Moxa W406-T-CE [51/74] Gprs connection

W315A/325A Linux User's Manual Managing Communications

4-17

GPRS Connection

GPRS is a packet-switched technology, which means that multiple users share the same transmission channel.

In addition, GPRS transmits only when there is outgoing data. The available bandwidth can be dedicated solely

to data communication when needed. In general, a GPRS network can be viewed as a special IP network that

offers IP connectivity to IP terminals.

The concepts of making GPRS connection are the same as the dial up service using PPP (refer to the section

“Dial-up Service-PPP” in chapter 4 for details). As the pppd daemon starts, it prepares the serial port settings

for communication. Then it runs an external program called chat, which sends AT commands to GSM/GPRS

module to establish connection. AT commands are just like the language between chat program and

GSM/GPRS module. The chat program waits for the string CONNECT to establish connection. After the

connection is established, pppd takes over the process to encapsulate TCP/IP packets.

The W315A/325A embedded computers provide a ready-to-use sccript gprscmd for fast connections (refer to

“Connecting to GPRS network” in chapter 2). We recommend that users use the gprscmd command

instead of rewriting their own connection command. Users who need to customize their own connection for

specific needs can edit the following files to meet their own connection standard.

Configuring the options for pppd

The option settings for pppd is located at /etc/ppp/peers/chtgprs. You can enable or disable an option by

removing or deleting the “#”

# File: /etc/ppp/peers/chtgprs

/dev/ttyS1 # modem port used

115200 # speed

defaultroute # use the cellular network for the default route

noipdefault

usepeerdns # use the DNS servers from the remote network

#nodetach # keep pppd in the foreground

#nocrtscts # hardware flow control

#lock # lock the serial port

noauth # don't expect the modem to authenticate itself

#local # don't use Carrier Detect or Data Terminal Ready

#persist

#demand

modem

#debug

# Use the next two lines if you receive the dreaded messages:

# No response to n echo-requests

# Serial link appears to be disconnected.

# Connection terminated.

lcp-echo-failure 4

lcp-echo-interval 65535

connect "/bin/chat -v -f /etc/chatscripts/chtgprs-connect"