Accessibility KPI in LTE – RF Optimization Part I
Accessibility KPI in LTE
Typically, the accessibility KPI refers to the initial establishment of the LTE E-RAB. Not only does this include the successful establishment of the default EPS bearer, but also takes into account the RRC Connection success rate as well as the successful establishment of an S1 signaling connection.
The RRC is used when an idle UE first communicates with the network, in order to set up a signaling path (an RRC connection) between the UE and the eNB. The resulting set of signaling radio bearers allows the UE & the eNB to exchange RRC and other signaling messages quickly and efficiently.
The RRC connection defines SRB 1, which runs over a Dedicated Control Channel (DCCH). SRB 1 is used for all subsequent RRC messages, as well as the initial set of NAS messages to the MME.
Once ciphering has been enabled, SRB 2 is established, and is used for any NAS messages from then on. Both SRB 1 and SRB 2 are dedicated bearers, and are identified by the UE’s assigned C-RNTI.
As part of the Attach procedure, the eNB and the MME have an S1-MME bearer path dedicated to the UE that is uniquely defined by the eNB UE S1AP ID and the MME UE S1AP ID.
The EPS bearer includes the following:
- EPS radio bearer between the UE and the eNB;
- S1-U EPS access bearer between the Enb and the S-GW;
- S5/S8 EPS bearer between the S-GW and the P-GW.
3GPP provides definitions for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) KPIs. Based on this specification, the initial EPS bearer success rate is defined as the product of the RRC success rate, the S1 Signaling Connection Establishment success rate, and the initial Evolved Radio Bearer (E-RAB) establishment success rate.
The RRC Connection establishment success rate is the ratio of successful RRC Connection establishments over the number of RRC Connection establishment attempts.
In most networks, the denominator would be pegged on reception of an RRC Connection request message and the numerator would be incremented when the eNodeB receives an RRC Connection Setup Complete message. Note that the cause value in the RRC Connection message would allow this KPI to be segregated based on cause.
The S1 signaling establishment success rate is based on the ability of the eNB and the MME to set up an S1 signaling connection dedicated to the specific UE.
The S1SIG.ConnEstabAtt counters are typically based on the number of initial UE Message messages sent by the eNB, while the S1SIG.ConnEstabSucc variable is based on the number of DL NAS transport messages and includes the assigned MME UE S1AP ID.
The initial bearer establishment success rate is defined based on a message exchange between the eNB and the MME. The eNB reception of the Initial Context Setup Request from the MME will typically be identified as an E-RAB establishment attempt.
The transmission of Initial Context Setup Response to the MME typically confirms the default bearer has been successfully established. In addition, a second KPI is also defined to measure the success rate of added EPS bearer establishments.