SMS contains more than just 160 characters of text. This page explains what else is there. It is still under construction, but comments are welcome!
SMS - Mobile Terminated MT SMS can be up to 163 bytes:
Header - 1 byte This identifies the type of message:
Service Centre Timestamp - 7 bytes.
This is YYMMDDHHMMSSZZ where ZZ is the timezone. Each nibble is swapped, e.g. 79=97. The timezone is in 1/4 hour units (somewhere near Pakistan is offset from GMT by something-and-a-quarter hours).
Originator Address - up to 12 bytes TON/NPI Length Address - nibble-swapped
Protocol Identifier - 1 Byte
Data Coding Scheme - 1 Byte
User Data Length - 1 Byte
User Data - up to 140 Bytes. This is coded according to the Data Coding Scheme.
SMS - Mobile Originated
Last Updated: 12th March 1997