Default Bearer Setup

Default Bearer Setup

Default Bearer Setup (aka E-UTRAN Initial Attach) procedure is followed to register UE to the network. The following diagram shows the UE attach procedure. This diagram only shows attach procedure for GTP based tunneling.

1. The UE initiates the Attach procedure by sending the Attach Request message to the eNodeB with following identities.

• IMSI or GUTI
• TAI (if available)
• UE Capabilities - indicates supported NAS and AS security algorithms by UE
• Attach Type - indicates EPS or combined EPS/IMSI
• ESM Message Container
• RRC Parameters

The ESM Message Container contains the following identities.

• Request Type
• PDN Type - indicates requested IP version (IPv4, IPv6 or IPv4v6)
• Protocol Configuration Options (PCO) - included by UE and sent transparently to MME to indicate Address allocation preference of UE (eg. after establishing default bearer using DHCP)
• Ciphered Options Transfer flag - must be set if PCO or APN or both are to be sent by UE
• KSI_ASME (if valid EPS security parameters available)
• NAS Sequence number (if valid EPS security parameters available)
• NAS-MAC (if valid EPS security parameters available)
• additional GUTI
• P-TMSI

The RRC parameters indicate the Selected Network and old GUMMEI.

2. The eNodeB derives the MME from RRC parameters. If the MME is not associated with the eNodeB, the eNodeB selects an MME using MME Selection function. The eNodeB forwards the Attach Request message in Initial UE message with following identities.

• Selected Network - indicates the PLMN
• TAI + ECGI - location of the cell

3. If the selected MME does not match the MME specified in GUTI, the new MME retrieves the old MME identity from the GUTI and sends an Identification Request message which includes the old GUTI and the Attach Request message. The old MME verifies the Attach Request message and then responds with Identification Response message. If the UE is not known in the old MME, it responds back with an appropriate error cause.

The additional GUTI allows the new MME to find any existing UE context stored in the new MME.

3a. If the UE is unknown in old and new MME, the new MME sends an Identity Request message to the UE to request the IMSI.

3b. The UE responds back with an Identity Response message which includes the IMSI.

If the Attach Request message was not integrity protected, or if integrity check failed, then authentication and NAS security setup to activate integrity protection are mandatory. The MME selects the NAS Security functions to protect all NAS messages. During this phase, the MME may also request ME (Mobile Equipment) Identity from the UE.

4a. The MME may send the Identity Check Request message to the EIR to check if the mobile equipment is not stolen or black-listed. This message includes the following identities.

• ME Identity
• IMSI

4b. The EIR responds with an Identity Check Ack message. Depending upon the result, the MME decides whether to continue with Attach procedure or reject the UE.

If the Ciphered Options Transfer flag was set by UE, the MME now requests Ciphered Options (PCO, APN or both) using Ciphered Options Request message and the UE responds with a Ciphered Options Response message. The PCO contains the username/password with PAP/CHAP parameters.

5. If there is no UE context in the MME, or if the UE sent an IMSI, or if the UE provided an old GUTI which does not refer to the new MME, then the MME sends an Update Location Request message to the HSS with following identities.

• MME Identity
• IMSI
• ME Identity
• MME Capabilities - indicates MME's support for regional access restrictions functionality
• Update Type - indicates Attach procedure

6. The HSS responds back with an Update Location Ack message which includes IMSI and Subscription Data. The Subscription Data contains one or more PDN subscription contexts. Each PDN subscription context contains an EPS subscribed QoS Profile and subscribed APN-AMBR. The MME validates the UE's presence in the TA (Tracking Area). If due to regional restrictions the UE is not allowed in the TA, or if subscription checking fails, the MME rejects the Attach Request with an appropriate cause. If the checks are successful, the MME creates a context for the UE.

The HSS can (optionally) provide PDN (IPv4 or IPv6) address to the UE which is included in the PDN subscribed context. The PDN subscription context can also contain the PDN GW identity.

If the UE did not provide an APN, the MME uses the PDN GW corresponding to the default APN provided by the HSS. If the UE provided an APN, the MME might use the PDN GW corresponding to that APN. If the HSS does not provide any PDN GW, the MME selects a PDN GW based on PDN GW selection function.

7. The MME selects a Serving-GW (S-GW) based on S-GW selection function and allocates an EPS Bearer Identity for the default bearer for the UE. Then it sends a Create Session Request message to the selected S-GW with following identities.

• IMSI
• MSISDN - if received from HSS
• MME TEID for Control Plane
• PDN-GW Address
• PDN Address (optional)
• APN
• RAT Type - E-UTRAN/GERAN/UTRAN
• Default EPS Bearer QoS
• PDN Type - IPv4, IPv6, IPv4v6
• APN-AMBR
• EPS Bearer Identity
• Protocol Configuration Options
• Handover Indication - if Request Type indicates "handover" instead of "Attach"
• ME Identity
• User Location Information - ECGI
• MS Info Change Reporting support indication
• Selection Mode - indicates whether subscribed APN received from HSS or non-subscribed APN received from UE was selected
• Charging characteristics
• Maximum APN Restriction
• Dual Address Bearer flag - set if PDN Type is IPv4v6
• protocol type on S5/S8 interface

8. The S-GW creates a new entry in its EPS Bearer table and sends a Create Session Request message to the PDN-GW with following entities.

• IMSI
• MSISDN - if received from HSS
• S-GW Address for User Plane
• S-GW TEID for User Plane
• S-GW TEID for Control Plane
• PDN Address (optional)
• APN
• RAT Type
• Default EPS Bearer QoS
• PDN Type
• APN-AMBR
• EPS Bearer Identity
• Protocol Configuration Options
• Handover Indication
• ME Identity
• User Location Information
• MS Info Change Reporting support indication
• Selection Mode
• Charging characteristics
• Maximum APN Restriction
• Dual Address Bearer flag - set if PDN Type is IPv4v6

9. If dynamic PCC is deployed, the PDN GW performs IP-CAN Session Establishment procedure with PCRF and obtains the default PCC rules for the UE. This may trigger establishment of multiple Dedicated Bearers in association with the Default Bearer. The PDN-GW (PCEF) provides the following identities to the PCRF.

• IMSI
• UE IP address
• User Location Information
• Serving Network
• RAT Type
• APN-AMBR
• Default EPS Bearer QoS

The PCRF may modify APN-AMBR and QoS parameters associated with the default bearer in response to the PDN-GW.

If dynamic PCC is not deployed and PDN-GW applies local QoS policy, this step is still followed to provide UE IP address to the PCRF. If UE IP address is not available when PDN-GW performs IP-CAN Session Establishment procedure with the PCRF, the PDN-GW initiates an IP-CAN Session Modification procedure when UE IP address is available.

10. The PDN-GW creates a new entry in its EPS Bearer table and generates a Charging ID. The PDN-GW sends a Create Session Response message to the S-GW with the following identities.

• PDN-GW address for User Plane
• PDN-GW TEID for User Plane
• PDN-GW TEID for Control Plane
• PDN Type
• PDN Address - if the PDN-GW received Create Session Request with PDN (UE IP) address (assigned by HSS), then the PDN-GW allocates the same IPv4 or IPv6 address to the UE
• EPS Bearer Identity
• EPS Bearer QoS
• Protocol Configuration Options
• Charging ID
• Prohibit Payload Compression
• APN Restriction
• Cause
• MS Info Change Reporting Action (Start) - if this is sent to S-GW, then the S-GW must report to PDN-GW whenever a UE's location change occurs
• APN-AMBR

11. The S-GW sends a Create Session Response message to the MME with following identities.

• S-GW address for User Plane
• S-GW TEID for User Plane
• S-GW TEID for Control Plane
• PDN Type
• PDN Address
• EPS Bearer Identity
• EPS Bearer QoS
• PDN-GW addresses and TEIDs for S5/S8 interface
• Protocol Configuration Options
• Prohibit Payload Compression
• APN Restriction
• Cause
• MS Info Change Reporting Action (Start)
• APN-AMBR

The MME determines the UE-AMBR to be used by the eNodeB based on the subscribed UE-AMBR and the APN-AMBR for the default APN.

12. The MME creates an Initial Context Setup Request message which contains an Attach Accept message. The Initial Context Setup Request message contains the following-

• Attach Accept message
• AS security context information for the UE
• Handover Restriction list
• EPS Bearer Identity
• EPS Bearer QoS
• UE-AMBR
• S-GW TEID for User Plane
• S-GW address for User Plane

The Attach Accept message contains the following-

• Session Management Request message
• APN
• (new) GUTI
• Protocol Configuration Options
• NAS Sequence Number
• NAS-MAC

The Session Management Request message contains EPS Bearer QoS parameter QCI and APN-AMBR.

13. The eNodeB sends an RRC Connection Reconfiguration message including EPS Radio Bearer Identity and Attach Accept message.

14. The UE sends an RRC Connection Reconfiguration Complete message to the eNodeB.

15. The eNodeB sends an Initial Setup Context Response message to the MME which includes the following identities.

• eNodeB TEID for User Plane
• eNodeB address for User Plane

16. The UE then sends a Direct Transfer message to the eNodeB which includes the Attach Complete message. The Attach Complete message contains the following identities.

• EPS Bearer Identity
• NAS Sequence number
• NAS-MAC

The Attach Complete message also contains the Session Management Response message.

17. The eNodeB forwards the Attach Complete message in an Uplink NAS Transport message. The Attach Complete message also contains the Session Management Response message.

18. Upon reception of Initial Setup Context Response message and Attach Complete message, the MME sends a Modify Bearer Request message to the S-GW including the following identities.

• EPS Bearer Identity
• eNodeB TEID for User Plane
• eNodeB address for User Plane
• Handover Indication

19. If Handover Indication is included in Modify Bearer Request message, the S-GW sends a Modify Bearer Request message to the PDN-GW to prompt the PDN-GW to tunnel packets from non-3GPP IP access to 3GPP IP access and immediately routing packets for default and dedicated bearers.

20. The PDN-GW acknowledges with Modify Bearer Response message to the S-GW.

21. The S-GW acknowledges by sending Modify Bearer Response message to the MME. After the MME receives the Modify Bearer Response message, and if the PDN-GW is different from the PDN-GW Identity it received in the PDN subscription context from the HSS, the MME sends Notify Request message to the HSS including the APN and PDN-GW Identity. The HSS stores the APN and PDN-GW Identity, and sends a Notify Response message to the MME.

UE-AMBR and APN-AMBR

APN-AMBR

The APN-AMBR defines the total bit rate that is allowed for all non-GBR bearers associated with a specific APN. The APN-AMBR limits the total non-GBR traffic for an APN, independent of the number of PDN connections and non-GBR bearers that are opened for that APN. For example, if an operator provides an APN for Internet, the operator may then limit the total bandwidth for that APN.

UE-AMBR

The UE-AMBR is defined per subscriber and defines the total bit rate allowed to be consumed for all non-GBR bearers of a UE. The subscription profile contains the UE-AMBR. The eNodeB enforces the UE-AMBR.

There are different AMBR values for uplink and downlink directions - UL APN-AMBR, DL APN-AMBR, and UL UE-AMBR, DL UE-AMBR.