This document will explain about the new bearer allocation on top of default bearer.
First we will cover the default bearer creation and then we will cover the allocation of dedicated bearer.
Default Bearer Creation:
Here I will elaborate more on GTP perspective for creating default bearer. Please refer the following points when attach request initiated by UE,
1. UE initiate a “Attach Request” towards eNB with the UE information such as IMSI, UE Core Network Capability, Attach Type, NAS information, RRC parameters etc.
2. ENB derives MME from RRC parameter and forward the “Attach Request” i.e. S1-MME message (Initial UE Message) towards MME with the Selected Network, CSG ID (Common subscriber group), and TAI+ECGI information.
3. MME will initiate a GTP-C signaling message towards S-GW to proceed with attach. This will help the EPS (Evolved Packet system) to create default bearer in the network. MME initiates Create Session Request with the following important information
a. IMSI
b. MME’s F-TEID for control plane (MME IP + TEID (Tunnel ID)). This will be used for future communication with MME.
c. PGW’s G-TEID for Control plane (P-GW IP + TEID (here it will be zero)). This will be used by S-GW to communicate with P-GW.
d. Bearer Context to be created. This is the bearer information to be created at MME. This will include the default bearer ID with TFT, QOS.
e. APN
f. Recovery
4. S-GW will process the “Create Session Request” initiated by MME and create the UE, eNB, and MME context and modify the Create session Request and send it to P-GW. Create session Request from S-GW will have the following important information
a. IMSI
b. S-GW’s F-TEID for control plane (S-GW IP + TEID (Tunnel ID)). This will be used for future communication with S-GW.
c. Bearer Context to be created. This is the bearer information to be created at S-GW. This will include the default bearer ID with TFT, QOS, S-GW user plane F-TEID
d. APN
e. Recovery
5. Once P-GW receives “Create Session Request” , it will create UE, S-GW context and communicate with PCRF for QOS and APN resolve. If that succeed, P-GW will respond back with “Create Session Response” towards S-GW. The Create Session Response will have following information:
a. Cause. This value will be used for Success/failure for the Create Session Request.
b. P-GW’s F-TEID for control plane (P-GW’s IP + TEID ). This information will be used for future signaling message communication with P-GW.
c. PAA (PDN Address Allocation).
d. Bearer Contexts Created. This information element will have Default EPS bearer ID, P-GW user plane F-TEID, TFT, Bearer Qos, Charging ID.
e. Charging GW
f. Recovery
6. Once S-GW receives “Create Session Response”, it will create P-GW context and forward/process the “Create Session Response “with the following information.
a. Cause. This value will be used for Success/failure for the Create Session Request.
b. S-GW’s F-TEID for control plane (S-GW’s IP + TEID ). This information will be used for future signaling message communication with S-GW.
c. PAA (PDN Address Allocation).
d. Bearer Contexts Created. This information element will have Default EPS bearer ID, P-GW user plane F-TEID, TFT, Bearer Qos, Charging ID.
e. Charging GW
f. Recovery
7. Now if downlink data comes from PDN it will get buffered in S-GW as user plane eNB information shared with S-GW.
8. MME shares the eNB information as part of attach procedure with “Modify Bearer request” with the following information
a. Bearer Contexts to be modified. This IE will have eNB’s user plane F-TEID.
9. Once S-GW receives the “Modify Bearer request” with eNB information. It will respond back with “Modify bearer Response”.
10. Now downlink data can be sent to eNB because S-GW has the information of eNB.
11. But uplink data can possible after step 6.
Dedicated Bearer Allocation:
Once the default bearer created the UE or network can opted for the dedicated bearer. To get more service from network, UE or NW may initiate the dedicated bearer allocation.
1. UE Initiated Dedicated Bearer Allocation:
a. UE Request “Bearer Resource Allocation” message towards MME for allocating dedicated bearer.
b. MME sends “Bearer Resource Command” with the default bearer ID to S-GW for a dedicated bearer allocation. That means MME commands to the P-GW to initiate a “Create Bearer Request” to start the dedicated bearer allocation. This command will provide the default bearer ID. This message will have the following information:
I. EBI i.e. Default EPS bearer ID.
II. PTI
III. TAD (TFT rule + TFT ID)
c. Once S-GW receives the “Bearer Resource Command” then the SGW or the PGW will determine if the UE is requesting an Allocation operation of bearer resources for a traffic flow aggregate based on the TFT operation code and the packet filter ID value in the Traffic Aggregate (TAD) IE and/or the presence of the EPS Bearer ID IE.
d. PCRF entity has to check the APN and QoS details for bearer approval.
e. After P-GW received “Bearer Resource Command”, it will initiate “Create Bearer Request” towards S-GW for dedicated bearer allocation. This request will have the following information.
I. EBI i.e. Default EPS bearer ID.
II. Bearer Context. This IE will contain the dedicated bearer id with value “0” (as MME is going to allocate the bearer ID), TFT, P-GW user plane F-TEID etc.
f. Once S-GW receives “Create Bearer Request”, it will propagate the message towards MME for dedicated bearer allocation.
g. MME does a bearer setup request with eNB after receiving “Create Bearer request” from S-GW.
h. Once the bearer setup request completes, MME sends back “Create Bearer Response” with the following information.
I. Cause. This is for success or failure.
II. Bearer Contexts. EBI. This ID will be assigned by MME with a value. eNB User plane F-TEID etc.
i. Once S-GW receives “Create Bearer Response” then it update the UE,eNB and MME context for the dedicated bearer and sends the “Create Bearer Response” towards P-GW.
2. NW Initiated Dedicated Bearer Allocation:
If the service provided by NW is not sufficient in a bearer then P-GW opted for dedicated bearer initiation.
a. P-GW will initiate “Create Bearer Request” towards S-GW for dedicated bearer allocation. This request will have the following information.
I. EBI i.e. Default EPS bearer ID.
II. Bearer Context. This IE will contain the dedicated bearer id with value “0” (as MME is going to allocate the bearer ID), TFT, P-GW user plane F-TEID etc.
b. Once S-GW receives “Create Bearer Request”, it will propagate the message towards MME for dedicated bearer allocation.
c. MME does a bearer setup request with eNB after receiving “Create Bearer request” from S-GW.
d. Once the bearer setup request completes, MME sends back “Create Bearer Response” with the following information.
I. Cause. This is for success or failure.
II. Bearer Contexts. EBI. This ID will be assigned by MME with a value. eNB User plane F-TEID etc.
e. Once S-GW receives “Create Bearer Response” then it update the UE,eNB and MME context for the dedicated bearer and sends the “Create Bearer Response” towards P-GW.
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