 peripherals, and Bluetooth, a wireless connection between a PC or cellular phone and its peripherals. Another advantage of polling is that no contention situations arise and each slave is guaranteed to get the transmission capacity that the master allocates to it. A disadvantage is the capacity that is wasted by the process of polling slaves that have nothing to transmit. 6.3 Data Communication Protocols The computers that communicate have to understand each other. They have to speak the same "language." This common language is defined as a data communication protocol. A detailed protocol specification that enables two different systems to communicate includes many communications rules such as how the letter "A" is presented as a binary word and what the voltage is of bit "1." As we will see, many specifications are needed to enable data communi- cations between systems. An ISO standard Open Systems Interconnection (OSI) gives guidelines about how this complicated set of communications rules is divided into smaller groups of rules and functions that are called lay- ers. This helps us concentrate on one group of functions (= protocol of a cer- tain layer) at a time and we do not have to think about the functions for which other layers are responsible. For example, if we are specifying the error detection code that belongs to the data link layer of OSI, we need not worry about the power levels of optical transmission or the shape of electrical pulses in the cable. These things are the problems of the lowest layer, called the physical layer in OSI. In the next section we review the OSI reference model that was stan- dardized by the ISO and try to clarify the importance of the principle of the layered structure in data communications. 6.3.1 Protocol Hierarchies To reduce the design complexity of computer communications hardware and software, the needed functionality is organized as a series of layers, each built on its predecessor (see Figure 6.8). Many proprietary protocols are in wide use in addition to the availa