hared transmission media and they Data Communications broadcast their data frames to the network, and only the destination machine, indicated by destination address in the frame, receives it. Figure 6.7 shows two physical topologies for a broadcast network, a bus and a star. Logi- cally both networks have a structure of a bus and each frame, transmitted by one station, reaches all other stations in the network. This type of broadcast channel is dynamically (i.e., on demand) allo- cated for hosts in one of two main ways. Devices using network may be autonomous or a master or central control computer may give permission to one device to transmit at a time. Computers in an Ethernet LAN are autono- mous; there is no central control computer that allocates network for users and anyone can use a free channel when needed, as we will see in Section 6.5. In this kind of decentralized channel allocation method, each computer has to decide itself whether or not to transmit. The traditional way to allocate a single channel between a mainframe computer and its terminals is for the main computer (master) to poll the ter- minals (slaves), each one in turn. The master sends regularly poll frames that contain identification of a slave and possibly data from master to slave. The slave responds to the poll frame with the data frame. The main advantage of this old principle is that operation of slaves is very straightforward and it is also used in some modern systems. Examples of Broadcast network, decentralized channel allocation autonomous computers, each has equal right to transmit on demand. Physical bus, logical bus Physical star, logical bus Broadcast network, centralized channel allocation, master polls each slave in turn. Poll for station D Response to poll frame Master Poll for station D Response to poll frame Master Figure 6.7 Dynamic broadcast channel allocation methods. Introduction to Telecommunications Network Engineering these systems include universal synchronous bus (USB), a high-speed interface between a PC and its