What is the UDH?
UDH stands for User Data Header and is specialised information that contains information relating to the message which is interpretted by the receiving device.
An example use of the UDH is message concatenation in which the UDH of each SMS tells the receiving device that the SMS forms part of a long message and what position it hols within that message.
Nokia Smart Messaging, EMS (Enhanced Messaging Service) also use the UDH to deliver non-textual content (e.g. ringtones, logos etc) and ETSI GSM 03.40 specifies how the UDH is formatted.