Motorola V186 - Cell Phone - GSM System Planner Manual page 117

Communications
MDLC over IP - Standard Modem
To avoid system setup for each modem/radio which supports packet data, a general
concept has been introduced for, whereby IP can connect to any modem or radio
supporting packet data.
A standard modem supporting packet data is a modem which requires an AT command
set to configure and PPP to initiate. It can connect to a PC using Microsoft Standard
Modem and RAS setup. A modem configuration file can be downloaded into the RTU
specifying the exact command set needed by the modem/radio. A default AT command
set is used in case this file is not downloaded. The same concept is used for circuit data
modem over dial port.
For information of downloading modem configuration files, refer to ACE3600 STS
Advanced Features Manual.
Connection to Standard modem is made using RS232 PPP over the operator
infrastructure. Since the operator infrastructure connects to LAN as well, a LAN-
connected RTU can communicate directly with these RTUs over that infrastructure, if
enabled by the operator.
Some modems have an internal fixed IP address for PPP connection. If so, only one
modem of the same vendor can be connected to RTU, since they all have the same IP
address. Other modems such as Motorola g18 do not have an internal IP address; in this
case several MDLC over IP ports can be configured to connect with them.
To verify if more than one modem can be used, try to connect two modems and see if you
get an error message: "IP Address in use by other ports".
MDLC over Null Modem
The RTU can connect to any device using PPP.
This connection is made using PPP and is basically the same as MDLC over Standard
modem. When the RTU is powered up, it sends a client string and expects a client-server
response. Only when it gets that response will it initiate PPP and poll for CD signal
(carrier detect). CD is constantly being polled, and if it drops, PPP is disconnected. The
user can opt to ignore CD using Advanced Link Layer parameters in the site
configuration. In this case, PPP is initiated upon power up. When connected, CD is polled
in order to stay connected. If it drops, then PPP is reconnected. By default, the RTU acts
as a Windows Null modem connection. It sends a client string and expects a client-server
response before initiating PPP. The user can override this behavior by downloading a
modem configuration file.
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