The modem must operate in two states in order to enable DUN:

• Local command state – The modem is off -line. It receives commands and provides status information to the host computer to which the modem is installed.
• Online state – In this state, the modem is transferring data between the host machine and a remote computer via the telephone system.

Three activities occur during the Local command and Online states. These include Dialing, Data exchange, and Answering. The modem would normally shift between the two states if, for example, the system tells it to go online and dial out to another unit, or if the modem receives an incoming call.

After the line connection has been established, a "handshaking sequence" takes place between the two modems and the computers. This is a flurry of short events that take place between the two systems to establish the readiness of the two modems and their computers to engage in data exchange. Dial-up modems send data over the serial telephone line in the form of an analog signal. Analog signals can be drawn as waves because they change gradually and continuously. Recall that digital communications are based on binary. In this system, the digital signals represent 1s and 0s. These signals must be converted to a waveform to travel across telephone (analog) lines. They are converted again back to the digital form (1s and 0s) by the receiving modem so that the receiving computer can understand the data.

The outcome of the handshake and the negotiation between the sending and receiving devices is the establishment of a common modulation that is supported by both computers. This completes the process of a session negotiation so that effective data exchange can occur.

Other devices may determine the actual speed of data transfer. As previously mentioned, there are small single-chip devices that run the COM port to which an external modem is attached. These are the Universal Asynchronous Receiver/Transmitter (UART) chips. The type of UART chip determines the top speed at which devices can communicate using the port. Current UART 16,450 and 16,550 chips enable speeds of up to 115,200bps, and the 16,650 chip supports a transmission rate of 230,400bps.

AT Commands
All modems require software to control the communication session. The set of commands that most modem software uses are known as the Hayes-compatible command set. They are named after the Hayes Microcomputer Products Company, which first defined them. The Hayes command set is based on a group of instructions that always begins with a set of attention characters (AT), followed by the command characters. Since these attention characters are always an important part of a Hayes command, the command set is usually referred to as the AT command set.

Simply put, the AT commands are modem control commands. The AT command set is used to issue dial, hang up, reset, and other instructions to the modem. Most user manuals that come with modems contain a complete listing of the AT command set. This is just a summary of the most commonly used AT commands. Note that the standard Hayes compatible code to dial is ATDxxxxx. Sometimes, a T is added to signify tone dialing. Students can use Figure to test their knowledge of these common modem AT commands.