E1 Interface and E1AP in 5G NR
The E1 interface is a control interface connecting the Control and User plane entities of CU in a gNB or en-gNB.
Definition
The E1 interface provides means for interconnecting a gNB-CU-CP and a gNB-CU-UP of a gNB within an NG-RAN, or for interconnecting a gNB-CU-CP and a gNB-CU-UP of an en-gNB within an E-UTRAN. The E1 Application Protocol (E1AP) supports the functions of the E1 interface (defined in Section 5 of TS 38.460) by signalling procedures defined in the present document.
E1AP is developed in accordance with the general principles stated in TS 38.401 [2] and TS 38.460 [3].
Other Important Points regarding E1 Interface
· The E1 interface is a control interface and is not used for user data forwarding.
· From a logical standpoint, the E1 is a point-to-point (p2p)interface between a gNB-CU-CP and a gNB-CU-UP. (A p2p logical interface should be feasible even in the absence of a physical direct connection between the endpoints.)
· the E1 interface separates Radio Network Layer and Transport Network Layer;
· the E1 interface enables the exchange of UE-associated information and non-UE-associated information;
E1 interface protocol structure
Figure 7.1–1 of 38.460 (also shown above) shows the protocol structure for E1 interface.
· The Transport Network Layer (TNL) is based on IP transport, comprising the SCTP on top of IP.
· The application layer signalling protocol, referred to as E1AP (E1 Application Protocol). The functions of E1AP are described in TS 38.460 section 5.
Difference Between E1-interface and E1AP
E1 is the interface itself (i.e. means of communication between CU-CP and CU-UP), where as E1AP encompasses the actual implementation of functions that cumulatively make the E1 interface.
E1AP Services
I would say that E1 interface is actually made up from the services provided by the E1AP. E1AP provides the signalling service between the gNB-CU-CP and the gNB-CU-UP that is required to fulfil the E1AP functions described in clause 7 of 38.463. E1AP services are divided into two groups:
The Services mentioned above are actually implemented via some functions which are described in 38.460 Section 5. Some of those functions are as below
A. E1 Functions
There are several functions that the E1 interface /E1AP should support.
A.1 E1 Interface Management Functions
This is a Non UE-associate service provided by the E1 interface.
I would say that the Interface management Functions have several subfunctions (see TS 38.460 Section 5 for further details). Some of the important sub-functions are as below
1. error sub-functions
The error indication function is used by the gNB-CU-UP or gNB-CU-CP to indicate to the gNB-CU-CP or gNB-CU-UP that an error has occurred.
2. reset sub-function
The reset function is used to initialize the peer entity after node setup and after a failure event occurred. This procedure can be used by both the gNB-CU-UP and the gNB-CU-CP.
3. Setup sub-function
The E1 setup function allows to exchange application-level data needed for the gNB-CU-UP and gNB-CU-CP to interoperate correctly on the E1 interface. The E1 setup is initiated by both the gNB-CU-UP and gNB-CU-CP.
4. gNB-CU-UP Configuration Update sub-function and gNB-CU-CP Configuration Update sub-function
These two functions are located in the respective entities. The gNB-CU-UP Configuration Update and gNB-CU-CP Configuration Update functions allow to update application level configuration data needed between the gNB-CU-CP and the gNB-CU-UP to interoperate correctly over the E1 interface.
The E1 setup and gNB-CU-UP Configuration Update functions allow the gNB-CU-UP to signal its capacity information to the gNB-CU-CP.
5. E1 gnB-CU-UP Status Indication sub-function
The E1 gNB-CU-UP Status Indication function allows to inform the overloaded or non-overloaded status over the E1 interface.
A.2 TEID allocations
This is the UE-associated service provided by the E1 interface
· The gNB-CU-UP is responsible for the allocation of the F1-U UL GTP TEID for each data radio bearer.
· The gNB-CU-UP is responsible for the allocation of the S1-U DL GTP TEID for each E-RAB and the NG-U DL GTP TEID for each PDU Session.
· The gNB-CU-UP is responsible for the allocation of the X2-U DL/UL GTP TEID or the Xn-U DL/UL GTP TEID for each data radio bearer.
A.3 E1 bearer context management function
This also seems like a UE-specific service provided by the E1 interface.
The establishment of the E1 bearer context is initiated by the gNB-CU-CP and accepted or rejected by the gNB-CU-UP based on admission control criteria (e.g., resource not available).
The modification of the E1 bearer context can be initiated by either gNB-CU-CP or gNB-CU-UP. The receiving node can accept or reject the modification. The E1 bearer context management function also supports the release of the bearer context previously established in the gNB-CU-UP. The release of the bearer context is triggered by the gNB-CU-CP either directly or following a request received from the gNB-CU-UP.
Some of the responsibilities of this function are:
- This function is used to setup and modify the QoS-flow to DRB mapping configuration. The gNB-CU-CP decides flowto- DRB mapping and provides the generated SDAP and PDCP configuration to the gNB-CU-UP. The gNB-CU-CP also decides the Reflective QoS flow to DRB mapping. The function is also used to send to the gNB-CU-UP the alternative QoS Parameters Sets when available for a QoS flow. For each PDU Session Resource to be setup or modified, the SNSSAI, shall be provided in the E1 bearer context setup procedure.
- This function is used for the gNB-CU-CP to send to the gNB-CU-UP transport layer information to be used for data forwarding e.g. during handovers.
- Others (see 5.1.2 of TS 38.460)
References
[1] TS 38.460 talks about E1 interface in a summarized manner.
[2] TS 38.463 talks about E1 Application Protocol (E1AP)
