Fax Modem (250-3581) MNP and V.42/VBIS Faxback Doc. # 5879 V.42/V.42 BIS DESCRIPTION V.42 supports two methods of error correction: LAPM and, as a fallback MNP 4. The FAX Modem offers a detection and negotiation technique for determining and establishing the best method of error correction between two FAX Modems. During detection, which occurs immediately after the establishment of the FAX Modem handshake, the communicating FAX Modems exchange a sequence of detection patterns to verify that both sides support the error-correction between two FAX Modems. During detection, which occurs immediately after the establishment of the FAX Modem handshake, the communicating FAX Modems exchange a sequence of detection patterns to verify that both sides support the error-correction technique. In the negotiation phase, the communicating FAX Modems exchange configuration information. The AT commands and S registers supporting V.42 bis are: ATWn -- Negotiation Progress Reporting AT&Q5 -- Enable Error Correction Mode AT/Tn -- Inactivity Timer Control AT%Cn -- Compression Control AT%Dn -- Set Dictionary Size (BTLZ) AT%Mn -- Select One/Two-Way Mode (BTLZ) AT%P -- Clear BTLZ Encoder Dictionary AT%Sn -- Set Maximum String Length (BTLZ) S36 -- V.42 Negotiation FALLBACK S46 -- Protocol Selection S48 -- V.42 Negotiation Action S82 -- Break Handling S86 -- Connection Failure Cause Code S95 -- Extended Result Codes. MICROCOM NETWORKING PROTOCOL (MNP) MNP DESCRIPTION MNP is a data link protocol that uses error detection algorithms to ensure data integrity. The FAX Modem supports classes 2 through 5. Class 5 uses a data compression algorithm to increase data throughout and is enabled by the AT%C1 command. If MNP5 is disabled, the FAX Modem defaults to class 4. The AT commands and S registers applicable to MNP are: ATWn -- Negotiation Progress Reporting. AT&Q5 -- Enable Error Correction Mode. AT\An -- Maximum MNP Block Size AT\Kn -- Break Control AT\Ln -- MNP Block Transfer Control AT\Nn -- Operation Mode Control AT\O -- Originate Reliable Link Control AT\Tn -- Inactivity Timer Control AT\U -- Accept Reliable Mode Control AT\Y -- Switch to Reliable Mode AT/Z -- Switch to Normal Mode AT%C -- Compression Control S36 -- Negotiation Failure Treatment S38 -- Delay Before Forced Hang-up. S40 -- MNP Bit Mapped Options S41 -- MNP Bit Mapped Options S46 -- Protocol Selection S95 -- Extended Result Codes MNP DATA MODE After establishing the MNP connection, the FAX Modem performs all of the functions during the Normal Data Mode with the following differences: 1. Retrains are performs as a result of retransmission rates. 2. When in command mode, during an MNP connection as a result of the escape sequence, the FAX Modem will continue to accept data and acknowledge frames. When the FAX Modem enters the data mode, the stored data will be delivered to the DTE. 3. Breaks are handled differently in MNP mode compared to non-MNP (moral and direct) modes as controlled by the \Kn command. MNP CLASS 5 DATA COMPRESSION This MNP mode may be enabled or disabled by using the AT%Cn command, and is in operation if an MNP Class 5 connection has been successfully established. If the FAX Modem establishes a class 5 connection, the FAX Modem increases it's throughput by compressing data into tokens before transmitting to the remote FAX Modem, and decompressing encoded received data before sending it to the DTE. The AT%C1 command has no effect if an MNP connection has already been established, except configuring to the FAX Modem for a future MNP Class 5 handshake attempt. PROGRAMMABLE INACTIVITY TIMER The FAX Modem can determine the length of time the FAX Modem will wait before disconnecting when no data is sent or received. In MNP mode, this inactivity timer is reset when data is either received from the DTE or is received from the telco line in the form of a Link Transfer PDU. This timer can be set using the AT\Tn command where n represents a value between 0 and 42 minutes. 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 DTE. In block mode, MNP sends data frames of 256 characters in length. Special communication software must be used when using block mode. AT\An MAXIMUM MNP BLOCK SIZE Sets maximum block size for MNP stream link connections. Use this command to transmit smaller blocks of data in a reliable link connection. AT\A0 = Sets maximum block size to 64 characters. AT\A1 = Sets maximum block size to 128 characters. AT\A2 = Sets maximum block size to 192 characters. AT\A3 = Sets maximum block size to 256 characters. AT\Bn TRANSMIT BREAK When this command is entered during a non-MNP connection, the FAX Modem sends a break signal to the remote FAX Modem. The length of the break is 100 times the n parameter value in milliseconds. If this command is entered in MNP mode, the FAX Modem sends a Link Attention PDU to the remote FAX Modem. AT\Gn FAX MODEM TO FAX MODEM FLOW CONTROL Enables or disables FAX Modem to FAX Modem flow control during a Normal mode connection. Since the reliable link (error correction) has its own method of flow control, the AT\Gn command is ignored when error correction is selected (AT&Q5). However, the DTE to FAX Modem flow control remains active during a reliable link. AT\G0 = Disables flow control (ON/XOFF). AT\G1 = Enables flow control (XON/XOFF). AT\Kn BREAK CONTROL Determines what the FAX Modem will do when a BREAK is received from the DTE or the remote FAX Modem. During MNP mode, the remote FAX Modem's Break control setting determines how the local FAX Modem will handle the BREAK. When a BREAK is received from the DTE during Normal or MNP mode, the FAX Modem takes the following action: AT\K0, 2, 4,= FAX Modem enters the command mode (waiting for AT) without sending a BREAK to the remote FAX Modem. AT\K1 = FAX Modem clears the terminal and FAX Modem buffers and sends a BREAK to the remote FAX Modem. AT\K3 = FAX Modem does not clear the buffer but sends a BREAK to the remote FAX Modem. AT\K5 = FAX Modem sends a BREAK to the remote FAX Modem in sequence with any transmitted data. When a BREAK is received from the remote FAX Modem during Normal mode, the FAX Modem takes the following action: AT\K0, 1 = FAX Modem clears the terminal and FAX Modem buffers and sends a BREAK to the local DTE. AT\K2, 3 = FAX Modem does not clear buffers but sends a BREAK to the local DTE. AT\K4, 5 = FAX Modem sends a BREAK in sequence with an Y data being buffered. When a BREAK is received from the DTE during Direct Mode, the FAX Modem takes the following action: AT\K0, 2, 3 = FAX Modem sends a BREAK to remote FAX Modem and enters command mode (waiting for AT). AT\K1, 3, 5 = FAX Modem sends a BREAK to remote FAX Modem. NOTE: In LAPM, breaks are "timed", meaning the FAX Modem attempts to preserve the duration of the break when transmitting it to the remote FAX Modem. IN MNP 4, breaks are not "timed", as MNP 4 has no facility for maintaining the duration of the signal; a long break is the same as a short break. AT\Ln MNP BLOCK TRANSFER CONTROL Determines whether the mode will use Block or Stream mode for MNP link. AT\L0 = Uses Stream mode for MNP link connections. AT\L1 = Uses Block mode for MNP link connections. AT\Nn OPERATION MODE CONTROL Select the operating mode the FAX Modem uses while connected (see AT&Qn command). AT\N0 = Selects Normal (speed buffering) mode. This option takes effect at physical connection time. AT\N1 = Selects Direct (pass through) mode. This option takes effect at physical connection time. If the parallel interface is selected (AEREN = low), the AT\N1 is interpreted as AT\N0 since the Direct mode is not available with the parallel interface. AT\N2 = Selects reliable link mode. This defines the FAX Modem-to-FAX Modem connection to require error correction. If an attempt to establish the reliable link fails, the FAX Modem disconnects. This command takes effect at physical connection time only. To establish MNP after a physical connection, use the AT\O, AT\U or AT\Y command. AT\N3 = Selects auto-reliable link mode. The FAX Modem will attempt an error-correction connection, but will fallback to Normal mode if unable to establish an MNP link. This command takes effect at physical connection time only. To establish MNP after a physical connection, use the AT\O, AT\U, or AT\Y command. AT\O ORIGINATE RELIABLE LINK CONTROL Forces the FAX Modem to originate an MNP connection regardless of whether the FAX Modem is in originate or answer mode. The FAX Modem will send up to two link requests. If the remote FAX Modem does not respond after the second request, the FAX Modem returns to its previous mode. This command can be executed only if a physical connection exists, and will return the NO CARRIER message if there is no connection, or an appropriate connect message if a reliable link connection already exists. This command must be the last command on a command line. AT\Y SWITCH TO RELIABLE MODE Causes the FAX Modem to try to establish a reliable link. Depending on whether the FAX Modem is in answer or originate mode, the FAX Modem either accepts a link request from the remote FAX Modem or sends a link request, respectively. If the MNP connection attempt fails, the FAX Modem returns to its previous state. This command can be executed only if a physical connection exists, and will return the NO CARRIER message, if there is no connection, or an appropriate connect message if a reliable link connection already exists. This command must be the last command on a command line. AT\Z SWITCH TO NORMAL MODE Causes the FAX Modem to switch to normal mode after a reliable link is established. Switching to normal mode erases any data that may be in the terminal and FAX Modem buffers. This command can be executed only if an MNP connection exists, and will return the NO CARRIER message if there is no connection, or an appropriate connect message if a reliable link connection previously existed. This command must be the last command on a command line. AT%Cn COMPRESSION CONTROL Determines whether or not the FAX Modem will use data compression. AT%C0 = Disable data compression. AT%C1 = Enable MNP 5 data compression. AT%C2 = Enable V.42 bis data compression. AT%C3 = Enable both V.42 bis and MNP 5 data compression. This command allows the data compression technique to be selected independently of the error-correction protocol. The combination of V.42 bis data compression and MNP 4 error correction enables users to benefit from the superior performance of V.42 bis data compression. Although the combination of MNP 5 data compression and LAPM error correction is not a good selection, it is also available to the user's discretion. This command affects register S46 which is used only to enable or disable V.42 bis data compression. AT%C0 and AT%C1 settings change the value in S46 to 136. AT%C2 and AT%C3 settings change the value in S46 to 138. AT%Dn SET DICTIONARY SIZE (V.42bis) Set the V.42 bis dictionary size AT%D0 = 512 AT%D1 = 1024 AT%D2 = 2048 AT%D3 = 4096 (one-way compression only).* * For two-way compression (AT%M3), the dictionary size is set for 2048 if AT%D3 is issued. AT%En ENABLE/DISABLE AUTO-RETRAIN Determines whether or not the FAX Modem automatically monitors the line quality and request a retrain when necessary during a 2400 bps connection. AT%En = Disables auto-retrain. AT%E1 = Enables auto-retrain. AT%L REPORT RECEIVED SIGNAL LEVEL Returns a value identifying the received signal level. The possible values are: 009 = Received level of-9 dBm\010 = Received level of-10 dBm 011 = Received level of-11 dBm . . 0043 = Received level of-43 dBm If the received level is greater than-9 dBm, 009 will be reported. If the received level is less than-43 dBm, 043 will be reported. AT%Mn SET ONE/TWO-WAY COMPRESSION MODE (V.42 bis) Selects the one/two-way compression mode when using V.42 bis compression. AT%M0 = Compression disabled. AT%M1 = Transmit compression only. AT%M2 = Receive compression only. AT%M3 = Two-way compression. AT%P CLEAR V.42 bis ENCODER DICTIONARY Resets the local FAX Modem V.42 bis encoder dictionary and sends a command code to the remote FAX Modem to reset the remote V.42 bis dictionary. AT%0 REPORT LINE SIGNAL QUALITY Returns the high-order byte of the calculated Eye Quality Monitor(EOM) value. The high byte can range from 0-127. However, when the value is 8 or greater, the FAX Modem will automatically retrain if enabled by the AT%E1 command. The value for a normal connection ranges from about 0 to 2 and approaches 8 for a progressively poorer connection. If a 2400 or 1200 baud connection does not exist, the FAX Modem returns ERROR. AT%Sn SET MAXIMUM STRING LENGTH (V.42 bis) Sets the maximum number of characters that can be compressed into one word. Range: 6-250 Default: 32 characters AT-Kn EXTENDED SERVICES Determines whether a LAPM connection will be converted to an MNP connection. When AT-K1 is on both FAX Modems, a LAPM connection will be converted to an MNP connection. AT-K0 = Disable extended services. AT-K1 = Enable extended services. AT*Hn LINK NEGOTIATION SPEED Sets the connection speed for link negotiation before upshift occurs between where FAX Modems poor telephone lines are anticipated. AT*H1 command should be used to set the connection speed to 1200 bps to facilitate link negotiation. AT*H0 = Link negotiation occurs at the highest supported speed. AT*H1 = Link negotiation occurs at 1200 bps. AT)Mn POWER LEVEL ADJUSTMENT FOR CELLULAR CONNECTION Adjusts the power level during link negotiation for reliable connections to reduce signaling distortion caused by cellular telephone equipment. AT)M0 = Auto adjustment (FAX Modem automatically adjusts power level if remote FAX Modem is set to AT)M1. AT)M1 = Force adjustment of power level. (SMC/all-07/05/94)