14400 Modem/ (250-3036B/38) Establishing MNP Faxback Doc. # 3656 Fax/Voice Connections The modem may be configured to execute the MNP handshake by using the AT%Q5, AT\N2 or AR\N3. Data Transfer MNP may be operated in either Block or Stream mode as selected by the AT\Ln command. In Stream mode, MNP sends data frames in varying lengths depending on the amount of time between characters coming from the PC. In Block mode, MNP sends data frames of 256 characters in length. Data Mode Data mode exists when a telco connection has been established between your modem and the remote modem. Speed Buffering (Normal Mode) Speed buffering allows a PC to send and receive data from a modem at a speed that differs from the speed of the physical, or modem to modem, connection. Speed buffering is implemented by using buffers to hold data until a PC or telco line is able to accept data from the modem. This data received from the PC for the telco line, and the modem buffer which holds data from the telco line for transmission to the PC. Speed buffering can operate at the selected telco speed with any permitted PC speed. Direct Mode The direct mode allows data to be transmitted and received directly from either the PC or remote modem. The direct mode is selected with the AT&QO or AT\NI command. When running direct mode, no flow control characters are recognized or transmitted, and the modem cannot perform error correction or data compression. The purpose of direct mode is to make the modem 'dumb' for compatibility with older style modems. Direct mode is not available when parallel interface is selected. In direct mode, any data received while the modem is in command mode is lost. Flow Control Flow control is the process by which a receiving buffer tells the transmitting source to start and stop transmission. There are two data paths which require flow control and each path has flow control options that can be selected. PC to Modem Flow Control The first path is between the PC and the local modem. If the speed of the telephone line connection differs from the speed of the PC, flow control must be used to ensure data integrity. 1. XON/XOFF Flow Control When receiving data from the PC, the modem sends the XOFF character (13H) to the PC when the terminal buffer is nearly full. This point is known as the high water mark. When the terminal buffer empties to a point called the low water mark, the modem sends the XON character (11H) to the PC to allow the buffer to fill. When transmitting data to the PC, the modem reacts to these flowcontrol characters from the PC by stopping transmission if an XOFF is received, or continuing transmission if an XON is received. XON/XOFF flow control is enabled with the AT&K4 command, or for transparent flow control with the AT&K5 command. Note: XON and XOFF characters are transmitted and detected according to the format that was sensed during speed and format sensing. 2. RTS/CTS Flow Control The modem controls the CTS output. When CTS is OFF, the PC is not allowed to send data. When CTS is ON, the PC can send data. When RTS is OFF, the modem will not send data to the PC. When RTS is ON, the modem is permitted to send data to the PC. RTS/CTS flow control is enabled by the AT&K3 command. Modem to Modem Flow Control The second data path is between local and remote modems over the telephone line. When receiving data from the remote modem, the local modem sends the XOFF character to the remote modem when the local modem buffer reaches the high water mark. When the modem buffer empties to the low water mark, the local modem sends the XON characters to allow the buffer to fill. It may be enabled or disabled with a \Gn command. When transmitting data to the remote modem, the local modem reacts to these flow control characters from the remote modem by stopping transmission if an XOFF is received, or by continuing transmission if an XON is received. Buffers The terminal and modem buffers each contain 1024 bytes. The high water mark is reached when the buffer is 50% full (512 characters). (LB/all-05/04/95)