Avocent ESP Serial Hub, Installer/User Manual, Page: 31 / 96

Chapter 3: Connection Methods, Attributes and Configurable Features 21
Modem emulation
ESP serial hubs may be used to replace costly POTS-line calls with cost-effective raw TCP
sessions. To accomplish this, the ESP hub provides a connection method called modem emulation.
Two ESP hubs work in conjunction to emulate the connection. One ESP hub will establish the
connection in response to AT commands. The other ESP hub will emulate a modem that has
received an incoming call.
The modem emulation connection method enables an ESP hub serial port to emulate modem dial
and answer functionality. The hub presents a Hayes-compatible modem interface to the attached
serial device by accepting AT modem commands and performing the required functions.
This connection method is valid only on RS-232 ports. For information about available adaptors
that can be used with modem emulation, see
Adaptors on page 77.
Table 3.5 describes this method’s configurable items for each serial port.
Idle TCP
buffering
Enables or disables buffering of received serial data while waiting for a TCP connection to be
established. When enabled, the most recent 20K bytes of data are held in an internal buffer.
Once the connection is established, the buffered serial data is sent to the remote endpoint of
the TCP connection.
The default value is disabled.
Table 3.4: Raw TCP Client Connection Method Configurable Items (Continued)
Item Description
Table 3.5: Modem Emulation Connection Method Configurable Items
Item Description
Enable/
Disable
Enables or disables the modem emulation connection method on the port. (See Exclusivity
and restrictions on page 15.)
The default value is disabled.
Allow
Incoming
Calls
Enables or disables incoming calls (that is, raw TCP sessions originating remotely). When
enabled, the ESP hub will listen for inbound raw TCP connection requests. See
Incoming
calls on page 28 for more information.
The default value is enabled.
Auto Answer
(Used only when Allow Incoming Calls is enabled.) Enables or disables auto answer. When
enabled, a connection will be accepted without interaction with the serial port. When disabled,
the ESP hub will send the appropriate string (RING or 2, depending on the ATV command or
configured Verbose Response value). Upon receipt of an ATA modem command, the call will
be accepted.
The default value is enabled.