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Operation
Asynchronous Operation
When you select Operating Mode 0 (&M0) the modem will operate asynchronously when the
modem is on-line (data mode - connected to the telephone line) and off-line (Command Mode -
disconnected from the telephone line). The modem can employ error correction (reliable) protocols
to ensure error-free delivery of asynchronous data sent between computers. The error control
methods are based on grouping data into frames with checksums determined by the contents of each
frame. The receiving modem checks the frames and sends acknowledgments to the transmitting
modem. When it detects a faulty frame, the receiving modem requests a retransmission. Frame
length varies according to the amount of data transmitted or the number of retransmissions
requested from the DTE.
The modem can achieve DTE speeds greater than the maximum VF speed when one of two
asynchronous compression protocols are used: ITU-T V.42 bis and MNP Level 5. They attempt to
increase throughput by compressing the information to be sent before actually sending it. The
modem is thus able to transmit more data in a given period of time. Characteristics of the individual
compression techniques are discussed below.
Note
Tests performed with the modem in asynchronous mode do not use error control. When a remote loop test
is initiated during an error control connection, error control is terminated so that transmission errors
caused by the transmission link can be detected. Refer to Chapter 6, Tests for a description of diagnostics.
Error Correcting Modes
The \N command selects one of five error correcting modes which use bidirectional data buffering
and flow control. This permits the modem to operate with different data rates set at the computer
serial port and the modem port. For each mode, the modem interprets \N in combination with the
%C command to determine what data compression protocol (if any) is available. Each mode is
described below.
Note
V.42 (LAPM)/MNP Auto-reliable Mode or V.42 (LAPM)/MNP Reliable Mode is typically the configura-
tion used most often. Experimentation and careful study of the command language will indicate what
works best for your application.
MNP Operation Overview
When enabled for MNP operation, the modem can be configured for compatibility with MNP
Levels 2 - 5 and MNP Levels 2 - 4. The range of levels or service classes allow its use with older
and recent equipment. The \N command determines whether the modem can use MNP Levels 2
- 4, described below. Once MNP Levels 2 - 4 are enabled, the &C command determines whether
Level 5 is available. When attempting an MNP reliable connection, modems will negotiate the
connection for the highest common MNP service level permitted by their configuration.
•
Level 2 Protocol - An asynchronous full-duplex protocol. Data frames and acknowledgments
can be transmitted simultaneously. The maximum frame size is 256 bytes.
•
Level 3 Protocol - A synchronous full-duplex protocol. Modem sends call setup
asynchronously, switching to synchronous mode when sending data (frame size: 256 bytes
max)
•
Level 4 Protocol
is automatically adjusted according to the line quality (based on number of retransmissions).
The frame header in Level 4 is shorter than in Level 3, which results in a slight improvement in
throughput - about 20% improvement in comparison to nominal data rate.
Note
Asynchronous DTE must be used for all levels of MNP error control.
060R122-000
Issue 15
Table 3-4
list the command combinations that set the various mode characteristics.
Provides synchronous framing with adaptive frame lengths. The frame length
-
SpectraComm Dual V.34 Modem
Installation and Operation
Asynchronous Operation
3-27
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