USSD BROWSING

The text-based internet services that Tilde users like to play 
with are all very interesting, but there's also more to text than 
the internet. If I was cooler I might now go on to talk about the 
Hellschreiber or something, but instead it's Unstructured 
Supplementary Service Data, or USSD.

I discovered this (or at least the details) after my mobile 
broadband internet provider (Telstra, the big ex state-owned telco 
that us Australians love to hate) discontinued the simple URL that 
you could visit to view your remaining internet data and expiry. 
Now you've got to create an account and log in, and I don't really 
want to do that because I'm kind-of ripping them off to get my 
cheap internet. But you could still get a balance via the #100# 
code on a mobile phone, and that turns out to be one of these USSD 
codes. It's a special protocol for phones to query the telco's 
system for a text response. The request code is of the structure 
*[code][*[selector]...]#, where eg. (with Telstra) *100# gives you 
a menu, then *100*2# selects the second option from that menu, and 
you can go down deeper through the menus with more *[number] 
sequences before the hash.

Looking through old forum posts, it seems that Telstra has had a 
couple of systems for presenting interesting content via this 
service, from the time before phones could access the internet. You 
used to be able to pull up news, weather, and some other stuff. 
Potentially an easy way to get some daily plain-text info without 
trying to strip it out of bloated websites. But I'm late to the 
party, these days this is all you get from *100#:

---------------------------------

Pre-Paid Mobile Broadband
Exp 25 May 2021

1. Recharge: credit/debit card
2. Recharge: voucher
3. Balance Details
4. Account Management

---------------------------------

Or *150# shows you your phone number. So yeah, back to just finding 
out my remaining internet data then...

I'm using a USB mobile broadband modem (one of the old style ones 
that doesn't just present as a USB ethernet adapter) so I need to 
use AT commands, where it turns out there's a special AT+CUSD 
command for USSD "browsing". There may be pre-baked ways to do this 
with software like ModemManager, but I was looking for a solution 
for the software already installed on the 8MB flash of my router 
running OpenWrt, so here's the script I came up with:

            -------------------

#!/bin/sh
#Get Telstra account usage/balance data
#Converts from byte codes because Telstra doesn't output strings in plain text for some reason.
# USSD code either specified on command line, or value set below is used.

#USSD Code:
# "*100#" shows the menu, adding "*3" selects the third menu item (can repeat for deeper menu levels)
[ "$1" ] && BALCODE="$1" || BALCODE='*100*3#'
TMP=/tmp/baldata.txt
MODEM=/dev/ttyUSB1

OLDTTY="`stty -g`"

stty -F $MODEM 9600 raw
chat -e -t 20 -r "$TMP" \
REPORT CUSD: \
'' AT \
OK\\r \
AT+CUSD=1,\""$BALCODE"\",15 \
OK\\r \
'' \
,72\\r \
 < $MODEM > $MODEM

stty "$OLDTTY"
echo 
"--------------------------------------------------------------------------"

if [ -f "$TMP" ]
then
 expr "`cat "$TMP"`" : '.*CUSD: [01],"\([0-9A-F]*\)",72' | sed 's/\([0-9A-F]\{2\}\)/\\\\\\x\1/gI' | xargs printf
 echo
 rm "$TMP"
else
 echo "--Error: No Response--"
fi

            -------------------

It turns out Telstra encode all the text in hexadecimal and then 
provide that as the ASCII string that's returned, so I found the 
sed + xargs + printf converter process on Stack Overflow (thank you 
Stack Overflow posters who always try to solve every problem with 
standard shell commands).

So with that I get a response like:

            -------------------

AT
OK
AT+CUSD=1,"*100*3#",15
OK

+CUSD: 
1,"0044006100740061003A00200032002E003900380036004700420020004500780070003A00320035002F00300035002F0032003000320031002000320033003A00350039003A00350039000A000A002800410045005300540029000A00300030002E00200048006F006D0065",72
--------------------------------------------------------------------
Data: 2.986GB Exp:25/05/2021 23:59:59

(AEST)
00. Home

            -------------------

The chat script doesn't have to echo to the terminal of course 
(remove the "-e" argument), but I quite like seeing the process 
going on. Also, I couldn't find any docs explaining what the ",72" 
at the end of the response string is supposed to mean, even though 
I'm using it to detect the end of the response in the chat script.
It seems to work reliably for Telstra.

As an extra bonus, here's a simple script for sending a text 
message from your computer via a mobile broadband modem:

            -------------------

#!/bin/sh
#Send SMS
# Syntax: sendsms.sh [phone number] [message]
MODEM=/dev/ttyUSB1
OLDTTY="`stty -g`"

stty -F $MODEM 9600 raw
chat -e \
'' AT \
OK\\r \
AT+CMGF=1 \
OK\\r \
AT+CMGS=\""$1"\" \
'> ' \
"$2"^z \
OK\\r \
 < $MODEM > $MODEM

stty "$OLDTTY"

            -------------------

 - The Free Thinker