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HAYES V-series V.42 User's Guide Notice: Hayes Microcomputer Products, Inc. reserves the right to make improvements to the product described in this manual at any time and without notice. Hayes, the Hayes logo, and Smartcom II are registered trademarks, and V-series, Smartcom III, Modem Enhancer, Smartmodem 9600, and Smartmodem 9600B are trademarks, of Hayes Microcomputer Products, Inc. MNP is a registered trademark, and Microcom Networking Protocol is a trademark, of Microcom, Inc. IBM is a registered trademark of International Business Machines Corporation. Apple and Macintosh are registered trademarks of Apple Computer Inc. copyright Hayes Microcomputer Products, Inc. ======================================================================= USING THIS GUIDE Hayes V-series V.42 products automatically attempt an error-control connection with the remote modem using CCITT standard V.42 protocol. If the remote system does not support V.42, or is configured in a way that it cannot make the connection, the V-series V.42 product willl attempt a connection using a method supported by the remote system. If you are using Hayes Smartcom II for the IBM PC or for the Apple Macintosh, version 3.1, or Smartcom III for the IBM PC, version 1.1, the software will perform all the necessary setup to make the best connection--whether to another V.42 compliant modem, another V-series system product, a Hayes Smartmodem product, or simply to a Hayes-compatible modem. If you will be using one of these Hayes communications packages with a V-series V.42 product, you need not read any further in this manual. If you are using software that requires you to configure the modem for your connections, this V.42 User's Guide and the User's Guide accompanying your V-series system product include complete explanations of the features and commands used for configuring and operating your V-series V.42 product. All the features of the V-series V.42 products are addressed via the Hayes Standard AT Command Set. Techniques for issuing AT commands and setting S-Registers are explained in the User's Guide accompanying your V-series system product. THE V.42 ADVANTAGE With the V-series system products, Hayes advanced modem communications by adding adaptive data compression, automatic feature negotiation, automatic speed buffering, and point-to-point error-control using an implementation of CCITT LAP-B (Link Access Protocol Balanced). The V-series V.42 products take that functionality one step further by including the CCITT V.42 standard for modem error-control connections. V.42 specifies LAP-M (Link Access Procedure for Modems) as the primary point-to point error-control protocol. The standard also includes an alternative protocol, Annex A, for compatibility with the installed base of modems supporting MNP (Microcom Networking Protocol) levels 2 through 4. This alternative protocol represents a static specification; all future development for enhancements to modem error-control will focus on the standard's primary protocol--LAP-M. Hayes V.42 products incorporate the following CCITT-prescribed features for LAP-M. * Benign detection phase * 8-bit address field default * Private parameter negotiation * Break length preserved * Modulo-128 I-frame sequence numbers * Poll/Final bit procedure * Address extension bit used * UI frame exchange for break signalling * XID frame exchange for negotiation * Separate primitives for ACK, NAK, and BUSY Hayes V-series V.42 products are fully-compliant with CCITT recommendation V.42 for point-to-point modem error-control. V.42 FEATURES AND COMMAND OPTIONS Control of the V.42 functions is provided by additonal S-Registers and an extension of the &Q5 error-control mode (described in the V-series system product User's Guide). These configurable options include: * Extension of V-series error-control mode to include V.42 protocols * Improved Automatic Feature Negotiation for a wider variety of connection types and negotiation progress reporting * Additional selections for fallback mode if desired connection is not made * User-selection of Adaptive Data Compression * V.42 recommended local flow control options * Greater control of break handling These features interact to provide the configuration that suits the particular environment(s) in which the V-series V.42 product is used. V-SERIES ERROR CONTROL MODE--&Q5 A Hayes V-series V.42 product can negotiate any one of several connection types, depending on the capabilities and configuration of the remote modem. This is accomplished through an extension of the V-series error-control mode--&Q5. In this factory-set configuration, the modem automatically attempts to make a connection using the V.42 standard. Hayes V-series V.42 products make connections using LAP-M at 1200 and 2400 bps and at 9600 bps when connecting with another Hayes V-series Smartmodem 9600 V.42 or V-series Smartmodem 9600B V.42. LAP-B is used when connections are made with either V-series system products or Hayes V-series X.25 products. Alternative protocol connections are available at 1200 and 2400 bps. When a V-series V.42 product is configured to make an MNP connection, it attempts an MNP Class 4 connection. From this initial attempt, the negotiation phase can also result in a Class 2 or Class 3 connection, depending on the capabilities of the other modem. If the V-series V.42 product is a V-series Modem Enhancer V.42 connected to a Smartmodem 1200 (which does not support synchronous communications), a Class 2 negotiation will be attempted. AUTOMATIC FEATURE NEGOTIATION Automatic feature negotiation offers a powerful technique for determining and establishing the best method of error-control between two modems. V-series system products analyze the capabilities of the remote modem, make an intelligent choice of the method of communication, and initiate the protocol. The feature negotiation process involves two phases: detection and negotiation. In the detection phase, the communicating modems exchange a rapid sequence of characters to verify that both sides support feature negotiation. This occurs immediately after the telephone connection is established. If the detection phase is completed, a negotiation follows. In the negotiation phase, the communicating modems exchange descriptive information (e.g., their configuration, the features and protocols supported) to determine the best connection possible. Desired connection types are selected with Registers S46 and S48. If the detection phase fails or negotiation does not achieve the desired connection type, the action to take (e.g., hang up, communicate in standard asynchronous mode --&Q0) can be specified with Register S36. Because MNP does not support a detection phase, selection of the alternative protocol must be made as a forced fallback with Registers S36 and S48. Feature Negotiation Action--S48 The process of feature negotiation determines the capabilities of the remote system. This process can be bypassed if desired, for example, when the capabilities of the remote modem are known and negotiation is unnecessary. S48 = 0 Negotiation disabled; modem presumes the remote modem is configured for and has the capabilities necessary for the prescribed connection mode, bypasses the detection and negotiation phases, and proceeds with the protocol selected with S46. S48 = 7 Negotiation enabled (factory setting). S48 = 128 Negotiation disabled; forces the fallback actions specified in S36 to be taken at once. Can be used to force the V.42 alternative protocol (compatible with MNP), bypassing the detection and negotiation phases. Protocol Selection--S46 The following protocol selections can be specified for the feature negotiation process. S46 = 0 Either LAP-M or LAP-B. S46 = 2 LAP-M with adaptive data compression or LAP-B with adaptive data compression (factory setting). S46 = 136 LAP-M only. S46 = 138 LAP-M with adaptive data compression. S46 = 1 LAP-B only. S46 = 3 LAP-B with adaptive data compression. Automatic Feature Negotiation determines whether or not compression is available for a connection type as part of the negotiation phase and enables or disables it as necessary. For example, in a connection with a non-Hayes modem supporting MNP, the V-series V.42 product determines compression is unavailable with the alternative protocol and automatically disables the feature. NEGOTIATION FALLBACK--S36 V-series V.42 products extend the range of options available for specifying the type of connection to attempt and what action to take if the desired connection cannot be made. S36 = 0 Modem hangs up. S36 = 1 Modem stays on-line and communication is initiated using a standard asynchronous connection (&Q0). S36 = 2 Reserved. S36 = 3 Modem stays on-line and communication is initiated using an asynchronous connection with ASB (automatic speed buffering). ASB is used for interfaces requiring a constant speed between the modem to computer/terminal. S36 = 4 Attempt to connect using V.42 Alternative Protocol (compatible with MNP levels 2-4); if negotiation fails, hang up. S36 = 5 Attempt to connect using V.42 Alternative Protocol (compatible with MNP levels 2-4); if negotiation fails, connect using a standard asynchronous connection (factory setting). S36 = 6 Reserved. S36 = 7 Attempt to connect using V.42 Alternative Protocol (compatible with MNP levels 2-4); if negotiation fails, connect using an asynchronous connection using ASB. These fallback options are initiated immediately on connection if Register S48 = 128. For example, a connection using the alternative protocol will be forced if S48 = 128 and S36 = 5 or 7. CONNECTION FAILURE CAUSE CODE--S86 When the modem issues a NO CARRIER result code, a value is written to this S-Register to help in determining the reason for the failed connection. Multiple occurrences may contribute to a NO CARRIER message; S86 records the first event that occurred. S86 = 0 Normal hang-up, no error occurred. S86 = 1,2,3 Reserved. S86 = 4 Physical carrier loss. S86 = 5 Feature negotiation failed to detect presence of another error-control modem at the other end. S86 = 6 Other error-control modem did not respond to feature negotiation message sent by this modem. S86 = 7 Other modem is synchronous-only; this modem is asynchronous-only. S86 = 8 Modems could not find a framing technique (sync or async) in common. S86 = 9 Modems could not find a protocol in common. S86 = 10 Feature negotiation message sent by other modem is incorrect. S86 = 11 Synchronous information (data or flags) not received from other modem. Modem waits 30 seconds before hanging up. S86 = 12 Normal disconnect initiated by other modem. S86 = 13 Other modem did not respond after many transmissions of the same message. Modem attempts connection 10 times before giving up. S86 = 14 Protocol violation occurred. ADAPTIVE DATA COMPRESSION Hayes V-series system products, including V-series V.42 products, can compress the data passed between the computer/terminal and the modem. Before data is sent across the line, the V-series V.42 product compresses the data. The V-series system product at the receiving end decompresses the data before sending it to the computer or terminal. This permits the speed between the computer/terminal and the modem to be higher than that between the two modems, effectively increasing data throughput. For optimum gain in throughput, the speed between the computer/terminal and the modem should be twice that of the speed between the two modems across the telephone line. (See the User's Guide accompanying your V-series system product for a description of DTE speed and DCE line speed.) LOCAL FLOW CONTROL Error-control and Automatic Speed Buffering connections require a method of local flow between the modem and the computer (the DTE interface) for data buffering. The V.42 recommendation specifies RTS/CTS and XON/XOFF as flow control methods. V-series V.42 products offer these two methods plus transparent XON/XOFF flow control. The factory setting, &K3--RTS/CTS flow control, is appropriate when hardware (cable between the modem and the computer/terminal) support RTS/CTS signals. Descriptions of the options available with the &K command are provided in the User's Guide accompanying your V-series system product. ASYNCHRONOUS FRAMING TECHNIQUE Connections between two error-control modems using either error-control/LAP-B or V.42 protocols are usually synchronous connections, even though the user interface is always asynchronous (refer to your V-series system product's User's Guide for descriptions of synchronous and asynchronous modes). NEGOTIATION PROGRESS MESSAGES In addition to the negotiation progress messages (protocol-related result codes) available with V-series system products, the following messages are implemented in V-series V.42 products. The W1 command enables negotiation progress messages. The factory setting, W0, disables the messages. Numeric Verbose Connection indicated 77 PROTOCOL: LAP-M V.42 LAP-M 78 PROTOCOL: LAP-M/HDX V.42 LAP-M half-duplex* 79 PROTOCOL: LAP-M/AFT V.42 LAP-M with AFT 80 PROTOCOL: ALT Alternative protocol * Hayes V-series half-duplex "ping-pong" protocol for 9600 and 4800 bps connections. BREAK HANDLING V.42 specifies three methods of break signal handling: in sequence, expedited, and destructive. Break signals provide a way for you to get the attention of the remote host. The break type used depends on your application. S82 = 128 "in sequence" signalling as data is sent and received; data integrity is maintained both ahead of and after the break (factory setting). S82 = 3 "expedited" signalling regardless of its sequence in data sent and received by each modem; data integrity is maintained both ahead of and after the break. S82 = 7 "destructive" signalling regardless of its sequence in data sent and received by each modem; data being processed by each modem at time break sent is destroyed. In LAP-M connections, breaks are "timed." The modem attempts to preserve the duration of the break when transmitting it to the remote modem. In alternative protocol connections, breaks are "not timed," as the alternative protocol has no facility for maintaining the duration of the signal; a long break is the same as a short break. In LAP-B connections, break signals are always in sequence and timed. Note: When timing is a factor, the detection and generation at the computer/terminal of break signals are in 10 millisecond increments: breaks not in even 10 millisecond intervals are rounded to the next increment. A break of less than 10 millisecond duration may not be detected; a break longer than 2.55 seconds will be transmitted with a 2.55 second duration.