One of the design principles of LTE was to streamline signaling as much as possible in order to simplify the system as much as possible and to execute procedures as quickly as possible. An interesting result of this is how messages of different protocol entities can be bundled into a single message that is sent over the air interface. Take the attach process as an example where the mobile device is ultimately assigned an IP address. In addition to reducing the number of steps required compared to GSM and UMTS, a single message is used to transmit the following towards the end of the procedure:
- An RRC Reconfiguration Message to establish a data channel (a DRB) for the user data;
- An Attach Accept message to tell the mobile that the attach was successful;
- An Activate Default Bearer Request message to tell the mobile to activate a logical bearer (for which a physical air interface bearer has just been configured with the RRC message above).
And all in one message on the air interface! In UMTS, those were all separate procedures with separate message exchanges. Pretty streamlined I would say! For details see 3GPP TS 23.401.