Tuesday, July 29, 2008

Getting online with GPRS in India

In order to get online using GPRS you would need:

  1. A device capable of handling GPRS; usually a cell phone or a GPRS modem. I've used a Nokia 3110c with some success. This particular cellphone model offers a USB port using which it may be connected to a host computer with a compatible USB cable.
  2. A subscription to GPRS service. Contact your cell service provider on the way to enable this on your cell phone. The examples below describe the configuration needed for using AirTel's GPRS service.
  3. The appropriate configuration for your OS. This post describes how to use GPRS using FreeBSD and NetBSD.

FreeBSD Configuration

  1. Ensure that the relevant drivers are present in the kernel. The Nokia 3110c is supported by the stock umodem(4) driver. Use kldload(8) to load this into your kernel if it is not already present:
    # kldload umodem
  2. On connecting the cellphone to the computer, you should see a message similar to the following on the console (or in /var/log/messages):
    ... kernel: ucom0: Nokia Nokia 3110c, rev x.yy/a.bb, addr 2, iclass 2/2
    ... kernel: ucom0: data interface 2, has CM over data, has break
    ... kernel: ucom0: status change notification available
    If all goes well, a new device entry such as /dev/cuaU0 should be present under /dev.
  3. Add the following to /etc/ppp/ppp.conf (note the leading whitespace on most lines):
     set device /dev/cuaU0
     enable dns
     set phone "*99***1#"
     set authname "airtel"
     set authkey "airtel"
      \"\" AT OK-AT-OK ATQ0V1E1S0=0&C1&D2+FCLASS=0 OK \
      AT+CGDCONT=1,\\\"IP\\\",\\\"airtelgprs.com\\\" OK \\dATDT\\T \
      add default HISADDR
      set ifaddr

    For the curious, the ppp(8) manual page fully describes the syntax and semantics of the configuration commands above.

    Note that the contents of the authname and authkey configuration items do not seem to matter. You could put the name of your pet dog there if you wish.

  4. Invoke the ppp(8) daemon in the usual way:
    % ppp airtel
    ppp> dial
    ... the prompt changes as PPP setup progresses

    And you should be done.

NetBSD Configuration

In NetBSD as in FreeBSD, this particular cellphone model is supported by the umodem(4) driver. Once recognized, the modem will be accessible using a path name such as /dev/ttyU0.

The following recipe uses the pppd(8) daemon. This recipe requires the NetBSD kernel to have been compiled with in-kernel PPP support.

  1. Create a file /etc/ppp/peers/airtel with the following contents:
    ttyU0 115200
    connect '/usr/sbin/chat -v -f /etc/ppp/chat-airtel -T "*99***1#"'
    user "airtel"
    password "airtel"
  2. Create a file /etc/ppp/chat-airtel with the following contents:
    "" "AT"
    OK "ATQ0V1E1S0=0&C1&D2+FCLASS=0"
    OK AT+CGDCONT=1,\"IP\",\"airtelgprs.com\"
    OK \dATDT\T
    TIMEOUT 40
  3. Start ppp using:
    # pppd call airtel

    Monitor /var/log/messages for error messages.

  4. Once the connection is setup, IP addresses for the ISP's DNS servers would be placed in /etc/ppp/resolv.conf. Add these to /etc/resolv.conf if needed.

Connection performance

In my experience, connection speeds vary widely from a few bytes/second to a peak of a few KB/sec. Connection latencies also tend to be variable, ranging from a few hundred milliseconds upwards. There are frequent long pauses, a minute or more in duration, during which no traffic flows (though the connection remains up).

Thus GPRS based connectivity is best used for batch tasks such as downloading and sending email.

1 comment:

  1. I didn't need the AT+CGDCONT=1, line.
    However, I got this:

    Aug 14 18:14:37 maverick pppd[415]: Could not determine remote IP address: defaulting to

    Seems to be faster although I have no numbers.