transmit. The UART typically handles the details of sending individual bits and storing received bits on the serial port. Two computers that want to exchange data must agree on whether both ends can transmit at once or whether the computers need to take turns. Most wired links between two computers are full duplex: both computers can transmit at the same time. Many wireless links are half duplex: the computers must take turns. A simplex link is one way only. A network with three or more computers sharing a data path must use a protocol that defines when a computer can transmit. A communication protocol can include the use of status and control lines. These lines can indicate when a transmitter has data to send or when a receiver is able to accept new data. The process of exchanging this information is called flow control. Hardware flow control uses dedicated lines for the signals. Devices can also use software flow control to provide the same information by sending defined codes, typically in the same path used for data. 7 Chapter 1 Additional status and control lines can provide other information such as the presence of a carrier frequency or a ring signal on a phone line. In serial networks where only one transmitter can be enabled at a time, a transmit-enable line at each computer can enable and disable the transmitters as needed. Message Protocols Serial communications often exchange messages that consist of blocks of data with defined formats. A message protocol can specify what type of data a message contains and how information is structured within the message. The computers in a network need a way to detect which computer is the intended receiver of transmitted data. Networks typically assign an address to each computer and include the receiver’s address in each message. For example, a very basic message might consist of two bytes: one byte to identify the receiver and one byte containing data. To enable a receiving computer to detect the start and end of a prom-electric.ru