HAYES V-series V.42 User's Guide

 

                     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.

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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.