# 陳毅 - 讀書會 - 21/07/14

:::info
- [x] 閱讀 Netmanias-04-LTE Identification III-Session and Bearer Identifiers
:::

## EPS Session
當 EPS Session 建立完成，這代表了三件事:
1. User 已經選擇好要通向哪一個 PDN
2. 該 PDN 中所使用的 IP Address 已經分配給使用者
- Static IP address allocation
- Dynamic IP address allocation
3. QoS 與計費策略已經確定
4. EPS Bearer 已經建立

## EPS Bearer

EPS session 負責傳送與處理 IP 封包的 flow (UE - P-GW - PDN)， EPS Bearer 則為 LTE 網路中封包傳送的管道。
EPS Bearer 由下面三個 bearer 串聯而成:
- Data Radio Bearer (DRB)
EPS bearer established over LTE-Uu interface. User traffic (IP packet) is delivered through a DRB.
Different DRBs are identified by their DRB ID, which is allocated by an eNB.

- S1 bearer
EPS bearer established over S1-U interface. User traffic is delivered through a GTP tunnel. Different
S1 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the
endpoints (eNB and S-GW) of the GTP tunnel.
- S5 bearer
EPS bearer established over S5 interface. User traffic is delivered through a GTP tunnel. Different S5
bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints
(S-GW and P-GW) of the GTP tunnel

## Types of EPS Bearers


- Additional EPS bearer can be established if the default EPS bearer itself is not sufficient enough to obtain QoS.

- The default one is never removed and keeps the user staying connected to the network unless the user detaches from the network.

- Dedicated EPS bearers are linked to a default EPS bearer. The linked bearers are represented by a Linked EPS Bearer Identity (LBI), indicating they are all associated with the same default EPS bearer.

## ID to identify PDN: PDN ID (APN)
An APN is a combination of a network ID and an operator ID. The network ID is used
when identifying PDNs such as Internet or Corporate VPNs or identifying services like IMS that the PDN
provides.
An APN is provisioned to an HSS as subscription information at the time of a user’s subscription.

## EPS Bearer IDs
> 由 MME 分配

An EPS bearer is virtual connection set between a UE and a P-GW to deliver user traffic over the LTE network.
Different bearers in the EPS bearer are identified by 4-bit EPS bearer IDs.


## E-RAB ID
> 由 MME 分配

E-RAB ID 與 EPS Bearer ID 存放相同的數值，預設的 EPS Bearer 會保持 UE 與網路之間的連線，當 UE 為空閒狀態時 (沒有封包流動)， E-RAB 會被停用且只留下 S-GW 通往 P-GW 的 S5 Bearer 。
直到有新的使用者連線， E-RAB 才會被重新建立，讓使用者的資料可以從 UE 通往 P-GW 。

## DRB ID
> 由 eNB 分配

DRB ID 與 EPS Bearer ID, E-RAB ID 存放相同的數值:
> During the setup procedure of the EPS bearer, the MME
requests the eNB for E-RAB setup, the eNB creates DRB for communication with the UE by assigning a DRB ID
and selecting logical channel configuration parameters based on the required QoS.

## ID to identify the endpoints of a GTP tunnel: TEID

> 上圖取自 **Netmanias-01-LTE Network Architecture - Basic**

GTP tunnels are identified by Tunnel Endpoint Identifiers
(TEID), in 32-bit integer, of two endpoints in uplink and downlink.
- DL S1 TEID
> 由 eNB 分配
- DL S5 TEID
> 由 S-GW 分配
- UL S1 TEID
> 由 S-GW 分配
- UL S5 TEID
> 由 P-GW 分配

## ID to connect a default EPS bearer and dedicated EPS bearers: LBI
> 由 MME 分配

因應不同的 QoS 需求， MME 可能會配置其他專用的 EPS Bearer 供 UE 使用，假設預設的 EPS Bearer 與專用的 EPS Bearer 都導向同一個 PDN ，那後者會有一個 LBI ，用來存放 Default EPS Bearer ID 。