Bluetoothtm Is a Low Cost Term Paper

Pages: 28 (7972 words)  ·  Bibliography Sources: 1+  ·  Level: College Senior  ·  Topic: Education - Computers

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[. . .] The Figure-1 shows the core Bluetooth™ specification:

Figure-1: The Bluetooth™ Protocol Stack

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The Bluetooth™ specification encompasses more than just the core specification. There are profiles, which give details of how applications should use the Bluetooth™ protocol stack, and a brand book explains how the Bluetooth™ brand should be used. The Bluetooth™ profiles guidelines on how application should use the Bluetooth™ protocol stack. The TCS (Telephony Control Protocol Specification) provides telephony services. The SDP (Service Discovery Protocol) lets Bluetooth™ devices discover what services other Bluetooth™ support. WAP and OBEX provide interfaces to the higher parts of the communication protocols. RFCOMM provides an RS232 like serial interface. Logic Link Control and Adaption multiplexes data from higher layers, and converts between different packet sizes. The Host Controller Interface handles communications between a separate host and a Bluettoth module. The Link Manager controls and configures links to other devices. The Baseband and Link controls the physical links via the radio, assembling packets and controlling frequency hopping. The Radio modulates and demodulates data transmission and reception on air.

The Radio

At the outset, note that the antenna is crucial since, it transmits and receives the radio waves, which Bluetooth™ wireless technology uses to communicate and hence, the crucial part in any Bluetooth™ implementation. The choice and positioning of antenna and surrounding environment needs close considerations. The Bluetooth™ devices operate in an unlicensed band situated at 2.4GHz. This band is reserved for general use by Industrial, Scientific, and Medical applications (ISM). The emission and interference specifications defined by ETSI ETS 300-328 in Europe or the FCC CFR47 Part 15 in USA.

Frequency Hopping

The mechanism of frequency hopping has been used to great effect as a means of secure and robust communication. Both these attributes are important for Bluetooth™ . It is always possible for radio channel to become temporarily blocked by an interference source and this is quite likely in busy ISM. Although Bluetooth™ provides a re-transmission scheme for lost data packets; it is altogether more efficient and robust to transmit the data on new channel, which is unlikely to also be blocked. The algorithm employed to calculate the hop sequence ensures maximum distance between adjacent hop channels in the sequence. Several active Bluetooth™ piconet may be within range of each other; with each piconet hopping independently with a pseudo random sequence based on each piconets identity / access code, collusions will be minimized. This is important as all Bluetooth™ devices only have 79 channels in which to operate. In an office or public environment, the number of active devices can vary quickly reach this limit, and a low correlation between non-communicating pair is essential.

The hop channel selection function is a straight forward mapping algorithm, which follows a different sequence depending on link control state (i.e., Enquiry, Paging, etc.,). It selects a particular phase in that sequence depending on parts of the supplied BD address (LAP and UAP). It then indexes through the sequence using the supplied Bluetooth™ CLK value. Due to the restriction on channel allocation, the algorithm may be truncated to work with only 23 channels in some countries rather than usual 79. The stream channel numbers generated is passed to the RF subsystem, where they are programmed into the channel synthesizer. For each of the 79 channels and 23 hop modes of operation, there are 4 sequences defined.

The Radio parameters

The Bluetooth™ specification gives minimum performance parameters for the RF system. However, care must be taken, as several of the parameters are only acceptable on paper and in particular do not address the real-world scenarios in which Bluetooth™ is to be employed. It is for this reason that many commercial Bluetooth™ RF devices significantly exceed the specified performance.

Bluetooth™ is specified to operate with maximum Bit Error Rate (BER) OF 0.1%. This gives a figure for receiver sensitivity of -- 70 dBm as described in the Bluetooth™ specification. In practice, receivers often exceed this by 10 or more dBm. Bluetooth™ used is simply GSFK with one symbol per bit, providing a gross bit rate of 1Mb/s from channel bandwidth of 1 MHz. By specifying tight constraints on symbol timing and drift rate, the task of recovering the received data stream is also simplified.

The specification does not give figures for synthesizer settling time. However, due to the high bandwidth processing operations in the lower layers of the protocol stack, synthesizer settling time is key performance parameter of any system. The lower layer's protocol processor must first decide what state to put the baseband link controller into and program various associated data into baseband. The baseband is then able to calculate the appropriate frequency hop channel number and program the synthesiser. This must of course occur the expected start of the data burst to allow the synthesizer to settle. This imposes a practical limit on the synthesizer settle time of around 180 ?s, and indeed many radios have settle times much lower, between 130?s to 170 ?s. The lower the settling time the fewer are the performance requirements placed on protocol processor.

However, there are some key design issues to be addressed in making such a low cost system reliable and robust, and some close attention must be given to system design. To facilitate the re-use and interoperability of radio parts with different baseband devices, an initiative named BlueRFTM has been launched to standardize the Bluetooth™ radio interface.

The Baseband

The definition of Baseband and Link Controller needs clarification before we move on to discussion on Basebond, proper. The Link Controller (LC) is responsible for carrying link level operations over several data packet durations in response to the Link Manager (LM) commands. Radio and Baseband represent the OSI Physical layer. The radio interfaces between on-air channel medium and the digital baseband, which formats data supplied by the LC for robust and reliable transmission over the channel and retrieves data from the channel for passing up the stack. The baseband is responsible for channel coding and decoding and low level timing control and management of the link within the domain of single data packet transfer.

The devices exists in two basic modes of operation either Slave or Master and communicate between each other in miniature networks known as piconets, which consists of a number of Slaves controlled by Master. The data links, which exist between devices, are classified into SCO for time bounded data such as audio and ACL for packet-based data. A number of different packet types exist and these offer a trade-off between reliability and data bandwidth. Bluetooth™ ensures that devices maintain time synchronization by repeatedly resynchronizing to the Master's transmissions. Since the frequency-hopping algorithm is based on the device clock, this also ensures that frequency hopping is in step.

The Service Discovery Protocol

A Bluetooth™ piconet is quite different from traditional LAN, rather than connecting to a network you connect to a device. The SDP provides means to browse through range of other services Bluetooth™ devices in the area can offer without using cable and wire. SDP relies on L2CAP links being established between SDP client and server. L2CAP just provides information for link details and SDP finds and connects to server.

SDP severs maintain a database record. Each service record provides information that a client needs to access a service. This information may include URL's for executable, documentation, and icons associated with the service. So a client may have to follow these URL's and retrieve information from elsewhere to be able to use the service. To use SDP, an L2CAP channel must be established between the SDP client and server. This channel has protocol service multiplexor reserved for SDP, so that any device can easily connect to the SDP service on another device. After the SDP information has been retrieved from the server, the client must establish a separate connection to use the service.

Services have Universally Unique Identifiers (UUID), which describe them. The services defined by the Bluetooth™ profiles have UUID's assigned by standard, but service providers can define their own services and assign own UUID's to those services. The UUID's are allocated by method that guarantees they will not be duplicated.

A UUID can be sent in a message asking a server if it supports the service identified by the UUID, Alternatively, instead of asking for a specific service, SDP can provide a mechanism for organizing services in trees, along with messages for browsing through the trees to look for a services.

SDP does not define the applications needed to drive the services discovery process nor does it define an interface to application. This is left to implementers. If required, it may be used alongside other service discovery methods, which provide application-programming interface. such as salutation or SLP (Service Location Protocol).

Wireless Access Protocol

WAP is defined and promoted by the WAP forum, which organizations may join for an annual fee. WAP provides a protocol stack similar to the… [END OF PREVIEW]

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