One of the big changes brought about by LTE is that when the mobile device connects to the network it also implicitly gets an IP address. This is called "Default EPS Bearer Activation". What seems to be a trivial change at first is actually a whole new way of thinking compared GSM and UMTS.
In 2G and 3G systems, the mobile registers to the network first and in order to get an IP address, a "PDP context activation procedure" has to follow. This is also what is known as establishing a "packet call". I never very much liked the term "packet call" which was probably created because of the still mainly circuit switched thinking at the time. So with the default bearer activated right from the start, the "packet call" and all the legacy behind it has become history.
To build on the default bearer there is also a procedure in LTE called "Dedicated EPS Bearer Activation" which is initiated for the network. But what is it needed for, most people ask at first, the mobile already has an IP address!? True, the mobile already has an IP address but the default bearer comes without any quality of service guarantees. For VoIP, IMS, VoLGA and other real time streaming applications, it would be good to have some QoS, especially on the air interface. This ensures that the base station and other network components deliver IP packets for those applications with a higher priority. Also on the mobile side, such IP packets should get a higher priority than other packets, especially when the bandwidth is limited. And that's what is done with a dedicated bearer.
In the network, the dedicated bearer is connected to a Traffic Flow Template (TFT), a concept that already exists in GSM and UMTS. In this template, think of it as a list of ip addresses and tcp/udp port combinations, describes which IP packets should be given a higher priority. The TFT is also forwarded to the mobile during the dedicated bearer activation and it helps the protocol stack to put the IP packets to or from a specific TCP/UDP port and/or IP addresses into a special QoS queue that is treated with a higher priority.
No need for the mobile to have an extra IP address for this higher priority traffic, as the protocol stack uses the Traffic Flow Template information to decide what to do with each IP packet. In other words for those who are familiar with GSM and UMTS, a dedicated bearer activation is pretty similar to a Secondary PDP context activation in 2G and 3G that can be used by the IMS for example to ensure real time data is delivered promptly.
For the details have a look at 3GPP TS 24.301, chapter 6.4 and table 18.104.22.168 a bit further down for the message details.