AL2094S Designers Guide
Doc. No. DG_2094E
Altec Electronic AG
Seite 12 / 30
2.4 Modem-on-Hold
The Modem-on-Hold (MOH) function (V.92 models only) enables the modem to place a data
call to the Internet on hold while using the same line to accept an incoming or place an
outgoing voice call. This feature is available only with a connection to a server supporting
MOH. MOH can be executed through either of two methods:
•
One method is to enable MOH through the +PMH command. With Call Waiting
Detection (+PCW command) enabled, an incoming call can be detected while online.
Using a string of commands, the modem negotiates with the server to place the data
connection on hold while the line is released so that it can be used to conduct a voice
call. Once the voice call is completed, the modem can quickly renegotiate with the
server back to the original data call.
•
An alternative method is to use communications software that makes use of the
Conexant Modem-on-Hold drivers. Using this method, the software can detect an
incoming call, place the data connection on hold, and switch back to a data connection.
2.5
Error Correction and Data Compression
2.5.1 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.
2.5.2 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.
2.5.3 V.44 Data Compression
V.44 data compression encodes pages and files associated with Web pages more efficiently
than V.42 bis. These files include WEB pages, graphics and image files, and document files.
V.44 can provide an effective data throughput rate up to DTE rate for a 56-kbps connection.
The improved performance amount varies both with the actual format and with the content of
individual pages and files.
2.5.4 V.42 bis Data Compression
V.42 bis data compression mode, enabled by the %Cn command or S46 register, operates
when a LAPM or MNP 10 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 2-KB dictionaries are used to store the strings. These
dictionaries are dynamically updated during normal operation.
2.5.5 MNP 5 Data Compression
MNP 5 data compression mode, enabled by the %Cn command, 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.
2.6
MNP 10 Data Throughput Enhancement
MNP 10 protocol and MNP Extended Services enhance performance under adverse channel
conditions such as those found in rural, long distance, or cellular environments. An MNP 10
connection is established when an MNP 2-4 connection is negotiated with a remote modem
supporting MNP 10.
MNP Extended Services.
The modem can quickly switch to MNP 10 operation when the
remote modem supports MNP 10 and both modems are configured to operate in V.42.
V.42 bis/MNP 5 Support.
V.42 bis/MNP 10 can operate with V.42 bis or MNP 5 data
compression.
