BINARY SMSs
Basically SMSs are small
number of packed bytes sent over the operator networks. Many people will
speculate the Text Messages are the only types of SMSs that exist, well they
are one of the many types that ideally exist, hence the term ‘texting’
So how do SMSs work and what
are the basic constructs of an SMS
SMSs use the concept of
‘ports’ just as a standard internet sockets does;
SMS messages have limits of
140-160 characters (depending on encoding type);
The body is not the only
thing you can edit in SMSs, there’s also UDH (User Header Data)
So J
Those Ports
Say you go to the my website http://0x7678.com/ you basically
called to port 80 of the webserver by convention. The connection will be
initialized on port 80 and then switched to a higher port to let other users
access the same port of the web server. Port 80, as stated by IANA refers to the
HTTP protocol, this means that a server, which is able to understand HTTP
protocol request, will be awakened and will be ready to answer and process HTTP
requests. The same happens with SMS messages. You can send an SMS to a specific
port of a phone and you will wake up a specific service on that device. Now,
just as , not all computers have a standard service (e.g web server) also not
all mobile devices have services listening to ports. (this is manufacturer
specific, so you will need to check your phone what is enabled to accept.
NOW >> BORING STUFF OUT
… WE GO ON TO BINARY SMSs
Ok long story short going
through the whole bit about how SMSs work is too tedious
But here we go ….
Ok so SMSs on default use 7
bits to handle a character. This means that you can write in an SMS only
characters on the basic ASCII char table … i.e 127 characters. If you want to
go onto more complex stuff and send more ‘interesting’ characters , then a
group of 8 bits is needed and the table of available chars get bigger. The
available space is 1120 bits per SMS, no more, no less. You can have 160 chars
using 7 bits or 140 chars using 8 bits.
NB: note this carefully … you
will find this letters looking alike but are very different, "
É " and this "
È " are very very different the
first is contained in the 7 bit basic ASCII and the second contained in the 8
bit larger ‘interesting’ table, so if you
use it without checking it, you wont have enough space so be very
precise.
UDH (User Data Header)
The UDH is what a ‘high level
developer’ can set while to do something more than a simple “text message”. A
UDH is very useful because you can send “invisible text messages” to mobile
application (where to “mobile applications” I mean those running on mobile
devices for example) or you can tell a device that the message will contain
special information. It’s very similar to an XML file: you have to tell the
parser what you are sending, and the content following the prolog which will be
handled by the parser itself.
The UDH is mainly used to
specify what ports our client (phone) will send SMS to. Its made by a set of
hex number which describe:
<how long the UDH
is><the format used to specify ports numbers><the port number
length><destination port number><source port number>
As a practical example, say I
want to create a UDH to send a WAP PUSH. Where the standard destination port
for WAP pushes is 2948, the UDH will be:
06 05 04 0B 84 23 F0
Where:
06 means
“hey the read the following 6 bytes”
05 is the
format for numbers, in this case hexadecimal numbers
04 will tell
the UDH that each port is represented using 4 characters
0B84 is the
destination port, 2948 (decimal representation) or 0B84 (hexadecimal
representation)
23F0 is the
source port, 9200 (decimal representation) or 23F0 (hexadecimal
representation).
NOTE: Use a
simple calculator to convert decimal numbers to hex: select “Dec”, put 2948 in
the calculator, then press the button “Hex”.
NOW REALLY >> BINARY SMSs
A binary SMS is an
XML-formatted textual SMS, which has been transformed with WBXML (a tag
transformer), this means that for each XML tag, a binary byte is associated.
E.g , the tag <SI> is converted as the binary character 
When you think WHY WBXML?
WBXML transformation is
smaller in the number of generated bytes than the verbose textual XML file
itself.
Note: many
tags are converted to bytes, but sometimes also contents (such as URL addresses
“OC” is more generic and stands for http:// so you can write the URL
in two ways.
<span>0D0x7678.com
</span>
or
<span>0Cwww.0x7678.com</span>
The first uses 9 chars (0D is
one byte), the second 13 chars
So far so good ….. ?
· Decide what we want to send
· Find the docs about that topic
· Find the XML structure of the message to be sent
· Customize the XML
· Convert the XML to WBXML
· Prepare the UDH
· Send the UDH and the BODY
NB
Binary SMSs have two
indicators whilst been sent, either a “Service Indication <SI>” or
“Service Load <SL>” the two have a difference only in <SI> prompts
the owner to the phone that content is coming through and you need to authorize
it...
[Cont. in Part 2....]
