Part II: Bluetooth's Technical Details

Versatility of user applications, supported by a variety of communication protocols make Bluetooth an ideal wireless solution for such appliances as cellular phones and personal digital assistants. Low-power in most cases the technology provides a portable, secure, and standard method to transfer information between devices. Comparable to other wireless technologies, Bluetooth permits the formation of small networks. The Bluetooth standards body has carefully constructed an open architecture of Bluetooth-specific protocols and links to generic protocols, such as IP. These factors have led market analysts to predict a $15 billion market by 2005.

	Interview with Red-M’s Graham Carter Jeffery Michalski caught up with Red-M’s product line manager, Graham Carter, the discuss the company’s recent Bluetooth product offerings. Click here to listen to the interview or read the transcript.

There are several defined Bluetooth profiles—vertical slices through the Bluetooth protocol stack—that form the basis of user applications, defining how they are to be implemented. Each profile defines the particular messages and procedures from the Bluetooth specifications and the air interface for specified services.

Defining the parameters of each profile reduces the risk of interoperability problems between products from different manufacturers. The main profiles are:

Generic Access Profile (GAP)

GAP defines how two unconnected Bluetooth units discover and establish a connection between one other. It defines operations that are generic and can be used by profiles referring to GAP and by devices implementing multiple profiles. GAP ensures that any two Bluetooth units, regardless of manufacturer and application, can exchange information via Bluetooth to discover what type of applications that they support. Bluetooth units not conforming to any other Bluetooth profile must conform to GAP to ensure basic interoperability and co-existence.

Service Discovery Application Profile (SDAP)

SDAP defines the investigation of services available to a Bluetooth unit. The profile handles the search for known and specific services and a general service search. It involves the Service Discovery User Application—obligatory in a Bluetooth unit for locating services. The SDUA interfaces with the Service Discovery Protocol that sends and receives service inquiries to and from other Bluetooth units. SDAP describes an application that interfaces with a specific Bluetooth protocol to take full advantage of it for the benefit of the end-user. The SDAP is dependent on the GAP, as it re-uses parts of GAP.

Serial Port Profile

The Serial Port Profile defines how to set-up virtual serial ports on two devices and connect them with Bluetooth. Through this profile, Bluetooth units have an emulation of a serial cable using RS-232 control signaling—RS-232 is a common interface standard for data communications equipment. The profile ensures that data rates up to 128-kbit/s can be used. The Serial Port Profile is also dependent on the GAP.

Generic Object Exchange Profile (GOEP)

GOEP defines the set of protocols and procedures to be used by applications handling object exchanges. Bluetooth usage models (many of the applications described above) are based on this profile, such as file transfer and synchronization. Applications using the GOEP assume that links and channels are established, as defined by the GAP. This profile describes the procedure for pushing data from one Bluetooth unit to another. The profile also describes how to pull data between units. The GOEP is dependent on the Serial Port Profile. Typical Bluetooth units using this profile will include notebook PCs, PDAs, mobile phones, and smart phones.

The SIG chose a frequency band ranging between 2.400- and 2.500-GHz for the Bluetooth air interface, ensuring worldwide operation, support for data and speech, and the chance to design small, low-power units. Bluetooth devices must be able to select a segment in the ISM band within which they can perform. The beauty of the ISM band is that it is open to any radio system, from cordless telephones to garage door openers and microwave ovens.

The total ISM band provides 83.5-MHz of spectrum, offering 79 Bluetooth channels, each providing around 500-kbit/s of data bandwidth. A Bluetooth device therefore has 500-kbit/s maximum interconnection bandwidth at any one time. Therefore, within an area of 300-m², up to 24 Bluetooth channels, not 79, could operate simultaneously—that is 24 Bluetooth applications through 24 different devices. Otherwise frequency hopping would be impractical and impossible to manage. This adds up to 12-Mbit/s of packet data, apart from any dedicated speech channels, each compressed to 32-kbit/s ADPCM.

The result means that Bluetooth density can be high, but service remains extremely flexible and efficient.

Table 1:  Worldwide Bluetooth bands and channels

The two different link types defined in Bluetooth are the Synchronous Connection Oriented (SCO) link for audio and the Asynchronous Connectionless Link (ACL) for data. The master unit controls all traffic in the piconet, allocating capacity for SCO links and handling a polling scheme for ACL links.

Before a unit has joined a piconet, it is in standby mode. It wakes up periodically and listens for messages every 1.28 seconds. The Bluetooth specifications determine a specific wake-up sequence and connection set-up procedure.

Slave units may only send information in the slave-to-master slot after being addressed in the preceding master-to-slave slot. If the master does not have any information to send in the master-to-slave slot, a packet with only an access code and header is sent. So every slave unit is addressed in a specific order and polling scheme and may only send upon being addressed. In this way, packet collisions between sending slave units are eliminated.

The specifications also include power-saving modes if there is no data transmission in a piconet. A slave can demand to be put on Hold mode or is put in Hold mode by the master. Data transfer starts immediately when the unit moves out of Hold mode. A temperature sensor, for example, would make good use of Hold mode. Sniff mode enables a slave to listen to the piconet at a reduced rate. In Park mode, a unit remains synchronized in the piconet but does not participate in the traffic.

If a mobile user wants to connect a number of Bluetooth units to a mobile phone, the best way to get high data transmission capacity is to form as many piconets as possible in one scatternet. Every connection will use a piconet's maximum capacity (721-kbit/s). The laws of probability imply that the number of collisions resulting in retransmission is so low that up to eight piconets are possible within one scatternet.

Bluetooth devices need built-in security to prevent eavesdropping and falsifying the message originator, so authentication and encryption have been added. Combined with frequency-hopping, these two techniques, plus the limited transmission range for a Bluetooth unit, ensure the technology is highly secure in normal mode, although additional security procedures can be invoked.

The ACL and the SCO links can be multiplexed to use the same RF link. Audio transmissions can occur between one or more Bluetooth units. Audio data does not go through the L2CAP layer but directly between two Bluetooth units after opening a Bluetooth link and a straightforward set-up.

Host Controller Interface (HCI)

HCI provides a uniform interface for accessing Bluetooth hardware capabilities. It contains a command interface to the baseband controller and link manager and access to hardware status. The interface also contains control and event registers.

Link Manager Protocol (LMP)

LMP is responsible for link set-up between Bluetooth units. It handles the control and negotiation of packet sizes used when transmitting data. The protocol also handles management of power modes, power consumption, and the state of a Bluetooth unit in a piconet. It also handles generation, exchange, and control of link and encryption keys for authentication and encryption.

Logical Link Control and Adaptation Protocol (L2CAP)

L2CAP is situated over the baseband layer and beside the LMP in the Bluetooth protocol stack. It provides connection-oriented and connectionless data services to upper layers. Its four main tasks are:

• Multiplexing—supporting protocol multiplexing since a number of protocols, such as SDP, can operate over L2CAP
• Segmentation and reassembly—data packets exceeding the Maximum Transmission Unit (MTU) must be segmented before being transmitted. This and the reverse functionality, reassemble, is performed by L2CAP
• Quality of Service—the establishment of an L2CAP connection allows the exchange of information regarding current QoS for the connection between the two Bluetooth units
• Groups—it supports a group abstraction, enabling implementations for mapping groups on to a piconet.

An L2CAP implementation must be uncomplicated with low overhead since it must be compatible with the limited computational resources in a small Bluetooth unit.

Service Discovery Protocol (SDP)

SDP defines how a Bluetooth client's application acts to discover available Bluetooth servers' services and their characteristics. It defines how a client can search for a service based on specific attributes without the client knowing anything of the available services. The protocol provides the means for the discovery of new services becoming available when the client enters an area where a Bluetooth server is operating. It also provides functionality for detecting when a service is no longer available.

RFCOMM

RFCOMM, or cable replacement protocol, is a serial port emulation protocol that covers applications that make use of the serial ports of the unit. It emulates RS-232 control and data signals over the Bluetooth baseband and provides transport capabilities for upper level services, such as OBEX, that use a serial line as the transport mechanism.

Telephony Control Protocol

Binary, TCS Binary, or TCS BIN, is a bit-oriented protocol that defines the call control signaling for establishing speech and data calls between Bluetooth units. It defines the signaling for establishing and releasing calls between Bluetooth units. It governs establishing a voice or data call in a point-to-point and point-to-multipoint configuration.

AT Commands (ATtention Commands) are supported for transmitting control signals for telephony control. They use the serial port emulation, RFCOMM, for transmission.

Some protocols are defined for adoption by the Bluetooth protocol stack; some adaptations are incomplete to date:

PPP

The IETF Point-to-Point Protocol in Bluetooth technology is designed to run over RFCOMM to accomplish point-to-point connections. PPP is a packet-oriented protocol and must, therefore, use its serial mechanisms to convert the packet data stream into a serial data stream.

TCP/UDP/IP

These standards, as defined to operate in Bluetooth units, allow them to communicate with other units connected, for instance, to the Internet. The Bluetooth unit can act as a bridge to the Internet. The TCP/IP/PPP protocol configuration is used for all Internet Bridge scenarios in Bluetooth 1.0 and for OBEX in future versions. The configuration is also available as transport for WAP.

OBEX

IrOBEX, shortly OBEX, is an optional application layer protocol designed to enable units supporting infrared communication to exchange a wide variety of data and commands in a resource-sensitive standardized fashion. The protocol uses a client-server model and is independent of the transport mechanism and transport API. It defines a folder-listing object, which is used to browse the contents of folders on remote device. RFCOMM is used as the main transport layer for OBEX.

Content Formats

The formats for transmitting vCard and vCalendar information are also defined in the Bluetooth specification. They define the format in which electronic business cards and personal calendar entries and scheduling information are transported. vCard and vCalendar are transferred by OBEX.

Wireless Application Protocol (WAP)

WAP is a wireless protocol specification that works across a variety of wide-area wireless network technologies bringing the Internet to mobile devices. Bluetooth can be used like other wireless network with regard to WAP, providing a bearer for transporting data between the WAP client and its adjacent WAP server. Bluetooth's ad hoc networking capability gives a WAP client unique possibilities regarding mobility compared with other WAP bearers.

WAP communications normally involve a client device that communicates with a Server/Proxy device using the WAP protocols. Bluetooth is expected to provide a bearer service as specified by the WAP architecture.

The WAP technology supports servers that push over Bluetooth. It opens new possibilities for distributing information to handheld devices on a location basis. For example, shops can push special price offers to a WAP client when it comes within Bluetooth range.

Figure 1:  A block diagram that demonstrates the physical data flow through the Bluetooth Protocol Stack and the different API categories.

Bluetooth Thin Client PAD

A Personal Access Device with GUI and thin client software that has little inherent functionality but takes all the intelligence and data it needs from other sources via Bluetooth. It can control items such as a VCR or TV directly; out of range it can still control them indirectly via other routes (set up automatically), such as a 3G handset or Bluetooth-enabled PC, which it will also use to communicate with the outside world.

Wireless Network Peripherals

Bluetooth will untether familiar desktop devices such as printers, scanners, and digital cameras, which remain accessible so long as they are within the PC's range. Cameras and scanners in particular benefit as they currently need large amounts of memory to hold images until invariably the user transfers them elsewhere. Bluetooth enables these devices to pass images quickly for long-term storage.

Wireless Headsets

A wireless, hands free connection to the mobile handset, which could be left untouched for incoming calls. This ultimate headset model enables the headset to act as audio input and outside interface for the remote unit (usually a phone or PC).

File Transfer

GOEP-based browsing of folders, directories, and even streaming media via Bluetooth to a remote user device, including push technology from content providers.

Synchronization

Automatic synchronization of data between, for example, a desktop PC, laptop, mobile phone, and personal organizer. Business card, calendar, and work information can be transferred and processed by cellular phones, computers, and PDAs using a common format and the Bluetooth protocol.

Three-In-One Phone

A cordless telephone handset may be connected to different service providers:

• Bluetooth connected to the PSTN at home on a fixed line charge; calls are made via a voice base station, both to other terminals and the outside world.
• Bluetooth connecting directly to other phones, acting as an intercom/walkie-talkie or handset extension—no charge is involved.
• As a cellular phone connected to the cellular infrastructure.
  
  

LAN/WAN Access

Data terminals use a LAN access point as a wireless connection to a LAN. When connected, the data terminals operate as if connected to a LAN via a dial-up link.

Bluetooth "White Goods"

This is the area in which Bluetooth proponents usually raise an eyebrow or two. Devices that have never communicated with humans or one another can provide information that makes life simpler or safer. A refrigerator keeps track of its contents, alerting the consumer when stocks are low. Likewise, a dustbin can read data as products are used up. A washing machine sends a message to say the cycle is complete. In each case, the communications protocol will be Bluetooth, and the receiving device will be a PAD, mobile terminal of some kind, or even a PC.

Internet Delivery

Although Bluetooth will not affect the Internet directly, it will be possible to access the Internet more easily via Bluetooth through devices such as the PAD and 3G personal terminals. Within an operator's access network, data could be cached locally on an access server to ensure a smooth flow of information to and from the user terminal.

	Bluetooth will be Huge Industry watchers are unanimous in forecasting the steep rise of mobile devices and data applications running over them for which Bluetooth will be a prime enabler. Click here to read what the industry watchers forecast for Bluetooth.

Bluetooth is not the only available short-range wireless communications technology. While some of these technologies directly compete with Bluetooth, others will complement Bluetooth.

802.11a (HyperLAN)

HyperLAN provides higher transmission capacity, greater range (up to 100-m) and supports more simultaneous users than Bluetooth, but makes fewer frequency hops that are more prone to interference. The units are physically larger, more expensive, and consume too much power to be viable for a handheld device, so it is unlikely to be incorporated into a handheld device.

802.11b

The U.S. specification for short-range digital radio, also in the ISM band. Bandwidth is up to 2-Mbit/s, a far lower frequency-hopping rate than Bluetooth, so signals are more prone to interference. Again, its far higher power consumption makes it unlikely to be incorporated into a handheld device.

DECT

While DECT is an established and proven technology, it is primarily designed as a voice service, not packet. It requires relatively expensive hardware, has a range up to 300-m and potential bandwidth of 384-kbit/s with voice carried as TDM data at 32-kbit/s. The chip set is not optimized for small devices.

GSM

A proven, global standard but not optimized for short-range use. Data is limited to 9.6-kbit/s. Bluetooth is designed to complement digital cellular networks providing local connectivity and application integration.

GPRS/UMTS

2.5G and 3G will offer up to 2-Mbit/s, but the same arguments apply as for GSM, with Bluetooth a natural complement to advanced 3G services.

Home RF

Developed from DECT and, like Bluetooth, operates at 2.4-GHz. It handles up to 127 units per network, while Bluetooth handles 8, at only 50 frequency hops/second, compared to Bluetooth's 1600.

Infrared (irDA)

4-Mbit/s maximum; inexpensive, low range (about 1-m), and line of sight only.

UWB (Ultra-Wideband Radio)

A new technology similar to radar; unproven and non-standard, but likely to have greater capacity and power consumption than Bluetooth—payloads possibly up to 1.25-Mbit/s with a 70-m range at 0.5-mW power consumption.

The kind of work being carried out in Oulu in association with Nokia will soon dovetail with some of the newer wireless products now starting to appear in the shops. Philips Electronics, for example, is now rolling out the first of its smart clothing products across Europe.

A jacket that integrates the functions of a personal stereo, a mobile phone, and an MP3 player has been created using a new type of textile fabric that carries electronic data. When the wearer receives a phone call, for example, the jacket interrupts any music the wearer may be listening to in order to receive the call. The $800 jacket is seen as the forerunner of more advanced fourth generation clothing that will literally make electronic communications part of the fabric of everyday life.

Nanish Bapna, analyst with Nomura, says "New 'smart' garments are very far from being a gimmick. They are landmark products in the way that the personal stereo or first mobile phones were. This is the start of the next stage of consumer electronics."