IBM Aptiva, Руководство пользователя

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How modems work 167
Connecting to other data
modems
Phone line noise can cause loss of data and connection
when modem speeds rise above 1200 bps. Over time,
many different software transmission protocols, such as
X-modem, MNP, and V.42bis, have been developed
solely to detect and correct the errors caused by phone
line noise. Not all of these protocols are compatible.
This may be the reason you are unable to connect to
another modem.
If your data modem uses any error detection and
correction protocol, the modem on the other end must
also have the same protocol. For example, if your data
modem uses X-modem (one of the early protocols), the
modem you’re connecting to must also use X-modem.
This is why you are asked to choose a transfer type in
your software before downloading a file from a bulletin
board.
Many different error detection and correction protocols
are commonly used today. Examples of the earliest error
detection and correction protocols include X-modem,
Y-modem, and Kermit. These protocols are performed
by software on your computer.
Recently developed protocols include the Microcom
Network Protocol (MNP) and V.42bis. V.42bis is the
newer of these two protocols. It has its own methods of
error detection and correction, and also incorporates the
MNP 4 and 5 protocols. As a result, modems with MNP 4
and 5 and modems with V.42bis can connect with each
other.
If you cannot connect to another modem, it may be
because the protocol you are using is not compatible
with the protocol of the modem on the other end. If
you’re having trouble connecting to other modems, try
following these guidelines.
•
If you know what protocols are available on the
modem you are connecting to, enable a protocol on
your modem that is supported by the other modem.
Usually V.42bis works best if both modems support
it.
•
If you frequently connect to bulletin boards, you
should buy a modem communications software
program that includes many different software
protocols. Different bulletin boards use different
protocols.
•
Many network services have their own proprietary
protocols. To connect to these services, you often
have to buy the protocol from the service. Other
network services rely on V.42bis or MNP 4 and 5.
Important:
All error detection and correction protocols com-
press data to avoid slowing the transmission rate.
However, combining protocols will not increase your
transmission speed or significantly reduce transmis-
sion errors.
For example, running V.42bis and X-modem together
will not give you a greater compression rate or trans-
mission speed than running V.42bis alone. Use only
one protocol.