This page contains the answers to the questionnaire in chapter 6 (Bluetooth)
All answers have been held as short as possible and require an
understanding and study of the corresponding chapter of the book.
Chapter 6: Bluetooth
Bluetooth transfer speeds depend on how many users exchange data in a Piconet, how much data is exchanged by the individual users at a certain time and what kind of multislot packets are used. In the ideal scenario with only two devices in which only one device has a lot of data to send, a peak data rate of 723 kbit/s can be achieved for one of the two devices.
FHSS (Frequency Hopping Spread Spectrum) sends each packet on a different channel (frequency). This avoids using the same channel for a prolonged amount of time which might already be in use by another network such as a Wireless LAN. FHSS also simplifies device configuration as no channel number has to be selected by the user in the Bluetooth settings. Bluetooth 1.2 introduces adaptive frequency hopping which avoids channels with high error rates caused by parallel transmissions from other networks. This reduces transmission errors and the influence on other networks in the same area while at the same time increasing the overall transmission speed.
The Inquiry procedure is invoked to search for unknown Bluetooth devices in the area. If a Bluetooth device is visible to other devices it responds to an inquiry packet it detects by sending its device ID. A paging on the other hand is used to directly establish contact with a Bluetooth device which is already known. If a Bluetooth device only wants to be accessible for devices with which it has previously communicated with, it only responds to paging messages and never to inquiries.
Bluetooth offers a number of power saving mechanisms and states: Connection hold: A device deactivates its transceiver for a certain time. Connection sniff: A device deactivates its transceiver for a certain time but checks at predefined times if the master device wants to resume communication. If not, the device automatically returns to the power save state. Connection park: The devices releases its device address and uses a very long timeout value before checking again if the master device would like to reestablish contact.
The link manager has the following tasks: Establishment of an ACL, SCO or eSCO connection, configuration of the connection, activation of the enhanced data rate mode, execution of a master-slave roll change procedure, pairing, authentication and ciphering management, adaptive frequency hopping management, and activation of different power save modes when appropriate.
The L2CAP layer’s protocol service multiplexer is used during connection establishment to select to which of several higher protocol layers to connect to. In addition, an individual connection ID is used on the L2CAP layer for each connection to identify packets. This allows two devices to establish several simultaneous connections between each other for different higher layer applications.
The service discovery database contains information about all services offered by a Bluetooth device. Other devices can query this database during connection establishment to detect which services are offered and how certain parameters have to be set in order to access them.
Each Bluetooth profile using the RFCOMM layer has to register with the Service Discovery database. If a remote device wants to use the service offered by the profile it has to query the data base in order to retrieve the RFCOMM channel number which has been assigned to this profile. As the number is dynamically allocated the database has to be queried for every new connection.
Authentication: Two Bluetooth devices are able to authenticate each other if they have previously been paired with each other.
Authorization: This is a security mechanism on the application level and allows to restrict access to applications to certain remote devices. This way it can be ensured that only some of the previously authenticated devices can access certain services. It might be desirable for example that only the notebook of a user can use the dial up connection profile of a phone. Other devices are barred from this profile but are allowed to transfer files from and to the mobile phone.
Bluetooth is a very versatile communication technology that can be used for a wide variety of different services. This ranges from services like electronic business cards to the establishment of an Internet connection. The Bluetooth standard defines a number of profiles to ensure interoperability on the application level. A profile specifies how a service is supposed to work and in which way remote devices can communicate with it.
The Dial-Up Network (DUN) profile uses the “Dial Up network” of the notebook on the client side. The server side, implemented for example on a mobile phone, simulates a modem which can be controlled via AT commands. The profile also specifies the ‘at-commands’ which have to be supported. This way, a circuit switched data connection can be established. The DUN profile can also be used to establish an Internet connection via GPRS and UMTS. This requires a number of enhancements which are not part of the DUN profile, but are covered by a number of 3GPP standards. On the mobile phone, a PPP server needs to be implemented that terminates the connection with the PPP client running on the notebook. In order to instruct the mobile phone’s modem emulation not to establish a circuit switched connection, ‘*99***1#’ has to be sent during the connection phase instead of a phone number. In addition, the mobile has to be instructed which Access Point Name to use for the GPRS or UMTS connection establishment. This can be done by using the “+cgdcont=1,…” ‘at-command’ in the advanced settings of the dial up network configuration dialog.
The object exchange (OBEX) profile has been designed for a fast and simple transmission of files and objects between two Bluetooth devices. The OBEX profile is the basis for the file transfer profile, the object push profile and the synchronization profile.
When using the hands-free profile, the hands-free set is only seen as a microphone and loudspeaker extension of the mobile phone. The connection to the network continues to be established by the mobile phone. The SIM access profile does just the opposite. With this profile, the mobile station is only used as a SIM card reader. All other functionalities including the GSM/UMTS transceiver are deactivated. The hands free set then uses the Bluetooth connection to access the SIM card and can perform all transactions between itself and the SIM card just as if the SIM card was directly inserted into the hands free set. Such hands-free sets are more expensive than those just using the hands-free profile, as they have to contain a complete mobile phone unit including the GSM/UMTS module. This has the advantage, however, that an external antenna can be used. Furthermore, the mobile phone can be configured for the use of both the SIM access profile and the headset profile. While the mobile phone is used in the car, the hands-free set takes over. Once the user leaves the car and takes the mobile phone with him, incoming calls can automatically be redirected to the Bluetooth headset once again. This can not be done as easily with a hands free set in the car supporting the hands-free profile as the phone is unable to decide for incoming calls to which device to establish contact.
In the PDA market it can be observed that more and more devices support both Bluetooth and Wireless LAN. Support of Wireless LAN is especially beneficial for use of public hotspots and mobile web browsing at home. The built in Bluetooth functionality on the other hand offers connectivity to other devices such as mobile phones which can thus be used as a gateway to the Internet in areas without Wireless LAN hotspots. Furthermore, Bluetooth can also be used to quickly and easily exchange data such as files, contacts and calendar entries.