A default bearer is created when UE attach to network for the first time, and default bearer remains as long as UE is attached with network. An UE can have more than one default bearer. Each default bearer comes with an APN name and PDN address (IP address) and it has assigned a QCI value 5 to 9 (i.e. Non GBR). When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached.
Dedicated bearer is an additional bearer which linked with a default bearer. Some LTE services wants guaranteed bit rate for that we need dedicated bearer. Dedicated bearer can be QCI value 1 to 9 (i.e. GBR or non-GBR), whereas default bearer can only be non-GBR. Dedicated Bearer uses the PDN address of Default Bearer it is linked to.
We can have a maximum of 11 Bearers or a combination of Default & Dedicated bearer where total number of bearers should not exceed 11.
Usually LTE networks with VoLTE implementations has two default and one dedicated bearer
Default bearer 1 – Used for signalling messages (sip signalling) related to IMS network. It uses QCI 5
Dedicated bearer – Used for VoLTE VoIP traffic. It uses QCI 1 and is linked to default bearer 1
Default bearer 2 – Used for all other smartphone traffic (video, chat, email, browser etc.)
Then the questions may come in mind, Why don’t we use dedicated bearer for IMS signalling? IMS network is separate network from normal internet. And it comes with its own APN. Therefore we need separate default bearer for IMS network. This also helps in separating IMS traffic from normal internet traffic as well.