i am thinking of setting up an sms center which send sms' to subscribers
Kind of a vague description, can you elaborate what are you actually looking for ?
If you are talking about Short Message Service Centres (SMSC), these basically receive messages via the SMPP (Short Message Peer to Peer) Protocol and forward the messages to the service providers via SS7 protocol via switches etc.
If on the other hand you just want to be an aggregator (middle man) who has connections to multiple SMSCs and sells them to your clients, you need more like an SMS Gateway, There are already a few Open Source SMS Gateways available, a popular one is Kannel . However It is not based on Java but in C. But if you are planning to implementing the gateway yourself in Java you could start by learning about SMPP. Also you will need an SMPP API which will do the job of handling all low level networking tasks so that you do not have to worry about constructing the SMPP protocol data units (PDUs).
Although there are many available I recommend OpenSMPP for that, cause it has been around for a while.