The controlling of a MODEM can be done by using instructions or AT commands. Here term AT in the command is the short form of an Attention. The command line used in every modem starts with ‘AT’ otherwise ‘at’ so these commands are named as AT commands.
For more information on AT command, refer to this link
TST stands for Ternary Search Tree. It is a special trie data structure where the child nodes of a standard trie are ordered as a binary search tree.
For more information on TST, refer to this link