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284 Chapter 5 • Communication Security: Web Based Services

Text Messaging and Short Message Service (SMS)

In addition to the IM software available for computers, text messaging also provides
the capability of sending electronic messages using software that’s bundled on many
different handheld technologies.These include wireless handheld devices like the
Blackberry, Palm Personal Digital Assistants (PDAs), two-way pagers, and cell
phones that support text messaging.Text messaging services may use protocols like
SMTP, but more often the Short Message Service (SMS) is used.

EXAM WARNING
Text messaging and SMSes are not on the Security+ exam. The exam
only focuses on IM technologies used on computers, such as Windows
Live Messenger (i.e., MSN Messenger), AIM, Yahoo! Messenger, Google
Talk, and other IM software that’s available. The information is provided
here for your personal reference, and to understand how the two tech-
nologies vary.

The SMS allows users of the service to send small electronic messages to one
another through a Short Message Service Center (SMSC).When a client sends a
text message, it is received by the SMSC, which attempts to send it on to the
intended recipient. If the recipient is unavailable (such as when their cell phone or
other device is turned off), the SMSC will do one of two things: it will either store
the message in a queue until the recipient goes online and then reattempt sending
it, or it will simply discard the message.
The messages sent using SMS are limited to 140 bytes, meaning that you can
send a message that contains 160 7-bit characters. However, despite the limitation,
longer messages can be sent using SMS in which each message is segmented over
multiple text messages. Information in the user data header identifies each message
as a segment of a longer message, so it can be reassembled by the recipient’s device
and displayed as a complete, longer message.
SMS also has the capability of sending binary data, and is commonly used to
distribute ring tones and logos to cell phone customers. Because of this capability,
programming code and configuration data can also be transmitted to a user’s device
using SMS, causing potential security problems.As we’ll see in the next section,
Java programs downloaded and installed on devices could contain malicious code,
as could other messages with attached files.

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