Chapter 10. Audio and Telephony Control

Support for voice or, more generically, audio is a distinguishing attribute of Bluetooth wireless communication. With support for both voice and data, the technology is well positioned to bridge the domains of computing and communications, as evidenced by the enthusiastic support for the Bluetooth technology within both industries. Several of the profiles address scenarios in which both a computing device and a telephony device are used. This chapter, our final in-depth examination of the core specification, deals with the components of the protocol stack that enable telephony and voice (audio) communication. The telephony control protocol is embodied by the TCS-BIN (or just TCS for short) layer, while audio can be carried natively over the baseband. TCS is based upon the existing ITU-T Q.931 protocol [ITU98], but even so it occupies over 60 pages in the specification. TCS is a binary encoding for packet-based telephony control and resides above the L2CAP layer of the stack. TCS-BIN is sufficient to realize the version 1.0 telephony profiles, although applications using AT commands over the RFCOMM serial port abstraction (including headset, dial-up networking and fax) might also accomplish a form of telephony control (this latter form of telephony control is not included as a separate entity in the version 1.0 specification; it is discussed further in subsequent sections here). Audio is not a layer of the protocol stack per se but rather a specific packet format that can be transmitted directly over the baseband layer. Since audio is frequently (although not exclusively) associated with telephony applications, it is discussed together with TCS in this chapter as a logical convenience. This chapter examines telephony functions, including audio, in Bluetooth wireless communication. As in preceding chapters we will not only provide highlights and interpretations of the specification but also touch upon the background information for these elements of the protocol stack, including the evolution of TCS-BIN.

Figure 10.1 depicts audio and TCS-BIN in the protocol stack; it also shows the component we call AT Command Telephony Control. This latter component is a remnant of what was once called TCS-AT and is explored further below. In general, when we refer simply to TCS we mean the TCS-BIN layer of the stack. TCS-BIN resides above L2CAP; audio communicates directly through the baseband; and AT command telephony control operates over RFCOMM. Telephony control applications can communicate directly with TCS-BIN and might also use AT command telephony control.