LTE Default Bearer concept

If you have started looking into LTE you will not doubt soon come against the concept of the default bearer. Even though in the begining I was a bit confused about it, it turns out it is quite simple to understand it, especially if you have some background in UMTS or GPRS as it is very similar to a Primary PDP context.

The picture above highlights the differences in terminology quite well, and as you can see besides the naming conventions the concepts are identical.

In UTMS/GPRS, the UE requests the establishment of a PDP Context. This creates a logical end to end "pipe" between the UE and the GGSN. The process of establishing a PDP context also assigns the UE an IP address. Once a primary PDP context is established, secondary PDP contexts can also be established which use the same IP address but require different QoS. Even though these secondary PDP contexts have been defined in the specs since the beginning of GPRS, they were never really implemented to my knowledge.

In LTE, the UE requests the establishment of a PDN Connection. This creates a logical end to end "pipe" between the UE and the PGW. The UE is also assigned an IP address (can be IPv4 or IPv6) and the default bearer is setup. This default bearer is always best effort. Unlike UMTS/GPRS where the UE can request a PDP context at any time, in LTE the PDN connection for the establishment of the default bearer is always requested at power on. This ensures that the UE always has an IP address which in a packet based system like LTE is a necessity. If now the UE requires some QoS that is different than best effort, a dedicated bearer can be setup. This will be a necessity when things like voice services are offered over LTE but could also be used when for example a streaming session is setup, or a Skype/Facetime session etc.

Finally if you are wondering how will the network know that a dedicated bearer is needed, this is done by deep packet inspection, most likely by the PCRF (Policy and Charging Rules Function) node.