Default Bearer of a UE (i.e UE1) can be Pre-emptable by another UE (i.e UE2) in Congestion?

Question

In LTE, when UE is first time attached to a network, it is usually allocated a default non-GBR bearer. Can this bearer be converted to a GBR bearer, without releasing the UE context at eNodeB?

Answer 1

Default bearer is associated with the ARP parameters. ARP parameters are considered while establishing a new bearer request.
If enough radio resources are not available to fulfill the request, it refuses to establish the bearer and send failure response.
Specification does not mandate which UE should be removed from the network in case of congestion.
It is totally implementation and requirement dependent. In few places you may have a subscribers of different classes such as gold, silver, bronze etc while in few places each user is considered at similar level.

Answer 2

Allocation-Retention-Priority consists Three parameters
Priority-Level
Pre-emption-Capability
Pre-emption-Vulnerability

In case of Resource congestion at RAN side,bearer having high priority level can grab the resource of the lower priority level.
So,as per your example,if the default bearer UE1 has lower priority as compared to bearer of UE2,than UE2 bearer will grab the resource of UE1 bearer.

PRE-EMPTION_CAPABILITY_ENABLED (0)
This value indicates that the service data flow or bearer is allowed to get resources that were already assigned to
another service data flow or bearer with a lower priority level.

PRE-EMPTION_VULNERABILITY_ENABLED (0)
This value indicates that the resources assigned to the service data flow or bearer can be pre-empted and
allocated to a service data flow or bearer with a higher priority level. This is the default value applicable if this
AVP is not supplied.

For details about this refer specs:ETSI TS 129 212 V8.14.0 (2012-01)