In the cellular phone industry, mobile phones and their networks sometimes support concatenated short message service (or concatenated SMS) to overcome the limitation on the number of characters that can be sent in a single SMS text message transmission (which is usually 160).
When the feature works properly, it is nearly transparent to the user, appearing as a single long text message.
Previously, due to incompatibilities between providers and lack of support in some phone models, there was not widespread use of this feature.
The number of parts that a multi-part or PDU mode SMS message may contain depends technically upon a header message but mostly upon the device sending or receiving the SMS and also upon the service provider.
A UDH can be used for various purposes and its contents and size varies accordingly, but a UDH for concatenating SMSes look like this: It is possible to use a 16 bit CSMS reference number in order to reduce the probability that two different concatenated messages are sent with identical reference numbers to a receiver.