Raspberry Pi sim900 Default Internet Access

I have set up a ITEAD sim900 GSM module to interface with raspberry pi. I believe I have established a gprs connection to AT&T though wvdial as I get these results.

    --> WvDial: Internet dialer version 1.61
    --> Initializing modem.
    --> Sending: AT+CGDCONT=1,"IP","Broadband"
    AT+CGDCONT=1,"IP","Broadband"
    OK
    --> Modem initialized.
    --> Sending: ATDT*99#
    --> Waiting for carrier.
    ATDT*99#
    CONNECT
    --> Carrier detected.  Starting PPP immediately.
    --> Starting pppd at Thu Aug 14 05:49:20 2014
    --> Pid of pppd: 2794

I have been looking all over the internet for some answers to a few questions that I have, but I can't seem to find any. Any help with the following questions will be greatly appreciated! Thanks!

I have three questions, and some may be stupid as I am VERY new to this field.

Am I actually connected to AT&T's GPRS network?
How can I make this module (serial port /dev/ttyAMA0) my default internet connection? What I mean is I want all internet traffic routed through this modem(web surfing, email etc.). I am connected to the Raspberry via ssh so I have to have either ethernet or wifi active to access the computer--I am currently using ethernet. After I connect through wvdial in the way shown above, and disable all other internet sources I have no access. It seems to still be looking to the active ethernet port for data(I could be wrong).
For my project I need to have the sim900 modem as the internet access point, but I also need to be able to connect to a LAN via wifi that has no internet access. Is this possible?

Finally i got the ( raspberrypi + ppp + gprs/gsm-modem ) working.

Some notes before start:

Make sure the power supply you used for raspberrypi is exact 5V and it can provide at-least 2A current without voltage drop-out.The SIM900 power-source must be 3.3V 2A
Set the SIM900 baud rate to 115200 via: AT+IPR=115200
Check the modem serial peripheral via: $ screen /dev/ttyAMA0 115200 type AT<enter> it will echo: OK. Hit ctrl+a k y to exit.

/etc/ppp/options-mobile

ttyAMA0
115200
lock
crtscts
modem
passive
novj
defaultroute
replacedefaultroute
noipdefault
usepeerdns
noauth
hide-password
persist
holdoff 10
maxfail 0
debug

Create the /etc/ppp/peers directory:

$ mkdir /etc/ppp/peers
$ cd /etc/ppp/peers

/etc/ppp/peers/mobile-auth

file /etc/ppp/options-mobile
user "your_usr"
password "your_pass"
connect "/usr/sbin/chat -v -t15 -f /etc/ppp/chatscripts/mobile-modem.chat"

/etc/ppp/peers/mobile-noauth

file /etc/ppp/options-mobile
connect "/usr/sbin/chat -v -t15 -f /etc/ppp/chatscripts/mobile-modem.chat"

Create the /etc/ppp/chatscripts directory:

$ mkdir /etc/ppp/chatscripts

/etc/ppp/chatscripts/mobile-modem.chat

ABORT 'BUSY'
ABORT 'NO CARRIER'
ABORT 'VOICE'
ABORT 'NO DIALTONE'
ABORT 'NO DIAL TONE'
ABORT 'NO ANSWER'
ABORT 'DELAYED'
REPORT CONNECT
TIMEOUT 6
'' 'ATQ0'
'OK-AT-OK' 'ATZ'
TIMEOUT 3
'OK' @/etc/ppp/chatscripts/pin
'OK\d-AT-OK' 'ATI'
'OK' 'ATZ'
'OK' 'ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0'
'OK' @/etc/ppp/chatscripts/mode
'OK-AT-OK' @/etc/ppp/chatscripts/apn
'OK' 'ATDT*99***1#'
TIMEOUT 30
CONNECT ''

/etc/ppp/chatscripts/my-operator-apn

AT+CGDCONT=1,"IP","<apn-name>"

/etc/ppp/chatscripts/pin.CODE

AT+CPIN=1234

/etc/ppp/chatscripts/pin.NONE

AT

/etc/ppp/chatscripts/mode.3G-only

AT\^SYSCFG=14,2,3fffffff,0,1

/etc/ppp/chatscripts/mode.3G-pref

AT\^SYSCFG=2,2,3fffffff,0,1

/etc/ppp/chatscripts/mode.GPRS-only

AT\^SYSCFG=13,1,3fffffff,0,0

/etc/ppp/chatscripts/mode.GPRS-pref

AT\^SYSCFG=2,1,3fffffff,0,0

The SYSCFG line in the mode.* files is device-dependent, and likely Huawei-specific, So You may use the mode.NONE file if your modem is SIM900. *

/etc/ppp/chatscripts/mode.NONE

AT

Make some symbolic links:

$ ln -s /etc/ppp/chatscripts/my-operator-apn /etc/ppp/chatscripts/apn
$ ln -s /etc/ppp/chatscripts/mode.NONE /etc/ppp/chatscripts/mode
$ ln -s /etc/ppp/chatscripts/pin.NONE /etc/ppp/chatscripts/pin

If you have to enter credentials use mobile-auth

$ mv provider provider.example
$ ln -s /etc/ppp/peers/mobile-noauth /etc/ppp/peers/provider

Check syslog in another console:

$ tail -f /var/log/syslog | grep -Ei 'pppd|chat'

Finally issue the pon command to see the result:

$ pon

The base instruction : https://wiki.archlinux.org/index.php/3G_and_GPRS_modems_with_pppd

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