p the response time goes down. Figure 6-3 is an example of a token passing ring topology. Token passing is a line access method, where control of the rights to access are distributed among all devices using the network. The only device that has the right to transmit is the device holding the "token" or Master RS-422 controller station station station station Slaves Fig. 6-2. A typical polled multidrop master/slave configuration. 84 TELECOMMUNICATIONS Receiver Transmitter Fig. 6-3. In token-passing ring topology, access rights are distributed. message. After the device has finished its transmission, it passes the token to another device. The token is passed from device to device in a logical sequence. Token passing is attractive because the transmission delay can be calculated for any network. This makes it ideal for use in real-time applications. The big drawback of the token passing method is that it is more complicated. Recovery circuits have to be built into each device to ensure that a sudden power loss will start the token message again. Another common type of broadcast topology often used in LAN networks is the bus. A bus network (Fig. 6-4) consists of a linear length of cable to which user stations are connected by cable inter- faces, or taps. A single cable can carry messages from each tap in both directions on either one channel, or two separate channels. 6.1.1 Front-End and Back-End Networks Data networks can be divided into two distinct groups based on the application requirements. Figure 6-5 shows a system with two CPUs, several users and shared storage resources. The user interface to the NETWORK STRUCTURES 85 station station station station Data highway Printer Floppy Operator's or related console device Control room Fig. 6-4. A bus network, typical configuration. processors is the frontend network, and the network to which the storage and input/output devices are attached is the backend net- work. Although a linear bus interconnection is shown in the figure, these networks can have any topology. Fr