EPS stands for Evolved packet system. whole LTE system is developed to provide all types services through packet switching. To achieve this GTP (GPRS tunneling protocol is being used). GTP is application layer protocol which differentiate session based on TEID (tunnil end identifier). UDP is used as a transport layer protocol. A bearer is a pipe that carry data from one node to another node. There are two types of GTP messages used one for control plane and one for user plane. EPS bearer has following components like TEID , Transport layer address, etc.
Please see following figure:
radio bearers GTP bearers GTP bearers
UE <------------------------------>enb<---------------------------------->SGW <---------------------->PGW<------->Internet
The pipes, which are carrying data from PGW to UE and vice versa called EPS bearer.
when data reaches at PGW from external world it process and create a GTP-U packet and sent to SGW.
SGW does the same thing it remove the GTP-U header received from PGW and create a new GTP-U header then forward packet to eNB. Once eNB receives GTP-U packet it removes header GTP-U header and then PDCP layer at enodeb performs IP header compression and ciphering and map to logical channel then message sent to UE through radio channel. radio bearers identified by bearer Identity (1-32) and logical channel identity(3-11).
Default bearer is one of the main bearer which is created at the time of initial attach procedure or at the time of new PDN connection. Default bearer represents a PDN connection and exists until UE gets detached from network or UE initiated PDN dis-connectivity explicitly or network force fully trigger release for the default bearer due to policy control. Default bearer is a non-GBR bearer and provide always on IP connectivity.
Dedicated bearer is created when the requested service can't be fulfilled through default bearer. Some services required a high level of QoS like voice call. so network create a dedicated bearer with required QoS . Dedicated bearer may be Non-GBR or GBR depend of qci(QoS class identifier) value. dedicated bearer can be created/release based on requirement but default bearer is created only all on IP connectivity and released only at the time of detach/PDN disconnection.
Note: Every dedicated bearer is always associated with default bearer(which is know as linked EPS bearer).