Hayes Microcomputer Products RCV56HCF - Manual (Page 20)

RCV56HCF PCI/CardBus Modem Designer’s Guide
ROCKWELL PROPRIETARY INFORMATION 1129 2-2
Fall Forward/Fallback (K56flex, V.34/V.32 bis/V.32)
During initial handshake, the modem will fallback to the optimal line connection within K56flex/V.34/V.32 bis/V.32 mode
depending upon signal quality if automode is enabled by the +MS command.
When connected in K56flex/V.34/V.32 bis/V.32 mode, the modem will fall forward or fallback to the optimal line speed within
the current modulation depending upon signal quality if fall forward/fallback is enabled by the %E1 command.
Retrain
The modem may lose synchronization with the received line signal under poor line conditions. If this occurs, retraining may
be initiated to attempt recovery depending on the type of connection.
The modem initiates a retrain if line quality becomes unacceptable if enabled by the %E command. The modem continues to
retrain until an acceptable connection is achieved, or until 30 seconds elapse resulting in line disconnect.
2.3 ERROR CORRECTION AND DATA COMPRESSION
V.42 Error Correction
V.42 supports two methods of error correction: LAPM and, as a fallback, MNP 4. The modem provides a detection and
negotiation technique for determining and establishing the best method of error correction between two modems.
MNP 2-4 Error Correction
MNP 2-4 is a data link protocol that uses error correction algorithms to ensure data integrity. Supporting stream mode, the
modem sends data frames in varying lengths depending on the amount of time between characters coming from the DTE.
V.42 bis Data Compression
V.42 bis data compression mode operates when a LAPM or MNP connection is established.
The V.42 bis data compression employs a “string learning” algorithm in which a string of characters from the DTE is encoded
as a fixed length codeword. Two dictionaries, dynamically updated during normal operation, are used to store the strings.
MNP 5 Data Compression
MNP 5 data compression mode operates during an MNP connection.
In MNP 5, the modem increases its throughput by compressing data into tokens before transmitting it to the remote modem,
and by decompressing encoded received data before sending it to the DTE.
2.4 MNP 10EC™ ENHANCED CELLULAR CONNECTION
A traditional landline modem, when used for high-speed cellular data transmission, typically encounters frequent signal
interference and degradation in the connection due to the characteristics of the analog cellular network. In this case, cellular-
specific network impairments, such as non-linear distortion, fading, hand-offs, and high signal-to-noise ratio, contribute to an
unreliable connection and lower data transfer performance. Implementations relying solely on protocol layer methods, such
as MNP 10, generally cannot compensate for the landline modem's degraded cellular channel performance.
The modem achieves higher cellular performance by implementing enhanced cellular connection techniques at both the
physical and protocol layers, depending on modem model. The modem enhances the physical layer within the modulation by
optimizing its responses to sudden changes in the cellular connection. The MNP 10EC protocol layer implemented in the
modem software improves data error identification/correction and maximizes data throughput by dynamically adjusting speed
and packet size based on signal quality and data error performance.
2.5 FAX CLASS 1 OPERATION
Facsimile functions operate in response to fax class 1 commands when +FCLASS=1.
In the fax mode, the on-line behavior of the modem is different from the data (non-fax) mode. After dialing, modem operation
is controlled by fax commands. Some AT commands are still valid but may operate differently than in data modem mode.
Calling tone is generated in accordance with T.30.
2.6 VOICE/AUDIO MODE
Voice and audio functions are supported by the Voice Mode. Voice Mode includes three submodes: Online Voice Command
Mode, Voice Receive Mode, and Voice Transmit Mode.