Default Bearer, Dedicated Bearer... What exactly is bearer ?

While trying to get a better understanding of VoLTE and IMS , I came across bearers stuff which I thought might be useful to share with you guys. Before we talk about what default and dedicated bearer is, let's understand what bearer itself is :
 
Bearer is just a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. Some flow of data might be provided guaranteed bit rate while other may face low transfer.  In short, bearer is a set of network parameter that defines data specific treatment   e.g. Person A will always get at least 256 Kbps download speed on his LTE phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Now let me talk briefly about Dedicated and Default bearer one by one.

Default Bearer in LTE

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. Default bearer  is best effort service. Each default bearer comes with an IP address. UE can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer. 

Dedicated Bearer

To put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services

Example

Usually LTE networks with VoLTE implementations has two default and one dedicated bearer

Default bearer 1: Used for signaling messages (sip signaling) 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)

Why dont we use dedicated bearer for ims signaling as well ?
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

How it is decided to send voip traffic (for example) on dedicated bearer?
Both UE and eNB has TFT which has rules for certain services. For example, incase of VoLTE voip traffic, the rule are defined on the basis of protocol number , destination network ip network etc

How is dedicated bearer linked to default bearer?
The value of "Linked EPS bearer identity" defined in setup info of dedicated bearer is used to link dedicated bearer to default bearer

Please also visit Quality of Service (QoS) in LTE to get more better understanding on bearers and QoS