (L2CAP), RFCOMM, and Service Discovery Protocol (SDP) are the base of all Bluetooth specifications.
The radio layer controls the specific requirements for a Bluetooth receiver. In the United States Bluetooth hops frequencies starting at 2.402 Ghz. The hops are 1 Mhz in distance, and produce a total of 79 hops before starting over. The upper frequency range stops at 2.480 Ghz. To protect the integrity of data a guard band is used on the upper and lower frequencies. Guard bands are specific frequencies that are designed to allow the nearby frequencies to “bleed” over and create interference without effecting the performance of another device. Part 15 of the FCC (Federal Communications Commission) rules and regulations for unlicensed devices state that a device may not cause harmful interference. Guard bands protect other devices from this interference, and bring the device itself into compliance with the FCC rules and regulations. The FCC is the government entity responsible for policing electronic devices, and all airborne frequencies.
The Baseband layer is implemented as a link controller that manages the physical aspects of Bluetooth like the channels and links to another device. The baseband manages these connections aside from all other actions of Bluetooth operation. The Baseband is also responsible for managing synchronous and asynchronous links, manages packets, and controls the paging and inquiry algorithms to allow interface with another device.
The Link Manager Protocol calls upon the Link Controller to manage the setup, authentication, and configuration of new links with a device. The link manager uses Protocol Data Units (PDU) to communicate with another device. The PDU is delivered based on the address contained in the 1 byte packet header.
The Host Controller Interface is basically a universal interface to easily call upon the functions of the Link Manager and the Baseband layer. The HCI is divided into three unique sections to access the Host, the Transport, and the Host Controller.
The Logical Link Control and Adaption Layer controls the data link for Bluetooth. The data link is capable of transmitting up to 64 kilobyte sized packets. These packets carry real-time voice traffic and act as a means of allowing multiplexed connections. A Multiplexed connection allows send and receive simultaneously among different specification layers within Bluetooth.
RFCOMM allows the device to support RS232 serial port emulation by invoking the L2CAP protocol. Serial ports transmit 1 Byte of data at a time, but the emulation mode in RFCOMM supports up to 60 simultaneous connections per device. The number of connections used together are determined by how the emulation is intended to be used. Different operations will require a different number of paired emulation threads to allow simultaneous send and receive within the connection.
The Service Discovery Protocol is responsible to probing each device to determine what profiles each device is capable of invoking. This information is passed to other layers in the device such as the Link Manager to establish a connection and invoke a specific type of function using the combination of the RFCOMM and the L2CAP protocols. You could say the SDP is responsible for tyeing all the other layers together.
The Bluetooth Profiles are responsible for commanding the other layers of the device in a very specific way. Set standards of communications within the Bluetooth profile allow for communication without the need to special software or interpreters, as long as the other device is capable of communicating using the specified profile. The most common profiles are HFP (Hands-Free Profile), A2DP (Stereo Audio Profile), and DUN (Dial up networking profile). Most cellular phones are now equipped with at least 2 out of the 3 profiles. In total, there are 25 standardized profiles within Bluetooth. Each of the profiles carry a unique function and application.