Googled and found this at http://www.rcrwireless.com/lte/lte-attach-procedure.html
The attach procedure for LTE is combined with (default) PDN (IP) connectivity. This can be called default PDN (IP) connectivity as default (or basic or initial) PDP context activation. This default connectivity is always of non-guaranteed bit rate type and is called “default bearer”. Additional PDP contexts or sessions (with different QoSs than default bearers) are called “dedicated bearers”. To get things started, the user equipment (UE) needs to initially register with the network to receive services that require registration. This registration is the main component of network attachment.
According to Wired n Wireless, establishing a default EPS bearer during the network attachment will enable IP connectivity for the UE of the EPS.
The attach procedure may trigger one or multiple dedicated bearer establishment procedures to designate a dedicated EPS bearer for that UE. The UE may also make a request for an IP address allocation during the LTE attach procedure. Once this happens, the next steps begin and bearers are largely involved. Wired n Wireless explains the bearer request and response messages below:
Create Default Bearer Request: MME —-> S-GW
MME selects a S-GW and allocates an EPS bearer identity for default bearer associated with UE. Next, it sends a “create default bearer request.” This message provides a subscribed APN AMBR for the APN. MSISDN is included if the attach type indicates handover. Then, selection mode indicates that a subscribed APN was selected. Charging characteristics indicates which kind of charging the bearer context is liable for. hen it sends Create Default Bearer Request with following IEs: IMSI, MSISDN, MME TEID for Control Plane, PDN GW address, PDN Address, APN, RAT Type, Default EPS bearer QoS, PDN Type, APN-AMBR, EPS Bearer Identity (EBI), Protocol COnfiguration Options, Handover Indication, ME identity, User Location Information (ECGI), MS Info Change Reporting Support Indication, Selection Mode, Charging Characteristics, Trace Reference, Trace Type, Trigger ID, OMC identity, Maximum APN Restriction, Dual Address Bearer Flag, Protocol over S5/S8.
Create Default Bearer Request : S-GW ——–>PDN-GW
The S-GW creates a new entry in its EPS bearer table and sends a create default bearer request message to PDN GW, indicated by the PDN GW address received in the previous step. This message contains IMSI, MSISDN, APN, S-GW address for User Plane, S-GW TEID of the User Plane, S-GW TEID of the Control plane, RAT Type, Default EPS bearer QoS, PDN Type, PDN Address, Subscribed APN-AMBR, EPS bearer identity, Protocol Configuration Options, Handover Indication, ME Identity, user Location Information (ECGI), MS Info change Reporting SUpport Indication, Selection Mode, Charging Characteristics, Trace Reference, Trace Type, Trigger ID, OMC Identity, Max APN Restriction
Dual Address Bearer Flag.
Create Default Bearer Response : PDN-GW ——> S-GW
The P-GW creates a new entry in its EPS bearer context table and generates a charging ID. This new entry allows the P-GW to route user plane PDU’s between the S-GW and the packet data network, allowing them to start charging. The PDN-GW returns a create default bearer response with following IE’s: PDN-GW address for the user plane, PDN GW TEID of the user plane, PDN GW TEID of the control plane, PDN Type, PDN address, EPS Bearer Identity, EPS bearer QoS, Protocol Configuration Options, Charging ID, Prohibit payload compression, APN Restriction Cause, MS Info Change Reporting Action, APN AMBR.
Create Default Bearer Response : S-GW ——–> MME
Finally, S-GW returns a create default bearer response to MME.
Call Flow -