Owner`s manual

PN 88300150
4/3/01 49
Auto-Reliable Mode (&E1)
In Auto-Reliable mode, the MT2834MR will, during the handshaking procedures at the start of the on-line
connection, automatically determine whether or not the modem with which it is communicating is using
the error correction. If the MT2834MR determines that the other modem is using V.42, it will switch itself
into the Reliable (V.42) mode of operation. If it is determined that the other modem is not using V.42, the
MT2834MR will remain in Normal mode.
The method the MT2834MR uses to determine if the V.42 modem involves the use of a “Link Request”.
When the MT2834MR is in Auto-Reliable mode and originates a call, it goes through normal handshaking
procedures just like any dial-up modem. After establishing the on-line connection, the MT2834MR
transmits a Link Request message to the answering modem. If the answering modem replies with an
appropriate V.42 acknowledgment response, the MT2834MR switches into Reliable mode. Otherwise, it
will stay in Normal mode. This V.42 handshaking procedure generally takes about five seconds.
When operating in V.42 Reliable mode, the MT2834MR uses its memory, or buffer, to store data as it is
received. During periods of error-caused retranmissions or compression slowdowns, this buffer may fill
up. To prevent buffer overflow and subsequent loss of data, the modem uses flow control to signal the
computer attached to its RS232C port that the modem buffer is close to being full. This causes the
computer to pause in its data transmission until the modem is able to empty its buffer sufficiently to
accept more data, at which time the modem signals the computer that it may resume transmission.
5.7 Introduction to Flow Control
Flow control refers to the techniques used by computer devices to stop and restart the flow of data from
each other. Flow control is necessary so that a device does not receive more data than it can handle. In
the case of the MT2834MR, there is a need for flow control in both directions. Flow control for data
passing from your computer to the modem is called Modem-Initiated Flow Control and flow control for
data passing from the modem to your computer is called Computer/Terminal-Initiated Pacing (see Figure
5-1).
The MT2834MR supports both hardware and software Modem Initiated Flow Control, and, on the
Computer/Terminal-Initiated Pacing side, supports hardware and software flow control, and a special
version used by Hewlett Packard compatible systems called ENQ/ACK Pacing. The MT2834MR allows
hardware and software pacing to be passed through the modem to the other end of the link so that your
computer or terminal can control data start/stop activity through your modem. This is called “Xon/Xoff
Pass-Through”.
<to be supplied>
Figure 5-1. Flow Control and Pacing
To state it simply, “Flow Control” is something the modem does to the computer, while “Pacing” is
something the computer does to the modem.
5.8 Modem-Initiated Flow Control
When operating in the V.42 Reliable mode, the MT2834MR uses its memory, or buffer, to store data as it
is received. During periods of error-caused retransmissions or compression slowdowns, this buffer may fill
up. To prevent buffer overflow and subsequent loss of data, the modem uses flow control to signal the
computer attached to its RS232C port that the modem buffer is close to being full. This causes the
computer to pause in its data transmission until the modem is able to empty its buffer sufficiently to
accept more data, at which time the modem signals the computer that it may resume transmission.
The MT2834MR gives you two choices for methods of modem-initiated flow control. (You also have a
third choice, which is to not use flow control at all.) One choice is “Xon/Xoff”, which uses special
characters in the data transmissions. The other is “Hardware Flow Control”, which uses the CTS output