Connecting To Other Data Modems - IBM Aptiva Hardware Handbook

Primarily, modem standards and recommendations in
the United States have been set by two organizations:
•
The International Telephone and Telegraph Corpora-
tion set the Bell standards.
•
The International Consultative Committee on Tele-
phony and Telegraphy (CCITT) set the V. (Vee Dot)
recommendations.
Data modems use these standards for speeds up to
28,800 bps:
•
Bell 103
•
Bell 212A
•
CCITT V.21, V.22, V.22bis,V.23, V.32, V.32bis, and V.34.
Many modems support all of these standards. Older
modems may support some, but not all of these stan-
dards.

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 correc-
tion 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
30 How modems work
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. These protocols
are performed by microcode. Microcode is software
built into the modem. 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 proto-
cols. 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
application that includes many different software
protocols. Different bulletin boards use different pro-
tocols.