Problems Encountered During Communications
Another set of problems can follow establishment of a data connection.
These may range from loss-of-carrier to data loss. The sections below discuss
these situations.
Data Loss or Corruption
A typical complaint about connections is that of lost or corrupted data.
A number of factors can affect the transmission of data.
Modem Settings
Garbled, or otherwise faulty data can occur from the time the connection
is made or just a momentary problem. If all the data that comes across is
incorrect, or if the modems handshake with each other successfully but no
data can be transferred, suspect an error in the setting of some communications
parameter. For example, both systems must use the same asynchronous character
format or synchronous framing and clocking scheme. Even if the local system
is operating properly, if the remote system uses a different format, the
garbled data may be exactly what is sent and not the result of any modem
or transmission error.
Line Noise
Occasional garbling or loss may be the result of noise or otherwise
poor quality of the telephone line. If a dial-up connection is too noisy,
the connection should be broken, and the connection re-attempted; you get
a different circuit each time. One that is acceptable for voice may not
be for data. With a leased line, the circuit is always the same. If you
experience line noise or line drops, you should contact (or the software
should recommend that the user contact) the vendor for the leased line.
Transmit and Receive Levels
Other factors that affect both leased and dial-up lines are the transmit
and receive levels. These settings determine the signal levels used by the
modem in each direction. Some Hayes modems permit these levels to be adjusted.
The range and availability of these adjustments is in large part controlled
by the local telephone system. For example, the recommended settings and
ranges are different for modems sold in the U.K. than for those sold in
the U.S. See the documentation accompanying the modem to determine whether
this capability is supported.
DTE Processor Restrictions
Some multitasking operating systems can occasionally lose small amounts
of data if the computer is heavily loaded and cannot allocate processing
time to the communications task frequently enough. In this case, the data
is corrupted by the DTE itself. This could also cause incomplete data transmission
to the remote system. DTE processor capabilities should be a concern when
developing software for data communications when the line speed is greater
than 9600 bits/s and the modem-to-DTE connection is 19200 bits/s or higher
(for example, when data compression is used). The modem provides exact transmission
of the data it receives, but if the DTE cannot keep up with the modem because
of other tasks or speed restrictions, precautions should be taken when writing
software or when adding modems with extra high speed capabilities into a
link.
One way to avoid the problem of data loss caused by the DTE is an upgraded
serial port such as Hayes Enhanced Serial Port (ESP(TM))card. This card
replaces the existing serial card, provides two ports, data buffering, and
a communication co-processor to take some of the load off of the DTE processor.
For information on this product line, contact Hayes Customer Service.
Buffer Overflow
Hayes V-series products provide a data buffer for overflow when the
modem is transmitting data to the DTE faster than it can process it, or
vice versa. The upper and lower limits of the buffer can be adjusted with
S49 and S50, although
the factory settings suit most situations.
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