The User Data Header (UDH) is a block of octets added to the beginning of the user data part of an SMS message. Due to the way an SMS message is limited to a maximum size of 140 bytes, the available user data length is reduced when a UDH is included. Its purpose is to inform the mobile handset of the type of data and data length of the user data part of an SMS message. Generally, the UDH is either used in combination with binary content known as smart messaging for ringtones, operator logos, WAP Push and vCards, or it is used for concatenated SMS. Binary content can also span across multiple message parts and therefore the UDH may contain both a smart message and a concatenation header.
